143 research outputs found
Online Midwinter Seminar (OMS) #1 Report
A report on the Mythopoeic Society\u27s first annual Online Midwinter Seminar (OMS) by Ben Dressler
Galaxy Harassment and the Evolution of Clusters of Galaxies
Disturbed spiral galaxies with high rates of star formation pervaded clusters
of galaxies just a few billion years ago, but nearby clusters exclude spirals
in favor of ellipticals. ``Galaxy harassment" (frequent high speed galaxy
encounters) drives the morphological transformation of galaxies in clusters,
provides fuel for quasars in subluminous hosts and leaves detectable debris
arcs. Simulated images of harassed galaxies are strikingly similar to the
distorted spirals in clusters at observed by the Hubble Space
Telescope.Comment: Submitted to Nature. Latex file, 7 pages, 10 photographs in gif and
jpeg format included. 10 compressed postscript figures and text available
using anonymous ftp from ftp://ftp-hpcc.astro.washington.edu/pub/hpcc/moore/
(mget *) Also available at http://www-hpcc.astro.washington.edu/papers
The fraction of early-type galaxies in low redshift groups and clusters of galaxies
We examine the fraction of early-type (and spiral) galaxies found in groups
and clusters of galaxies as a function of dark matter halo mass. We use
morphological classifications from the Galaxy Zoo project matched to halo
masses from both the C4 cluster catalogue and the Yang et al (2007) group
catalogue. We find that the fraction of early-type (or spiral) galaxies remains
constant (changing by less than 10%) over three orders of magnitude in halo
mass (13<log MH/Msol/h<15.8). This result is insensitive to our choice of halo
mass measure, from velocity dispersions or summed optical luminosity.
Furthermore, we consider the morphology-halo mass relations in bins of galaxy
stellar mass M*, and find that while the trend of constant fraction remains
unchanged, the early-type fraction amongst the most massive galaxies (11<log
M*/Msol/h <12) is a factor of three greater than lower mass galaxies
(10<logM*/Msol/h<10.7). We compare our observational results with those of
simulations presented in De Lucia et al (2011), as well as previous
observational analyses, and semi-analytic bulge (or disc) dominated galaxies
from the Millennium Simulation. We find the simulations recover similar trends
as observed, but may over-predict the abundances of the most massive bulge
dominated (early-type) galaxies. Our results suggest that most morphological
transformation is happening on the group scale before groups merge into massive
clusters. However, we show that within each halo a morphology-density relation
remains: it is summing the total fraction to a self-similar scaled radius which
results in a flat morphology-halo mass relationship.Comment: 9 page, 5 figures, modified to match accepted version (MNRAS
Merger and Ring Galaxy Formation Rates at z<=2
We compare the observed merger rate of galaxies over cosmic time and the
frequency of collisional ring galaxies (CRGs), with analytic models and halo
merger and collision rates from a large cosmological simulation. In the Lambda
cold dark matter (LCDM) model we find that the cosmic {\it merger fraction}
does not evolve strongly between 0.2<z<2, implying that the observed decrease
of the cosmic star formation rate since z~1 might not be tied to a disappearing
population of major mergers. Halos hosting massive galaxies undergo on average
~2 mergers from z~2 up to present day, reflecting the late assembly time for
the massive systems and the related downsizing problem. The cosmic {\it merger
rate} declines with redshift: at the present time it is a factor of 10 lower
than at z~2, in reasonable agreement with the current available data. The rate
of CRG formation derived from the interactions between halo progenitors up to
z=2 is found to be a good tracer of the cosmic merger rate. In the LCDM model
the rate of CRGs as well as the merger rate do not scale as (1+z)^m, as
suggested by previous models. Our predictions of cosmic merger and CRG rates
may be applied to forthcoming surveys such as GOODS and zCOSMOS.Comment: Accepted for a publication on MNRAS. More references added and a
comparison with previous work
Galaxy Zoo: Passive Red Spirals
We study the spectroscopic properties and environments of red spiral galaxies
found by the Galaxy Zoo project. By carefully selecting face-on, disk dominated
spirals we construct a sample of truly passive disks (not dust reddened, nor
dominated by old stellar populations in a bulge). As such, our red spirals
represent an interesting set of possible transition objects between normal blue
spirals and red early types. We use SDSS data to investigate the physical
processes which could have turned these objects red without disturbing their
morphology. Red spirals prefer intermediate density regimes, however there are
no obvious correlations between red spiral properties and environment -
environment alone is not sufficient to determine if a spiral will become red.
