2,207 research outputs found
Spatially resolved stellar populations in the isolated elliptical NGC 821
We present the analysis of Lick absorption-line indices from three separate
long-slit spectroscopic observations of the nearby isolated elliptical galaxy
NGC 821. The three data sets present a consistent picture of the stellar
population within one effective radius, in which strong gradients are evident
in both luminosity-weighted age and metallicity. The central population
exhibits a young age of ~4 Gyr and a metallicity ~3 times solar. At one
effective radius the age has risen to ~12 Gyr and the metallicity fallen to
less than 1/3 solar. The low metallicity population around one effective radius
appears to have an exclusively red horizontal branch, with no significant
contribution from the blue horizontal branch evident in some globular clusters
of the same age and metallicity. Despite the strong central age gradient, we
demonstrate that only a small fraction (<10%) of the galaxy's stellar mass can
have been created in recent star formation events. We consider possible star
formation histories for NGC 821 and find that the most likely cause of the
young central population was a minor merger or tidal interaction that caused
NGC 821 to consume its own gas in a centrally concentrated burst of star
formation 1 to 4 Gyr ago.Comment: 11 pages. Accepted for publication in MNRA
Potential Economic Damage from Introduction of Brown Tree Snakes, Boiga irregularis (Reptilia: Colubridae), to the Islands of Hawaiâi.
v. ill. 23 cm.QuarterlyThe Brown Tree Snake (Boiga irregularis) has caused ecological and economic damage to Guam, and the snake has the potential to colonize other islands in the Pacific Ocean. This study quantifies the potential economic damage if the snake were translocated, established in the state of Hawaiâi, and causing damage at levels similar to those on Guam. Damages modeled included costs of medical treatments due to snakebites, snake-caused power outages, and decreased tourism resulting from effects of the snake. Damage caused by presence of the Brown Tree Snake on Guam was used as a guide to estimate potential economic damage to Hawaiâi from both medical- and power outageârelated damage. To predict tourism impact, a survey was administered to Hawaiian tourists that identified tourist responses to potential effects of the Brown Tree Snake. These results were then used in an input-output model to predict damage to the state economy. Summing these damages resulted in an estimated total potential annual damage to Hawaiâi of between 2.14 billion. This economic analysis provides a range of potential damages that policy makers can use in evaluation of future prevention and control programs
VLA Limits for Intermediate Mass Black Holes in Three Globular Clusters
The observational evidence for central black holes in globular clusters has
been argued extensively, and their existence has important consequences for
both the formation and evolution of the cluster. Most of the evidence comes
from dynamical arguments, but the interpretation is difficult, given the short
relaxation times and old ages of the clusters. One of the most robust
signatures for the existence of a black hole is radio and/or X-ray emission. We
observed three globular clusters, NGC6093 (M80), NGC6266 (M62), and NGC7078
(M15), with the VLA in the A and C configuration with a 3-sigma noise of 36, 36
and 25 microJy, respectively. We find no statistically-significant evidence for
radio emission from the central region for any of the three clusters. NGC6266
shows a 2-sigma detection. It is difficult to infer a mass from these upper
limits due to uncertainty about the central gas density, accretion rate, and
accretion model.Comment: 5 pages, accepted for publication in the Astronomical Journa
Consistent simulation of bromine chemistry from the marine boundary layer to the stratosphere â Part 2: Bromocarbons
In this second part of a series of articles dedicated to a detailed analysis of bromine chemistry in the atmosphere we address one (out of two) dominant natural sources of reactive bromine. The two main source categories are the release of bromine from sea salt and the decomposition of bromocarbons by photolysis and reaction with OH. Here, we focus on C<sub>1</sub>-bromocarbons. We show that the atmospheric chemistry general circulation model ECHAM5/MESSy realistically simulates their emission, transport and decomposition from the boundary layer up to the mesosphere. We included oceanic emission fluxes of the short-lived bromocarbons CH<sub>2</sub>Br<sub>2</sub>, CH<sub>2</sub>ClBr, CHClBr<sub>2</sub>, CHCl<sub>2</sub>Br, CHBr<sub>3</sub> and of CH<sub>3</sub>Br. The vertical profiles and the surface mixing ratios of the bromocarbons are in general agreement with the (few available) observations, especially in view of the limited information available and the consequent coarseness of the emission fields. For CHBr<sub>3</sub>, CHCl<sub>2</sub>Br and CHClBr<sub>2</sub> photolysis is the most important degradation process in the troposphere. In contrast to this, tropospheric CH<sub>2</sub>Br<sub>2</sub>, CH<sub>3</sub>Br and CH<sub>2</sub>ClBr are more efficiently decomposed by reaction with OH. In the free troposphere approximately 40% of the C<sub>1</sub>-bromocarbons decompose by reaction with OH. Our results indicate that bromoform contributes substantial amounts of reactive bromine to the lower stratosphere and thus should not be neglected in stratospheric simulations
