We present the first wide area (19 deg2), deep (≈120–150 μJy beam−1), high-resolution (5.6 × 7.4 arcsec) LOFAR High Band Antenna image of the Boötes field made at 130–169 MHz. This image is at least ...an order of magnitude deeper and 3–5 times higher in angular resolution than previously achieved for this field at low frequencies. The observations and data reduction, which includes full direction-dependent calibration, are described here. We present a radio source catalogue containing 6 276 sources detected over an area of 19 deg2, with a peak flux density threshold of 5σ. As the first thorough test of the facet calibration strategy, introduced by van Weeren et al., we investigate the flux and positional accuracy of the catalogue. We present differential source counts that reach an order of magnitude deeper in flux density than previously achieved at these low frequencies, and show flattening at 150-MHz flux densities below 10 mJy associated with the rise of the low flux density star-forming galaxies and radio-quiet AGN.
We present an analysis of 16 galaxy clusters, one group, and one galaxy drawn from the Chandra Data Archive. These systems possess prominent X-ray surface brightness depressions associated with ...cavities or bubbles that were created by interactions between powerful radio sources and the surrounding hot gas. The central galaxies in these systems harbor radio sources with luminosities ranging between approx2 x 10 super(38) and 7 x 10 super(44) ergs s super(-1). The cavities have an average radius of approx10 kpc, and they lie at an average projected distance of approx20 kpc from the central galaxy. The minimum energy associated with the cavities ranges from pV approx 10 super(55) ergs in galaxies, groups, and poor clusters to pV approx 10 super(60) ergs in rich clusters. We evaluate the hypothesis that cooling in the hot gas can be quenched by energy injected into the surrounding gas by the rising bubbles. We find that the instantaneous mechanical luminosities required to offset cooling range between 1pV and 20pV per cavity. Nearly half of the systems in this study may have instantaneous mechanical luminosities large enough to balance cooling, at least for a short period of time, if the cavities are filled with a relativistic gas. We find a trend or upper envelope in the distribution of central X-ray luminosity versus instantaneous mechanical luminosity, with the sense that the most powerful cavities are found in the most X-ray-luminous systems. Such a trend would be expected if many of these systems produce bubbles at a rate that scales in proportion to the cooling rate of the surrounding gas. Finally, we use the X-ray cavities to measure the mechanical power of radio sources over six decades of radio luminosity, independently of the radio properties themselves. We find that the ratio of the instantaneous mechanical (kinetic) luminosity to the 1.4 GHz synchrotron luminosity ranges typically between a few and roughly a few thousand for luminous radio sources but can be several thousand for weaker sources. This wide range implies that the 1.4 GHz synchrotron luminosity is an unreliable gauge of the mechanical power of radio sources.
We present an analysis of deep Chandra X-ray observations of the galaxy cluster MS 0735.6+7421, which hosts the most energetic radio active galactic nucleus (AGN) known. Our analysis has revealed two ...cavities in its hot atmosphere with diameters of 200–240 kpc. The total cavity enthalpy, mean age, and mean jet power are 9 × 1061 erg, 1.6 × 108 yr, and 1.7 × 1046 erg s-1, respectively. The cavities are surrounded by nearly continuous temperature and surface brightness discontinuities associated with an elliptical shock front of Mach number 1.26 (1.17–1.30) and age of 1.1 × 108 yr. The shock has injected at least 4 × 1061 erg into the hot atmosphere at a rate of 1.1 × 1046 erg s-1. A second pair of cavities and possibly a second shock front are located along the radio jets, indicating that the AGN power has declined by a factor of 30 over the past 100 Myr. The multiphase atmosphere surrounding the central galaxy is cooling at a rate of 40 M⊙yr-1, but does not fuel star formation at an appreciable rate. In addition to heating, entrainment in the radio jet may be depleting the nucleus of fuel and preventing gas from condensing out of the intracluster medium. Finally, we examine the mean time intervals between AGN outbursts in systems with multiple generations of X-ray cavities. We find that, like MS0735, their AGN rejuvenate on a time-scale that is approximately 1/3 of their mean central cooling time-scales, indicating that jet heating is outpacing cooling in these systems.
