554 research outputs found
A volume-limited sample of X-ray galaxy groups and clusters - II. X-ray cavity dynamics
We present the results of our study of a volume-limited sample (z <= 0.071)
of 101 X-ray galaxy groups and clusters, in which we explore the X-ray cavity
energetics. Out of the 101 sources in our parent sample, X-ray cavities are
found in 30 of them, all of which have a central cooling time of less than3
Gyr. New X-ray cavities are detected in three sources. We focus on the subset
of sources that have a central cooling time of less than 3 Gyr, whose active
galactic nucleus (AGN) duty cycle is approximately 61 percent (30/49). This
rises to over 80 percent for a central cooling time of less than 0.5 Gyr. When
projection effects and central radio source detection rates are considered, the
actual duty cycle is probably much higher. In addition, we show that data
quality strongly affects the detection rates of X-ray cavities. After
calculating the cooling luminosity and cavity powers of each source with
cavities, it is evident that the bubbling process induced by the central AGN
has to be, on average, continuous, to offset cooling. We find that the radius
of the cavities, r, loosely depends on the ambient gas temperature as T^0.5,
above about 1.5 keV, with much more scatter below that temperature. Finally, we
show that, at a given location in a group or cluster, larger bubbles travel
faster than smaller ones. This means that the bubbles seen at larger distances
from cluster cores could be the result of the merging of several smaller
bubbles, produced in separate AGN cycles.Comment: Accepted for publication in MNRAS; 26 pages (including 10 pages of
images), 8 figures, 2 tables. Higher resolution images will be available as
online materia
Inhibitors of \u3cem\u3eN\u3csup\u3eα\u3c/sup\u3e\u3c/em\u3e-acetyl-l-ornithine Deacetylase: Synthesis, Characterization and Analysis of their Inhibitory Potency
A series of N α-acyl (alkyl)- and N α-alkoxycarbonyl-derivatives of l- and d-ornithine were prepared, characterized, and analyzed for their potency toward the bacterial enzyme N α-acetyl-l-ornithine deacetylase (ArgE). ArgE catalyzes the conversion of N α-acetyl-l-ornithine to l-ornithine in the fifth step of the biosynthetic pathway for arginine, a necessary step for bacterial growth. Most of the compounds tested provided IC50 values in the μM range toward ArgE, indicating that they are moderately strong inhibitors. N α-chloroacetyl-l-ornithine (1g) was the best inhibitor tested toward ArgE providing an IC50 value of 85 μM while N α-trifluoroacetyl-l-ornithine (1f), N α-ethoxycarbonyl-l-ornithine (2b), and N α-acetyl-d-ornithine (1a) weakly inhibited ArgE activity providing IC50 values between 200 and 410 μM. Weak inhibitory potency toward Bacillus subtilis-168 for N α-acetyl-d-ornithine (1a) and N α-fluoro- (1f), N α-chloro- (1g), N α-dichloro- (1h), and N α-trichloroacetyl-ornithine (1i) was also observed. These data correlate well with the IC50 values determined for ArgE, suggesting that these compounds might be capable of getting across the cell membrane and that ArgE is likely the bacterial enzymatic target
Probing the extreme realm of AGN feedback in the massive galaxy cluster, RX J1532.9+3021
We present a detailed Chandra, XMM-Newton, VLA and HST analysis of one of the
strongest cool core clusters known, RX J1532.9+3021 (z=0.3613). Using new, deep
90 ks Chandra observations, we confirm the presence of a western X-ray cavity
or bubble, and report on a newly discovered eastern X-ray cavity. The total
mechanical power associated with these AGN-driven outflows is (22+/-9)*10^44
erg/s, and is sufficient to offset the cooling, indicating that AGN feedback
still provides a viable solution to the cooling flow problem even in the
strongest cool core clusters. Based on the distribution of the optical
filaments, as well as a jet-like structure seen in the 325 MHz VLA radio map,
we suggest that the cluster harbours older outflows along the north to south
direction. The jet of the central AGN is therefore either precessing or
sloshing-induced motions have caused the outflows to change directions. There
are also hints of an X-ray depression to the north aligned with the 325 MHz
jet-like structure, which might represent the highest redshift ghost cavity
discovered to date. We further find evidence of a cold front (r=65kpc) that
coincides with the outermost edge of the western X-ray cavity and the edge of
