Browsing by Subject "Active galactic nuclei"

Now showing 1 - 4 of 4

Bridging star-forming galaxy and AGN ultraviolet luminosity functions at z = 4 with the SHELA wide-field survey
(2018-12-06) Stevans, Matthew Louis, Jr.; Finkelstein, Steven L.
This thesis presents a joint analysis of the rest-frame ultraviolet (UV) luminosity functions of continuum-selected star-forming galaxies and galaxies dominated by active galactic nuclei (AGNs) at z ~ 4. These 3,740 z ~ 4 galaxies are selected from broad-band imaging in nine photometric bands over 18 deg² in the Spitzer/HETDEX Exploratory Large Area Survey (SHELA) field. The large area and moderate depth of our survey provide a unique view of the intersection between the bright end of the galaxy UV luminosity function (M [subscript AB] < -22) and the faint end of the AGN UV luminosity function. We do not separate AGN-dominated galaxies from star-formation-dominated galaxies, but rather fit both luminosity functions simultaneously. These functions are best fit with a double power-law (DPL) for both the galaxy and AGN components, where the galaxy bright-end slope has a power-law index of -3:80 ± 0.10, and the corresponding AGN faint-end slope is α [subscript AGN] = -1.49 +0.30/-0.21. We cannot rule out a Schechter-like exponential decline for the galaxy UV luminosity function, and in this scenario the AGN luminosity function has a steeper faint-end slope of -2.08 +0.18/-0.11. Comparison of our galaxy luminosity function results with a representative cosmological model of galaxy formation suggests that the molecular gas depletion time must be shorter, implying that star formation is more efficient in bright galaxies at z = 4 than at the present day. If the galaxy luminosity function does indeed have a power-law shape at the bright end, the implied ionizing emissivity from AGNs is not inconsistent with previous observations. However, if the underlying galaxy distribution is Schechter, it implies a significantly higher ionizing emissivity from AGNs at this epoch.
Computational and astrophysical studies of black hole spacetimes
(2004) Bonning, Erin Wells; Matzner, Richard A. (Richard Alfred), 1942-
This dissertation addresses three problems of interest concerning astrophysical black holes, namely the numerical solution of Einstein’s equations for a spacetime containing two orbiting and coalescing black holes, the simulation of a light curve from an accretion disk near the innermost orbit around a spinning black hole, and determining relations between central black hole mass and host galaxy properties in active galactic nuclei. I first address the problem of setting the initial conditions for the Cauchy formulation of general relativity. I present the solution of the constraint equations via a conformal decomposition and discuss the construction of the background fields as superposed Kerr-Schild black holes. The constraint equations are solved for two black holes with arbitrary linear and angular momenta. The binding energy and spin-spin coupling of the two holes are computed in the initial data slice and analyzed. I discuss the extent to which the superposed Kerr-Schild initial data limits extraneous radiation and estimate the accuracy of determinations of the innermost stable circular orbit through sequences of initial data. The second topic concerns the time variability of isotropically radiating material orbiting in an idealized accretion disk around a spinning black hole. I solve the geodesic equations for photon propagation from the surface of the disk to an observer for different orbital parameters. The general relativistic effects upon the signal received are calculated, including the energy shift, relativistic time delay, and gravitational lensing. I produce light curves showing the change in flux over time due to the relativistic effects. Applications of this model to stellar-mass systems as well as super-massive black holes are discussed. Lastly, I discuss the relationship between a galaxy’s central black hole and its evolutionary history. In particular I examine the correlations among host galaxy luminosity, stellar velocity dispersion, and central black hole mass in active galactic nuclei. I derive black hole masses and stellar velocity dispersions from quasar broad and narrow emission lines, respectively. The utility of using the narrow line emitting gas as a surrogate for stellar velocity dispersion is investigated through examining host magnitudes and narrow [O III] line widths for low redshift quasars.
