Browsing by Subject "Galaxies"
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Item Characterizing the multicomponent density structure of galaxies(2012-12) Dhar, Barun KumarAbridged) Quantifying the 3D spatial structure of galaxies forms a key element in furthering our understanding of the formation and evolution of galaxies and consequently the universe. What we observe, however, is a 2D projection (on the plane of the sky) of the intensity of light of galaxies. Inferring the intrinsic 3D distribution is critical for the understanding of galaxies, but technically difficult. Despite 80 years of research, we still do not have a consensus on a model that can describe the 2D projected light of galaxies over their entire spatial extent.In the first part of this work, I summarize existing models and then show how one can efficiently model the high resolution light profiles of galaxies to quantify both the 2D and the 3D distributions and also infer the existence of multiple components in a galaxy. Using data of 23 galaxies from the Virgo Cluster, I present models of their light profiles with mean residuals of ~ 3% (that is consistent with measurement errors) over a radial range spanning an order of up to 10^5 -- an unprecedented accuracy over such large spatial extent. Consequences of such models are discussed, especially an evidence for an universality in the functional form that describes both the baryonic and dark matter distributions. In the second part of this work, I demonstrate a novel way of obtaining projections of intrinsic quantities when the standard method does not yield analytical solutions, i.e. a procedure to perform otherwise difficult analytical integrations.Item Dusty Lyman-alpha emitters as seen by Spitzer(2014-06) Dolan, Kyle ArthurWe present an analysis of Spitzer data for a large sample of low-redshift Lyα-emitting galaxies discovered by GALEX. Using the Donley et al. (2012) AGN selection region in color-color space IRAC photometry, we determined from our sample that the LAEs at z~0.3 have an AGN fraction of ~24%. The total bolometric LIR for the sample was found using χ2 fitting along with template SEDs, and we found that the galaxies ranged from 108.05LSun to 1011.57LSun, with a median LIR value of 1010.39LSun. LIR and LLyα for our sample do not appear to be correlated, unlike the high-LIIRLyα-emitting objects examined by Colbert et al. (2006) and Nilsson & Mller (2009), which may mean that their samples of LAEs, with LIR/LSun > 1012, are qualitatively different from our own. The SFR values for the sample have a median value of 5.63 MSun yr-1, in agreement with the value of 6MSun yr-1 found in Cowie et al. (2011). Also, we find that most of the sample has a contribution of SFRIR to SFRTotal that is greater than 60%, indicating that these LAEs have a significant amount of dust extinction, and SFRUV alone is also not a good indicator for SFRTotal. From comparing dust extinction to UV continuum slope , it was found that LAEs do not follow the same curves predicted for SF or SB galaxies, indicating that LAEs at low redshift may experience more variation in their native UV spectra, making it impossible to recover their dust attenuation from their UV slope. This would suggest that LAEs may consist of stellar populations of varying ages, leading to more intrinsic variation in their UV slope.Item Investigating the role of the baryon-dark matter transition in galaxy-scale gravitational lenses with ramifications for galaxy structure and cosmology(2020-06) Gomer, MatthewGravitational lensing is a powerful tool to study the structure of galaxies and cosmology, however the constraints from lensing are subject to degeneracies and cannot provide a unique solution. The lens model informs the ultimate choice of solution, and so it is critical that the lens model accurately reflects galaxy structure. Probably the most commonly-used lens model is a power-law ellipse+shear model. We show that this ellipse+shear model is unable to statistically explain the angular distribution of quad image systems. Considering additional complications to the azimuthal structure of a lens, we show that the observed angular distribution cannot be explained by $\Lambda$CDM substructure, but can be explained by a transition region between two mass components representing baryons and dark matter. The combination of offset centers, misaligned position angles, and Fourier components introduces enough asymmetry in a lens to explain the observed population. Because lensing is used to measure $H_0$, it is important to know the potential biasing effects that simplifying assumptions implicit to modeling can create. We therefore study the effect of the radial profile assumption (that the profile is a power law) and the azimuthal shape assumption (that the lens is ellipse+shear) on the recovery of $H_0$. To do so, we create mock lenses which are more complicated than the model, then fit their images with the model. For the radial structure, we find that when two-component lenses are fit with a power law, they return biased values of $H_0$. Worse, the bias does not match the analytical prediction, making it more difficult to account for. Stellar kinematic information, which in practice is used to inform the solution by providing a measure of mass, does not correctly inform the unbiased value of $H_0$ because the power-law model is inaccurate. For the azimuthal structure, different types of shape complications have different effects, but the recovered value of $H_0$ can be biased substantially, especially if the profiles are offset from one another. Finally, we discover that the image distance ratios of observed quads are statistically different from mock quads, indicating additional complications to the structure of lenses which have not yet been accounted for.Item The massive Star Population in M101(2014-08) Grammer, SkylerAn increasing