We used a low-dispersion prism (LDP) in IMACS on the 6.5 m Baade (Magellan I) telescope to obtain low resolution spectroscopy of galaxies with 18 < zAB < 23.3 mag over a ∼0.2 deg<super>2</super> field. In contrast to spectra obtained through a traditional grating, the resolution of LDP spectra was a strong function of wavelength, with the spectral resolving power of the LDP varying from R ∼ 66 at 5000 Å to R ∼ 15 at 8500 Å. Stellar population synthesis models were fit to the relative flux calibrated LDP spectra and Subaru Suprime-Cam V Riz photometry in order to measure redshifts. We measured ∼10,000 redshifts to a precision of ∼1% in (1 + z).
Given the emerging consensus regarding the key role of stellar mass in shaping the star formation histories (SFHs) of galaxies, we aimed to study stellar mass limited samples. Our z-band spectroscopic selection was critical in assembling samples to low mass limits at 0.6 < z</italic> < 0.9. Our final sample consisted of ∼1100 galaxies at 0.6 < <italic>z < 0.9 above a mass limit of M > 1.8 × 1010 [special characters omitted] (log M/[special characters omitted] > 10.25). This mass limit roughly corresponds to 0.2M ☆, thus reaching well below the knee of the Schechter mass function and also a factor of two below the threshold mass above which galaxy properties appear to be primarily determined by stellar mass.
The rest-frame color of a galaxy serves as trace for the age of its stellar populations. We investigated the rest-frame u – g color of galaxies as a function of local density in a redshift slice (0.80 < z</italic> < 0.87) containing the cluster RX J0152.7-1357 as well as several other groups. We find that the fraction of red galaxies increases at higher densities, reaching ∼90–100% in the cores of moderate mass groups, the same value as found in the center of RX J0152.7-1357, and suggesting the presence of older stellar populations in these high density environments.
Because dust obscuration can make younger star forming galaxies (SFGs) appear red, we investigated the obscured star formation rates (SFRs) of galaxies in our sample by utilizing a stacking analysis with Spitzer MIPS 24 µm imaging. We found the median specific SFRs (SSFRs) of galaxies, at a fixed mass, decline by an order of magnitude over the full range of environments studied at 0.80 < <italic>z < 0.87: from the low density field, to the cores of groups and a cluster. This result suggests that the elevated fraction of red galaxies at high densities is indicative of galaxies with older stellar populations rather than dusty obscured SF.
We extended our analysis of galaxy SFRs by expanding the redshift interval of study to 0.6 < <italic>z < 0.9 and also employed three different SFR indicators: (1) mid-IR fluxes from MIPS 24 µm imaging, (2) SFRs from SED fitting, which leverage newly acquired Magellan IMACS B-band imaging, and (3) [OII]λ3727 Å emission line luminosities from the LDP spectra. With all three SFR indicators, we found the median SFRs and SSFRs of galaxies above M > 1.8 × 1010 [special characters omitted] to decline at higher densities. The median SFRs and SSFRs of galaxies were an order of magnitude lower in the cores of groups and a cluster compared to the field. To determine the role of SFGs in driving the SSFR-density relation, we utilized a color-color selection with a U – V vs. V – J diagram to distinguish quiescent and SFGs.
We conclude that with mass-limited samples, group and cluster halos at 0.6 < <italic>z < 0.9 represent sites where SF is quenched and galaxies are moved onto the red-sequence in large proportions compared to the field. This is similar to the general trend at z ∼ 0. These larger halos are predicted to grow in mass by more than a factor of two below z ∼ 1. As a consequence, the growth of structure in the universe, on group scales in particular, contributes towards the decline in SF and buildup of the red-sequence observed over the last half of cosmic time. (Abstract shortened by UMI.)
|School:||University of California, Santa Cruz|
|School Location:||United States -- California|
|Source:||DAI-B 71/11, Dissertation Abstracts International|
|Keywords:||Galaxy clusters, Star formation|
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