Resolved Dwarf Galaxy Searches within ∼5 Mpc with the Vera Rubin Observatory and Subaru Hyper Suprime-Cam^*

The Λ+cold dark matter (ΛCDM) model for structure formation is successful on large scales (≳10 Mpc) and provides a good match to the structure in the cosmic microwave background and the large-scale distribution of galaxies. In this model, galaxies grow hierarchically within dark matter halos (e.g., Springel et al. 2006), but quantitatively verifying this on small galaxies has met with challenges, particularly with respect to the faint end of the galaxy luminosity function (for a recent review, see Bullock & Boylan-Kolchin 2017). These challenges have been widely discussed in the literature and include the "missing satellites problem" (e.g., Klypin et al. 1999; Moore et al. 1999), "too big to fail" (e.g., Boylan-Kolchin et al. 2011, 2012), and the apparent planes of satellites around nearby galaxies (e.g., Pawlowski et al. 2012; Ibata et al.2013; Müller et al. 2018).

Significant progress has been made in addressing these small-scale ΛCDM challenges on the theoretical front with the inclusion of realistic baryonic physics into galaxy-scale simulations (e.g., Brooks et al. 2013; Sawala et al. 2016; Wetzel et al. 2016; Samuel et al. 2021; Engler et al. 2021), which roughly match the properties of the Milky Way (MW) satellite system. Likewise, the number and diversity of satellites around the MW continues to expand (for instance, most recently, Mau et al. 2020; Cerny et al. 2021; see Simon 2019 for a recent review). The Local Group, and the MW in particular, will continue to be an important testing ground for understanding the astrophysics and cosmological implications of the very faintest dwarf galaxy satellites (e.g., Munshi et al. 2019; Nadler et al. 2021, among others).

Ultimately, to fully test the ΛCDM model and the important astrophysics relevant for the formation and evolution of dwarf galaxies (e.g., stellar and supernova feedback, reionization, tidal and ram pressure stripping, etc.), studies of faint satellite systems beyond the Local Group are necessary in order to sample primary halos with a range of masses, morphologies, and environments. Observationally, this work is now underway using wide-field imaging data sets centered around primary galaxies with a range of masses (e.g., Chiboucas et al. 2013; Crnojević et al. 2014; Sand et al. 2014, 2015a; Carlin et al. 2016; Crnojević et al. 2016; Toloba et al. 2016; Danieli et al. 2017; Smercina et al. 2018; Bennet et al. 2019; Crnojević et al. 2019; Bennet et al. 2020; Carlsten et al. 2021; Davis et al. 2021; Drlica-Wagner et al. 2021), as well as spectroscopic surveys around MW analogs at larger distances (Geha et al. 2017; Mao et al. 2021). Numerical studies are also addressing the scatter in satellite properties seen in MW-like galaxies, as well as their physical origins (e.g., Font et al. 2021). Searches for faint galaxies in the field are also uncovering a multitude of dwarf galaxy systems using several techniques (e.g., Sand et al. 2015b; Tollerud et al. 2015; Bennet et al. 2017; Leisman et al. 2017; Bennet et al. 2018; Greco et al. 2018; Zaritsky et al. 2019; Tanoglidis et al. 2021, among many others).

In this work, we will investigate the prospects for identifying faint dwarf galaxies in the nearby universe (D ≲ 5 Mpc) using resolved stellar populations with current and near-future ground-based instrumentation. Successful wide-field searches are already underway in this area of parameter space with at least four instruments: the Megacam imager on the Canada–France–Hawaii Telescope (CFHT; Boulade et al. 2003), the Megacam imager on the Magellan Clay telescope (McLeod et al. 2015), the Dark Energy Camera (DECam) on the Blanco telescope (Flaugher et al. 2015), and the Hyper Suprime-Cam (HSC) on the Subaru telescope (Miyazaki et al. 2018). This work will accelerate and expand to the entire southern sky with the advent of the Vera C. Rubin Observatory (Rubin) Legacy Survey of Space and Time (LSST). Also on the horizon is the Nancy Grace Roman Space Telescope, which will be an excellent platform for discovering resolved dwarf galaxies in the Local Volume (Akeson et al. 2019) and the subject of a future paper.

