The Local Volume galaxy census and HST SNAP surveys

TL;DR

This study analyzes HST images of dwarf galaxies outside the Local Group, determining new distances and identifying probable satellite galaxies, while highlighting variability in survey productivity.

Contribution

It provides new distance measurements for two dwarf galaxies and identifies their likely association with larger spirals, revealing insights into the Local Volume galaxy census.

Findings

Two new galaxy distances determined via the Tip of the Red Giant Branch.

Identification of probable dwarf satellites of NGC 4826 and NGC 4736.

Significant variability in the effectiveness of different SNAP surveys.

Abstract

We examined F814W and F606W images of dwarf galaxies from the Hubble Space Telescope archive, which were obtained under the HST SNAP programs 17159 to 17797. Among 58 observed dwarfs located outside the Local Group, we found only a few objects that were confidently resolved into stars. We determined two new distances for the galaxies: dw1252+2215 ($5.32\pm0.20$ Mpc) and dw1234+3952 ($4.34\pm0.16$ Mpc) via the Tip of the Red Giant Branch. They turned out to be new probable dwarf satellites of the nearby luminous spiral galaxies NGC 4826 and NGC 4736, respectively. We also note that recent SNAP surveys vary in their productivity in measuring new galaxy distances by more than an order of magnitude.

Tables

Table 1. Table 1: Properties of nearby dwarf galaxies

Parameter	Unit	dw1252+2215	dw1234+3952	dw0954+6821
TRGB	mag	24.60$\pm$0.08	24.11$\pm$0.08	24.00$\pm$0.08
M(F814W)	mag	–4.08	–4.11	–4.10
$A_I$	mag	0.053	0.024	0.161
${\left(m-M\right)}_0$	mag	28.17	28.19	27.94
Distance	Mpc	5.32$\pm$0.20	4.34$\pm$0.16	3.87$\pm$0.15
$g$	mag	19.86$\pm$0.13	18.46$\pm$0.10	(20.28)
$r$	mag	19.33$\pm$0.13	17.88$\pm$0.10	—
$M_g$	mag	-8.9	-9.8	(-8.0)

Table 2. Table 2: Five recent SNAP surveys aimed at measuring distances to nearby galaxies

SNAP	PI.	Targets	Observed	NewDist	Efficiency
(1)	(2)	(3)	(4)	(5)	(6)
12546	Tully	132	45	42	93%
13442	Tully	128	45	43	96%
15922	Tully	153	80	53	66%
17158	Bell	55	20	2	10%
17797	Bell	103	33	1	3%

Full text

The Local Volume galaxy census and HST SNAP surveys

I.D.Karachentsev1, M.I.Chazov1, V.E.Karachentseva2

We examined F814W and F606W images of dwarf galaxies from the Hubble Space Telescope archive, which were obtained under the HST SNAP programs 17159 to 17797. Among 58 observed dwarfs located outside the Local Group, we found only a few objects that were confidently resolved into stars. We determined two new distances for the galaxies: dw1252+2215 ($5.32\pm 0.20$ Mpc) and dw1234+3952 ($4.34\pm 0.16$ Mpc) via the Tip of the Red Giant Branch. They turned out to be new probable dwarf satellites of the nearby luminous spiral galaxies NGC 4826 and NGC 4736, respectively. We also note that recent SNAP surveys vary in their productivity in measuring new galaxy distances by more than an order of magnitude.

Key words: galaxies — dwarf galaxies, galaxies — distances

One of the priority tasks of observational cosmology on small scales is the creation and replenishment of a representative sample of galaxies with measured radial velocities and accurate distances in the nearest fixed volume of the Universe. Analysis of deviations in the radial velocities of galaxies from the ideal (unperturbed) Hubble flow $V=H_{0}\times D$, where $D$ is the distance to a galaxy and $H_{0}$ is the Hubble parameter, allows us to determine the mass of dark matter contained in local attractors. Radial velocities of galaxies measured by optical and radio telescopes have a characteristic accuracy of $\sim~(3-10)$ km s*-1*. Mass determinations of distances to nearby galaxies use a method based on a calibrated luminosity of the Tip of the Red Giant Branch (TRGB) [1, 2]. To apply this method, a stellar color-magnitude diagram based on images taken in F814W and F606W filters on the Hubble Space Telescope (HST) is used. The accuracy of the TRGB method is about 5%, allowing one to record the position of a galaxy inside or outside a nearby group.

