Shedding light on the lives of star forming galaxies

Abstract

This thesis examines two separate aspects related to the evolution of star forming galaxies using spectroscopic and photometric measurements taken from large area sky surveys. The first part of the thesis focuses on the local evolution of star formation rate density, and the second part explores the universality of the stellar initial mass function (IMF). The first part of this thesis is motivated by the published measurements of the low z H luminosity (LH) functions,, that show a large dispersion in the local number density of sources (0.5 1 Mpc-3dex-1), and correspondingly in the SFR density. The possible causes for these discrepancies include limited volume sampling, biases arising from survey sample selection, and different methods of correcting for dust obscuration and AGN contamination. The Galaxy And Mass Assembly (GAMA) survey and Sloan Digital Sky Survey (SDSS) provide deep spectroscopic observations over a wide sky area enabling detection of a large sample of star-forming galaxies spanning 0.001  SFR H (solar masses per year) 100 with which to robustly measure the evolution of the SFR density in the lowz universe. The large number of high SFR galaxies present in our sample allows an improved measurement of the bright end of the luminosity function, indicating that the decrease in  at bright luminosities is best described by a Saunders functional form rather than the traditional Schechter function. This result is consistent with other published luminosity functions in the FIR and radio. For GAMA and SDSS we find the rband apparent magnitude limit, combined with the subsequent requirement for H detection leads to an incompleteness due to missing bright H sources with faint rband magnitudes. To correct for this incompleteness, we use the lowest redshift LHabsolute (rband) magnitude and LHstellar mass distributions as a reference to model the higherz bivariate luminosity functions, thereby approximating the contribution from the missing optically faint star forming galaxies to the local star formation rate and stellar mass densities. The second part of this thesis is motivated by the recent extragalactic studies indicating variations in the stellar IMF. The IMF describes the distribution in stellar masses produced from a burst of star formation. For more than fifty years, the implicit assumption underpinning most areas of research involving the IMF has been that it is universal, regardless of time and environment. We measure the highmass IMF slope for a sample of lowtomoderate redshift galaxies from the GAMA survey. The large range in luminosities and galaxy masses of the sample permits the exploration of underlying IMF dependencies. A strong IMFstar formation rate dependency is discovered, which shows that highly star forming galaxies form proportionally more massive stars (they have IMFs with flatter powerlaw slopes) than galaxies with low star formation rates. This has a significant impact on a wide variety of galaxy evolution studies, all of which rely on assumptions about the slope of the IMF. Our result is supported by, and provides an explanation for, the results of numerous recent explorations suggesting a variation of or evolution in the IMF.