167 research outputs found
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. 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/M) 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)
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 Myr 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 (), 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 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 and 1 dex in log M*).Comment: 18 pages, 18 figures, accepted by A
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