# JWST’s PEARLS: dust attenuation and gravitational lensing in the backlit-
galaxy system VV 191

William C. Keel Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35404, USA  Rogier A. Windhorst School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Department of Physics, Arizona State University, Tempe, AZ 85287-1504, USA  Rolf A. Jansen School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Seth H. Cohen School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Jake Summers  School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Benne Holwerda Department of Physics and Astronomy, University of Louisville, Natural Science 102, 40292 KY Louisville, USA  Sarah T. Bradford  Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35404, USA  Clayton D. Robertson Department of Physics and Astronomy, University of Louisville, Natural Science 102, 40292 KY Louisville, USA  Giovanni Ferrami School of Physics, University of Melbourne, Parkville, VIC 3010, Australia  ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia  Stuart Wyithe School of Physics, University of Melbourne, Parkville, VIC 3010, Australia  ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia  Haojing Yan Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA  Christopher J. Conselice Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.  Simon P. Driver International Centre for Radio Astronomy Research (ICRAR) and the International Space Centre (ISC), The University of Western Australia, M468, 35 Stirling Highway, Crawley, WA 6009, Australia  Aaron Robotham  International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, M468, 35 Stirling Highway, Crawley, WA 6009, Australia  Norman A. Grogin Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Christopher N.A. Willmer Department of Astronomy / Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA  Anton M. Koekemoer Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Brenda L. Frye Department of Astronomy / Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA  Nimish P. Hathi Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Russell E. Ryan, Jr. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Nor Pirzkal  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Madeline A. Marshall National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada  Dan Coe AURA for the European Space Agency (ESA), Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,vvv MD 21218, USA  Jose M. Diego Instituto de Física de Cantabria, Edificio Juan Jordá, Avenida de los Castros s/n, E-39005 Santander, Cantabria, Spain  Instituto de Física de Cantabria (CSIC-UC). Avenida. Los Castros, s/n. 39005 Santander, Spain  Thomas J. Broadhurst Department of Physics, University of the Basque Country UPV/EHU, E-48080 Bilbao, Spain  DIPC, Basque Country UPV/EHU, E-48080 San Sebastian, Spain  Ikerbasque, Basque Foundation for Science, E-48011 Bilbao, Spain  Michael J. Rutkowski Department of Physics and Astronomy, Minnesota State University, Mankato, Mankato, MN 56001  Lifan Wang Department of Physics and Astronomy, Texas A& M University, Mitchell Physics Building (MPHY) 4242 TAMU, College Station, TX 77843-4242  S. P. Willner Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  Andreea Petric Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21210, USA  Cheng Cheng Chinese Academy of Sciences, National Astronomical Observatories, CAS, Beijing 100101, China  Adi Zitrin Physics Department, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 8410501, Israel William Keel<EMAIL_ADDRESS>
(Received [)

###### Abstract

We derive the spatial and wavelength behavior of dust attenuation in the
multiple-armed spiral galaxy VV 191b using backlighting by the superimposed
elliptical system VV 191a in a pair with an exceptionally favorable geometry
for this measurement. Imaging using JWST) and HST spans the wavelength range
0.3-4.5µm with high angular resolution, tracing the dust in detail from 0.6 to
1.5 µm . Distinct dust lanes continue well beyond the bright spiral arms, and
trace a complex web, with a very sharp radial cutoff near 1.7 Petrosian radii.
We present attenuation profiles and coverage statistics in each band at radii
14-21 kpc. We derive the attenuation law with wavelength; the data both within
and between the dust lanes clearly favor a stronger reddening behavior
($R=A_{V}/E_{B-V}\approx 2.0$ between 0.6 and 0.9 µm, approaching unity by 1.5
µm) than found for starbursts and star-forming regions of galaxies. Power-law
extinction behavior $\propto\lambda^{-\beta}$ gives $\beta=2.1$ from 0.6-0.9
µm. $R$ decreases at increasing wavelengths ($R\approx 1.1$ between 0.9 and
1.5 µm), while $\beta$ steepens to 2.5. Mixing regions of different column
density flattens the wavelength behavior, so these results suggest a different
grain population than in our vicinity. The NIRCam images reveal a lens arc and
counterimage from a background galaxy at $z\approx 1$, spanning $90^{\circ}$
azimuthally at 2.8″ from the foreground elliptical galaxy nucleus, and an
additional weakly-lensed galaxy. The lens model and imaging data give a
mass/light ratio $M/L_{B}=7.6$ in solar units within the Einstein radius 2.0
kpc.

Interstellar dust extinction (837) — Spiral galaxies (1560) — Strong
gravitational lensing (1643)

††software:  MultiDrizzle/DrizzlePac (Koekemoer et al., 2003; Fruchter et al.,
2010), Python(AstroConda/Astropy) (Astropy Collaboration, 2013, 2018):
http://www.astropy.org, Matplotlib (Hunter, 2007), IRAF, IDL, lenstronomy
(Birrer & Amara , 2018; Birrer et al., 2021), EAZY (Brammer et al., 2008).
††facilities: James Webb Space Telescope (Near InfraRed Camera); Hubble Space
Telescope (Wide Field Camera 3) Mikulski Archive for Space Telescopes:
https://archive.stsci.edu

Submitted to AJ ]

1in

##  1 Introduction

Dust is a key ingredient in both observed and physical properties of galaxies.
It obscures and scatters stellar light, and enables star formation by acting
as a catalyst for molecular gas formation. It is the ingredient in the ISM
that most dramatically changes how we perceive a galaxy’s starlight, and the
site of much of the astrochemistry. On broad scales, correction for the
effects of dust is an important part of many techniques for estimating
astronomical distances.

The cumulative effects of grains are often traced via attenuation (combined
extinction and scattering) from the ultraviolet to near-infrared regimes, and
thermal emission in the far-infrared. Interpretation of the UV-optical-IR
(UVOIR) attenuation signature is limited by the need to understand the local
radiation field and three-dimensional mix of stars and dust, both of which are
features of the many routines designed to model the observed spectral energy
distributions (SEDs) of a few individual galaxies in great detail (e.g.,
Holwerda et al., 2012; De Looze et al., 2012; Gentile et al., 2015; Verstappen
et al., 2013; Hughes et al., 2014; Allaert et al., 2015; De Geyter et al.,
2014, 2015; Mosenkov et al., 2016, 2018) or populations with preset priors
(e.g. Driver et al., 2018; Bellstedt et al., 2020, 2021; Thorne et al., 2021;
Zou et al., 2022). In contrast to the extinction-based techniques for probing
dust effects, the use of far-IR emission is independent of the need for any
background source, but becomes progressively less sensitive for colder dust
temperatures, requires knowledge of the emissivity behavior of grains much
smaller than the wavelengths in question, and is generally observed at large
angular diffraction limits.

Star-by-star observations within the Milky Way, and recently elsewhere in the
Local Group, have been used to measure wavelength dependence of extinction
(absorption plus scattering). A widely-used parametrization is the CCM curve
(Cardelli et al., 1989), which is characterized by the ratio of total to
selective extinction in the $V$ band, $R=A_{V}/E_{B-V}$111where $A_{V}$ is the
extinction in magnitudes at $V$ and $E_{B-V}$ is the color excess - difference
in extinctions - between $B$ and $V$ bands, the strength of the 2175-Å bump,
and UV slope. In the near-infrared bands, a power-law form in wavelength has
been found useful at least from 0.5–5$\mu$m,
$A_{\lambda}\propto\lambda^{-\beta}$ with estimates of $\beta$ ranging from
2.07 ((Wang & Chen, 2019) to 1.65 (using the summary data in Table 3 of Zhang
& Yuan 2022). It has long been clear that there is not a universal extinction
law, since variations are seen between Galactic environments as well as
between our part of the Milky Way and, for example, the Magellanic Clouds.

In the context of almost all observations of distant galaxies, the extinction
law (reflecting the grain properties) is inextricably mixed with geometric
factors involving the relatave distributions of emitting sources and grains
(Calzetti 2001, Chevallard et al. 2013), so we are properly deriving the
attenuation behavior which combines both factors. Many studies have derived
attenuation laws from the color behavior of galaxy samples and radiative-
transfer models, or from models for the spectral energy distributions (SEDs)
of galaxies compared to their observed properties, sometimes with a luminosity
constraint including far-infrared emission (e.g., Salim et al. 2018). The
Calzetti et al. (1994) attenuation law, derived for the inner regions of
starburst galaxies, has proven to be of surprisingly wide applicability, a
reasonable fit to the behavior of whole galaxies as well as the outer regions
of some star-forming galaxies. However, its applicability into the infrared is
limited because, as defined, this relation goes to zero at 2.2 $\mu$m.

Generically, attenuation laws for galaxies or large parts of galaxies will be
flatter (have a smaller wavelength dependence) than the extinction laws
applying to individual stars, due to the mixing of regions of differing
extinction within the area measured (and possibly significant effects of
scattering into the beam). This is true for the Calzetti et al. (1994) form
versus the Cardelli et al. (1989) curve, and for the range of near-IR indices
$\beta=1.0-1.4$ found for integrated galaxy light by Salim et al. (2018)
compared to the steeper values $\beta\approx 1.8$ found for single stars as
noted above. Our HST and JWST data allow a new approach to this issue in the
$0.6-4.4$ $\mu$m range.

In earlier work, we have explored the use of backlit (occulting) galaxy pairs
to improve our understanding of attenuation in galaxies. In these cases, a
background galaxy backlights a large part of a foreground system, providing
spatially continuous light (ideally from a smooth E or S0 galaxy) so that all
the dust structure can be detected. While limited to a small (random) subset
of galaxies, this technique preserves the angular resolution of UVOIR imaging,
avoids the bias of small background objects toward preferential detection in
more transparent spatial “windows (Holwerda et al., 2007), and works equally
well independent of grain temperature (specifically even for the coldest
grains, whose emissivity can be very low for far-IR and submillimeter
detection).

