# PHANGS-JWST First Results: Measuring PAH Properties across the multiphase
ISM

Jérémy Chastenet Sterrenkundig Observatorium, Ghent University, Krijgslaan
281-S9, 9000 Gent, Belgium  Jessica Sutter Center for Astrophysics and Space
Sciences, Department of Physics, University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093, USA  Karin Sandstrom Center for
Astrophysics and Space Sciences, Department of Physics, University of
California, San Diego
9500 Gilman Drive, La Jolla, CA 92093, USA  Francesco Belfiore INAF — Arcetri
Astrophysical Observatory, Largo E. Fermi 5, I-50125, Florence, Italy  Oleg V.
Egorov Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität
Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany  Kirsten L. Larson
AURA for the European Space Agency (ESA), Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, MD 21218, USA  Adam K. Leroy Department of
Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, Ohio
43210, USA  Center for Cosmology and Astroparticle Physics, 191 West Woodruff
Avenue, Columbus, OH 43210, USA  Daizhong Liu Max-Planck-Institut für
Extraterrestrische Physik (MPE), Giessenbachstr. 1, D-85748 Garching, Germany
Erik Rosolowsky Department of Physics, University of Alberta, Edmonton,
Alberta, T6G 2E1, Canada  David A. Thilker Department of Physics and
Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA  Elizabeth
J. Watkins Astronomisches Rechen-Institut, Zentrum für Astronomie der
Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
Thomas G. Williams Sub-department of Astrophysics, Department of Physics,
University of Oxford, Keble Road, Oxford OX1 3RH, UK  Max-Planck-Institut für
Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany  Ashley. T. Barnes
Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121,
Bonn, Germany  F. Bigiel Argelander-Institut für Astronomie, Universität Bonn,
Auf dem Hügel 71, 53121 Bonn, Germany  Médéric Boquien Centro de Astronomía
(CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile
Mélanie Chevance Institut für Theoretische Astrophysik, Zentrum für Astronomie
der Universität Heidelberg,
Albert-Ueberle-Strasse 2, 69120 Heidelberg, Germany  Cosmic Origins Of Life
(COOL) Research DAO, coolresearch.io  Daniel A. Dale Department of Physics and
Astronomy, University of Wyoming, Laramie, WY 82071, USA  J. M. Diederik
Kruijssen Cosmic Origins Of Life (COOL) Research DAO, coolresearch.io  Eric
Emsellem European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748
Garching, Germany  Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de
Recherche Astrophysique de Lyon UMR5574, F-69230 Saint-Genis-Laval France
Kathryn Grasha Research School of Astronomy and Astrophysics, Australian
National University, Canberra, ACT 2611, Australia  ARC Centre of Excellence
for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia  Brent Groves
International Centre for Radio Astronomy Research, University of Western
Australia, 7 Fairway, Crawley, 6009 WA, Australia  Hamid Hassani Department of
Physics, University of Alberta, Edmonton, Alberta, T6G 2E1, Canada  Annie
Hughes IRAP, Université de Toulouse, CNRS, CNES, UPS, (Toulouse), France
Kathryn Kreckel Astronomisches Rechen-Institut, Zentrum für Astronomie der
Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
Sharon E. Meidt Sterrenkundig Observatorium, Ghent University, Krijgslaan
281-S9, 9000 Gent, Belgium  Hsi-An Pan Department of Physics, Tamkang
University, No.151, Yingzhuan Road, Tamsui District, New Taipei City 251301,
Taiwan  Miguel Querejeta Observatorio Astronómico Nacional (IGN), C/Alfonso
XII, 3, E-28014 Madrid, Spain  Eva Schinnerer Max-Planck-Institut für
Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany  Cory M. Whitcomb
Ritter Astrophysical Research Center, University of Toledo, Toledo, OH 43606,
USA

###### Abstract

Ratios of polycyclic aromatic hydrocarbon (PAH) vibrational bands are a
promising tool for measuring the properties of the PAH population and their
effect on star formation. The photometric bands of the MIRI and NIRCam
instruments on JWST provide the opportunity to measure PAH emission features
across entire galaxy disks at unprecedented resolution and sensitivity. Here
we present the first results of this analysis in a sample of three nearby
galaxies: NGC 628, NGC 1365, and NGC 7496. Based on the variations observed in
the 3.3, 7.7, and 11.3 $\mu$m features, we infer changes to the average PAH
size and ionization state across the different galaxy environments. High
values of F335MPAH/F1130W and low values of F1130W/F770W are measured in H II
regions in all three galaxies. This suggests that these regions are populated
by hotter PAHs, and/or that the PAH ionization fraction is larger. We see
additional evidence of heating and/or changes in PAH size in regions with
higher molecular gas content as well as increased ionization in regions with
higher H$\alpha$ intensity.

