Powerful High-redshift Radio Galaxies Research Articles

A new sample of southern radio galaxies: host-galaxy masses and star-formation rates

ABSTRACT In this study we define a new sample of distant powerful radio galaxies in order to study their host galaxy properties and provide targets for future observations of H i absorption with new radio telescopes and to understand the fuelling and feedback from such sources. We have cross-matched the Sydney University Molonglo Sky Survey (SUMSS) radio catalogue at 843 MHz with the VISTA Hemisphere Survey (VHS) near-infrared catalogue using the likelihood ratio technique. Photometric redshifts from the Dark Energy Survey are then used to assign redshifts to the radio source counterparts. We found a total of 249 radio sources with photometric redshifts over a 148 deg2 region. By fitting the optical and near-infrared photometry with spectral synthesis models we determine the stellar mass and star formation rates of the radio sources, finding typical stellar masses of 1011–1012 M⊙ for the powerful high-redshift radio galaxies. We also find a population of low-mass blue galaxies. However, by comparing the derived star formation rates to the radio luminosity, we suggest that these sources are false positives in our likelihood ratio analysis. Follow-up, higher resolution (≲5 arcsec) radio imaging would help alleviate these mid-identifications, as the limiting factor in our cross-identifications is the low resolution (∼45 arcsec) of the SUMSS radio imaging.

Massive galaxies on the road to quenching: ALMA observations of powerful high redshift radio galaxies

We present 0.″3 (band 6) and 1.″5 (band 3) ALMA observations of the (sub)millimeter dust continuum emission for 25 radio galaxies at 1 < z < 5.2. Our survey reaches a rms flux density of ∼50 μJy in band 6 (200–250 GHz) and ∼20 μJy in band 3 (100–130 GHz). This is an order of magnitude deeper than single-dish 850 μm observations, and reaches fluxes where synchrotron and thermal dust emission are expected to be of the same order of magnitude. Combining our sensitive ALMA observations with low-resolution radio data from ATCA, higher resolution VLA data, and infrared photometry from Herschel and Spitzer, we have disentangled the synchrotron and thermal dust emission. We determine the star-formation rates and AGN infrared luminosities using our newly developed Multi-resolution and multi-object/origin spectral energy distribution fitting code (MR-MOOSE). We find that synchrotron emission contributes substantially at λ ∼ 1 mm. Through our sensitive flux limits and accounting for a contribution from synchrotron emission in the mm, we revise downward the median star-formation rate by a factor of seven compared to previous estimates based solely on Herschel and Spitzer data. The hosts of these radio-loud AGN appear predominantly below the main sequence of star-forming galaxies, indicating that the star formation in many of the host galaxies has been quenched. Future growth of the host galaxies without substantial black hole mass growth will be needed to bring these objects on the local relation between the supermassive black holes and their host galaxies. Given the mismatch in the timescales of any star formation that took place in the host galaxies and lifetime of the AGN, we hypothesize that a key role is played by star formation in depleting the gas before the action of the powerful radio jets quickly drives out the remaining gas. This positive feedback loop of efficient star formation rapidly consuming the gas coupled to the action of the radio jets in removing the residual gas is how massive galaxies are rapidly quenched.

Defying jet-gas alignment in two radio galaxies atz~ 2 with extended light profiles: Similarities to brightest cluster galaxies

We report the detection of extended warm ionized gas in two powerful high-redshift radio galaxies, NVSS J210626-314003 at z=2.10 and TXS 2353-003 at z=1.49, that does not appear to be associated with the radio jets. This is contrary to what would be expected from the alignment effect, a characteristic feature of distant, powerful radio galaxies at z> 0.6. The gas also has smaller velocity gradients and line widths than most other high-z radio galaxies with similar data. Both galaxies are part of a systematic study of 50 high-redshift radio galaxies with SINFONI, and are the only two that are characterized by the presence of high surface-brightness gas not associated with the jet axis and by the absence of such gas aligned with the jet. Both galaxies are spatially resolved with ISAAC broadband imaging covering the rest-frame R band, and have extended wings that cannot be attributed to line contamination. We argue that the gas and stellar properties of these galaxies are more akin to gas-rich brightest cluster galaxies in cool-core clusters than the general population of high-redshift radio galaxies at z>2. In support of this interpretation, one of our sources, TXS 2353-003, for which we have H\alpha\ narrowband imaging, is associated with an overdensity of candidate H\alpha\ emitters by a factor of 8 relative to the field at z=1.5. We discuss possible scenarios of the evolutionary state of these galaxies and the nature of their emission line gas within the context of cyclical AGN feedback.

