In the run up to routine observations with the upcoming generation of radio facilities, the nature of sub-mJy radio population has been hotly debated. Here, we describe multi-frequency data designed to probe the emission mechanism that dominates in these faint radio sources. Our analysis is based on observations of the Lockman Hole using the Giant Metrewave Radio Telescope (GMRT) -the deepest 610-MHz imaging yet reported -together with 1.4-GHz imaging from the Very Large Array (VLA), well matched in resolution and sensitivity to the GMRT data: σ 610 MHz ∼ 15 μJy beam −1 , σ 1.4 GHz ∼ 6 μJy beam −1 , full width at half-maximum (FWHM) ∼ 5 arcsec. The GMRT and VLA data are cross-matched to obtain the radio spectral indices for the faint radio emitters. Statistical analyses show no clear evolution for the median spectral index, α 610 MHz 1.4 GHz (where S ν ∝ ν α ), as a function of flux density. α 610 MHz 1.4 GHz is found to be approximately −0.6 to −0.7, based on an almost unbiased 10σ criterion, down to a flux level of S 1.4 GHz 100 μJy. The fraction of inverted spectrum sources (α 610 MHz 1.4 GHz > 0) is less than 10 per cent. The results suggest that the most prevalent emission mechanism in the sub-mJy regime is optically thin synchrotron, ruling out a dominant flat spectrum or ultra-steep spectrum radio population. The spectral index distribution has a significant scatter, α ≈ 0.4-0.5, which suggests a mixture of different populations at all flux levels. Spectroscopic classification of radio sources with X-ray emission has allowed us to estimate that the fraction of radio-quiet active galactic nuclei (AGN) at 30 μJy S 1.4 GHz < 300 μJy is roughly 25 ± 10 per cent, suggesting that star-forming galaxies dominate the sub-mJy regime.

On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-size radio sources have been found, so-called radio relics [1,2]. These relics are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach number collisionless shocks generated by cluster-cluster merger events [3]. A longstanding problem is how low-Mach number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here we report on the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411-3412. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters.

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In this article we present deep, high-resolution radio interferometric observations at 153 MHz to complement the extensively studied NOAO Boötes field. We provide a description of the observations, data reduction and source catalog construction. From our single pointing GMRT observation of ∼12 h we obtain a high-resolution (26 × 22 ) image of ∼11.3 square degrees, fully covering the Boötes field region and beyond. The image has a central noise level of ∼1.0 mJy beam −1 , which rises to 2.0-2.5 mJy beam −1 at the field edge, placing it amongst the deepest ∼150 MHz surveys to date. The catalog of 598 extracted sources is estimated to be ∼92 percent complete for >10 mJy sources, while the estimated contamination with false detections is <1 percent. The low rms position uncertainty of 1.24 facilitates accurate matching against catalogs at optical, infrared and other wavelengths. Differential source counts are determined down to 10 mJy. There is no evidence for flattening of the counts towards lower flux densities as observed in deep radio surveys at higher frequencies, suggesting that our catalog is dominated by the classical radio-loud AGN population that explains the counts at higher flux densities. Combination with available deep 1.4 GHz observations yields an accurate determination of spectral indices for 417 sources down to the lowest 153 MHz flux densities, of which 16 have ultra-steep spectra with spectral indices below −1.3. We confirm that flattening of the median spectral index towards low flux densities also occurs at this frequency. The detection fraction of the radio sources in NIR K S -band is found to drop with radio spectral index, which is in agreement with the known correlation between spectral index and redshift for brighter radio sources.

We present results from a study of X‐shaped sources based on observations using the Giant Metrewave Radio Telescope (GMRT). These observations were motivated by our low‐frequency study of 3C 223.1, an X‐shaped radio source, which showed that the wings (or low surface brightness jets) have flatter spectral indices than that of the active lobes (or high surface brightness jets), a result not easily explained by most models. We have now obtained GMRT data at 240 and 610 MHz for almost all the known X‐shaped radio sources and have studied the distribution of the spectral indices across the sources. While the radio morphologies of all the sources at 240 and 610 MHz show the characteristic X‐shape, the spectral characteristics of the X‐shaped radio sources seem to fall into three categories, namely, sources in which (i) the wings have flatter spectral indices than the active lobes have, (ii) the wings and the active lobes have comparable spectral indices, and (iii) the wings have steeper spectral indices than the active lobes have. We discuss the implications of the new observational results on the various formation models that have been proposed for X‐shaped sources.

This paper presents the first high-resolution and high-sensitivity study of the radio properties of optically selected type 2 quasars. We used the Very Large Array at 8.4 GHz to observe 59 sources drawn from the Sloan Digital Sky Survey sample of Zakamska et al. (2003). The detection rate of our survey is 59% (35/59), comparable to the detection rate in FIRST at 1.4 GHz. Ongoing star formation, although present, contributes negligible radio emission at the current sensitivity limit. Comparing the radio powers with the [O III] λ5007 luminosities, we find that roughly 15%±5% of the sample can be considered radio-loud. Intriguingly, the vast majority of the detected sources in our sample fall in a region intermediate between those traditionally occupied by radio-loud and radio-quiet quasars. Moreover, most of these "radio-intermediate" sources tend to have flat or inverted radio spectra, which we speculate may be caused by free-free absorption by ionized gas in the narrow-line region. The incidence of flat-spectrum sources in type 2 quasars appears to be much higher than in type 1 quasars, in apparent violation of the simple orientation-based unified model for active galaxies.

