published_or_final_version / Physics / Master / Master of Philosophy

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We have examined the late-type stellar content of 3 disk fields of NGC 2403 using VRI CCD photometry. The AGB luminosity function has been constructed and differs strongly with that of the LMC. Notably, even fewer bright AGB stars are seen in NGC 2403 than in the LMC. The shape of the AGB luminosity function for the innermost field, however, indicates that a recent burst of star formation may have occurred. Red and blue supergiants and HII regions are also identified in the field to support this. The relative distance modulus between NGC 2403 and the LMC is derived to be 8.90 and the true distance modulus to NGC 2403 is (m - M)₀ = 27.40 ± 0.24. The carbon to M star number for the innermost field is 8/17 which corresponds to [Fe/H] = -0.35. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

We constrain the recent star formation histories of the host galaxies of eight optical/UV-detected tidal disruption events (TDEs). Six hosts had quick starbursts of <200 Myr duration that ended 10-1000 Myr ago, indicating that TDEs arise at different times in their hosts' post-starburst evolution. If the disrupted star formed in the burst or before, the post-burst age constrains its mass, generally excluding O, most B, and highly massive A stars. If the starburst arose from a galaxy merger, the time since the starburst began limits the coalescence timescale and thus the merger mass ratio to more equal than 12: 1 in most hosts. This uncommon ratio, if also that of the central supermassive black hole (SMBH) binary, disfavors the scenario in which the TDE rate is boosted by the binary but is insensitive to its mass ratio. The stellar mass fraction created in the burst is 0.5%-10% for most hosts, not enough to explain the observed 30-200x. boost in TDE rates, suggesting that the host's core stellar concentration is more important. TDE hosts have stellar masses 10(9.4)-10(10.3) M circle dot,consistent with the Sloan Digital Sky Survey volume-corrected, quiescent Balmer-strong comparison sample and implying SMBH masses of 10(5.5)-10(7.5) M circle dot, Subtracting the host absorption line spectrum, we uncover emission lines; at least five hosts have ionization sources inconsistent with star formation that instead may be related to circumnuclear gas, merger shocks, or post-AGB stars.

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Our aim in this thesis is to use the clustering of QSOs to investigate large- scale structure and cosmology. We are particularly concerned with estimating the cosmological parameters which govern the evolution of structure in the Universe. We first investigate how QSOs trace the distribution of 'normal' galaxies by measuring the correlation between a sample of ~ 150 QSOs and faint, b(_j) < 23 galaxies. At z < 1.5 we find that the cross-correlation amplitude is marginally negative. This low signal clearly rules out models in which QSOs inhabit rich environments. The environments of QSOs are more similar to those of average galaxies. The slight negative correlation can be explained by gravitational lensing, but this has no effect on our conclusions concerning QSO environments. We determine the clustering properties of a combined sample of > 1500 QSOs including the LBQS and Durham/AAT QSO surveys. This data set has a clustering amplitude Ɛ(10 h(^-1) Mpc) = 0.83 ± 0.29 for Ωₒ = 1 at z = 1.27. On ~ 100 – 1000 h(^-1) Mpc scales the limit on detected signals in Ɛ is ±0.025. A model of clustering evolution which includes the effect of bias was used to compare QSO clustering to the clustering of low redshift galaxies and Seyfert galaxies. If Seyferts and QSOs are similarly clustered, then the data prefer a low Ωₒ or high bias for QSOs and galaxies. In contrast, comparisons to the CMB measurements of COBE assuming a CDM-type power spectrum suggest low bias. This might be taken as evidence for low do, but the data is still consistent with Ωₒ = 1 and b(_gp) ~ b(gp) ~ 2..We consider the possibility that nearby galaxy clusters can gravitationally lense background QSOs. We apply the lensing hypothesis to the result of Boyle et al., (1988) and find that cluster masses required are too large. A small dust component could retrieve the lensing model and allow more reasonable mass estimates for clusters from this method. The requirement for a new, deep, wide-field, QSO survey is clear. We discuss the construction of the candidate catalogue for the 2dF QSO Redshift Survey, which will contain ~ 25000 QSOs. We calibrate the photographic plates used for the candidate catalogue and assess the sources of errors and incompleteness. From preliminary spectroscopic observations we conclude that the completeness of the 2dF catalogue is ~ 71.1 ± 7.1%, compared with an estimated completeness of ~ 80%. We propose to substantially increase the catalogue completeness (to ~ 90%), by the introduction of UKST r plates into our candidate catalogue.

Thesis (Ph. D.)--University of Hawaii at Manoa, 2004. / Includes bibliographical references (leaves 187). / Also available by subscription via World Wide Web / xxiii, 187 leaves, bound ill. 29 cm

We present a sub-kiloparsec localization of the sites of supermassive black hole (SMBH) growth in three active galactic nuclei (AGNs) at z similar to 3 in relation to the regions of intense star formation in their hosts. These AGNs are selected from Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) observations in the Hubble Ultra-Deep Field and COSMOS, with the centimetric radio emission tracing both star formation and AGN, and the sub/millimeter emission by dust tracing nearly pure star formation. We require radio emission to be >= 5 x more luminous than the level associated with the sub/millimeter star formation to ensure that the radio emission is AGN-dominated, thereby allowing localization of the AGN and star formation independently. In all three galaxies, the AGNs are located within the compact regions of gas-rich, heavily obscured, intense nuclear star formation, with R-e = 0.4-1.1 kpc and average star formation rates of similar or equal to 100-1200 M(circle dot)yr(-1). If the current episode of star formation continues at such a rate over the stellar mass doubling time of their hosts, similar or equal to 0.2 Gyr, the newly formed stellar mass will be of the order of 10(11)M(circle dot). within the central kiloparsec region, concurrently and cospatially with significant growth of the SMBH. This is consistent with a picture of in situ galactic bulge and SMBH formation. This work demonstrates the unique complementarity of VLA and ALMA observations to unambiguously pinpoint the locations of AGNs and star formation down to similar or equal to 30 mas, corresponding to; 230 pc at z = 3.

The physical mechanisms that quench star formation, turning blue star-forming galaxies into red quiescent galaxies, remain unclear. In this Letter, we investigate the role of gas supply in suppressing star formation by studying the molecular gas content of post-starburst galaxies. Leveraging the wide area of the Sloan Digital Sky Survey, we identify a sample of massive intermediate-redshift galaxies that have just ended their primary epoch of star formation. We present Atacama Large Millimeter/submillimeter Array CO(2-1) observations of two of these post-starburst galaxies at z similar to 0.7 with M-* similar to 2 10(11) M-circle dot. Their molecular gas reservoirs of (6.4 +/- 0.8) x 10(9) M-circle dot and (34.0 +/- 1.6) x 10(9) M-circle dot are an order of magnitude larger than comparable-mass galaxies in the local universe. Our observations suggest that quenching does not require the total removal or depletion of molecular gas, as many quenching models suggest. However, further observations are required both to determine if these apparently quiescent objects host highly obscured star formation and to investigate the intrinsic variation in the molecular gas properties of post-starburst galaxies.