916 research outputs found
The Stellar Content Near the Galactic Center
High angular resolution J, H, K, and L' images are used to investigate the
stellar content within 6 arcsec of SgrA*. The data, which are complete to K ~
16, are the deepest multicolor observations of the region published to date.Comment: 34 pages, including 12 figure
Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys - IV. Influence of mergers in the evolution of faint field galaxies from z~1
HST images of a sample of 285 galaxies with measured z from the CFRS and
Autofib-LDSS redshift surveys are analysed to derive the evolution of the
merger fraction out to z~1. We have performed visual and machine-based merger
identifications, as well as counts of bright pairs of galaxies with magnitude
differences less than 1.5 mag. We find that the pair fraction increases with z,
with up to ~20% of the galaxies being in physical pairs at z~0.75-1. We derive
a merger fraction varying with z as (1+z)^{3.2 +/- 0.6}, after correction for
line-of-sight contamination, in excellent agreement with the merger fraction
derived from the visual classification of mergers for which m = 3.4 +/- 0.6.
After correcting for seeing effects on the ground-based selection of survey
galaxies, we conclude that the pair fraction evolves as (1+z)^{2.7 +/- 0.6}.
This implies that an average L* galaxy will have undergone 0.8 to 1.8 merger
events from z=1 to 0, with 0.5 to 1.2 merger events occuring in a 2 Gyr time
span at z~0.9. This result is consistent with predictions from semi-analytical
models of galaxy formation. From the simple co-addition of the observed
luminosities of the galaxies in pairs, physical mergers are computed to lead to
a brightening of 0.5 mag for each pair on average, and a boost in star
formation rate of a factor of 2, as derived from the average [O II] equivalent
widths. Mergers of galaxies are therefore contributing significantly to the
evolution of both the luminosity function and luminosity density of the
Universe out to z~1.Comment: 14 pages, 6 PS figures included. Accepted for publication in MNRA
Magellanic Cloud X-ray Sources: III. Completion of a ROSAT Survey
This paper concludes a series of three papers presenting ROSAT
High-Resolution Imager (HRI) observations of unidentified Einstein and
serendipitous ROSAT X-ray sources in the direction of the Magellanic Clouds.
Accurate positions and fluxes have been measured for these sources. Optical
photometry and spectroscopy were obtained to search for identifications in
order to determine the physical nature of these sources. The present paper
includes new data for 24 objects; identifications are given or confirmed for 30
sources. For six sources optical finding charts showing the X-ray positions are
provided. The results from this program are summarized, showing the populations
of luminous X-ray sources in the Magellanic Clouds are quite different from
those in the Galaxy.Comment: 28 pages, 2 figures; to appear in Astronomical Journa
Geographies of the COVID-19 pandemic
The spread of the novel coronavirus (SARS-CoV-2) has resulted in the most devastating global public health crisis in over a century. At present, over 10 million people from around the world have contracted the Coronavirus Disease 2019 (COVID-19), leading to more than 500,000 deaths globally. The global health crisis unleashed by the COVID-19 pandemic has been compounded by political, economic, and social crises that have exacerbated existing inequalities and disproportionately affected the most vulnerable segments of society. The global pandemic has had profoundly geographical consequences, and as the current crisis continues to unfold, there is a pressing need for geographers and other scholars to critically examine its fallout. This introductory article provides an overview of the current special issue on the geographies of the COVID-19 pandemic, which includes 42 commentaries written by contributors from across the globe. Collectively, the contributions in this special issue highlight the diverse theoretical perspectives, methodological approaches, and thematic foci that geographical scholarship can offer to better understand the uneven geographies of the Coronavirus/COVID-19. </jats:p
New limits on a cosmological constant from statistics of gravitational lensing
We present new limits on cosmological parameters from the statistics of
gravitational lensing, based on the recently revised knowledge of the
luminosity function and internal dynamics of E/S0 galaxies that are essential
in lensing high-redshift QSOs. We find that the lens models using updated
Schechter parameters for such galaxies, derived from the recent redshift
surveys combined with morphological classification, are found to give smaller
lensing probabilities than earlier calculated. Inconsistent adoption of these
parameters from a mixture of various galaxy surveys gives rise to systematic
biases in the results. We also show that less compact dwarf-type galaxies which
largely dominate the faint part of the Schechter-form luminosity function
contribute little to lensing probabilities, so that earlier lens models
overestimate incidents of small separation lenses. Applications of the lens
models to the existing lens surveys indicate that reproduction of both the
lensing probability of optical sources and the image separations of optical and
radio lenses is significantly improved in the revised lens models. The
likelihood analyses allow us to conclude that a flat universe with
Omega=0.3(+0.2-0.1) and Omega+Lambda=1 is most preferable, and a
matter-dominated flat universe with Lambda=0 is ruled out at 98 % confidence
level. These new limits are unaffected by inclusion of uncertainties in the
lens properties.Comment: 30 pages, 9 ps figures, AASTeX, ApJ in pres
RXJ0142.0+2131: I. The galaxy content of an X-ray-luminous galaxy cluster at z=0.28
We present a photometric and spectroscopic study of stellar populations in
the X-ray-luminous cluster of galaxies RXJ0142.0+2131 at z=0.280. This paper
analyses the results of high signal-to-noise spectroscopy, as well as g'-, r'-,
and i'-band imaging, using the Gemini Multi-Object Spectrograph on Gemini
North. Of 43 spectroscopic targets, we find 30 cluster members over a range in
color. Central velocity dispersions and absorption-line strengths for lines in
the range 3700A < lambda_rest < 5800A are derived for cluster members, and are
compared with a low-redshift sample of cluster galaxies, and single stellar
population (SSP) models. We use a combination of these indicators to estimate
luminosity-weighted mean ages, metallicities ([M/H]), and alpha-element
abundance ratios ([alpha/Fe]).
