4,129 research outputs found
A Richness Study of 14 Distant X-ray Clusters From the 160 Square Degree Survey
We have measured the surface density of galaxies toward 14 X-ray-selected
cluster candidates at redshifts greater than z=0.46, and we show that they are
associated with rich galaxy concentrations. We find that the clusters range
between Abell richness classes 0-2, and have a most probable richness class of
one. We compare the richness distribution of our distant clusters to those for
three samples of nearby clusters with similar X-ray luminosities. We find that
the nearby and distant samples have similar richness distributions, which shows
that clusters have apparently not evolved substantially in richness since
redshift z =0.5. We compare the distribution of distant X-ray clusters in the
L_x--richness plane to the distribution of optically-selected clusters from the
Palomar Distant Cluster Survey. The optically-selected clusters appear overly
rich for their X-ray luminosities when compared to X-ray-selected clusters.
Apparently, X-ray and optical surveys do not necessarily sample identical mass
concentrations at large redshifts. This may indicate the existence of a
population of optically rich clusters with anomalously low X-ray emission. More
likely, however, it reflects the tendency for optical surveys to select
unvirialized mass concentrations, as might be expected when peering along
large-scale filaments.Comment: The abstract has been abridged. Accepted for publication in the
Astrophysical Journa
Open journal systems and undergraduate research: How an open journal system can facilitate digital literacy and a research culture.
In August 2016 Leeds Beckett University launched its first open access undergraduate student journal, Critical reflections: a student journal on contemporary sociological issues. The journal is the first of its kind at the university and includes student reflections upon everyday scenarios viewed through a theoretical–sociological lens. As well as delivering many opportunities for the students, the purpose of the journal corresponds with the strategic objectives of the university as a whole. Firstly, to grow and develop a research culture by recognising the contribution of undergraduate research, and the impact this has upon the digitally literate graduate. Secondly, to allow students to capitalise upon their time at university by facilitating the experience of writing for publication, and the consequent effect upon both employability and their academic career development. The following summarises the beginnings of the journal and its relevance to the student experience, as well as outlining practical considerations for librarians, information specialists and academic staff
Simulating the Hot X-ray Emitting Gas in Elliptical Galaxies
We study the chemo-dynamical evolution of elliptical galaxies and their hot
X-ray emitting gas using high-resolution cosmological simulations. Our Tree
N-body/SPH code includes a self-consistent treatment of radiative cooling, star
formation, supernovae feedback, and chemical enrichment. We present a series of
LCDM cosmological simulations which trace the spatial and temporal evolution of
heavy element abundance patterns in both the stellar and gas components of
galaxies. X-ray spectra of the hot gas are constructed via the use of the
vmekal plasma model, and analysed using XSPEC with the XMM EPN response
function. Simulation end-products are quantitatively compared with the
observational data in both the X-ray and optical regime. We find that radiative
cooling is important to interpret the observed X-ray luminosity, temperature,
and metallicity of the interstellar medium of elliptical galaxies. However,
this cooled gas also leads to excessive star formation at low redshift, and
therefore results in underlying galactic stellar populations which are too blue
with respect to observations.Comment: 6 pages, 3 figures, to appear in the proceedings of "The IGM/Galaxy
Connection - The Distribution of Baryons at z=0", ed. M. Putman & J.
