High Redshift Radio Galaxies (original) (raw)

Erratum: Radio galaxies at z? 2.5: Results from Keck spectropolarimetry

Astronomy and Astrophysics, 2001

In classifying the ensemble of powerful extragalactic radio sources, considerable evidence has accumulated that radio galaxies and quasars are orientation-dependent manifestations of the same parent population: massive spheroidal galaxies containing correspondingly massive black holes. One of the key factors in establishing this unification has been the signature of a hidden quasar detected in some radio galaxies in polarized light. The obscuration of our direct view of the active nucleus usually, but not necessarily exclusively, by a thick nuclear disk or torus can act conveniently as a "natural coronograph" that allows a much clearer view of the host of a radio galaxy than of a quasar. In this study, we exploit the opportunity to eliminate the quasar glare by performing sensitive spectropolarimetry with the Keck II telescope of a sample of radio galaxies with redshifts around 2.5. This represents the epoch when quasars were many times more common that they are now and is likely to be the period during which their host galaxies were being assembled into what become the most massive galaxies in the Universe today. We show that dust-reflected quasar light generally dominates the restframe ultraviolet continuum of these sources and that a highly clumped scattering medium results in almost grey scattering of the active galactic nucleus photons. The observations, however, do not exclude a substantial star formation rate averaged over a Gyr of evolution. The sub-mm reradiation from the scattering dust is likely to represent only a small fraction (∼10%) of the total far infrared luminosity. An analysis of the emission lines excited in the interstellar medium of the host galaxy by the hard quasar radiation field reveals evidence of a dramatic chemical evolution within the spheroid during this epoch. Secondary nitrogen production in intermediate mass stars produces a characteristic signature in the Nv/Civ and Nv/Heii line ratios which has been seen previously in the broad line region of quasars at similar redshifts. We find intriguing correlations between the strengths of the Lyα and Nv emission lines and the degree of ultraviolet continuum polarization which may represent the dispersal of dust associated with the chemical enrichment of the spheroid.

The nuclear radio-optical properties of intermediate-redshift FR�II radio galaxies and quasars

Astronomy and Astrophysics, 2004

We extend the study of the nuclei of 3CR objects as seen with the HST to higher redshift FR II radio sources (0.4 < z < 0.6). Our results reflect what has been found for FR II of lower redshift. The position of the nuclei on the plane formed by optical and radio luminosities is related to their optical spectral classification: the nuclei of both high and low excitation galaxies lie on the correlation found for FR I sources, while broad-lined objects have a significant optical excess. The nuclear properties of these sources are better understood when considering the equivalent width of the [OIII] emission line with respect to their optical luminosities. Even in the range of redshift considered here, low excitation galaxies show peculiar nuclear properties, more similar to those observed in FR I. This confirms that not all FR II are unified with quasars. Our findings have important implications for the FR II-quasar unification scheme: by reconsidering the classification of all 3CR radio galaxies with z < 1 in the light of their nuclear properties, we derive the opening angle of the obscuring torus for different redshift bins. We find that the covering factor of the obscuring structure decreases as redshift increases (θ ∼ 44 • for z < 0.3 to θ ∼ 56 • for 0.5 < z < 1). We argue that this behavior may be interpreted in the framework of the receding torus model, in which the opening angle of the torus increases as the luminosity of the accretion disk around the central black hole increases.

Optical off-nuclear spectra of quasar hosts and radio galaxies

Monthly Notices of the Royal Astronomical Society, 2000

We present optical (∼ 3200Å to ∼ 9000Å ) off-nuclear spectra of 26 powerful active galaxies in the redshift range 0.1 ≤ z ≤ 0.3, obtained with the Mayall and William Herschel 4-meter class telescopes. The sample consists of radio-quiet quasars, radio-loud quasars (all with −23 ≥ M V ≥ −26) and radio galaxies of Fanaroff & Riley Type II (with extended radio luminosities and spectral indices comparable to those of the radio-loud quasars). The spectra were all taken approximately 5 arcseconds offnucleus, with offsets carefully selected so as to maximise the amount of galaxy light falling into the slit, whilst simultaneously minimising the amount of scattered nuclear light. The majority of the resulting spectra appear to be dominated by the integrated stellar continuum of the underlying galaxies rather than by light from the non-stellar processes occurring in the active nuclei, and in many cases a 4000Å break feature can be identified. The individual spectra are described in detail, and the importance of the various spectral components is discussed. Stellar population synthesis modelling of the spectra will follow in a subsequent paper .

