2,233 research outputs found
The Detectability of AGN Cavities in Cooling-Flow Clusters
Chandra X-ray Observatory has revealed X-ray cavities in many nearby cooling
flow clusters. The cavities trace feedback from the central active galactic
nulceus (AGN) on the intracluster medium (ICM), an important ingredient in
stabilizing cooling flows and in the process of galaxy formation and evolution.
But, the prevalence and duty cycle of such AGN outbursts is not well
understood. To this end, we study how the cooling is balanced by the cavity
heating for a complete sample of clusters (the Brightest 55 clusters of
galaxies, hereafter B55). In the B55, we found 33 cooling flow clusters, 20 of
which have detected X-ray bubbles in their ICM. Among the remaining 13, all
except Ophiuchus could have significant cavity power yet remain undetected in
existing images. This implies that the duty cycle of AGN outbursts with
significant heating potential in cooling flow clusters is at least 60 % and
could approach 100 %, but deeper data is required to constrain this further.Comment: 4 pages, 2 figures; to appear in the proceedings of "The Monsters'
Fiery Breath", Madison, Wisconsin 1-5 June 2009, Eds. Sebastian Heinz & Eric
Wilcots; added annotation to the figur
Jet Interactions with the Hot Halos of Clusters and Galaxies
X-ray observations of cavities and shock fronts produced by jets streaming
through hot halos have significantly advanced our understanding of the
energetics and dynamics of extragalactic radio sources. Radio sources at the
centers of clusters have dynamical ages between ten and several hundred million
years. They liberate between 1E58-1E62 erg per outburst, which is enough energy
to regulate cooling of hot halos from galaxies to the richest clusters. Jet
power scales approximately with the radio synchrotron luminosity to the one
half power. However, the synchrotron efficiency varies widely from nearly unity
to one part in 10,000, such that relatively feeble radio source can have
quasar-like mechanical power. The synchrotron ages of cluster radio sources are
decoupled from their dynamical ages, which tend to be factors of several to
orders of magnitude older. Magnetic fields and particles in the lobes tend to
be out of equipartition. The lobes may be maintained by heavy particles (e.g.,
protons), low energy electrons, a hot, diffuse thermal gas, or possibly
magnetic (Poynting) stresses. Sensitive X-ray images of shock fronts and
cavities can be used to study the dynamics of extragalactic radio sources.Comment: 10 pages, 3 figures, invited review, "Extragalactic Jets: Theory and
Observation from Radio to Gamma Ray, held in Girdwood, Alaska, U.S.A. 21-24
May, 2007, minor text changes; one added referenc
A Deep Chandra Observation of the AGN Outburst and Merger in Hickson Compact Group 62
We report on an analysis of new Chandra data of the galaxy group HCG 62, well
known for possessing cavities in its intragroup medium (IGM) that were inflated
by the radio lobes of its central active galactic nucleus (AGN). With the new
data, a factor of three deeper than previous Chandra data, we re-examine the
energetics of the cavities and determine new constraints on their contents. We
confirm that the ratio of radiative to mechanical power of the AGN outburst
that created the cavities is less than 10^-4, among the lowest of any known
cavity system, implying that the relativistic electrons in the lobes can supply
only a tiny fraction of the pressure required to support the cavities. This
finding implies additional pressure support in the lobes from heavy particles
(e.g., protons) or thermal gas. Using spectral fits to emission in the
cavities, we constrain any such volume-filling thermal gas to have a
temperature kT > 4.3 keV. For the first time, we detect X-ray emission from the
central AGN, with a luminosity of L(2-10 keV) = (1.1 +/- 0.4) x 10^39 erg s^-1
and properties typical of a low-luminosity AGN. Lastly, we report evidence for
a recent merger from the surface brightness, temperature, and metallicity
structure of the IGM.Comment: Accepted to MNRAS, 14 pages, 9 figure
A technique to record the sedentary to walk movement during free living mobility : a comparison of healthy and stroke populations
Background
Hesitation between moving from a sedentary posture (lying/sitting) to walking is a characteristic of
mobility impaired individuals, as identified from laboratory studies. Knowing the extent to which this
hesitation occurs during everyday life would benefit rehabilitation research. This study aimed to
quantify this transition hesitation through a novel approach to analysing data from a physical activity
monitor based on a tri-axial accelerometer and compare results from two populations; stroke
patients and age-matched unimpaired controls.
Methods
Stroke patients living at home with early supported discharge (n=34, 68.9YO ± 11.8) and age matched
controls (n=30, 66.8YO ± 10.5) wore a physical activity monitor for 48hrs. The outputs from
the monitor were then used to determine the transitions from sedentary to walking. The time delay
between a sedentary posture ending and the start of walking classified four transition types: 1)
fluent (<=2s), 2) hesitant (>2s<=10s), 3) separated (>10s) and 4) a change from sedentary with no
registered walking to a return to sedentary.
Results
Control participants initiated walking after a sedentary posture on 92% of occasions. Most
commonly (43%) this was a fluent transition. In contrast stroke patients walked after changing from
a sedentary posture on 68% of occasions with only 9% of transitions classed as fluent, (p<0.05).
