Photometric and Spectroscopic Observations of Star formation in Nearby Galaxies

2017-05-22T05:17:27Z (GMT) by Amelia Fraser-McKelvie
There are populations of galaxies that don’t follow the normal correlations between observable properties such as star formation rate, stellar mass, gas fraction, colour, luminosity, and morphology. In this thesis, we present detailed studies of three populations of such galaxies that may challenge existing theories of galaxy evolution: brightest cluster galaxies (BCGs), massive central void galaxies, and passive spiral galaxies.<br><br> Local Universe BCGs are expected to be largely quiescent, based on both their mass and location within the cosmic web. We quantify the overall fraction of starbursting BCGs in the local Universe by determining the 12 μm star formation rates (SFRs) of 384 BCGs using the AllWISE survey. To ensure a homogeneous and uniformly massive sample, we select BCGs in L<sub>x</sub> > 10<sup>44</sup> erg s<sup>-1</sup> clusters. For the 0.0 < z < 0.2 Universe, just four BCGs possess SFRs < 10 M<sub>ʘ</sub> yr<sup>-1</sup> . The stellar populations of 98% of 0.0 < z < 0.2 BCGs are effectively passively evolving, translating to only modest galaxy growth in the current epoch. Our results show the main mass growth method of BCGs at current times is dry mergers, in agreement with previous literature. We find a trend of increasing SFR with z for low star forming BCGs (SFR < 1 M<sub>ʘ</sub> yr-<sup>1</sup>), though do not have the sample size to see this same trend in highly star forming BCGs. This low starburst fraction demonstrates that the BCG starburst phase (exemplified by Perseus A) is an atypical state for a low redshift BCG.<br> <br>We present integral field spectroscopy (IFS) using the Wide Field Spectrograph (WiFeS) of nine central void galaxies with M<sub>*</sub> > 10<sup>10</sup> M<sub>ʘ</sub>. While the majority of these galaxies are evolving secularly, we find a range of morphologies, merger histories and stellar population distributions, though similarly low H alpha-derived SFRs of < 1 M<sub>ʘ</sub> yr-<sup>1</sup> . One of our galaxies hosts an active galactic nucleus, and two have kinematic disruptions to their gas that are not seen in their stellar component, likely due to recent merger activity. The diversity of our void galaxies, despite their similar masses and environments, means they are still assembling, with minor mergers playing an important role. A comparison to a mass-matched sample of galaxies from all environments shows that despite environmental differences, galaxies of mass M<sub>*</sub> > 10<sup>10</sup> M<sub>ʘ</sub> have similarly low SFRs (< 3 M<sub>ʘ</sub> yr<sup>-1</sup>). We infer mass quenching is the dominant quenching mechanism for high-mass, isolated central galaxies.<br> <br>We have identified a population of passive spiral galaxies from photometry and IFS. We selected z < 0.035 spiral galaxies that have WISE colours consistent with little mid-infrared emission from warm dust. Photometry of these 51 spiral galaxies in ultraviolet, optical and mid-infrared show these galaxies have colours consistent with passive galaxies. Ten galaxies form a spectroscopic pilot study and were observed using WiFeS to check for signs of nebular emission from star formation. We see no evidence of the substantial nebular emission found in previous red spiral samples. An environmental study finds that passive spiral galaxies live in a variety of environments and span 2 dex in stellar mass. An unusually high ansa bar fraction leads us to question whether internal structure may be playing a role in their quenching