Searching for large-scale structures around high-redshift radio galaxies with Herschel

This paper presents the first results of a far-infrared search for protocluster-associated galaxy overdensities using the SPIRE instrument on-board the {\it Herschel} Space Observatory. Large ($\sim$400 arcmin$^{2}$) fields surrounding 26 powerful high-redshift radio galaxies ($2.0 < z < 4.1$; $L_{\rm 500 MHz} > 10^{28.5}$ WHz$^{-1}$) are mapped at 250, 350 and 500\mic to give a unique wide-field sample. On average the fields have a higher than expected, compared to blank fields, surface density of 500\mic sources within 6 comoving Mpc of the radio galaxy. The analysis is then restricted to potential protocluster members only, which are identified using a far-infrared colour selection; this reveals significant overdensities of galaxies in 2 fields, neither of which are previously known protoclusters. The probability of finding 2 overdensities of this size by chance, given the number of fields observed is $5 \times 10^{-4}$. Overdensities here exist around radio galaxies with $L_{\rm 500 MHz} \gtrsim 10^{29}$ WHz$^{-1}$ and $z < 3$. The radial extent of the average far-infrared overdensity is found to be $\sim$6 comoving Mpc. Comparison with predictions from numerical simulations shows that the overdensities are consistent with having masses $> 10^{14}$Msolar. However, the large uncertainty in the redshift estimation means that it is possible that these far-infrared overdensities consist of several structures across the redshift range searched.

Radio jets in galaxies with actively accreting black holes: new insights from the SDSS

In the local Universe, the majority of radio-loud active galactic nuclei (AGN) are found in massive elliptical galaxies with old stellar populations and weak or undetected emission lines. At high redshifts, however, almost all known radio AGN have strong emission lines. This paper focuses on a subset of radio AGN with emission lines (EL-RAGN) selected from the Sloan Digital Sky Survey. We explore the hypothesis that these objects are local analogues of powerful high-redshift radio galaxies. The probability for a nearby radio AGN to have emission lines is a strongly decreasing function of galaxy mass and velocity dispersion and an increasing function of radio luminosity above 1025 W Hz−1. Emission-line and radio luminosities are correlated, but with large dispersion. At a given radio power, radio galaxies with small black holes have higher [O iii] luminosities (which we interpret as higher accretion rates) than radio galaxies with big black holes. However, if we scale the emission-line and radio luminosities by the black hole mass, we find a correlation between normalized radio power and accretion rate in Eddington units that is independent of black hole mass. There is also a clear correlation between normalized radio power and the age of the stellar population in the galaxy. Present-day EL-RAGN with the highest normalized radio powers are confined to galaxies with small black holes. High-redshift, high radio-luminosity AGN would be explained if big black holes were similarly active at earlier cosmic epochs. To investigate why only a small fraction of emission-line AGN become radio-loud, we create matched samples of radio-loud and radio-quiet AGN and compare their host galaxy properties and environments. The main difference lies in their environments; our local density estimates are a factor of 2 larger around the radio-loud AGN. We propose a scenario in which radio-loud AGN with emission lines are located in galaxies where accretion of both cold and hot gas can occur simultaneously. At the present day, these conditions are only satisfied for low-mass galaxies in dense environments, but they are likely to apply to most galaxies with massive black holes at higher redshifts.

