# DISTANCES TO 10 NEARBY GALAXIES OBSERVED WITH THE HUBBLE SPACE TELESCOPE

I.D.Karachentsev  Special Astrophysical Observatory, the Russian Academy of
Sciences, Nizhnij Arkhyz, Karachai-Cherkessian Republic, Russia 369167
<EMAIL_ADDRESS>N.A.Tikhonov  Special Astrophysical Observatory, the Russian
Academy of Sciences, Nizhnij Arkhyz, Karachai-Cherkessian Republic, Russia
369167<EMAIL_ADDRESS>
###### Abstract

Images of 10 galaxies in F814W and F606W filters obtained on the Hubble Space
Telescope (HST) are used to construct color-magnitude diagrams for the star
population of these galaxies. The distances to the galaxies are estimated from
the luminosity of the tip of the red giant branch. The galaxies examined here
have radial velocities from 250 to 760 km/s relative to the centroid of the
Local Group and distances ranging from 3.7 to 13.0 Mpc. Several other observed
galaxies with low radial velocities are noted at distances beyond the limit of
13 Mpc.

galaxies— dwarf galaxies: distances — galaxies

##  1 Introduction

Considerable advances in the study of the kinematics and dynamics of the Local
universe over the last two decades have resulted from massive determination of
highly accurate distances to nearby galaxies with the aid of the Hubble Space
Telescope (HST). Using the luminosity of the tip of the red giant branch
(TRGB) (Lee et al. 1993) it has become possible to measure distances with an
error of $\sim 5$% for more than 400 galaxies in the Local volume (LV) within
distances of D$\approx$11 Mpc around us. For many galaxies their membership in
nearby groups has been established and based on the virial velocities of the
satellites the masses of the dark halo surrounding the bright central galaxies
(Karachentsev and Kashibadze 2021) has been determined. The average density of
the dark matter concentrated in the halos of the local galaxies turned out to
be equal to 0.08 in units of the critical density (Karachentsev and Telikova
2018), which forms only a quarter of the global cosmic density of matter
$\Omega_{m}\approx 0.31$. This result seems paradoxical since the average
density of stellar mater in the Local volume is practically indistinguishable
from the average global stellar density. The deviation of the radial
velocities of the galaxies from the ideal Hubble flow $V=H_{0}D$, where
$H0=73$ km/s/Mpc is the Hubble parameter, also provided precise evidence of
the existence of a collective motion of the local galaxies toward a nearby
rich cluster in the Virgo constellation, as well as of their participation in
the systematic expansion of the neighborhoods of the Local void. Observational
data on the distances of the galaxies in the LV presented in the Updated
Nearby Galaxy Catalog (UNGC) (Karachentsev et al. 2013) and Extragalactic
Distance Database (EDD) (Anand et al. 2021) are the basis for testing
cosmological models on small-scales.

The latest significant supplement to the data on the distances of nearby
galaxies were the observations on the HST with the ACS camera in the SNAP
15922 program (PI, R. B. Tully). Of the 153 targets in the program,
observations were carried out for 80 galaxies. Accurate TRGB-distances were
obtained for 53 galaxies (Anand et al 2021, Karachentsev et al. 2020a,
Karachentsev et al. 2020b, Karachentsev et al. 2021, Karachentsev et al.
2022a, Karachentsev et al. 2022b, Karachentsev et al. 2023) and documented in
the EDD data base (edd.ifa.hawaii.edu). Independent measurements of TRGB-
distances for 24 dwarf galaxies from the SNAP15922 survey were made by
Tikhonov and Galazutdinova (2022); for half of these galaxies the distances
were determined for the first time. Among the observed objects, cases remained
in which the guiding of the telescope was not ideal, along with galaxies with
a TRGB position near the limit of the observations and several blue compact
galaxies with a high stellar concentration. The examination of these complex
cases is the subject of our paper.

##  2 Observations and data processing

Images of these galaxies were obtained with the ACS HST camera with exposures
of 760 s in both the F814W and F606W filters. The choice of exposure was
determined by the feasibility of carrying out a cycle of observations within a
single orbital period of the HST in the SNAP-regime. The standard program
packages DAOPHOT II (Stetson 1987, Stetson 1994) and DOLPHOT 2.0 (Dolphin
2016) were used for stellar photometry. The results of the photometry of the
stars underwent customary testing in accord with the “$\chi$” and “SHARP”
criteria, in order to exclude diffuse objects. The details of the method are
discussed in Tikhonov et al. (2019). Images of the 10 galaxies that we have
studied are shown as a mosaic in Fig. 1. The size of each image in the F606W
filter is 1.67′. Some large galaxies only fall partially into this format.