Dust physics (2229), Interstellar dust (836), Polycyclic aromatic hydrocarbons
(1280)

††facilities: JWST (NIRCam, MIRI), MUSE, ALMA

##  1 Introduction

The mid-infrared (mid-IR) emission features at 3.3, 6.2, 7.7, 8.6, 11.3, 12.6,
and 17 $\mu$m first seen in Milky Way star–forming regions (Gillett et al.,
1973; Merrill et al., 1975) and later observed in the spectrum of star-forming
galaxies (e.g., Smith et al., 2007) have been attributed to the vibrational
modes of large carbonaceous molecules (Allamandola et al., 1989; Draine & Li,
2001; Tielens, 2008a), referred to as polycyclic aromatic hydrocarbons (PAHs,
Leger & Puget, 1984; Allamandola et al., 1985), or in some models, to the same
modes of surface H atoms in the aromatic mantle of dust grains (Jones et al.,
2017). PAHs are important in a variety of processes in the ISM, including
photoelectric heating of neutral gas (Bakes & Tielens, 1994; Wolfire et al.,
1995; Weingartner & Draine, 2001) and the formation of H2 (Le Page et al.,
2009; Le Bourlot et al., 2012).

The characteristics of these molecules, including their size, charge,
hydrogenation, and structure, determine the efficiency of these processes
(e.g. the charge state of PAHs determines the efficiency of the photoelectric
effect; Tielens, 2008a), and set the relative strength of the various mid-IR
vibrational bands (Maragkoudakis et al., 2020). Intensive laboratory and
theoretical work continues to identify the contributing species to each of
these emission features or complexes of features (e.g. Boersma et al., 2014;
Bauschlicher et al., 2018; Mattioda et al., 2020; Kerkeni et al., 2022).
Observations of PAH band ratios can use this information to infer properties
like average size and ionization by modeling the distribution of PAHs that
contribute to the observed emission (e.g. Draine et al., 2021; Rigopoulou et
al., 2021; Maragkoudakis et al., 2022; Kerkeni et al., 2022).

Some of these features are now generally agreed to be produced by specific
types of PAHs. For instance, the 3.3 $\mu$m feature is characteristic of
small, neutral PAHs, while the 7.7 $\mu$m complex traces vibrational bands
produced by larger, positively charged ions, and the 11.3 $\mu$m is
representative of a population of grains that are both larger and neutral
(e.g. Galliano et al., 2008; Boersma et al., 2016, 2018; Maragkoudakis et al.,
2020; Draine et al., 2021; Rigopoulou et al., 2021; Maragkoudakis et al.,
2022). The ratios of these bands can therefore constrain quantities such as
the relative amounts of small to large PAHs (in terms of number of carbon
atoms) and the fraction of ionized to neutral PAHs.

Determining how these ratios vary across a galaxy can also provide important
context for how the PAHs are influencing the surrounding ISM. For example,
higher fractions of ionized PAHs could lead to a decrease in the photoelectric
heating efficiency, as highly charged grains will have higher ionization
potentials (Tielens, 2008b). This has been observed in variations in the
relative strength of far-infrared cooling lines compared to total infrared
luminosity as a function of PAH feature ratios, suggesting the properties of
the PAHs influence the balance of ISM heating and cooling (Croxall et al.,
2012; McKinney et al., 2021; Sutter & Fadda, 2022). As both a source and sink
for free electrons, PAHs also play a large role in setting the ionization
balance (Li, 2020; Bakes & Tielens, 1994). Understanding the properties of the
PAHs as traced by band ratios will therefore help establish how these
important molecules influence ISM conditions.

The James Webb Space Telescope (JWST) opens a new era in PAH studies by
simultaneously offering a diverse suite of filters covering many key PAH
features, and offering unprecedented resolution in the mid-IR.

While previously restricted to Local Group galaxies (see e.g. Sandstrom et
al., 2012), it is now possible to directly probe these ratios across the disks
of nearby galaxies, at small physical scales (tens of parsecs), with JWST.
With the NIRCam instrument (Rieke et al., 2005) covering the 3.3 $\mu$m
feature, and the MIRI instrument (Rieke et al., 2015) covering the 7.7 and
11.3 $\mu$m complexes, we can investigate the properties of PAHs across the
extent of $D\lesssim 20$ Mpc galaxies at $\sim 10-50$ pc resolution. The
sensitivity of JWST further enables measurements of the PAH properties in
multiple phases of the ISM (e.g. diffuse atomic gas, molecular clouds without
massive star forming regions, molecular clouds with associated H II regions,
diffuse ionized gas), not just the bright star–forming complexes that were
visible to Spitzer IRS.

