The clustering of radio galaxies at z~0.55 from the 2SLAQ LRG survey

We examine the clustering properties of low-power radio galaxies at redshift 0.4<z<0.8, using data from the 2SLAQ Luminous Red Galaxy (LRG) survey. We find that radio-detected LRGs (with optical luminosities of 3-5L* and 1.4GHz radio powers between 1e24 and 1e26 W/Hz) are significantly more clustered than a matched sample of radio-quiet LRGs with the same distribution in optical luminosity and colour. The measured scale length of the 2pt auto-correlation function, r0, is 12.3+/-1.2 1/h Mpc and 9.02+/-0.52 1/h Mpc for the radio-detected and radio-quiet samples respectively. Using the halo model framework we demonstrate that the radio-loud LRGs have typical halo masses of 10.1+/-1.4 x10^13 1/h M_sun compared to 6.44+/-0.32 x10^13 1/h M_sun for the radio-quiet sample. A model in which the radio-detected LRGs are almost all central galaxies within haloes provides the best fit, and we estimate that at least 30% of LRGs with the same clustering amplitude as the radio-detected LRGs are currently radio-loud. Our results imply that radio-loud LRGs typically occupy more massive haloes than other LRGs of the same optical luminosity, so the probability of finding a radio-loud AGN in a massive galaxy at z~0.55 is influenced by the halo mass in addition to the dependence on optical luminosity. If we model the radio-loud fraction of LRGs, F_rad, as a function of halo mass M, then the data are well-fitted by a power law of the form F_rad \propto M^(0.65+/-0.23). The relationship between radio emission and clustering strength could arise either through a higher fuelling rate of gas onto the central black holes of galaxies in the most massive haloes (producing more powerful radio jets) or through the presence of a denser IGM (providing a more efficient working surface for the jets, thus boosting their radio luminosity).Comment: Accepted for publication in MNRA

The Large Area Radio Galaxy Evolution Spectroscopic Survey (LARGESS): survey design, data catalogue and GAMA/WiggleZ spectroscopy

© 2016 The Authors. We present the Large Area Radio Galaxy Evolution Spectroscopic Survey (LARGESS), a spectroscopic catalogue of radio sources designed to include the full range of radio AGN populations out to redshift z ~ 0.8. The catalogue covers ~800 deg 2 of sky, and provides optical identifications for 19 179 radio sources from the 1.4 GHz Faint Images of the Radio Sky at Twenty-cm (FIRST) survey down to an optical magnitude limit of i mod < 20.5 in Sloan Digital Sky Survey (SDSS) images. Both galaxies and point-like objects are included, and no colour cuts are applied. In collaboration with the WiggleZ and Galaxy And Mass Assembly (GAMA) spectroscopic survey teams, we have obtained new spectra for over 5000 objects in the LARGESS sample. Combining these new spectra with data from earlier surveys provides spectroscopic data for 12 329 radio sources in the survey area, of which 10 856 have reliable redshifts. 85 per cent of the LARGESS spectroscopic sample are radio AGN (median redshift z = 0.44), and 15 per cent are nearby star-forming galaxies (median z = 0.08). Low-excitation radio galaxies (LERGs) comprise the majority (83 per cent) of LARGESS radio AGN at z < 0.8, with 12 per cent being high-excitation radio galaxies (HERGs) and 5 per cent radioloud QSOs. Unlike the more homogeneous LERG and QSO sub-populations, HERGs are a heterogeneous class of objects with relatively blue optical colours and a wide dispersion in mid-infrared colours. This is consistent with a picture in which most HERGs are hosted by galaxies with recent or ongoing star formation as well as a classical accretion disc

Optical Properties of High-Frequency Radio Sources from the Australia Telescope 20 GHz (AT20G) Survey