Figure 1: Images of the galaxies in the F606W filter obtained on the ACS
camera of the Hubble Space Telescope. The size of the pictures is
$1.67^{\prime}\times 1.67^{\prime}$; north and west are indicated by arrows.

The resulting color-magnitude diagrams (CMD) for each of the 10 galaxies are
shown in the left-hand frames of Fig. 2. The right-hand panels of the figure
reproduce the constructed luminosity function of the galaxies on a linear
scale. The luminosity function was constructed using stars with a color index
in the range $1.0<(V-1)<1.7$, corresponding to the range of color of the stars
in the red giant branch. The position of the TRGB of the galaxies is indicated
by a horizontal line. It was determined from the jump in the luminosity
function of the stars, for which the Sobel function (Madore and Freedman
1995), the maxima of which correspond to sharp changes in the number of stars,
was used. This method has known difficulties that have been repeatedly
discussed in the literature. These difficulties show up when the TRGB-galaxies
lie near the photometric limit (in our case $I_{\rm lim}\approx 27.0^{m}$) or
if the number of measured stars in a dwarf galaxy is low.

Figure 2: CM diagrams for the galaxies examined here (left frames). The
horizontal lines indicate the position of the TRGB. The right hand panels show
the luminosity function of the red stars on a linear scale.

Yet another factor that makes it difficult to determine the position of the
TRGB is the presence in a galaxy of subsystem of AGB-stars with luminosities
that are only slightly brighter than the luminosity of the tip of the red
giant branch. In order to attenuate the role of this effect, for photometry we
chose stars that are on the margins of the galaxy, where the relative
contribution of AGB-stars is smaller than the contribution of the RGB-
population. The measured distances for the galaxies examined here are listed
in Table 1, where (1) is the name of the galaxy in the sao.ru/lv/lvgdb data
base; (2) is the number of the galaxy in the HyperLEDA catalog (Makarov et al
2014); (3) are the equatorial coordinates in the epoch J2000.0; (4) is the
radial velocity in km/s relative to the centroid of the Local group; (5) is
the position of the tip of the red giants branch (mag.); (6) is the
interstellar absorption in the direction of the galaxy (mag.) according to
Schlafly and Finkbeiner (2011); (7) the position of the TRGB (mag.), which
depends weakly on the average color index (F814W–F606W) in the population of
the RGB (Rizzi et al. 2007); (8,9) is the modulus of the distance of a galaxy
(mag.) and the linear distance in Mpc. The galaxies in the table are ranked in
order of their estimated distances.

Table 1: TRGB properties of the observed galaxies. Galaxy | PGC | RA(2000.0)Dec | $V_{\rm LG}$ | $I_{\rm TRGB}$ | $A_{I}$ | $M_{I}$ | $DM$ | $D$
---|---|---|---|---|---|---|---|---
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9)
[KK2000]58 | 2815824 | 134600.8-361944 | +255 | 23.85 | 0.09 |  $-$4.11 | 27.87 | 3.75
NGC 7713 | 71866 | 233615.0-375620 | +696 | 25.45 | 0.02 |  $-$4.10 | 29.53 | 8.05
UGC 5829 | 31923 | 104241.9+342656 | +592 | 25.69 | 0.04 |  $-$4.11 | 29.76 | 8.95
Mrk 1265 | 32413 | 104940.4+225019 | +534 | 25.89 | 0.04 |  $-$4.05 | 29.90 | 9.55
ESO 373-007 | 27104 | 093245.4-331444 | +556 | 26.10 | 0.20 |  $-$4.05 | 29.95 | 9.77
NGC 6684 | 62453 | 184857.9-651024 | +720 | 26.27 | 0.10 |  $-$3.88 | 30.05 | 10.23
UGC 12588 | 71368 | 232442.4+412053 | +723 | 26.33 | 0.22 |  $-$4.11 | 30.22 | 11.07
Mrk 475 | 52358 | 143905.4+364822 | +677 | 26.22 | 0.02 |  $-$4.11 | 30.31 | 11.53
KKH 68 | 2807141 | 113053.3+140846 | +753 | 26.47 | 0.06 |  $-$4.07 | 30.48 | 12.47
KDG 178 | 42413 | 124010.0+323931 | +763 | 26.47 | 0.02 |  $-$4.12 | 30.57 | 13.00

A comparison of the distances estimated in this way by different authors shows
that the typical error in measuring $D$ is $\sim 5$%. This error rises to
7–10% as the TRGB of the galaxy approaches the photometric limit. Wherein,
there is sometimes also a systematic error owing to confusion in the position
of the tip of the branches of the RGB and AGB stars.

It is clear that the data obtained here on the estimates of the distances of
the galaxies require more detailed commentary.