In this letter we deploy this new capability to measure the resolved
variations in PAH band ratios in three of the first targets of the PHANGS-JWST
survey (Lee et al., 2022). We make maps of the F335MPAH/F770W, F335MPAH/F1130W
and F1130W/F770W over wide areas to trace how the properties of the PAHs vary
in comparison with multiwavelength tracers of the phase of the ISM.

##  2 Data

###  2.1 JWST-MIRI maps tracing the $7.7$ and $11.3~{}\mu$m bands

We use the MIRI F770W and F1130W maps of three early targets in the PHANGS-
JWST Treasury program #2107 (PI: Lee): NGC 628, NGC 1365, and NGC 7496.
Description of the data reduction for the MIRI images can be found in Lee et
al. (2022). Since we are interested in the diffuse PAH emission throughout the
maps, the background level and the uncertainty in the maps are critical. The
background level was tied to larger archival Spitzer and WISE maps of the
targets with well-covered “off” galaxy regions (see Appendix A and B in Leroy
et al., subm.). As shown in Figure 1, the F770W and F1130W were ideally
designed to sample the 7.7 and 11.3 $\mu$m PAH emission complexes. The filter
transmission curves are shown as colored lines with labels on the right, and
the shaded areas help visualize the fraction of each complex that is included
in the filters used here. For visual comparison purposes, we indicate with
vertical lines the ‘clip points’ used in Draine et al. (2021). In this Letter,
the F770W and F1130W values we use are instrument’s filter-integrated fluxes.

Figure 1: Visual representation of some of the NIRCam, and the MIRI filters,
with their normalized transmission curves, in color, and associated integrated
area for the three main filters used in this study. The vertical lines and the
shaded regions mark the wavelength cuts for the clip points defined in Draine
et al. (2021) that they used to integrate the feature luminosities. The
associated complexes are labels on top. The black solid and dashed lines are
representative mid-IR spectra from Draine & Li (2007, $U=1.0$, $b_{\rm
C}=0.05$), and Draine et al. (2021, ‘mMMP’ with $U=1.0$, ‘st’ ionization and
‘std’ size), for comparison.

All maps were convolved to the point spread function (PSF) of the F1130W
filter (${\rm FWHM}\sim 0.36^{\prime\prime}$) using a standard convolution
procedure, with kernels generated following the approach of Aniano et al.
(2011). All maps were also aligned to the same astrometric grid, with pixels
size $\sim 0.11^{\prime\prime}$.

Although the F770W and F1130W bands are dominated by the 7.7 $\mu$m and 11.3
$\mu$m PAH emission features, both can also include some hot dust and/or
continuum emission, especially towards bright H II regions. Whitcomb et al.
(2022) used PAHFIT (Smith et al., 2007) to show that the Spizter/IRS spectra
of the star-forming regions in the SINGS sample, which are among the brightest
in these galaxies, may have $\sim 30\%$ continuum contamination in F1130W and
$\sim 10\%$ in F770W, although these measurements rely on models of mid-IR
emission that will likely be updated with new JWST results. Note that H II
regions are the place where contamination from hot dust is the highest.
Naturally, a more detailed analysis and correction will be the subject of
further study.

In addition to continuum contamination, it is also possible that silicate
absorption at 9.7 $\mu$m could reduce the F1130W flux. Smith et al. (2007)
show that this absorption should not play a large role in most star-forming
galaxies. Additionally, Groves et al. (submitted) also find that a median
$E(B-V)\sim 0.2-0.3$ for H II regions in the four targets in this work, a
relatively low value. At this point, we therefore do not considered silicate
absorption in this early work.

