The Evolution of Star Formation Activity in Cluster Galaxies Over 0.15 < z < 1.5

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Wagner, Cory
Galaxies , Astronomy , Elliptical galaxies , High-redshift , Astrophysics , Galaxy evolution , Galaxy clusters
In this thesis, we explore 7.5 billion years of evolution in cluster galaxy star formation activity using a sample of 11 high-redshift (1 < z < 1.5) clusters from the IRAC Shallow Cluster Survey, and 25 low-redshift (0.15 < z < 1) clusters from The Cluster Lensing And Supernova survey with Hubble. We compare cluster galaxy star formation to that of the field over 0.15 < z < 1.5 using ~8000 galaxies from the UltraVISTA survey. Mid-infrared star formation rates are measured using Spitzer 24 μm data for isolated high-redshift galaxies. We calculate rest-frame ultraviolet star formation rates for low-redshift cluster members using Hubble Space Telescope observations. Using publically available mid-infrared and ultraviolet data for our field sample, we empirically derive scaling relations to adjust low-redshift cluster galaxy ultraviolet star formation rates to mid-infrared levels. We classify cluster galaxy morphology by visual inspection, and use quantitatively measured morphologies for field galaxies. Cluster late-type galaxies at z > 1 show enhanced star formation activity relative to the field, and account for nearly 90% of the overall star formation activity in high-redshift clusters. While high-redshift early-type galaxies are substantially quenched relative to cluster late-types, they still contribute ~13% of the total cluster star formation activity. With early-type fractions increasing from 34 to 56% from z ~ 1.5 → 1.16, we find that new cluster early-type galaxies are likely being formed around z ~ 1.4. The fraction of early-type galaxies that are star-forming drops from 29 to 11% over this period, yet their specific star formation rates are roughly constant. These factors suggest that the events that created these new galaxies, possibly mergers, were both recent and gas-rich. With typical coverages of 50% of z < 1 cluster virial radii, we can only probe the cores of low-redshift clusters. We find that in this regime, the star formation activity of cluster galaxies is quenched relative to the field. We compare the mean star formation rate of cluster galaxies to the results of Alberts et al. (2014), who fit the mean star formation rate evolution over 0.3 < z < 1.5, and measured star formation rates by stacking 250 μm Herschel images. We find excellent agreement between the Herschel-based fit and both our Spitzer-derived and ultraviolet → infrared star formation rates.
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