Radio galaxies of the local universe (original) (raw)
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Monthly Notices of the Royal Astronomical Society, 2004
The results of analysis of HST I−band imaging of a sample of 41 radio galaxies spanning three orders of magnitude in radio luminosity at redshift z ≃ 0.5 are presented. These results represent the first stage in a coordinated programme to study the connection between radio luminosity and host-galaxy properties, black-hole masses and cluster environments in radio galaxies spanning a wide range in radio luminosity over a restricted range in redshift. The full sample is comprised of objects drawn from four complete, low-frequency selected radio samples with progressively fainter flux-density limits (3CRR, 6CE, 7CRS and the new TexOx-1000 sample). Modelling of the HST imaging data shows that the host galaxies have surface-brightness distributions consistent with those expected for classic ellipticals (Sérsic parameter, β ≃ 0.25), with β in the range 0.17 < β < 0.30, and a mean of < β >= 0.23 ± 0.01. The luminosities of the host galaxies are found to be comparable with those of galaxies drawn from the bright end of the local cluster galaxy luminosity function, spanning the range 0.7L ⋆ < L < 10L ⋆ , with a mean of 3.2 ± 0.3L ⋆ , after correcting for the effects of passive evolution. In addition, the radio galaxies are shown to follow a Kormendy (µ e −r e ) relation indistinguishable from that of both powerful low-redshift radio galaxies and local Abell brightest cluster galaxies. Combining our new results with those in the literature it is found that the scalelengths and Kormendy relations of 3C-class radio galaxies do not vary significantly over the redshift range 0.0 < z < 0.8, providing no evidence for dynamical evolution of this class of host galaxy within this redshift interval. Converting the host-galaxy luminosities into black-hole mass estimates, using the local M bh − M bulge correlation, predicts that the radio galaxies harbour central black holes with masses in the range 10 8.1 M ⊙ < M bh < 10 9.5 M ⊙ , with a geometric mean of < M bh >= 10 8.87±0.04 M ⊙ . Finally, a significant (≃ 3σ) correlation is found between black-hole mass and 151-MHz radio luminosity for those objects in the sample with either high-excitation nuclear spectra (HEG) or classical double (CD) radio structures.
The 2dF Galaxy Redshift Survey: the population of nearby radio galaxies at the 1-mJy level
Monthly Notices of the Royal Astronomical Society, 2002
We use redshift determinations and spectral analysis of galaxies in the 2dF Galaxy Redshift Survey to study the properties of local radio sources with S ≥ 1 mJy. 557 objects (hereafter called the spectroscopic sample) drawn from the FIRST survey, corresponding to 2.3 per cent of the total radio sample, are found in the 2dFGRS catalogue within the area 9 h 48 m ∼ < RA(2000) ∼ < 14 h 32 m and −2.77 • ∼ < dec(2000) ∼ < 2.25 • , down to a magnitude limit b J = 19.45. The excellent quality of 2dF spectra allows us to divide these sources into classes, according to their optical spectra.
The Spitzer High-redshift Radio Galaxy Survey
The Astrophysical …, 2010
We present results from a comprehensive imaging survey of 70 radio galaxies at redshifts 1 < z < 5.2 using all three cameras onboard the Spitzer Space Telescope. The resulting spectral energy distributions unambiguously show a stellar population in 46 sources and hot dust emission associated with the active nucleus in 59. Using a new restframe S 3 µm /S 1.6 µm versus S 5 µm /S 3 µm criterion, we identify 42 sources where the restframe 1.6 µm emission from the stellar population can be measured. For these radio galaxies, the median stellar mass is high, 2 × 10 11 M ⊙ , and remarkably constant within the range 1 < z < 3. At z > 3, there is tentative evidence for a factor of two decrease in stellar mass. This suggests that radio galaxies have assembled the bulk of their stellar mass by z ∼ 3, but confirmation by more detailed decomposition of stellar and AGN emission is needed.
Stellar populations in a complete sample of local radio galaxies
Monthly Notices of the Royal Astronomical Society, 2005
We investigate the nature of the continuum emission and stellar populations in the inner 1-3 kiloparsecs of a complete sample of twenty-four southern radio galaxies, and compare the results with a control sample of eighteen non-active early-type galaxies. Twelve of the radio galaxies are classified as Fanaroff-Riley type I (FR I), eight as FR II and four as intermediate or undefined type (FR x). Optical long-slit spectra are used to perform spectral synthesis as a function of distance from the nucleus at an average sampling of 0.5-1.0 kpc and quantify the relative contributions of a blue featureless continuum and stellar population components of different ages. Our main finding is a systematic difference between the stellar populations of the radio and control sample galaxies: the former have a larger contribution from an intermediate age (1 Gyr) component, suggesting a connection between the present radio activity and a starburst which occurred ∼1 Gyr ago. In addition, we find a correlation between the contribution of the 1 Gyr component and the radio power, suggesting that more massive starbursts have led to more powerful radio emission. A similar relation is found between the radio power and the mean age of the stellar population, in the sense that stronger nuclear activity is found in younger galaxies. We also find that the stellar populations of FR I galaxies are, on average, older and more homogeneous than those of FR IIs. Significant population gradients were found in only four radio galaxies, which are also those with more than 10 % of their total flux at 4020Å contributed by age components younger than 100 Myr and/or a featureless continuum (indistinguishable from a 3 Myr old stellar population).