This technique has been applied to a range of spirals, with the sample size
growing as survey data improved the potential sample size. Small samples in
ground-based images were considered by Keel (1983), White & Keel (1992), White
et al. (2000), and using HST data by Elmegreen et al. (2001), Keel & White
(2001a), Keel & White (2001b), and Holwerda et al. (2009). This technique was
extended into the ultraviolet, mostly using GALEX data, by Keel et al. (2014),
with more detailed treatment of the uncertainties introduced by imperfect
symmetry in the galaxies. Addition of spectroscopic information can improve
the separation of contributions from the two galaxies (Domingue et al., 2000)
and provide a finer wavelength sampling of the attenuation (Holwerda et al.,
2013). When both far-infrared (FIR) and backlighting measurements are
possible, Domingue et al. (1999) found broad consistency in the dust masses
derived in these complementary ways. These studies have provided estimates of
attenuation and dust masses, hints of fractal dust structure in well-resolved
spiral arms, and has shown that the Calzetti et al. (1994) effective
attenuation law applies to dust across many spiral disks as well as in the
star-forming regions for which it was initially derived. Comparison with
simulations indicates that this behavior with wavelength, quite different from
that characteristic of extinction measured star-by-star in nearby galaxies,
results from the mix of optical depths within each resolution element. Keel &
White (2001a) showed, using HST data, that the derived attenuation law becomes
steeper as the resolution element becomes smaller, across size ranges 50-200
pc, fitting with this idea. A handful of galaxies are known to host dust lanes
or patches so far from their centers that the grains must be very cold ($<15$
K) (Holwerda et al., 2009), which is in the right sense to account for a low-
temperature component of the overall emission of galaxies including Herschel
data (e.g., Bourne et al., 2012; Ciesla et al., 2012; Smith et al., 2012;
Cortese et al., 2012, 2014; Clark et al., 2015; Beeston et al., 2018).

“Prime Extragalactic Areas for Reionization and Lensing Science” (PEARLS) is a
_JWST_ GTO program (PI: R. Windhorst) designed to provide the community with
medium-deep imaging in up to eight near-infrared filters (Windhorst et al.,
2023). As one of the main program goals is to trace the evolution and assembly
history of galaxies over cosmic time, the impact of extinction and scattering
by interstellar dust grains on the observed tracers of such evolution poses a
potential source of bias and uncertainty. To assess this issue, one of the
PEARLS targets is a pair of galaxies with a fortuitous chance alignment on the
sky: a face-on spiral galaxy partially backlit by an elliptical galaxy. In
such a configuration, the properties of the dust in the foreground spiral
galaxy can be measured in an absolute sense, and to very low dust column
densities, and in both arm and interarm regions. This target, VV 191, serves
as a GTO pilot program to demonstrate the power of _JWST_ for future GO
surveys that target a larger and more diverse sample of galaxies. In addition,
unknown to us when selecting the target and clearly revealed by the JWST data,
one of the components of the galaxy system observed here strongly lenses a
distant background galaxy, illustrating multiple configurations and outcomes
of overlapping galaxy images at once.

Previous work on dust in backlit galaxies has concentrated on grand-design
spirals, often 2-armed systems, where application of symmetry to model the
foreground galaxy is manageable. VV 191 projects the outer disk against such a
bright part of the background galaxy that we can complement earlier work by
studying the disk attenuation of a multiple-armed spiral, where the dust lanes
have flocculent structure. VV 191 was initially recognized as an occulting,
rather than merging, system during the Galaxy Zoo survey for such pairs (Keel
et al., 2013). It nearly matches the schematic ideal for analysis of such a
pair, with a face-on spiral roughly half-projected against a background E or
S0 galaxy (itself nearly circular in projection), and the spiral nearly face-
on (photometric axial ratio within 0.15 of circular at all radii). The
backlighting near the edge of the disk is so strong that uncertainties in
correcting for the spiral’s own light barely contribute to the error budget in
retrieving the attenuation. The analysis technique and uncertainty analysis
are described in detail in section 2.6.

We present here a combination of HST and JWST imaging studies for VV 191,
showing the power of carrying this technique into the near-infrared (NIR),
using a galaxy pair with nearly ideal geometry for this analysis. Adding the
near-IR bands was expected to provide more leverage for measuring the
reddening slope, and give accurate attenuation measurements in regions that
might be effectively saturated at much shorter wavelengths. As it happened,
the F090W and F150W JWST data also provide higher spatial resolution of dust
structures.

##  2 Observations and data analysis

###  2.1 The galaxy pair VV 191 and its overall properties

The VV atlas222English translation hosted at
https://ned.ipac.caltech.edu/level5/VV_Cat/frames.html (Vorontsov-Velyaminov,
1959) shows VV 191b as the spiral to the northeast, and VV 191a as the
background elliptical galaxy. NED uses the same convention; we are observing
attenuation by VV 191b against the light of VV 191a. Because existing values
are compromised by blending, we redetermined the Petrosian radius (in the SDSS
convention) for the spiral VV 191b after subtracting a model for the
elliptical galaxy and using only the non-backlit portion of the galaxy, giving
12.5″=12.5 kpc in the F606W filter (essentially $r$ band). This is a large
galaxy, and we detect backlit dust beyond a radius 20 kpc (1.7 Petrosian
radii). Virtually the entire radial range where we trace backlit dust (14-20
kpc) is outside the radius where even the NIRCam images show distinct spiral
arms in starlight, making modeling of the spiral’s light unusually tractable;
the interstellar medium still maintains a spiral pattern in this outer zone.
Symmetry and continuity show that long, organized dust lanes occur just inside
the stellar arms, but are not confined to these locations.

VV 191 was originally selected as having such a large redshift difference that
interaction between the components could be ruled out. SDSS DR7 gave a
redshift for one component of $z=0.051$, while NED quoted one for the other
galaxy of $z=0.037$. However, after the STARSMOG snapshot program observed it,
by the release of SDSS DR15, SDSS measured the second galaxy and gave an
accordant value (current values are $z=0.0513$ for the elliptical and
$z=0.0514$ for the spiral). Evidence for whether these galaxies might be
affected by mutual gravitational interaction must then be sought from the
galaxy images themselves. Neither galaxy shows the kind of asymmetry which
suggest interactions within radii $\approx 20$ kc, indicating that they are at
least the sum of these radii apart along the line of sight. This redshift with
the current consensus cosmology gives an angular scale of 1.008 kpc
arcsecond-1 (Wright, 2006). The galaxy nuclei are
20$.\mkern-5.0mu^{\prime\prime}$4 apart (20.6 kpc in projection). The FWHM
resolution in our images projects to values from 60 pc (F090W) to 180 pc
(F444W).

###  2.2 HST imaging

An initial HST image of VV 191 was obtained as part of the STARSMOG snapshot
program (HST program 13695), which targeted potential backlit-galaxy systems
from the Galaxy Zoo and GAMA samples. Using the Wide-Field Camera 3 (WFC3)
with F606W filter, we obtained a total exposure time of 900 seconds, in two
subexposures with a small dither motion between. This passband, centered near
6060 Å , is broadly similar to SDSS $r$.

Hoping to measure the slope of the attenuation law over a broad wavelength
range, we also obtained WFC3 images in the UV F336W and F225W bands under HST
program 15106 (PI Holwerda). A single-orbit (2749-s) pair of dithered
exposures was obtained in F336W, while three single-orbit exposures totalling
8874 s were carried out in F225W band. As expected, the signal levels from the
red elliptical galaxy were low enough to make adequate treatment of cosmic-ray
events a challenge. We were able to retrieve a transmission map with usable
precision for a small dust-lane region near the elliptical-galaxy core in
F336W, while for F225W we did not achieve the requisite signal-to-noise ratio
in view of the faintness of the background in the UV. F225W is very sensitive
in detecting regions of relatively unreddened star formation, and we find a
useful correlation with the dust emission seen in the NIRCam LW channels.

Figure 1: The overall geometry of VV 191, illustrated with a composite of the
HST F606W and F336W data in the blue channel, JWST NIRCam F090W in the green,
and NIRCam F356W in red. Each is shown with a logarithmic intensity mapping,
starting from slightly below the sky level to closely match SDSS-style sinh
mapping. The field shown is $29.6\times 55.0$″. The image is shown rotated by
$27^{\circ}$ clockwise from cardinal orientation in this display, to align
with the separation of the galaxies. This shows the backlit dust lanes in
front of the elliptical VV 191a, as well as a probable gravitationally lensed
arc and counterimage behind VV 191a, and additional background galaxies with a
wide range of colors and likely redshifts.

###  2.3 JWST imaging

VV 191 was observed with
NIRCam333http://ircamera.as.arizona.edu/nircam/in_instrument_overview.php on 2
July 2021, quite early in the GO+GTO mission phase, as part of the PEARLS GTO
program (1176). Both components of VV 191 fit in a single short-wavelength
(SW) detector quadrant, centered on the dust-overlap region, allowing freedom
of orientation and simpler reduction. Planned total exposures were 901 seconds
in each of the F090W/F356W and F150W/F444W filter pairs, all using MEDIUM8
readout with 3 groups and 3 dithers. The filters were chosen to combine high
throughput with a long wavelength baseline; the 3.3 µm PAH feature would
appear within the F356W band, so we could in principle detect emission if
strong enough (although we detect no such emission associated with the outer
dust lanes). The NIRCam bands we observed are broadly comparable in wavelength
to SDSS $z$, $H$, and the WISE [3.4] and [4.6] or Spitzer [3.6] and [4.5]
passbands.

