Theses and Dissertations from UMD
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Item Gas and Star Formation at the Peak of Cosmic Star Forming Activity(2021) Lenkic, Laura; Bolatto, Alberto; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gas and star formation in galaxies are intimately linked to one another. Molecular hydrogen gas is the material out of which stars form, while the process of forming stars, in turn, depletes the reservoirs of gas in galaxies and builds up their stellar mass. Observations of star formation in galaxies over time indicate that they must form stars for timescales longer than would be expected from their gas content and star formation rates, indicating that processes that replenish the star forming fuel must be present. The focus of this thesis is on two components of this qualitative picture: the molecular hydrogen gas content of galaxies over time, and the link between gas and star formation in galaxies resembling those observed at the epoch of most active star formation. First, I present a systematic search for serendipitous carbon monoxide emitting sources in the second Plateau de Bure High-z Blue-Sequence Survey (PHIBSS2). These observations presented an opportunity to quantify the mass density of molecular gas in galaxies as a function of time, and to link this to the star formation history of the Universe. I use a match-filter technique to systematically detect 67 serendipitous sources, after which I characterize their properties, creating a catalog of their redshifts, line widths, fluxes, estimations of the detection reliability, and completeness of the detection algorithm. I find that these serendipitous sources are unrelated to the primary sources that were targeted by PHIBSS2, and use the catalog to construct luminosity functions spanning a redshift range from $\sim 0.3-5$. From these luminosity functions, I place constraints on the molecular hydrogen content in galaxies over cosmic time. My work presents one of the first attempts to use existing observations for this measurement and yields results that are consistent with previous studies, while demonstrating the scientific power of large, targeted surveys. Next, I study a sample of rare, nearby galaxies that are most similar to those we observe at the peak of cosmic star forming activity that occured $\sim 10$ billion years ago. These galaxies are drawn from the DYnamics of Newly Assembled Massive Objects (DYNAMO) survey, and their proximity to us allows for very detailed studies of their massive star forming clumps. I use observations from the Hubble Space Telescope (HST) to measure colors that are sensitive to stellar population age and extinction. From these measurements, I find that clumps in DYNAMO galaxies have colors that are most consistent with very young centers and outskirts that appear systematically older, by as much as 150~Myr in some cases. I attribute this age difference to the presence of ongoing star formation in the centers of clumps that maintains the population of massive, short-lived stars and gives rise to colors consistent with young ages. Furthermore, I find that within the disks of their host galaxies, younger clumps are preferentially located far from galaxy centers, while older clumps are preferentially located closer to the centers. These results are consistent with hydrodynamic simulations of high-redshift clumpy galaxies that predict clumps form in the outskirts of galaxies via a violent disk instability, and as they age, migrate to the centers of galaxies where they merge and contribute to the growth of galactic bulges. Building on this study, I combine observations of DYNAMO galaxies from the HST and the Atacama Large Millimeter/sub-millimeter Array (ALMA) to trace molecular hydrogen gas and star formation. I link these observations to measurements of the molecular gas velocity dispersions to test theories of star formation. I find that compared to local samples of ``normal'' star forming galaxies, DYNAMO systems have consistently high velocity dispersions, molecular gas surface densities, and star formation rate surface densities. Indeed, throughout their disks, DYNAMO galaxies are comparable to the centers of local star forming galaxies. Stellar bar driven gas flows into the centers of galaxies in these local samples may give rise to the high observed velocity dispersions, and gas and star formation rate surface densities. For DYNAMO galaxies, the widespread elevated values of these parameters may be driven by galactic-scale gas inflows, which is predicted by theories. Finally, current theories of star formation, such as the feedback regulated model, assume that turbulence dissipates on timescales proportional to the angular velocity of a galaxy (eddy or crossing time). Yet, I find such models have difficulty reproducing the DYNAMO measurements, and thus conclude that the turbulent dissipation timescale in DYNAMO galaxies must scale with galactocentric radius.Item The Cosmic Near-Infrared Background: From the Dark Ages to the Present(2014) Helgason, Kari; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Cosmic Infrared Background (CIB) is made up of the collective light from galaxies and quasars built-up over the entire cosmic history. It plays an important role in