The redshift dependence of spectral index in powerful radio galaxies

We present and discuss in this paper the rest frame radio spectra (1-25 GHz) of a sample of fourteen radio galaxies at z > 2 from the newly defined MRC/1Jy complete sample of 558 radio sources. These galaxies are among the most powerful radio sources known and range in luminosity from 1028-1028.8 watt Hz-1 at 1 GHz. We find that the median rest frame spectral index of this sample of galaxies at z > 2 is significantly steeper than that of a matched luminosity sample of 3CRR galaxies which are at a much lower redshift (0.85 < z < 1.7). This indicates that spectral index correlates primarily with redshift, at least in the luminosity range considered here. The difference between the distributions of rest frame spectral curvatures for the two samples does not appear to be statistically significant. We suggest a new explanation for the steeper spectra of radio galaxies at high redshift involving steeper electron energy spectra at injection. Electron energy spectra are expected to steepen in a first-order Fermi acceleration process, at both non-relativistic and relativistic shock fronts, as the upstream fluid velocity decreases. This may well be the case at high redshifts: the hotter and denser circum-galactic medium at high redshifts could result in slower speeds for the hotspot and the jet material behind it. The smaller sizes of radio sources at higher redshifts provide support to this scenario

The Molonglo reference catalog 1 Jy radio source survey. III. Identification of a complete quasar sample

We present a new complete sample of 111 radio quasars (including six BL Lac objects) selected from the Molonglo Reference Catalog (MRC) at 408 MHz. The sample, which we call the Molonglo Quasar Sample (MQS), forms part of a complete survey of 557 MRC radio sources with S408 ≥ 0.95 Jy in the declination range -30 ° < δ < -20 ° , b > 20 ° but excluding the R.A. range 14h03m-20h20m. Quasar classifications are based on high-resolution radio images, deep optical identifications, and follow-up spectroscopy of sources in the strip. The relatively low radio frequency of the finding survey and the complete optical identification of quasars to faint magnitudes ensure that the MQS is relatively free from orientation biases that affect most other samples of radio-loud quasars. The MQS is therefore particularly well suited to investigating the effects of radio axis orientation on quasar properties. This paper describes in detail the formation of the MQS and presents basic radio and optical data, including VLA images of extended radio sources in the sample and a complete set of optical finding charts

Tunable-filter imaging of quasar fields at z~1. I. A cluster around MRC B0450-221

Using a combination of multicolour broad- and narrow-band imaging techniques and follow-up spectroscopy, we have detected an overdensity of galaxies in the field of quasar MRC B0450-221, whose properties are consistent with a cluster at the quasar redshift z=0.9. An excess of red galaxies (V-I>2.2, I-K'>3.8) is evident within 1' of the quasar, with the colours expected for galaxies at z=0.9 that have evolved passively for 3 Gyr or more. A number of line-emitting galaxies (nine candidates with equivalent widths EW>70A) are also detected in the field using the TAURUS Tunable Filter (TTF). Three have been confirmed spectroscopically to indeed lie at z=0.9. The TTF candidates with the strongest [O II] line emission cluster in a group which lies 200-700 kpc away from the quasar and the red galaxy excess, and therefore most likely on the outskirts of the cluster. These observations are the first in a series probing quasar environments at z~1 with TTF.Comment: Accepted for publication in AJ. 25 pages, 24 figs (large files in jpg or gif format), uses emulateapj.st

Recurrent radio outbursts at the center of the NGC1407 galaxy group

We present deep Giant Metrewave Radio Telescope (GMRT) radio observations at 240, 330 and 610 MHz of the complex radio source at the center of the NGC1407 galaxy group. Previous GMRT observations at 240 MHz revealed faint, diffuse emission enclosing the central twin-jet radio galaxy. This has been interpreted as an indication of two possible radio outbursts occurring at different times. Both the inner double and diffuse component are detected in the new GMRT images at high levels of significance. Combining the GMRT observations with archival Very Large Array data at 1.4 and 4.9 GHz, we derive the total spectrum of both components. The inner double has a spectral index \alpha=0.7, typical for active, extended radio galaxies, whereas the spectrum of the large-scale emission is very steep, with \alpha=1.8 between 240 MHz and 1.4 GHz. The radiative age of the large-scale component is very long, ~300 Myr, compared to ~30 Myr estimated for the central double, confirming that the diffuse component was generated during a former cycle of activity of the central galaxy. The current activity have so far released an energy which is nearly one order of magnitude lower than that associated with the former outburst. The group X-ray emission in the Chandra and XMM-Newton images and extended radio emission show a similar swept-back morphology. We speculate that the two structures are both affected by the motion of the group core, perhaps due to the core sloshing in response to a recent encounter with the nearby elliptical galaxy NGC1400.Comment: 15 pages, 12 figures and 5 tables. Accepted for publication in Ap