###  2.2 JWST-NIRCam maps tracing the 3.3 $\mu$m band

We use the 3.3 $\mu$m emission maps from Sandstrom et al. (subm.). They use a
combination of the NIRCam F335M (measuring the main PAH emission feature), and
the F300M plus F360M filters to remove starlight continuum. They base their
continuum removal approach on the prescription from Lai et al. (2020), with
additional corrections to avoid over-subtraction of the PAH emission in
regions where the map is ISM emission dominated rather than stellar continuum
dominated in the F360M filter111Note that an additional factor would be
required to convert F335MPAH from MJy sr-1 into an integrated band intensity
(c.f. Lai et al., 2020).. Lai et al. (2020) show contamination from emission
lines is negligible in the F335M band. For full details on the removal of
contamination from starlight in the F335M band, we refer to Sandstrom et al.
(subm.), which includes robust discussion of any potential remaining
contamination which could artificially raise the F335MPAH. We refer to these
final maps that isolate the emission from the 3.3 $\mu$m emission complex as
F335MPAH. We convolve these maps to the F1130W PSF using the same approach as
the other maps, as described above.

###  2.3 Ancillary data

We combine CO, Hi, and H$\alpha$ measurements to trace the conditions of the
ISM in our targets. We use the same CO/Hi/H$\alpha$ data as the companion
letter on PAH fraction (Chastenet et al., subm.). We trace the molecular gas
content using CO data from the PHANGS-ALMA survey (Leroy et al., 2021a, b). We
convert ${\rm{}^{12}CO~{}(2-1)}$ measurements to H2 surface density using the
Milky Way CO-to-H2 conversion factor $\alpha_{\rm CO}=4.35$ M⊙ pc-2 (K km
s${}^{-1})^{-1}$, and a ${}^{12}\mathrm{CO}(2-1)$ to ${}^{12}\mathrm{CO}(1-0)$
line ratio $R_{21}=0.65$ (den Brok et al., 2021; Leroy et al., 2022). We use
Hi data from the PHANGS-MeerKAT survey (C. Eibensteiner et al., in prep) and
THINGS (Walter et al., 2008) surveys, assuming optically thin 21-cm emission,
and using a conversion factor of $0.020$ M⊙ pc-2 (K km s${}^{-1})^{-1}$ (Leroy
et al., 2012, see also Walter et al. 2008). We do not convolve the JWST and
other datasets to match the $11^{\prime\prime}-15^{\prime\prime}$ Hi
resolution. We assume that the Hi is smoothly distributed below the MeerKAT
and THINGS resolution, as suggested by observations of more nearby targets
(Leroy et al., 2013). There are currently no available 21-cm observations for
NGC 1365, so we assume it to have a uniform neutral gas surface density of 8
M⊙ pc-2 everywhere, based on the observed flatness of the atomic gas
distribution in the disks of galaxies (e.g. Schruba et al., 2011; Bigiel &
Blitz, 2012; Kennicutt & Evans, 2012; Wong et al., 2013). We trace ionized gas
using the H$\alpha$ data (not extinction corrected) from the PHANGS-MUSE
observations (Emsellem et al., 2022).

###  2.4 Noise properties and masks

After convolution to the F1130W PSF, we measure a background standard
deviation of $\sim 0.065$ MJy sr-1 in the MIRI F770W map of NGC7496. It is the
only target that provides sufficiently empty space to measure a background
without significant contamination from the source. We use that value to
perform signal-to-noise (S/N) cuts in all three galaxies, and we exclude
pixels with ${\rm S/N\leq 3}$ in the MIRI maps. All MIRI maps have similar
noise from the pipeline, verified with moderate masking, so it therefore is
reasonable to assume the convolved maps shows similar noise. We consequently
base our noise estimate off the convolved map of NGC 7496.

After convolution, we measure a noise in the F335MPAH maps of $\sim 0.014$ MJy
sr-1 in NGC 7496. Similarly, we use that value in all three targets to perform
cuts and remove pixels with ${\rm S/N\leq 3}$ in the F335MPAH maps. Figures 2
through 4 show the remaining pixels in the three targets.

Since NGC 1365 and NGC 7496 have saturated central sources that cause
contamination of the surrounding maps by the PSF wings, we use the instrument
PSF of the F1130W filter to mask the saturated and contaminated pixels in
these galaxy centers.

##  3 Band ratios

We present maps of the F335MPAH/F770W, F335MPAH/F1130W and F1130W/F770W band
ratios in NGC 628 (Figure 2), NGC 1365 (Figure 3), and NGC 7496 (Figure 4).
The color scale for each band ratio is the same across the three figures, with
limits chosen using the 5th and 95th percentiles of all values. We overlay the
H II regions cataloged in Groves et al. (subm.); Santoro et al. (2022) with
black contours.

