511 research outputs found
Charting the evolution of the ages and metallicities of massive galaxies since z=0.7
The stellar populations of intermediate-redshift galaxies can shed light onto
the growth of massive galaxies in the last 8 billion years. We perform deep,
multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70
galaxies in the E-CDFS with redshift 0.6522.7 and
stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous
low-redshift studies, we constrain the stellar mass, mean stellar age and
stellar metallicity of individual galaxies from stellar absorption features. We
characterize for the first time the dependence of stellar metallicity and age
on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming
galaxies separately. These relations for the whole sample have a similar shape
as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex
in metallicity, at odds with simple passive evolution. We find that no
additional star formation and chemical enrichment are required for z=0.7
quiescent galaxies to evolve into the present-day quiescent population.
However, this must be accompanied by the quenching of a fraction of z=0.7
Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those
of quiescent galaxies, thus increasing the scatter in age without affecting the
metallicity distribution. However rapid quenching of the entire population of
massive star-forming galaxies at z=0.7 would be inconsistent with the
age/metallicity--mass relation for the population as a whole and with the
metallicity distribution of star-forming galaxies only, which are on average
0.12 dex less metal-rich than their local counterparts. This indicates chemical
enrichment until the present in at least a fraction of the z=0.7 massive
star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure
HD 101088, An Accreting 14 AU Binary in Lower Centaurus Crux With Very Little Circumstellar Dust
We present high resolution (R=55,000) optical spectra obtained with MIKE on
the 6.5 m Magellan Clay Telescope as well as Spitzer MIPS photometry and IRS
low resolution (R~60) spectroscopy of the close (14 AU separation) binary, HD
101088, a member of the ~12 Myr old southern region of the Lower Centaurus Crux
(LCC) subgroup of the Scorpius-Centaurus OB association. We find that the
primary and/or secondary is accreting from a tenuous circumprimary and/or
circumsecondary disk despite the apparent lack of a massive circumbinary disk.
We estimate a lower limit to the accretion rate of > 1x10^-9 solar masses per
year, which our multiple observation epochs show varies over a timescale of
months. The upper limit on the 70 micron flux allows us to place an upper limit
on the mass of dust grains smaller than several microns present in a
circumbinary disk of 0.16 moon masses. We conclude that the classification of
disks into either protoplanetary or debris disks based on fractional infrared
luminosity alone may be misleading.Comment: 8 pages, 2 figures, ApJ accepte
Fundamental Parameters of four Massive Eclipsing Binaries in Westerlund 1
We present fundamental parameters of 4 massive eclipsing binaries in the
young massive cluster Westerlund 1. The goal is to measure accurate masses and
radii of their component stars, which provide much needed constraints for
evolutionary models of massive stars. Accurate parameters can further be used
to determine a dynamical lower limit for the magnetar progenitor and to obtain
an independent distance to the cluster. Our results confirm and extend the
evidence for a high mass for the progenitor of the magnetar.Comment: 2 pages, to appear in the proceedings of the IAUS 282 on "From
Interacting Binaries to Exoplanets:Essential Modelling Tools" (Tatranska
Lomnica, July 18-22, 2011), Cambridge University Pres
The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA) I: The Sample and the Spectra
We introduce Project MEGaSaURA: The Magellan Evolution of Galaxies
Spectroscopic and Ultraviolet Reference Atlas. MEGaSaURA comprises
medium-resolution, rest-frame ultraviolet spectroscopy of N=15 bright
gravitationally lensed galaxies at redshifts of 1.68z3.6, obtained with
the MagE spectrograph on the Magellan telescopes. The spectra cover the
observed-frame wavelength range \AA ; the average
spectral resolving power is R=3300. The median spectrum has a signal-to-noise
ratio of per resolution element at 5000 \AA . As such, the MEGaSaURA
spectra have superior signal-to-noise-ratio and wavelength coverage compared to
what COS/HST provides for starburst galaxies in the local universe. This paper
describes the sample, the observations, and the data reduction. We compare the
measured redshifts for the stars, the ionized gas as traced by nebular lines,
and the neutral gas as traced by absorption lines; we find the expected bulk
outflow of the neutral gas, and no systemic offset between the redshifts
measured from nebular lines and the redshifts measured from the stellar
continuum. We provide the MEGaSaURA spectra to the astronomical community
through a data release.Comment: Resubmitted to AAS Journals. Data release will accompany journal
publication. v2 addresses minor comments from refere
Wide-Field Chandra X-Ray Observations of AGN in Abell 85 & Abell 754
To better understand the mechanism or mechanisms that lead to AGN activity
today, we measure the X-ray AGN fraction in a new sample of nearby clusters and
examine how it varies with galaxy properties, projected cluster-centric radius,
and cluster velocity dispersion. We present new wide-field Chandra X-ray
Observatory observations of Abell 85, Abell 754 and the background cluster
Abell 89B out to their virial radii. Out of seventeen X-ray sources associated
with galaxies in these clusters, we classify seven as X-ray AGN with L_{X,B} >
10^{41} erg/s. Only two of these would be classified as AGN based on their
optical spectra. We combine these observations with archival data to create a
sample of X-ray AGN from six z < 0.08 clusters and find that 3.4+1.1/-0.8% of
M_R 10^{41} erg/s. We find that
more X-ray AGN are detected in more luminous galaxies and attribute this to
larger spheriods in more luminous galaxies and increased sensitivity to lower
Eddington-rate accretion from black holes in those spheroids. At a given X-ray
luminosity limit, more massive black holes can be accreting less efficiently,
yet still be detected. If interactions between galaxies are the principal
drivers of AGN activity, then the AGN fraction should be higher in lower
velocity dispersion clusters and the outskirts of clusters. However, the
tendency of the most massive and early-type galaxies to lie in the centers of
the richest clusters could dilute such trends. While we find no variation in
the AGN fraction with projected cluster-centric radius, we do find that the AGN
fraction increases significantly from 2.6+1.0/-0.8% in rich clusters to
10.0+6.2/-4.3% in those with lower velocity dispersions.Comment: Accepted by Astrophysical Journal, 17 pages using emulateapj.cls, 10
B & W Figures (degraded): Full resolution paper available at
http://www.astronomy.ohio-state.edu/~sivakoff/AGN/XAGN_A85_A754.pd
Motor-cognitive dual-task deficits in individuals with early-mid stage Huntington's disease
Background. Huntington’s disease (HD) results in a range of cognitive and motor impairments that progress throughout the disease stages; however, little research has evaluated specific dual-task abilities in this population, and the degree to which they may be related to functional ability. Objectives. The purpose of this study was to a) examine simple and complex motor-cognitive dual-task performance in individuals with HD, b) determine relationships between dual-task walking ability and disease-specific measures of motor, cognitive and functional ability, and c) examine the relationship of dual-task measures to falls in individuals with HD. Methods. Thirty-two individuals with Huntington’s disease were evaluated for simple and complex dual-task ability using the Walking While Talking Test. Demographics and disease-specific measures of motor, cognitive and functional ability were also obtained. Results. Individuals with HD had impairments in simple and complex dual-task ability. Simple dual-task walking was correlated to disease-specific motor scores as well as cognitive performance, but complex dual-task walking was correlated with total functional capacity, as well as a range of cognitive measures. Number of prospective falls was strongly correlated to dual-task measures. Conclusions. Our results suggest that individuals with HD have impairments in cognitive-motor dual-task ability that are related to disease progression and specifically functional ability. Dual-task measures appear to evaluate a unique construct in individuals with early to mid-stage HD, and may have value in improving the prediction of falls risk in this population
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