Accretion Disk Temperatures and Continuum Colors in QSOs
Accretion disks around supermassive black holes are widely believed to be the
dominant source of the optical-ultraviolet continuum in many classes of active
galactic nuclei (AGN). We study here the relationship between the continuum
colors of AGN and the characteristic accretion disk temperature (T_max). Based
on NLTE models of accrection disks in AGN computed as described by Hubeny et
al. (2000), we find that continuum intensity ratios for several pairs of
wavelengths between 1350 and 5100 A should show a trend of bluer colors for
higher T_max, notwithstanding random disk inclinations. We compare this
theoretical expectation with observed colors of QSOs in the Sloan Digital Sky
Survey,deriving black hole mass and thence T_max from the width of the Mg II
broad emission line. The observed colors generally do not show the expected
trend and in some cases show a reverse trend of redder colors with increasing
T_max. The cause of this discrepancy does not appear to be dust reddening or
galaxy contamination but may relate to the accretion rate, as the offset
objects are accreting above ~30 % of the Eddington limit. The derived disk
temperature depends primarily on line width, with little or no dependence on
luminosity.Comment: 7 pages, 7 figures, accepted for publication in ApJ, uses
emulateapj.cl
Magnetic Phase Diagrams of Erbium
The magnetic phase diagrams of erbium in the magnetic fieldâtemperature plane have been constructed for applied magnetic fields along the a and b axes. For an a-axis applied field our HâT phase diagrams determined from magnetization and magnetoresistance data are in good agreement and consistent with that of Jehan et al. for temperatures below 50 K. A splitting of the basal plane NĂ©el temperature (TNâ„) above 3.75 T introduces two new magnetic phases. Also a transition from a fan to a canted fan phase as suggested by Jehan et al. is observed in an increasing field below TC. Our phase diagram for a b-axis applied field constructed from magnetization data is very similar to the phase diagram of Watson and Ali using magnetoresistance measurements. However, the anomaly at 42 K reported by Watson and Ali is not observed in the present study. No splitting of the TNâ„ transition is observed in either work for a field applied along the b axis
A planar extrapolation of the correlation problem that permits pairing
It was observed previously that an SU(N) extension of the Hubbard model is
dominated, at large N, by planar diagrams in the sense of 't Hooft, but the
possibility of superconducting pairing got lost in this extrapolation. To allow
for this possibility, we replace SU(N) by U(N,q), the unitary group in a vector
space of quaternions. At the level of the free energy, the difference between
the SU(N)and U(N,q) extrapolations appears only to first nonleading order in N.Comment: 8 pages, 2 figure
Soft X-Ray Sources at the Centers of the Elliptical Galaxies NGC 4472 and NGC 4649
Analysis of recent Chandra observations of the elliptical galaxies NGC 4472
and NGC 4649 has revealed faint soft X-ray sources at their centers. The
sources are located to within 1'' of the optical centers of the galaxies. They
are most likely associated with the central supermassive black holes. Interest
in these and several other similar objects stems from the unusually low
luminosity of the supermassive black holes embedded in dense interstellar
medium. Our Chandra sources have very soft spectra. They are detectable only
below ~0.6 keV and have luminosities in the 0.2-2.5 keV energy band of ~ 6 *
10^{37} erg/s and ~1.7 * 10^{38} erg/s in NGC 4649 and NGC 4472, respectively.Comment: Shortened version of the paper published in Astronomy Letter
Quantifying the fast outflow in the luminous Seyfert galaxy PG1211+143
We report two new XMM-Newton observations of PG1211+143 in December 2007,
again finding evidence of the fast outflow of highly ionised gas first detected
in 2001. Stacking the new spectra with those from two earlier XMM-Newton
observations reveals strong and broad emission lines of FeXXV and OVIII,
indicating the fast outflow to be persistent and to have a large covering
factor. This finding confirms a high mass rate for the ionised ouflow in
PG1211+143 and provides the first direct measurement of a wide angle,
sub-relativistic outflow from an AGN transporting mechanical energy with the
potential to disrupt the growth of the host galaxy. We suggest PG1211+143 may
be typical of an AGN in a rapid super-Eddington growth phase.Comment: Accepted for publication in MNRAS. Extended version with new figures
and table
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