###  2.4 Data Reduction

Reduction of the NIRCam data used pipeline version 1.8.0 and reference files
corresponding to CRDS context pmap-0995, as detailed by Windhorst et al.
(2023). This set of calibration data improves flat-fielding and the uniformity
of photometric zero points (especially between NIRCam detectors) compared to
our initial look with pmap-0913, while the newer pipeline routines are much
better at rejecting ‘snowball” decay events which previously left circular
artifacts. We paid particular attention to cross-checks of the flux scale
(important only for the serendipitous lensing targets discussed below) and
treatment of the “1/$f$” readout noise, which manifests as gradual baseline
drifts for each row along the $x$ direction (Rauscher et al., 2012). All
detectors at each wavelength were drizzle-combined into single mosaic images,
although our analysis here uses only the single short-wavelength (SW) detector
encompassing the galaxies in VV 191 and matching cropped regions on the long-
wavelength detectors. Cosmic rays can be rejected almost perfectly with the
multiple readout strategy of the NIRCam detectors.

The dust analysis relies only on differential measurements. For colors used in
estimating photometric redshifts of newly-found lensed objects, especially
since our initial analysis used an early version of the NIRCam photometric
zero points, we used additional data for consistency checks on the magnitude
scales. We combined the SDSS photometry and spectrum of VV 191a itself with
the known consistent behavior of the spectral energy distributions (SEDs) of
giant elliptical galaxies. We adopted the broadband data from Brown et al.
(2014), and 1.5-µm spectrum from François et al. (2019). The scatter in
broadband optical-IR colors for luminous ellipticals suggests that this
procedure should be accurate to 5% in flux, comparable to the initial
precision of JWST absolute calibration (Rigby et al., 2022). In Table 1, we
compare the derived magnitudes for the core 3″-diameter region of VV 191a from
NIRCam data with “new” zero points (pmap-995) and corresponding values
starting from SDSS and HST data, extrapolating redward with a composite SED as
noted above. The scatter between the two sets of color indices is
$\sigma=0.10$ magnitude, broadly consistent with the stated 0.05-magnitude
accuracy of the NIRCam zero points and the scatter among elliptical galaxies
in NIR colors ($\sigma=0.06-0.07$ magnitude depending on filter pair) from
Brown et al. (2014).

Table 1: Consistency color checks on VV 191a nucleus

Color | AB (NIRCam) | AB (external) | NIRCam–external | Source
---|---|---|---|---
F606W-F090W | 0.99 | 0.88 | 0.11 | SDSS spectrum
F090W-H | 0.57 | 0.62 | -0.05 |  Brown et al. (2014), François et al. (2019)
H-F356W | -1.00 | -1.09 | 0.09 | Brown et al. (2014)
F356W-F444W | -0.43 | -0.51 | -0.08 | Brown et al. (2014)

F150W transformed to H using data in François et al. (2019). F090W is
consistent with SDSS $z$.

###  2.5 Additional data processing

We resampled the HST F225W, F336W, and (lower-resolution) F606W data to match
the coordinate grid of the NIRCam drizzled mosaics, which were internally very
well aligned through use of Gaia stars. In view of the change in how world
coordinate system (WCS) information is included in the headers between HST and
JWST data, we computed linear transformations between these images using sets
of $\approx 10$ well-peaked objects in common. When needed to match point-
spread functions (PSFs) between data sets, we deal only with the Gaussian-like
core. Residual effects of the diffraction spikes do appear in our attenuation
maps at levels below 1%, setting the precision floor in some regions near the
core of the elliptical galaxy. The spikes normally appear at dynamic range of
order 300 for the longer-wavelength bands, and the Airy rings matter in the
longer-wavelength bands only for dynamic range $>20$ (a level which our dust
measurements reach only in the shorter-wavelength bands).

The pipeline processing reduced the effects of “$1/f$” noise in our data, but
the large angular size of the galaxies in VV 191 meant that the most-used ways
to use changes in sky level to track these variations along readout rows were
not completely effective in the F090W and F150W data. We were able to
substantially mitigate the residual “$1/f$” effects in a heuristic way, by
subtracting models for the large-scale structure of each galaxy, taking the
median along readout rows with clipping of high and low values at $\pm 0.1$
MJy/sr, and subtracting this result from the data in each affected NIRCam
filter after rotating to match the coordinate frame of the drizzled
combination. This success is due to the elliptical models incorporating
essentially all galaxy structures which are not much smaller than the
251-pixel median window used to track the readout variations.

We illustrate the geometry of the VV 191 system with a composite of the HST
and JWST data sets in Fig. 1.

###  2.6 Analysis technique

Schematically, the fraction $T$ of transmitted light at each point in the
foreground galaxy is derived from the observed intensity $I$ and model-based
estimates of the foreground intensity $F$ and background intensity $B$ (if
observed by themselves, as judged from the non-overlapping parts of each
galaxy) according to

$T={{(I-F)}\over{B}}.$ $None$

Transmission $T$ is closer to the observations than the more physically
interesting optical depth $\tau$, and has better-behaved errors, so we start
with $T$. To the extent that extinction-free sections of the background galaxy
can be identified for the modeling, this technique provides an absolute zero
point for attenuation.

When the galaxies in a backlighting system are close together, scattering may
potentially alter the intensity and color of light coming through the
foreground dusty disk. In this case, however, the extent of symmetric
isophotes of the two galaxies indicates that they are not close enough for
tidal influences to alter these (and have not done so for several crossing
times). White et al. (2000) presented a calculation for a similar geometry and
set of brightness profiles, showing that the differential scattering effects
across the foreground disk (that is, the contribution which would not be
subtracted by modeling the galaxy intensity profile) can be neglected as long
as the galaxies are farther apart than the sum of their radii to the relevant
region, almost certainly true in this case since the galaxies’ isophotes are
very elliptically symmetric (outside the overlapping regions) to still larger
radii. In addition, since scattered light is bluer than either attenuated or
direct starlight, it would produce a trend in the color-attenuation relation
with projected distance from background nucleus.

We modelled each galaxy using the procedure to fit ellipses to isophotes of
Jedrzejewski (1987), as implemented in the isophote package of STSDAS. The
result is a smooth intensity model of each galaxy matching these isophotes
(whose centers may not necessarily coincide, although fixing the center may
improve results when the arc of fitted data is short). This was done
iteratively when needed, with progressively improved correction for each
galaxy’s effect on the other, both by subtracting the current model for the
galaxy not being measured, and resulting improved masking of discrete
features. This works very well for the background early-type galaxy. It is
obviously a cruder approximation for the foreground spiral, usable in this
case because the background light is so bright in comparison to the outer
parts of the disk and the outer parts of the disk lack smooth stellar arms to
further complicate modelling. The form of Equation 1 shows how errors in each
model affect the derived transmission (point by point). Differences between
the images and models gives typical (RMS) uncertainties in regions of
interest; there is not a straightforward way to show their effects graphically
when averaging spatially, because structure in the foreground galaxy has
spatial correlations. The background galaxy in this system is demonstrably
very smooth, leading to very small uncertainties where its light dominates
over the foreground disk. The signal-to-noise ratio in the transmission image
depends the brightness and symmetry of both galaxies, generally being best in
the most strongly-backlit foreground region where there is significant dust
attenuation. Pixel masks for fitting isophotes were produced for each image,
iteratively improved by alternately subtracting the latest model for each
galaxy before fitting the other. Including the lens arc, other background
galaxies, star clusters, and foreground stars, typically 5% of pixels in the
region of interest ($14^{\circ}$ on either side of the line between galaxy
nuclei and radii 11-24.7″from the center of the spiral) were masked.

Steps in this process are illustrated in Figure 2 using the NIRCam F090W
image. The longer-wavelength NIRCam images show speckled residuals in the
brightest parts of the elliptical galaxy after model subtraction; we suspect
this arises from image reconstruction with only 3 dither pointings, but this
does not affect areas of interest for either dust or lensing studies.

Figure 2: Steps in the dust analysis, shown using the NIRCam F090W data. Upper
left, original image ($I$ in Eqn. 1). Upper right, smooth and symmetric model
for the foreground spiral ($F$ in Eqn. 1) . Lower left, model for the
background elliptical ($B$ in Eqn. 1). Lower right, derived transmission map
for the foreground galaxy ($T=(I-F)/B$), which is meaningful only in regions
where the errors due to galaxy structure are small (as shown in Section 3).

###  2.7 Uncertainty estimation

Understanding the uncertainties in our attenuation maps informs our
understanding of how far to analyze them. Since fine structure and the
accuracy of modeling are more important contributions to the error budget than
Poisson statistics or readout noise in some regions, we take an empirical
approach to quantify the visual impression in our transmission (attenuation)
maps. Starting with the model-subtracted data in each filter, we constructed a
simple circular model of how each galaxy’s structure, plus detection
statistics, contributes to the overall uncertainties in the attenuation
measurements. After subtracting our smooth model for each galaxy, we averaged
standard-deviation values in annuli of each galaxy. For the half least
affected by overlap, this standard deviation was evaluated in 15∘ sectors,
omitting sectors including background galaxies, stars, or very bright star-
forming regions, before deriving the average. For each galaxy, we fit a smooth
empirical function of radius $r$ to these averaged standard deviations
$\sigma$ using combinations of convenient basic functions (Gaussian,
exponential, linear, truncated power law) as interpolating devices, producing
two-dimensional realizations of these function to compute the expected
uncertainty per resolution element at each point in the attenuation maps. The
signal-to-noise ratio of attenuation measurement reaches 20 (per $2\times
2$-pixel resolution element) in two dark clouds in the F090W data. Throughout
the region we analyze, the quality of the fit to the elliptical galaxy
contributes more to the error budget than the mean foreground light for the
outer disk of the spiral, allowing unusually well-constrained measurements of
the effects of dust.

