AGN Heating Research Articles

A Spectacular Hα Complex in Virgo: Evidence for a Collision between M86 and NGC 4438 and Implications for the Collisional ISM Heating of Ellipticals

Deep wide-field Hα+[N II] imaging around the Virgo Cluster giant elliptical galaxy M86 reveals a highly complex and disturbed ISM/ICM. The most striking feature is a set of Hα filaments which clearly connect M86 with the nearby disturbed spiral NGC 4438 (23' = 120 kpc projected away), providing strong evidence for a previously unrecognized collision between them. Spectroscopy of selected regions shows a fairly smooth velocity gradient between M86 and NGC 4438, consistent with the collision scenario. Such a collision would impart significant energy into the ISM of M86, probably heating the gas and acting to prevent the gas from cooling to form stars. We propose that cool gas stripped from NGC 4438 during the collision and deposited in its wake is heated by shocks, ram pressure drag, or thermal conduction, producing most of the Hα filaments. Some Hα filaments are associated with the well-known ridge of bright X-ray emission to the NW of the nucleus, suggesting that the collision is responsible for peculiarities of M86 previously ascribed to other effects. M86 is radio-quiet; thus AGN heating is unlikely to play a significant role. The M86 system has implications for understanding the role of gravitational interactions in the heating of the ISM in ellipticals, and how collisions in clusters transform galaxies.

Obscuring Active Galactic Nuclei with Nuclear Starburst Disks

We assess the potential of nuclear starburst disks to obscure the Seyfert-like AGNs that dominate the hard X-ray background at z ~ 1. Over 1200 starburst disk models, based on the theory developed by Thompson et al., are calculated for five input parameters: the black hole mass, the radial size of the starburst disk, the dust-to-gas ratio, the efficiency of angular momentum transport in the disk, and the gas fraction at the outer disk radius. We find that a large dust-to-gas ratio, a relatively small starburst disk, a significant gas mass fraction, and efficient angular momentum transport are all important to produce a starburst disk that can potentially obscure an AGN. The typical maximum star formation rate in the disks is ~10 M☉ yr−1. Assuming no mass loss due to outflows, the starburst disks feed gas onto the black hole at rates sufficient to produce hard X-ray luminosities of 1043-1044 erg s−1. The starburst disks themselves should be detectable at mid-infrared and radio wavelengths; at z = 0.8, the predicted fluxes are ~1 mJy at 24 μm and ~10-30 μJy at 1.4 GHz. Thus, we predict a large fraction of radio/X-ray matches in future deep radio surveys. The starburst disks should be easily distinguished from AGNs in future 100 μm surveys by the Herschel Space Observatory with expected fluxes of ~5 mJy. Any AGN-obscuring starbursts will be associated with hot dust, independent of AGN heating, resulting in observable signatures for separating galactic and nuclear star formation. Finally, because of the competition between gas and star formation, nuclear starbursts will be associated with lower luminosity AGNs. Thus, this phenomenon is a natural explanation for the observed decrease in the fraction of obscured AGNs with luminosity.

Search for cold gas along radio lobes in the cooling core galaxies MS0735.6+7421 and M87

We report CO observations towards MS0735.6+7421 a distant cooling core galaxy, and towards M87, the nearest cooling core in the center of the Virgo cluster. Both galaxies contain radio cavities that are thought to be responsible for the heating that can regulate or stop the cooling of the surrounding gas. In this feedback process, there could still be some gas cooling along filaments, along the borders of the radio cavities. Molecular gas is known to exist in clusters with cooling cores, in long and thin filaments that can be formed behind the rising bubbles inflated by the central AGN. CO emission was searched for at several locations along the radio lobes of those two galaxies, but only upper limits were found. These correspond to cold gas mass limits of a few 109 for each pointing in MS0735.6+7421, and a few 106 in M87. This non detection means that either the cooling is strongly reduced by the AGN feedback or that the gas is cooling in very localized places like thin filaments, possibly diluted in the large beam for MS0735.6+7421. For M87, the AGN heating appears to have stopped the cooling completely.

