Date of Award
Doctor of Philosophy (PhD)
Astrophysical & Planetary Sciences
John T. Stocke
The purpose of this thesis is to conduct a survey to discover new H I and molecular absorption systems at radio frequencies at significant cosmological distances. While such systems offer significant advantages for studying molecular gas at high redshift, only 5 molecular absorbers are known, all at z < 1. Various attempts to find more have failed. Finding more such systems will help us study the physical properties of the gas out of which stars form in the early universe. A particularly important use of molecular absorbers is to test whether there is any variation in fundamental physical constants over cosmic time. Radio frequencies have significant advantage over optical frequencies for this purpose. Because the host galaxies of radio-loud AGN are almost exclusively elliptical galaxies, which are low in neutral gas, we have selected a rare sample of 80 radio-loud AGN that have non-elliptical morphologies in the optical. These could be late-type galaxies, interacting systems or intervening galaxies well foreground to the radio source, all of which potentially have plenty of cold gas. We performed NIR photometry and confirmed that most of the objects are neither unobscured quasars nor elliptical galaxies. In fact, many have late-type galaxy SEDs in the optical and near-IR (NIR) requiring that the AGN are completely "buried" even in the NIR. We have also obtained sub-arc second (VLA and VLBA) radio maps that show that most of the radio sources are compact enough for high optical depth absorption lines to be present. This sample includes a substantial number of Compact Symmetric Objects (CSOs) and Medium-sized Symmetric Objects (MSOs), which are believed to be the "infant" stage of classic double-lobe radio galaxies. Up to now, we have found 5 H I absorption systems at z = 0.2--0.9 but NO molecular absorbers. One of the detections is in an intervening system, and others are all associated absorption systems (z_abs=z_agn). We find a high detection rate among compact sources, especially CSOs, and radio galaxies (instead of radio quasars). We demonstrate that the most promising candidates to find high-z HI absorbers are optical-faint/radio-loud AGN, for which it is not easy to obtain redshifts. Because "blind" searches are nearly impossible considering the severe radio frequency interference (RFI) at sub-GHz frequencies, the lack of accurate spectroscopic redshifts for these faint sources has proven to be a significant obstacle for this research.
Yan, Ting, "A Search for Redshifted Absorption Lines at Radio Frequencies" (2013). Astrophysical & Planetary Sciences Graduate Theses & Dissertations. 35.