As a consistency check, we also evaluated the Gaussian scatter of derived
attenuations in “blank” parts of each zone we analyze in Section 2.6. Since
resampling was involved in drizzle combination aligning HST to JWST data, and
smoothing occurs when matching PSF core widths, we reduce pixel-to-pixel
correlations via sparse sampling (every 3 pixels in each axis) for this
exercise. The two approaches give results usually agreeing at the 30% level in
uncertainty for the NIRCam data, and at the factor-of-two level for the HST
data

We use these pixel-by-pixel uncertainty estimates in selecting regions for
detailed analysis, and in fitting the slope of the reddening law between
different filter bands. Deeper interpretation of these values will be limited
by spatial correlations in the unmodelled galaxy structure, although for VV
191 much of the backlit dust is in regions where the diffuse foreground spiral
pattern is too faint to affect the pixel scatter measurably.

##  3 Spatial structure of dust attenuation

Important results came from the HST F606W snapshot image alone, informing the
selection of this pair for JWST observations. Most notable, the network of
dust lanes (both those tracing the spiral pattern and weaker lanes cross-
cutting in a “‘dust web”) has a sharp outer boundary, so pronounced that it
appears in the azimuthally averaged transmission profile (Bradford 2016). This
appears in HST and JWST data (Figure 3). These profiles also show dips in
transmission where dust arms cross the sector of usable signal-to-noise. For
reference, the region analyzed for the profiles in Fig. 3 is outlined in Fig.
4. The region more than 21.5″from the spiral core, with no evidence of dust
attenuation, has a mean derived transmission within 0.4% of unity in all five
bands from F606W to F444W, furnishing a check on the accuracy and
applicability of the galaxy models. However, there are ripples in the “zero-
attenuation” level, including unphysical values greater than unity, at levels
up to 1.5% and closely matching in bands from F150W to F444W. This amplitude
is greater than plausible flat-fielding uncertainties in NIRCam444B.
Sunnquist, private communication.. We suspect these reflect mismatches between
galaxy structure and our elliptically-symmetric models; this is an aspect of
the low-level systematics which we attempt to deal with in fitting the
reddening behavior in Section 4. The fact that they are consistent between
filters and confined in radius gives reason to think that the systematic
errors are similarly confined in radius within the foreground disk so will be
approximately constant across each of the regions we fit.

Figure 3: Azimuthally averaged transmission in F606W and all four NIRCam
passbands, from the zone of highest signal-to-noise ratio in the transmission
map (13∘ of azimuth in the spiral disk, on either side of the line between
spiral and elliptical nuclei). This quantifies the sharp outer edge to the
dust lanes seen in Fig. 2, which is so complete that it is obvious even when
averaged over all structure in the disk. Labelled curves show the various
filter bands; F356W and F444W are distinguished by line thickness to reduce
crowding. Unmasked pixels have been averaged in 10-pixel or 0.3″radial bins.
The radial location of the background elliptical-galaxy nucleus is shown for
reference. The standard deviation of the mean in each bin would be only
slightly wider than the plotted lines, so the structure reflects real
variation within the galaxy. With a $26^{\circ}$ sector width, spiral features
give attenuation signatures where they cross it. At this redshift $z=0.051$,
1″= 1.008 kpc. Matching ripples in the profiles (most pronounced at radii 16.5
and 20.5″) for F150W, F356W, and F444W may indicate genuine asymmetric
structure in one of the galaxies, since each image was independently masked
and modeled. Their amplitude would suggest an accuracy of 1.5% from such
structure. This motivates our fitting techniques for reddening behavior to
incorporate such local systematic factors as nuisance parameters.

We illustrate these features across our observed wavelength range with the
two-dimensional transmission maps from both HST and JWST data in Figure 5.
These illustrate the concentration of dust signatures to a system of lanes,
some of which cut nearly perpendicular to the pitch angle of the overall
spiral pattern (in what we term the dust web, some parts of which fit with
spiral spurs of features as more often described). The shaping of these
structures so far out in the disk appears to include processes beyond the
differential rotation which shapes so much of spiral patterns.

As a broad summary of the results, Figure 6 shows the cumulative distributions
of transmission in broad radial bins. This is relevant to, for example, priors
on color measurements of supernovae in galaxies of this type, and this
technique offers the possibility of extension to a broader range of galaxy
types and impact parameters. Figure 6 provides a complementary view to the
averaged radial profiles in Figure 3, also useful in assessing the probability
of a line of sight having a particular level of transmission all the way
through the disk. This compares the cumulative (fractional) distribution of
area with attenuation greater than a given level (transmission less than that
value) for the three filters with the best dust measurements. This covers the
well-measured range from 14.5-21 kpc, divided into inner and outer halves as
well as showing the entire range.

Figure 4: Sector of the disk of VV 191b used for the measurements in Fig. 3,
overlaid on the F090W attenuation map. The orientation is the same as in Fig.
2, with north $24^{\circ}$ clockwise from the right. Circular segments show
fiducial radii from the nucleus of the spiral VV 191b, while radial lines show
the azimuthal boundaries of the measured region with highest signal-to-noise
ratio (center $\pm 13^{\circ}$). Black areas indicate distant background
galaxies, gravitationally lensed images, or foreground star-forming regions
masked from the fit here and in determining the wavelength behavior of
attenuation. The center of the elliptical galaxy VV 191a is shown with a plus
sign.

Figure 5: Transmission (and attenuation) maps from HST and JWST data, spanning
the observed range 0.33-4.4$\mu$m. These represent our estimates of the
transmitted light at each point; regions with more dust, giving less
transmitted light, appear darker. The edge of the dust disk almost exactly
crosses the background nucleus (marked with a black plus sign). The
combination of better galaxy symmetry and better S/N makes the dust features
best detectable at 0.9µm, despite the larger attenuation at shorter
wavelengths. The background galaxy is so dim at 0.33$\mu$m that we can derive
only spatially averaged quantities from that filter; for display, these data
are median-smoothed by 0.15″ here. All brightness scales are the same, from
0.3 to 1.3, as shown in the scale bar (in transmission or residual intensity,
to put unity at a conveniently visible level). The center of the spiral lies
off the frame to the upper right. Each image section spans $15.0\times 15.9$″.
Bright background objects and foreground stellar associations appear, which
were masked for measurements of the dust effects. The F444W images shows
ringlike artifacts to the right, from modeling the spiral disk, where masking
during isophotal fits left poor residuals.

Figure 6: Cumulative distributions of area with transmission less than the
value on the $x$ axis for the three best-measured filter bands within the
best-measured disk sector as shown in Fig. 4. The mapped radial range 14.5-21
kpc is shown as a whole (solid line) and broken into inner (dot-dashed) and
outer (dashed) halves, as marked on the F606W panel. Pixels affected by
unresolved sources, background galaxies, and emission from star-forming
regions have been masked.

##  4 The broadband attenuation law in VV 191b

The slope of the reddening-attenuation relation (reddening law) combines
information on the grain populations and spatial distribution, when observed
at the resolutions available for galaxies (in contrast to sight lines to
individual stars, where scattering and blending of regions with different
column densities are negligible). We compare the distributions of points in
the galaxy to the reddening trajectories predicted for various ratios of
total-to-selective extinction $R=A_{V}/E_{B-V}$ or power-law index $\beta$.
For a range of $A_{V}$ values, we calculate the fraction of the flux from an
elliptical galaxy emerging in each of our passbands, incorporating photon
weighting (Hogg, 2022) within the filter bands by folding in the SDSS spectrum
of the core of VV 191a, the 1.5-µm spectrum of a typical luminous elliptical
from François et al. (2019), and in the near-UV F336 band, a composite of data
for NGC 4472, incorporating averaged IUE spectra (Norgaard-Nielsen &
Kjaergaard 1981, Oke et al. 1981, Burstein et al. 1988) plus a Kitt Peak
large-aperture spectrum extending near the atmospheric cutoff, all resampled
to the redshift of VV 191a. Both photon weighting and finite passband widths
mean that progressively more flux is detected at increasing values of $A_{V}$
than would be predicted from the central wavelength alone.

The linear resolution of these data ranges from 60–180 pc. Compared to the
fractal-like distribution in the backlit spiral in AM 0500$-$620 (Keel &
White, 2001a), these values are in the regime where we might expect
significant “graying” of the attenuation law as regions of different optical
depth are mixed within each resolution element.

To the extent that a large part of the elliptical galaxy is unobscured so our
model reflects its true brightness profile, this technique measures absolute
attenuation, sensitive to gray attenuation as well as reddening.

We measure the slope of the attenuation (reddening) law between each two of
the three filters with the best attenuation measurements (F606W, F090W,
F150W), matching the PSF core to the F606W data, by varying the slope between
the filters (CCM parameter $R$ or power-law index $\beta$) and fitting to the
curve formed by the transmissions predicted for the two bands as the total
extinction is varied (for bookkeeping and comparison, we use $A_{V}$,
following the prescription of Cardelli et al. (1989) since the Calzetti et al.
(1994) form is not defined in our redder filter bands). Fitting attenuation
laws to these clouds of points poses several specific challenges. The
predicted transmission varying a given parameter ($R$ or $\beta$) for each
filter pair is a one-parameter family anchored at both (1,1) and (0,0), as
shown in Fig. 7. In the regime of small attenuation (transmission $\approx
1$), even small systematic errors in the data values (Section 3) will exercise
undue influence on the fitting results. Such small systematics would be
multiplicative if due to a model shortfall for the elliptical galaxy or
additive if due to structure in the spiral; they are always small in these
data, less than 4%, so we cannot distinguish and treat them as multiplicative
for convenience. We explored several approaches to deriving these parameters,
recognizing that still more detailed modeling may be warranted, going beyond
the scope of this analysis and to be explored in future work. Working in a
low-attenuation regime into the near-IR bands, the fitting is less well
constrained than in, for example, the HST observations of NGC 3314 reported by
Keel & White (2001b), where dust lanes with larger attenuations were targeted
including shorter wavelengths and only values relative to the diffuse
surrounding dust were retrieved, rather than the absolute attenuations we seek
in this work.