Red spirals are a small fraction of spirals at low masses, but are a
significant fraction at large stellar masses - massive galaxies are red
independent of morphology. We confirm that red spirals have older stellar popns
and less recent star formation than the main spiral population. While the
presence of spiral arms suggests that major star formation cannot have ceased
long ago, we show that these are not recent post-starbursts, so star formation
must have ceased gradually. Intriguingly, red spirals are ~4 times more likely
than normal spirals to host optically identified Seyfert or LINER, with most of
the difference coming from LINERs. We find a curiously large bar fraction in
the red spirals suggesting that the cessation of star formation and bar
instabilities are strongly correlated. We conclude by discussing the possible
origins. We suggest they may represent the very oldest spiral galaxies which
have already used up their reserves of gas - probably aided by strangulation,
and perhaps bar instabilities moving material around in the disk.Comment: MNRAS in press, 20 pages, 15 figures (v3
Anticoagulation in Patients With COVID-19: JACC Review Topic of the Week.
Clinical, laboratory, and autopsy findings support an association between coronavirus disease-2019 (COVID-19) and thromboembolic disease. Acute COVID-19 infection is characterized by mononuclear cell reactivity and pan-endothelialitis, contributing to a high incidence of thrombosis in large and small blood vessels, both arterial and venous. Observational studies and randomized trials have investigated whether full-dose anticoagulation may improve outcomes compared with prophylactic dose heparin. Although no benefit for therapeutic heparin has been found in patients who are critically ill hospitalized with COVID-19, some studies support a possible role for therapeutic anticoagulation in patients not yet requiring intensive care unit support. We summarize the pathology, rationale, and current evidence for use of anticoagulation in patients with COVID-19 and describe the main design elements of the ongoing FREEDOM COVID-19 Anticoagulation trial, in which 3,600 hospitalized patients with COVID-19 not requiring intensive care unit level of care are being randomized to prophylactic-dose enoxaparin vs therapeutic-dose enoxaparin vs therapeutic-dose apixaban. (FREEDOM COVID-19 Anticoagulation Strategy [FREEDOM COVID]; NCT04512079).Dr Farkouh has received research grants from Amgen, Novo Nordisk,
and Novartis. Dr Stone has received speaker honoraria from Infraredx; has served as a consultant to Valfix, TherOx, Robocath, HeartFlow, Ablative Solutions, Miracor, Neovasc, Abiomed, Ancora,
Vectorious, Elucid Bio, Occlutech, CorFlow, Apollo Therapeutics,
Impulse Dynamics, Cardiomech, Gore, and Amgen; and has equity/
options from Ancora, Cagent, Applied Therapeutics, Biostar family of
funds, SpectraWave, Orchestra Biomed, Aria, Cardiac Success, Valfix,
and Xenter. Dr Godoy is supported by the Frederick Banting and
Charles Best Canada Graduate Scholarship (Doctoral Research Award)
from the Canadian Institutes of Health Research. All other authors
have reported that they have no relationships relevant to the contents of this paper to disclose.S
Conditional deletion of WT1 in the septum transversum mesenchyme causes congenital diaphragmatic hernia in mice
Congenital diaphragmatic hernia (CDH) is a severe birth defect. Wt1-null mouse embryos develop CDH but the mechanisms regulated by WT1 are unknown. We have generated a murine model with conditional deletion of WT1 in the lateral plate mesoderm, using the G2 enhancer of the Gata4 gene as a driver. 80% of G2-Gata4Cre;Wt1fl/fl embryos developed typical Bochdalek-type CDH. We show that the posthepatic mesenchymal plate coelomic epithelium gives rise to a mesenchyme that populates the pleuroperitoneal folds isolating the pleural cavities before the migration of the somitic myoblasts. This process fails when Wt1 is deleted from this area. Mutant embryos show Raldh2 downregulation in the lateral mesoderm, but not in the intermediate mesoderm. The mutant phenotype was partially rescued by retinoic acid treatment of the pregnant females. Replacement of intermediate by lateral mesoderm recapitulates the evolutionary origin of the diaphragm in mammals. CDH might thus be viewed as an evolutionary atavismEspaña, Ministerio de EconomĂa BFU2014- 52299-PJunta de AndalucĂa P11-CTS-0756