##  3 Individual cases

[KK2000]58. This dwarf spheroidal galaxy was first resolved into stars in
images obtained with the earthbound 8-m VLT telescope (Müller et al. 2019).
The authors determined a distance to the galaxy of $3.36\pm 0.11$ Mpc, which
placed this galaxy on the leading edge of the group around Cen A (NGC 5128).
Our estimate of the distance, $3.75\pm 0.18$, corresponds better to the
average distance of the group Cen A (3.68 Mpc). Near the center of the dSph-
galaxy there is a globular stellar cluster, the radial velocity of which was
measured by Fahrion et al. (2020). On the images obtained from ACS HST the
globular cluster is well resolved into stars of the red giant branch (Fig. 3).
The position of the TRGB in the cluster is consistent with the estimate of the
TRGB over the entire body of the dwarf galaxy.

Figure 3: Left: CM diagram for stars in the globular cluster near the center
of the dSph-galaxy [KK2000]58 in an aperture of radius $2^{\prime\prime}$.
Right: CM diagram for stars in the neighboring area with the same aperture.

NGC 7713. According to the NASA Extragalactic Database (NED;
ned.ipac.caltech.edu), this spiral galaxy of a late type has 23 estimates of
distance by the Tully-Fisher method (Tully and Fisher 1977) which relate the
luminosity of the galaxy to the amplitude of its rotation. The average value
of the distance from it is $9.05\pm 1.38$ Mpc, which agrees, to within the
standard deviation, with our estimate of $8.05\pm 0.40$ Mpc. At a projected
distance of 1.9∘ from it (or 270 kpc) there is another spiral galaxy of a late
type, IC 5332, with a radial velocity $V_{\rm LG}=716$ km/s and distance
$D=9.01$ Mpc (Anand et al 2021) that indicate a possible physical association
of these galaxies.

UGC5829=DDO 84=VV 794. An irregular (Im) galaxy with a very peculiar structure
that has gotten the name “spider” (Fig. 4). More than half the baryon mass of
the galaxy belongs to the gaseous component, which evidently triggers numerous
sources of star formation in it. UGC 5829 = KIG 434 shows up in the Catalog of
Isolated Galaxies (Karachentseva 1973). The nearest relatively massive galaxy
NGC 3432 with $V_{\rm LG}=578$ km/s and $D=9.14$ Mpc (Anand et al 2021) is at
a projection distance of 3.0∘ or 470 kpc. UGC 5829 is a distinct example of
how a distorted peculiar galactic structure can be created, not by an external
perturbation, but by strictly internal properties of a galaxy.

Figure 4: An image of the peculiar galaxy UGC 5829 (VV 794, “Spider”), taken
from the digital sky survey DECaLS. The size of the field is $6^{\prime}\times
6^{\prime}$, north is upward, and east, to the left.

Mrk 1265. This blue compact dwarf galaxy is in the aureole of a bright star in
the Milky Way. The compactness of the structure, abundance of blue stars, and
presence of an aureole of the projected star lower the accuracy of the
photometry. According to our estimate, the distance to the galaxy is
$D=9.55\pm 0.96$ Mpc, which is somewhat greater than the distance estimate
$D_{\rm NAM}=7.8$ Mpc from the radial velocity taking into account the local
field of the velocities in the model of the Numerical Action Method (NAM,
Shaya et al. 2017).

ESO 373-007=AM 0930-330=[KK 2000]31. The two estimates of the distance of this
dIrr-galaxy, 8.32 Mpc according to Tully-Fisher and 9.65 Mpc in the NAM model
are in good agreement with our estimate $D=9.77\pm 0.98$ Mpc via the TRGB.
Nearby, at a projected distance of $15^{\prime}$ or 40 kpc there is a brighter
Sd-galaxy ESO 373-008 with a radial velocity of $V_{\rm LG}=620$ km/s and a
TF-distance of 9.68 Mpc. The two galaxies evidently form an isolated pair.

NGC 6684. This galaxy is of an early type S0a, in which structure a bar and a
ring can are seen. A star loop and radial jet can be seen on the periphery.
The NED presents 14 crude estimates of the distance by various approximate
methods in a range from 5.5 to 15.5 Mpc. The TRGB distance that we have
measured, $10.23\pm 1.9$ Mpc, lies right in the middle of this interval. At a
distance $2.6^{\circ}$ from NGC 6684 there is a spiral galaxy NGC 6744 with
several satellites. Its TRGB-distance, 9.51 Mpc (Anand et al 2021) and radial
velocity $V_{\rm LG}=720$ km/s show that these galaxies are probably members
of a single diffuse association.