We present an analysis of the energetics and particle content of the lobes of 24 radio galaxies at the cores of cooling clusters. The radio lobes in these systems have created visible cavities in the ...surrounding hot, X-ray- emitting gas, which allow direct measurement of the mechanical jet power of radio sources over six decades of radio luminosity, independently of the radio properties themselves. We find that jet (cavity) power increases with radio synchrotron power approximately as image, where image depending on the bandpass of measurement and state of the source. However, the scatter about these relations caused by variations in radiative efficiency spans more than 4 orders of magnitude. A number of factors contribute to this scatter, including aging, entrainment, variations in magnetic field strengths, and the partitioning of energy between electrons and nonradiating heavy particles. After accounting for variations in synchrotron break frequency (age), the scatter is reduced by sim50%, yielding the most accurate scaling relation available between the lobe radio power and the jet (cavity) power. Furthermore, we place limits on the magnetic field strengths and particle content of the radio lobes using a variety of X-ray constraints. We find that the lobe magnetic field strengths vary between a few to several tens of microgauss depending on the age and dynamical state of the lobes. If the cavities are maintained in pressure balance with their surroundings and are supported by internal fields and particles in equipartition, the ratio of energy in electrons to heavy particles (k) must vary widely from approximately unity to 4000, consistent with heavy (hadronic) jets.
We present an analysis of the growth of black holes through accretion and bulges through star formation in 33 galaxies at the centers of cooling flows. Most of these systems show evidence of cavities ...in the intracluster medium (ICM) inflated by radio jets emanating from their active galactic nuclei (AGNs). We present a new and extensive analysis of X-ray cavities in these systems. We find that AGNs are energetically able to balance radiative losses (cooling) from the ICM in more than half of our sample. We examine the relationship between cooling and star formation and find that the star formation rates are approaching or are comparable to X-ray and far-UV limits on the rates of gas condensation onto the central galaxy. The vast gulf between radiative losses and the sink of cooling material, which has been the primary objection to cooling flows, has narrowed significantly. Using the cavity (jet) powers, we place strong lower limits on the rate of growth of the central black holes, and we find that they are growing at an average rate of 60.1 M sub( )yr super(-1), with some systems growing as quickly as 61 M sub( )yr super(-1). We find a trend between bulge growth (star formation) and black hole growth that is approximately in accordance with the slope of the local (Magorrian) relation between black hole and bulge mass, but the scatter suggests that bulges and black holes do not necessarily grow in lockstep. Bondi accretion can power the low-luminosity sources, provided the nuclear gas density rises as 6r super(-1) to the Bondi radius, but is probably too feeble to fuel the most powerful outbursts.
