Infrared Insights on the Nature and Evolution of Star-Forming Galaxies

Abstract

The peak of the star formation rate (SFR) of the Universe is widely accepted to be at 1 < z < 3, after which the SFR declined by more than an order of magnitude to the present level. The mechanisms resulting in the decline and the nature of individual star-forming galaxies at the peak of galaxy evolution, however, are poorly understood. This thesis summarizes an effort to understand both the statistical properties of star-forming galaxies and the physical conditions in individual galaxies at 0 < z < 3. I have studied the star formation (SF) sizes of local and high-z ultraluminous infrared galaxies (ULIRGs) using Pa-alpha, 24 micron and radio continuum observations and discovered that high-z ULIRGs have extended SF regions over 3-10 kpc, similar to local lower LIR SF galaxies, but with a scaled-up star formation rate surface density, ∑(SFR). Local ULIRGs, in contrast, are compact and invariably merger-triggered starbursts. A major implication to galaxy evolution is that there is a route besides major mergers to trigger very high levels of SF activity at z ~ 2, a conclusion further supported by our morphological study. I also find star formation rate surface density to be a good indicator of the IR galaxy spectral energy distribution universally and use this fact to develop a new SFR estimator using single-band 24 micron observations. The resulting indicator predicts IR luminosity and SFR within 0.15 dex of the values measured with far-IR photometry. This affords the deepest unobscured probe of SF at 0 < z < 3. According to my separate study, the spread of extinction values of SF galaxies is larger than previously known from optical observations and also indicates a large variety of dust distribution scenarios, from a uniform mixture that resembles the extinction screen assumption to inhomogeneous mixtures, which could undermine the assumptions commonly used to correct for extinction at high-z and necessitates the use of unobscured SF tracers. Lastly, I present the luminosity functions of galaxies and their evolution measured from IR observations out to z = 1.2.