Cores Of Radio Galaxies Research Articles

10C continued: a deeper radio survey at 15.7 GHz

We present deep 15.7-GHz observations made with the Arcminute Microkelvin Imager Large Array in two fields previously observed as part of the Tenth Cambridge (10C) survey. These observations allow the source counts to be calculated down to 0.1 mJy, a factor of five deeper than achieved by the 10C survey. The new source counts are consistent with the extrapolated fit to the 10C source count, and display no evidence for either steepening or flattening of the counts. There is thus no evidence for the emergence of a significant new population of sources (e.g. starforming) at 15.7 GHz flux densities above 0.1 mJy, the flux density level at which we expect starforming galaxies to begin to contribute. Comparisons with the de Zotti et al. model and the SKADS Simulated Sky show that they both underestimate the observed number of sources by a factor of two at this flux density level. We suggest that this is due to the flat-spectrum cores of radio galaxies contributing more significantly to the counts than predicted by the models.

The faint radio source population at 15.7 GHz – II. Multi-wavelength properties

A complete, flux density limited sample of 96 faint ($> 0.5$ mJy) radio sources is selected from the 10C survey at 15.7 GHz in the Lockman Hole. We have matched this sample to a range of multi-wavelength catalogues, including SERVS, SWIRE, UKIDSS and optical data; multi-wavelength counterparts are found for 80 of the 96 sources and spectroscopic redshifts are available for 24 sources. Photometric reshifts are estimated for the sources with multi-wavelength data available; the median redshift of the sample is 0.91 with an interquartile range of 0.84. Radio-to-optical ratios show that at least 94 per cent of the sample are radio loud, indicating that the 10C sample is dominated by radio galaxies. This is in contrast to samples selected at lower frequencies, where radio-quiet AGN and starforming galaxies are present in significant numbers at these flux density levels. All six radio-quiet sources have rising radio spectra, suggesting that they are dominated by AGN emission. These results confirm the conclusions of Paper I that the faint, flat-spectrum sources which are found to dominate the 10C sample below $\sim 1$ mJy are the cores of radio galaxies. The properties of the 10C sample are compared to the SKADS Simulated Skies; a population of low-redshift starforming galaxies predicted by the simulation is not found in the observed sample.

Associated 21-cm absorption towards the cores of radio galaxies

We present the results of Giant Metrewave Radio Telescope observations to detect H i in absorption towards the cores of a sample of radio galaxies. From observations of a sample of 16 sources, we detect H i in absorption towards the core of only one source, the Fanaroff–Riley type II (FR II) radio galaxy 3C 452 which has been reported earlier by Gupta & Saikia. In this paper we present the results for the remaining sources which have been observed to a similar optical depth as for a comparison sample of compact steep-spectrum (CSS) and gigahertz peaked-spectrum (GPS) sources. We also compile available information on H i absorption towards the cores of extended radio sources observed with angular resolutions of a few arcsec or better. The fraction of extended sources with detection of H i absorption towards their cores is significantly smaller (7/47) than the fraction of H i detection towards CSS and GPS objects (28/49). For the cores of extended sources, there is no evidence of a significant correlation between H i column density towards the cores and the largest linear size of the sources. The distribution of the relative velocity of the principal absorbing component towards the cores of extended sources is not significantly different from that of the CSS and GPS objects. However, a few of the CSS and GPS objects have blueshifted components ≳1000 km s−1, possibly due to jet–cloud interactions. With the small number of detections towards cores, the difference in the detection rate between FR I (4/32) and FR II (3/15) sources is within the statistical uncertainties.

CONTRIBUTION OF GAMMA-RAY-LOUD RADIO GALAXIES’ CORE EMISSIONS TO THE COSMIC MeV AND GeV GAMMA-RAY BACKGROUND RADIATION

The Fermi gamma-ray satellite has recently detected gamma-ray emissions from radio galaxy cores. From these samples, we first examine the correlation between the luminosities at 5 GHz, L_{5GHz}, and at 0.1-10 GeV, L_{gamma}, of these gamma-ray loud radio galaxies. We find that the correlation is significant with L_{gamma} \propto L_{5GHz}^{1.16} based on a partial correlation analysis. Using this correlation and the radio luminosity function (RLF) of radio galaxies, we further explore the contribution of gamma-ray loud radio galaxies to the unresolved extragalactic gamma-ray background (EGRB). The gamma-ray luminosity function is obtained by normalizing the RLF to reproduce the source count distribution of the Fermi gamma-ray loud radio galaxies. We find that gamma-ray loud radio galaxies will explain ~25% of the unresolved Fermi EGRB flux above 100 MeV and will also make a significant contribution to the EGRB in the 1-30 MeV energy band. Since blazars explain 22% of the EGRB above 100 MeV, radio loud active galactic nuclei (AGNs) population explains ~47% of the unresolved EGRB. We further make an interpretation on the origin of the EGRB. The observed EGRB spectrum at 0.2-100 GeV does not show an absorption signature by the extragalactic background light. Thus, the dominant population of the origin of EGRB at very high energy (>30 GeV) might be nearby gamma-ray emitting sources or sources with very hard gamma-ray spectrum.

