Theses and Dissertations from UMD
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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 Simulations of Small Mass Structures in the Local Universe to Constrain the Nature of Dark Matter(2014) Polisensky, Emil; Ricotti, Massimo; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I use N-body simulations of the Milky Way and its satellite population of dwarf galaxies to probe the small-scale power spectrum and the properties of the unknown dark matter particle. The number of dark matter satellites decreases with decreasing mass of the dark matter particle. Assuming that the number of dark matter satellites exceeds or equals the number of observed satellites of the Milky Way, I derive a lower limit on the dark matter particle mass of mWDM > 2.1 keV for a thermal dark matter particle, with 95% confidence. The recent discovery of many new dark matter dominated satellites of the Milky Way in the Sloan Digital Sky Survey allows me to set a limit comparable to constraints from the complementary methods of Lyman-&alpha forest modeling and X-ray observations of the unresolved cosmic X-ray background and of halos from dwarf galaxy to cluster scales. I also investigate the claim that the largest subhalos in high resolution dissipationless cold dark matter (CDM) simulations of the Milky Way are dynamically inconsistent with observations of its most luminous satellites. I quantify the effects of the adopted cosmological parameters on the satellite densities and show the tension between observations and simulations adopting parameters consistent with WMAP9 is greatly diminished. I explore warm dark matter (WDM) cosmologies for 1-4 keV thermal relics. In 1 keV cosmologies subhalos have circular velocities at kpc scales 60% lower than their CDM counterparts, but are reduced by only 10% in 4 keV cosmologies. Recent reports of a detected X-ray line in emission from galaxy clusters has been argued as evidence of sterile neutrinos with properties similar to a 2 keV thermal relic. If confirmed, my simulations show they would naturally reconcile the densities of the brightest satellites and be consistent with the abundance of ultra-faint dwarfs. I conclude by using N-body simulations of a large set of dark matter halos in different CDM and WDM cosmologies to demonstrate that the spherically averaged density profile of dark matter halos has a shape that depends on the power spectrum of initial conditions. Virialization isotropizes the velocity dispersion in the inner regions of the halo but does not erase the memory of the initial conditions in phase space. I confirm that the slope of the inner density profile in CDM cosmologies depends on the halo mass with more massive halos exhibiting steeper profiles. My simulations support analytic models of halo structure that include angular momentum and argue against a universal form for the density profile.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.Item A DEEP X-RAY SURVEY OF THE LOCKMAN HOLE NORTHWEST(2005-08-26) Yang, Yuxuan; Mushotzky, Richard F; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)I present the X-ray analysis of the Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole Northwest field. The contiguous solid angle of the survey is about 0.4 sqr degree and the flux limits are 5x10^-16 erg/cm^2/s in the 0.4-2 keV band and 3x10^-15 erg/cm^2/s in the $2-8$~keV band. The survey bridges the gap between deep pencil beam surveys, and shallower, larger area surveys, allowing a better probe of the X-ray sources that contribute most of the 2--10 keV cosmic X-ray background. A total of 525 X-ray point sources and 4 extended sources have been found. The number counts, X-ray spectra evolution, X-ray variability of the X-ray sources are presented. We show 3 of the 4 extended sources are likely galaxy clusters or galaxy groups. We report the discovery of a gravitational lensing arc associated with one of these sources. I present the spatial correlation function analysis of non-stellar X-ray point sources in the CLASXS and Chandra Deep Field North (CDFN). I calculate both redshift-space and projected correlation functions in comoving coordinates.The correlation function for the CLASXS field over scales of 3 Mpc < s < 200 Mpc can be modeled as a power-law of the form xi(s)=(s/s_0)^{-gamma}, with gamma = 1.6^{+0.4}_{-0.3} and s_0 = 8.05^{+1.4}_{-1.5} Mpc. The redshift-space correlation function for CDFN on scales of 1 Mpc <s < 100 Mpc is found to have a similar correlation length, but a shallower slope. The real-space correlation functions are derived from the projected correlation functions. By comparing the real- and redshift-space correlation functions, we are able to estimate the redshift distortion parameter beta = 0.4 +/- 0.2 at an effective redshift z = 0.94. We found the clustering does not dependence significantly on X-ray color or luminosity. A mild evolution in the clustering amplitude is found, indicating a rapid increase of bias with redshift. The typical mass of the dark matter halo derived from the bias estimates show little change with redshift. The average halo mass is found to be log(M_{halo}/M_sun}) ~ 12.4.