114 research outputs found
Structure of Ten Free N-Glycans in Ripening Tomato Fruit (Arabinose Is a Constituent of a Plant N-Glycan)
Testing the Limits of AGN Feedback and the Onset of Thermal Instability in the Most Rapidly Star-forming Brightest Cluster Galaxies
We present new, deep, narrow- and broadband Hubble Space Telescope observations of seven of the most star-forming brightest cluster galaxies (BCGs). Continuum-subtracted [OII] maps reveal the detailed, complex structure of warm (T ~ 104 K) ionized gas filaments in these BCGs, allowing us to measure spatially resolved star formation rates (SFRs) of ~60-600 M ⊙yr-1. We compare the SFRs in these systems and others from the literature to their intracluster medium cooling rates ( ), measured from archival Chandra X-ray data, finding a best-fit relation of + (-3.22 ± 0.38) with an intrinsic scatter of 0.39 ± 0.09 dex. This steeper-than-unity slope implies an increasingly efficient conversion of hot (T ~ 107 K) gas into young stars with increasing , or conversely a gradual decrease in the effectiveness of AGN feedback in the strongest cool cores. We also seek to understand the physical extent of these multiphase filaments that we observe in cluster cores. We show, for the first time, that the average extent of the multiphase gas is always smaller than the radii at which the cooling time reaches 1 Gyr, the t cool/t ff profile flattens, and that X-ray cavities are observed. This implies a close connection between the multiphase filaments, the thermodynamics of the cooling core, and the dynamics of X-ray bubbles. Interestingly, we find a one-to-one correlation between the average extent of cool multiphase filaments and the radius at which the cooling time reaches 0.5 Gyr, which may be indicative of a universal condensation timescale in cluster core
Intracluster medium of the merging cluster Abell 3395
We present a detailed imaging and spectral analysis of the merging
environment of the bimodal cluster A3395 using X-ray and radio observations.
X-ray images of the cluster show five main constituents of diffuse emission :
A3395 NE, A3395 SW, A3395 NW, A3395 W, and a filament connecting NE to W. X-ray
surface-brightness profiles of the cluster did not show any shock fronts in the
cluster. Temperature and entropy maps show high temperature and high entropy
regions in the W, the NW, the filament and between the NE and SW subclusters.
The NE, SW and W components have X-ray bolometric luminosities similar to those
of rich clusters of galaxies but have relatively higher temperatures.
Similarly, the NW component has X-ray bolometric luminosity similar to that of
isolated groups but with much higher temperature. It is, therefore, possible
that all the components of the cluster have been heated by the ongoing mergers.
The NE subcluster is the most massive and luminous constituent and other
subclusters are found to be gravitationally bound to it. The W component is
most probably either a clump of gas stripped off the SW due to ram pressure or
a separate subcluster that has merged or is merging with the SW. No X-ray
cavities are seen associated with the Wide Angle Tailed (WAT) radio source near
the centre of the SW subcluster. Minimum energy pressure in the radio
emission-peaks of the WAT galaxy is comparable with the external thermal
pressure. The radio spectrum of the WAT suggests a spectral age of ~10Myr
A Powerful AGN Outburst in RBS 797
Utilizing ks of Chandra X-ray Observatory imaging, we present an
analysis of the intracluster medium (ICM) and cavity system in the galaxy
cluster RBS 797. In addition to the two previously known cavities in the
cluster core, the new and deeper X-ray image has revealed additional structure
associated with the active galactic nucleus (AGN). The surface brightness
decrements of the two cavities are unusually large, and are consistent with
elongated cavities lying close to our line-of-sight. We estimate a total AGN
outburst energy and mean jet power of erg and
erg s, respectively, depending on the
assumed geometrical configuration of the cavities. Thus, RBS 797 is apparently
among the the most powerful AGN outbursts known in a cluster. The average mass
accretion rate needed to power the AGN by accretion alone is
yr. We show that accretion of cold gas onto the AGN at this level is
plausible, but that Bondi accretion of the hot atmosphere is probably not. The
BCG harbors an unresolved, non-thermal nuclear X-ray source with a bolometric
luminosity of erg s. The nuclear emission is
probably associated with a rapidly-accreting, radiatively inefficient accretion
flow. We present tentative evidence that star formation in the BCG is being
triggered by the radio jets and suggest that the cavities may be driving weak
shocks () into the ICM, similar to the process in the galaxy
cluster MS 0735.6+7421.Comment: Accepted to ApJ; 20 pages, 11 low-resolution figure
Cluster Radio Halos at the crossroads between astrophysics and cosmology in the SKA era
Giant Radio Halos (RH) are diffuse, Mpc-sized, synchrotron radio sources
observed in a fraction of merging galaxy clusters. The current scenario for the
origin of RHs assumes that turbulence generated during cluster mergers
re-accelerates pre-existing fossil and/or secondary electrons in the
intra-cluster-medium (ICM) to the energies necessary to produce the observed
radio emission. Moreover, more relaxed clusters could host diffuse "off state"
halos produced by secondary electrons. In this Chapter we use Monte Carlo
simulations, that combine turbulent-acceleration physics and the generation of
secondaries in the ICM, to calculate the occurrence of RHs in the Universe,
their spectral properties and connection with properties of the hosting
clusters. Predictions for SKA1 surveys are presented at low (100-300 MHz) and
mid (1-2 GHz) frequencies assuming the expected sensitivities and spatial
resolutions of SKA1. SKA1 will step into an unexplored territory allowing us to
study the formation and evolution of RHs in a totally new range of cluster
masses and redshift, allowing firm tests of the current theoretical hypothesis.
