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Register a new userThe M101 Satellite Luminosity Function and the Halo to Halo Scatter Among Local Volume Hosts
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We have obtained deep Hubble Space Telescope (HST) imaging of 19 dwarf galaxy candidates in the vicinity of M101. Advanced Camera for Surveys HST photometry for 2 of these objects showed resolved stellar populations and Tip of the Red Giant Branch derived distances consistent with M101 group membership. The other 17 were found to have no resolved stellar populations, meaning they are background low surface brightness (LSB) galaxies. It is notable 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 careful in drawing conclusions about group membership without robust distance estimates. In this work we update the satellite luminosity function 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 24.5pm7.7 for the median local Milky Way (MW)-mass host. This makes M101 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 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 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 recent 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 LambdaCDM model.
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Low-mass satellites around Milky Way (MW)-like galaxies are important probes of small scale structure and galaxy formation. However, confirmation of satellite candidates with distance measurements remains a key barrier to fast progress in the Local Volume (LV). We measure the surface brightness fluctuation (SBF) distances to recently cataloged candidate dwarf satellites around 10 massive hosts within $D<12$ Mpc to confirm association. The satellite systems of these hosts are complete and mostly cleaned of contaminants down to $M_g{sim}-9$ to $-10$, within the area of the search footprints. Joining this sample with hosts surveyed to comparable or better completeness in the literature, we explore how well cosmological simulations combined with common stellar to halo mass relations (SHMR) match observed satellite luminosity functions in the classical satellite luminosity regime. Adopting a SHMR that matches hydrodynamic simulations, the predicted overall satellite abundance agrees well with the observations. The MW is remarkably typical in its luminosity function amongst LV hosts. Contrary to recent results, we find that the host-to-host scatter predicted by the model is in close agreement with the scatter between the observed systems, once the different masses of the observed systems are taken into account. However, we find significant evidence that the observed systems have more bright and fewer faint satellites than the SHMR model predicts, necessitating a higher normalization of the SHMR around halo masses of $10^{11}$ msun than present in common SHMRs. These results demonstrate the utility of nearby satellite systems in inferring the galaxy-subhalo connection in the low-mass regime.
Rapid advance has been made recently in accurate distance measurements for nearby ($D < 11$ Mpc) galaxies based on the magnitude of the tip of red giant branch stars resolved with the Hubble Space Telescope. We use observational properties of galaxies presented in the last version of Updated Nearby Galaxy Catalog to derive a halo mass of luminous galaxies via orbital motion of their companions. Our sample contains 298 assumed satellites with known radial velocities around 25 Milky Way-like massive galaxies and 65 assumed satellites around 47 fainter dominant galaxies. The average total mass-to-$K$-band luminosity ratio is $31pm6 M_odot/L_odot$ for the luminous galaxies, increasing up to $sim200 M_odot/L_odot$ toward dwarfs. The bulge-dominated luminous galaxies are characterized with $langle{}M_T/L_Krangle = 73pm15 M_odot/L_odot$, while the disc-dominated spirals have $langle{}M_T/L_Krangle = 17.4pm2.8 M_odot/L_odot$. We draw attention to a particular subsample of luminous spiral galaxies with signs of declining rotation curve, which have a radial velocity dispersion of satellites less than 55 km/s and a poor dark matter halo with $langle{}M_T/L_Krangle = 5.5pm1.1 M_odot/L_odot$. We note that a fraction of quenched (dSph, dE) companions around Milky Way-like galaxies decreases with their linear projected separation as $0.75 exp(-R_p/350,mathrm{kpc})$.