To date, the sample of galaxies in the Local Volume contains $\sim 1500$ candidates with estimated distances within 12 Mpc [3]. For 550 of them, TRGB distances have already been measured and collected in the Extragalactic Distance Database [4]. More than a quarter of all TRGB distances in the Local Volume were obtained in the SNAP survey mode, when images of galaxies in both filters, F814W and F606W, were exposed during one HST orbital period in gaps between scheduled observations. Using such observational data, it is possible to measure TRGB distances up to $\sim 12$ Mpc within one orbital period, that determines the size of the Local Volume.

During the last observing cycles on HST, two SNAP proposals were accepted for execution: SNAP 17158, “The lowest luminosity galaxy candidates ever discovered outside the Milky Way” (PI E.Bell, 55 orbits) and SNAP 17797 “Galactic underdogs: assessing the true satellite galaxy population in the Local Void” (PI E.Bell, 103 orbits). The purpose of these programs was to refine the luminosity function of galaxies at its faint end by measuring TRGB distances to satellites around nearby luminous galaxies (M 31, M 81, M 64, M 94, etc.), situated in the Local Volume.

The discovery of new satellites could resolve the well-known paradox that the expected number of satellites in the standard $\Lambda$CDM cosmological model with masses less than $10^{6-7}M_{\odot}$ is in the tens of times greater than their observed number [5, 6]. Successful searches for dwarf galaxies, especially of a low surface brightness, in modern deep sky surveys [7–10] have essentially reduced this contradiction. Note that the weakening of this paradox also occurred due to the realization that a some satellites disappear in the process of evolution, being absorbed by massive host galaxies [11]. Nevertheless, a significant discrepancy between the expected and observed number of low-mass galaxies still remains.

We have carried out a visual inspection of images of all observed galaxies available in the HST archive for both the programs. Due to the circumstances of the SNAP surveys, images of only 20 galaxies were obtained within the framework SNAP 17158 and 38 galaxies from SNAP 17797. Typical exposures in F814W and F606W filters were about 750 sec. We excluded 18 putative satellites of M 31 from consideration, since the images of these nearby objects with a scarce population require more sophisticated analysis. The color-magnitude diagrams for stars in four ultra-faint satellites of M 81 have already been discussed by the authors of SNAP 17158 program [12]. These dwarf systems were previously discovered by ground-based observations with the 8-meter Subaru telescope [13–15]. New data from the SNAP survey [12] confirm the membership of these dwarfs in the M 81 group, but do not lead to elaboration of their distances.

Based on our analysis of images of 20 galaxies from SNAP 17158 and 33 galaxies from SNAP 17797, we came to the following conclusions.

$\bullet$ For two galaxies, dw1252+2215 (J12 52 05.8 +22 15 54) and dw1234+3952 = SMDG 1234+3952 (J12 34 22.4 +39 52 43), which are well resolved into stars, it is possible to determine their distances. The color-magnitude diagrams for them are shown in Figure 1. For photometry of the stellar population, the DOLPHOT package was used [16]. The position of the Tip of the Red Giant Branch (TRGB) was estimated using the method developed by Makarov et al. [17]. The horizontal line in the figure shows the measured value of the TRGB. The results of the distance determination for both the galaxies are presented in Table 1. In it, we have added a third well-resolved galaxy, dw0954+6821 (J09 54 07 +68 21 51), located in the M 81 group, the CMD for which was already discussed in [12]. The rows of the table indicate: (1) the apparent magnitude of TRGB; (2) the absolute magnitude of TRGB, estimated from the calibrated relation M(F814W) = –4.06 + 0.20 [$<$F606W–F814W$>$ – 1.23], justified in [2]; (3) Galactic extinction in the $I$-band from [18]; (4,5) the obtained distance modulus and linear distance of the galaxy with an indication of its standard error; (6,7) integrated $g$- and $r$-magnitudes of the galaxies according to [19]; in the case of dw0954+6821, the average integrated apparent magnitude is indicated according to the data of [15] and [12]; (8) the absolute magnitude of the galaxy.

