1,486 research outputs found
Chaotic cold accretion onto black holes
Using 3D AMR simulations, linking the 50 kpc to the sub-pc scales over the
course of 40 Myr, we systematically relax the classic Bondi assumptions in a
typical galaxy hosting a SMBH. In the realistic scenario, where the hot gas is
cooling, while heated and stirred on large scales, the accretion rate is
boosted up to two orders of magnitude compared with the Bondi prediction. The
cause is the nonlinear growth of thermal instabilities, leading to the
condensation of cold clouds and filaments when t_cool/t_ff < 10. Subsonic
turbulence of just over 100 km/s (M > 0.2) induces the formation of thermal
instabilities, even in the absence of heating, while in the transonic regime
turbulent dissipation inhibits their growth (t_turb/t_cool < 1). When heating
restores global thermodynamic balance, the formation of the multiphase medium
is violent, and the mode of accretion is fully cold and chaotic. The recurrent
collisions and tidal forces between clouds, filaments and the central clumpy
torus promote angular momentum cancellation, hence boosting accretion. On
sub-pc scales the clouds are channelled to the very centre via a funnel. A good
approximation to the accretion rate is the cooling rate, which can be used as
subgrid model, physically reproducing the boost factor of 100 required by
cosmological simulations, while accounting for fluctuations. Chaotic cold
accretion may be common in many systems, such as hot galactic halos, groups,
and clusters, generating high-velocity clouds and strong variations of the AGN
luminosity and jet orientation. In this mode, the black hole can quickly react
to the state of the entire host galaxy, leading to efficient self-regulated AGN
feedback and the symbiotic Magorrian relation. During phases of overheating,
the hot mode becomes the single channel of accretion (with a different cuspy
temperature profile), though strongly suppressed by turbulence.Comment: Accepted by MNRAS: added comments and references. Your feedback is
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Where does the gas fueling star formation in BCGs originate?
We investigate the relationship between X-ray cooling and star formation in
brightest cluster galaxies (BCGs). We present an X-ray spectral analysis of the
inner regions, 10-40 kpc, of six nearby cool core clusters (z<0.35) observed
with Chandra ACIS. This sample is selected on the basis of the high star
formation rate (SFR) observed in the BCGs. We restrict our search for cooling
gas to regions that are roughly cospatial with the starburst. We fit single-
and multi-temperature mkcflow models to constrain the amount of isobarically
cooling intracluster medium (ICM). We find that in all clusters, below a
threshold temperature ranging between 0.9 and 3 keV, only upper limits can be
obtained. In four out of six objects, the upper limits are significantly below
the SFR and in two, namely A1835 and A1068, they are less than a tenth of the
SFR. Our results suggests that a number of mechanisms conspire to hide the
cooling signature in our spectra. In a few systems the lack of a cooling
signature may be attributed to a relatively long delay time between the X-ray
cooling and the star burst. However, for A1835 and A1068, where the X-ray
cooling time is shorter than the timescale of the starburst, a possible
explanation is that the region where gas cools out of the X-ray phase extends
to very large radii, likely beyond the core of these systems.Comment: to appear in A&
On the connection between turbulent motions and particle acceleration in galaxy clusters
Giant radio halos are Mpc-scale diffuse radio sources associated with the
central regions of galaxy clusters. The most promising scenario to explain the
origin of these sources is that of turbulent re-acceleration, in which MeV
electrons injected throughout the formation history of galaxy clusters are
accelerated to higher energies by turbulent motions mostly induced by cluster
mergers. In this Letter, we use the amplitude of density fluctuations in the
intracluster medium as a proxy for the turbulent velocity and apply this
technique to a sample of 51 clusters with available radio data. Our results
indicate a segregation in the turbulent velocity of radio halo and radio quiet
clusters, with the turbulent velocity of the former being on average higher by
about a factor of two. The velocity dispersion recovered with this technique
correlates with the measured radio power through the relation , which implies that the radio power is
nearly proportional to the turbulent energy rate. Our results provide an
observational confirmation of a key prediction of the turbulent re-acceleration
model and possibly shed light on the origin of radio halos.Comment: Submitted to ApJ Letter
Unification of X-ray winds in Seyfert galaxies: from ultra-fast outflows to warm absorbers
The existence of ionized X-ray absorbing layers of gas along the line of
sight to the nuclei of Seyfert galaxies is a well established observational
fact. This material is systematically outflowing and shows a large range in
parameters. However, its actual nature and dynamics are still not clear. In