We have obtained deep Hubble Space Telescope (HST) imaging of four faint and ultra-faint dwarf galaxy candidates in the vicinity of M101 - Dw21, Dw22, Dw23 and Dw35, originally discovered by Bennet et al. (2017). Previous distance estimates using the surface brightness fluctuation technique have suggested that these four dwarf candidates are the only remaining viable M101 satellites identified in ground based imaging out to the virial radius of M101 (D~250 kpc). Advanced Camera for Surveys imaging of all four dwarf candidates shows no associated resolved stellar populations, indicating that they are thus background galaxies. We confirm this by generating simulated HST color magnitude diagrams of similar brightness dwarfs at the distance of M101. Our targets would have displayed clear, resolved red giant branches with dozens of stars if they had been associated with M101. With this information, we construct a satellite luminosity function for M101, which is 90% complete to M_V=-7.7 mag and 50% complete to M_V=-7.4 mag, that extends into the ultra-faint dwarf galaxy regime. The M101 system is remarkably poor in satellites in comparison to the Milky Way and M31, with only eight satellites down to an absolute magnitude of M_V=-7.7 mag, compared to the 14 and 26 seen in the Milky Way and M31, respectively. Further observations of Milky Way analogs are needed to understand the halo-to-halo scatter in their faint satellite systems, and connect them with expectations from cosmological simulations.
Models of galaxy formation in a cosmological context predict that massive disk galaxies should have structured extended stellar halos. Recent studies in integrated light, however, report a few galaxies, including the nearby disk galaxy M101, that have no measurable stellar halos to the detection limit. We aim to quantify the stellar content and structure of M101s outskirts by resolving its stars. We present the photometry of its stars based on deep F606W and F814W images taken with Hubble Space Telescope as part of the GHOSTS survey. The constructed CMDs of stars reach down to two magnitudes below the tip of the red giant branch. We derived radial number density profiles of the bright red giant branch (RGB) stars. The mean color of the RGB stars at $R sim$ 40 -- 60 kpc is similar to those of metal-poor globular clusters in the Milky Way. We also derived radial surface brightness profiles using the public image data provided by the Dragonfly team. Both the radial number density and surface brightness profiles were converted to radial mass density profiles and combined. We find that the mass density profiles show a weak upturn at the very outer region, where surface brightness is as faint as $mu_gapprox 34$ mag arcsec$^{-1}$. An exponential disk + power-law halo model on the mass density profiles finds the total stellar halo mass of $M_{halo}=8.2_{-2.2}^{+3.5}times 10^7M_odot$. The total stellar halo mass does not exceed $M_{halo} = 3.2 times 10^8$ $M_{odot}$ when strongly truncated disk models are considered. Combining the halo mass with the total stellar mass of M101, we obtain the stellar halo mass fraction of $M_{halo}/M_{gal} = 0.20_{-0.08}^{+0.10}%$ with an upper limit of 0.78%. We compare the halo properties of M101 with those of six GHOSTS survey galaxies as well as the Milky Way and M31 and find that M101 has an anemic stellar halo.
White dwarfs are the fossils left by the evolution of low-and intermediate-mass stars, and have very long evolutionary timescales. This allows us to use them to explore the properties of old populations, like the Galactic halo. We present a population synthesis study of the luminosity function of halo white dwarfs, aimed at investigating which information can be derived from the currently available observed data. We employ an up-to-date population synthesis code based on Monte Carlo techniques, that incorporates the most recent and reliable cooling sequences for metal poor progenitors as well as an accurate modeling of the observational biases. We find that because the observed sample of halo white dwarfs is restricted to the brightest stars only the hot branch of the white dwarf luminosity function can be used for such purposes, and that its shape function is almost insensitive to the most relevant inputs, like the adopted cooling sequences, the initial mass function, the density profile of the stellar spheroid, or the adopted fraction of unresolved binaries. Moreover, since the cut-off of the observed luminosity has not been yet determined only lower limits to the age of the halo population can be placed. We conclude that the current observed sample of the halo white dwarf population is still too small to obtain definite conclusions about the properties of the stellar halo, and the recently computed white dwarf cooling sequences which incorporate residual hydrogen burning should be assessed using metal-poor globular clusters.
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