Radio galaxies in the 2SLAQ Luminous Red Galaxy Survey: I. The evolution of low-power radio galaxies to z~0.7

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 12/2006; 381(1). DOI: 10.1111/j.1365-2966.2007.12231.x
Source: arXiv


We have combined optical data from the 2dF-SDSS (Sloan Digital Sky Survey) LRG (Luminous Red Galaxy) and QSO (quasi-stellar object) (2SLAQ) redshift survey with radio measurements from the 1.4 GHz VLA (Very Large Array) FIRST (Faint Images of the Radio Sky at Twenty-cm) and NVSS (NRAO VLA Sky Survey) surveys to identify a volume-limited sample of 391 radio galaxies at redshift 0.4 < z < 0.7. By determining an accurate radio luminosity function for luminous early-type galaxies in this redshift range, we can investigate the cosmic evolution of the radio-galaxy population over a wide range in radio luminosity.
The low-power radio galaxies in our LRG sample (those with 1.4 GHz radio luminosities in the range 1024 to 1025 W Hz−1, corresponding to Fanaroff–Riley I (FR I) radio galaxies in the local Universe) undergo significant cosmic evolution over the redshift range 0 < z < 0.7, consistent with pure luminosity evolution of the form (1 +z)k, where k= 2.0 ± 0.3. Our results appear to rule out (at the 6–7σ level) models in which low-power radio galaxies undergo no cosmic evolution. The most powerful radio galaxies in our sample (with radio luminosities above 1026 W Hz−1) may undergo more rapid evolution over the same redshift range.
The evolution seen in the low-power radio-galaxy population implies that the total energy input into massive early-type galaxies from active galactic nucleus (AGN) heating increases with redshift, and was at least 50 per cent higher at z∼ 0.55 (the median redshift of the 2SLAQ LRG sample) than in the local universe.

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    ABSTRACT: We have identified 7824 radio sources from the 1.4 GHz NRAO VLA Sky Survey (NVSS) with galaxies brighter than K= 12.75 mag in the Second Incremental Data Release of the 6 degree Field Galaxy Survey (6dFGS DR2). The resulting sample of redshifts and optical spectra for radio sources over an effective sky area of 7076 deg2 (about 17 per cent of the celestial sphere) is the largest of its kind ever obtained. NVSS radio sources associated with galaxies in the 6dFGS span a redshift range 0.003 < z < 0.3 and have median . Through visual examination of 6dF spectra we have identified the dominant mechanism for radio emission from each galaxy. 60 per cent are fuelled by star formation and 40 per cent are fuelled by an active galactic nucleus (AGN) powered by a supermassive black hole. We have accurately determined the local radio luminosity function (RLF) at 1.4 GHz for both classes of radio source and have found it to agree well with other recent determinations. From the RLF of star-forming galaxies we derive a local star formation density of 0.022 ± 0.001 M⊙ yr−1 Mpc−3, in broad agreement with recent determinations at radio and other wavelengths. We have split the RLF of radio-loud AGNs into bins of absolute K-band magnitude (MK) and compared this with the underlying K-band galaxy luminosity function of all 6dFGS galaxies to determine the bivariate radio-K-band luminosity function. We verify that radio-loud AGNs preferentially inhabit the brightest and hence most massive host galaxies and show that the fraction of all galaxies which host a radio-loud AGN scales as fradio-loud∝L2.1K for fradio-loud < 0.3, indicative of a similarly strong scaling with black hole mass and stellar mass.
    Monthly Notices of the Royal Astronomical Society 03/2007; 375(3). DOI:10.1111/j.1365-2966.2006.11353.x · 5.11 Impact Factor
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    ABSTRACT: We present an analysis of the optical spectra of a volume-limited sample of 375 radio galaxies at redshift 0.4 < z < 0.7 from the 2dF-SDSS (Sloan Digital Sky Survey) Luminous Red Galaxy (LRG) and QSO (quasi-stellar object) (2SLAQ) redshift survey. We investigate the evolution of the stellar populations and emission-line properties of these galaxies. By constructing composite spectra and comparing with a matched sample of radio-quiet sources from the same survey, we also investigate the effect on the galaxy of the presence of an active nucleus. The composite spectra, binned by redshift and radio luminosity, all require two components to describe them, which we interpret as an old and a younger population. We found no evolution with redshift of the age of the younger population in radio galaxies, nor were they different from the radio-quiet comparison sample. Similarly, there is no correlation with radio power, with the exception that the most powerful radio sources (P1.4 > 1026W Hz−1) have younger stars and stronger emission lines than the less powerful sources. This suggests that we have located the threshold in radio power where strong emission lines ‘switch on’, at radio powers of around 1026 W Hz−1. Except for the very powerful radio galaxies, the presence of a currently active radio active galactic nucleus (AGN) does not appear to be correlated with any change in the observed stellar population of a luminous red galaxy at z∼ 0.5.
    Monthly Notices of the Royal Astronomical Society 11/2007; 539(2). DOI:10.1111/j.1365-2966.2007.12741.x · 5.11 Impact Factor
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    ABSTRACT: In this paper the cosmic evolution of the space density of Fanaroff & Riley Class I (FRI) radio sources is investigated out to z ~ 1, in order to understand the origin of the differences between these and the more powerful FRIIs. High resolution radio images are presented of the best high redshift FRI candidate galaxies, drawn from two fields of the Leiden Berkeley Deep Survey, and previously defined in Rigby, Snellen & Best (2007, Paper I). Together with lower resolution radio observations (both previously published in Paper I and, for a subset of sources, also presented here) these are used to morphologically classify the sample. Sources which are clearly resolved are classified by morphology alone, whereas barely or unresolved sources were classified using a combination of morphology and flux density loss in the higher resolution data, indicative of resolved out extended emission. The space densities of the FRIs are then calculated as a function of redshift, and compared to both measurements of the local value and the behaviour of the more powerful FRIIs. The space density of FRI radio sources with luminosities (at 1.4 GHz) > 10^25 W/Hz is enhanced by a factor of 5-9 by z ~ 1, implying moderately strong evolution of this population; this enhancement is in good agreement with models of FRII evolution at the same luminosity. There are also indications that the evolution is luminosity dependent, with the lower powered sources evolving less strongly. Comment: 27 pages, 9 figures, accepted for publication in MNRAS
    Monthly Notices of the Royal Astronomical Society 12/2007; 385(1). DOI:10.1111/j.1365-2966.2008.12839.x · 5.11 Impact Factor
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