number of non-terminal giant eruptions are being observed by modern supernova and transient surveys. Very little is known about the origin of these giant eruptions and their progenitors which are presumably very-massive, evolved stars such as luminous blue variables, hypergiants, and supergiants. Motivated by the small number of progenitors positively associated with these giant eruptions, we have begun a survey of the luminous and evolved massive star populations in several nearby galaxies. We aim to identify the likely progenitors of the giant eruptions, study the spatial variations in the stellar populations, and examine the relationship between massive star populations and their environment.The work presented here is focused on stellar populations in the relatively nearby, giant, spiral galaxy M101 from sixteen archival BVI HST/ACS images. We create a catalog of stars in the direction to M101 with photometric errors < 10% for V < 24.5 and 50% completeness down to V = 26.5 even in regions of high stellar crowding. Using color and magnitude criteria we have identified candidate luminous OB type stars and blue supergiants, yellow supergiants, and red supergiants for future observation. We examine their spatial distributions across the face of M101 and find that the ratio of blue to red supergiants decreases by two orders of magnitude over the radial extent.From our catalog, we derive the the star formation history (SFH) for the stellar populations in five 2' wide annuli by fitting the color-magnitude diagrams. Binning the SFH into time frames corresponding to populations traced by Halpha, far ultraviolet (FUV), and near ultraviolet (NUV) emission, we show that the fraction of stellar populations young enough to contribute in Halpha is 15% " 35% in the inner regions, compared to less than 5% in the outer regions. This provides a sufficient explanation for the lack of Halpha emission at large radii. We also model the blue to red supergiant ratio in our five annuli, examine the effects that a metallicity gradient and variable SFH have on the predicted ratios, and compare to the observed values. We find that the radial behavior of our modeled blue to red supergiant ratios is highly sensitive to both spatial variations in the SFH and metallicity. Incorporating the derived SFH into the modeled ratios, we are able to reproduce the observed values at large radii (low metallicity), but at small radii (high metallicity) the modeled and observed ratios are discrepant. Though photometry has proven to be a powerful tool to identify candidate evolved massive stars and their effects on their host galaxy, spectroscopy is necessary to study the physical properties of individual stars. We observed moderate-resolution optical spectra for 56 of the brightest stars in the direction to M101 using the Multiple Mirror Telescope. We also created light curves for each target using multi-epoch UBV R images from the Large Binocular Telescope. We separate the spectroscopially confirmed members of M101 into four groups: hot supergiants, intermediate supergiants, emission-line stars, and LBVs. Several stars in each group are discussed in detail. Of the spectroscopically confirmed members, we find that eight meet our criterion for variability. We present light curves for the known LBV candidates, V2, V4, and V9, and introduce a new candidate: 9492 14 11998. Additionally, we identify 20 new variables in M101. Lacking spectra, we separated the variables, by their photometric properties, into three groups: hot, intermediate, and cool. We find two hot stars with V -band variability of ±1 magnitude; we flag these stars as LBV candidates. Of the intermediate and cool variables, we identify several stars with low- to moderate-amplitude variability (0.1"0.5 magnitudes).Item Model-free analysis of quadruply imaged gravitationally lensed systems.(2015-04) Woldesenbet, AddishiwotGravitational lensing has proven to be a very valuable tool as a probe to better understand our universe. Parametric modeling of one multiple image gravitational lens system at a time is a common practice in the field of lensing. Instead of individual lens modeling, an alternative approach is to use symmetries in different spaces to make conclusions about families of lenses. The latter method is the focus of this thesis. Three types of lenses are defined based on whether they do or do not obey two-fold and double mirror symmetries. The analysis concentrates on quadruply imaged systems, or quads, and uses only the relative polar angles of quads around the center of the lens. The analysis is statistical in nature, and model-free because its conclusions relate to whole classes of models, instead of specific models. The work done here is twofold. Firstly, exploratory analysis is done to check for possible existence of degeneracies. Type I lenses which obey both symmetries mentioned above are found to form a nearly invariant surface in the 3D space of relative image angles. In the same space, lenses that break the double mirror symmetry, grouped as Type II, form two distinct surfaces. In addition, degeneracy in this class of lenses is discovered. A preliminary study of the last group of lenses, Type III, that break both symmetries, is done. Secondly, quad distributions in the 3D space from each of the three families were compared to observed galaxy-lens quads. Three quarters of observed quads were inconsistent with the distribution of quads of Type I lenses. Type II lenses reproduce most individual lens systems but fail to reproduce the population properties of observed quads. Preliminary exploration of Type III lenses shows a very promising agreement with observations. Examples of Type IIIs are lenses with substructure (with clump masses larger than those responsible for flux ratio anomalies in quads), and lenses with luminous or dark nearby perturber galaxies, or line of sight structures.