The Discovery of 1000 km/s Outflows in Massive Post-starburst Galaxies at z=0.6

Numerical simulations suggest that active galactic nuclei (AGNs) play an important role in the formation of early-type galaxies by expelling gas and dust in powerful galactic winds and quenching star formation. However, the existence of AGN feedback capable of halting galaxy-wide star formation has yet to be observationally confirmed. To investigate this question, we have obtained spectra of 14 post-starburst galaxies at z~0.6 to search for evidence of galactic winds. In 10/14 galaxies we detect Mg II 2796,2803 absorption lines which are blueshifted by 490 - 2020 km/s with respect to the stars. The median blueshift is 1140 km/s. We hypothesize that the outflowing gas represents a fossil galactic wind launched near the peak of the galaxy's activity, a few 100 Myr ago. The velocities we measure are intermediate between those of luminous starbursts and broad absorption line quasars, which suggests that feedback from an AGN may have played a role in expelling cool gas and shutting down star formation.Comment: 5 pages, 2 figures, accepted to ApJ Letter

Shining A Light On Galactic Outflows: Photo-Ionized Outflows

We study the ionization structure of galactic outflows in 37 nearby, star forming galaxies with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We use the O I, Si II, Si III, and Si IV ultraviolet absorption lines to characterize the different ionization states of outflowing gas. We measure the equivalent widths, line widths, and outflow velocities of the four transitions, and find shallow scaling relations between them and galactic stellar mass and star formation rate. Regardless of the ionization potential, lines of similar strength have similar velocities and line widths, indicating that the four transitions can be modeled as a co-moving phase. The Si equivalent width ratios (e.g. Si IV/Si II) have low dispersion, and little variation with stellar mass; while ratios with O I and Si vary by a factor of 2 for a given stellar mass. Photo-ionization models reproduce these equivalent width ratios, while shock models under predict the relative amount of high ionization gas. The photo-ionization models constrain the ionization parameter (U) between -2.25 < log(U) < -1.5, and require that the outflow metallicities are greater than 0.5 Z$_\odot$. We derive ionization fractions for the transitions, and show that the range of ionization parameters and stellar metallicities leads to a factor of 1.15-10 variation in the ionization fractions. Historically, mass outflow rates are calculated by converting a column density measurement from a single metal ion into a total Hydrogen column density using an ionization fraction, thus mass outflow rates are sensitive to the assumed ionization structure of the outflow.Comment: 30 pages, 17 tables, 14 figures. Accepted for publication in MNRA

The ages and metallicities of galaxies in the local universe

We derive stellar metallicities, light-weighted ages and stellar masses for a magnitude-limited sample of 175,128 galaxies drawn from the Sloan Digital Sky Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of these parameters using a large library of model spectra at medium-high resolution, covering a comprehensive range of star formation histories. The constraints we derive are set by the simultaneous fit of five spectral absorption features, which are well reproduced by our population synthesis models. By design, these constraints depend only weakly on the alpha/Fe element abundance ratio. Our sample includes galaxies of all types spanning the full range in star formation activity, from dormant early-type to actively star-forming galaxies. We show that, in the mean, galaxies follow a sequence of increasing stellar metallicity, age and stellar mass at increasing 4000AA-break strength (D4000). For galaxies of intermediate mass, stronger Balmer absorption at fixed D4000 is associated with higher metallicity and younger age. We investigate how stellar metallicity and age depend on total galaxy stellar mass. Low-mass galaxies are typically young and metal-poor, massive galaxies old and metal-rich, with a rapid transition between these regimes over the stellar mass range 3x10^9<M/Msun<3x10^10. Both high- and low-concentration galaxies follow these relations, but there is a large dispersion in stellar metallicity at fixed stellar mass, especially for low-concentration galaxies of intermediate mass. Despite the large scatter, the relation between stellar metallicity and stellar mass is similar to the correlation between gas-phase oxygen abundance and stellar mass for star-forming galaxies. [abriged]Comment: 22 pages, 14 figures, accepted for publication on MNRAS, data available at http://www.mpa-garching.mpg.de/SDSS

