Discovery of Isolated, Quenched, and Likely Backsplash Dwarf Galaxies near M101

TL;DR

The paper reports the discovery of three ultra-faint, quiescent dwarfs in the outskirts of the M101 group, with Sha DG-I and Sha DG-III identified as strong backsplash candidates and Sha DG-II likely a faint satellite of a nearby SMC-mass host. It leverages multi-wavelength archival data (HSC-SSP, CFHTLS, GALEX, Apertif) and targeted follow-up (TTT2) to measure structural parameters, provisional TRGB distances, and quiescence indicators (UV, Hα, and HI limits). The results favor a backsplash origin for two dwarfs and place them at projected distances of a few hundred kpc from M101, while placing M101’s satellite population in the context of ΛCDM and halo–environment connections via SHMR and tidal indices. The work highlights environmental density as a key factor in shaping subhalo abundances and advocates future radial velocity and proper motion measurements to decisively classify backsplash vs field origins, with Rubin LSST poised to expand the sample for robust cosmological tests.

Abstract

I report the discovery of three faint, semi-resolved quiescent dwarf galaxies, two of which are strong backsplash candidates associated with the nearby satellite-sparse spiral M101 (D ~ 6.7 Mpc). The galaxies lie within the magnitude range MV ~ -7.70 to -8.2 and half-light radii rh ~ 110 to 204 pc. Shapiro DG-I (Sha DG-I/MAGE1412+5650) is a concurrently discovered and isolated galaxy. Shapiro DG-II (Sha DG-II) is a fainter dwarf and potential companion to the SMC-mass galaxy NGC 5585 (D ~ 6.84 Mpc). Shapiro DG-III (Sha DG-III) is an isolated dwarf on the edge of the ultra-faint regime. Hydrodynamical simulations suggest a backsplash population of galaxies that have been environmentally stripped by interactions with a host and ejected from the system, though they have not yet been definitively observed in the local universe. Considering their quenched stellar populations, indicated by the lack of coincident GALEX emission, relative distances, and fitting models, the galaxies are consistent with a backsplash origin and are pending follow-up. Analysis is performed to test whether the system's population can be explained by standard cosmology. A potential correlation is found between satellite abundances and halo masses calibrated by tidal indices in a sample of nearby MW-like galaxies, suggesting the importance of environmental density in the formation of dark matter subhaloes, though a larger sample is required. M101's sparse satellite system fits well in the relation and is in agreement with ΛCDM expectations.

Figures (11)

• Figure 1: $z/i,r,g$ cutout images of Shapiro DG-I/MAGE1412+5650 (left), Shapiro DG-II (center), and Shapiro DG-III (right) from Subaru Hyper-Suprime-Cam SSP, HSCLA, and the Canada-France-Hawaii Telescope Legacy Survey. N is up and E is left. The FOV is 54 $\times$ 54 arcsec.
• Figure 2: Cutout images of Sha DG-I/MAGE1412+5650 (left column), Shapiro DG-II (center column), and Shapiro DG-III (right column) from HSC-SSP and HSCLA (top row) and GALEX UV (bottom row). N is up and E is left. The FOV is 54 $\times$ 54 arcsec.
• Figure 3: Map centered on M101, shown as the center box, of the projected positions of associated galaxies in a $\sim400 \times400\,\mathrm{ kpc}$ field. The confirmed M101 dwarf satellites are shown as blue stars. The dwarf galaxies discovered in this work are highlighted as the red stars. The red lines extending from each candidate represent its major axis position alignment. The kernel density estimate of the confirmed satellites is shown as the blue to white contour. The arrow extending from M101 represents the geometric centroid of the confirmed satellites. The virial radius of M101, $\sim250 \,\mathrm{ kpc}$, is displayed as the blue dotted circle surrounding M101. Unrelated background galaxies from Bennet_2017Bennet_2019 are not shown. NGC 5585, the likely host galaxy of Sha DG-II, is shown as a grey circle. Its estimated virial radius based on its mass Hunter_2025 is displayed as a grey dotted circle.
• Figure 4: The best-fit models of the candidate backsplash galaxies Sha DG-I/MAGE1412+5650 (top), Sha DG-II (center), and Sha DG-III (bottom). The left panel displays the original selected image from CFHTLS or HSC. The best-fit models generated using Astropy for the galaxies are displayed in the center. The model of Sha DG-I/MAGE1412+5650 is generated as the weighted sum of two iterations. The residual images created by subtracting the models from the image are shown in the right panel. Most of the resolved stars remain visible in the residuals.
• Figure 5: The luminosity function of the HSC SSP $z$ band image. In the top panel, the count of resolved stars is measured within each bin at a size of $0.25\,\mathrm{mag}$. The flattened luminosity function is calculated using these bins and is displayed with a Gaussian filter at an amount $\sigma$. In the bottom panel, the edge-response of the TRGB is calculated using a Sobel edge detection filter, and the result is similarly displayed with Gaussian smoothing. The maximum edge at $z \,\mathrm{mag} = 25.14$ is displayed with the dotted red line.