The existence of binary supermassive black holes (SBHs) is predicted by models of hierarchical galaxy formation. To date, only a single binary SBH has been imaged, at a projected separation of 7.3 parsecs. Here we report the detection of a candidate dual SBH with projected separation of 0.35 pc in the gas-rich interacting spiral galaxy NGC 7674 (Mrk 533).This peculiar Seyfert galaxy possesses a ∼0.7 kpc Z-shaped radio jet; the leading model for the formation of such sources postulates the presence of an uncoalesced binary SBH created during the infall of a satellite galaxy.Using very long baseline interferometry (VLBI), we imaged the central region of Mrk 533 at radio frequencies of 2, 5, 8 and 15 GHz. Two, possibly inverted-spectrum radio cores were detected at 15 GHz only; the 8 − 15 GHz spectral indices of the two cores are ≥ −0.33 and ≥ −0.38 (±30%), consistent with accreting SBHs. We derive a jet speed ∼ 0.28c from multiepoch parsec-scale data of the hotspot region, and a source age ≥ 8.2 × 10 3 yrs. AGN are believed to result from mass accretion on to supermassive black holes (SBHs) with masses 10 6 M < ∼ M • < ∼ 10 9 M ; the emission lines in the spectra are believed to be emitted from gas clouds orbiting the SBH−accretion disk system in regions of sub-parsec size (the broad emission line region, BLR) or at distances of hundreds to thousands of parsecs (the narrow emission line region, NLR). While Mrk 533 is a nearly face-on spiral galaxy, its optical spectrum shows a broad Hβ line in polarized emission, 4 implying that the view of the central AGN, specifically the BLR is obscured, presumably by a dusty torus.Hierarchical models of galaxy formation predict that spiral galaxies merge to form elliptical galaxies, leaving two or more SBHs in the center of the merged elliptical. 5 The host galaxies of nearby Seyfert galaxies, which are spiral in nature, are expected to evolve via minor mergers. 6 Since small galaxies are believed less likely to contain massive SBHs than massive galaxies, 7 it is not a priori clear whether Seyfert galaxies should ever host binary SBHs. Nevertheless we report here the detection of sub-parsec-scale dual radio cores which we interpret as a binary SBH in the interacting Seyfert galaxy Mrk 533.Mrk 533 has radio jets on the ∼700 parsec scale. 8 In this paper, we present multi-frequency parsec-scale data on Mrk 533 acquired with the Very Long Baseline Array (VLBA), showing the presence of two compact radio components. We adopt the following cosmological parameters: H 0 = 73 km s −1 Mpc −1 , Ω matter = 0.27 and Ω vacuum = 0.73. At the distance of Mrk 533, 1 arcsecond corresponds to 0.533 kpc. The radio spectral index, α, is defined such that flux density, F ν , at frequency ν is F ν ∝ ν α . et al. 8 argued that the AGN in Mrk 533 was located along a line connecting the two strongest sources, which they labeled "C" and "W", and that those two, kiloparsec-2 scale features were radio lobes or hot spots created by the AGN. However, no radio core was detected by them in their very long ...

Based on new and archival Chandra observations of the Sombrero galaxy (M 104=NGC 4594), we study the X-ray emission from its nucleus and the extended X-ray emission in and around its massive stellar bulge. We find that the 0.3-8 keV luminosity of the nucleus appears constant at ∼2.4 × 10 40 ergs s −1 , or ∼10 −7 of its Eddington luminosity, on three epochs between December 1999 and April 2008, but drops by a factor of two in the November 2008 observation. The 2-6 keV unresolved emission from the bulge region closely follows the K-band star light and most likely arises from unresolved stellar sources. At lower energies, however, the unresolved emission reaches a galactocentric radius of at least 23 kpc, significantly beyond the extent of the star light, clearly indicating the presence of diffuse hot gas. We isolate the emission of the gas by properly accounting for the emission from unresolved stellar sources, predominantly cataclysmic variables and coronally active binaries, whose quasi-universal X-ray emissivity was recently established. We find a gas temperature of ∼0.6 keV with little variation across the field of view, except for a lower temperature of ∼0.3 keV along the stellar disk. The metal abundance is not well constrained due to the limited counting statistics, but is consistent with metal-enrichment by Type Ia supernovae. We measure a total intrinsic 0.3-2 keV luminosity of ∼2×10 39 ergs s −1 , which corresponds to only one percent of the available energy input by Type Ia supernovae in the bulge, but is comparable to the prediction by the latest galaxy formation models for disk galaxies as massive as Sombrero. However, such numerical models do not fully account for internal feedback processes, such as nuclear feedback and stellar feedback, against accretion from the intergalactic medium. On the other hand, we find no evidence for either the nucleus or the very modest star-forming activities in the disk to be a dominant heating -2source for the diffuse gas. We also show that neither the expected energy released by Type Ia supernovae nor the expected mass returned by evolved stars is recovered by observations. We argue that in Sombrero a galactic-scale subsonic outflow of hot gas continuously removes much of the "missing" energy and mass input from the bulge region. The observed density and temperature distributions of such an outflow, however, continues to pose challenges to theoretical studies.