Rosenberg; High resolution version is available at
http://astronomy.swin.edu.au/staff/dkawata/research/papers.htm
A Disk Galaxy of Old Stars at z ~ 2.5
We describe observations of a galaxy in the field of the radio
galaxy 4C 23.56, photometrically selected to have a spectral-energy
distribution consistent with an old stellar population at the redshift of the
radio galaxy. Exploration of redshift--stellar-population-reddening constraints
from the photometry indicates that the galaxy is indeed at a redshift close to
that of 4C23.56, that the age of the most recent significant star formation is
roughly >~2 Gyr, and that reddening is fairly modest, with more reddening
required for the younger end of stellar age range. From analysis of a deep
adaptive-optics image of the galaxy, we find that an r^1/4-law profile, common
for local spheroidal galaxies, can be excluded quite strongly. On the other
hand, a pure exponential profile fits remarkably well, while the best fit is
given by a Sersic profile with index n=1.49. Reconstruction of the
two-dimensional form of the galaxy from the best-fit model is consistent with a
disk galaxy with neither a significant bulge component nor gross azimuthal
structure. The assembly of roughly 2L* of old stars into such a configuration
this early in the history of the universe is not easily explainable by any of
the currently popular scenarios for galaxy formation. A galaxy with these
properties would seem to require smooth but rapid infall of the large mass of
gas involved, followed by a burst of extremely vigorous and efficient star
formation in the resulting disk.Comment: 8 pages, 6 figures, emulateapj.sty, accepted for publication in The
Astrophysical Journa
Anatomy of Cirrus Clouds: Results from the Emerald Airborne Campaigns
2000 FLORIDA AVE NW, WASHINGTON, USA, DC,
2000
Cellphone Laws and Teens\u27 Calling While Driving: Analysis of Repeated Cross-Sectional Surveys in 2013, 2015, 2017, and 2019
BACKGROUND: Distracted driving among teens is a public health and safety concern. Most states in the U.S. have sought to restrict cellphone use while driving by enacting laws. This study examines the difference in prevalence of self-reported calling while driving (CWD) between states with different cellphone bans.
METHODS: Demographics and CWD data were extracted from state Youth Risk Behavior Surveys (YRBS) from 14 states in 2013, 2015, 2017, and 2019. The state YRBS is conducted every 2 years with a representative sample of 9th through 12th grade students attending public school. States were grouped by type of cellphone law(s): no ban (the absence of both handheld calling ban and young driver ban), young driver ban (a ban on all forms of cellphone use while driving, for young drivers only), or concurrent ban (a young driver ban plus a ban on handheld calling for all drivers irrespective of age). Poisson regression models with robust variance were used to estimate prevalence ratios comparing CWD prevalence across ban types.
RESULTS: In total, 157,423 high school students participated in the surveys, and 65,044 (45%) participants reached the minimum age to obtain an intermediate license and drove during the 30 days prior the survey. Approximately 53% of participants reported CWD at least once during the previous 30 days, and the percentages varied widely by states (range: 51-55%). Compared to students from states with no ban, those from states with concurrent bans were 19%(95% CI: 14-24%) less likely to engage in CWD. Students in states with concurrent bans were 23% less likely to engage in CWD compared to students in states with young driver bans (95% CI:17-27%).
CONCLUSIONS: Engaging in CWD is common among teen drivers. The concurrent implementation of a handheld calling ban and a young driver ban was associated with a lower prevalence of CWD
Isolated, Massive Supergiants near the Galactic Center
We have carried out a pilot project to assess the feasibility of using radio,
infrared, and X-ray emission to identify young, massive stars located between 1
and 25 pc from the Galactic center. We first compared catalogs compiled from
the Very Large Array, the Chandra X-ray Observatory, and 2MASS. We identified
two massive, young stars: the previously-identified star that is associated
with the radio HII region H2, and a newly-identified star that we refer to as
CXOGC J174516.1-290315. The infrared spectra of both stars exhibit very strong
Br-gamma and He I lines, and resemble those of massive supergiants that have
evolved off of the main sequence, but not yet reached the Wolf-Rayet phase. We
estimate that each star has a bolometric luminosity >10^6 L_sun. The detection
of these two sources in X-rays is surprising, because stars at similar
evolutionary states are not uniformly bright X-ray sources. Therefore, we
suggest that both stars are in binary systems that contain either OB stars
whose winds collide with those of the luminous supergiants, or compact objects
that are accreting from the winds of the supergiants. We also identify X-ray
emission from a nitrogen-type Wolf-Rayet star and place upper limits on the
X-ray luminosities of three more evolved, massive stars that previously have
been identified between 1 and 25 pc from Sgr A*. Finally, we briefly discuss
the implications that future searches for young stars will have for our
understanding of the recent history of star formation near the Galactic center.