Figure 2: PAH band ratios in NGC 628. The maps show all pixels passing ${\rm
S/N\geq 3}$ in all bands (see Section 2.4). The F335MPAH refers to the 3.3
$\mu$m maps developed by Sandstrom et al. (subm.). The contour show the H II
regions from the nebula catalog by Groves et al. (subm.); Santoro et al.
(2022). (The maps were arbitrarily rotated from north-up to allow for better
visualization.)  Figure 3: Same as Figure 2 for NGC 1365. The central pixels
are masked due to saturation, using the PSF at F1130W. The visible stripes are
due to $1/f$ noise in the NIRCam bands. (The maps were arbitrarily rotated
from north-up to allow for better visualization.) Figure 4: Same as Figure 2
for NGC 7496. The central pixels are masked due to saturation, using the PSF
at F1130W. The visible stripes are due to $1/f$ noise in the NIRCam bands.

Overall, each ratio shows variations both within and across galaxies, spanning
values within a factor of 4–5 for F335MPAH/F770W and F335MPAH/F1130W, and less
than a factor of 2 in the F1130W/F770W maps. Table 1 lists the mean, median
and ${\rm 16^{th}-84^{th}}$ percentile ranges of the three ratios in each
galaxy. These values are also well-illustrated in Figure 5. The mean and
median values in F335MPAH/F770W and F335MPAH/F1130W increase from NGC 628, to
NGC 1365, and to NGC 7496 which shows the highest ratios. The ${\rm
16^{th}-84^{th}}$ percentile ranges between NGC 628 and NGC 7496 barely
overlap, indicating a real offset between these two galaxies in the
F335MPAH/F770W and F335MPAH/F1130W ratios. On the other hand, all three
galaxies show relatively similar mean and median values of the F1130W/F770W
ratio, with significant overlap in ${\rm 16^{th}-84^{th}}$ ranges. This
suggests that, in terms of band ratios, the main difference between each
galaxy is driven by the emission from the 3.3 $\mu$m complex, traced by
F335MPAH (see also Sandstrom et al., subm.).

Galaxy | F335MPAH/F770W | F335MPAH/F1130W | F1130W/F770W
---|---|---|---
Mean | Median |  ${\rm 16^{th}-84^{th}}$ perc. | Mean | Median |  ${\rm 16^{th}-84^{th}}$ perc. | Mean | Median |  ${\rm 16^{th}-84^{th}}$ perc.
NGC 628 | 0.043 | 0.039 | $0.028-0.054$ | 0.032 | 0.029 | $0.02-0.042$ | 1.38 | 1.37 | $1.20-1.56$
NGC 1365 | 0.061 | 0.053 | $0.035-0.084$ | 0.042 | 0.037 | $0.023-0.058$ | 1.48 | 1.48 | $1.27-1.69$
NGC 7496 | 0.067 | 0.057 | $0.040-0.092$ | 0.049 | 0.041 | $0.029-0.065$ | 1.41 | 1.40 | $1.22-1.60$
Table 1: Means, medians and ${\rm 16^{th}}$ and ${\rm 84^{th}}$ percentiles
for the three ratios F335MPAH/F770W (left), F335MPAH/F1130W (center), and
F1130W/F770W (right), in each galaxy. While the ${\rm 16^{th}-84^{th}}$
percentile ranges of F1130W/F770W overlap between targets, the other ratios
with F335MPAH seem to show more distinct values.

The ratio maps show higher values of the F335MPAH/F770W and F335MPAH/F1130W
ratios in H II regions. This is clearly visible in NGC 628 and NGC 7496, and
slightly less obvious in NGC 1365 though still distinguishable. This indicates
enhanced 3.3 $\mu$m emission in these regions compared to the other two PAH
features. In all galaxies, the F1130W/F770W maps show darker colors in H II
regions, pointing to increased average PAH ionization likely driven by the
proximity of young stellar populations. This is similar to what is seen in
Dale et al. (2022), which examines the properties of the PAHs within compact
stellar clusters and finds a positive correlation between cluster age and PAH
ionization. In the galaxy NGC 1365, darker colors in the right panel of Figure
3 (i.e., lower values of F1130W/F770W), tracing more ionized PAHs, can be
found in the dust lanes. This is also visible in NGC 7496.