order to gain insights into these important issues we performed a literature
search for papers reporting the parameters of the soft X-ray warm absorbers
(WAs) in 35 type 1 Seyferts and compared their properties to those of the
ultra-fast outflows (UFOs) detected in the same sample. The fraction of sources
with WAs is >60%, consistent with previous studies. The fraction of sources
with UFOs is >34%, >67% of which also show WAs. The large dynamic range
obtained when considering all the absorbers together allows us, for the first
time, to investigate general relations among them. In particular, we find
significant correlations indicating that the closer the absorber is to the
central black hole, the higher the ionization, column, outflow velocity and
consequently the mechanical power. The absorbers continuously populate the
whole parameter space, with the WAs and the UFOs lying always at the two ends
of the distribution. This strongly suggest that these absorbers, often
considered of different types, could actually represent parts of a single
large-scale stratified outflow observed at different locations from the black
hole. The observed parameters and correlations are consistent with both
radiation pressure through Compton scattering and MHD processes contributing to
the outflow acceleration, the latter playing a major role. Most of the
absorbers, especially the UFOs, have a sufficiently high mechanical power to
significantly contribute to AGN feedback.Comment: Manuscript updated to match the MNRAS published version. Link to the
related INAF news: http://www.media.inaf.it/2013/02/05/warm-absorbers
The stripping of a galaxy group diving into the massive cluster A2142
Structure formation in the current Universe operates through the accretion of
group-scale systems onto massive clusters. The detection and study of such
accreting systems is crucial to understand the build-up of the most massive
virialized structures we see today. We report the discovery with XMM-Newton of
an irregular X-ray substructure in the outskirts of the massive galaxy cluster
Abell 2142. The tip of the X-ray emission coincides with a concentration of
galaxies. The bulk of the X-ray emission of this substructure appears to be
lagging behind the galaxies and extends over a projected scale of at least 800
kpc. The temperature of the gas in this region is 1.4 keV, which is a factor of
~4 lower than the surrounding medium and is typical of the virialized plasma of
a galaxy group with a mass of a few 10^13M_sun. For this reason, we interpret
this structure as a galaxy group in the process of being accreted onto the main
dark-matter halo. The X-ray structure trailing behind the group is due to gas
stripped from its original dark-matter halo as it moves through the
intracluster medium (ICM). This is the longest X-ray trail reported to date.
For an infall velocity of ~1,200 km s-1 we estimate that the stripped gas has
been surviving in the presence of the hot ICM for at least 600 Myr, which
exceeds the Spitzer conduction timescale in the medium by a factor of >~400.
Such a strong suppression of conductivity is likely related to a tangled
magnetic field with small coherence length and to plasma microinstabilities.
The long survival time of the low-entropy intragroup medium suggests that the
infalling material can eventually settle within the core of the main cluster.Comment: 11 pages, 7 figures, accepted for publication in A&
A textbook example of ram-pressure stripping in the Hydra A/A780 cluster
In the current epoch, one of the main mechanisms driving the growth of galaxy clusters is the continuous accretion of group-scale halos. In this process, the ram pressure applied by the hot intracluster medium on the gas content of the infalling group is responsible for stripping the gas from its dark-matter halo, which gradually leads to the virialization of the infalling gas in the potential well of the main cluster. Using deep wide-field observations of the poor cluster Hydra A/A780 with XMM-Newton and Suzaku, we report the discovery of an infalling galaxy group 1.1 Mpc south of the cluster core. The presence of a substructure is confirmed by a dynamical study of the galaxies in this region. A wake of stripped gas is trailing behind the group over a projected scale of 760 kpc. The temperature of the gas along the wake is constant at kT ~ 1.3 keV, which is about a factor of two less than the temperature of the surrounding plasma. We observe a cold front pointing westwards compared to the peak of the group, which indicates that the group is currently not moving in the direction of the main cluster, but is moving along an almost circular orbit. The overall morphology of the group bears remarkable similarities with high-resolution numerical simulations of such structures, which greatly strengthens our understanding of the ram-pressure stripping process
Human leucocyte antigen diversity: a biological gift to escape infections, no longer a barrier for haploidentical hemopoietic stem cell transplantation
