Links between Galaxy Structure and Stellar Populations

Abstract

In this thesis we analyse the observational relations between galaxy structure and global stellar population properties to determine the dependencies between a galaxy's star formation and mass assembly histories. Stellar population parameters correlate with a range of galaxy properties, but it is unclear which relations are causal and which are the result of another underlying trend. The well-established correlations between mass and other galaxy properties are often considered evidence for mass driving a galaxy's evolution. However, we find that, at fixed mass, stellar population properties show significant dependence on size, indicating that the size of a galaxy is also an important property tracing, and possibly influencing, its evolution. The focus of the thesis is to quantitatively compare trends between various stellar population properties and key galaxy structural parameters, in particular the galaxy's mass (M), gravitational potential (M/R) and surface density (M/R^2), to determine which relations are intrinsically tighter and are therefore more likely to reflect a causal relation. We start by analysing a sample of 625 early-type galaxies (ETGs) from the SAMI survey. We show that, compared to correlations with mass, metallicity [Z/H] correlates strongly with M/R, while age correlates best with M/R^2. For [alpha/Fe], a proxy for star formation duration, we find comparable results for M/R and M/R^2, with both being significantly stronger correlations than the [alpha/Fe]--M relation. First, we concur with previous studies in finding that gravitational potential is the primary regulator of global metallicity by determining the escape velocity required for metal-rich supernova ejecta to escape the system and avoid being recycled into later stellar generations. Second, to explain the age and [alpha/Fe] correlations with M/R^2, we propose two possible mechanisms: (a)~the correlations arise as results of compactness-driven quenching mechanisms, and/or (b)~as fossil records of the \Sigma_SFR ~ \Sigma_gas relation in their disk-dominated progenitors. To test these conclusions, we study the ages and metallicities of 2085 star-forming galaxies (SFGs) from the SDSS Legacy survey. By investigating whether these relations are also present in earlier phases of galaxy evolution, we narrow the range of possible physical mechanisms responsible for producing them. As with the trends found in ETGs, we find that in SFGs age correlates best with M/R^2 and [Z/H] correlates best with M/R. Showing that the age--M/R^2 relation exists in star-forming galaxies demonstrates it must originate before quenching. We conclude that the age--M/R^2 relation is consistent with compact galaxies forming earlier, perhaps driven by higher gas fractions in the early Universe causing older galaxies to form more compactly during their in-situ formation phase. Lastly we investigate the change in age and metallicity relations for quiescent galaxies from intermediate redshift (0.60<z<0.76), using the LEGA-C survey, to z<0.11 using the SAMI survey. We find that, as for their low-redshift counterparts, the metallicity of quiescent galaxies at 0.60<z<0.76 correlates with M/R. This supports the hypothesis that the relation arises because the gravitational potential regulates the gas escape velocity. On the other hand, we find no correlation between age and M/R^2 in the LEGA-C sample, despite there being a strong relation at low redshift. We consider this change in the age--M/R^2 relation in the context of the redshift evolution of the star-forming and quiescent populations in the mass--size plane and find our results can be explained as a consequence of galaxies forming more compactly at higher redshift, and remaining compact throughout their evolution. The age--M/R^2 relation at z=0 results from the build-up of the quiescent and star-forming populations from galaxies that formed over a range of redshifts and therefore with a range of surface densities. Show more