The Massive Hosts of Radio Galaxies Across Cosmic Time
The Astrophysical …, 2007
We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1 < z < 5.2. Using IRAC (3.6 − 8.0 µm), IRS (16 µm) and MIPS (24 − 160 µm) imaging, we decompose the rest-frame optical to infrared spectral energy distributions into stellar, AGN, and dust components and determine the contribution of host galaxy stellar emission at rest-frame H− band. Stellar masses derived from rest-frame near-IR data, where AGN and young star contributions are minimized, are significantly more reliable than those derived from rest-frame optical and UV data. We find that the fraction of emitted light at rest-frame H− band from stars is > 60% for ∼ 75% the high redshift radio
The 2dF galaxy redshift survey: clustering properties of radio galaxies
Monthly Notices of the Royal Astronomical Society, 2004
The clustering properties of local, S 1.4GHz ≥ 1 mJy, radio sources are investigated for a sample of 820 objects drawn from the joint use of the FIRST and 2dF Galaxy Redshift surveys. To this aim, we present 271 new b J ≤ 19.45 spectroscopic counterparts of FIRST radio sources to be added to those already introduced in Magliocchetti et al. (2002). The two-point correlation function for the local radio population is found to be entirely consistent with estimates obtained for the whole sample of 2dFGRS galaxies. From measurements of the redshift-space correlation function ξ(s) we derive a redshift-space clustering length s 0 = 10.7 +0.8 −0.7 Mpc, while from the projected correlation function Ξ(r T ) we estimate the parameters of the real-space correlation function ξ(r) = (r/r 0 ) −γ , r 0 = 6.7 +0.9 −1.1 Mpc and γ = 1.6 ± 0.1, where h = 0.7 is assumed. Different results are instead obtained if we only consider sources that present signatures of AGN activity in their spectra. These objects are shown to be very strongly correlated, with r 0 = 10.9 +1.0 −1.2 Mpc and γ = 2 ± 0.1, a steeper slope than has been claimed in other recent works. No difference is found in the clustering properties of radio-AGNs of different radio luminosity. Comparisons with models for ξ(r) show that AGN-fuelled sources reside in dark matter halos more massive than ∼ 10 13.4 M ⊙ , higher the corresponding figure for radio-quiet QSOs. This value can be converted into a minimum black hole mass associated with radio-loud, AGN-fuelled objects of M min BH ∼ 10 9 M ⊙ . The above results then suggest -at least for relatively faint radio objects -the existence of a threshold black hole mass associated with the onset of significant radio activity such as that of radio-loud AGNs; however, once the activity is triggered, there appears to be no evidence for a connection between black hole mass and level of radio output.
BLAST: the far-infrared/radio correlation in distant galaxies
Monthly Notices of the Royal Astronomical Society, 2010
We investigate the correlation between far-infrared (FIR) and radio luminosities in distant galaxies, a lynchpin of modern astronomy. We use data from the Balloon-borne Large Aperture Submillimetre Telescope (BLAST), Spitzer, the Large Apex BOlometer Cam-erA (LABOCA), the Very Large Array (VLA) and the Giant Metre-wave Radio Telescope (GMRT) in the Extended Chandra Deep Field South (ECDFS). For a catalogue of BLAST 250-µm-selected galaxies, we re-measure the 70-870-µm flux densities at the positions of their most likely 24-µm counterparts, which have a median [interquartile] redshift of 0.74 [0.25, 1.57]. From these, we determine the monochromatic flux density ratio, q 250 (= log 10 [S 250µm /S 1,400MHz ]), and the bolometric equivalent, q IR . At z ≈ 0.6, where our 250-µm filter probes rest-frame 160-µm emission, we find no evolution relative to q 160 for local galaxies. We also stack the FIR and submm images at the positions of 24-µm-and radio-selected galaxies. The difference between q IR seen for 250-µm-and radio-selected galaxies suggests star formation provides most of the IR luminosity in < ∼ 100-µJy radio galaxies, but rather less for those in the mJy regime. For the 24-µm sample, the radio spectral index is constant across 0 < z < 3, but q IR exhibits tentative evidence of a steady decline such that q IR ∝ (1 + z) −0.15±0.03 -significant evolution, spanning the epoch of galaxy formation, with major implications for techniques that rely on the FIR/radio correlation. We compare with model predictions and speculate that we may be seeing the increase in radio activity that gives rise to the radio background.
Radio Detection of Radio-Quiet Galaxies
The Astronomical Journal, 2008
We investigate the radio emission of ∼ 185,000 quiescent (optically unclassifiable) galaxies selected from the Sloan Digital Sky Survey (SDSS). By median-stacking FIRST cutouts centered on the optically-selected sources, we are able to reach flux densities down to the 10s of µJy. The quiescent galaxy sample is composed of two subgroups inhabiting vastly different regimes: those targeted for the SDSS MAIN Galaxy Sample (∼55%), and those targeted for the Luminous Red Galaxy (LRG) sample (∼45%). To investigate the star-formation of these quiescent galaxies, we calibrate a radio-SFR conversion using a third sample of star-forming galaxies. We confirm a tight power-law dependence for the star-forming (SF) sample, where L 1.4GHz ∼ (SFR) 1.37. Comparing this SFR-indicator with indicators in the optical and UV, we derive conflicting SFR estimates for the MAIN sample quiescent galaxies. These radio-derived SFRs intersect those calculated using the 4000-Å break (D4000) around an SFR of 1 M ⊙ yr −1 and agree to within a factor of 3 over the range of SFRs. However, we find that the radio-derived SFRs are too high relative to the SFRs estimated for similar populations of galaxies using analysis of UV emission, implying either contamination of the radio by Active Galactic Nuclei (AGN) or incomplete dust modeling. If AGN activity is dominant in these galaxies, then a relation between AGN radio luminosity and galaxy mass is required to explain the observed trends. For the LRGs, on the other hand, we find the radio luminosity to be relatively high (compared to the SF galaxies) and independent of SFR as derived from D4000, indicating an AGN component dominates their radio emission. AGN-based radio emission often implies the existence of radio jets, providing evidence of a mechanism for low-level feedback in these quiescent LRGs.