Since the beginning of life, every multicellular organism appeared to have a complex innate immune system although the adaptive immune system, centred on lymphocytes bearing antigen receptors generated by somatic recombination, arose in jawed fish approximately 500 million years ago. The major histocompatibility complex MHC, named the Human leucocyte antigen (HLA) system in humans, represents a vital function structure in the organism by presenting pathogen-derived peptides to T cells as the main initial step of the adaptive immune response. The huge level of polymorphism observed in HLA genes definitely reflects selection, favouring heterozygosity at the individual or population level, in a pathogen-rich environment, although many are located in introns or in exons that do not code for the antigen-biding site of the HLA. Over the past three decades, the extent of allelic diversity at HLA loci has been well characterized using high-resolution HLA-DNA typing and the number of new HLA alleles, produced through next-generation sequencing methods, is even more rapidly increasing. The level of the HLA system polymorphism represents an obstacle to the search of potential compatible donors for patients affected by haematological disease proposed for a hematopoietic stem cell transplant (HSCT). Data reported in literature clearly show that antigenic and/or allelic mismatches between related or unrelated donors and patients influences the successful HSCT outcome. However, the recent development of the new transplant strategy based on the choice of haploidentical donors for HSCT is questioning the role of HLA compatibility, since the great HLA disparities present do not worsen the overall clinical outcome. Nowadays, NGS has contributed to define at allelic levels the HLA polymorphism and solve potential ambiguities. However, HLA functions and tissue typing probably need to be further investigated in the next future, to understand the reasons why in haploidentical transplants the presence of a whole mismatch haplotype between donors and recipients, both the survival rate and the incidence of acute GvHD or graft rejection are similar to those reported for unrelated HSCTs
Deep Chandra observations of the stripped galaxy group falling into Abell 2142
In the local Universe, the growth of massive galaxy clusters mainly operates
through the continuous accretion of group-scale systems. The infalling group in
Abell 2142 is the poster child of such an accreting group, and as such, it is
an ideal target to study the astrophysical processes induced by structure
formation. We present the results of a deep (200 ks) observation of this
structure with Chandra, which highlights the complexity of this system in
exquisite detail. In the core of the group, the spatial resolution of Chandra
reveals the presence of a leading edge and a complex AGN-induced activity. The
morphology of the stripped gas tail appears straight in the innermost 250 kpc,
suggesting that magnetic draping efficiently shields the gas from its
surroundings. However, beyond kpc from the core, the tail flares and
the morphology becomes strongly irregular, which could be explained by a
breaking of the drape, e.g. because of turbulent motions. The power spectrum of
surface-brightness fluctuations is relatively flat (),
which indicates that thermal conduction is strongly inhibited even beyond the
region where magnetic draping is effective. The amplitude of density
fluctuations in the tail is consistent with a mild level of turbulence with a
Mach number . Overall, our results show that the processes
leading to the thermalization and mixing of the infalling gas are slow and
relatively inefficient.Comment: Accepted for publication in A&
Deep Chandra observations of the stripped galaxy group falling into Abell 2142
In the local Universe, the growth of massive galaxy clusters mainly operates through the continuous accretion of group-scale systems. The infalling group in Abell 2142 is the poster child of such an accreting group, and as such, it is an ideal target to study the astrophysical processes induced by structure formation. We present the results of a deep (200 ks) observation of this structure with Chandra that highlights the complexity of this system in exquisite detail. In the core of the group, the spatial resolution of Chandra reveals a leading edge and complex AGN-induced activity. The morphology of the stripped gas tail appears straight in the innermost 250 kpc, suggesting that magnetic draping efficiently shields the gas from its surroundings. However, beyond ~ 300 kpc from the core, the tail flares and the morphology becomes strongly irregular, which could be explained by a breaking of the drape, for example, caused by turbulent motions. The power spectrum of surface-brightness fluctuations is relatively flat (P2D ∝ k⁻²∙³ which indicates that thermal conduction is strongly inhibited even beyond the region where magnetic draping is effective. The amplitude of density fluctuations in the tail is consistent with a mild level of turbulence with a Mach number M3D ~ 0:1 -0:25. Overall, our results show that the processes leading to the thermalization and mixing of the infalling gas are slow and relatively inefficient
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