289 research outputs found
Chaotic cold accretion on to black holes in rotating atmospheres
Chaotic cold accretion (CCA) profoundly differs from classic black hole
accretion models. Using 3D high-resolution simulations, we probe the impact of
rotation on the hot and cold accretion flow in a typical massive galaxy. In the
hot mode, with or without turbulence, the pressure-dominated flow forms a
geometrically thick rotational barrier, suppressing the accretion rate to ~1/3
of the Bondi rate. When radiative cooling is dominant, the gas loses pressure
support and quickly circularizes in a cold thin disk. In the more common state
of a turbulent and heated atmosphere, CCA drives the dynamics if the gas
velocity dispersion exceeds the rotational velocity, i.e., turbulent Taylor
number < 1. Extended multiphase filaments condense out of the hot phase via
thermal instability and rain toward the black hole, boosting the accretion rate
up to 100 times the Bondi rate. Initially, turbulence broadens the angular
momentum distribution of the hot gas, allowing the cold phase to condense with
prograde or retrograde motion. Subsequent chaotic collisions between the cold
filaments, clouds, and a clumpy variable torus promote the cancellation of
angular momentum, leading to high accretion rates. The simulated sub-Eddington
accretion rates cover the range inferred from AGN cavity observations. CCA
predicts inner flat X-ray temperature and density profiles, as
recently discovered in M 87 and NGC 3115. The synthetic H{\alpha} images
reproduce the main features of cold gas observations in massive ellipticals, as
the line fluxes and the filaments versus disk morphology. Such dichotomy is key
for the long-term AGN feedback cycle. As gas cools, filamentary CCA develops
and boosts AGN heating; the cold mode is thus reduced and the rotating disk
remains the sole cold structure. Its consumption leaves the atmosphere in hot
mode with suppressed accretion and feedback, reloading the cycle.Comment: 18 pages, 21 figures, published in A&A; fully revised version with
new major results related to H{\alpha} and X-ray observation
Spitzer Observations of Transient, Extended Dust in Two Elliptical Galaxies: New Evidence of Recent Feedback Energy Release in Galactic Cores
Spitzer observations of extended dust in two optically normal elliptical
galaxies provide a new confirmation of buoyant feedback outflow in the hot gas
atmospheres around these galaxies. AGN feedback energy is required to prevent
wholesale cooling and star formation in these group-centered galaxies. In NGC
5044 we observe interstellar (presumably PAH) emission at 8 microns out to
about 5 kpc. Both NGC 5044 and 4636 have extended 70 microns emission from cold
dust exceeding that expected from stellar mass loss. The sputtering lifetime of
this extended dust in the ~1keV interstellar gas, ~10^7 yrs, establishes the
time when the dust first entered the hot gas. Evidently the extended dust
originated in dusty disks or clouds, commonly observed in elliptical galaxy
cores, that were disrupted, heated and buoyantly transported outward. The
surviving central dust in NGC 5044 and 4636 has been disrupted into many small
filaments. It is remarkable that the asymmetrically extended 8 micron emission
in NGC 5044 is spatially coincident with Halpha+[NII] emission from warm gas. A
calculation shows that dust-assisted cooling in buoyant hot gas moving out from
the galactic core can cool within a few kpc in about ~10^7 yrs, explaining the
optical line emission observed. The X-ray images of both galaxies are
disturbed. All timescales for transient activity - restoration of equilibrium
and buoyant transport in the hot gas, dynamics of surviving dust fragments, and
dust sputtering - are consistent with a central release of feedback energy in
both galaxies about 10^7 yrs ago.Comment: 13 pages. Accepted by ApJ; minor typos correcte
PAH Spectroscopy from 1-5 m
The PAH model predicts many weak emission features in the 1-5 m region
that can resolve significant questions that it has faced since its inception in
the mid-80s. These features contain fundamental information about the PAH
population that is inaccessible via the much stronger PAH bands in the 5-20
m region. Apart from the 3.3 m band and plateau, PAH spectroscopy
across most of the 1-5 m region has been unexplored due to its low
intrinsic intensity. ISO and Akari covered some of this wavelength range, but
lacked the combined sensitivity and resolution to measure the predicted bands
with sufficient fidelity. The spectroscopic capabilities of the NIRSpec
instrument on board JWST will make it possible to measure and fully
characterize many of the PAH features expected in this region. These include
the fundamental, overtone and combination C-D and CN stretching bands
of deuterated PAHs, cyano-PAHs (PAH-C N), and the overtones and
combinations of the strong PAH bands that dominate the 5-20 m region.