Most of the baryons in galaxy clusters reside between the galaxies in a hot, tenuous gas. The densest gas in their centres should cool and accrete onto giant central galaxies at rates of 10-1,000 ...solar masses per year. No viable repository for this gas, such as clouds or new stars, has been found. New X-ray observations, however, have revealed far less cooling below X-ray temperatures than expected, altering the previously accepted picture of cooling flows. As a result, most of the gas must be heated to and maintained at temperatures above ∼2 keV (ref. 3). The most promising heating mechanism is powerful radio jets emanating from supermassive black holes in the central galaxies of clusters. Here we report the discovery of giant cavities and shock fronts in a distant (z = 0.22) cluster caused by an interaction between a radio source and the hot gas surrounding it. The energy involved is ∼6 × 1061 erg, the most powerful radio outburst known. This is enough energy to quench a cooling flow for several Gyr, and to provide ∼1/3 keV per particle of heat to the surrounding cluster.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
ABSTRACT
We report on a detailed spatial and spectral analysis of the large-scale X-ray emission from the merging cluster Cygnus A. We use 2.2 Ms Chandra and 40 ks XMM–Newton archival data sets to ...determine the thermodynamic properties of the intracluster gas in the merger region between the two subclusters in the system. These profiles exhibit temperature enhancements that imply significant heating along the merger axis. Possible sources for this heating include the shock from the ongoing merger, past activity of the powerful AGN in the core, or a combination of both. To distinguish between these scenarios, we compare the observed X-ray properties of Cygnus A with simple, spherical cluster models. These models are constructed using azimuthally averaged density and temperature profiles determined from the undisturbed regions of the cluster and folded through marx to produce simulated Chandra observations. The thermodynamic properties in the merger region from these simulated X-ray observations were used as a baseline for comparison with the actual observations. We identify two distinct components in the temperature structure along the merger axis, a smooth, large-scale temperature excess we attribute to the ongoing merger, and a series of peaks where the temperatures are enhanced by 0.5–2.5 keV. If these peaks are attributable to the central AGN, the location, and strength of these features imply that Cygnus A has been active for the past 300 Myr injecting a total of ∼1062 erg into the merger region. This corresponds to ∼10 per cent of the energy deposited by the merger shock.
A 227 ks Chandra X-ray image of the hot plasma in the Hydra A cluster has revealed an extensive cavity system. The system was created by a continuous outflow or a series of bursts from the nucleus of ...the central galaxy over the past 200-500 Myr. The cavities have displaced 10% of the plasma within a 300 kpc radius of the central galaxy, creating a swiss-cheese-like topology in the hot gas. The surface brightness decrements are consistent with empty cavities oriented within 40 degree of the plane of the sky. The outflow has deposited upward of 10 super(61) ergs into the cluster gas, most of which was propelled beyond the inner similar to 100 kpc cooling region. The supermassive black hole has accreted at a rate of approximately 0.1-0.25 M unk yr super(-1) over this time frame, which is a small fraction of the Eddington rate of a similar to 10 super(9) M unk black hole, but is dramatically larger than the Bondi rate. Given the previous evidence for a circumnuclear disk of cold gas in Hydra A, these results are consistent with the AGN being powered primarily by infalling cold gas. The cavity system is shadowed perfectly by 330 MHz radio emission. Such low-frequency synchrotron emission may be an excellent proxy for X-ray cavities and thus the total energy liberated by the supermassive black hole.
ABSTRACT
The Chandra X‐ray Observatory has revealed X‐ray bubbles in the intracluster medium (ICM) of many nearby cooling‐flow clusters. The bubbles trace feedback that is thought to couple the ...central active galactic nucleus (AGN) to the ICM, helping to stabilize cooling flows and govern the evolution of massive galaxies. However, the prevalence and duty cycle of such AGN outbursts is not well understood. To this end, we study how cooling is balanced by bubble heating for complete samples of clusters (the Brightest 55 clusters of galaxies, hereafter B55, and the HIghest X‐ray FLUx Galaxy Cluster Sample, HIFLUGCS). We find that the radio luminosity of the central galaxy only exceeds 2.5 × 1030 erg s−1 Hz−1 in cooling‐flow clusters. This result implies a connection between the central radio source and the ICM, as expected if AGN feedback is operating. Additionally, we find a duty cycle for radio‐mode feedback, the fraction of time that a system possesses bubbles inflated by its central radio source, of ≳69 per cent for the B55 and ≳63 per cent for the HIFLUGCS. These duty cycles are lower limits since some bubbles are likely missed in existing images. We used simulations to constrain the bubble power that might be present and remain undetected in the cooling‐flow systems without detected bubbles. Among theses systems, almost all could have significant bubble power. Therefore, our results imply that the duty cycle of AGN outbursts with the potential to heat the gas significantly in cooling‐flow clusters is at least 60 per cent and could approach 100 per cent.
PDF
