Discovery of Diffuse Dwarf Galaxy Candidates around M101

Abstract

We have conducted a search of a 9 deg 2 region of the CFHTLS around the Milky Way analog M101 (D∼7 Mpc), in order to look for previously unknown low surface brightness galaxies. This search has uncovered 38 new low surface brightness dwarf candidates, and confirmed 11 previously reported galaxies, all with central surface brightness µ(g,0)>23mag/arcsec 2 , potentially extending the satellite luminosity function for the M101 group by ∼1.2 magnitudes. The search was conducted using an algorithm that nearly automates the detection of diffuse dwarf galaxies. The candidates small size and low surface brightness means that the faintest of these objects would likely be missed by traditional visual or computer detection techniques. The dwarf galaxy candidates span a range of −7.1 ≥ M g ≥ −10.2 and half light radii of 118-540 pc at the distance of M101, and they are well fit by simple Sérsic surface brightness profiles. These properties are consistent with dwarfs in the Local Group, and to match the Local Group luminosity function ∼10-20 of these candidates should be satellites of M101. Association with a massive host is supported by the lack of detected star formation and the over density of candidates around M101 compared to the field. The spatial distribution of the dwarf candidates is highly asymmetric, and concentrated to the northeast of M101 and therefore distance measurements will be required to determine if these are genuine members of the M101 group.

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

“…In the current work we present HST results for 19 of the dwarf candidates reported by Bennet et al (2017), four of which were first identified by Karachentsev et al (2015). We list these objects in Table 1 and 2.…”

“…Meanwhile the other 17 targets appear as unresolved diffuse emission, indicating that the TRGB is too faint to be detected in our HST imaging, and that they are in the background. Beyond this, there are a further 23 diffuse dwarfs from Bennet et al (2017) that were not imaged with HST, and we statistically assess their M101 membership status in Section 4.3. The spatial distribution of all of the resolved and diffuse dwarfs around M101 are shown in Figure 1.…”

“…Twenty-three dwarf candidates from the Bennet et al (2017) sample were not observed by HST and therefore have unknown distances, aside from Dw26 which was confirmed to be a background galaxy via HI observations (Bennet et al 2017). The untargeted objects have mean properties (e.g.…”

“…Another method to estimate the distances of the untargeted LSB objects in Bennet et al (2017) is through the surface brightness fluctuations (SBF) method reported in Carlsten et al (2019b). These distances show a total of four possible M101 dwarfs (Dw21, Dw22, Dw23 & Dw35) among the non-targeted LSB objects from Bennet et al (2017), as their reported distances are consistent with the M101 group to within 1 σ. However there are significant uncertainties on these distances and no firm conclusions about the group membership of these objects were drawn by Carlsten et al (2019b).…”

A correlation between the number of satellites and the bulge-to-total baryonic mass ratio extending beyond the Local Group

“…These studies confirmed in total 8 satellites for this galaxy with the faintest having a V-band absolute magnitude of M V = −8.2 mag. Most of these candidates have been found in a wide survey using also the CFHT byBennet et al (2017) and some of them byMerritt et al (2014),Javanmardi et al (2016), andMüller et al (2017b). These surveys cover almost isotropically the 260 kpc virial radius of M101 (seefigure 1inCarlsten et al 2019b).…”