815 research outputs found
Halpha and 4000 Angstrom Break Measurements for ~3500 K-selected Galaxies at 0.5<z<2.0
We measure spectral features of ~3500 K-selected galaxies at 0.5<z<2.0 from
high quality medium-band photometry using a new technique. First, we divide the
galaxy sample in 32 subsamples based on the similarities between the full
spectral energy distributions (SEDs) of the galaxies. For each of these 32
galaxy types we construct a composite SED by de-redshifting and scaling the
observed photometry. This approach increases the signal-to-noise ratio and
sampling of galaxy SEDs and allows for model-independent stellar population
studies. The composite SEDs are of spectroscopic quality, and facilitate -- for
the first time -- Halpha measurement for a large magnitude-limited sample of
distant galaxies. The linewidths indicate a photometric redshift uncertainty of
dz<0.02x(1+z). The composite SEDs also show the Balmer and 4000 Angstrom
breaks, MgII absorption at ~2800 Angstrom, the dust absorption feature at 2175
Angstrom, and blended [OIII]+Hbeta emission. We compare the total equivalent
width of Halpha, [NII], and [SII] (W_Halpha+) with the strength of the 4000
Angstrom break (D(4000)) and the best-fit specific star formation rate, and
find that all these properties are strongly correlated. This is a reassuring
result, as currently most distant stellar population studies are based on just
continuum emission. Furthermore, the relation between W_Halpha+ and Dn(4000)
provides interesting clues to the SFHs of galaxies, as these features are
sensitive to different stellar ages. We find that the correlation between
W_Halpha+ and D(4000) at 0.5<z<2.0 is similar to z~0, and that the suppression
of star formation in galaxies at z<2 is generally not abrupt, but a gradual
process.Comment: Accepted for publication in ApJ; high-resolution version can be
downloaded at https://www.cfa.harvard.edu/~mkriek/papers
X-ray properties of K-selected galaxies at 0.5<z<2.0: Investigating trends with stellar mass, redshift and spectral type
We examine how the total X-ray luminosity correlates with stellar mass,
stellar population, and redshift for a K-band limited sample of ~3500 galaxies
at 0.5<z<2.0 from the NEWFIRM Medium Band Survey in the COSMOS field. The
galaxy sample is divided into 32 different galaxy types, based on similarities
between the spectral energy distributions. For each galaxy type, we further
divide the sample into bins of redshift and stellar mass, and perform an X-ray
stacking analysis using the Chandra COSMOS (C-COSMOS) data. We find that full
band X-ray luminosity is primarily increasing with stellar mass, and at similar
mass and spectral type is higher at larger redshifts. When comparing at the
same stellar mass, we find that the X-ray luminosity is slightly higher for
younger galaxies (i.e., weaker 4000\AA breaks), but the scatter in this
relation is large. We compare the observed X-ray luminosities to those expected
from low and high mass X-ray binaries (XRBs). For blue galaxies, XRBs can
almost fully account for the observed emission, while for older galaxies with
larger 4000\AA breaks, active galactic nuclei (AGN) or hot gas dominate the
measured X-ray flux. After correcting for XRBs, the X-ray luminosity is still
slightly higher in younger galaxies, although this correlation is not
significant. AGN appear to be a larger component of galaxy X-ray luminosity at
earlier times, as the hardness ratio increases with redshift. Together with the
slight increase in X-ray luminosity this may indicate more obscured AGNs or
higher accretion rates at earlier times.Comment: 9 pages, 9 figures, ApJ accepte
Massive quenched galaxies at z~0.7 retain large molecular gas reservoirs
The physical mechanisms that quench star formation, turning blue star-forming
galaxies into red quiescent galaxies, remain unclear. In this Letter, we
investigate the role of gas supply in suppressing star formation by studying
the molecular gas content of post-starburst galaxies. Leveraging the wide area
of the SDSS, we identify a sample of massive intermediate-redshift galaxies
that have just ended their primary epoch of star formation. We present ALMA
CO(2-1) observations of two of these post-starburst galaxies at z~0.7 with M* ~
2x10^11 Msun. Their molecular gas reservoirs of (6.4 +/- 0.8) x 10^9 Msun and
(34.0 +/- 1.6) x 10^9 Msun are an order of magnitude larger than
comparable-mass galaxies in the local universe. Our observations suggest that
quenching does not require the total removal or depletion of molecular gas, as
many quenching models suggest. However, further observations are required both
to determine if these apparently quiescent objects host highly obscured star
formation and to investigate the intrinsic variation in the molecular gas
properties of post-starburst galaxies.Comment: Accepted for publication in ApJ Letters (6 pages, 5 figures
Ages of massive galaxies at from 3D-HST rest-frame optical spectroscopy
We present low-resolution near-infrared stacked spectra from the 3D-HST
survey up to and fit them with commonly used stellar population