A sensitive search for CO J = 1–0 emission in 4C 41.17: high-excitation molecular gas at z = 3.8

We report sensitive imaging observations of the CO J=1-0 line emission in the powerful high-redshift radio galaxy 4C 41.17 (z=3.8) with the NRAO Very Large Array (VLA), conducted in order to detect the large concomitant H_2 gas reservoir recently unveiled in this system by De Breuck et al (2005) via the emission of the high excitation J=4-3 line. Our observations fail to detect the J=1-0 line but yield sensitive lower limits on the R_43=(4-3)/(1-0) brightness temperature ratio of R_43 ~ 0.55 - >1.0 for the bulk of the H_2 gas mass. Such high ratios are typical of the high-excitation molecular gas phase ``fueling'' the star formation in local starbursts, but quite unlike these objects, much of the molecular gas in 4C 41.17 seems to be in such a state, and thus participating in the observed starburst episode. The widely observed and unique association of highly excited molecular gas with star forming sites allows CO line emission with large (high-J)/(low-J) intensity ratios to serve as an excellent ``marker'' of the spatial distribution of star formation in distant dust-obscured starbursts, unaffected by extinction.

Detection of CO J = 1–0 in the z = 3.79 radio galaxy 4C 60.07

We report on the detection of the lowest CO transition in the powerful high-redshift radio galaxy 4C 60.07 at . The CO emission is distributed in two spatially and kinematically distinct components as was previously known from the observations of the higher excitation CO line. The total molecular gas mass in 4C 60.07 inferred from the CO emission is , sufficient to fuel the inferred star-formation rate of ∼1600 yr-1 for 108 yrs. From our high-resolution CO VLA maps we find the dynamical mass of 4C 60.07 to be comparable to that of a giant elliptical at the present time. A significant fraction of the mass is in the form of molecular gas suggesting that 4C 60.07 is in an early state of its evolution. The merging nature of 4C 60.07 along with its large dynamical mass imply that this system is a giant elliptical caught in its formative stages.

The spatial clustering of radio sources in NVSS and FIRST; implications for galaxy clustering evolution

We have measured the angular correlation function, $w(\theta)$, of radio sources in the 1.4 GHz NVSS and FIRST radio surveys. Below ~$6\arcmin$ the signal is dominated by the size distribution of classical double radio galaxies, an effect underestimated in some previous studies. We model the physical size distribution of FRII radio galaxies to account for this excess signal in $w(\theta)$. The amplitude of the true cosmological clustering of radio sources is roughly constant at $A\simeq1\times10^{-3}$ for flux limits of 3–40 mJy, but has increased to $A\simeq7\times10^{-3}$ at 200 mJy. This can be explained if powerful (FRII) radio galaxies probe significantly more massive structures compared to radio galaxies of average power at $z\sim1$. This is consistent with powerful high-redshift radio galaxies generally having massive (forming) elliptical hosts in rich (proto-)cluster environments. For FRIIs we derive a spatial (comoving) correlation length of $r_0=14\pm3$ h -1 Mpc. This is remarkably close to that measured for extremely red objects (EROs) associated with a population of old elliptical galaxies at $z\sim1$ by [CITE]. Based on their similar clustering properties, we propose that EROs and powerful radio galaxies may be the same systems seen at different evolutionary stages. Their r 0 is ~$2\times$ higher than that of QSOs at a similar redshift, and comparable to that of bright ellipticals locally. This suggests that r 0 (comoving) of these galaxies has changed little from $z\sim1$ to $z=0$, in agreement with current Λ CDM hierarchical merging models for the clustering evolution of massive early-type galaxies. Alternatively, the clustering of radio galaxies can be explained by the galaxy conservation model. This then implies that radio galaxies of average power are the progenitors of the local field population of early-types, while the most powerful radio galaxies will evolve into a present-day population with r 0 comparable to that of local rich clusters.

