Allison Merritt

FIRST RESULTS FROM THE DRAGONFLY TELEPHOTO ARRAY: THE APPARENT LACK OF A STELLAR HALO IN THE MASSIVE SPIRAL GALAXY M101

We use a new telescope concept, the Dragonfly Telephoto Array, to study the low surface brightness outskirts of the spiral galaxy M101. The radial surface brightness profile is measured down to mu_g ~ 32 mag/arcsec^2, a depth that approaches the sensitivity of star count studies in the Local Group. We convert surface brightness to surface mass density using the radial g-r color profile. The mass density profile shows no significant upturn at large radius and is well-approximated by a simple bulge + disk model out to R = 70 kpc, corresponding to 18 disk scale lengths. Fitting a bulge + disk + halo model we find that the best-fitting halo mass M_halo ~ 1.7 x 10^8 M_sun. The total stellar mass of M101 is M_tot,* ~ 5.3 x 10^10 Msun, and we infer that the halo mass fraction f_halo = M_halo / M_tot,* ~ 0.003. This mass fraction is lower than that of the Milky Way (f_halo ~ 0.02) and M31 (f_halo ~ 0.04). All three galaxies fall below the f_halo - M_tot,* relation predicted by recent cosmological simulations that trace the light of disrupted satellites, with M101s halo mass a factor of ~10 below the median expectation. However, the predicted scatter in this relation is large, and more galaxies are needed to better quantify this possible tension with galaxy formation models. Dragonfly is well suited for this project: as integrated-light surface brightness is independent of distance, large numbers of galaxies can be studied in a uniform way.

THE DISCOVERY OF SEVEN EXTREMELY LOW SURFACE BRIGHTNESS GALAXIES IN THE FIELD OF THE NEARBY SPIRAL GALAXY M101

Dwarf satellite galaxies are a key probe of dark matter and of galaxy formation on small scales and of the dark matter halo masses of their central galaxies. They have very low surface brightness, which makes it difficult to identify and study them outside of the LocalGroup. We used a low surface brightness-optimized telescope, the Dragonfly Telephoto Array, to search for dwarf galaxies in the field of the massive spiral galaxy M101. We identify seven large, low surface brightness objects in this field, with effective radii of 10−30 arcseconds and central surface brightnesses of µg ∼ 25.5−27.5 mag arcsec −2 . Given their large apparent sizes and low surface brightnesses, these objects would likely be missed by standard galaxy searches in deep fields. Assuming the galaxies are dwarf satellites of M101, their absolute magnitudes are in the range −11.6 . MV . −9.3 and their effective radii are 350 pc − 1.3 kpc. Their radial surface brightness profiles are well fit by Sersic profiles with a very low Sersic index (n ∼ 0.3−0.7). The properties of the sample are similar to those of well-studied dwarf galaxies in the Local Group, such as Sextans I and Phoenix. Distance measurements are required to determine whether these galaxies are in fact associated with M101 or are in its foreground or background. Subject headings: cosmology: observations — galaxies: dwarf — galaxies: halos — galaxies: evolution

THE DRAGONFLY NEARBY GALAXIES SURVEY. I. SUBSTANTIAL VARIATION IN THE DIFFUSE STELLAR HALOS AROUND SPIRAL GALAXIES

Galaxies are thought to grow through accretion; as less massive galaxies are disrupted and merge over time, their debris results in diffuse, clumpy stellar halos enveloping the central galaxy. Here we present a study of the variation in the stellar halos of galaxies, using data from the Dragonfly Nearby Galaxies Survey (DNGS). The survey consists of wide field, deep (

A galaxy lacking dark matter

\mu_{g} > 31

Low Metallicities and Old Ages for Three Ultra-diffuse Galaxies in the Coma Cluster

mag arcsec

Extensive Globular Cluster Systems Associated with Ultra Diffuse Galaxies in the Coma Cluster

^{-2}

Evidence of Absence of Tidal Features in the Outskirts of Ultra Diffuse Galaxies in the Coma Cluster

) optical imaging of nearby galaxies using the Dragonfly Telephoto Array. Our sample includes eight spiral galaxies with stellar masses similar to that of the Milky Way, inclinations of

Future Prospects: Deep Imaging of Galaxy Outskirts Using Telescopes Large and Small

16-90

The Dragonfly Nearby Galaxies Survey. IV. A Giant Stellar Disk in NGC 2841

degrees and distances between

First Results from Project Dragonfly

7-18