Galaxy Zoo: The Environmental Dependence of Bars and Bulges in Disc Galaxies
We present an analysis of the environmental dependence of bars and bulges in
disc galaxies, using a volume-limited catalogue of 15810 galaxies at z<0.06
from the Sloan Digital Sky Survey with visual morphologies from the Galaxy Zoo
2 project. We find that the likelihood of having a bar, or bulge, in disc
galaxies increases when the galaxies have redder (optical) colours and larger
stellar masses, and observe a transition in the bar and bulge likelihoods, such
that massive disc galaxies are more likely to host bars and bulges. We use
galaxy clustering methods to demonstrate statistically significant
environmental correlations of barred, and bulge-dominated, galaxies, from
projected separations of 150 kpc/h to 3 Mpc/h. These environmental correlations
appear to be independent of each other: i.e., bulge-dominated disc galaxies
exhibit a significant bar-environment correlation, and barred disc galaxies
show a bulge-environment correlation. We demonstrate that approximately half
(50 +/- 10%) of the bar-environment correlation can be explained by the fact
that more massive dark matter haloes host redder disc galaxies, which are then
more likely to have bars. Likewise, we show that the environmental dependence
of stellar mass can only explain a small fraction (25 +/- 10%) of the
bar-environment correlation. Therefore, a significant fraction of our observed
environmental dependence of barred galaxies is not due to colour or stellar
mass dependences, and hence could be due to another galaxy property. Finally,
by analyzing the projected clustering of barred and unbarred disc galaxies with
halo occupation models, we argue that barred galaxies are in slightly
higher-mass haloes than unbarred ones, and some of them (approximately 25%) are
satellite galaxies in groups. We also discuss implications about the effects of
minor mergers and interactions on bar formation.Comment: 20 pages, 18 figures; references updated; published in MNRA
Ultraviolet Imaging of the z=0.23 Cluster Abell 2246
We present deep ultraviolet observations of a field containing the cluster
Abell 2246 (z=0.225) which provide far-ultraviolet (FUV) images of some of the
faintest galaxies yet observed in that bandpass. Abell 2246 lies within the
field of view of Ultraviolet Imaging Telescope (UIT) observations of the quasar
HS1700+64, which accumulated over 7100 seconds of UIT FUV exposure time during
the Astro-2 mission in March 1995. For objects found on both the FUV and
ground-based V-band images, we obtain FUV (l ~ 1520 A) photometry and V-band
photometry, as well as mid-UV (l ~ 2490 A) photometry from UIT Astro-1
observations and ground-based I-band photometry. We find five objects in the
images which are probably galaxies at the distance of Abell 2246, with FUV
magnitudes (m(FUV)) between 18.6 and 19.6, and V magnitudes between 18.4 and
19.6. We find that their absolute FUV fluxes and colors imply strongly that
they are luminous galaxies with significant current star formation, as well as
some relatively recent, but not current, (> 400 Myr ago) star formation. We
interpret the colors of these five objects by comparing them with local
objects, redshift-corrected template spectra and stellar population models,
finding that they are plausibly matched by 10-Gyr-old population models with
decaying star formation, with decay time constants in the range 3 Gyr < t < 5
Gyr, with an additional color component from a single burst of moderate ( ~
400-500 Myr) age. From derived FUV luminosities we compute current star
formation rates. We compare the UV properties of Abell 2246 with those of the
Coma cluster, finding that Abell 2246 has significantly more recent star
formation, consistent with the Butcher-Oemler phenomenon.Comment: Accepted for publication in the Astronomical Journal, June 1998. 17
Pages AAS latex, includes 4 bitmap .jpg format images and 4 other figures.
PDF, Embedded Gzipped PS version (1.9Mb) TeX source and figures available at
http://www.astro.virginia.edu/~bd4r/galaxies.htm
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