Our current understanding of radio-loud AGN comes predominantly from studies at frequencies of 5 GHz and below. With the recent completion of the Australia Telescope 20 GHz (AT20G) survey, we can now gain insight into the high-frequency radio properties of AGN. This paper presents supplementary information on the AT20G sources in the form of optical counterparts and redshifts. Optical counterparts were identified using the SuperCOSMOS database and redshifts were found from either the 6dF Galaxy survey or the literature. We also report 144 new redshifts. For AT20G sources outside the Galactic plane, 78.5% have optical identifications and 30.9% have redshift information. The optical identification rate also increases with increasing flux density. Targets which had optical spectra available were examined to obtain a spectral classification. There appear to be two distinct AT20G populations; the high luminosity quasars that are generally associated with point-source optical counterparts and exhibit strong emission lines in the optical spectrum, and the lower luminosity radio galaxies that are generally associated with passive galaxies in both the optical images and spectroscopic properties. It is suggested that these different populations can be associated with different accretion modes (cold-mode or hot-mode). We find that the cold-mode sources have a steeper spectral index and produce more luminous radio lobes, but generally reside in smaller host galaxies than their hot-mode counterparts. This can be attributed to the fact that they are accreting material more efficiently. Lastly, we compare the AT20G survey with the S-cubed semi-empirical (S3-SEX) models and conclude that the S3-SEX models need refining to correctly model the compact cores of AGN. The AT20G survey provides the ideal sample to do this.Comment: Accepted for publication in MNRA

A small area faint KX redshift survey for QSOs in the ESO Imaging Survey Chandra Deep Field South

In this paper we present preliminary spectroscopic results from a small area faint K-excess (KX) survey, and compare KX selection against UVX selection. The aim of the KX method is to produce complete samples of QSOs flux-limited in the K band, in order to minimize any selection bias in samples of QSOs from the effects of reddening and extinction. Using the photometric catalogue of the ESO Imaging Survey Chandra Deep Field South (48 arcmin^2) we have identified compact objects with J-K colours redder than the stellar sequence, that are brighter than K=19.5. We have obtained spectra of 33 candidates, using the LDSS++ spectrograph on the AAT. Amongst the 11 bluer candidates, with V-J<3, three are confirmed as QSOs. Identification of the 22 redder candidates with V-J>3 is substantially incomplete, but so far no reddened QSOs have been found. Near-infrared spectroscopy will be more effective in identifying some of these targets. Only two UVX (U-B<-0.2) sources brighter than K=19.5 are found which are not also KX selected. These are both identified as galactic stars. Thus KX selection appears to select all UVX QSOs. The surface density of QSOs in the blue subsample (V-J<3) at K<19.5 is 325^+316_-177 deg^-2. Because identification of the red subsample (V-J>3) is substantially incomplete, the 2sigma upper limit on the density of reddened QSO is large, <1150 deg^-2. As anticipated, at these faint magnitudes the KX sample includes several compact galaxies. Of the 14 with measured redshifts, there are roughly equal numbers of early and late type objects. Nearly all the early type galaxies are found in a single structure at z=0.66.Comment: 9 pages, 8 figures. Accepted for publication in MNRA

IFU observations of luminous type II AGN - I. Evidence for ubiquitous winds

We present observations of 17 luminous (log(L[O III]/L) > 8.7) local (z < 0.11) type II AGN. Our aim is to investigate the prevalence and nature of AGN-driven outflows in these galaxies by combining kinematic and ionization diagnostic information. We use non-parametric methods (e.g. W80, the width containing 80 per cent of the line flux) to assess the line widths in the central regions of our targets. The maximum values of W80 in each galaxy are in the range 400–1600 km s−1, with a mean of 790 ± 90 km s−1. Such high velocities are strongly suggestive that these AGN are driving ionized outflows. Multi-Gaussian fitting is used to decompose the velocity structure in our galaxies. 14/17 of our targets require three separate kinematic components in the ionized gas in their central regions. The broadest components of these fits have FWHM = 530–2520 km s−1, with a mean value of 920 ± 50 km s−1. By simultaneously fitting both the Hβ/[O III] and Hα/[N II] complexes, we construct ionization diagnostic diagrams for each component. 13/17 of our galaxies show a significant (>95 per cent) correlation between the [N II]/Hα ratio and the velocity dispersion of the gas. Such a correlation is the natural consequence of a contribution to the ionization from shock excitation and we argue that this demonstrates that the outflows from these AGN are directly impacting the surrounding ISM within the galaxies. Key words: galaxies: active – galaxies: evolution – galaxies: kinematics and dynamic