Here we combine image-level simulations of resolved, faint dwarf galaxies at varying distances, luminosities, ellipticities, and sizes with deep ground-based images from the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS; Crnojević et al. 2014; Sand et al. 2014; Crnojević et al. 2016; Toloba et al. 2016; Crnojević et al. 2019; Hughes et al. 2021). As we discuss, these data are of nearly the same quality and depth as those expected from the 10 yr LSST and can be used to rigorously quantify the detectability of faint dwarf galaxies in the Local Volume using state-of-the-art techniques. By comparing these recovery statistics with the sample of known nearby galaxy hosts (see Table 1 and Figure 1), we can forecast the ensemble of dwarf luminosity functions that will be uncovered in the LSST (and HSC) era. First, in Section 2, we present the landscape of resolved dwarf studies. In Section 3, we describe our methodology for creating mock observations of resolved, faint dwarf galaxies (Section 3.1) and introduce the matched-filter technique as a powerful tool to search both compact and extended systems (Section 3.2). In Section 4, we present the results of our experiments, providing our recovery fraction as a function of the dwarf parameters. In Section 5, we discuss our results and provide a preview of faint dwarf galaxy discoveries that will be possible in the LSST (and HSC) era. Finally, we summarize our key results in Section 6.

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For most of this work, we focus on the identification of faint dwarf galaxies with predominantly old, metal-poor stellar populations within roughly the virial radius of larger host galaxies. We focus on the luminosity range (M_V ≳ −11 mag), where the Updated Nearby Galaxy Catalog (Karachentsev et al.2013) suggests that incompleteness becomes significant in the existing sample of galaxies. In this luminosity range, most dwarf satellites around more massive hosts have been quenched and stripped of their gas (e.g., Spekkens et al. 2014; Karunakaran et al. 2020; Putman et al. 2021, Karunakaran et al. 2021) and thus do not have younger stellar populations (exceptions include Antlia and Antlia B, likely satellites of NGC 3109). It is worth pointing out that even gas-rich dwarfs have a majority of their mass formed at old ages (e.g., Weisz et al. 2011) and thus would be detectable using methods that just focus on the detectability of old stars. Furthermore, any dwarf with a younger stellar population will be easier to detect due to the presence of relatively rare blue stars, so the presentation in this work can be considered conservative. The discovery space for faint dwarf galaxies in the field, with typically younger stellar populations on average, will be broader than that presented here.

To gauge the distance range that we can identify and study faint dwarf galaxies in resolved stars, we plot the color–magnitude diagram (CMD) of the MW satellite Draco (M_V = −8.7 mag; ∼2.5 × 10⁵ L_⊙; Martin et al. 2008) in Figure 2, using data from the Sloan Digital Sky Survey (SDSS; Ahumada et al. 2020). Draco is a faint member of the MW's "classical" satellites, which were discovered prior to the era of digital sky surveys, and its CMD is typical; it displays a clear red giant branch (RGB) due to an old, metal-poor stellar population, as well as a prominent horizontal branch (HB). In Figure 2, we also mark the position of the tip of the red giant branch (TRGB) in the r band, as determined from theoretical isochrones (M_r = −3.0 mag; Sand et al. 2014).

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Current and planned deep-imaging surveys around nearby galaxies will ultimately have different faint-end magnitude limits, although they will typically reach r ≈ 27th mag or fainter. LSST expects a 5σ point-source image depth of (g, r, i) = (27.4, 27.5, 26.8) mag after the 10 yr survey (Ivezić et al. 2019). Meanwhile, current resolved dwarf search programs with HSC reach a 50% completeness limit of (g, i) = (27.5, 26.7) mag (e.g., Carlin et al. 2019), although even deeper depths are possible with longer exposure times in good conditions (e.g., Tanaka et al. 2017; Müller et al. 2019). Here we will use the best image quality fields of the PISCeS program, which searches for resolved dwarfs and other halo substructures around NGC 253 and Cen A with the Magellan Megacam, with corresponding 50% completeness limits of (g, r) = (27.83, 27.35) and (27.51, 27.01) mag, respectively, at a signal-to-noise ratio detection threshold of three. These limits include stringent cuts made to select point sources, as we describe below, and are thus difficult to encapsulate with simple metrics. These same fields have a final image FWHM of (g, r) = (05, 06) and (05, 05), respectively. By using data in both Cen A (b = 19°) and NGC 253 (b = −88°), we can examine the effects of MW foreground contamination on dwarf detection (see Section 4.4).

Adopting the deep limits from the PISCeS program fields, we mark the corresponding depths at three fiducial distances, D = 1.5, 3.5, and 5 Mpc, on the Draco CMD in Figure 2. Current and next-generation deep surveys will be able to probe ∼2 mag below the TRGB at 5 Mpc and correspondingly deeper for closer distances, clearly making resolved stellar searches for faint satellites (and other substructures) feasible. Resolved dwarf searches beyond ∼5 Mpc will likely have diminishing returns, although they should be pursued through other venues, such as the Roman Space Telescope (Akeson et al. 2019).

