The M101 Satellite Luminosity Function and the Halo to Halo Scatter Among Local Volume Hosts
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Draft version December 12, 2019 Typeset using LATEX twocolumn style in AASTeX62 The M101 Satellite Luminosity Function and the Halo to Halo Scatter Among Local Volume Hosts P. Bennet,1 D. J. Sand,2 D. Crnojevic,´ 3, 1 K. Spekkens,4, 5 A. Karunakaran,5 D. Zaritsky,2 and B. Mutlu-Pakdil2 1Physics & Astronomy Department, Texas Tech University, Box 41051, Lubbock, TX 79409-1051, USA 2Steward Observatory, University of Arizona, 933 North Cherry Avenue, Rm. N204, Tucson, AZ 85721-0065, USA 3University of Tampa, 401 West Kennedy Boulevard, Tampa, FL 33606, USA 4Department of Physics and Space Science, Royal Military College of Canada P.O. Box 17000, Station Forces Kingston, ON K7K 7B4, Canada 5Department of Physics, Engineering Physics and Astronomy, Queens University, Kingston, ON K7L 3N6, Canada (Received 11th June, 2019; Revised 20th August, 2019; Accepted 19th September, 2019) Submitted to ApJ 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 MV =−8.2. M101 is a sparsely populated system with only 9 satellites down to MV ≈−8, as compared to 26 for M31 and 24.5±7.7 for the median 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 (MV =−9.1). M101 and M94 share several properties that mark them as unusual compared to the other Local Volume 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. Keywords: Dwarf galaxies, Luminosity function, Galaxy evolution, HST photometry, Galaxy stellar arXiv:1906.03230v2 [astro-ph.GA] 11 Dec 2019 halos, Galaxy groups 1. INTRODUCTION baryons { the so-called Λ Cold Dark Matter (ΛCDM) model for structure formation (e.g. Planck Collabora- Observations on large scales (&10 Mpc) are consis- tent with a Universe dominated by dark energy and tion et al. 2016). Despite this success, challenges re- cold dark matter, along with a small contribution from main on smaller, subgalactic scales where straightfor- ward expectations for the faint end of the galaxy lumi- nosity function (LF) are not met (see Bullock & Boylan- Corresponding author: Paul Bennet Kolchin 2017, for a recent review) including the `miss- [email protected] ing satellites problem' (e.g. Moore et al. 1999; Klypin 2 Bennet et al. et al. 1999), `too big to fail' (e.g. Boylan-Kolchin et al. lites around the MW is ∼3σ discrepant from zero), this 2011, 2012) and the apparent planes of satellites around provides additional motivation to explore the faint end nearby galaxies (e.g. Pawlowski et al. 2012; Ibata et al. of the LF where the numbers of satellites is larger and 2013; M¨ulleret al. 2018). therefore produce more robust statistics. Despite the Significant progress has been made in reconciling these large observational effort, the number of Milky Way-like small-scale ΛCDM `problems' on both the theoretical systems studied is still small, and further work is needed (Brooks et al. 2013; Sawala et al. 2016; Wetzel et al. to quantify the observed range in substructure proper- 2016) and observational (e.g. Torrealba et al. 2018a,b; ties. Koposov et al. 2018, most recently around the Milky Here we present Hubble Space Telescope follow-up to Way, MW) fronts, although the focus has been on the 19 dwarf galaxy candidates recently discovered around Local Group and its satellite system. Ultimately, to fully M101 (to which we assume a distance of D=7 Mpc test the ΛCDM model for structure formation, studies throughout this work; Lee & Jang 2012; Tikhonov et al. beyond the Local Group are necessary in order to sam- 2015), both to determine their membership status and ple primary halos with a range of masses, morpholo- to construct a satellite LF. M101 is an excellent system gies and environments. This work is now beginning for comparing with our own Local Group, as its stellar 10 in earnest using wide-field imaging datasets, as well as mass (∼5.3×10 M ; van Dokkum et al. 2014) is sim- spectroscopy, centered around primary galaxies with a ilar to that of the MW to within the uncertainties (e.g. range of masses (e.g. Chiboucas et