In particular, the combination of SKA1-LOW and SUR will allow the discovery of
~1000 ultrasteep- spectrum halos and to detect for the very first time "off
state" RHs. We expect that at least ~2500 giant RHs will be discovered by
SKA1-LOW surveys up to z~0.6. Remarkably these surveys will be sensitive to RHs
in a cluster mass range (down to ~10^14 solar masses) and redshifts (up to ~1)
that are unexplored by current observations. SKA1 surveys will be highly
competitive with present and future SZ-surveys in the detection of
high-redshift massive objects.Comment: 13 pages, 6 figures, to appear in proceedings of "Advancing
Astrophysics with the Square Kilometre Array" PoS(AASKA14)07
New scaling relations in cluster radio halos and the re-acceleration model
In this paper we derive new expected scaling relations for clusters with
giant radio halos in the framework of the re-acceleration scenario in a
simplified, but physically motivated, form, namely: radio power (P_R) vs size
of the radio emitting region (R_H), and P_R vs total cluster mass (M_H)
contained in the emitting region and cluster velocity dispersion (sigma_H) in
this region.
We search for these correlations by analyzing the most recent radio and X-ray
data available in the literature for a well known sample of clusters with giant
radio halos. In particular we find a good correlation between P_R and R_H and a
very tight ``geometrical'' scaling between M_H and R_H. From these correlations
P_R is also expected to scale with M_H and sigma_H and this is confirmed by our
analysis. We show that all the observed trends can be well reconciled with
expectations in the case of a slight variation of the mean magnetic field
strength in the radio halo volume with M_H. A byproduct correlation between R_H
and sigma_H is also found, and can be further tested by optical studies. In
addition, we find that observationally R_H scales non-linearly with the virial
radius of the host cluster, and this immediately means that the fraction of the
cluster volume which is radio emitting increases with cluster mass and thus
that the non-thermal component in clusters is not self-similar.Comment: 11 pages, 12 figures, accepted for publication in MNRA
Clusters of galaxies : observational properties of the diffuse radio emission
Clusters of galaxies, as the largest virialized systems in the Universe, are
ideal laboratories to study the formation and evolution of cosmic
structures...(abridged)... Most of the detailed knowledge of galaxy clusters
has been obtained in recent years from the study of ICM through X-ray
Astronomy. At the same time, radio observations have proved that the ICM is
mixed with non-thermal components, i.e. highly relativistic particles and
large-scale magnetic fields, detected through their synchrotron emission. The
knowledge of the properties of these non-thermal ICM components has increased
significantly, owing to sensitive radio images and to the development of
theoretical models. Diffuse synchrotron radio emission in the central and
peripheral cluster regions has been found in many clusters. Moreover
large-scale magnetic fields appear to be present in all galaxy clusters, as
derived from Rotation Measure (RM) studies. Non-thermal components are linked
to the cluster X-ray properties, and to the cluster evolutionary stage, and are
crucial for a comprehensive physical description of the intracluster medium.