To put this in context with the Local Volume, we compile a list of nearby galaxies (D ≲ 5 Mpc) around which faint satellite galaxies could be found (see Table 1, taken from the Updated Nearby Galaxy Catalog; Karachentsev et al. 2013). This list is divided into three categories based on the K-band luminosities: the MW luminosity group (log(L_K /L_⊙) ≳ 10), Large Magellanic Cloud (LMC) analogs (9 ≲ log(L_K /L_⊙) ≲ 10), and Small Magellanic Cloud (SMC) analogs (8.0 ≲ log(L_K /L_⊙) ≲ 9.0). Similarly, we plot a collection of nearby galaxies in supergalactic coordinates in Figure 1, where we also demarcate the D = 1.5, 3.5, and 5 Mpc fiducial distances we consider in this work. For each galaxy in Table 1, we also note their Galactic coordinates and extinction in the V band (A_V ) as determined from the Schlafly & Finkbeiner (2011) calibration of the Schlegel et al. (1998) dust maps. Both extinction and foreground MW star counts may strongly affect prospects for Local Volume dwarf discovery (see Section 4.4). In addition, we derive their stellar masses (M_⋆) from the K-band luminosity by assuming a K-band mass-to-light ratio of unity and then infer the total halo mass of each galaxy by using the M_⋆–M_halo relationship given by Moster et al. (2010), along with the corresponding virial radius based on the Bryan & Norman (1998) definition (i.e., the radius such that the mean enclosed halo density is 104 times the critical density of the universe, ρ_c = $3{H}_{0}^{2}/8\pi G$). Finally, we also include the tidal index parameter (see Karachentsev et al. 2013) for each galaxy in Table 1 as an indicator of local environment or density contrast. Recent work has indicated a tentative relationship between satellite richness and environment (Bennet et al. 2019), and future Local Volume surveys are required to investigate this further.

In this section, we describe our methodology for implanting simulated dwarf galaxies directly into our chosen PISCeS deep imaging fields as a proxy for similarly deep imaging surveys that may be performed with Rubin, HSC, or similar facilities. We implant dwarfs with a variety of luminosities, sizes, ellipticities, and surface brightnesses broadly consistent with the dwarf galaxy population identified in the Local Group. We focus on three different fiducial distances—1.5, 3.5, and 5 Mpc—to study these resolved, simulated dwarfs. Within a given region of sky, we also place dwarfs in different spatial positions to sample the effects that varied stellar backgrounds may have on dwarf detection. Additionally, we place a subset of dwarfs into our low Galactic latitude "Cen A" data to understand the effects of MW foreground contamination and extinction. We detect this simulated dwarf ensemble with a matched-filter technique to understand our recovery fraction as a function of the dwarf parameters described above.

3.1. Imaging Data and Mock Dwarf Observations

As mentioned earlier, we utilize the imaging data from the PISCeS survey, in which the halos of Cen A and NGC 253 have been targeted with the Megacam imager on the Magellan Clay 6.5 m telescope (McLeod et al. 2015) out to a projected radius of ∼150 kpc. Megacam has an ∼24' × 24' field of view and a binned pixel scale of 016, similar to HSC (017) and Rubin (02). Our chosen NGC 253 deep imaging field was observed for 8 × 300 s in g and 7 × 300 s in r, and the Cen A field was observed for 6 × 300 s in each of the g and r bands. The data were reduced in a standard way by the Smithsonian Astrophysical Observatory Telescope Data Center (see Crnojević et al. 2016 for further details on our survey strategy and observational methods). In this paper, we mostly use the best image quality field of NGC 253, which is 90% complete at (g, r) = (26.98, 26.35) mag and 50% complete at (g, r) = (27.83, 27.35) mag, as assessed by artificial star tests (see below). We make use of the deepest Cen A pointing (90% complete at (g, r) = (26.53, 26.04) mag, 50% complete at (g, r) = (27.51, 27.01) mag) to assess the effects of MW disk contamination in detecting dwarf galaxies (see Section 4.4). Both fields are approximately R ∼ 100 kpc distant in projection from NGC 253 and Cen A, so halo contamination is negligible.

