236 research outputs found
Stochastic heating of cooling flows
It is generally accepted that the heating of gas in clusters of galaxies by
active galactic nuclei (AGN) is a form of feedback. Feedback is required to
ensure a long term, sustainable balance between heating and cooling. This work
investigates the impact of proportional stochastic feedback on the energy
balance in the intracluster medium. Using a generalised analytical model for a
cluster atmosphere, it is shown that an energy equilibrium can be reached
exponentially quickly. Applying the tools of stochastic calculus it is
demonstrated that the result is robust with regard to the model parameters,
even though they affect the amount of variability in the system.Comment: 7 pages, 6 figures, accepted by MNRAS,
http://www.astro.soton.ac.uk/~gbp/pub/pavlovski_stochh.pd
Cold gas in the Intra Cluster Medium: implications for flow dynamics and powering optical nebulae
We show that the mechanical energy injection rate generated as the
intra-cluster medium (ICM) flows around cold clouds may be sufficient to power
the optical and near infra-red emission of nebulae observed in the central
regions of a sample of seven galaxy clusters. The energy injection rate is
extremely sensitive to the velocity difference between the ICM and cold clouds,
which may help to explain why optical and infra-red luminosity is often larger
than expected in systems containing AGNs. We also find that mass recycling is
likely to be important for the dynamics of the ICM. This effect will be
strongest in the central regions of clusters where there is more than enough
cold gas for its evaporation to contribute significantly to the density of the
hot phase.Comment: 8 pages, 2 figures, accepted for publication in MNRA
Investigating the properties of AGN feedback in hot atmospheres triggered by cooling-induced gravitational collapse
Radiative cooling may plausibly cause hot gas in the centre of a massive
galaxy, or galaxy cluster, to become gravitationally unstable. The subsequent
collapse of this gas on a dynamical timescale can provide an abundant source of
fuel for AGN heating and star formation. Thus, this mechanism provides a way to
link the AGN accretion rate to the global properties of an ambient cooling
flow, but without the implicit assumption that the accreted material must have
flowed onto the black hole from 10s of kiloparsecs away. It is shown that a
fuelling mechanism of this sort naturally leads to a close balance between AGN
heating and the radiative cooling rate of the hot, X-ray emitting halo.
Furthermore, AGN powered by cooling-induced gravitational instability would
exhibit characteristic duty cycles (delta) which are redolent of recent
observational findings: delta is proportional to L_X/sigma_{*}^{3}, where L_X
is the X-ray luminosity of the hot atmosphere, and sigma_{*} is the central
stellar velocity dispersion of the host galaxy. Combining this result with
well-known scaling relations, we deduce a duty cycle for radio AGN in
elliptical galaxies that is approximately proportional to M_{BH}^{1.5}, where
M_{BH} is the central black hole mass. Outburst durations and Eddington ratios
are also given. Based on the results of this study, we conclude that
gravitational instability could provide an important mechanism for supplying
fuel to AGN in massive galaxies and clusters, and warrants further
investigation.Comment: Accepted for publication in MNRAS. 8 page
Testing angular velocity as a new metric for metabolic demands of slow-moving marine fauna: a case study with Giant spider conchs Lambis truncata
BackgroundQuantifying metabolic rate in free-living animals is invaluable in understanding the costs of behaviour and movement for individuals and communities. Dynamic body acceleration (DBA) metrics, such as vectoral DBA (VeDBA), are commonly used as proxies for the energy expenditure of movement but are of limited applicability for slow-moving species. It has recently been suggested that metrics based on angular velocity might be better suited to characterise their energetics. We investigated whether a novel metric—the ‘Rate of change of Rotational Movement (RocRM)’, calculated from the vectoral sum of change in the pitch, roll and yaw/heading axes over a given length of time, is a suitable proxy for energy expenditure.ResultsWe found that RocRM can be used as an alternative energy expenditure proxy in a slow-moving benthic invertebrate. Eleven Giant spider conchs Lambis truncata (collected in the Red Sea) were instrumented with multiple channel (Daily Diary) tags and kept in sealed chambers for 5 h while their oxygen consumption, V̇O2, was measured. We found RocRM to be positively correlated with V̇O2, this relationship being affected by the time-step (i.e. the range of the calculated differential) of the RocRM. Time steps of 1, 5, 10 and 60 s yielded an explained variability of between 15 and 31%. The relationship between V̇O2 and VeDBA was not statistically significant, suggesting RocRM to provide more accurate estimations of metabolic rates in L. truncata.ConclusionsRocRM proved to be a statistically significant predictor of V̇O2 where VeDBA did not, validating the approach of using angular-based metrics over dynamic movement-based ones for slower moving animals. Further work is required to validate the use of RocRM for other species, particularly in animals with minimally dynamic movement, to better understand energetic costs of whole ecosystems. Unexplained variability in the models might be a consequence of the methodology used, but also likely a result of conch activity that does not manifest in movement of the shell. Additionally, density plots of mean RocRM at each time-step suggest differences in movement scales, which may collectively be useful as a species fingerprint of movement going forward
Mass transport by buoyant bubbles in galaxy clusters
We investigate the effect of three important processes by which AGN-blown
bubbles transport material: drift, wake transport and entrainment. The first of
these, drift, occurs because a buoyant bubble pushes aside the adjacent
material, giving rise to a net upward displacement of the fluid behind the
bubble. For a spherical bubble, the mass of upwardly displaced material is
roughly equal to half the mass displaced by the bubble, and should be ~
10^{7-9} solar masses depending on the local ICM and bubble parameters. We show
that in classical cool core clusters, the upward displacement by drift may be a
key process in explaining the presence of filaments behind bubbles. A bubble
also carries a parcel of material in a region at its rear, known as the wake.