UGC 12588. This dIrr-galaxy is seen face-on and lies in a zone of substantial
galactic extinction. Near it, at a projected distance of $42^{\prime}$ there
is a brighter spiral galaxy NGC 7640 with $V_{\rm LG}=668$ km/s and a TRGB
distance of 8.43 Mpc (Anand et al 2021). Our estimate of the TRGB-distance of
UGC 12588 is $11.07\pm 1.1$ Mpc, which is substantially greater than the
distance of the neighboring galaxy. To test the possible physical coupling of
these galaxies it is necessary to make deeper observations of UGC 12588.

Mrk 475. This blue compact galaxy lies far from the other galaxies with close
radial velocities. The dense stellar field in the main body of the galaxy
lowers the accuracy of the performed photometry. Our estimate of the TRGB-
distance, $11.53\pm 1.15$ Mpc, turned out to be considerably higher than the
kinematic estimate 9.18 Mpc (NAM).

KKH 68=AGC 212837. The position of the TRGB for this dIrr-galaxy is only
$0.5^{m}$ higher than the photometric limit, which makes our estimate of the
distance $D_{\rm TRBG}=12.47\pm 1.25$ Mpc unreliable. This galaxy lies at an
angular distance of $2.9^{\circ}$ from the spiral Sb-galaxy NGC 3627 (M 66)
with TRGB distance of $D_{\rm TRBG}=11.12\pm 0.56$ Mpc (Hoyt et al. 2019),
which is the brightest member of the group in the Leo constellation. Probably
KKH68 belongs to the peripheral members of this group.

KDG 178=BTS 147. This gas-rich dwarf galaxy lies only at 26′ from the bright
spiral galaxy NGC 4631, for which $V_{\rm LG}=581$ km/s and $D_{\rm
TRBG}=7.35\pm 0.10$ Mpc (EDD). Judging from the distance $13.0\pm 1.3$ Mpc,
which we obtained, KDG 178 lies behind the group of dwarfs around NGC 4631.
The large radial distance of this object from the group NGC 4631 is indirectly
confirmed by relatively wide width of the 21-cm line, $W_{50}=52$ km/s, which
corresponds to a TF-distance of more than 11 Mpc.

Besides the cases enumerated above, we also note several galaxies observed in
the SNAP 15922 program but which appear to be definitely distant. Jerjen, et
al. (2000) estimated a distance of 4.29 Mpc for the galaxy ESO 219-010 (PGC
44110)by the method of fluctuations in surface brightness. This galaxy is
essentially unresolved into stars in the images taken with the ACS. In terms
of texture, a distance of this galaxy can be roughly estimated as $\simeq 15$
Mpc.

Two dwarf galaxies in the profile of the Virgo cluster, EVCC 67 and UGC 7983
with radial velocities, respectively, of 458 and 565 km/s, turned out to be
unresolved in the images taken with ACS. They are evidently members of the
Virgo cluster at a distance of 16.5 Mpc.

The isolated dIrr-galaxy KKH 46 (PGC 2807128) has a radial velocity $V_{\rm
LG}=409$ km/s and a width of the 21-cm line of $W_{50}=25$ km/s. It is in the
so-called “Anomalous velocity zone”, (Tully 1988) where the galaxies with
distances about 16 Mpc have large negative peculiar velocities of about –700
km/s.

##  4 Concluding comments

Using the images obtained with the ACS-camera on the Hubble Space Telescope in
the F814W and F606W filters, we have carried out PSF-photometry and
constructed color-magnitude diagrams for the stellar population of 10 nearby
galaxies. The positions of the tips of the red giant branches have been
determined based on the jump in the luminosity function of the red stars and
these have been used to estimate the distances to the galaxies. Besides the
spiral galaxies NGC6684 and NGC 7713, the other objects are related to dIrr,
dIm, BCD, and dSph dwarf galaxies. The radial velocities of the galaxies
relative to the centroid of the Local group lie within an interval from 250 to
770 km/s and the distances that we have determined lie within a range from 3.7
to 13.0 Mpc with a median of 10.0 Mpc. We also have noticed several galaxies
with radial velocities $V_{\rm LG}<600$ km/s, which are scarcely resolved into
stars and lie at distances greater than 13 Mpc. These galaxies are either
members of the Virgo cluster or lie within a zone of anomalously high negative
peculiar velocities. The resulting data supplement the picture of the field of
the peculiar velocities of the galaxies in the Local volume, that is produced
by nearby attractors with different dark halo mass.

This work was based on observations taken with the NASA/ESA Hubble Space
Telescope under contract NAS5-26555. The HyperLEDA, NED, and EDD data bases
have been used in this paper. This work was supported by a grant from the
Ministry of Science and Higher Education of the Russian Federation, No.
075-15-2022-262 (13.MNPMU.21.0003).

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