5,906 research outputs found
Solving the Cooling Flow Problem through Mechanical AGN Feedback
Unopposed radiative cooling of plasma would lead to the cooling catastrophe,
a massive inflow of condensing gas, manifest in the core of galaxies, groups
and clusters. The last generation X-ray telescopes, Chandra and XMM, have
radically changed our view on baryons, indicating AGN heating as the balancing
counterpart of cooling. This work reviews our extensive investigation on
self-regulated heating. We argue that the mechanical feedback, based on massive
subrelativistic outflows, is the key to solving the cooling flow problem, i.e.
dramatically quenching the cooling rates for several Gyr without destroying the
cool-core structure. Using a modified version of the 3D hydrocode FLASH, we
show that bipolar AGN outflows can further reproduce fundamental observed
features, such as buoyant bubbles, weak shocks, metals dredge- up, and
turbulence. The latter is an essential ingredient to drive nonlinear thermal
instabilities, which cause the formation of extended cold gas, a residual of
the quenched cooling flow and, later, fuel for the feedback engine. Compared to
clusters, groups and galaxies require a gentler mechanical feedback, in order
to avoid catastrophic overheating. We highlight the essential characteristics
for a realistic AGN feedback, with emphasis on observational consistency.Comment: Accepted by AN; 4 pages, 2 figure
The self-regulated AGN feedback loop: the role of chaotic cold accretion
Supermassive black hole accretion and feedback play central role in the
evolution of galaxies, groups, and clusters. I review how AGN feedback is
tightly coupled with the formation of multiphase gas and the newly probed
chaotic cold accretion (CCA). In a turbulent and heated atmosphere, cold clouds
and kpc-scale filaments condense out of the plasma via thermal instability and
rain toward the black hole. In the nucleus, the recurrent chaotic collisions
between the cold clouds, filaments, and central torus promote angular momentum
cancellation or mixing, boosting the accretion rate up to 100 times the Bondi
rate. The rapid variability triggers powerful AGN outflows, which quench the
cooling flow and star formation without destroying the cool core. The AGN
heating stifles the formation of multiphase gas and accretion, the feedback
subsides and the hot halo is allowed to cool again, restarting a new cycle.
Ultimately, CCA creates a symbiotic link between the black hole and the whole
host via a tight self-regulated feedback which preserves the gaseous halo in
global thermal equilibrium throughout cosmic time.Comment: 4 pages, 1 figure; accepted for publication (IAUS 319
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
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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IRS II: a framework and infrastructure for semantic web services
In this paper we describe IRS–II (Internet Reasoning Service) a framework and implemented infrastructure, whose main goal is to support the publication, location, composition and execution of heterogeneous web services, augmented with semantic descriptions of their functionalities. IRS–II has three main classes of features which distinguish it from other work on semantic web services. Firstly, it supports one-click publishing of standalone software: IRS–II automatically creates the appropriate wrappers, given pointers to the standalone code. Secondly, it explicitly distinguishes between tasks (what to do) and methods (how to achieve tasks) and as a result supports capability-driven service invocation; flexible mappings between services and problem specifications; and dynamic, knowledge-based service selection. Finally, IRS–II services are web service compatible – standard web services can be trivially published through the IRS–II and any IRS–II service automatically appears as a standard web service to other web service infrastructures. In the paper we illustrate the main functionalities of IRS–II through a scenario involving a distributed application in the healthcare domain
The Impact of Radio AGN Bubble Composition on the Dynamics and Thermal Balance of the Intracluster Medium
Feeding and feedback of active galactic nuclei (AGN) are critical for
understanding the dynamics and thermodynamics of the intracluster medium (ICM)
within the cores of galaxy clusters. While radio bubbles inflated by AGN jets
could be dynamically supported by cosmic rays (CRs), the impact of CR-dominated
jets are not well understood. In this work, we perform three-dimensional
simulations of CR-jet feedback in an isolated cluster atmosphere; we find that
CR jets impact the multiphase gas differently than jets dominated by kinetic
energy. In particular, CR bubbles can more efficiently uplift the cluster gas
and cause an outward expansion of the hot ICM. Due to adiabatic cooling from
the expansion and less efficient heating from CR bubbles by direct mixing, the
ICM is more prone to local thermal instabilities, which will later enhance
chaotic cold accretion onto the AGN. The amount of cold gas formed during the
bubble formation and its late-time evolution sensitively depend on whether CR
transport processes are included or not. We also find that low-level, subsonic
driving of turbulence by AGN jets holds for both kinetic and CR jets;
nevertheless, the kinematics is consistent with the Hitomi measurements.