Optical Spectroscopy and Nebular Oxygen Abundances of the Spitzer/SINGS Galaxies

We present intermediate-resolution optical spectrophotometry of 65 galaxies obtained in support of the Spitzer Infrared Nearby Galaxies Survey (SINGS). For each galaxy we obtain a nuclear, circumnuclear, and semi-integrated optical spectrum designed to coincide spatially with mid- and far-infrared spectroscopy from the Spitzer Space Telescope. We make the reduced, spectrophotometrically calibrated one-dimensional spectra, as well as measurements of the fluxes and equivalent widths of the strong nebular emission lines, publically available. We use optical emission-line ratios measured on all three spatial scales to classify the sample into star-forming, active galactic nuclei (AGN), and galaxies with a mixture of star formation and nuclear activity. We find that the relative fraction of the sample classified as star-forming versus AGN is a strong function of the integrated light enclosed by the spectroscopic aperture. We supplement our observations with a large database of nebular emission-line measurements of individual HII regions in the SINGS galaxies culled from the literature. We use these ancillary data to conduct a detailed analysis of the radial abundance gradients and average HII-region abundances of a large fraction of the sample. We combine these results with our new integrated spectra to estimate the central and characteristic (globally-averaged) gas-phase oxygen abundances of all 75 SINGS galaxies. We conclude with an in-depth discussion of the absolute uncertainty in the nebular oxygen abundance scale.Comment: ApJS, in press; 52 emulateapj pages, 12 figures, and two appendices; v2: final abundances revised due to minor error; conclusions unchange

Clocking the Evolution of Post-Starburst Galaxies: Methods and First Results

Detailed modeling of the recent star formation histories (SFHs) of post-starburst (or "E+A") galaxies is impeded by the degeneracy between the time elapsed since the starburst ended (post-burst age), the fraction of stellar mass produced in the burst (burst strength), and the burst duration. To resolve this issue, we combine GALEX ultraviolet photometry, SDSS photometry and spectra, and new stellar population synthesis models to fit the SFHs of 532 post-starburst galaxies. In addition to an old stellar population and a recent starburst, 48% of the galaxies are best fit with a second recent burst. Lower stellar mass galaxies (log M$_\star$/M$_\odot<10.5$) are more likely to experience two recent bursts, and the fraction of their young stellar mass is more strongly anti-correlated with their total stellar mass. Applying our methodology to other, younger post-starburst samples, we identify likely progenitors to our sample and examine the evolutionary trends of molecular gas and dust content with post-burst age. We discover a significant (4$\sigma$) decline, with a 117-230 Myr characteristic depletion time, in the molecular gas to stellar mass fraction with the post-burst age. The implied rapid gas depletion rate of 2-150 M$_\odot$yr$^{-1}$ cannot be due to current star formation, given the upper limits on the current SFRs in these post-starbursts. Nor are stellar winds or SNe feedback likely to explain this decline. Instead, the decline points to the expulsion or destruction of molecular gas in outflows, a possible smoking gun for AGN feedback.Comment: Accepted for publication in Ap

Absorption-line probes of the prevalence and properties of outflows in present-day star-forming galaxies

We analyze star forming galaxies drawn from SDSS DR7 to show how the interstellar medium (ISM) Na I 5890, 5896 (Na D) absorption lines depend on galaxy physical properties, and to look for evidence of galactic winds. We combine the spectra of galaxies with similar geometry/physical parameters to create composite spectra with signal-to-noise ~300. The stellar continuum is modeled using stellar population synthesis models, and the continuum-normalized spectrum is fit with two Na I absorption components. We find that: (1) ISM Na D absorption lines with equivalent widths EW > 0.8A are only prevalent in disk galaxies with specific properties -- large extinction (Av), high star formation rates (SFR), high star formation rate per unit area ($\Sigma_{\rm SFR}$), or high stellar mass (M*). (2) the ISM Na D absorption lines can be separated into two components: a quiescent disk-like component at the galaxy systemic velocity and an outflow component; (3) the disk-like component is much stronger in the edge-on systems, and the outflow component covers a wide angle but is stronger within 60deg of the disk rotation axis; (4) the EW and covering factor of the disk component correlate strongly with dust attenuation, highlighting the importance that dust shielding may play the survival of Na I. (5) The EW of the outflow component depends primarily on $\Sigma_{\rm SFR}$ and secondarily on Av; (6) the outflow velocity varies from ~120 to 160km/s but shows little hint of a correlation with galaxy physical properties over the modest dynamic range that our sample probes (1.2 dex in log$\Sigma_{\rm SFR}$ and 1 dex in log M*).Comment: 18 pages, 18 figures, accepted by A