the radio mini-halo. The common location of the cold front with the edge of the
radio mini-halo supports the idea that the latter originates from electrons
being reaccelerated due to sloshing induced turbulence. Alternatively, its
coexistence with the edge of the X-ray cavity may be due to cool gas being
dragged out by the outburst. We confirm that the central AGN is highly
sub-Eddington and conclude that a >10^10M_Sun or a rapidly spinning black hole
is favoured to explain both the radiative-inefficiency of the AGN and the
powerful X-ray cavities.Comment: Accepted for publication to ApJ (minor corrections), 16 pages, 16
figures, 5 tables. Full resolution at http://www.stanford.edu/~juliehl/M1532
Compressive strength, microstructure and hydration products of hybrid alkaline cements
Ordinary Portland cement (OPC) is the dominant binder in the construction industry with a global
production that currently reaches a total of 3 Gt per year. As a consequence, the cement industry’s
contribution to the total worldwide CO2
emissions is of about 7% of the total emissions. Publications
on the field of alkali-activated binders (also termed geopolymers), state that this new material is,
potentially, likely to become an alternative to Portland cement. However, recent LCA studies show
that the environmental performance of alkali-activated binders depends, to great extent, of their
composition. Also, researchers report that these binders can be produced in a more eco-efficient
manner if the use of sodium silicate is avoided. This is due to the fact that the referred component
is associated to a high carbon footprint. Besides, most alkali-activated cements suffer from severe
efflorescence, a reaction originated by the fact that the alkaline and/or soluble silicates that are added
during processing cannot be totally consumed. This paper presents experimental results on hybrid
alkaline cements. The compressive strength results and the efflorescence observations show that some
of the new mixes already exhibit a promising performance
Is there a giant Kelvin-Helmholtz instability in the sloshing cold front of the Perseus cluster?
Deep observations of nearby galaxy clusters with Chandra have revealed concave ‘bay’ structures in a number of systems (Perseus, Centaurus and Abell 1795), which have similar X-ray and radio properties. These bays have all the properties of cold fronts, where the temperature rises and density falls sharply, but are concave rather than convex. By comparing to simulations of gas sloshing, we find that the bay in the Perseus cluster bears a striking resemblance in its size, location and thermal structure, to a giant (≈50 kpc) roll resulting from Kelvin–Helmholtz instabilities. If true, the morphology of this structure can be compared to simulations to put constraints on the initial average ratio of the thermal and magnetic pressure, β = pth/pB, throughout the overall cluster before the sloshing occurs, for which we find β = 200 to best match the observations. Simulations with a stronger magnetic field (β = 100) are disfavoured, as in these the large Kelvin–Helmholtz rolls do not form, while in simulations with a lower magnetic field (β = 500), the level of instabilities is much larger than is observed. We find that the bay structures in Centaurus and Abell 1795 may also be explained by such features of gas sloshing
The Evolution of the Intracluster Medium Metallicity in Sunyaev-Zel'dovich-Selected Galaxy Clusters at 0 < z < 1.5
We present the results of an X-ray spectral analysis of 153 galaxy clusters
observed with the Chandra, XMM-Newton, and Suzaku space telescopes. These
clusters, which span 0 < z < 1.5, were drawn from a larger, mass-selected
sample of galaxy clusters discovered in the 2500 square degree South Pole
Telescope Sunyaev Zel'dovich (SPT-SZ) survey. With a total combined exposure
time of 9.1 Ms, these data yield the strongest constraints to date on the
evolution of the metal content of the intracluster medium (ICM). We find no
evidence for strong evolution in the global (r<R500) ICM metallicity (dZ/dz =
-0.06 +/- 0.04 Zsun), with a mean value at z=0.6 of = 0.23 +/- 0.01 Zsun
and a scatter of 0.08 +/- 0.01 Zsun. These results imply that >60% of the
metals in the ICM were already in place at z=1 (at 95% confidence), consistent
with the picture of an early (z>1) enrichment. We find, in agreement with
previous works, a significantly higher mean value for the metallicity in the
