Bamford, Steven Peter
The evolution of massive disc galaxies with environment and redshift.
PhD thesis, University of Nottingham.
This thesis examines the evolution of massive disc galaxies as a function of cosmic time and environment by analysing a sample of luminous disc galaxies, located in the field and rich clusters at intermediate redshifts. The data utilised for this study are two-dimensional optical spectra obtained with the FORS2 instrument on the VLT, along with imaging from a variety of sources. From these we measure absolute rest-frame B-band magnitudes, stellar scalelengths (r_d,phot), rotation velocities (V_rot), emission-line scalelengths (r_d,spec) and emission-line equivalent widths, resulting in estimates of gas-phase oxygen abundance, current star formation rate (SFR) and dust extinction.
We investigate evolution of the field Tully-Fisher relation (TFR) using a sample of 89 galaxies covering the redshift range 0.1-1. We find evidence that these luminous spiral galaxies are increasingly offset from the local TFR with redshift, reaching a brightening of -1.0+-0.5 mag, at a given V_rot, by z ~ 1. We argue that, due to likely selection effects, this observed evolution represents an upper limit.
Previous studies have used an observed correlation between TFR residuals and V_rot to argue that low mass galaxies have evolved significantly more than those with higher mass. However, we demonstrate that such a correlation does not necessarily indicate a physical difference in the evolution of galaxies with different V_rot.
Interpreting the luminosity evolution derived from the TFR as due to evolution in the SFR of these luminous spiral galaxies, we find that SFR(z) is proportional to (1+z)^(1.7+-1.1). Although the uncertainties are large, this evolution, which is probably an upper limit, appears to be slower than that derived for the overall field galaxy population. This suggests that the rapid evolution of the SFR density of the universe observed since z ~ 1 is not in general driven by the evolution of the SFR in individual bright spiral galaxies.
The measured emission-line equivalent widths, diagnostic ratios, oxygen abundances, star formation rates and dust extinctions for a sample of 40 luminous, massive (V_rot > 80 km/s), star-forming, field disc galaxies, with redshifts z=0.2-0.8, cover similar ranges to those observed across a large sample of local galaxies. However, at a given galaxy luminosity, many of our galaxies have oxygen abundances significantly lower than local galaxies with similar luminosities. The galaxies in this luminous, metal-poor subsample exhibit physical conditions similar to those of local faint and metal-poor star-forming galaxies. Lower-metallicity systems are ~2 mag brighter, and have star formation rates an order of magnitude higher, compared with similar metallicity galaxies today. Oxygen abundances are not found to correlate with the emission scale length size of galaxies, and the rotation velocity--metallicity relation, while perhaps present, is unclear. This suggests that massive field galaxies at intermediate redshifts are diverse in terms of their interstellar gas properties and stellar content.
To examine variations in the TFR with environment, matched samples of 58 field and 22 cluster galaxies are constructed, selected in a homogeneous manner and covering similar ranges in redshift (0.25 < z < 1.0) and luminosity (M_B < -19.5 mag). The distributions of M_B, V_rot and scalelength are found to be comparable for the two samples. However, we find that the TFR of the cluster galaxies is systematically offset with respect to the field sample by -0.7+-0.2 mag. This offset is significant at 3-sigma and persists when we account for an evolution of the field TFR with redshift. Tests are performed to investigate potential differences in the observed emission lines and derived parameters of the cluster and field samples. However, no such differences which could account for the offset are found.
Offsets are also found between cluster and field samples in the relations of M_B and V_rot versus r_d,phot and r_d,spec, although these are difficult to interpret. Our cluster galaxies are found to have ratios of emission-line to stellar scalelengths (r_d,spec / r_d,phot) significantly lower than for our field galaxies: 0.88 +- 0.08 versus 1.15 +- 0.05, respectively. This indicates that star formation is more centrally concentrated in the cluster galaxies. The comparison of interstellar gas properties between 16 bright, star-forming, cluster disc galaxies at intermediate redshifts (0.3 < z < 0.6, <z> = 0.42) and their counterparts in the coeval field, reveals that both samples are generally similar. However, on average the cluster galaxies have emission-line equivalent widths that are significantly lower than for the field galaxies. A contrasting fraction of the distant cluster galaxies, though, appears to have much higher emission-line equivalent widths, comparable to the highest seen in the field. This tentatively implies a bimodality in the star formation rates per unit luminosity of distant cluster galaxies, which is not present for our field sample. However we find no substantial difference in the long term star formation histories of these cluster and field galaxies, as indicated by their gas-phase metallicities.
The most likely explanation for the results of our cluster versus field comparison is that spiral galaxies entering intermediate-redshift clusters often experience a short-lived enhancement of their star formation rate, followed by a decline, which we would expect to be accompanied by a transformation to S0 morphology.
Thesis (University of Nottingham only)
||Q Science > QB Astronomy
||UK Campuses > Faculty of Science > School of Physics and Astronomy
||05 Sep 2006
||15 Sep 2016 02:18
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