The relative prominence of the 3.3 $\mu$m feature in H II regions could
suggest that PAHs in these regions are smaller, and/or hotter than in the
surrounding diffuse ISM. The emission in the 3.3 $\mu$m is usually attributed
to small PAHs. In this case, the higher F335MPAH/F770W and F335MPAH/F1130W
would suggest smaller PAHs in H II regions. But the seemingly higher fraction
of smaller PAHs in harsh environment is not supported by theoretical studies,
which have shown that smaller PAHs have shorter lifetimes in a hot gas (e.g.,
Micelotta et al., 2010). However, it is important to note that in the vicinity
of H II regions with harder radiation fields, PAHs will be hotter due to an
increase in the average energy of the photons. An increase in dust temperature
naturally shifts the power in the emission spectrum towards shorter
wavelengths; in this case, hotter PAHs would lead to more power in the 3.3
$\mu$m feature, interpreted as an _apparent_ shift towards smaller sizes
(Draine et al., 2021). This shift is clearly demonstrated in Figure 21, panel
d of Draine et al. (2021), in which the value of F${}_{\rm
clip}(3.3)$/F${}_{\rm clip}(11.2)$ irradiated by a Galactic radiation field
(mMMP) is approximately a factor of two lower than the F${}_{\rm
clip}(3.3)$/F${}_{\rm clip}(11.2)$ irradiated by a young stellar population
(BC03, 3Myr). We note that the shift reported by the models is smaller than
the shift we see in the H II regions across the galaxies presented here,
suggesting potential additional factors not accounted for in the Draine et al.
(2021) models.

The combined analysis of these three galaxies also indicates that the
F1130W/F770W ratios are lowest in H II regions, the star-forming arms, and gas
and dust lanes that may include shocked gas. This argues that the fraction of
ionized PAHs surrounding sites of active star formation are is on average
higher than in the diffuse ISM. This is further discussed in Egorov et al.
(2022), which examines the PAH emission in H II regions. The relative
consistency of the ratios in the interarm ISM where detected, despite sampling
three different galaxies, implies that PAHs permeate the diffuse ISM and have
similar properties throughout this ISM phase.

Note that we attribute the high F335MPAH/F770W and F335MPAH/F1130W in the
inner 200 pc ($\sim 4^{\prime\prime}$) of NGC 628 to the very low ISM
contribution in that region, and not a change in PAH properties (e.g., Dale et
al., 2006; Hoyer et al., 2022).

##  4 Variation of band ratios with ISM environment

In Figure 5, we show the variations of the three ratios, F335MPAH/F770W (top
row), F335MPAH/F1130W (middle row), and F1130W/F770W (bottom row), as a
function of $I_{\rm H{\alpha}}/\Sigma_{\rm H{\sc I}+H_{2}}$ in units of ${\rm
erg~{}s^{-1}~{}kpc^{-2}~{}(M_{\odot}~{}pc^{-2}})^{-1}$ (left column), and
fraction of molecular gas (right column). The symbols are the medians in each
bin of the x-axis, and the error bars show 3 standard errors of the mean.

All ratios show clear variations with the ionized gas content, as shown in the
first column. The first two ratios are sensitive to the global size of the
PAHs, and increase with $I_{\rm H{\alpha}}/\Sigma_{\rm H{\sc I}+H_{2}}$. This
suggests that the PAH population decreases to lower sizes as the ionized gas
becomes more prominent, though heating could also drive this observed change
(as discussed in Section 3). The F1130W/F770W ratio, on the other hand,
decreases at high values of $I_{\rm H{\alpha}}/\Sigma_{\rm H{\sc I}+H_{2}}$,
which suggests that the PAHs are more ionized with a higher fraction of
ionized gas to total gas. Both these trends are consistent with the expected
first order effect from hot gas, that should be able to fragment PAHs or kick
electrons. However, there is also a known negative effect of ionized gas on
the global PAH abundance (e.g. Micelotta et al., 2010; Chastenet et al.,
2019). The last column of Figure 5 shows the variations of the “abundance
ratio”, $R_{\rm PAH}={\rm(F770W+F1130W)/F2100W}$ tracing the global PAH
abundance222To first order, (7.7+11.3)/21 traces PAH abundance, as was seen by
the equivalent 8/24 $\mu$m that was frequently used in Spitzer observations
(e.g., Engelbracht et al., 2006, 2008; Draine & Li, 2007). In our early
results, we are focused on observed ratios, rather than modeling. Any exact
relationship between the PAH fraction and the ratio used to measure $R_{\rm
PAH}$ will be left to future studies. (see also Letters from Chastenet et al.,
subm.; Egorov et al., 2022, in this Issue). In these panels, neither of the
ratios shows a strong trend with $R_{\rm PAH}$. Chastenet et al. (subm.) show
that $R_{\rm PAH}$ clearly decreases with $I_{\rm H{\alpha}}/\Sigma_{\rm H{\sc
I}+H_{2}}$, but no clear variation is seen with size or ionization tracers.