characterizing the evolution of galaxies and contains information on other sources inaccessible to direct detection. In this dissertation, I seek to understand current CIB measurements in terms of all sources emitting since the era of the first stars. First, I model the CIB arising from known galaxy populations using 233 measured UV, optical and NIR luminosity functions from a variety of surveys spanning a wide range of redshifts. Our empirical approach, in conjunction with a halo model describing the clustering of galaxies, allows us to compute the fluctuations of the unresolved CIB and compare to current measurements. I find that fluctuations from known galaxy populations are unable to account for the large scale CIB clustering signal seen by current space observatories, and this discrepancy continues to diverge out to larger angular scales. This suggests that known galaxy populations are not responsible for the bulk of the fluctuation signal seen in the measurements and favors a new population of faint and highly clustered sources. I also empirically reconstruct the evolving extragalactic background light from galaxies and derive the associated opacity of the universe to high energy photons out to z~4. Covering the whole range from UV to mid-IR (0.15-25 micron), I provide for the first time a robust empirical calculation of the photon-photon optical depth out to several TeV. In the absence of significant contributions to the cosmic diffuse background from unknown populations, such as the putative first stars and black holes, the universe appears to be largely transparent to gamma-rays at all Fermi/LAT energies out to z~2 whereas becoming opaque to TeV photons already at z~0.2. In addition, I study contributions from extragalactic populations to a recently discovered cross-correlation signal of the CIB fluctuations with the Cosmic X-ray Background (CXB). I model the X-ray emission from AGN, normal galaxies and hot gas residing in virialized structures, calculating their CXB contribution and spatial coherence with all infrared emitting counterparts. At small angular scales the coherence between the CIB and the CXB can be explained by galaxies and AGN. However, at large angular scales I find the net contribution from these populations only to account for a fraction of the measured CIBxCXB signal. The discrepancy suggests that the signal originates from the same unknown source population producing the CIB clustering signal out to ~1 deg.Item The Fossils of the First Galaxies in the Local Universe(2011) Bovill, Mia Sauda; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We argue that, at least a fraction of the newly discovered population of ultra-faint dwarfs in the Local Group constitute the fossil relics of a once ubiquitous population of dwarf galaxies formed before reionization with maximum circular velocities, vmax<\sub> < 20 km s−1<\super>, where vmax<\sub> = M1/3. To follow the evolution and distribution of the fossils of the first galaxies on Local Volume, 5 − 10 Mpc, scales, we have developed a new method for generating initial conditions for ΛCDM N-body simulations which provides the necessary dynamic range. The initial distribution of particles represents the position, velocity and mass distribution of the dark and luminous ha- los extracted from pre-reionization simulations. We find that ultra-faint dwarfs have properties compatible with well preserved fossils of the first galaxies and are able to reproduce the observed luminosity-metallicity relation. However, because the brightest pre-reionization dwarfs form preferentially in overdense regions, they have merged into non-fossil halos with vmax<\sub> > 20−30 km s−1<\super>. Hence, we find a luminosity threshold of true-fossils of < 106 Lsolar<\sub>, casting doubts on the classification of some clas- sical dSphs as fossils. We also argue that the ultra-faints at R < 50 kpc, have had their stellar properties significantly modified by tides, and that a large population of fossils remains undetected due to log(SigmaV<\sub> ) < −1.4. Next, we show that fossils of the first galaxies have galactocentric distributions and cumulative luminosity func- tions consistent with observations. We predict there are ∼ 300 luminous satellites orbiting within Rvir<\sub> of the Milky Way, ∼ 50 − 70% of which are fossils. Despite our multidimensional agreement at low luminosities, our primordial model produces an overabundance of bright dwarf satellites (LV<\sub> > 105<\super> Lsolar<\sub>), with this "bright satellite problem" most evident in the outer parts of the Milky Way. We estimate that, although relatively bright (LV<\sub> > 105 Lsolar<\sub>), these ghostly primordial populations are very diffuse, producing primordial populations with surface brightnesses below survey detection limits. Although we cannot yet present unmistakable evidence for the existence of the fossils of first galaxies in the Local Group, we suggest observational strategies to prove their existence. (i) The detection of "ghost halos" of primordial stars around isolated dwarfs. (ii) The existence of a yet unknown popu- lation of ∼ 150 Milky Way ultra-faints with half-light radii rhl<\sub> ≈ 100 − 1000 pc and luminosities LV<\sub> < 104<\super> Lsolar<\sub>, detectable by future deep surveys.