$\bullet$ The distance of the galaxy dw1252+2215 coincides within the measurement error with the distance of the luminous spiral galaxy NGC 4826, $5.3\pm 0.1$ Mpc, derived in [19]. Being at a projected separation of $R_{p}=116$ kpc, the dwarf galaxy is a new probable satellite of it. Note that Carlsten et al. [20] give a similar distance of 5.42 Mpc for dw1252+2215 based on its surface brightness fluctuations.

$\bullet$ The dwarf galaxy dw1234+3952 = SMDG1234+3952 is included in the Zaritsky catalog of ultra-diffuse galaxies [21]. Judging by its distance, this dwarf is a new satellite of the spiral galaxy M 94 = NGC 4736, which has a distance of $4.41\pm 0.09$ Mpc [4]. The projection separation of both the galaxies is $R_{p}=260$ kpc. It is curious that Zaritsky et al. [21] give an improbably large value of heliocentric radial velocity for the SMDG1234+3952, $V_{h}=6658\pm 16$ km s*-1*.

$\bullet$ Among the other observed galaxies, the dwarf galaxy dw1311+4051 appears to be the most resolved into stars. According to observations with the FAST radio telescope, it has a heliocentric radial velocity of $V_{h}=604\pm 1$ km s*-1* [22] and a corresponding kinematic distance of 9.3 Mpc. The hydrogen mass of this dwarf, $\log(M_{HI}/M_{\odot})=6.97$, is three times greater than the total mass of its stars. The galaxy’s CM-diagram shows a population of blue stars, which complicates the TRGB distance estimate. This dwarf is a probable satellite of the spiral galaxy NGC 5055 with $D=9.04\pm 0.09$ Mpc [4], being at a projection separation of $R_{p}=229$ kpc from it.

$\bullet$ Judging by the texture of the images resolved to extremely faint stars, 12 dwarf galaxies have distances $D<12$ Mpc, i.e. are confirmed as the true members of the Local Volume sample. Of these, three objects: dw0852+3249, dw1046+1358 and dw1116+1356 are new satellites of galaxies NGC 2683, NGC 3377, and NGC 3627, respectively.

$\bullet$ Five dwarf galaxies represented in the UNGC catalog [4]: dw0315-4112, dw0321-4149, dw0846+3300, dw1116+5537 and dw1122+5515 have a structure that indicates their belonging to the distant background, but not to satellites of massive galaxies in the Local Volume.

It would be interesting to compare the performance of different SNAP programs in measuring new distances. Table 2 contains the main findings from the five recent SNAP surveys that measured new distances of galaxies in the Local Volume. The first two columns indicate the name of the SNAP program and its Principal Investigator. The third and fourth columns represent the number of requested targets and the number of observed ones. The last two columns show the number of new galaxy distance estimates and, accordingly, the efficiency of the HST observations.

As one can see, the success in measuring new distances for galaxies in the Local Volume varies significantly among different SNAPs. The low efficiency of the last two programs is obviously due to a bad choice of objects whose distances could be determined with short exposures in a SNAP survey. Many targets in these programs were selected from Table 8 in the publication by Carlsten et al [20], where dubious candidates for satellites of nearby massive galaxies have been collected. The experience of the undertaken comparison may be useful in preparing subsequent SNAP proposals.

I.D.K. and M.I.C. acknowledge support from the Russian Science Foundation grant No.24–12–00277.

1 Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhny Arkhyz, Russia, e-mail: idkarach@gmail.com

2 Main Astronomical Observatory, National Academy of Sciences of Ukraine, Kiev, 03143, Ukraine

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