The Hot Interstellar Medium in Normal Elliptical Galaxies. III. The Thermal Structure of the Gas

This is the third paper in a series analyzing X-ray emission from the hot interstellar medium in a sample of 54 normal elliptical galaxies observed by Chandra. We focus on a subset of 36 galaxies with sufficient signal to compute radial temperature profiles. We distinguish four qualitatively different types of profile: positive gradient (outwardly rising), negative gradients (falling), quasi-isothermal (flat), and hybrid (falling at small radii and rising at larger radii). We measure the mean logarithmic temperature gradients in two radial regions: from 0 to 2 J-band effective radii RJ (excluding the central point source), and from 2 to 4 RJ. We find the outer gradient to be uncorrelated with intrinsic host galaxy properties, but strongly influenced by the environment: galaxies in low-density environments tend to show negative outer gradients, while those in high-density environments show positive outer gradients, suggesting the influence of circumgalactic hot gas. The inner temperature gradient, however, is largely unaffected by the environment, but strongly correlated with intrinsic host galaxy characteristics: negative inner gradients are more common for smaller, optically faint, low radio luminosity galaxies, whereas positive gradients are found in bright galaxies with stronger radio sources. There is no evidence for bimodality in the distribution of inner or outer gradients. We propose three scenarios to explain the inner temperature gradients: (1) weak AGNs heat the ISM locally, while higher luminosity AGNs heat the system globally through jets inflating cavities at larger radii; (2) the onset of negative inner gradients indicates a declining importance of AGN heating relative to other sources, such as compressional heating or supernovae; or (3) the variety of temperature profiles are snapshots of different stages of a time-dependent flow, cyclically reversing the temperature gradient over time.

Where do ‘red and dead’ early-type void galaxies come from?

Void regions of the Universe offer a special environment for studying cosmology and galaxy formation, which may expose weaknesses in our understanding of these phenomena. Although galaxies in voids are observed to be predominately gas rich, star forming and blue, a sub-population of bright red void galaxies can also be found, whose star formation was shut down long ago. Are the same processes that quench star formation in denser regions of the Universe also at work in voids? We compare the luminosity function of void galaxies in the 2dF Galaxy Redshift Survey, to those from a galaxy formation model built on the Millennium Simulation. We show that a global star formation suppression mechanism in the form of low luminosity AGN heating is sufficient to reproduce the observed population of void early-types. Radio mode heating is environment independent other than its dependence on dark matter halo mass, where, above a critical mass threshold of approximately M_vir~10^12.5 M_sun, gas cooling onto the galaxy is suppressed and star formation subsequently fades. In the Millennium Simulation, the void halo mass function is shifted with respect to denser environments, but still maintains a high mass tail above this critical threshold. In such void halos, radio mode heating remains efficient and red galaxies are found; collectively these galaxies match the observed space density without any modification to the model. Consequently, galaxies living in vastly different large-scale environments but hosted by halos of similar mass are predicted to have similar properties, consistent with observations.

On the evolution of cooling cores in X-ray galaxy clusters

(Abridged) To define a framework for the formation and evolution of the cooling cores in X-ray galaxy clusters, we study how the physical properties change as function of the cosmic time in the inner regions of a 4 keV and 8 keV galaxy cluster under the action of radiative cooling and gravity only. The cooling radius, R_cool, defined as the radius at which the cooling time equals the Universe age at given redshift, evolves from ~0.01 R200 at z>2, where the structures begin their evolution, to ~0.05 R200 at z=0. The values measured at 0.01 R200 show an increase of about 15-20 per cent per Gyr in the gas density and surface brightness and a decrease with a mean rate of 10 per cent per Gyr in the gas temperature. The emission-weighted temperature diminishes by about 25 per cent and the bolometric X-ray luminosity rises by a factor ~2 after 10 Gyrs when all the cluster emission is considered in the computation. On the contrary, when the core region within 0.15 R500 is excluded, the gas temperature value does not change and the X-ray luminosity varies by 10-20 per cent only. The cooling time and gas entropy radial profiles are well represented by power-law functions. The behaviour of the inner slopes of the gas temperature and density profiles are the most sensitive and unambiguous tracers of an evolving cooling core. Their values after 10 Gyrs of radiative losses, T_gas ~ r^0.4 and n_gas ~ r^(-1.2) for the hot (cool) object, are remarkably in agreement with the observational constraints available for nearby X-ray luminous cooling core clusters. Because our simulations do not consider any AGN heating, they imply that the feedback process does not greatly alter the gas density and temperature profiles as generated by radiative cooling alone.