These bands will reveal the amount of D tied up in PAHs, the PAH D/H ratio, the
D distribution between PAH aliphatic and aromatic subcomponents, and delineate
key stages in PAH formation and evolution on an object-by-object basis and
within extended objects. If cyano-PAHs are present, these bands will also
reveal the amount of cyano groups tied up in PAHs, determine the N/C ratio
within that PAH subset, and distinguish between the bands near 4.5 m that
arise from CD versus CN.Comment: 13 page, 5 figures, 2 table
AMUSE-VIRGO. III: mid-infrared photometry of early-type galaxies and limits on obscured nuclear emission
We complete our census of low-level nuclear activity in Virgo Cluster
early-type galaxies by searching for obscured emission using Spitzer Space
Telescope mid-infrared (MIR) imaging at 24mu. Of a total sample of 95
early-type galaxies, 53 objects are detected, including 16 showing
kiloparsec-scale dust in optical images. One dimensional and two dimensional
surface photometry of the 37 detections without extended dust features reveals
that the MIR light is more centrally concentrated than the optical light as
traced by Hubble Space Telescope F850LP-band images. No such modeling was
performed for the sources with dust detected in the optical images. We explore
several possible sources of the MIR excess emission, including obscured nuclear
emission. We find that radial metallicity gradients in the stellar population
appear to be a natural and most likely explanation for the observed behavior in
a majority of the sources. Alternatively, if the concentrated MIR emission were
due to nuclear activity, it would imply a MIR-to-X luminosity ratio ~5-10 for
the low luminosity AGN detected in X-rays by our survey. This ratio is an order
of magnitude larger than that of typical low-luminosity AGN and would imply an
unusual spectral energy distribution. We conclude that the black holes found by
our survey in quiescent early-type galaxies in Virgo have low bolometric
Eddington ratios arising from low accretion rates and/or highly radiatively
inefficient accretion.Comment: 17 pages, 8 figures; accepted for publication in Ap
Deep Chandra Observations of A2495: A Possible Sloshing-regulated Feedback Cycle in a Triple-offset Galaxy Cluster
We present the analysis of new, deep Chandra observations (130 ks) of the galaxy cluster A2495. This object is known for the presence of a triple offset between the peaks of the intracluster medium (ICM), the brightest cluster galaxy (BCG), and the warm gas glowing in Hα line. The new Chandra data confirm that the X-ray emission peak is located at a distance of ∼6.2 kpc from the BCG, and at ∼3.9 kpc from the Hα emission peak. Moreover, we identify two generations of X-ray cavities in the ICM, likely inflated by the central radio galaxy activity. Through a detailed morphological and spectral analysis, we determine that the power of the active galactic nucleus (AGN) outbursts (P cav = 4.7 ± 1.3 × 1043 erg s−1) is enough to counterbalance the radiative losses from ICM cooling (L cool = 5.7 ± 0.1 × 1043 erg s−1). This indicates that, despite a fragmented cooling core, A2495 still harbors an effective feedback cycle. We argue that the offsets are most likely caused by sloshing of the ICM, supported by the presence of spiral structures and a probable cold front in the gas at ∼58 kpc east of the center. Ultimately, we find that the outburst interval between the two generations of X-ray cavities is of the order of the dynamical sloshing timescale, as already hinted from the previous Chandra snapshot. We thus speculate that sloshing may be able to regulate the timescales of AGN feedback in A2495, by periodically fueling the central AGN
A BCG with Offset Cooling:Is the AGN Feedback Cycle Broken in A2495?
We present a combined radio/X-ray analysis of the poorly studied galaxy cluster A2495 (z = 0.07923) based on new EVLA and Chandra data. We also analyze and discuss Hα emission and optical continuum data retrieved from the literature. We find an offset of ∼6 kpc between the cluster brightest cluster galaxy (BCG) (MCG+02-58-021) and the peak of the X-ray emission, suggesting that the cooling process is not taking place on the central galaxy nucleus. We propose that sloshing of the intracluster medium (ICM) could be responsible for this separation. Furthermore, we detect a second, ∼4 kpc offset between the peak of the Hα emission and that of the X-ray emission. Optical images highlight the presence of a dust filament extending up to ∼6 kpc in the cluster BCG and allow us to estimate a dust mass within the central 7 kpc of 1.7 × 105 M . Exploiting the dust-to-gas ratio and the L Hα-M mol relation, we argue that a significant amount (up to 109 M ) of molecular gas should be present in the BCG of this cluster. We also investigate the presence of ICM depressions, finding two putative systems of cavities; the inner pair is characterized by t age ∼ 18 Myr and P cav ∼ 1.2 × 1043 erg s-1, the outer one by t age ∼ 53 Myr and P cav ∼ 5.6 × 1042 erg s-1. Their age difference appears to be consistent with the freefall time of the central cooling gas and with the offset timescale estimated with the Hα kinematic data, suggesting that sloshing is likely playing a key role in this environment. Furthermore, the cavities' power analysis shows that the active galactic nucleus energy injection is able to sustain the feedback cycle, despite cooling being offset from the BCG nucleus.</p
AGN Driven Weather and Multiphase Gas in the Core of the NGC 5044 Galaxy Group
A deep Chandra observation of the X-ray bright group, NGC 5044, shows that
the central region of this group has been strongly perturbed by repeated AGN
outbursts. These recent AGN outbursts have produced many small X-ray cavities,
cool filaments and cold fronts. We find a correlation between the coolest X-ray
emitting gas and the morphology of the Ha filaments. The Ha filaments are
oriented in the direction of the X-ray cavities, suggesting that the warm gas
responsible for the Halpha emission originated near the center of NGC 5044 and
was dredged up behind the buoyant, AGN-inflated X-ray cavities. A detailed
spectroscopic analysis shows that the central region of NGC 5044 contains
spatially varying amounts of multiphase gas. The regions with the most
inhomogeneous gas temperature distribution tend to correlate with the extended
235 MHz and 610 MHz radio emission detected by the GMRT. This may result from
gas entrainment within the radio emitting plasma or mixing of different
temperature gas in the regions surrounding the radio emitting plasma by AGN
induced turbulence. Accounting for the effects of multiphase gas, we find that
the abundance of heavy elements is fairly uniform within the central 100 kpc,
with abundances of 60-80% solar for all elements except oxygen, which has a
significantly sub-solar abundance. In the absence of continued AGN outbursts,
the gas in the center of NGC 5044 should attain a more homogeneous distribution
of gas temperature through the dissipation of turbulent kinetic energy and heat
conduction in approximately 10e8 yr. The presence of multiphase gas in NGC 5044
indicates that the time between recent AGN outbursts has been less than
approximately 10e8 yr
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