These data are not accurate enough to discriminate between CCM and power-law
forms from 0.6-1.5 $\mu$m; there are values of each parameter which give
virtually identical attenuation behavior in each filter pair over the
attenuation range we observe although the value of $A_{V}$ between equivalent
parts of the two curves differs significantly at large attenuations).

Respecting the scatter of pixel values, which may be large compared to the
changes we seek to measure, cases of asymmetric scatter in one or both
filters, and the two-dimensional nature of the scatter since it may be
comparable in each filter of a fitting pair, we experimented with several ways
to fit reddening slopes to data from each pair of filters with significant
dust measures over large regions (F606W, F090W, F150W). Ordinary $\chi^{2}$
minimization gives results very sensitive to small systematic offsets in the
zero-attenuation level, since each fitting function by itself gives a one-
parameter family anchored at both ends. We first tried fit to the curves
bisecting the lowest-transmission sections of each point cloud, with a
heuristic cut to reduce the influence of systematics at high transmissions,
and also used binned versions of each point cloud to attempt fits following
the ridgelines as defined by contour plots. Our final fits for the reddening
behavior combine features of these approaches, with explicit allowance for
systematics in galaxy removal. As well as the reddening slope (separately for
$R$ and $\beta$, we fit for a multiplicative factor near unity (in practice
$\pm 3\%$) for each filter as nuisance parameters. In some regions, there may
be an additional component of fine-scale structure in the foreground galaxy
(that is, surface-brightness fluctuations). Inspection shows that actual noise
and real structure (due to the distribution of luminous stars) contribute
about equally in the long-wavelength F356W and F44W bands in the inner zones
we fit, so the color of the brighter and dimmer spots depends on the stellar
population. This would be manifested in the two-filter data by a component
near unity transmission with a different slope than the larger-scale reddening
behavior; we try to avoid this effect by fitting only points with transmission
$<0.98$ in the bluer passband of each pair.

Our fits start by summing the pixel values of transmission in a filter pair
into a grid, normally summed in bin of 0.01 in each transmission value.
Passing a $3\times 3$-pixel Laplacian gradient filter over such a smoothed
array allows numerical retrieval of the ridgeline of each distribution, where
the attenuation parameter and systematic scaling can be fit in a way analogous
to $\chi^{2}$ minimization, minimizing the 2-dimensional deviations of each
point on the gradient-derived ridgelines from the transmission curve, although
the expected scatter is now far removed from the individual pixel
uncertainties. This approach therefore incorporates the 2-dimensional nature
of the uncertainties for each pixel, and gives a quantitative way of assessing
the accuracy of the best-fit parameters. We present the results from this
gradient-fit approach in in Tables 2 and 3 for $R$ and $\beta$ respectively.

These fits have been done for three regions spanning the arms as defined by
dust lanes and two “interarm” regions (Figure 8), which together largely fill
the region of highest S/N for attenuation. The fitting procedure is
illustrated in Figure 7. We list the best-fit values for each region and
filter pair using each approach: values of $R$ in Table 2 and power-law index
$\beta$ in Table 3. The 2-band fits use pixel masks produced independently for
each filter (and independent of the pixel masks used in fitting the galaxy
models), to reject stellar associations in VV 191b and background galaxies.
When matching resolutions, the masked regions are propagated to include all
pixels whose value would be affected by more than 0.05% by a masked region in
either filter band after the convolution, well below the intensity
uncertainties in transmission values. The point clouds, ridgelines, and fitted
2-color transmission curves for each pair of the three filters with widespread
dust detection and each measured region are shown in Fig. 9. This illustrates
how similar the best-fit $R$ and $\beta$ curves are for each filter pair. The
behavior for $R<1$, formally found for the F090-F150 filter pair, becomes
highly nonlinear (and poorly determines from existing data), so we list $R=1$
as a minimum value and show this in Fig. 9.

Figure 7: Process for fitting reddening slope $R$ or $\beta$, comparing the
distribution of points in transmission, after matching PSF core widths to the
F606W value, in various pairs of filter bands with curves predicted by
particular reddening forms. Derived transmission values are shown in the F606W
and F090W filters, for all unmasked pixels in zone 1 of Figure 8. Curves in
the left panels use the CCM reddening law with values of $R=1-4$ spaced by 1.
Right panels use a power law in extinction
$A_{\lambda}\propto\lambda^{-\beta}$ for $\beta=4$ to 1 in steps of 1. This
zone emphasizes arm attenuation and therefore does not show the cloud of
points near (1,1) indicating an area of no attenuation, although the cloud of
most common values should still fall along the best-fit curve in the absence
of systematic offsets. Gray attenuation would trace a straight line from the
origin through (1,1), $\beta=0$ or an infinite value for $R$. The middle
panels show logarithmically-spaced contours of the point distribution (after
summing in bins of 0.01 on each axis), with thin lines marking the extreme
curves from the top panels for reference. The bottom panels show this
distribution in gray scale after filtering with a $3\times 3$-bin Laplacian to
trace its ridgeline, as a valley in the Laplacian gradient. The adopted fits
to the ridgeline are shown as lines over the contour plots and as points over
the Laplacian together with the best-fit curve for each parametrization. While
a best fit can be found for either, each set of highly-constrained model
curves shows systematic departures from the ridgeline of these data.

Across all regions and filter pairs, the value of $R=A_{V}/E_{B-V)}$ is very
low, 1.94 from F606W-F090W and even smaller for filter pairs involving F150W
(Table 2).This change with wavelength shows that the common fitting functions
do not capture the broadband behavior in this situation (so a global fit
across the well-measured bands from 0.6-1.5$\mu$m is not useful at this
point).

We carried out analogous fitting for the index of a power-law attenuation law,
where $A_{\lambda}=A_{V}(\lambda/\lambda_{V})^{-\beta}$, with results listed
in Table 3. As with the CCM form, for each zone and filter pair, we did fits
to the ridgeline of the point clouds as derived from the Laplacian transform
of the gridded points, and as consistency tests, compare eyeball fits both to
the point cloud and to the ridgeline of the smoothed distribution of data
points. We include weighted means for the entire data set and separately for
the arm and interarm zones, derived by analysis of all points in each category
rather than by averaging the zone values. For adequate signal-to-noise ratio
in tracing the gradients, some zones (indicated with asterisks) had their
gridded point distribution smoothed by a Gaussian of $\sigma=0.03$ in each
transmission value rather than the default 0.02. Comparison of zones where
both Gaussian values give useful results indicate that the scatter introduced
by this choice is slightly larger than the listed uncertainty of the fit. Very
broadly, regions where regions of different attenuation are blended within
each resolution element will show flatter measured attenuation (large $R$, or
$\beta$ closer to zero). The fitted values show trends of steeper slopes (more
positive values of $\beta$) at longer wavelengths, smaller projected radii
within the galaxy, and in interarm regions compared to the spiral arms.

Our fitting procedure gives useful bounds on the parameter values best fitting
each data set. However, in most cases the attenuation tracks in these diagrams
predicted by either CCM or power-law forms do not completely describe the
data. Typically, the data exhibit more curvature in the reddening tracks than
the fits - that is, more reddening than predicted at small attention and less
than predicted at large attenuation. This is broadly consistent with larger-
attenuation pixels including areas with different extinction within each
resolution element, but the situation invites further, more detailed modeling.

Table 2: Gradient-Fit Results to Slope $R$ of the Attenuation Law

Region | Radius | Area | F606W-F090W | F090W-F150W | F606W-F150W
---|---|---|---|---|---
| (arcsec) | (pixels) |  |  |
1 | 19.4 | 15693 | $2.16\pm 0.05$ | $1.00\pm 0.10^{*}$ | $1.23\pm 0.06$
2 | 16.5 | 22013 | $2.66\pm 0.11$ | $1.18\pm 0.08^{*}$ | $1.14\pm 0.06$
3 | 10.5 | 28695 | $1.20\pm 0.05$ | $1.12\pm 0.07^{*}$ | $1.10\pm 0.04$
4 | 18.0 | 17168 | $2.14\pm 0.05$ | $1.11\pm 0.10^{*}$ | $1.00\pm 0.10$
5 | 12.5 | 24734 | $1.31\pm 0.07$ | $1.13\pm 0.06^{*}$ | $1.25\pm 0.05$
Mean | 14.6 | 107303 | $1.94\pm 0.06$ | $1.13\pm 0.07$ | $1.10\pm 0.06$
Arms | 14.6 | 66401 | $2.07\pm 0.05$ | $1.00\pm 0.05$ | $1.13\pm 0.04$
Interarm | 147 | 41902 | $1.27\pm 0.07$ | $1.15\pm 0.06$ | $1.00\pm 0.04$

*Fitted with $\sigma=0.03$ smoothing of binned point distribution; otherwise, $\sigma=0.02$.