Formation of cold filaments in cooling flow clusters

Emission-lines in the form of filamentary structures is common in bright clusters characterized by short cooling times. In the Perseus cluster, cold molecular gas, tightly linked to the Hα filaments, has been recently revealed by CO observations. In order to understand the origin of these filamentary structures, we have investigated the evolution of the hot ICM gas perturbed by the AGN central activity in a Perseus like cluster. Using very-high resolution TreeSPH simulations combined with a multiphase model and a model of plasma bubbles, we have been able to follow the density and temperature evolution of the disturbed ICM gas around the bubbles. Our simulations show that a fraction of the gas present at the center of clusters is trapped and entrained by the rising buoyant bubble to higher radius where the AGN heating is less efficient. The radiative cooling makes it cool in a few tens of Myr below , forming cold filamentary structures in the wake and in the rim of the bubbles. The predicted cold gas formed outside the cluster center is in agreement with the total CO mass and density profile of the observed molecular gas as well as with the kinematics of the Hα filaments. This scenario explains the Hα and CO filaments observed in luminous clusters without contradicting the observed lack of gas. It also emphasizes that if the AGN feedback provides some heating (negative feedback) it also perturbs the ICM, increasing its cooling (positive feedback).

Mass distribution in the most X-ray-luminous galaxy cluster RX J1347.5-1145 studied withXMM-Newton

We report on the analysis of XMM-Newton observations of RX J1347.5-1145 (z=0.451), the most X-ray-luminous galaxy cluster. We present a detailed total and gas mass determination up to large distances (~1.7 Mpc), study the scaling properties of the cluster, and explore the role of AGN heating in the cluster cool core. By means of spatially resolved spectroscopy we derive density, temperature, entropy, and cooling time profiles of the intra-cluster medium. We compute the total mass profile of the cluster in the assumption of hydrostatic equilibrium. If the disturbed south-east region of the cluster is excluded from the analysis, our results on shape, normalization, scaling properties of density, temperature, entropy, and cooling time profiles are fully consistent with those of relaxed, cool core clusters. We compare our total and gas mass estimates with previous X-ray, lensing, dynamical, and SZ studies. We find good agreement with other X-ray results, dynamical mass measurements, weak lensing masses and SZ results. We confirm a discrepancy of a factor ~2 between strong lensing and X-ray mass determinations and find a gross mismatch between our total mass estimate and the mass reconstructed through the combination of both strong and weak lensing. We explore the effervescent heating scenario in the core of RX J1347.5-1145 and find support to the picture that AGN outflows and heat conduction are able to quenching radiative cooling.

X‐Ray Thermal Coronae of Galaxies in Hot Clusters: Ubiquity of Embedded Mini–Cooling Cores

We present a systematic investigation of X-ray thermal coronae in 157 early-type and 22 late-type galaxies from a survey of 25 hot (kT > 3 keV), nearby (z 60% in LKs > 2L* galaxies. These embedded coronae in hot clusters are generally smaller, less luminous, and less massive than coronae in poor environments, demonstrating the negative effects of hot cluster environments on galactic coronae. Nevertheless, these coronae still manage to survive ICM stripping, evaporation, rapid cooling, and powerful AGN outflows, making them a rich source of information about gas stripping, microscopic transport, and feedback processes in the cluster environment. Heat conduction across the boundary of the coronae has to be suppressed by a factor of 100, which implies that the X-ray gas in early-type galaxies is magnetized and the magnetic field plays an important role in energy transfer. The luminous, embedded coronae, with high central density (0.1-0.4 cm-3), are miniversions of group and cluster cooling cores. As the prevalence of coronae of massive galaxies implies a long lifetime (several Gyr), there must be a heat source inside coronae to offset cooling. While we argue that AGN heating may not generally be the heat source, we conclude that SN heating can be enough as long as the kinetic energy of SNe can be efficiently dissipated. Diffuse thermal coronae have also been detected in at least 8 of 22 late-type galaxies in our sample. The fraction of luminous X-ray AGNs (>1041 ergs s-1) is not small (~5%) in our sample.

