Shedding light on the lives of star forming galaxies

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 univariate and bivariate luminosity functions, and star formation rate density, while the second part aims to explore the universality of the stellar initial mass function. The first part of this thesis is motivated by the published measurements of the low–z Hα luminosity functions, Φ, 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 < SFRHα (M yr−1) < 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 Φ 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 Hα detection leads to an incompleteness due to missing bright Hα sources with faint r–band magnitudes. To correct for this incompleteness, we use the lowest redshift LHα–Mr and LHα–M distributions as a reference to model the higher–z bivariate LFs, thereby approximating the contribution from the missing optically faint star forming galaxies to the local star formation rate and stellar mass densities. Furthermore, we present the univariate Mr andM LFs of Hα SF galaxies obtained by integrating the bivariate LFs along the LHα axis. As our sample is selected on the basis of detected Hα emission, a direct tracer of on–going star formation in galaxies, this sample represents a true star forming galaxy sample, and is drawn from both photometrically classified blue and red sub– populations. We find that not all members of the GAMA blue population, conventionally called star formers, have detected Hα emission, and approximately 20–30% at all stellar masses of the GAMA red population, conventionally called passive galaxies, are in fact star forming. This would mean blue galaxies, although they may not have current star formation, have undergone a recent burst of star formation, while the red galaxies may be dominated by old stars but still have some on–going star formation. The second part of this thesis is motivated by the recent extragalactic studies indicating variations in the stellar initial mass function (IMF). The stellar 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.