The second column shows the variations of the three ratios with the fraction
of molecular gas, $f_{\rm H_{2}}\equiv\Sigma_{\mathrm{H_{2}}}/(\Sigma_{\rm
H{\sc I}}+\Sigma_{\mathrm{H_{2}}})$. Both ratios tracing the size of PAHs
(F335MPAH/F770W, and F335MPAH/F1130W) show a steep decrease with $f_{\rm
H_{2}}$ in NGC 7496, and a much shallower trend in NGC 0628 and NGC 1365. In
the bottom-center panel, however, while all curves show similar trends, only
NGC 1365 differs from NGC 628, while NGC 7496 show identical values over a
large range of $f_{\rm H_{2}}$. While NGC 1365 clearly has a higher CO
content, NGC 628 and NGC 7496 show similar $L_{\rm CO}$ values (Leroy et al.,
2021b). Additionally, note that both NGC 1365 and NGC 7496 host an AGN in
their center. The variations shown in the center column could indicate that:
(1) the presence of an AGN has a stronger effect on the size of PAHs rather
than their ionization, and (2) the CO content will have the opposite effect,
with a higher impact on ionization rather than size. However, these two
Seyfert galaxies show different trends, making it difficult to clearly
determine the effect of an AGN on the PAH population at this stage.

Figure 5: Running medians of the F335MPAH/F770W (top), F335MPAH/F770W
(middle), and F1130W/F770W (bottom), in bins of $I_{\rm H{\alpha}}/\Sigma_{\rm
H{\sc I}+H_{2}}$ (left), fraction of molecular gas, $f_{\rm H_{2}}$ (right),
and the abundance ratio $R_{\rm PAH}$ = (F700W+F1130W)/F2100W.

In Figure 6 we show the 2D histograms of all three ratios with each other,
with the associated running medians and error bars showing 3 standard errors
of the mean. In the bottom panel, the F335MPAH/F770W and F335MPAH/F1130W
appear clearly correlated, which is expected as they are both tracers of the
global size distribution of PAHs333We note that the bulk of the
F335MPAH/F1130W distribution is lower than the one found in Sandstrom et al.
(subm.). This is likely due to the convolution to F1130W resolution, which
leads to including more pixels in the diffuse regions, with lower F335MPAH
values.. In the top row, the F1130W/F770W as a function of F335MPAH/F770W
(left) shows a rather flat distribution. In the right panel, F1130W/F770W
seems to gently decrease with F335MPAH/F1130W (though the large scatter can be
consistent with a flat trend). The lack of a clear trend between F1130W/F770W
and the other ratios is interesting as it suggests that ionization and size do
not strongly vary together. A clear interpretation of the difference between
the two panels in the top row requires a clear understanding of the different
populations traced by F335MPAH/F770W and F335MPAH/F1130W.

Figure 6: 2D histograms of the F335MPAH/F770W vs F1130W/F770W (top), and
F335MPAH/F770W vs F335MPAH/F1130W (bottom). Both ratios in the bottom panel
trace the size of PAHs, and show a clear correlation. In the top panel, the
x-axis traces size, while the y-axis traces ionization.

##  5 Conclusions

Data from JWST provide exciting opportunities for understanding the ISM
conditions of galaxies outside the Milky Way at $\sim$10 pc resolution. In
particular, the MIRI and NIRCam instruments are able to capture the strength
of important PAH features which can be used to assess the properties of these
dust grains. Studies of the PAH population in nearby galaxies have shown the
PAHs have an important impact on other evolution process, as they can be used
as a star formation rate tracer and are heavily responsible for the
photoelectric heating of the gas. Through analysis of the ratio of the 11.3
$\mu$m to the 7.7 $\mu$m PAH features, changes in the ionization fraction of
the PAHs can be determined, while the strength of the 3.3 $\mu$m compared to
the 7.7 $\mu$m or 11.3 $\mu$m features provides information on the average PAH
size.

We present maps of these ratios across nearby galaxies NGC 628, NGC 1365, and
NGC 7496. Based on the relative consistency of these ratio across each galaxy
disk, we conclude that the population of PAHs is fairly uniform. H II regions
and star-forming spiral arms show lower 11.3/7.7 and higher F335MPAH/F1130W
values, indicating the presence of smaller, highly charged grains in these
regions. The trends in these ratios are further discussed in Dale et al.
(2022), where each ratio is measured in a sample of over 1000 stellar
clusters. In addition, the trends observed between the state of the gas
measured by $f_{\rm H_{2}}$ and $I_{\rm H{\alpha}}/\Sigma_{\rm H{\sc
I}+H_{2}}$ show that local gas conditions can greatly impact the sizes and
ionization levels of the PAHs: more ionized (traced by F1130W/F770W) and
smaller PAHs (traced by F335MPAH/F770W and F335MPAH/F1130W) are present in
regions showing high H$\alpha$ intensity.