Chemodynamical Simulations of Elliptical Galaxies

We review recent developments in the field of chemodynamical simulations of elliptical galaxies, highlighting (in an admittedly biased fashion) the work conducted with our cosmological N-body/SPH code GCD+. We have demonstrated previously the recovery of several primary integrated early-type system scaling relations (e.g. colour-magnitude relation, L-x-T-x-[Fe/H](x)) when employing a phenomenological AGN heating scheme in conjunction with a self-consistent treatment of star formation, supernovae feedback, radiative cooling, chemical enrichment, and stellar/X-ray population synthesis. Here we emphasise characteristics derived from the full spatial information contained within the simulated dataset, including stellar and coronal morphologies, metallicity distribution functions. and abundance gradients.

Investigating AGN heating in a sample of nearby clusters

We analyse those objects in the Brightest 55 sample of clusters of galaxies which have a short central cooling time and a central temperature drop. Such clusters are likely to require some form of heating. Where clear radio bubbles are observed in these clusters, their energy injection is compared to the X-ray cooling rate. Of the 20 clusters requiring heating, at least 14 have clear bubbles, implying a duty cycle for the bubbling activity of at least 70 per cent. The average distance out to which the bubbles can offset the X-ray cooling, r heat , is given by r heat /r cool = 0.86 ± 0.11 where r cool is defined as the radius as which the radiative cooling time is 3 Gyr. 10 out of 16 clusters have r hcat /r cool ? 1, but there is a large range in values. The clusters which require heating but show no clear bubbles were combined with those clusters which have a radio core to form a second subsample. Using r heat = 0.86r cool we calculate the size of an average bubble expected in these clusters. In five cases (3C129.1, A2063, A2204, A3112 and A3391) the radio morphology is bi-lobed and its extent similar to the expected bubble sizes. A comparison between the actual bubble size and the maximum expected if they were to offset the X-ray cooling exactly, R max , shows a peak at R bubble ∼ 0.7R max with a tail extending to larger R bubble /R max . The offset from the expected value of R bubble ∼ R max may indicate the presence of a non-thermal component in the innermost intracluster medium of most clusters, with a pressure comparable to the thermal pressure.

The Hot Gas Content of Low‐Luminosity Early‐Type Galaxies and the Implications Regarding Supernova Heating and Active Galactic Nucleus Feedback

We have analyzed Chandra observations of 18 low-luminosity early-type galaxies with LB 3 × 1010 L☉B. Thermal emission from hot gas with temperatures between 0.2 and 0.8 keV comprises 5%-70% of the total 0.5-2.0 keV emission from these galaxies. We find that the total X-ray luminosity from LMXBs (resolved plus the power-law component of the unresolved emission) scales roughly linearly with the K-band luminosity of the galaxies with a normalization comparable to that found in more luminous early-type galaxies. All of the galaxies in our sample are gas-poor, with gas masses much less than that expected from the accumulation of stellar mass loss over the lifetime of the galaxies. The average ratio of gas mass to stellar mass in our sample is Mgas/M* = 0.001, compared to more luminous early-type galaxies that typically have Mgas/M* = 0.01. The time required to accumulate the observed gas mass from stellar mass loss in these galaxies is typically 3 × 108 yr. Since the cooling time of the gas is longer than the replenishment time, the gas cannot be condensing out of the hot phase and forming stars, implying that the gas is most likely being expelled from these galaxies in a wind. The one exception to this is NGC 4552, which is the most optically luminous galaxy in our sample and has the highest gas content. Using recent estimates of the Type Ia supernova rate and AGN heating rate in early-type galaxies, we find that, on average, heating by Type Ia supernovae should exceed AGN heating in galaxies with LB 3 × 1010 L☉B. We also find that heating by Type Ia supernovae is energetically sufficient to drive winds in these galaxies, even if the present Type Ia supernova rate is overestimated by a factor of 2 or the present stellar mass-loss rate is underestimated by a factor of 2.