Demographics and evolution of super massive black holes in quasars and galaxies
(2008-08) Salviander, Sarah Triplett, 1971-; Shields, Gregory A.
This dissertation addresses the co-evolutionary relationship between central super-massive black holes and host galaxies. This relationship is suggested by observed correlations between black hole mass (M[subscript BH]) and properties of the host galaxy bulge. We first discuss investigation of the relationship between MBH and host galaxy velocity dispersion, [sigma subscript asterisk], for quasars in the Sloan Digital Sky Survey (SDSS). We derive MBH from the broad emission line width and continuum luminosity, and [sigma subscript asterisk] from the width of narrow forbidden emission lines. For redshifts z < 0.5, our results agree with the locally-observed M[subscript BH]- [sigma subscript asterisk] relationship. For 0.5 < z < 1.2, the M[subscript BH]- [sigma subscript asterisk] relationship appears to evolve with redshift in the sense that bulges are too small for their black holes. Part of this apparent trend can be attributed to observational biases, including a Malmquist bias involving the quasar luminosity. Accounting for these biases, we find approximately a factor of two evolution in the M[subscript BH]- [sigma subscript asterisk] relationship between the present and redshift z [approximately equal] 1. The second topic involves a search for the largest velocity dispersion galaxies in the SDSS. Black holes in quasars can have M[subscript BH]exceeding 5 billion M[mass compared to the sun], implying [sigma subscript asterisk] > 500 km s−1 by the local M[subscript BH]- [sigma subscript asterisk] relationship. We present high signal-to-noise HET observations for eight galaxies at redshift z < 0.3 from the SDSS showing large [sigma subscript asterisk] while appearing to be single galaxies in HST images. The maximum velocity dispersion we find is [sigma subscript asterisk] = 444 km s−1, suggesting either that quasar black hole masses are overestimated or that the black hole - bulge relationship changes at high black hole mass. The third topic involves work contributed to co-authored papers, including: (1) evidence for recoiling black holes in SDSS quasars, (2) the [sigma][O III] - [sigma subscript asterisk] relationship in active galactic nuclei (AGN), and (3) accretion disk temperatures and continuum colors in quasars. Lastly, we discuss research in progress, including: (1) possible physical influences on the width of narrow emission lines of SDSS AGN, including the gravitational effect of the black hole, and (2) a search for binary AGN in the SDSS using double-peaked [O III] emission lines.
Emission-line properties of active galactic nuclei and an experiment in integrated, guided-inquiry science classes and implications for teaching astronomy
(2012-08) Ludwig, Randi Renae; Kopp, Sacha; Robinson, Edward Lewis, 1945-; Hemenway, Mary Kay; Prather, Edward; Shields, Gregory; Wills, Derek
This dissertation examines two broad topics -- emission line properties of active galactic nuclei (AGN) and the effect of hands-on, integrated science courses on student understanding of astronomy. To investigate trends in overall properties of emission lines in AGN, we apply principal component analysis (PCA) to the fluxes in the H [beta] - (O III) region of a sample of 9046 spectroscopically-identified broad-line AGN from the Sloan Digital Sky Survey (SDSS) Data Release 5 with a redshift range of 0.1 < z < 0.56. After performing independent spectral PCA on subsets defined effectively by their (O III) equivalent width (EW), we find only the weakest (O III) objects retain the optical Fe II - (O III) anticorrelation and the correlation of EW[subscript O III] with H [beta] linewidth that have previously been found in high-luminosity AGN. The objects with strongest EW[subscript O III] do not differ from the entire data set significantly in other spectral and derived properties, such as luminosity, redshift, emission line shapes, Eddington ratio, continuum slope, and radio properties. However, our findings are consistent with previous suggestions that (O III) emission is primarily a function of covering factor of the narrow-line region. To investigate the other side of the Fe II - (O III) anticorrelation, we examine the effect of changes in the gas-phase abundance of Fe on observed variation in Fe II. Using AGN spectra from the SDSS in the redshift range of 0.2 < z < 0.35, we measure the Fe/Ne abundance of the narrow-line region (NLR) using the (Fe VII)/(Ne V) line intensity ratio. We find no significant difference in the abundance of Fe relative to Ne in the NLR as a function of Fe II/H [beta]. However, the (N II)/(S II) ratio increases by a factor of 2 with increasing Fe II strength. This indicates a trend in N/S abundance ratio, and by implication in the overall metallicity of the NLR gas, with increasing Fe II strength. We propose that the wide range of Fe II strength in AGN largely results from the selective depletion of Fe into grains in the low ionization portion of the broad-line region. We utilize photoionization models to show that the strength of the optical Fe II lines varies almost linearly with gas-phase Fe abundance, while the ultraviolet Fe II strength varies more weakly, as seen observationally. After examining the emission line properties of large samples of fairly typical AGN, we investigated the newly expanded regime of low-mass AGN (M[subscript BH] [less than or approximately equal to] 10⁶ M[subscript sun]) with respect to their emission line properties at a smaller scale. We utilize the high spectral resolution and small aperture of our Keck data of 27 low-mass AGN, taken with the Echellette Spectrograph and Imager, to isolate the NLRs of these low-mass black holes. Some of these low-luminosity objects plausibly represent examples of the low-metallicity AGN described by Groves et al. (2006), based on their (N II)/H[alpha] ratios and their consistency with the Kewley & Ellison (2008) mass-metallicity relation. We also find that these low-mass AGN have steeper UV continuum slopes than more-massive AGN based on their He II/H[beta] ratio. Overall, NLR emission lines in these low-mass AGN exhibit trends similar to those seen in AGN with higher-mass BHs, such as increasing blueshifts and broadening with increasing ionization potential. Additionally, we see evidence of an intermediate line region whose intensity correlates with L/L[subscript Edd] in these objects, as seen in higher-mass AGN. We highlight the interesting trend that, at least in these low-mass BHs, the (O III) EW is highest in symmetric NLR lines with no blue wing. This trend of increasing (O III) EW with line symmetry could be explained by a high covering factor of lower ionization gas in the NLR. We also investigate effective methods for teaching astronomy and connections between astronomical topics in student learning and understanding. After developing the curriculum for a hands-on, learner-centered astronomy course (Hands-on-Science, hereafter HoS) aimed at pre-service elementary teachers, we measure student performance in HoS compared to traditional, large lecture courses (hereafter Astro101). We utilize distractor-driven multiple choice assessments in order to quantitatively assess student understanding and evaluate the persistence or correction of common misconceptions in astronomy. We find that for the topics included in the HoS curriculum, HoS students have a higher average post-test score, and higher normalized gains, than the Astro101 students. We cannot pinpoint the exact cause of this student achievement because of the multitude of nontraditional practices incorporated into the HoS implementation. Increased time-on-task, a classroom environment structured around student discussion, or focus on conceptual understanding could each be key factors in the high achievement of HoS students. We conclude that the HoS students are better prepared in astronomy for their future careers as elementary school teachers by HoS courses than they would have been in traditional, introductory astronomy courses. When we compare directly between topics covered in both HoS and Astro101, we find that HoS students have normalized gains that are a factor of 2-4 higher than those of Astro101 students. Therefore, we conclude that curricula similar to the HoS approach would benefit Astro101 students as well, particularly for topics which are most impacted by the HoS method, such as Moon phases and seasons. Lastly, a PCA of the changes in HoS student scores reveals that there is very little systematic student variation apart from the trends apparent in the mean changes in the sample. Thus, we do not find groupings of questions that some subsets of students systematically learn more readily than others. Another way to interpret this result is that the HoS curriculum and methodology indiscriminately help all kinds of pre-service elementary teachers, despite presumptive differences in their own learning styles and strengths.