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 $2 ; 10 38 and 7 ; 10 44 ergs s À1 . The cavities have an average radius of $10 kpc, and they lie at an average projected distance of $20 kpc from the central galaxy. The minimum energy associated with the cavities ranges from pV $ 10 55 ergs in galaxies, groups, and poor clusters to pV $ 10 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 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 $0.1 M yr À1, with some systems growing as quickly as $1 M yr À1. 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 $r À1 to the Bondi radius, but is probably too feeble to fuel the most powerful outbursts.

We present source catalogs for the 4 Ms Chandra Deep Field-South (CDF-S), which is the deepest Chandra survey to date and covers an area of 464.5 arcmin 2 . We provide a main Chandra source catalog, which contains 740 X-ray sources that are detected with WAVDETECT at a false-positive probability threshold of 10 −5 in at least one of three X-ray bands (0.5-8 keV, full band; 0.5-2 keV, soft band; and 2-8 keV, hard band) and also satisfy a binomial-probability source-selection criterion of P < 0.004 (i.e., the probability of sources not being real is less than 0.004); this approach is designed to maximize the number of reliable sources detected. A total of 300 main-catalog sources are new compared to the previous 2 Ms CDF-S main-catalog sources. We determine X-ray source positions using centroid and matched-filter techniques and obtain a median positional uncertainty of ≈ 0.42 ′′ . We also provide a supplementary catalog, which consists of 36 sources that are detected with WAVDETECT at a false-positive probability threshold of 10 −5 , satisfy the condition of 0.004 < P < 0.1, and have an optical counterpart with R < 24. Multiwavelength identifications, basic optical/infrared/radio photometry, and spectroscopic/photometric redshifts are provided for the X-ray sources in the main and supplementary catalogs. 716 (≈ 97%) of the 740 main-catalog sources have multiwavelength counterparts, with 673 (≈ 94% of 716) having either spectroscopic or photometric redshifts. The 740 main-catalog sources span broad ranges of full-band flux and 0.5-8 keV luminosity; the 300 new main-catalog sources span similar ranges although they tend to be systematically lower. Basic analyses of the X-ray and multiwavelength properties of the sources indicate that > 75% of the main-catalog sources are AGNs; of the 300 new main-catalog sources, about 35% are likely normal and starburst galaxies, reflecting the rise of normal and starburst galaxies at the very faint flux levels uniquely accessible to the 4 Ms CDF-S. Near the center of the 4 Ms CDF-S (i.e., within an off-axis angle of 3 ′ ), the observed AGN and galaxy source densities have reached 9800 +1300 −1100 deg −2 and 6900 +1100 −900 deg −2 , respectively. Simulations show that our main catalog is highly reliable and is reasonably complete. The mean backgrounds (corrected for vignetting and exposure-time variations) are 0.063 and 0.178 count Ms −1 pixel −1 (for a pixel size of 0.492 ′′ ) for the soft and hard bands, respectively; the majority of the pixels have zero background counts. The 4 Ms CDF-S reaches on-axis flux limits of ≈ 3.2 × 10 −17 , 9.1 × 10 −18 , and 5.5 × 10 −17 erg cm −2 s −1 for the full, soft, and hard bands, respectively. An increase in the CDF-S exposure time by a factor of ≈ 2-2.5 would provide further significant gains and probe key unexplored discovery space.

LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

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 approximately 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 approximately 6 x 10(61) erg, the most powerful radio outburst known. This is enough energy to quench a cooling flow for several Gyr, and to provide approximately 1/3 keV per particle of heat to the surrounding cluster.