Effervescent heating: constraints from nearby cooling flow clusters observed with XMM-Newton

Aims. We have used deprojected radial density and temperature profiles of a sample of 16 nearby CF clusters observed with XMM-Newton to test whether the effervescent heating model can satisfactorily explain the dynamics of CF clusters. Methods. For each cluster we derived the required extra heating as a function of cluster-centric distance for various values of the unknown parameters M (mass deposition rate) and f c (conduction efficiency). We fitted the extra heating curve using the AGN effervescent heating function and derived the AGN parameters L (the time-averaged luminosity) and r 0 (the scale radius where the bubbles start rising in the ICM). Results. While we do not find any solution with the effervescent heating model for only one object, we do show that AGN and conduction heating are not cooperating effectively for half of the objects in our sample. For most of the clusters we find that, when a comparison is possible, the derived AGN scale radius r 0 and the observed AGN jet extension have the same order of magnitude. The AGN luminosities required to balance radiative losses are substantially lowered if the fact that the AGN deposits energy within a finite volume is taken into account. For the Virgo cluster, we find that the AGN power derived from the effervescent heating model is in good agreement with the observed jet power.

Oviin Elliptical Galaxies: Indicators of Cooling Flows

Early-type galaxies often contain a hot X-ray-emitting interstellar medium [(3-8) × 106 K] with an apparent radiative cooling time much less than a Hubble time. If unopposed by a heating mechanism, the gas will radiatively cool to temperatures 104 K at a rate proportional to LX/TX, typically 0.03-1 M☉ yr-1. We can test whether gas is cooling through the 3 × 105 K range by observing the O VI doublet, whose luminosity is proportional to the cooling rate. Here we report on a study of an unbiased sample of 24 galaxies, obtaining Far Ultraviolet Spectroscopic Explorer spectra to complement the X-ray data of ROSAT and Chandra. The O VI line emission was detected in about 40% of the galaxies and at a luminosity level similar to the prediction from the cooling flow model. There is a correlation between and X, although there is significant dispersion about the relationship, where the O VI is brighter or dimmer than expected by a factor of 3 or more. If the cooling flow picture is to be retained, then this dispersion requires that cooling flows be time-dependent, as might occur by the activity of an AGN. However, of detected objects, those with the highest or lowest values of /X are not systematically hot or cool, as one might predict from AGN heating.

An Observationally Motivated Framework for AGN Heating of Cluster Cores

The cooling flow problem is a long-standing puzzle that has received considerable recent attention, in part because the mechanism that quenches cooling flows in galaxy clusters is likely to be the same mechanism that sharply truncates the high end of the galaxy luminosity function. Most of the recent models for halting cooling in clusters have focused on AGN heating, but the actual heating mechanism has remained mysterious. Here we present a framework for AGN heating derived from a Chandra survey of gas entropy profiles within cluster cores. This set of observations strongly suggests that the inner parts of cluster cores are shock-heated every ~108 yr by intermittent AGN outbursts, driven by a kinetic power output of ~1045 ergs s-1 and lasting at least 107 yr. Beyond ~30 kpc these shocks decay to sound waves, releasing buoyant bubbles that heat the core's outer parts. Between heating episodes, cooling causes the core to relax toward an asymptotic pure cooling profile. The density distribution in this asymptotic profile is sufficiently peaked that the AGN shock does not cause a core entropy inversion, allowing the cluster core to retain a strong iron abundance gradient, as observed.