A combined low-radio frequency/X-ray study of galaxy groups I. Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz

We present new Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz of 18 X-ray bright galaxy groups. These observations are part of an extended project, presented here and in future papers, which combines low-frequency radio and X-ray data to investigate the interaction between central active galactic nuclei (AGN) and the intra-group medium (IGM). The radio images show a very diverse population of group-central radio sources, varying widely in size, power, morphology and spectral index. Comparison of the radio images with Chandra and XMM-Newton X-ray images shows that groups with significant substructure in the X-ray band and marginal radio emission at >= 1 GHz host low-frequency radio structures that correlate with substructures in IGM. Radio-filled X-ray cavities, the most evident form of AGN/IGM interaction in our sample, are found in half of the systems, and are typically associated with small, low- or mid-power double radio sources. Two systems, NGC5044 and NGC4636, possess multiple cavities, which are isotropically distributed around the group center, possibly due to group weather. In other systems the radio/X-ray correlations are less evident. However, the AGN/IGM interaction can manifest itself through the effects of the high-pressure medium on the morphology, spectral properties and evolution of the radio-emitting plasma. In particular, the IGM can confine fading radio lobes in old/dying radio galaxies and prevent them from dissipating quickly. Evidence for radio emission produced by former outbursts that coexist with current activity is found in six groups of the sample.Comment: Accepted for publication in the Astrophysical Journal Supplement Series, 26 pages, 18 figures. A version with high-quality figures is http://www.astro.umd.edu/~simona/giacintucci_hr.pd

A GMRT multifrequency radio study of the isothermal core of the poor galaxy cluster AWM 4

We present a detailed radio morphological study and spectral analysis of the wide-angle-tail radio source 4C +24.36 associated with the dominant galaxy in the relaxed galaxy cluster AWM 4. Our study is based on new high sensitivity GMRT observations at 235 MHz, 327 MHz and 610 MHz, and on literature and archival data at other frequencies. We find that the source major axis is likely oriented at a small angle with respect to the plane of the sky. The wide-angle-tail morphology can be reasonably explained by adopting a simple hydrodynamical model in which both ram pressure (driven by the motion of the host galaxy) and buoyancy forces contribute to bend the radio structure. The spectral index progressively steepens along the source major axis from $\alpha \sim$0.3 in the region close to the radio nucleus to beyond 1.5 in the lobes. The results of the analysis of the spectral index image allow us to derive an estimate of the radiative age of the source of $\sim$ 160 Myr. The cluster X-ray emitting gas has a relaxed morphology and short cooling time, but its temperature profile is isothermal out to at least 160 kpc from the centre. Therefore we seek evidence of energy ejection from the central AGN to prevent catastrophic cooling. We find that the energy injected by 4C +24.36 in the form of synchrotron luminosity during its lifetime is far less than the energy required to maintain the high gas temperature in the core. We also find that it is not possible for the central source to eject the requisite energy in the intracluster gas in terms of the enthalpy of buoyant bubbles of relativistic fluid, without creating discernible large cavities in the existing X-ray XMM-Newton observations.Comment: 15 pages, 12 figures, accepted for publication in Ap

The Molonglo reference catalog 1 Jy radio source survey. II. Radio structures of galaxy identifications

This is the second in a series of papers discussing the radio, optical, and near-IR properties of a large and complete sample of radio sources selected from the Molonglo Reference Catalog (MRC) at 408 MHz. The sample consists of 557 radio sources from the MRC that have S0.408≥0.95 Jy, -30 ° <δ< -20°, and |b| ≥ 20° but excluding the right ascension range of 14h03m_20h20m. While 111 of these are identified with quasars (including six with BL Lac objects), a vast majority of the rest are identified with galaxies. Only a small fraction (≤2%) appear to have no optical or near-IR counterparts down to an r magnitude of ~24 or a K magnitude of ~19. A similar fraction have ambiguous identifications. This paper presents the radio structures of the 446 radio sources in the sample that are either identified with galaxies or that have remained unidentified. Most of the structural information given in the paper is based on high-resolution (~1"-10") observations made with the Very Large Array at a frequency of 4.86 GHz. Several of the large or diffuse sources were also observed at 1.41 GHz. Contour plots of the radio images are also presented for the well-resolved sources. A substantial fraction (~20%) of the sources in the galaxy sample is found to be of the "compact steep spectrum" variety with α 4.86 0.408>0.5 and linear size l≤20 kpc (for H0 = 50 km s-1 Mpc-1; q0 = 0.5)