Ultraluminous Infrared Galaxies: Power Sources and Ages Along the Merger Sequence

Citation

Murphy, Thomas Williams, Jr. (2000) Ultraluminous Infrared Galaxies: Power Sources and Ages Along the Merger Sequence. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JR80-9T40. https://resolver.caltech.edu/CaltechETD:etd-08292008-130426

Abstract

Ultraluminous infrared galaxies (ULIRGs) are the most luminous galaxies in the local universe, with power outputs above 10¹² times that of our sun. Almost all ULIRGs are found to be associated with collisions between large, gas-rich galaxies. With most of the energy output emerging at far-infrared wavelengths, these galaxies are understood to be extremely dusty. As such, the nature of the power source within the dust shroud is not clearly understood. The two most likely explanations involve either a massive burst of star formation or the presence of an active galactic nucleus (AGN) in the form of a dust-enshrouded quasar.

This work presents two separate approaches to investigating the nature of ultraluminous infrared galaxies. The first is a spectrosciopic survey of 33 ULIRGs in the near-infrared band around a wavelength of 2 microns, where dust extinction is much less severe than in visible light. The aim of this study is to search for evidence of buried AGN, as indicated by velocity-broadened hydrogen recombination lines as well as the presence of the high excitation [Si VI] emission line. This survey finds that signs of AGN are rare in ULIRGs, with the most natural conclusion being that the majority of ULIRGs are powered by starburst phenomena.

The second approach presented involves the design, construction, and use of a novel instrument built specifically to study the spatial and kinematical properties of gas within the morphologically complex ULIRGs. A single long slit is generally not capable of capturing the rich phenomenology found in the complex environments of ULIRGs. Data from the Palomar Integral Field Spectrograph reveals that several ULIRGs are found to exist at times very early in the galaxy encounter process. The possibility that ultraluminous activity may develop so early in the merger history suggests a non-trivial luminosity evolution among galaxy mergers, wherein the less powerful class of luminous infrared galaxies may represent a different phase of the same overall phenomenon.

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