centers of cool core clusters versus non-cool core clusters. We find weak
evidence for evolution in the central metallicity of cool core clusters (dZ/dz
= -0.21 +/- 0.11 Zsun), which is sufficient to account for this enhanced
central metallicity over the past ~10 Gyr. We find no evidence for metallicity
evolution outside of the core (dZ/dz = -0.03 +/- 0.06 Zsun), and no significant
difference in the core-excised metallicity between cool core and non-cool core
clusters. This suggests that strong radio-mode AGN feedback does not
significantly alter the distribution of metals at r>0.15R500. Given the
limitations of current-generation X-ray telescopes in constraining the ICM
metallicity at z>1, significant improvements on this work will likely require
next-generation X-ray missions.Comment: 11 pages, 8 figures, 2 tables. Submitted to ApJ. Comments welcome
Very Large Array observations of the mini-halo and AGN feedback in the Phoenix cluster
(Abridged) The relaxed cool-core Phoenix cluster (SPT-CL J2344-4243) features
an extremely strong cooling flow, as well as a mini-halo. Strong star-formation
in the brightest cluster galaxy indicates that AGN feedback has been unable to
inhibit this cooling flow. We have studied the strong cooling flow in the
Phoenix cluster by determining the radio properties of the AGN and its lobes.
In addition, we use spatially resolved observations to investigate the origin
of the mini-halo. We present new Very Large Array 1-12 GHz observations of the
Phoenix cluster which resolve the AGN and its lobes in all four frequency
bands, and resolve the mini-halo in L- and S-band. Using our L-band
observations, we measure the total flux density of the radio lobes at 1.5 GHz
to be mJy, and the flux density of the mini-halo to be
mJy. Using L- and X-band images, we produce the first spectral index maps of
the lobes from the AGN and measure the spectral indices of the northern and
southern lobes to be and , respectively.
Similarly, using L- and S-band data, we map the spectral index of the
mini-halo, and obtain an integrated spectral index of .
We find that the mini-halo is most likely formed by turbulent re-acceleration
powered by sloshing in the cool core due to a recent merger. In addition, we
find that the feedback in the Phoenix cluster is consistent with the picture
that stronger cooling flows are to be expected for massive clusters like the
Phoenix cluster, as these may feature an underweight supermassive black hole
due to their merging history. Strong time variability of the AGN on
Myr-timescales may help explain the disconnection between the radio and the
X-ray properties of the system. Finally, a small amount of jet precession
likely contributes to the relatively low ICM re-heating efficiency of the
mechanical feedback.Comment: 12 pages, 14 figures. Accepted for publication in A&
The Evolution of Environmental Quenching Timescales to
Using a sample of 4 galaxy clusters at and 10 galaxy
clusters at , we measure the environmental quenching
timescale, , corresponding to the time required after a galaxy is accreted
by a cluster for it to fully cease star formation. Cluster members are selected
by a photometric-redshift criterion, and categorized as star-forming,
quiescent, or intermediate according to their dust-corrected rest-frame colors
and magnitudes. We employ a "delayed-then-rapid" quenching model that relates a
simulated cluster mass accretion rate to the observed numbers of each type of
galaxy in the cluster to constrain . For galaxies of mass , we find a quenching timescale of 1.24 Gyr
in the cluster sample, and 1.50 Gyr at . Using values
drawn from the literature, we compare the redshift evolution of to
timescales predicted for different physical quenching mechanisms. We find
to depend on host halo mass such that quenching occurs over faster timescales
in clusters relative to groups, suggesting that properties of the host halo are
responsible for quenching high-mass galaxies. Between and , we
find that evolves faster than the molecular gas depletion timescale and
slower than an SFR-outflow timescale, but is consistent with the evolution of
the dynamical time. This suggests that environmental quenching in these
galaxies is driven by the motion of satellites relative to the cluster
environment, although due to uncertainties in the atomic gas budget at high
redshift, we cannot rule out quenching due to simple gas depletion
- …