Emission-line properties of active galactic nuclei and an experiment in integrated, guided-inquiry science classes and implications for teaching astronomy


Emission-line properties of active galactic nuclei and an experiment in integrated, guided-inquiry science classes and implications for teaching astronomy

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Title: Emission-line properties of active galactic nuclei and an experiment in integrated, guided-inquiry science classes and implications for teaching astronomy
Author: Ludwig, Randi Renae
Abstract: 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.
Department: Astronomy
Subject: Active galaxies Active galactic nuclei Emission lines Science courses Astronomy Teaching Student understanding Learning
URI: http://hdl.handle.net/2152/ETD-UT-2012-08-6269
Date: 2012-08

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