Details

Autor(en) / Beteiligte

• Wu, Ronin

Titel

Tracing Star Formation in the Mid-infrared

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2012

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• One of the most important missions of the 20th century in astrophysics is the launch of the Spitzer Space Telescope in August 2003. Since then, with its unprecedented sensitivity and spatial resolution in the infrared, this earth--trailing satellite has significantly improved our understanding of star--formation history and the composition of the interstellar medium from an extragalactic perspective. In this thesis, I present the statistical studies of several star--formation tracers and properties of star--forming galaxies using the photometric and spectroscopic data taken by the Spitzer Space Telescope. The first approach I take in understanding the mid-infrared star–forming tracers is studying the aromatic features at 7.7μm. Aromatic features are the dominant emission lines in the mid-infrared spectra of star–forming galaxies, but these features are much weaker in galaxies of low–luminosity. I combine the infrared and the optical imaging data to demonstrate this trend. Comparison with optical spectroscopic data shows that the strength of the 7.7μm aromatic feature is closely related to the stellar mass of galaxies. Our analysis shows that both oxygen abundance and radiation hardness affect the strength of this feature. However, the generally low oxygen abundance in low–luminosity galaxy interstellar environments, which prevents the synthesis of aromatic molecules, appears to have the stronger effect. The second approach I take is the analysis of the Spitzer SDSS Statistical Spectroscopic Survey (S5), a multi-wavelength study of ∼300 homogeneously selected star–forming galaxies at redshifts 0.05 < z < 0.1. The S5 galaxies span the color range of the Sloan Digital Sky Survey and span two orders of magnitudes in stellar mass. The key goal of the survey is to provide a deeper understanding of the properties of warm interstellar medium by comparing the optical and mid-infrared spectra of galaxies that are structurally similar. The S5 survey contains a significant fraction of galaxies that show composite properties of star–forming and AGN. I explain the data analysis for this survey and present comparisons of optical and mid-infrared AGN/star–forming diagnostics and discuss possible heating mechanisms for aromatic features and molecular hydrogen in galaxies of different types. Molecular hydrogen is the simplest and most abundant molecule in the Universe. Stars are formed from molecular gas through gravitational condensation. The abundance of molecular hydrogen is a direct indicator for the star–forming ability of galaxies. In the third and last part of this thesis, I present the first mass function of molecular hydrogen directly measured using the molecule's rotational lines in the mid-infrared for galaxies in the homogeneously selected, S5 sample.