Exploring star formation in high-mass galaxies in the low-z universe
We present a quantitative analysis of high-mass, low-z galaxies in order to investigate the 'downsizing' scenario of galaxy evolution. High-mass, low-z galaxies with ongoing star formation, antithetical to the 'downsizing' model, are identified using the 22�m data (W4 band) from the Wide-field Infrared Survey Explorer (WISE). A cluster and field sample is chosen to investigate any possible environmental effects. The cluster sample is based upon the GMBCG catalog, which contains 55,424 brightest cluster galaxies (BCGs) at 0.1 [less than or about] z [less than or about] 0.55 identified in the Sloan Digital Sky Survey (SDSS). We identify 389 W4-detected BCGs (W4BCGs) that have median SFRs of [about]50 M[symbol]/yr based upon their total IR luminosity (LIR), which is attributed to dust-enshrouded star formation. BCGs with such high SFRs are found in "cool-core" clusters and the star formation is thought to be fueled by a "cooling flow." Using Chandra X-ray data, it is shown that a subset of BCGs do reside within coolcores, but their mass deposition rates cannot account for the SFR. For comparison, a field sample is drawn from the Max-Planck Institute for Astrophysics - John Hopkins University (MPA-JHU) "value-added" SDSS DR7 catalog of spectrum measurements. A set of 1,244 high-mass, elliptical field galaxies within the same redshift range as the W4BCG catalog are identified for comparison. The median mass for the field sample is lower than the W4BCGs (Log(M/M[symbol])=10.9 and 11.2 respectively), as are their SFRs. However, the specific star formation rate (sSFR), the star formation rate per stellar mass, is comparable for both groups (Log(sSFR)[about]-9.7). This possibly reveals that there is no environmental dependence on the sSFR for these W4-detected galaxies. While a possible mechanism responsible for the SFR was identified for the W4BCGs, the process responsible for the star formation in the field sample requires further investigation.
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