ChandraandXMM‐NewtonObservations of a Sample of Low‐Redshift FR I and FR II Radio Galaxy Nuclei

We present spectral results from Chandra and XMM-Newton observations of a sample of 22 low-redshift (z < 0.1) radio galaxies and consider whether the core emission originates from the base of a relativistic jet, or an accretion flow, or contains contributions from both. We find correlations between the unabsorbed X-ray, radio, and optical fluxes and luminosities of FR I-type radio-galaxy cores, implying a common origin in the form of a jet. On the other hand, we find that the X-ray spectra of FR II-type radio galaxy cores are dominated by absorbed emission, with NH ≳ 1023 atoms cm-2, which is likely to originate in an accretion flow. We discuss several models that may account for the different nuclear properties of FR I- and FR II-type cores and also demonstrate that both heavily obscured, accretion-related and unobscured, jet-related components may be present in all radio galaxy nuclei. Any absorbed, accretion-related components in FR I-type galaxies have low radiative efficiencies.

A View through Faraday’s Fog. II. Parsec‐Scale Rotation Measures in 40 Active Galactic Nuclei

Results from a survey of the parsec-scale Faraday rotation measure (RM) properties for 40 quasars, radio galaxies, and BL Lac objects are presented. Core RMs for quasars vary from approximately 500 to several thousand rad m-2. Quasar jets have RMs that are typically 500 rad m-2 or less. The cores and jets of the BL Lac objects have RMs similar to those found in quasar jets. The jets of radio galaxies exhibit a range of RMs from a few hundred to almost 10,000 rad m-2 for the jet of M87. Radio galaxy cores are generally depolarized, and only one of four radio galaxies (3C 120) has a detectable RM in the core. Several potential identities for the foreground Faraday screen are considered, and we believe the most promising candidate for all the active galactic nucleus types considered is a screen in close proximity to the jet. This constrains the path length to approximately 10 pc, and magnetic field strengths of approximately 1 μG can account for the observed RMs. For 27 out of 34 quasars and BL Lac objects, their optically thick cores have good agreement with a λ2 law. This requires the different τ = 1 surfaces to have the same intrinsic polarization angle independent of the observed frequency and distance from the black hole.

Very Long Baseline Array Polarimetry of Three Powerful Radio Galaxy Cores

We present sensitive, high dynamic range, Very Long Baseline Array (VLBA) polarimetric observations of the cores of three powerful radio galaxies: 3C 166, 3C 236, and 3C 390.3. Significant polarization is detected in one source (3C 166) allowing us to map out the Faraday rotation measure (RM) distribution and projected magnetic field direction. The inner jet of 3C 166 is found to have a rest frame RM of -2300 rad m-2, similar to those found in quasar cores. No polarized flux was detected from the other two sources, but in both, counterjets are seen. The counterjet in 3C 236 was previously known, but the detection in 3C 390.3 is a new discovery. We suggest that the low fractional polarization in radio galaxy cores is the result of Faraday depolarization by ionized gas associated with the accretion disk. The lower polarization of radio galaxy cores compared to quasars is then naturally explained by unified models as a result of the viewing angle.

Parsec-scale radio cores in early-type galaxies

We find compact (<0.03 arcsec) radio-continuum cores in about 70 per cent of radio-emitting elliptical and S0 galaxies over a wide range in total radio power (1021–1026 WHz–1 at 5 GHz). The cores usually have a flat or rising spectrum between 2.3 and 8.4 GHz, with a median spectral index of + 0.3. Even at low luminosities, the radio emission from most elliptical galaxies appears to be powered by a parsec-scale ‘engine’ like those in classical radio galaxies and quasars. The core and total radio power are related (⁠|${P_{{\rm{core}}}}\, \propto \,P_{{\rm{total}}}^{0.7}$| on average), and the parsec-scale cores of radio galaxies are typically one hundred times more powerful than those in ‘normal’ giant elliptical galaxies.