We assume that the stellar populations of the dwarf galaxies we wish to investigate are mainly composed of old, metal-poor stars and therefore well described by a single population. We construct our model galaxies by sampling from a 10 Gyr old stellar population with an overall metallicity of [Fe/H] = −2.0 (using a Dartmouth isochrone; Dotter et al. 2008), assuming a Salpeter initial mass function. The stellar profiles of the dwarfs are generally well described by a single exponential model (e.g., Martin et al. 2008; Muñoz et al. 2018). We use an exponential profile with elliptical half-light radii (r_h ) ranging from 0.02 to 5 kpc and corresponding ellipticities ¹³ of = [0, 0.3, 0.5]. We intentionally focus on generating simulated galaxies that bracket the observed central surface brightness range of nearby dwarfs (e.g., by examining plots of M_V versus r_h for Local Group and Local Volume dwarf spheroidals and ultrafaints). As we expect to resolve HB stars at a distance of 1.5 Mpc (see Figure 2), we use a PARSEC isochrone (age = 10 Gyr, [Fe/H] = −2.0; Bressan et al. 2012) for our simulations and include HB stars at this distance. Our pipeline then adds Galactic extinction to the stars on a source-by-source basis by interpolating the Schlegel et al. (1998) extinction maps and the Schlafly & Finkbeiner (2011) correction coefficients and then injects these stars into the Megacam images with the DAOPHOT routine ADDSTAR (Stetson 1987, 1994). Based on the luminosity function, ∼85%–95% of the galaxy's light comes from stars of r ≲ 34.0 mag; therefore, we choose to include stars with a magnitude of r ≲ 34.0 mag for each galaxy realization. We experiment with 1 mag brighter and fainter cuts and find that our choice does not change the results. Overall, we simulate dwarfs with absolute magnitudes in the range −11 mag ≲ M_V ≲ −4 mag, dependent on the distance being examined, in order to understand the potential for dwarf galaxy detection over a broad range of luminosity.

For each luminosity and size bin, we simulate three different shapes ( = [0, 0.3, 0.5]) and place each simulated galaxy at five different positions in the field, resulting in 15 simulated systems. Figure 3 shows example simulated galaxies with three different ellipticities, which are placed in different sky positions in the NGC 253/Cen A fields (top/bottom). Because of implantation of stars well below the detection limits of the image, these dwarfs also include unresolved light. By performing multiple simulations for each size/luminosity bin, we account for "CMD shot noise" (Martin et al. 2008). A total of 2815 galaxies are generated in this manner. Our simulations are summarized in Table 2.

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Table 2. A Summary of Our Simulations

D	MV	rh	Ellipticity	Ndwarf	Detectability	LG Analog
(Mpc)	(mag)	(pc)			(%)
(1)	(2)	(3)	(4)	(5)	(6)	(7)
1.5	−8	20, 30, 50, 80, 120, 200, 300, 500, 800, 1200, 2000	0, 0.3, 0.5	975	100	And XXIX
	−7	20, 30, 50, 80, 120, 200, 300, 500, 800, 1200			100	Eri II, And XI
	−6	20, 30, 50, 80, 120, 200, 300, 500			100	Hercules
	−5	20, 30, 50, 80, 120, 200			92	Leo IV, Hydra II
	−4	20, 30, 50, 80, 120, 200			0	Gru II, Pisces II
3.5	−11	3000, 5000	0, 0.3, 0.5	1180	100 a	DDO113, Cetus
	−10	50, 80, 120, 200, 300, 500, 800, 1200, 2000, 3000			100	Antlia B, And III
	−9	50, 80, 120, 200, 300, 500, 800, 1200, 2000			100	Draco, Carina
	−8	50, 80, 120, 200, 300, 500, 800, 1200			95	And XXIX
	−7	50, 80, 120, 200, 300			45	Eri II, And XI
	−6	50, 80, 120, 200, 300			0	Hercules
5	−11	2000, 3000	0, 0.3, 0.5	660	100 a	DDO113, Cetus
	−10	80, 120, 200, 300, 500, 800, 1200, 2000, 3000			100	Antlia B, And III
	−9	80, 120, 200, 300, 500, 800, 1200, 2000			97	Draco, Carina
	−8	80, 120, 200, 300, 500, 800, 1200			50	And XXIX
	−7	80, 120, 200, 300, 500, 800			0	Eri II, And XI

Notes. Column 1: fiducial distance at which simulated galaxies are injected. Column 2: absolute V-band magnitude of simulated galaxies. Column 3: elliptical half-light radii values that we use in our simulations. Column 4: ellipticity values used for each M_V and r_h value. Column 5: total number of simulated galaxies for each fiducial distance. Column 6: detection completeness for our simulated galaxies (injected into the NGC 253, low Galactic latitude data) in the range of 26 mag arcsec⁻² ≲ μ_V,0 ≲ 29 mag arcsec⁻². Column 7: Local Group analogs that detectability ratio (%) approximately refers to.

^a For M_V = −11, we only simulate galaxies with μ_V,0 > 29 mag arcsec⁻². Given that our simulated galaxies with μ_V,0 = 30 mag arcsec⁻² have an 80% recovery rate, we extrapolate the detectability here for the range of 26 mag arcsec⁻² ≲ μ_V,0 ≲ 29 mag arcsec⁻².