Stellar populations in the nuclear regions of nearby radio galaxies

Monthly Notices of the Royal Astronomical Society, 2001

We present optical spectra of the nuclei of seven luminous (P 178MHz > ∼ 10 25 W Hz −1 Sr −1 ) nearby (z < 0.08) radiogalaxies, which mostly correspond to the FR II class. In two cases, Hydra A and 3C 285, the Balmer and λ4000Å break indices constrain the spectral types and luminosity classes of the stars involved, revealing that the blue spectra are dominated by blue supergiant and/or giant stars. The ages derived for the last burst of star formation in Hydra A are between 7 and 40 Myr, and in 3C 285 about 10 Myr. The rest of the narrow-line radiogalaxies (four) have λ4000Å break and metallic indices consistent with those of elliptical galaxies. The only broad-line radiogalaxy in our sample, 3C 382, has a strong featureless blue continuum and broad emission lines that dilute the underlying blue stellar spectra. We are able to detect the Ca II triplet in absorption in the seven objects, with good quality data for only four of them. The strengths of the absorptions are similar to those found in normal elliptical galaxies, but these values are both consistent with single stellar populations of ages as derived from the Balmer absorption and break strengths, and, also, with mixed young+old populations.

Radio-Excess IRAS Galaxies. IV. Optical Spectroscopy

The Astronomical Journal, 2006

This is the fourth in our series of papers discussing the nature of radio-excess galaxies, which have radio emission associated with an active nucleus but which do not fit into the traditional categories of either radio-loud or radio-quiet active galaxies. In this paper, we present optical spectra of our sample of FIR-luminous radio-excess galaxies. Optical emission line ratio diagnostics are used to determine the dominant source of the ionizing radiation. We find that radio excess is an excellent indicator of the presence of an active nucleus. The radio-excess sample contains a much higher fraction of AGN than samples selected on FIR luminosity alone, or using other criteria such as warm FIR colors. Several objects have ambiguous classifications and are likely to be composite objects with mixed excitation. The type of optical spectrum appears to be associated with the radio-loudness: our results suggest that radio-loud objects may be more 'pure' AGN than radio-intermediate objects. We find strong evidence for interaction between the radio plasma and the surrounding gas. Broad, structured optical emission lines are observed and a relative blueshift is measured between the [O III] λ5007 and Hα lines in several sources. Jet energy fluxes are inferred from the [O III] λ5007 luminosities using a shock model for the interaction between the radio jet and the line-emitting gas. The jet energy fluxes of the radioexcess objects are lower than in powerful radio sources, consistent with our previous results. We conclude that the jets of radio-intermediate sources are intrinsically weaker than those in sources with more powerful radio emission. A significant fraction of the sample spectra show post-starburst stellar continuum, with A-star absorption lines. Post-starburst stellar populations are consistent with the large fraction of merging or disturbed host galaxies in the sample. The ages of the radio sources are significantly less than those of A stars indicating that, if the radio sources are associated with merging activity, there is a delay between the interaction and the initiation of the radio activity.

Far‐Infrared Observations of Radio Quasars and FR II Radio Galaxies

The Astrophysical Journal, 2005

We report MIPS photometry of 20 radio-loud quasars and galaxies at 24 and 70 µm (and of five at 160 µm). We combine this sample with additional sources detected in the far infrared by IRAS and ISO for a total of 47 objects, including 23 steep spectrum Type I AGNs: radio-loud quasars and broad line radio galaxies; and 24 Type II AGNs: narrow line and weak line FR II radio galaxies. Of this sample, the far infrared emission of all but 3C 380 appears to be dominated by emission by dust heated by the AGN and by star formation. The AGN appears to contribute more than 50% of the far infrared luminosity in most of sources. It is also expected that the material around the nucleus is optically thin in the far infrared. Thus, the measurements at these wavelengths can be used to test the orientation-dependent unification model. As predicted by the model, the behavior of the sources is consistent with the presence of an obscuring circumnuclear torus; in fact, we find it may still have significant optical depth at 24 µm. In addition, as expected for the radio-loud quasars, there is a significant correlation between the low frequency radio (178 MHz) and the 70 µm emission, two presumably isotropic indicators of nuclear activity. This result is consistent with the simple unified scheme. However, there is a population of radio galaxies that are underluminous at 70 µm compared with the radio-loud quasars and hence are a challenge to the simple unified model.

