The M101 Satellite Luminosity Function and the Halo–Halo Scatter among Local Volume Hosts

Abstract

We have obtained deep Hubble Space Telescope (HST) imaging of 19 dwarf galaxy candidates in the vicinity of M101. Advanced Camera for Surveys (ACS) HST photometry for 2 of these objects showed resolved stellar populations and Tip of the Red Giant Branch (TRGB) derived distances (D∼7 Mpc) consistent with M101 group membership. The remaining 17 were found to have no resolved stellar populations, meaning they are either part of the background NGC 5485 group or are distant low surface brightness (LSB) galaxies. It is noteworthy that many LSB objects which had previously been assumed to be M101 group members based on projection have been shown to be background objects, indicating the need for future diffuse dwarf surveys to be very careful in drawing conclusions about group membership without robust distance estimates. In this work we update the satellite luminosity function (LF) of M101 based on the presence of these new objects down to M V =−8.2. M101 is a sparsely populated system with only 9 satellites down to M V ≈−8, as compared to 26 for M31 and 24.5±7.7 for the median Milky Way (MW)-mass host in the Local Volume. This makes M101 by far the sparsest group probed to this depth, though M94 is even sparser to the depth it has been examined (M V =−9.1). M101 and M94 share several properties that mark them as unusual compared to the other local MW-mass galaxies examined: they have a very sparse satellite population but also have high star forming fractions among these satellites; such properties are also found in the galaxies examined as part of the SAGA survey. We suggest that these properties appear to be tied to the wider galactic environment, with more isolated galaxies showing sparse satellite populations which are more likely to have had recent star formation, while those in dense environments have more satellites which tend to have no ongoing star formation. Overall our results show a level of halo-to-halo scatter between galaxies of similar mass that is larger than is predicted in the ΛCDM model.

The Satellite Luminosity Function of M101 into the Ultra-faint Dwarf Galaxy Regime

“…Follow-up HST imaging of 19 dwarf galaxy candidates confirmed that DwA and Dw9 are M101 group members, verified by their TRGB distance, and that the remainder of their sample are all background objects . Using the collected M101 dataset, Bennet et al (2019) constructed a satellite luminosity function for M101 that is complete to M V ≈−8, and showed that M101 has a very sparse satellite population in contrast to the MW and M31. Further, Bennet et al (2019) speculated that this may be due to the relative isolation of M101, as a comparable system with few satellites, M94, has a similarly isolated environment (Smercina et al 2018).…”

“…Images are 0.6' × 0.6' for the new candidates; north is up and east is to the left. For contrast, in the bottom set of panels we show colorized HST cutouts from two confirmed M101 satellites presented inBennet et al (2019) -DwA (MV =−9.5) and Dw9 (MV =−8.2); these images are 1.0' × 1.0' due to the larger size of these objects. In this case, each dwarf shows a clear, associated point source overdensity, indicating that we are resolving it into stars.…”

Abundance of dwarf galaxies around low-mass spiral galaxies in the Local Volume

“…The Local Volume (D < 11 Mpc, Kraan-Korteweg & Tammann 1979;Karachentsev et al 2004Karachentsev et al , 2013 hosts over 30 large galaxies with total luminosities in excess of M tot ≈ −20 K mag. Several surveys have targeted the more prominent of these giant galaxies like M 83 (Müller et al 2015), Centaurus A (Crnojević et al 2014(Crnojević et al , 2016Müller et al 2017;Taylor et al 2018), and others (Merritt et al 2014;Karachentsev et al 2015;Javanmardi et al 2016;Park et al 2017;Smercina et al 2018;Bennet et al 2019;Carlsten et al 2020;Davis et al 2020). Furthermore, dwarf galaxy surveys reach even more distant galaxy clusters (Venhola et al 2017;Wittmann et al 2019), groups (Geha et al 2017;Cohen et al 2018;Habas et al 2020), and the field (Greco et al 2018;Prole et al 2019).…”

“…Its LF seems to be inconsistent with predictions from highresolution dark matter simulations, having far too few satellites for such a massive galaxy. In addition, Bennet et al (2019) argued for a strong variation in the LF derived from the environments of the giant galaxies in the Local Volume. This scatter seems to be larger than what is expected from the concordance model (but see also Carlsten et al 2020 for a different view).…”

Dwarfs from the Dark (Energy Survey): a machine learning approach to classify dwarf galaxies from multi-band images

“…Follow-up observations of many dwarf galaxy candidates from some of these surveys with better instruments have revealed an unavoidable contamination of false detections, i.e. E-mail: oliver.muller@astro.unistra.fr by confusion with background galaxies, foreground galactic cirrus, or instrumental noise (Chiboucas et al 2013;Merritt et al 2016;Müller et al 2018Müller et al , 2019Bennet et al 2019). A prime example is the galaxy Cen 8/KK 198, which has been regarded as dwarf galaxy associated with the Centaurus group for more than two decades (Cote 1996;Jerjen et al 2000;Karachentsev et al 2013;Müller et al 2017), until VLT observations have uncovered it as a low-surface brightness spiral galaxy (Müller et al 2019(Müller et al , 2021.…”