RXJ0142.0+2131 is a relatively poor cluster and lacks galaxies with high
central velocity dispersions. Although the red sequence and the Faber-Jackson
relation are consistent with pure passive evolution of the early-type
population with a formation redshift of z_form = 2, the strengths of the 4000A
break and scaling relations between metal line indices and velocity dispersion
reject this model with high significance. By inverting SSP models for the
Hbeta_G, Mgb, and line indices, we calculate that, at a given velocity
dispersion and metallicity, galaxies in RXJ0142.0+2131 have luminosity-weighted
mean ages 0.14 +- 0.07 dex older than the low-redshift sample. We also find
that [alpha/Fe] in stellar populations in RXJ0142.0+2131 is 0.14 +- 0.03
greater than at low redshift. All scaling relations are consistent with these
estimated offsets. (abridged)Comment: AJ, accepted. 31 pages, 13 figures, uses emulateapj.cls.
High-resolution figures available on request from first autho
The infrared imaging spectrograph (IRIS) for TMT: the science case
The InfraRed Imaging Spectrograph (IRIS) is a first-light instrument being
designed for the Thirty Meter Telescope (TMT). IRIS is a combination of an
imager that will cover a 16.4" field of view at the diffraction limit of TMT (4
mas sampling), and an integral field unit spectrograph that will sample objects
at 4-50 mas scales. IRIS will open up new areas of observational parameter
space, allowing major progress in diverse fields of astronomy. We present the
science case and resulting requirements for the performance of IRIS.
Ultimately, the spectrograph will enable very well-resolved and sensitive
studies of the kinematics and internal chemical abundances of high-redshift
galaxies, shedding light on many scenarios for the evolution of galaxies at
early times. With unprecedented imaging and spectroscopy of exoplanets, IRIS
will allow detailed exploration of a range of planetary systems that are
inaccessible with current technology. By revealing details about resolved
stellar populations in nearby galaxies, it will directly probe the formation of
systems like our own Milky Way. Because it will be possible to directly
characterize the stellar initial mass function in many environments and in
galaxies outside of the the Milky Way, IRIS will enable a greater understanding
of whether stars form differently in diverse conditions. IRIS will reveal
detailed kinematics in the centers of low-mass galaxies, allowing a test of
black hole formation scenarios. Finally, it will revolutionize the
characterization of reionization and the first galaxies to form in the
universe.Comment: to appear in Proc. SPIE 773
Microlensing in the double quasar SBS1520+530
We present the results of a monitoring campaign of the double quasar
SBS1520+530 at Maidanak observatory from April 2003 to August 2004. We obtained
light curves in V and R filters that show small-amplitude \Delta m~0.1 mag
intrinsic variations of the quasar on time scales of about 100 days. The data
set is consistent with the previously determined time delay of \Delta
t=(130+-3) days by Burud et al. (2002). We find that the time delay corrected
magnitude difference between the quasar images is now larger by (0.14+-0.03)
mag than during the observations by Burud et al. (2002). This confirms the
presence of gravitational microlensing variations in this system.Comment: 6 pages, 7 figures. Accepted for publication in A&
Constraints on Thermal Emission Models of Anomalous X-ray Pulsars
Thermal emission from the surface of an ultramagnetic neutron star is
believed to contribute significantly to the soft X-ray flux of the Anomalous
X-ray Pulsars. We compare the detailed predictions of models of the surface
emission from a magnetar to the observed properties of AXPs. In particular, we
focus on the combination of their luminosities and energy-dependent pulsed
fractions. We use the results of recent calculations for strongly magnetized
atmospheres to obtain the angle- and energy-dependence of the surface emission.
We include in our calculations the effects of general relativistic photon
transport and interstellar extinction. We find that the combination of the
large pulsed fractions and the high luminosities of AXPs cannot be accounted
for by surface emission from a magnetar with two antipodal hot regions or a
temperature distribution characteristic of a magnetic dipole. This result is
robust for reasonable neutron star radii, for the range of magnetic field
strengths inferred from the observed spin down rates, and for surface
temperatures consistent with the spectral properties of AXPs. Models with a
single hot emitting region can reproduce the observations, provided that the
distance to one of the sources is ~30% less than the current best estimate, and
allowing for systematic uncertainties in the spectral fit of a second source.
Finally, the thermal emission models with antipodal emission geometry predict a
characteristic strong increase of the pulsed fraction with photon energy, which
is apparently inconsistent with the current data. The energy-dependence of the
pulsed fraction in the models with one hot region shows a wider range of
behavior and can be consistent with the existing data. Upcoming high-resolution
observations with Chandra and XMM-Newton will provide a conclusive test.Comment: 25 preprint pages, 7 color figures, ApJ, in pres
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