COLA II - Radio and Spectroscopic Diagnostics of Nuclear Activity in Galaxies

2002

We present optical spectroscopic observations of 93 galaxies taken from the infra-red selected COLA (Compact Objects in Low Power AGN) sample. These are all galaxies for which we have previously obtained low resolution radio observations and high resolution (< 0.05 ′′ ) Long Baseline Array (LBA) snapshots. The sample spans the range of far-IR luminosities from normal galaxies to LIRGs, and contains a significant number of galaxies involved in galaxy-galaxy interactions. Of the galaxies observed, 78 (84%) exhibit emission lines indicating that they are either AGN or actively forming stars (starburst galaxies). Using a newly-developed theoretically-based optical emission-line scheme to classify the spectra, we find 15% of the emission-line galaxies are Seyferts, 77% are starbursts, and the rest are either borderline AGN/starburst or show ambiguous characteristics. We find little evidence for an increase in the fraction of AGN in the sample as a function of far-IR luminosity, in contrast to previous studies, but our sample covers only a small range in infrared luminosity (10.5 ≤ L F IR ≤11.7) and thus a weak trend may be masked. Instead, as the infrared luminosity increases so does the fraction of metal-rich starbursts; objects which on more traditional diagnostic diagrams would have been classified as weak LINERs (Low Ionization Narrow Emission Line Regions). As a whole the Seyfert galaxies exhibit a small, but statistically significant, radio excess on the radio-FIR correlation compared to the galaxies classified as starbursts. Compact (<0.05") radio cores are detected in 55% of the Seyfert galaxies, and these galaxies exhibit a significantly larger radio excess than the Seyfert galaxies in which compact cores were not detected. Our results indicate that there may be two distinct populations of Seyferts, "radioexcess" Seyferts, which exhibit extended radio structures and compact radio cores, and "radio-quiet" Seyferts, in which the majority of the radio emission can be attributed to star-formation in the host galaxy. No significant difference is seen between the IR and optical spectroscopic properties of Seyferts with and without radio cores.

The Nuclear to Host Galaxy Relation of High‐Redshift Quasars

The Astrophysical Journal, 2007

We present near-infrared imaging obtained with ESO VLT+ISAAC of the host galaxies of a sample of low luminosity quasars in the redshift range 1 < z < 2, aimed at investigating the relationship between the nuclear and host galaxy luminosities at high redshift. This work complements our previous systematic study to trace the cosmological evolution of the host galaxies of high luminosity quasars ). The new sample includes 15 low luminosity quasars, of which nine are radio-loud (RLQ) and six are radio-quiet (RQQ). They have similar distribution of redshift and optical luminosity, and together with the high luminosity quasars they cover a large range (∼4 mag) of the quasar

The near-infrared properties of the host galaxies of radio quasars

Astronomy and Astrophysics, 2003

We present K-band images of 31 radio quasars selected from the B3-VLA quasar sample. A new method has been developed to detect and restore the host galaxies of these quasars. We have detected the host galaxies of 16 of them. Using these data together with previous data from the B3-VLA quasars (Carballo et al. 1998, hereafter Paper I) and literature data from other samples, we have analyzed a sample of 69 hosts of radio quasars, covering a redshift range between 0 < z < 3. The host galaxies are large (r e ∼ 15 kpc) and luminous elliptical galaxies (∼75% of them brighter than L * K), with an evolution similar to that of radio galaxies. A significant fraction (∼40%) of them shows evidence of a possible collision/merging process. They follow a µ e −r e relation similar to that of normal elliptical galaxies. The morphological and photometric similarities between these galaxies and radio galaxies in this wide range of redshifts is a good test of the reliability of unification schemes. All of them show little evolution from z = 3 to the present epoch. Their K − z distribution is consistent with a no-evolution model, with a fraction of the dispersion due to differences in radio power. We have found a correlation between the quasar radio power and the host luminosity. We have also found a correlation between the host and nuclear source luminosities. These correlations could be induced by a physical relation between the central black hole and the bulge mass (Magorrian et al. 1998).

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.