Browsing by Author "Gilbank, David"

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    Exploring star formation in dwarf galaxies at Z1
    (2014) Ramraj, Riona; Gilbank, David; Blyth, SL
    This dissertation presents the first H α measurements of star formation rates (SFRs) for dwarf galaxies at z~1 (when the Universe was approximately half its current age) using the Hubble Space Telescope (HST).We target a sample derived from the Redshift One LDSS3 Emission Line Survey (ROLES), which used [OII]3727 as a tracer of star formation to target very low stellar masses (M~10 8:5M) at this epoch. In this dissertation, we study these low mass ROLES galaxies using near-infrared slitless spectroscopy from HST. We target and measure the luminosity of the H α emission line. Even though H_ is a more direct SFR indicator, H α derived SFRs still need to be corrected for dust which is normally done using Balmer decrement measurements that are not available for our spectra. We find that using SED-fitted dust estimates, in the V-band, underestimates the extinction in high mass galaxies, in agreement with a recent result obtained by Price et al. (2013). We instead derive an empirical mass-dependent dust correction to take into account the extra extinction needed in high mass galaxies. We compare our dust corrected H α SFRs to the [OII] SFRs from ROLES. The [OII] SFRs were calculated using a mass-dependent empirical correction derived for galaxies in the Local Universe (Gilbank et al. 2010a). We find that the empirically corrected [OII] SFRs agree quite well with our dust corrected H α SFRs, having a slope of 1.6±0.7. This confirms that the mass-dependent empirical correction to [OII] works at z~1. We study the H α SSFR-mass relation for galaxies. We confirm previous results that at similar redshifts, low mass galaxies form their stars later and on longer timescales than high mass galaxies. However, our study probes down to lower limiting SFRs and lower stellar masses than any other previous studies, placing tighter constraints on galaxy evolution at this epoch.
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    Studying the environmental dependence of star formation properties of galaxies in the Abell 1437 galaxy cluster and its surrounding large scale structure
    (2014) Kotze, J P; Kraan-Korteweg, Renée C; Woudt, Patrick Alan; Väisänen, Petri; Gilbank, David
    It has become clear that the local galaxy environment plays a crucial role in the evolution of galaxies. Recent studies show a strong bimodal distribution of galaxy properties, with red, passive galaxies dominating the bimodal distribution in dense environments, compared to blue, actively star forming galaxies dominating the bimodal distribution in low density environments. Blue, star-forming galaxies are continuously transformed into red, passive galaxies through one or more processes while being accreted into higher density environments. The processes responsible for the observed transformation remain uncertain. We address these issues by performing an in-depth study of large-scale structure surrounding the galaxy cluster Abell 1437 at z = 0.135. We consider the colour and specific star formation rate distributions of galaxies as a function of environment. In this thesis we develop techniques to derive environmental samples which consist of a cluster, groups, filament and field. The large-scale structure surrounding the cluster is characterised through the use of a Friends-of-Friends algorithm based on spectroscopic data. This provides a reference from which we construct a photometric environmental catalogue using methods developed to find groups and define filamentary structure. To accomplish this, we construct a ugrizJHK multi-band dataset using a custom pipeline to derive a seeing matched photometric catalogue from Sloan Digital Sky Survey (SDSS) and UKIRT Deep Sky Survey (UKIDSS) imaging data. The photometric dataset is complemented by a near uniformly sampled spectroscopic dataset from the SDSS and Two Degree Field (2dF) survey. Stellar masses and star formation rates for the environmental samples are derived using the state-of-the-art magphys SED fitting code. The environmental samples we derive yield, within the uncertainties, similar galaxy populations than typically found in clusters, groups and the field. Red fractions were computed for the cluster, groups, filament and field sample for which we found 0.90 ± 0.13, 0.79 ± 0.01, 0.69 ± 0.02 and 0.58 ± 0.01 respectively. This is the first filament red fraction measurement made in this way. We studied the passive fraction of galaxies as a function of environment using specific star formation rates. Passive fractions were computed using two different definitions of passiveness, 1/tH (log(sSFR) = −10.07) and the minimum in the bimodal sSFR distribution (log(sSFR) = −11.0) as the division between active and passive galaxies. We found that the log(sSFR) = −11.0 bimodal divider is a natural division between active and passive galaxies and does not suffer from density-dependent selection effects. Quiescent fractions derived from the environmental samples reproduce previously known trends of star formation rates with environment. The filament environment is one of the least well studied environments since their low galaxy density makes them difficult to detect. We studied the fractional excess of star forming galaxies along the filament between Abell 1437 and the neighbouring region of overdensities, and find a significant increase in star formation activity. Although there have been hints of excess star formation in filaments previously, we claim our results to be of highest significance thus far.