They play an important role in the cluster formation and evolution. We review
here the observational properties of diffuse non-thermal sources detected in
galaxy clusters: halos, relics and mini-halos. We discuss their classification
and properties. We report published results up to date and obtain and discuss
statistical properties. We present the properties of large-scale magnetic
fields in clusters and in even larger structures: filaments connecting galaxy
clusters. We summarize the current models of the origin of these cluster
components, and outline the improvements that are expected in this area from
future developments thanks to the new generation of radio telescopes.Comment: Accepted for the publication in The Astronomy and Astrophysics
Review. 58 pages, 26 figure
Hot Gas in Galaxy Groups: Recent Observations
Galaxy groups are the least massive systems where the bulk of baryons begin
to be accounted for. Not simply the scaled-down versions of rich clusters
following self-similar relations, galaxy groups are ideal systems to study
baryon physics, which is important for both cluster cosmology and galaxy
formation. We review the recent observational results on the hot gas in galaxy
groups. The first part of the paper is on the scaling relations, including
X-ray luminosity, entropy, gas fraction, baryon fraction and metal abundance.
Compared to clusters, groups have a lower fraction of hot gas around the center
(e.g., r < r_2500), but may have a comparable gas fraction at large radii
(e.g., r_2500 < r < r_500). Better constraints on the group gas and baryon
fractions require sample studies with different selection functions and deep
observations at r > r_500 regions. The hot gas in groups is also iron poor at
large radii (0.3 r_500 - 0.7 r_500). The iron content of the hot gas within the
central regions (r < 0.3 r_500) correlates with the group mass, in contrast to
the trend of the stellar mass fraction. It remains to be seen where the missing
iron in low-mass groups is. In the second part, we discuss several aspects of
X-ray cool cores in galaxy groups, including their difference from cluster cool
cores, radio AGN heating in groups and the cold gas in group cool cores.
Because of the vulnerability of the group cool cores to radio AGN heating and
the weak heat conduction in groups, group cool cores are important systems to
test the AGN feedback models and the multiphase cool core models. At the end of
the paper, some outstanding questions are listed.Comment: 31 pages, 9 figures, to appear in the focus issue on "Galaxy
Clusters", New Journal of Physics,
http://iopscience.iop.org/1367-2630/focus/Focus%20on%20Galaxy%20Cluster
A giant radio halo in the cool core cluster CL1821+643
Giant radio haloes are Mpc-size sources found in some merging galaxy clusters. The synchrotron emitting electrons are thought to be (re)accelerated by plasma turbulence induced by the merging of two massive clusters. Cool core galaxy clusters have a low-temperature core, likely an indication that a major merger has not recently occurred. CL1821+643 is one of the strongest cool core clusters known so far. Surprisingly, we detect a giant radio halo with a largest linear size of ~1.1 Mpc. We discuss the radio and X-ray properties of the cluster in the framework of the proposed models for giant radio haloes. If a merger is causing the radio emission, despite the presence of a cool core, we suggest that it should be off-axis, or in an early phase, or a minor one
Extreme genetic fragility of the HIV-1 capsid
Genetic robustness, or fragility, is defined as the ability, or lack thereof, of a biological entity to maintain function in the face of mutations. Viruses that replicate via RNA intermediates exhibit high mutation rates, and robustness should be particularly advantageous to them. The capsid (CA) domain of the HIV-1 Gag protein is under strong pressure to conserve functional roles in viral assembly, maturation, uncoating, and nuclear import. However, CA is also under strong immunological pressure to diversify. Therefore, it would be particularly advantageous for CA to evolve genetic robustness. To measure the genetic robustness of HIV-1 CA, we generated a library of single amino acid substitution mutants, encompassing almost half the residues in CA. Strikingly, we found HIV-1 CA to be the most genetically fragile protein that has been analyzed using such an approach, with 70% of mutations yielding replication-defective viruses. Although CA participates in several steps in HIV-1 replication, analysis of conditionally (temperature sensitive) and constitutively non-viable mutants revealed that the biological basis for its genetic fragility was primarily the need to coordinate the accurate and efficient assembly of mature virions. All mutations that exist in naturally occurring HIV-1 subtype B populations at a frequency >3%, and were also present in the mutant library, had fitness levels that were >40% of WT. However, a substantial fraction of mutations with high fitness did not occur in natural populations, suggesting another form of selection pressure limiting variation in vivo. Additionally, known protective CTL epitopes occurred preferentially in domains of the HIV-1 CA that were even more genetically fragile than HIV-1 CA as a whole. The extreme genetic fragility of HIV-1 CA may be one reason why cell-mediated immune responses to Gag correlate with better prognosis in HIV-1 infection, and suggests that CA is a good target for therapy and vaccination strategies
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