This work highlights the diagnostic power of JWST MIRI and NIRCam observations
of nearby galaxies. By probing previously inaccessible spatial scales and
emission levels, we can better assess the physical mechanisms that link PAH
grains and their local environment. Further work to determine how these PAH
feature ratios are influenced by stellar population age, gas–phase
metallicity, and other galactic-scale and ISM properties will shed light on
the delicate interplay between stars, gas, and dust in galaxies.

## Acknowledgments

We thank the anonymous referee for their careful reading and comments that
helped improve the clarity of the paper. This work was carried out as part of
the PHANGS collaboration, associated with JWST program 2107. This work is
based on observations made with the NASA/ESA/CSA JWST. Some/all of the data
presented in this paper were obtained from the Mikulski Archive for Space
Telescopes (MAST) at the Space Telescope Science Institute, which is operated
by the Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS 5-03127. The specific observations analyzed can be accessed via
http://dx.doi.org/10.17909/9bdf-jn24 (catalog 10.17909/9bdf-jn24). Based on
observations collected at the European Southern Observatory under ESO
programmes 094.C-0623 (PI: Kreckel), 095.C-0473, 098.C-0484 (PI: Blanc),
1100.B-0651 (PHANGS-MUSE; PI: Schinnerer), as well as 094.B-0321 (MAGNUM; PI:
Marconi), 099.B-0242, 0100.B-0116, 098.B-0551 (MAD; PI: Carollo) and
097.B-0640 (TIMER; PI: Gadotti). This paper makes use of the following ALMA
data: ADS/JAO.ALMA#2012.1.00650.S, ADS/JAO.ALMA#2013.1.01161.S,
ADS/JAO.ALMA#2015.1.00925.S, ADS/JAO.ALMA#2015.1.00956.S,
ADS/JAO.ALMA#2017.1.00392.S, ADS/JAO.ALMA#2017.1.00766.S,
ADS/JAO.ALMA#2017.1.00886.L, ADS/JAO.ALMA#2018.1.01651.S.
ADS/JAO.ALMA#2018.A.00062.S. ALMA is a partnership of ESO (representing its
member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST
and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the
Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and
NAOJ.

JC acknowledges support from ERC starting grant #851622 DustOrigin. EJW
acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation) – Project-ID 138713538 – SFB 881 (“The Milky Way System”,
subproject P1). HAP acknowledges support by the National Science and
Technology Council of Taiwan under grant 110-2112-M-032-020-MY3. MC gratefully
acknowledges funding from the DFG through an Emmy Noether Research Group
(grant number CH2137/1-1). COOL Research DAO is a Decentralized Autonomous
Organization supporting research in astrophysics aimed at uncovering our
cosmic origins. JMDK gratefully acknowledges funding from the European
Research Council (ERC) under the European Union’s Horizon 2020 research and
innovation programme via the ERC Starting Grant MUSTANG (grant agreement
number 714907). TGW acknowledges funding from the European Research Council
(ERC) under the European Union’s Horizon 2020 research and innovation
programme (grant agreement No. 694343). MB acknowledges support from FONDECYT
regular grant 1211000 and by the ANID BASAL project FB210003. KK, OE
gratefully acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) in the form of an Emmy Noether Research Group
(grant number KR4598/2-1, PI Kreckel). FB would like to acknowledge funding
from the European Research Council (ERC) under the European Union’s Horizon
2020 research and innovation programme (grant agreement No.726384/Empire). MQ
acknowledges support from the Spanish grant PID2019-106027GA-C44, funded by
MCIN/AEI/10.13039/501100011033. ER and HH acknowledge the support of the
Natural Sciences and Engineering Research Council of Canada (NSERC), funding
reference number RGPIN-2022-03499. KG is supported by the Australian Research
Council through the Discovery Early Career Researcher Award (DECRA) Fellowship
DE220100766 funded by the Australian Government. KG is supported by the
Australian Research Council Centre of Excellence for All Sky Astrophysics in 3
Dimensions (ASTRO 3D), through project number CE170100013. AKL gratefully
acknowledges support by grants 1653300 and 2205628 from the National Science
Foundation, by award JWST-GO-02107.009-A, and by a Humboldt Research Award
from the Alexander von Humboldt Foundation.

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