al. 2013; Sand et al. McMillan 2011). M101 also has an `anemic' low mass 2014, 2015a; Crnojevi´cet al. 2014a; M¨ulleret al. 2015; stellar halo (van Dokkum et al. 2014) that nonetheless Crnojevi´cet al. 2016; Carlin et al. 2016; Toloba et al. shows signs of past galaxy interactions (e.g. Mihos et al. 2016; Bennet et al. 2017; Danieli et al. 2017; Smercina 2013). Measuring the diversity of satellite populations et al. 2017; Geha et al. 2017; Smercina et al. 2018; Crno- around MW-like systems is a main driver for this work. jevi´cet al. 2019). Field searches are also uncovering a An outline of the paper follows. In Section2, we give plethora of faint dwarf galaxy systems using a variety of context and an overview of recent dwarf galaxy searches techniques (e.g. Tollerud et al. 2015; Sand et al. 2015b; around M101, and how our 19 dwarf targets were se- Leisman et al. 2017; Greco et al. 2018; Bennet et al. lected for follow-up. In Section3 we describe the HST 2018; Zaritsky et al. 2019). photometry and artificial star tests. In Section4 we One opportunity is to measure the dispersion in sub- present the properties of the dwarf populations around structure properties among Milky Way-like halos, par- M101; we also discuss the statistical properties of the tially to understand if the Local Group has unusual population of M101 dwarf candidates that were not ob- substructure properties, and to help guide simulations served by HST. Next, in Section5 we discuss the lu- which are addressing ΛCDM’s so-called problems. Ini- minosity function of the M101 system and compare it tial results in this arena are exciting { the Satellites to other nearby Local Volume galaxies and those found Around Galactic Analogs (SAGA; Geha et al. 2017) sur- in the SAGA survey. We also compare the dwarf star vey has found that the halo to halo scatter in bright forming fractions within these groups and explore a po- satellite numbers is higher than expected from abun- tential correlation between host environment and star dance matching expectations. SAGA also found many formation fraction within the satellites. Finally, we sum- examples of star forming dwarf satellites, in contrast marize and conclude in Section6. with the dwarf population in the Local Group. Addi- tionally, a recent search for faint satellites around M94 2. DWARF CANDIDATES AROUND M 101 (D=4.2 Mpc), another Milky Way analogue, found only Although traditionally thought to have a relatively two satellites with M >4×105 M in comparison to the ? poor satellite galaxy population (e.g. Bremnes et al. eight systems found around the Milky Way (Smercina 1999), dwarf galaxy searches around M101 have been et al. 2018)1ote there are also 14 additional dwarf candi- reinvigorated by the surge in interest in the low sur- dates with velocities consistent with M94 (Karachentsev face brightness (LSB) universe. Using a set of specially et al. 2013), however these objects were outside of of the designed telephoto lenses, the Dragonfly team identi- search radius in Smercina et al.(2018). At the bright fied seven new diffuse dwarf galaxy candidates in a ∼9 end of the satellite LF the number of objects is small and deg2 region around M101 (Merritt et al. 2014). Out therefore the statistical power is low (e.g. the 8 satel- of these seven dwarf candidates, three were identified as true M101 dwarfs based on their HST-derived tip 1 N of the red giant branch (TRGB) distance (M101 DF1, M101 DF2, and M101 DF3; Danieli et al. 2017); the re- 3 maining four were found to be background sources, per- sample of the entire diffuse dwarf candidate population haps associated with the elliptical galaxy NGC5485, at around M101. After determining the membership status D∼27 Mpc (Merritt et al. 2016). Other small telescope of these dwarf candidates, we construct the satellite LF searches have also identified M101 dwarf candidates in for M101, and compare it with other MW analogues to recent years, with Karachentsev et al.(2015) reporting get an initial measure of the halo to halo scatter in this on four additional objects (DwA through DwD; see also population.