Ages of Type Ia Supernovae Over Cosmic Time

We derive empirical models for galaxy mass assembly histories, and convolve these with theoretical delay time distribution (DTD) models for Type Ia supernovae (SNe Ia) to derive the distribution of progenitor ages for all SNe Ia occurring at a given epoch of cosmic time. In actively star-forming galaxies, the progression of the star formation rate is shallower than a $t^{-1}$ SN Ia DTD, so mean SN Ia ages peak at the DTD peak in all star-forming galaxies at all epochs of cosmic history. In passive galaxies which have ceased star formation through some quenching process, the SN Ia age distribution peaks at the quenching epoch, which in passive galaxies evolves in redshift to track the past epoch of major star formation. Our models reproduce the SN Ia rate evolution in redshift, the relationship between SN Ia stretch and host mass, and the distribution of SN Ia host masses in a manner qualitatively consistent with observations. Our model naturally predicts that low-mass galaxies tend to be actively star-forming while massive galaxies are generally passive, consistent with observations of galaxy "downsizing". Consequently, the mean ages of SNe Ia undergo a sharp transition from young ages at low host mass to old ages at high host mass, qualitatively similar to the transition of mean SN Ia Hubble residuals with host mass. The age discrepancy evolves with redshift in a manner currently not accounted for in SN Ia cosmology analyses. We thus suggest that SNe Ia selected only from actively star-forming galaxies will yield the most cosmologically uniform sample, due to the homogeneity of young SN Ia progenitor ages at all cosmological epochs.Comment: 15 pages, 15 figures, accepted for publication in MNRA

The Velocity Dispersion Function for Quiescent Galaxies in the Local Universe

We investigate the distribution of central velocity dispersions for quiescent galaxies in the SDSS at $0.03 \leq z \leq 0.10$. To construct the field velocity dispersion function (VDF), we construct a velocity dispersion complete sample of quiescent galaxies with Dn4000$> 1.5$. The sample consists of galaxies with central velocity dispersion larger than the velocity dispersion completeness limit of the SDSS survey. Our VDF measurement is consistent with previous field VDFs for $\sigma > 200$ km s$^{-1}$. In contrast with previous results, the VDF does not decline significantly for $\sigma < 200$ km s$^{-1}$. The field and the similarly constructed cluster VDFs are remarkably flat at low velocity dispersion ($\sigma < 250$ km s$^{-1}$). The cluster VDF exceeds the field for $\sigma > 250$ km s$^{-1}$ providing a measure of the relatively larger number of massive subhalos in clusters. The VDF is a probe of the dark matter halo distribution because the measured central velocity dispersion may be directly proportional to the dark matter velocity dispersion. Thus the VDF provides a potentially powerful test of simulations for models of structure formation.Comment: Accepted for publication in ApJ. 10 pages, 8 figures. Comments welcom

A Complete Spectroscopic Census of Abell 2029: A Tale of Three Histories

A rich spectroscopic census of members of the local massive cluster Abell 2029 includes 1215 members of A2029 and its two infalling groups, A2033 and a Southern Infalling Group (SIG). The two infalling groups are identified in spectroscopic, X-ray and weak lensing maps. We identify active galactic nuclei (AGN), star-forming galaxies, E+A galaxies, and quiescent galaxies based on the spectroscopy. The fractions of AGN and post-starburst E+A galaxies in A2029 are similar to those of other clusters. We derive the stellar mass ($M_{*}$)-metallicity of A2029 based on 227 star-forming members; A2029 members within $10^{9} M_{\odot} < M _{*} < 10^{9.5} M_{\odot}$ are more metal rich than SDSS galaxies within the same mass range. We utilize the spectroscopic index $D_{n}4000$, a strong age indicator, to trace past and future evolution of the A2029 system. The median $D_{n}4000$ of the members decreases as the projected clustercentric distance increases for all three subsystems. The $D_{n}4000 - M_{*}$ relations of the members in A2029 and its two infalling groups differ significantly indicating the importance of stochastic effects for understanding the evolution of cluster galaxy populations. In the main cluster, an excess around $D_{n}4000 \sim 1.8$ indicates that some A2029 members became quiescent galaxies 2-3 Gyr ago consistent with the merger epoch of the X-ray sloshing pattern.Comment: 17 pages, 11 figures, submitted to Ap