Table 3: Gradient-Fit Results to Power-law Slope $\beta$ of the Attenuation
Law

Region | Radius | Area | F606W-F090W | F090W-F150W | F606W-F150W
---|---|---|---|---|---
| (arcsec) | (pixels) |  |  |
1 | 19.4 | 15693 | $1.82\pm 0.08$ |  $2.90\pm 0.10$* | $2.67\pm 0.09$
2 | 16.5 | 22013 | $1.68\pm 0.10$ | $2.53\pm 0.07$ | $2.88\pm 0.08$
3 | 10.5 | 28695 |  $3.69\pm 0.12$* | $2.68\pm 0.10$ | $3.23\pm 0.10$
4 | 18.0 | 17168 | $1.87\pm 0.10$ |  $1.0\pm 0.2$* | $3.08\pm 0.12$
5 | 12.5 | 24734 |  $3.27\pm 0.12$* |  $1.90\pm 0.05$* | $3.00\pm 0.10$
Mean | 14.6 | 107303 | $2.12\pm 0.12$ | $2.48\pm 0.12$ | $3.20\pm 0.20$
Arm | 14.6 | 66401 | $2.00\pm 0.10$ | $3.12\pm 0.07$ | $3.04\pm 0.06$
Interarm | 14.7 | 41902 | $3.47\pm 0.10$ | $2.04\pm 0.09$ | $3.54\pm 0.08$

*Fitted with $\sigma=0.03$ smoothing of binned point distribution; otherwise, $\sigma=0.02$.

Figure 8: The F090W attenuation map with masked regions shown (black) and the
five zones chosen for fitting attenuation slopes in Table 2. Left to right,
they are regions 1, 4, 2, 5, and 3. Zones 1,2, and 3 are arm regions with
organized dust lanes, while zones 4 and 5 are interarm regions with minimal
coverage by distinct lanes. This subimage has north to the right, and covers
an area $18.4\times 21.7$″. Zones are marked by subtracting constant values
for each from the attenuation map before display. The small area with a dashed
outline labelled UV is the region used for F336W measurements, where the
brightest background and large attenuation combine to give usable signal-to-
noise ratios.

Figure 9: Point clouds for the zones in Fig. 8, with the trajectories
corresponding to the adopted best-fit values of $R=A_{V}/E_{B-V}$ and power-
law index $\beta$ as in Tables 2 and 3 superimposed. The derived transmission
(residual intensity) in each pair of the filter bands F606W, F090W, F150W is
shown pixel by pixel after matching PSF widths; the central panels for F606W
vs. F150W give the most leverage in fitting the overall reddening law. Typical
uncertainties per point are shown for each panel; since these are mostly
function of distance from the two galaxy centers, they are nearly the same
within each zone. The wide gray bands mark the ridgelines of the point clouds
as traced with the Laplacian. The fitted slope values from Tables 2 and 3 are
shown as curves, with the CCM values of $R$ shown as solid curves and the
power-law slope results for $\beta$ as dashed curves.

The mean (all-region) values of $R$ (and $\beta$) in Tables 2 and 3 imply
extinctions in these filter bands of $A_{F606W}/A_{V}=0.83$,
$A_{F090W}/A_{V}=0.30$, and $A_{F150W}/A_{V}=0.047$. Slope fits to the longer-
wavelength bands versus F606W give limits (combining all 5 measured zones)
$A_{F356W}/A_{F606W}<0.035$ and $A_{F444W}/A_{F606W}<0.021$, which imply
nonrestrictive limits on the power-law index $\beta>0.4$ and $\beta>0.7$
respectively. Implicitly, the lack of relation between flux in these bands and
attenuation measured at shorter wavelength shows the nondetection of emission
from hot dust or PAH molecules in these (cold) dust lanes.

Because the red color of the background galaxy limits the ultraviolet S/N so
severely, we use only a subset of 3510 pixels in fitting region 1 for F336W
encompassing the Y-shaped dust lane projected closest to the nucleus of VV
191a, and constructed the 2D grid for fitting the ridgeline of the F336W-F606W
attenuation distribution using bin sizes of 0.05 in each filter. As shown in
Figure 10, the extinction ratio between F336W and F606W bands is best
described by $R=1.7\pm 0.4$ whether one fits the ridgeline, a running mean
when sorted by F606W transmission, or by bisecting the cloud of individual
pixel values. However, the gradient measures of the estimated ridgeline value
show evidence of a further decrease at larger attenuations, more
characteristic of a screen than the more likely mix of extinction values. The
low values of $R$ in some of our fits may need to be understood as indicative,
because the behavior becomes so nonlinear for such low $R$ and there is little
data on reddening behavior which is this strong.

Figure 10: Fits to transmission measurements in the small region used for
F336W, comparing F336W and F606W transmission values. Individual pixels re
show as points; the broad gray line traces the ridgeline of this point cloud
using the Laplacian approach, from data in bins of 0.05 in each coordinate.
For comparison, the jagged line shows a running mean over 199 points when
sorted by F606W transmission, reflecting the much smaller error in F606W. The
curves show the results of CCM models with the indicated $R$ values; we claim
only that the data are consistent with $R=1.7\pm 0.4$ and strongly limited by
the signal-to-noise ratio in F336W.

The range of reddening behavior in galaxies as functions of galaxy type,
metallicity, local environment, and averaging scale-size is surely broad and
not yet understood. The most comparable previous results are from backlit
galaxies observed with HST at linear resolutions $\approx 100$ pc. For disk
dust in fairly luminous spiral disks, some systems show Milky Way-like
reddening. The broad dusty arm in AM 1316$-$241 (Keel & White, 2001a) gives
$R_{V}=3.4\pm 0.2$, and multiple arm segments in NGC 3314 (Keel & White,
2001b) yield $R_{V}=3.5\pm 0.3$ with no radial trend. However, the spiral arms
in AM 0500$-$620 (Keel & White, 2001a) have $R_{V}=2.5\pm 0.4$, while the
foreground system in NGC 1275 has a very different, almost grey extinction law
($R>4$, formally much greater, from Keel & White 2001a). This galaxy is moving
at high velocity through the intracluster medium of the Perseus cluster, so
its grains may have been exposed to alteration by the hot gas. Comparison of
spectral slopes in star-forming regions led Calzetti et al. (1994) to propose
a reddening law which has seen wide application, giving consistent results not
only for the starburst regions where it was first derived, but for the outer
parts of spirals in the UV (Keel et al., 2014). Further work (Calzetti et al.,
2000) found a value $R_{V}=4.05$ for starburst regions in anchoring the
parameters of this relation, which certainly responds to both the grain
properties and the relative distributions of stars and dust in these intense
star-forming regions.

These kinds of measurements, summing over substantial regions, unavoidably mix
regions of various optical depths, so this mix affects the results along with
the (astrophysically more interesting) properties of the grain populations. We
see the effects of this mixing by fitting power-law reddening slopes $\beta$
after block-averaging the transmission maps by factors 2, 4, and 8,
propagating the masked regions in each case cross the 5 zones shown in Figure
8). In the mean, $\beta$, measured between F606 and F090W, decreases by 0.41
between 2 and 8-pixel averaging steps, and decreases by a mean of 0.36 between
F606W and F150W. Decreasing $\beta$ moves in the direction of grayer behavior,
as expected for mixing regions of different attenuation. As the number of
points decreases in this averaging, our ridgeline procedure ceases to be
useful, so the block-averaged data were fitted using minimization about the
predicted curves for the points themselves. Thus these are strictly comparable
to each other but not necessarily to the ridgeline fits used for the full-
resolution maps.

The extinction behavior of the grains themselves is best derived when we can
study their effects star by star, along essentially a single line of sight.
This is how the classical Milky Way extinction curve was determined (albeit
from a more limited part of the Galaxy than one might like), and has been done
in the Magellanic Clouds and M31 (Clayton et al., 2015) as well; the reddening
law in the SMC in particular becomes much steeper than in the Milky Way going
into the ultraviolet. Farther afield, supernovae (especially when observed
polarimetrically) offer ways to isolate the effects of the grain properties,
often implying $R\approx 2$ (Wang & Wheeler, 2008; Patat et al., 2015). There
is clearly wide scope to improve our understanding of how the properties and
distribution of grains vary within and among galaxies. Our results for VV
191b, probing the outskirts of disk dust and extending to wavelengths where
this behavior has been poorly known, show stronger reddening (steeper
attenuation law with wavelength) than otherwise seen in galaxies, even with a
possible role for mixtures of attenuation below the resolution limit
flattening the observed behavior. This might hint at a very different
population of grains, more weighted to smaller grains than in our
neighborhood.

##  5 Gravitational Lensing

Our first inspection of the NIRCam images revealed an object 2.8″from the
nucleus of the elliptical galaxy VV 191a, with a bright core and outskirts
forming an arc nearly centered on the foreground galaxy. It is much redder
than the elliptical galaxy, ruling out any sort of disrupted dwarf satellite
and suggesting a gravitationally lensed image of a background galaxy. In this
case VV 191 is now seen to show both dust backlighting and lensing
(retroactively earning its place in a project whose overall acronym includes
“Lensing”).

This object is shown in Figure 11 after subtracting the model for the
foreground elliptical as described in section 2.6. It is so red that only its
core was obvious in our HST F606W exposure. Galaxy-galaxy lens arcs often show
two images, and indeed a similarly red, slightly resolved object appears on
the opposite side of the foreground nucleus from the arc, at projected radius
1.03″ . We quantify the colors using aperture photometry (with radius 0.5″)
for the brightest part of the arc and for the foreground nucleus, flux
integrated within a segment of an annulus for the entire arc, and integrated
Gaussian fits for the counterimage. As we used data from an early version of
the calibration pipeline while the photometric calibration was still in flux,
we confirmed our understanding of the flux scale using the core of the
elliptical galaxy VV 191a and the consistent SEDs of luminous elliptical
galaxies (Table 1). The possible counterimage is so small that we measure its
count rates (WFC3) or signal in mJy (NIRCam) by fitting two-dimensional
Gaussian profiles using IRAF imexamine. These results are shown in Table 5,
with ratios shown in magnitudes as well for quick interpretation. The listed
errors include fluctuations in the residual background after model
subtraction, which often exceed those from photon statistics.