Discussion/Conclusion
A new data analysis technique reports the frequency of walking following a change in sedentary
position in stroke patients and healthy controls and characterises this transition according to the
time delay before walking. This technique creates opportunities to explore everyday mobility in
greater depth
An Energetic AGN Outburst Powered by a Rapidly Spinning Supermassive Black Hole or an Accreting Ultramassive Black Hole
Powering the 10^62 erg nuclear outburst in the MS0735.6+7421 cluster central
galaxy by accretion implies that its supermassive black hole (SMBH) grew by
~6x10^8 solar masses over the past 100 Myr. We place upper limits on the amount
of cold gas and star formation near the nucleus of <10^9 solar masses and <2
solar masses per year, respectively. These limits imply that an implausibly
large fraction of the preexisting cold gas in the bulge must have been consumed
by its SMBH at the rate of ~3-5 solar masses per year while leaving no trace of
star formation. Such a high accretion rate would be difficult to maintain by
stellar accretion or the Bondi mechanism, unless the black hole mass approaches
10^11 solar masses. Its feeble nuclear luminosities in the UV, I, and X-ray
bands compared to its enormous mechanical power are inconsistent with rapid
accretion onto a ~5x10^9 solar mass black hole. We suggest instead that the AGN
outburst is powered by a rapidly-spinning black hole. A maximally-spinning,
10^9 solar mass black hole contains enough rotational energy, ~10^62 erg, to
quench a cooling flow over its lifetime and to contribute significantly to the
excess entropy found in the hot atmospheres of groups and clusters. Two modes
of AGN feedback may be quenching star formation in elliptical galaxies centered
in cooling halos at late times. An accretion mode that operates in gas-rich
systems, and a spin mode operating at modest accretion rates. The spin
conjecture may be avoided in MS0735 by appealing to Bondi accretion onto a
central black hole whose mass greatly exceeds 10^10 solar mass. The host
galaxy's unusually large, 3.8 kpc stellar core radius (light deficit) may
witness the presence of an ultramassive black hole.Comment: Accepted for publication in ApJ. Modifications: adopted slightly
higher black hole mass using Lauer's M_SMBH vs L_bulge relation and adjusted
related quantities; considered more seriously the consequences of a
ultramassive black hole, motivated by new Kormendy & Bender paper published
after our submission; other modifications per referee comments by Ruszkowsk
Exploring Zeptosecond Quantum Equilibration Dynamics: From Deep-Inelastic to Fusion-Fission Outcomes in Ni+Ni Reactions
Energy dissipative processes play a key role in how quantum many-body systems
dynamically evolve towards equilibrium. In closed quantum systems, such
processes are attributed to the transfer of energy from collective motion to
single-particle degrees of freedom; however, the quantum many-body dynamics of
this evolutionary process are poorly understood. To explore energy dissipative
phenomena and equilibration dynamics in one such system, an experimental
investigation of deep-inelastic and fusion-fission outcomes in the
Ni+Ni reaction has been carried out. Experimental outcomes have
been compared to theoretical predictions using Time Dependent Hartree Fock and
Time Dependent Random Phase Approximation approaches, which respectively
incorporate one-body energy dissipation and fluctuations. Excellent
quantitative agreement has been found between experiment and calculations,
indicating that microscopic models incorporating one-body dissipation and
fluctuations provide a potential tool for exploring dissipation in low-energy
heavy ion collisions.Comment: 11 pages, 9 figures, 1 table, including Supplemental Material -
Version accepted for publication in Physical Review Letter
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
Leveraging 3D Technology for Students with Autism: An innovative university-community collaboration for skill development and vocational exploration
This article describes a university-community collaboration in which an inter-professional team partnered to provide students with autism spectrum disorder (ASD) a paid job opportunity to apply 3D modelling skills for a local construction company. Providing meaningful vocational opportunities to improve the transition to adulthood for individuals with ASD is imperative, as individuals with ASD have unemployment rates that are some of the highest of all disabilities. This novel evidence-supported educational program was designed to develop 3D technology skills, explore vocational careers and promote social engagement through shared interests for transition-age youth with ASD. Both parents and students reported many successful outcomes, including increase in student self-confidence, social and technology skill development and the opportunity for vocational exploration by these young people. Implications of the case study are reported in relation to university-community partnerships and the critical role of community collaboration in addressing the high rates of unemployment in individuals with autism
A study of high-redshift AGN feedback in SZ cluster samples
We present a study of active galactic nucleus (AGN) feedback at higher redshifts (0.3 < z < 1.2) using Sunyaev-Zel'dovich selected samples of clusters from the South Pole Telescope and Atacama Cosmology Telescope surveys. In contrast to studies of nearby systems, we do not find a separation between cooling flow (CF) clusters and non-CF clusters based on the radio luminosity of the central radio source (cRS). This lack may be due to the increased incidence of galaxy-galaxy mergers at higher redshift that triggers AGN activity. In support of this scenario, we find evidence for evolution in the radio-luminosity function of the cRS, while the lower luminosity sources do not evolve much, the higher luminosity sources show a strong increase in the frequency of their occurrence at higher redshifts. We interpret this evolution as an increase in high-excitation radio galaxies (HERGs) in massive clusters at z > 0.6, implying a transition from HERG-mode accretion to lower power low-excitation radio galaxy (LERG)-mode accretion at intermediate redshifts. Additionally, we use local radio-to-jet power scaling relations to estimate feedback power and find that half of the CF systems in our sample probably have enough heating to balance cooling. However, we postulate that the local relations are likely not well suited to predict feedback power in high-luminosity HERGs, as they are derived from samples composed mainly of lower luminosity LERGs
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