Finally, we carefully discuss the key observable signatures of each bubble
model, focusing on gamma-ray emission (and related comparison with Fermi), as
well as thermal Sunyaev-Zel'dovich constraints.Comment: accepted to Ap
Spectral Energy Distribution Mapping of Two Elliptical Galaxies on sub-kpc scales
We use high-resolution Herschel-PACS data of 2 nearby elliptical galaxies,
IC1459 & NGC2768 to characterize their dust and stellar content. IC1459 &
NGC2768 have an unusually large amount of dust for elliptical galaxies (1-3 x
10^5 Msun), this dust is also not distributed along the stellar content. Using
data from GALEX (ultraviolet) to PACS (far-infrared), we analyze the spectral
energy distribution (SED) of these galaxies with CIGALEMC as a function of the
projected position, binning images in 7.2" pixels. From this analysis, we
derive maps of SED parameters, such as the metallicity, the stellar mass, the
fraction of young star and the dust mass. The larger amount of dust in FIR maps
seems related in our model to a larger fraction of young stars which can reach
up to 4% in the dustier area. The young stellar population is fitted as a
recent (~ 0.5 Gyr) short burst of star formation for both galaxies. The
metallicities, which are fairly large at the center of both galaxies, decrease
with the radial distance with fairly steep gradient for elliptical galaxies.Comment: 14 pages, 26 figures, to be published in Ap
Know Your Enemy: Stealth Configuration-Information Gathering in SDN
Software Defined Networking (SDN) is a network architecture that aims at
providing high flexibility through the separation of the network logic from the
forwarding functions. The industry has already widely adopted SDN and
researchers thoroughly analyzed its vulnerabilities, proposing solutions to
improve its security. However, we believe important security aspects of SDN are
still left uninvestigated. In this paper, we raise the concern of the
possibility for an attacker to obtain knowledge about an SDN network. In
particular, we introduce a novel attack, named Know Your Enemy (KYE), by means
of which an attacker can gather vital information about the configuration of
the network. This information ranges from the configuration of security tools,
such as attack detection thresholds for network scanning, to general network
policies like QoS and network virtualization. Additionally, we show that an
attacker can perform a KYE attack in a stealthy fashion, i.e., without the risk
of being detected. We underline that the vulnerability exploited by the KYE
attack is proper of SDN and is not present in legacy networks. To address the
KYE attack, we also propose an active defense countermeasure based on network
flows obfuscation, which considerably increases the complexity for a successful
attack. Our solution offers provable security guarantees that can be tailored
to the needs of the specific network under consideratio
LineSwitch: Efficiently Managing Switch Flow in Software-Defined Networking while Effectively Tackling DoS Attacks
Software Defined Networking (SDN) is a new networking architecture which aims
to provide better decoupling between network control (control plane) and data
forwarding functionalities (data plane). This separation introduces several
benefits, such as a directly programmable and (virtually) centralized network
control. However, researchers showed that the required communication channel
between the control and data plane of SDN creates a potential bottleneck in the
system, introducing new vulnerabilities. Indeed, this behavior could be
exploited to mount powerful attacks, such as the control plane saturation
attack, that can severely hinder the performance of the whole network.
In this paper we present LineSwitch, an efficient and effective solution
against control plane saturation attack. LineSwitch combines SYN proxy
techniques and probabilistic blacklisting of network traffic. We implemented
LineSwitch as an extension of OpenFlow, the current reference implementation of
SDN, and evaluate our solution considering different traffic scenarios (with
and without attack). The results of our preliminary experiments confirm that,
compared to the state-of-the-art, LineSwitch reduces the time overhead up to
30%, while ensuring the same level of protection.Comment: In Proceedings of the 10th ACM Symposium on Information, Computer and
Communications Security (ASIACCS 2015). To appea
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