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We treat the images with simulated dwarf galaxies in the same way as the unaltered images and perform point-spread function photometry using the DAOPHOT and ALLFRAME software suite (Stetson 1987, 1994), following the same methodology described in Crnojević et al. (2016), with small adjustments. We remove objects that are not point sources by culling our catalogs of outliers in χ² versus magnitude, magnitude error versus magnitude, and sharpness versus magnitude space. ¹⁴ Instrumental magnitudes are then calibrated to the DES DR1 catalog (Abbott et al. 2018) for the NGC 253 field and the SDSS system for the Cen A field (Crnojević et al. 2016). The final calibrated catalogs are dereddened on a star-by-star basis using the Schlegel et al. (1998) reddening maps with the coefficients from Schlafly & Finkbeiner (2011). In Figures 4–6, we display example simulated galaxies for each fiducial distance along with their observed CMDs.

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Before moving forward, we perform additional tests on a subset of our simulations for verification and validation of our pipeline. First, we fit an exponential profile to the two-dimensional distribution of stars consistent with each simulated dwarf by using the maximum-likelihood technique of Martin et al. (2008) as implemented by Sand et al. (2009). We compare our recovered structural parameters back to the true inputs and confirm that they are consistent within the uncertainties. Then, we derive the absolute magnitudes of our simulated galaxies following the same procedure as in Mutlu-Pakdil et al. (2018). In short, we build a well-populated CMD, including our completeness and photometric uncertainties, with the same isochrones used for our mock observations and their associated luminosity functions. We then randomly select the same number of stars from this artificial CMD as was found from our exponential profile fits. We sum the flux of these stars and extrapolate the flux of unaccounted stars using the adopted luminosity function. We calculate 100 realizations in this way and take the mean as our absolute magnitudes and the standard deviation as our uncertainties. We verify that our recovered luminosities and the true inputs agree well within the recovered uncertainties. As our focus in this paper is solely on detection, we will present the details of our investigation in a future paper. This will allow us to improve our current tools and properly interpret resolved stellar population studies in the Local Volume in the next-generation surveys.

3.2. Dwarf Recovery Using a Matched-filter Technique

Figures 7–9 report the results of our artificial resolved dwarf tests at distances of 1.5, 3.5, and 5 Mpc, respectively. The figures show the size–luminosity space probed by our tests, ¹⁵ where the colored blocks present the detection efficiency map of our simulated dwarfs. Each block represents ∼15 simulated dwarfs, on average. The blue hatched regions labeled as "100% Detected (Extrapolated)" denote the approximate regions where dwarfs should be easily detected and hence have not been explored in our simulations. Similarly, orange hatched regions labeled as "0% Detected (Extrapolated)" represent the approximate regions where we extrapolate that dwarfs will be undetectable. For convenience, in Table 2, we report the detection efficiency of each luminosity bin for 26 mag arcsec⁻² ≲ μ_V,0 ≲ 29 mag arcsec⁻² at each fiducial distance. We also note Local Group analogs with similar properties in each row. Our experiments show that current and next-generation deep surveys will push the discovery frontier for new dwarf galaxies to fainter magnitudes, lower surface brightnesses, and larger distances.

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4.1. Distance of 1.5 Mpc

4.2. Distance of 3.5 Mpc

4.3. Distance of 5 Mpc

4.4. Effects of MW Contamination and Extinction

The results presented here so far come from the chosen NGC 253 pointing, which is located at a high Galactic latitude (b = −88°) such that the foreground extinction is very low (E(B − V) = 0.02). However, there are many galaxies within 5 Mpc that are located at a low Galactic latitude with higher extinction (see Table 1). Therefore, we repeat a subset of dwarf simulations by using the best imaging field from PISCeS of Cen A (b = 19°, E(B − V) = 0.12) to assess the effects of MW disk contamination and extinction in detecting dwarf galaxies. Because the extinction is highly correlated to the Galactic latitude, our experiment here probes their combined effects.

Figure 10 presents our Cen A simulation results for a distance of 3.5 Mpc. For comparison, we redisplay our results of the NGC 253 field at the same distance (Figure 8) in panel (a). For the size–luminosity space where all distant dwarfs are located (orange symbols), detection completeness stays the same, with a very high recovery rate (100%). However, compared to our NGC 253 experiments, the overall detection signal is ∼20% lower, on average, in the Cen A data. The effects become especially important for systems with a detection significance of ≲7σ in our NGC 253 experiments, e.g., low surface brightness systems (μ_V,0 ≳ 29 mag arcsec⁻²) and objects fainter than M_V ≳ −8. For example, the detectability of compact objects (e.g., M_V ∼ −8, r_h ∼ 120 pc) decreases to 55% from 95%, while systems like And XI (M_V = −6.9, r_h = 157 pc) decrease to 10% from 45%.