The mass of the wake is comparable to the drift mass and increases the average
density of the bubble, trapping it closer to the cluster centre and reducing
the amount of heating it can do during its ascent. Moreover, material dropping
out of the wake will also contribute to the trailing filaments. Mass transport
by the bubble wake can effectively prevent the build-up of cool material in the
central galaxy, even if AGN heating does not balance ICM cooling. Finally, we
consider entrainment, the process by which ambient material is incorporated
into the bubble. AbridgedComment: Accepted for publication in MNRAS. 17 pages, 4 figures, 2 tables.
Formatted for letter paper and adjusted author affiliations
Observable consequences of kinetic and thermal AGN feedback in elliptical galaxies and galaxy clusters
We have constructed an analytical model of AGN feedback and studied its
implications for elliptical galaxies and galaxy clusters. The results show that
momentum injection above a critical value will eject material from low mass
elliptical galaxies, and leads to an X-ray luminosity, , that is
, depending on the AGN fuelling mechanism, where
is the velocity dispersion of the hot gas. This result agrees well
with both observations and semi-analytic models. In more massive ellipticals
and clusters, AGN outflows quickly become buoyancy-dominated. This necessarily
means that heating by a central cluster AGN redistributes the intracluster
medium (ICM) such that the mass of hot gas, within the cooling radius, should
be , where is the gravitational acceleration at the cooling radius. This
prediction is confirmed using observations of seven clusters. The same
mechanism also defines a critical ICM cooling time of Gyr, which is
in reasonable agreement with recent observations showing that star formation
and AGN activity are triggered below a universal cooling time threshold.Comment: Accepted for publication in MNRAS, 11 pages, 2 figures and 2 table
On the exclusion of intra-cluster plasma from AGN-blown bubbles
Simple arguments suggest that magnetic fields should be aligned tangentially
to the surface of an AGN-blown bubble. If this is the case, charged particles
from the fully ionised intra-cluster medium (ICM) will be prevented,
ordinarily, from crossing the boundary by the Lorentz force. However, recent
observations indicate that thermal material may occupy up to 50% of the volume
of some bubbles. Given the effect of the Lorentz force, the thermal content
must then be attributed to one, or a combination, of the following processes:
i) the entrainment of thermal gas into the AGN outflow that inflated the
bubble; ii) rapid diffusion across the magnetic field lines at the ICM/bubble
interface; iii) magnetic reconnection events which transfer thermal material
across the ICM/bubble boundary. Unless the AGN outflow behaves as a magnetic
tower jet, entrainment may be significant and could explain the observed
thermal content of bubbles. Alternatively, the cross-field diffusion
coefficient required for the ICM to fill a typical bubble is roughly 10^16 cm^2
s^-1, which is anomalously high compared to predictions from turbulent
diffusion models. Finally, the mass transfer rate due to magnetic reconnection
is uncertain, but significant for plausible reconnection rates. We conclude
that entrainment into the outflow and mass transfer due to magnetic
reconnection events are probably the most significant sources of thermal
content in AGN-blown bubbles.Comment: Accepted for publication in MNRAS, 8 pages, 1 figur
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