We estimate photometric redshifts for the lensed components based on the
magnitudes in Table 5, using the EAZY code (Brammer et al., 2008). The arc has
$z_{phot}=0.90\pm 0.07$ (60% probability) with secondary solutions
$z_{phot}=0.86\pm 0.07$ (26%) and $z_{phot}=1.17\pm 0.08$ (14%). Our initial
analysis derived much larger values; the updated NIRCam zero points,
reflecting greater sensitivity in the longer-wavelength bands and especially
F444W than predicted, give a blue spectral slope between F356W and F444W which
matches the long side of the generic 1.6$\mu$m peak in galaxy SEDs. The 4000-Å
break is now seen to fall between F606W and F090W bands. The filter selection,
driven by the dust analysis, leaves a large gap between F150W and F356W which
drives the large remaining uncertainty in photometric redshifts. The
counterimage has colors consistent with the same redshift, albeit with larger
uncertainties. This modest redshift fits with the small angular sizes of the
lensed components, since the derived redshift range is near the maximum in
angular-size distance; at $z=0.9$ the angular scale in the consensus cosmology
is already within 10% of its peak value at $z=1.6$.

Figure 11: Lensed arc and counterimage, shown after subtraction of the model
for the foreground elliptical galaxy, with intensity scaling as appropriate
for each. The top row shows HST WFC3 F606W and JWST NIRCam F090W and F150W
bands, with NIRCam F356W and F444W at the bottom. The nucleus of the
elliptical galaxy VV 191a is marked with a plus sign on the single-filter
images. Other features discussed in the text or in Table 5 are labelled on the
F444W image: the arc, counterimage CI, diffuse feature, and weakly lensed
galaxy A. FG indicates an outlying star-forming region in the foregrounf
galaxy. At lower right is a logarithmically-scaled composite of F150W (blue),
F356W (green), and F444W (red) data to show color structure in the arc and
relative colors of arc, counterimage, and weakly lensed galaxy A (which is
shown at higher contrast and magnified by a factor 1.5 in the inset for
F444W). The arc has radius 2.83 ″. The circle on the F356W image shows the
modelled Einstein radius 2.01″. Model subtraction leaves signatures of dust
from the outer disk of VV 191b visible to the right of the elliptical nucleus,
along with sub-percent residuals from fitting the elliptical-galaxy model near
the core and minor contributions from the hexagonal PSF structure. At the
shortest wavelengths, foreground dust lanes cross the right side of the arc
and affect its observed spatial profile.

We reproduced the image configuration, including the counterimage, with a two-
component model for the lens: a singular isothermal ellipsoid (SIE) and an
external shear. An initial optimization used the image centroids and flux
ratio of the arc and counterimage in F444W as constraints. Then with those
outputs as initial conditions, we performed a second optimization (using a
particle swarm optimization algorithm in both instances) over the F444W band
image (after subtraction of the foreground elliptical galaxy, see Figure 11),
including a model for the source light. Since there is a $\approx 0.5$ mag
color gradient between the core and outer region within the lensed arc, we
modeled the source light with a point source and two extended components (disk
and bulge) using two Sersic profiles. These profiles are constrained to be
concentric during the optimization. The best fit model parameters values
obtained after the second optimization are listed in Table 4. The strong
lensing modeling was done using the
lenstronomy555https://github.com/sibirrer/lenstronomy software package (Birrer
& Amara  2018, Birrer et al. 2021).

This model is able to reproduce both the image positions and the flux ratio in
the band F444W (see Table 5). The result is shown in Figure 12, along with the
lens geometry which requires the source to be near the cusp of the tangential
caustic.

Tracing the color of the arc as a function of angle around the lensing nucleus
shows a color gradient, with the core 0.44 magnitude redder between F090W and
F444W than the surrounding $20^{\circ}$ away. This might reflect a reddened
star-forming core or AGN, where the outer regions of the source are where the
outer regions of the source and the central redder component of the source are
blended in the counterimage. This color gradient further motivates our two-
component model for the lensed source.

The model optimization yields a reconstructed source with a bulge half-light
radius of $R_{hl}=0.09"$. The physical size depends on the angular-diameter
distance, and for the arc best fit photo-z source redshift
$R_{hl}(z=0.9)\approx 0.75$ kpc. Similarly, the effective velocity dispersion
for the isothermal lens model is $\sigma_{SIE}(z=0.9)\approx 250$ km/s which
is very close to the measured velocity dispersion of $\simeq 270\pm 7km/s$
dispersion from the SDSS for VV 191a, described above, though for a detailed
comparison a modest correction for the light profile is required (note that
this quantity is not the same as the measured dynamical value Treu & Koopmans
2002) because the measured dispersion in the fixed-diameter fiber of SDSS
spectroscopy is inherently intensity weighted. The apparent physical size of
the counterimage in the lens plane is $\approx 90$ pc.

We can take advantage of the symmetry of the lens and the low redshift of the
lensing galaxy VV 191a ($z=0.0513$ corresponding to an angular diameter
distance of only 208Mpc) to obtain a precise mass projected within the
Einstein radius of 2.01″. This is model independent, related only to
fundamental constants and the distances involved. The poorly-known redshift of
the lensed galaxy does not affect this calculation as the lens distance
$d_{L}$ is so small that the distance ratio $d_{LS}/d_{S}$ is almost unity,
for example for the photometric redshift estimates, the ratio is negligibly
different for $z=0.8$ and $z=1.2$ with values of $0.922$ and $0.942$
respectively. This results in a projected mass within the Einstein radius of
$M(<2.01")\approx 1.1\times 10^{11}M_{\odot}$ and $1\arcsec=1.008kpc$ at the
redshift of VV191a. We can compare this to the projected light from our
profile fit for VV 191a summed within the same $2.01\arcsec$ radius.

To estimate the mass-to-light ratio we convert the HST F606W (which is close
to SDSS $r$) luminosity to the $g$ band using the observation that $(g-r)$
colors of low-z luminous elliptical galaxies are always close to 0.80, and use
$(g-B)$ transformations from Jordi et al. 2006 for elliptical galaxies.
Applying the Galactic foreground extinction estimate from Schlafly &
Finkbeiner (2011) for the F606W data, we obtain $L_{B}(<2.01")=1.45\times
10^{10}L_{\odot,B}$, which yields a central mass-to-light ratio of
$M/L_{B}=7.6M_{\odot}/L_{\odot,B}$. This value can be compared to the higher-
redshift elliptical galaxy lens mass-to-light ratio in the B-band from Treu &
Koopmans (2002) of $M/L_{B}=8\pm 1M_{\odot}/L_{\odot}$, as well as the results
from Gerhard et al. (2001) for local E/S0 galaxies, which have sample mean
values near 7.2 from both dynamical models and individual metallicities plus
Salpeter IMF. Because the lens is comparatively nearby, these values depend
only weakly on the distance to the background source. For example, were the
source as distant as $z=3$, $M/L_{B}$ would decrease by 8%.

Strong lensing model parameters
---
SIE lens | SHEAR lens
$\theta_{E}$ $["]$ |  $\phi_{SIE}$ [deg] | $q$ | $\gamma_{1}$ | $\gamma_{2}$
$2.01$ | $-72$ | $0.93$ | $0.042$ | $-0.075$
Sérsic source
Disk | Bulge |
$R_{S}$ $["]$ |  $n_{S}$ $(^{*})$ |  $R_{S}$ $["]$ |  $n_{S}$ $(^{*})$ | centersrc
0.21 | 1 | 0.09 | 4 | ($-0.436,-0.854$)
Table 4: Best fit lens and source component parameters. The SIE component is
characterized by the Einstein radius $\theta_{E}$, the inclination angle
$\phi_{SIE}$, and the axial ratio $q$. The shear is defined by the two
components $(\gamma_{1},\gamma_{2})$. The extended source is modeled with two
Sérsic profiles, defined by the half-light radius $R_{S\acute{e}rsic}$, the
power-law index $n_{S\acute{e}rsic}$ and the center in the source plane. The
parameters marked with $(^{*})$ are kept fixed during the optimization. The
center of the lens components is kept fixed at $(0,0)$.

Figure 12: (top left) Observed data in band F444W, after subtraction of the elliptical galaxy lens. (top right) Lensing model: a lens with a Singular Isothermal Ellipsoid (SIE) mass distribution and external shear, with an extended source behind it. (bottom left) Lensing geometry reconstruction: the source (black star) is near the cusp of the tangential caustic (green line) and produces two images, of which A is the brightest. A is stretched into an arc due to its proximity to the critical line (red). (bottom right) Image-Model pixel-by-pixel differences in log scale.  Table 5: Photometry of the VV191a Possible Lensing Features Measurement | WFC3 | NIRCAM | NIRCAM | NIRCAM | NIRCAM
---|---|---|---|---|---
| F606W | F090W | F150W | F356W | F444W
Arc integrated: |  |  |  |  |
AB mag | $23.16\pm 0.3$ | $20.71\pm 0.05$ | $19.73\pm 0.05$ | $19.03\pm 0.05$ | $19.39\pm 0.05$
Arc core: |  |  |  |  |
AB mag | $23.65\pm 0.15$ | $21.16\pm 0.05$ | $19.95\pm 0.05$ | $19.41\pm 0.05$ | $19.77\pm 0.05$
Counterimage CI: |  |  |  |  |
Arc/counterimage ratio | — | 8.09 | 9.63 | 6.92 | 6.49
AB mag | – | $22.98\pm 0.05$ | $21.89\pm 0.05$ | $21.13\pm 0.05$ | $21.42\pm 0.05$
Weak lens candidate A: |  |  |  |  |
AB mag | — | $25.80\pm 0.20$ | $22.94\pm 0.05$ | $21.44\pm 0.05$ | $21.26\pm 0.05$
Diffuse component: |  |  |  |  |
AB mag | — | $23.51\pm 0.25$ | $22.52\pm 0.18$ | $22.11\pm 010$ | $22.63\pm 0.10$

The image configuration is quite similar to that seen in the strongly
magnified systems IRAS F10214+4724 (Eisenhardt et al., 1996) and 3C 220.3
(Haas et al., 2014). In exploring mass and lensing models, we note the SDSS
DR15 velocity dispersion $272\pm 7$ km s-1 with the BOSS fiber aperture of
diameter 2″.