(abridged)Comment: 9 pages, including 8 figures. Submitted to ApJ, and modified in
response to referee's repor
Hydrodynamical description of a hadron-quark first-order phase transition
Solutions of hydrodynamical equations are presented for the equation of state
of the Var der Waals type allowing for the first order phase transition.
Attention is focused on description of the hadron-quark phase transition in
heavy ion collisions. It is shown that fluctuations dissolve and grow as if the
fluid is effectively very viscous. Even in spinodal region germs are growing
slowly due to viscosity and critical slowing down. This prevents enhancement of
fluctuations in the near-critical region, which is frequently considered as a
signal of the critical point in heavy ion collisions.Comment: 4 pages, 4 figure
Gravitational Stability of Circumnuclear Disks in Elliptical Galaxies
A significant fraction of nearby elliptical galaxies are known to have high
density gas disks in their circumnuclear (CN) region (0.1 to a few kpc). Yet,
ellipticals, especially luminous ones, show little signs of recent star
formation (SF). To investigate the possible cause of the dearth of SF in these
systems, we study the gravitational stability of CN gas disks embedded within
the potentials of both the stellar bulge and the central massive black hole
(BH) in ellipticals. We find that CN disks in higher mass galaxies are
generally more stable than those in lower mass galaxies, because higher mass
galaxies tend to have more massive BHs and more centrally concentrated stellar
density profiles. We also consider the case in which the central stellar
density profile has a core, which is often observed for ellipticals whose total
stellar mass is higher than about 10^11 Msun. Such a cored stellar density
profile leads to more unstable CN disks than the power-law density profile
characteristic of less massive galaxies. However, the more massive BHs in
high-mass galaxies act to stabilize the CN disk. Our results demonstrate that
the gravitational potentials of both the central BH and the stellar component
should be taken into account when studying the properties of CN disks, as their
stability is sensitive to both the BH mass and the stellar density profile. Our
results could explain the observed trend that less luminous ellipticals have a
greater tendency to exhibit ongoing SF than giant ellipticals.Comment: 8 pages, 5 figures, accepted for publication in Ap
Hierarchical Formation of Galaxies with Dynamical Response to Supernova-Induced Gas removal
We reanalyze the formation and evolution of galaxies in the hierarchical
clustering scenario. Using a semi-analytic model (SAM) of galaxy formation
described in this paper, which we hereafter call the Mitaka model, we
extensively investigate the observed scaling relations of galaxies among
photometric, kinematic, structural and chemical characteristics. In such a
scenario, spheroidal galaxies are assumed to be formed by major merger and
subsequent starburst, in contrast to the traditional scenario of monolithic
cloud collapse. As a new ingredient of SAMs, we introduce the effects of
dynamical response to supernova-induced gas removal on size and velocity
dispersion, which play an important role on dwarf galaxy formation. In previous
theoretical studies of dwarf galaxies based on the monolithic cloud collapse
given by Yoshii & Arimoto and Dekel & Silk, the dynamical response was treated
in the extremes of a purely baryonic cloud and a baryonic cloud fully supported
by surrounding dark matter. To improve this simple treatment, in our previous
paper, we formulated the dynamical response in more realistic, intermediate
situations between the above extremes. While the effects of dynamical response
depend on the mass fraction of removed gas from a galaxy, how much amount of
the gas remains just after major merger depends on the star formation history.
A variety of star formation histories are generated through the Monte Carlo
realization of merging histories of dark halos, and it is found that our SAM
naturally makes a wide variety of dwarf galaxies and their dispersed
characteristics as observed. (Abridged)Comment: 24 pages including 29 figures, using emulateapj.cls; accepted for
publication in Ap
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