The SAMI Galaxy Survey: Revising the Fraction of Slow Rotators in IFS Galaxy Surveys

The fraction of galaxies supported by internal rotation compared to galaxies stabilized by internal pressure provides a strong constraint on galaxy formation models. In integral field spectroscopy surveys, this fraction is biased because survey instruments typically only trace the inner parts of the most massive galaxies. We present aperture corrections for the two most widely used stellar kinematic quantities $V/\sigma$ and $\lambda_{R}$. Our demonstration involves integral field data from the SAMI Galaxy Survey and the ATLAS$^{\rm{3D}}$ Survey. We find a tight relation for both $V/\sigma$ and $\lambda_{R}$ when measured in different apertures that can be used as a linear transformation as a function of radius, i.e., a first-order aperture correction. We find that $V/\sigma$ and $\lambda_{R}$ radial growth curves are well approximated by second order polynomials. By only fitting the inner profile (0.5$R_{\rm{e}}$), we successfully recover the profile out to one $R_{\rm{e}}$ if a constraint between the linear and quadratic parameter in the fit is applied. However, the aperture corrections for $V/\sigma$ and $\lambda_{R}$ derived by extrapolating the profiles perform as well as applying a first-order correction. With our aperture-corrected $\lambda_{R}$ measurements, we find that the fraction of slow rotating galaxies increases with stellar mass. For galaxies with $\log M_{*}/M_{\odot}>$ 11, the fraction of slow rotators is $35.9\pm4.3$ percent, but is underestimated if galaxies without coverage beyond one $R_{\rm{e}}$ are not included in the sample ($24.2\pm5.3$ percent). With measurements out to the largest aperture radius the slow rotator fraction is similar as compared to using aperture corrected values ($38.3\pm4.4$ percent). Thus, aperture effects can significantly bias stellar kinematic IFS studies, but this bias can now be removed with the method outlined here.Comment: Accepted for Publication in the Monthly Notices of the Royal Astronomical Society. 16 pages and 11 figures. The key figures of the paper are: 1, 4, 9, and 1

Is the observed high-frequency radio luminosity distribution of QSOs bimodal?

The distribution of QSO radio luminosities has long been debated in the literature. Some argue that it is a bimodal distribution, implying that there are two separate QSO populations (normally referred to as 'radio-loud' and 'radio-quiet'), while others claim it forms a more continuous distribution characteristic of a single population. We use deep observations at 20 GHz to investigate whether the distribution is bimodal at high radio frequencies. Carrying out this study at high radio frequencies has an advantage over previous studies as the radio emission comes predominantly from the core of the AGN, hence probes the most recent activity. Studies carried out at lower frequencies are dominated by the large scale lobes where the emission is built up over longer timescales (10^7-10^8 yrs), thereby confusing the sample. Our sample comprises 874 X-ray selected QSOs that were observed as part of the 6dF Galaxy Survey. Of these, 40% were detected down to a 3 sigma detection limit of 0.2-0.5 mJy. No evidence of bimodality is seen in either the 20 GHz luminosity distribution or in the distribution of the R_20 parameter: the ratio of the radio to optical luminosities traditionally used to classify objects as being either radio-loud or radio-quiet. Previous results have claimed that at low radio luminosities, star formation processes can dominate the radio emission observed in QSOs. We attempt to investigate these claims by stacking the undetected sources at 20 GHz and discuss the limitations in carrying out this analysis. However, if the radio emission was solely due to star formation processes, we calculate that this corresponds to star formation rates ranging from ~10 solar masses/yr to ~2300 solar masses/yr.Comment: 13 pages, 11 figures. Accepted for publication in Ap