synthesis models: BC03 (Bruzual & Charlot, 2003), FSPS10 (Flexible Stellar
Population Synthesis, Conroy & Gunn 2010), and FSPS-C3K (Conroy, Kurucz,
Cargile, Castelli, in prep). The accuracy of the grism redshifts allows the
unambiguous detection of many emission and absorption features, and thus a
first systematic exploration of the rest-frame optical spectra of galaxies up
to . We select massive galaxies (), we
divide them into quiescent and star-forming via a rest-frame color-color
technique, and we median-stack the samples in 3 redshift bins between
and . We find that stellar population models fit the observations well
at wavelengths below rest-frame, but show systematic residuals
at redder wavelengths. The FSPS-C3K model generally provides the best fits
(evaluated with a statistics) for quiescent galaxies, while BC03
performs the best for star-forming galaxies. The stellar ages of quiescent
galaxies implied by the models, assuming solar metallicity, vary from 4 Gyr at
to 1.5 Gyr at , with an uncertainty of a factor of 2
caused by the unknown metallicity. On average the stellar ages are half the age
of the Universe at these redshifts. We show that the inferred evolution of ages
of quiescent galaxies is in agreement with fundamental plane measurements,
assuming an 8 Gyr age for local galaxies. For star-forming galaxies the
inferred ages depend strongly on the stellar population model and the shape of
the assumed star-formation history.Comment: 13 pages, 15 figures, accepted for publication in Ap
Trained and Amphetamine-Induced Circling Behavior in Lesioned, Transplanted Rats
Rats were trained to turn for water
reinforcement and then were given unilateral 6-
hydroxydopamine lesions. After lesion, rats
showed deficits in trained turning both contraand
ipsilateral to the side of the lesion, with
contralateral turning more severely impaired.
The lesioned rats were then transplanted with
fetal mesencephalic dopamine tissue into
striatum. A control group of lesioned rats were
sham transplanted. Four weeks after transplant,
1.5 mg/kg D-amphetamine challenge injections
were used to test the functioning of the
transplants. In the control rats, D-amphetamine
induced ipsilateral turning; in transplanted rats,
D-amphetamine slowed the rate of ipsilateral
turning or reversed the direction of
amphetamine-induced rotation. Only rats which
reversed their, amphetamine-induced turn
direction after transplant were used for the rest
of the experiment. Trained turning was assessed
at 4, 8, 12 and 16 weeks post transplant.
Transplants did not improve learned
performance at any time post transplant. When
D-amphetamine was administered in
conjunction with the trained turning sessions, a
low dose (0.12 mg/kg) enhanced contralateral
trained turn rates, without affecting ipsilateral
turn rates. Higher doses of amphetamine
reduced ipsilateral turn rate in the transplanted
animals. The results of this study suggest that
transplants alone do not reinstate performance
of conditioned rotation
The Age Spread of Quiescent Galaxies with the NEWFIRM Medium-band Survey: Identification of the Oldest Galaxies out to z~2
With a complete, mass-selected sample of quiescent galaxies from the NEWFIRM
Medium-Band Survey (NMBS), we study the stellar populations of the oldest and
most massive galaxies (>10^11 Msun) to high redshift. The sample includes 570
quiescent galaxies selected based on their extinction-corrected U-V colors out
to z=2.2, with accurate photometric redshifts, sigma_z/(1+z)~2%, and rest-frame
colors, sigma_U-V~0.06 mag. We measure an increase in the intrinsic scatter of
the rest-frame U-V colors of quiescent galaxies with redshift. This scatter in
color arises from the spread in ages of the quiescent galaxies, where we see
both relatively quiescent red, old galaxies and quiescent blue, younger
galaxies towards higher redshift. The trends between color and age are
consistent with the observed composite rest-frame spectral energy distributions
(SEDs) of these galaxies. The composite SEDs of the reddest and bluest
quiescent galaxies are fundamentally different, with remarkably well-defined
4000A- and Balmer-breaks, respectively. Some of the quiescent galaxies may be
up to 4 times older than the average age- and up to the age of the universe, if
the assumption of solar metallicity is correct. By matching the scatter
predicted by models that include growth of the red sequence by the
transformation of blue galaxies to the observed intrinsic scatter, the data
indicate that most early-type galaxies formed their stars at high redshift with
a burst of star formation prior to migrating to the red sequence. The observed
U-V color evolution with redshift is weaker than passive evolution predicts;
possible mechanisms to slow the color evolution include increasing amounts of
dust in quiescent galaxies towards higher redshift, red mergers at z<1, and a
frosting of relatively young stars from star formation at later times.Comment: 20 pages, 20 figures, accepted for publication in Ap
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