197 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
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