Dissertation/Thesis Abstract

Characterizing Assembly Histories in the Local Universe with the Dragonfly Telephoto Array
by Merritt, Allison Taylor, Ph.D., Yale University, 2017, 127; 10783454
Abstract (Summary)

This thesis explores the ways in which observations of the low surface brightness universe can inform our views of galaxy evolution and, specifically, the extent to which galaxies assemble stellar mass through mergers and accretion events. I first 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 (μg > 31 mag aresec-2) optical imaging of nearby galaxies using the Dragonfly Telephoto Array. The sample in question includes eight spiral galaxies with stellar masses similar to that of the Milky Way, inclinations of 16 – 90 degrees and distances between 7–18 Mpc. I construct stellar mass surface density profiles from the observed g-band surface brightness in combination with the gr color as a function of radius, and compute the halo fractions from the excess stellar mass (relative to a disk+bulge fit) beyond 5 half-mass radii. I find a mean halo fraction of 0.009 ± 0.005 and a large RMS scatter of 1.01[special characters omitted] dex. The peak-to-peak scatter of the halo fraction is a factor of > 100 – while some galaxies feature strongly structured halos resembling that of M31, three of the eight have halos that are completely undetected in our data. I conclude from this sample that spiral galaxies as a class exhibit a rich variety in stellar halo properties, implying that their assembly histories have been highly non-uniform. I find no convincing evidence for an environmental or stellar mass dependence of the halo fraction in the sample.

In addition to being the future building blocks of the stellar halos of galaxies, 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, however, which makes it difficult to identify and study them outside of the Local Group. I used the Dragonfly Telephoto Array to search for dwarf galaxies in the field of the massive spiral galaxy M101, and identified seven large, low surface brightness objects in this field, with effective radii of 10-30 arcseconds and central surface brightnesses of μg,0 ~ 25.5 – 27.5 mag aresec -2. Given their large apparent sizes and low surface brightnesses, these objects would likely be missed by standard galaxy searches in deep fields. 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.

Finally, follow-up observations of these low surface brightness objects with the Hubble Space Telescope subsequently revealed that three of the seven objects were bonafide satellite galaxies of the M101 group. I show that, unexpectedly, the other four galaxies are ultra-diffuse galaxies in a group environment. The galaxies have effective radii of 10 – 38 and central surface brightnesses of 25.6 – 27.7 mag aresec-2 in the g-band. They remain persistently unresolved even with the spatial resolution of HST/ACS, which implies distances of D > 17.5 Mpc. I show that the galaxies are most likely associated with a background group at ~ 27 Mpc containing the massive ellipticaLs NGC 5485 and NGC 5473. At this distance, the galaxies have sizes of 2.6-4.9 kpc, and are classified as UDGs. They are similar to the populations that have been revealed in clusters such as Coma, Virgo, and Fornax, yet have on average even lower surface brightness. The discovery of four UDGs in a galaxy group demonstrates that the UDG phenomenon is not exclusive to cluster environments. Furthermore, their morphologies seem less regular than those of the cluster populations, which may suggest a different formation mechanism or be indicative of a threshold in surface density below which UDGs are unable to maintain stability.

Indexing (document details)
Advisor: van Dokkum, Pieter
School: Yale University
School Location: United States -- Connecticut
Source: DAI-B 79/05(E), Dissertation Abstracts International
Subjects: Astronomy
Keywords: Galaxies
Publication Number: 10783454
ISBN: 9780355709209
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