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To fully test the ΛCDM paradigm and constrain the physics governing galaxy formation and evolution at the smallest scales, we need comprehensive investigations into the abundance and properties of dwarf galaxies around primary galaxies with different masses, morphologies, and environments. In Table 1, we compile a list of nearby galaxies within ≈5 Mpc, which we can study in detail with resolved stellar populations with current state-of-the-art telescope facilities. Within the MW luminosity group, beyond the Local Group, dedicated deep wide-field surveys exist for Cen A (Crnojević et al. 2016, 2019 with Magellan/Megacam), NGC 253 (Sand et al. 2014; Toloba et al. 2016; B. Mutlu-Pakdil et al. 2021, in preparation, with Magellan/Megacam), M94 (Smercina et al. 2018 with Subaru/HSC), and M81 (including the M82 region; Chiboucas et al. 2009, 2013 with CFHT/MegaCam; Okamoto et al. 2019 with Subaru/HSC). The average seeing FWHM of these surveys is ∼08 . With systematic resolved stellar searches in these deep surveys, the faintest dwarfs discovered so far are CenA-MM17-Dw10 (M_V = −7.8, r_h = 250 pc; Crnojević et al. 2019) in the Cen A group and d0944+69 (M_V = −8.1, r_h = 130 pc; Chiboucas et al. 2013) in the M81 group, and they are consistent with our experiments at a distance of 3.5 Mpc (see Figures 8 and 10).

In Figure 11 (top panel), we show a comparison of the observed SLFs of these nearby galaxies to the theoretical expectations from Dooley et al. (2017b). For the observed luminosity functions, we adopt the Crnojević et al. (2019) compilation, which includes objects within 300 kpc of each host. These SLFs should be considered as a lower limit due to incomplete spatial coverage; while the MW SLF suffers due to our limited ability to detect satellites near the Galactic plane, other systems have not yet been mapped out to their virial radii (see Table 1; most deep wide-field surveys are limited to a radius of approximately 150 kpc). Note that we make no attempt to correct any luminosity function for incompleteness. The region where the Cen A and M81 SLFs become incomplete (in luminosity) are shown with open symbols and dashed lines (as reported by Crnojević et al. 2019 and Chiboucas et al. 2013). For theoretical predictions, we use the satellite stellar mass functions of Dooley et al. (2017b; see their Figure 5), which were derived by applying the abundance-matching models of Garrison-Kimmel et al. (2017) and Brook et al. (2014) and a reionization model to the dark matter–only Caterpillar simulation suite. We derive the luminosity functions from the stellar mass functions by assuming a stellar mass-to-light ratio of 1 and calculating V-band absolute magnitudes from luminosities. As stated by Dooley et al. (2017b), numbers predicted by the Brook et al. models represent the low end of the possibilities for dwarf satellite galaxies, and values from the Garrison-Kimmel et al. model are closer to median expectations. We observe a clear, large scatter in the observed luminosity functions at fixed magnitude, and we need a larger sample of dwarf satellites to determine the typical halo-to-halo scatter and ultimately constrain the models. In Figure 11, we also mark our ≳90% detection completeness limit ¹⁶ for each fiducial distance. With the near-future surveys, it will be possible to go significantly further down the luminosity functions of nearby galaxies.

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As is obvious from their high A_V values, several galaxies located in the zone of Galactic obscuration (e.g., IC 342, Circinus) are not suitable for such resolved studies. More promising targets are M83, M64, and NGC 4945, and future dedicated surveys of their halos will extend our general knowledge of substructure not only to new systems but also to new environments. The tidal index parameter, Θ₅, can be used as a measure of environment (Karachentsev et al. 2013), and Θ₅ < 0 indicates an isolated galaxy. Interestingly, recent studies indicate a tentative relationship between satellite richness and environment, suggesting that isolated MW-like galaxies have fewer satellites and a higher overall star formation fraction than their counterparts in dense environments (Bennet et al. 2019). This is also in general agreement with the SAGA spectroscopic survey results (Geha et al. 2017; Mao et al. 2021), which focused on isolated MW-like galaxies at D = 20–40 Mpc and found a high fraction of star-forming dwarfs in their sample (note that they are limited to M_V ≲ −12). These satellite population trends with environment may indicate that external processes such as tidal/ram pressure stripping are a physical driver of dwarf galaxy evolution. However, these results hinge on two nearby and relatively isolated galaxies: M94 (D = 4.41 Mpc, Θ₅ = 0.1) and M101 (D = 6.95 Mpc, Θ₅ = 1.3). ¹⁷ As shown in Section 4, a complete census of dwarf galaxies around M83, M64, and NGC 4945 is possible down to M_V ≈ −7 or −8. In particular, as M64 (D = 4.41 Mpc, Θ₅ = −0.7) and M83 (D = 4.90 Mpc, Θ₅ = 0.8) are in low-density environments, they provide us with a unique opportunity to extend the range of environments probed by the existing surveys of nearby galaxies and quickly establish whether or not recent satellite environmental trends are valid or if a new physical mechanism is necessary to explain the recently measured halo-to-halo scatter at the faint end of the SLF.