An additional faint red galaxy appears at right in Figure 11, 5.25″from the
foreground nucleus. This object is elongated tangential to the foreground
galaxy (best seen in F444W), in a region where the arc model predicts that
weak lensing should stretch the image by about a factor 1.6 without generating
a counterimage, so this is a second plausibly lensed background system. It is
also very red; the F356W and F444W magnitudes are more reliable because the
F090W and F150W images are confused with a blue star image $0.18$″away (which
is too faint to be a major contaminant in the longer bands). Uncertainty due
to deblending star and galaxy fluxes is indicated by colons for these
passbands in Table 5. EAZY gives a primary photometric redshift $z=2.06\pm
0.10$ (55% likelihood) with a secondary (34%) peak at $z=1.7\pm 0.1$.

A similar EAZY fit gives photometric redshift estimate $z=0.73\pm 0.18$ for
the diffuse component outside the western end of the lensed arc, allowing but
not requiring its source to be physically associated with that of the arc.

Galaxy-galaxy gravitational lensing was a later discovery than lensing by
groups and clusters, since the Einstein radii are smaller and the light of the
lensed source may be blended with that from the lens galaxy. However, the more
detailed work needed to find such lenses has paid off in tracing the mass
distributions of (especially early-type) galaxies over a substantial redshift
range, with detection techniques ranging from high-redshift AGN spectra in
low-redshift galaxies (Huchra et al., 1985), through multiple redshift systems
in SDSS spectra (the SLACS survey, Bolton et al. 2004, 2005, 2006) and
searches for lens structures in HST images (a recent summary of the many
advances in finding such lens systems is given by O’Donnell et al. 2022). The
difficulty of recognizing the lensed components in VV191 from even HST data in
the optical, together with their prominence in the NIRCam images, suggests
that large numbers of galaxy-galaxy lenses will appear (even serendipitously)
in JWST galaxy studies. Such samples will improve our understanding of the
mass distribution in the lens galaxies over cosmic time, in favorable cases
also telling us about the masses of supermassive black holes. In VV 191a, the
lack of a detectable third image (not seen either in the data we show here or
the HST near-UV data) is consistent with the SMBH masses typically expected at
this luminosity, as is the mass model.

##  6 Summary and Conclusions

Combining HST and JWST imaging, we have used the backlighting of the face-on
spiral galaxy VV 191b by a luminous elliptical to trace the structure and
reddening law of dust in its outer disk. The unusually favorable geometry of
this system allows us to map the dust attenuation with spatial resolution
$\approx 60$ pc, spanning radii 12-21 kpc (much of this range, beyond the
stellar spiral arms), and with high-quality mapping from 0.6-1.5$\mu$m.

The distinct dust lanes end sharply at radius 21 kpc (1.7 Petrosian radii),
and exhibit not only the usual pattern following the pitch angle of the bright
spiral arms but additional lanes cutting at a variety of angles to this.

We fit curves for various values of total-to-selective extinction $R$ to the
PSF-matched attenuation values measured pixel-by-pixel in multiple filters
from 0.6-1.5 µm, to estimate the slope of the effective reddening law. Most
regions can be described by $R\approx 2.0$ using a CCM reddening law, or with
power-law index $\beta\approx 2.0$, with longer wavelengths showing smaller
$R$ and more positive $\beta$ (less-gray extinction). Limits to the
attenuation at longer wavelengths 3.5-4.4$\mu$m are 0.02-0.035 $A_{V}$.
Although even sub-percent systematics in modeling the galaxy brightness
profiles are important for small attenuation values, we see some evidence that
small-attenuation regions have larger $R$ than regions with more attenuation,
which could be a sign that clumpy structure affects strong-attenuation areas
more strongly. The most striking feature of these measurements is how strongly
reddening the behavior is, even in the likely presence of mixing effects
combining region of different attenuation in each resolution element, which we
in fact detect by measuring our transmission maps after various degrees of
averaging. Broadly, this might suggest a population strongly dominated by
small grains, such as has been suggested in the outer disk of the Milky Way by
Zasowski et al. (2009) based on stellar colors in the near-infrared. It may be
important in shaping the grain population the the dust features we sample in
VV 191b are as far as 7.5 kpc from regions of ongoing star formation as traced
in the UV or by hot dust.

This investigation of dust far from star-forming regions may be relevant to
the extinction and reddening in cosmologically-interesting SN Ia, which may
explode anywhere in a galaxy (in contrast to the concentration of core-
collapse supernova near their formation regions). Supernovae probe extinction
much more precisely than our spatially-averaged measurements, approaching the
point-source probes provided by ordinary stars. Thorp & Mandel (2022) use a
hierarchical Bayesian approach to address the possibility of unphysical ranges
of $R$ retrieved in supernova samples due to observational error, finding
$R=2.2-2.6$ for type Ia supernova in the Carnegie Supernova project. They
contrast this with $R=1.9$ from fits to individual SN from Johansson et al.
(2021). All the values are significantly lower than the frequently-used Milky
Way (local) mean $R\approx 3.5$, in the same sense as our results for the
outer disk of VV 191b.

The high quality of the NIRCam data from 0.9 to 1.5 µm means that the NIR is
now the most sensitive regime to trace backlit dust in galaxies, even with its
greater attenuation at shorter wavelengths. These observations detect dust
lanes farther out than the spiral pattern is seen in starlight even with the
deep IR images. F150W is surprisingly valuable for its leverage in fitting the
slope of the effective reddening law (F070W+F150W would be a fast survey pair
with NIRCam, with longer wavelengths observed at no extra cost in observing
time). The Galaxy Zoo overlapping-galaxy catalog alone (Keel et al., 2013)
includes a number of deeper overlap pairs within the distance $z=0.05$ of this
one, which would be at least as well resolved and allow construction of
composite attenuation profiles which could be valuable in constructing priors
for supernova photometry into the near-IR, and improving our understanding of
the energy budget in disk galaxies. Complementing such efforts, these
optical/NIR attenuation maps could provide a comparison set for data tracing
the PAH emission bands farther into the infrared, and far-IR or submillimeter
tracers of the overall dust emission, to refine our understanding of how the
components of the grain population change in various environments.

The JWST images further revealed a galaxy-galaxy gravitational lensing
instance through the elliptical galaxy VV 191a, with a background system at
$z\approx 0.9$ appearing as a $90^{\circ}$ arc and counterimage. An additional
distant background galaxy appears weakly lensed with modeled axial ratio 1.6.
All these can be reproduced simultaneously with a lens modeling including a
singular isothermal sphere and shear (likely associated with the adjacent
spiral galaxy). The small redshift of the lens allows a determination of the
mass/light ratio within its Einstein radius almost independent of the source
redshift, $M/L_{B}=7.6$ in solar units.

VV 191 vividly demonstrates a facet of backlit-galaxy dust analysis which
becomes important only at significant redshifts: large redshift differences
increase the role of gravitational lensing in distorting the background
sources, so using galaxy symmetry in modeling is more effective for small
redshift offsets between the two galaxies.

We dedicate this paper to Prof. Hendrik van de Hulst, who during his long and
very productive career at Sterrewacht Leiden had a mission to understand
cosmic dust. This project is based on observations made with the NASA/ESA _H_
ubble Space Telescope and the NASA/ESA/CSA _James Webb Space Telescope_ ,
obtained from the Mikulski Archive for Space Telescopes, which is a
collaboration between the Space Telescope Science Institute (STScI/NASA), the
Space Telescope European Coordinating Facility (ST-ECF/ESA), and the Canadian
Astronomy Data Centre (CADC/NRC/CSA). The IUE spectra were retrieved from the
INES service of LAEFF in Spain. RAJ, RAW, and SHC acknowledge support from
NASA _JWST_ Interdisciplinary Scientist grants NAG5-12460, NNX14AN10G and
80NSSC18K0200 from GSFC. CNAW acknowledges funding from the JWST/NIRCam
contract NASS-0215 to the University of Arizona. We acknowledge support from
the ERC Advanced Investigator Grant EPOCHS (788113), as well as a studentship
from STFC. AZ acknowledges support by Grant No. 2020750 from the United
States-Israel Binational Science Foundation (BSF) and Grant No. 2109066 from
the United States National Science Foundation (NSF), and by the Ministry of
Science & Technology, Israel. This research was supported by the Australian
Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions
(ASTRO 3D), through project number CE170100013. We thank our Program
Coordinator, Tony Roman, for his expert help scheduling this program. We thank
the Galaxy Zoo volunteers for finding so many of these backlit-galaxy systems
(particularly Galaxy Zoo users kodemunkey and Goniners for first pointing out
this pair). The data presented in this paper were obtained from the Mikulski
Archive for Space Telescopes (MAST) at the Space Telescope Science Institute.
The specific observations analyzed can be accessed via
https://doi.org/10.17909/78wg-t688 (catalog 10.17909/78wg-t688).

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