In addition to the MW luminosity group, it is essential to explore dwarf satellites around low-mass galaxies to better constrain the M_⋆–M_halo relationship. Cosmologically motivated models predict two to five and one to three satellites with M_V ≲ −7.7 around LMC- and SMC-sized galaxies, respectively (Dooley et al. 2017a). In Figure 11 (bottom panel), we highlight our ≳90% detection completeness limit for each distance fiducial. With the current and near-future ground-based instrumentation, we can test these predictions by performing systematic resolved stellar searches around the LMC/SMC analogs listed in Table 1. If we only focus on systems with an A_V value of <0.5, there are 19 LMC analogs and 36 SMC analogs within 5 Mpc that are suitable for such studies. There is already a significant ongoing effort to map out the halos of several low-mass galaxies and search for their satellite populations, e.g., the Pan-Andromeda Archaeological Survey around M33 (for which only ∼half of the virial radius was mapped with CFHT/MegaCam, and two possible satellites have been reported so far; Martin et al. 2009, 2013; Martinez-Delgado et al. 2021), the Magellanic Analog Dwarf Companions And Stellar Halos (MADCASH) Survey (which is an ongoing DECam+HSC deep imaging campaign around a dozen relatively isolated nearby low-mass galaxies, where three satellites have been reported to date: one around NGC 2403, one around NGC 4214, and one around NGC 3109; Sand et al. 2015a; Carlin et al. 2016, 2021), and DELVE-DEEP (which aims to obtain deep DECam imaging around Sextans B, IC 5152, NGC 55, and NGC 300; Drlica-Wagner et al. 2021). While the current campaigns focus on field dwarf galaxies, near-future studies present a great opportunity to target all 55 low-mass galaxies within 5 Mpc and explore the environmental effects on substructure properties at this mass scale. Based on the predictions of Dooley et al. (2017b), near-future studies are expected to find ∼(24–63, 14–110, 19–129) new satellites with M_V ≲ (−5, −8, −9) out to ∼(1.5, 3.5, 5) Mpc when these LMC/SMC analogs are mapped out to their virial radii. These studies will also be able to detect fainter dwarfs down to M_V ≈ −7 and −8, with ∼50% completeness, out to 3.5 and 5 Mpc, respectively (see Table 2). In particular, M33, M32, IC 1613, NGC 205, and NGC 3109 are excellent examples where we can establish secure satellite populations down to M_V ≈ −5 (due to their proximity), providing strong quantitative constraints on the M_⋆–M_halo relationship.

While we mostly focus on the discovery space of dwarf satellites around Local Volume galaxies (≲5 Mpc), LSST and other deep imaging surveys will also enable discovery of isolated dwarf galaxies that to date have been impossible without wide-area coverage. In particular, LSST will expand the sky coverage and volume over which our matched-filter technique can be applied. Isolated dwarf galaxies provide a unique reference group to disentangle environmental galaxy formation processes, as they live in fields isolated from galaxy groups where the environmental effect on galaxy processes is expected to be minimal (e.g., Dickey et al. 2019).

In this work, we examine the distance range where we can probe with resolved stars from the ground in the next decade. For the distances where this is not possible, surface brightness fluctuations provide a complementary tool to identify dwarf galaxies out to ∼25 Mpc (Greco et al. 2021). New and upcoming ground-based imaging surveys will make it possible to explore larger volumes with greater sensitivity, providing a large, rigorous census of faint galaxies across a wide range of environments, which will be crucial for our understanding of dark matter and galaxy formation.

In this work, we investigate the prospects for identifying resolved, faint dwarf galaxies within 5 Mpc with current and near-future ground-based instrumentation (e.g., Rubin, HSC). We perform image-level simulations of resolved, faint dwarf galaxies at three fiducial distances—1.5, 3.5, and 5 Mpc—with varying luminosities, ellipticities, sizes, stellar backgrounds, and galactic latitudes, utilizing the deep and high-quality imaging from the PISCeS data set. Then, we rigorously quantify the detectability of faint dwarf galaxies using a matched-filter technique. Our primary results may be summarized as follows.

• 1.  
  The matched-filter technique is quite powerful for identifying both compact and extended systems, but it becomes unreliable when systems are so extended that the background overwhelms the dwarf stars or so compact that the dwarf stars cannot be resolved due to image crowding.
• 2.  
  We probe the effects of local stellar densities in dwarf detection by placing dwarfs in different spatial positions. Varied stellar backgrounds affect the peak S in the matched-filter maps at about the 15% level, on average. This becomes particularly important for extended systems and faint dwarfs, where the number density of dwarf stars is low.
• 3.  
  We assess the effects of the ellipticity in dwarf detection by creating mock observations with three different ellipticities, i.e., = 0, 0.3, and 0.5 (see Figure 3). At fixed r_h and M_V , we find that rounder compact systems have a higher detectability ratio than their elongated analogs, and that elongated larger systems are relatively easier to detect than their round counterparts.
• 4.  
  At a fiducial distance of 1.5 Mpc, current and next-generation deep surveys will be able to resolve HB stars (see Figure 2), making it possible to uncover ultrafaint satellite dwarfs like Hercules, Leo IV, and Hydra II at the edge of the Local Group. A secure census of ultrafaint satellite dwarfs will be possible down to μ_V,0 ∼ 30 mag arcsec⁻² for M_V = [−7, −6] and μ_V,0 ∼ 29 mag arcsec⁻² for M_V = −5 (see Figure 7).
• 5.  
  At a distance of 3.5 Mpc, similar-depth surveys will be able to probe ∼2.5 mag below the TRGB, enabling a secure census of faint dwarf galaxies to M_V ≲ −7 (see Figure 5).
• 6.  
  At a distance of 5 Mpc, it will be possible to reach ∼2 mag below the TRGB, enabling a secure census of faint dwarf galaxies down to M_V ≈ −8. The detection completeness is very high for M_V ≲ −9 down to μ_V,0 ∼ 30 mag arcsec⁻², and it stays high for M_V ≈ −8 in the range 26 mag arcsec⁻² ≲ μ_V,0 ≲ 30 mag arcsec⁻² (see Figure 6).
• 7.  
  We perform our experiments primarily on a pointing close to NGC 253, which is located at a high Galactic latitude (b = −88°, E(B − V) = 0.02). To assess the effects of MW disk contamination and extinction in detecting dwarf galaxies, we repeat a subset of dwarf simulations by using the Cen A data (b = 19°, E(B − V) = 0.12). The overall detection signal is ∼20% lower, on average, in our Cen A experiments. The effects become especially important for systems with a detection significance of 7σ in our NGC 253 experiments, e.g., low surface brightness systems (μ_V,0 ≳ 29 mag arcsec⁻²) and objects fainter than M_V ≈ −8 (see Figure 10).
• 8.  
  Within the MW luminosity group, beyond the Local Group, the next ideal targets are M83, M64, and NGC 4945, which can extend our knowledge of substructure to new environments and quickly establish whether recent tentative relationships between satellite richness and environment are valid (Bennet et al. 2019).
• 9.  
  Near-future studies present a great opportunity to target all 55 suitable low-mass galaxies within 5 Mpc (see Table 1 with A_V < 0.5) and explore the environmental effects on substructure properties at this mass scale. In particular, M33, M32, IC 1613, NGC 205, and NGC 3109 are great examples where we can establish secure satellite populations down to M_V ≈ −5.

It is worth remembering that our simulations represent a set of idealized experiments. Here we assume that dwarf galaxies are pure single-stellar populations, following a smooth exponential stellar profile. While these assumptions are reasonable for Local Volume dwarfs, they do not fully capture the potentially complex structure of dwarf galaxies, as studies of Local Group dwarfs have revealed (e.g., Sand et al. 2012; Muñoz et al. 2018; Mutlu-Pakdil et al. 2018, among others). In addition, we assume that the underlying stellar population of the host halo is low or negligible. However, the completeness of the dwarf search is expected to be lower in the inner regions of the parent galaxy due to halo contamination. The effect would be strongest for the largest, low surface brightness dwarfs for a given luminosity. That said, simulations with realistic baryonic effects predict few subhalos within 20 kpc (e.g., Garrison-Kimmel et al. 2017). Future work should explore these effects and carefully characterize detection limits near the parent galaxy.

Notwithstanding the above caveats, our experiments show that near-future studies, which will probe substantially deeper than previous data sets, will push the discovery frontier for new dwarf galaxies to fainter magnitudes, lower surface brightnesses, and larger volumes. These discoveries will extend the faint end of the SLF of numerous nearby galaxies with a range of masses, morphologies, and environments, which will allow us to quantify the statistical fluctuations in satellite abundances around hosts and parse environmental effects as a function of host properties. Ultimately, the goal is to understand galaxy formation and test the ΛCDM model on small scales.

We thank the anonymous referee for constructive comments that helped to improve this paper. We are grateful to P. Bennet for useful discussions and tabulated data that were used in this paper.

B.M.P. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-2001663. J.D.S. acknowledges support from NSF grant AST-1412792. J.S. acknowledges support from NSF grant AST-1812856 and the Packard Foundation. D.J.S. acknowledges support from NSF grants AST-1821967 and 1813708.

Facility: Magellan:Clay (Megacam). -

Software: Astropy (Astropy Collaboration et al. 2013, 2018), The IDL Astronomy User's Library (Landsman 1993), DAOPHOT (Stetson 1987, 1994).