Absorbers and globular cluster formation in powerful high-redshift radio galaxies

A radiative hydrodynamic simulation for a typical, powerful high redshift radio galaxy is presented. The jet is injected at one third the speed of light into a 10000 times denser, homogeneous medium. In the beginning of the simulation, the bow shock consists of a spherical shell that is similar to a spherical blast wave. This shell cools radiatively down to approximately 10,000 K, providing after 6 Mio yrs a neutral column of 3.8 x 10^21 cm^-2 around the whole system. The shell starts to fragment and forms condensations. This absorbing screen will cover a smaller and smaller fraction of the radio source, and therefore the emission line region, and eventually form stars in typically 10,000 globular clusters of one million solar masses. Approximately 10^9 solar masses are entrained into the radio cocoon. This gas, cooling and illuminated from the radio source, could be the emission line gas observed in high redshifted radio galaxies and radio loud quasars. The neutral column behind the bow shock can account for the absorption found in almost all of the small sources. The globular cluster excess of approximately 10^4 systems found in present day brightest cluster galaxies (BCGs), which are believed to be the vestiges of these objects, is consistent with the presented scenario.

An Alignment Effect in FR I Radio Galaxies:U‐Band Polarimetry of the Abell 2597 Cluster Central Galaxy

We have obtained U-band polarimetry of the spatially extended optical continuum associated with the FR I radio source PKS 2322-122, located in the Abell 2597 cluster central galaxy. We find a 3 σ upper limit to the degree of optical continuum polarization of less than 6%. The accuracy of the measurement is limited primarily by our ability to measure the amount of diluting galactic starlight. This limit is inconsistent with the continuum being primarily scattered light or synchrotron radiation. We can therefore exclude models that attribute the continuum to scattered light from an active nucleus that is hidden from direct view. Our result does not support the unification paradigm for BL Lac objects and FR I radio sources. Essentially all of the data pertaining to the continuum along the radio source—the blue lobes—indicate that they are regions of recent star formation. The spatial coincidence between the lobes and disturbances in the radio source suggests that star formation may have been induced by an interaction between the radio source and the cool (less than 104 K) surrounding gas. This result, in addition to the results of a similar study of the A1795 cluster central galaxy, shows that under the appropriate conditions FR I radio sources may be capable of inducing significant episodes of star formation in elliptical galaxies. We compare the rest-frame U-band polarized luminosities and 1.4 GHz radio powers of A2597 and A1795 to those for several high-redshift radio galaxies exhibiting the alignment effect. We find that if the polarized luminosities of radio galaxies scale in proportion to their radio luminosities, we would not have detected a polarized signal in either A2597 or A1795. We suggest that the strength of the active galactic nucleus is the fundamental property distinguishing some powerful high-redshift radio galaxies exhibiting the alignment effect from the lower power FR I radio galaxies in cooling flows that exhibit the lobe phenomenon. While both FR I and FR II radio sources seem to be capable of triggering star formation, FR I radio sources seem to be incapable of producing a large enough polarized luminosity to contribute significantly to the aligned continuum radiation.

Distant Radio Galaxies: Probes of the Formation of Massive Galaxies

In this paper we review some of the evidence that the hosts of powerful high redshift (1 ≲ z ≲ 5) radio galaxies (HzRGs) are the progenitors of present day brightest cluster galaxies (BCGs). On the basis of HST imaging we argue that the scenario describing the formation of BCGs consists of at least two important stages. At z &gt; 2 a significant fraction of the stellar mass of a BCG is formed during a massive burst of star-formation. By z ~ 1, well developed massive ellipticals are already observed and from then until the present epoch, the total mass in stars of the BCG will further grow by a factor of order 3, mainly through the accretion of cluster ellipticals.

HST, radio and infrared observations of 28 3CR radio galaxies at redshift z ∼ 1 — II. Old stellar populations in central cluster galaxies

Hubble Space Telescope images of 3CR radio galaxies at redshifts 0.6 < z < 1.8 have shown a remarkable variety of structures, generally aligned along the radio axis, indicating that the radio source strongly influences the optical appearance of these galaxies. In this paper we investigate the host galaxies underlying this aligned emission, combining the HST data with ground-based infrared images. An investigation of the spectral energy distributions of the galaxies shows that the contribution of the aligned blue component to the K-band light is generally small (∼10 per cent). The radial intensity profiles of the galaxies are well matched at radii ≲ 35 kpc by de Vaucouleurs' law, demonstrating that the K-band light is dominated by that of an elliptical galaxy. There is no evidence for a nuclear point source, in addition to the de Vaucouleurs profile, with a contribution ≳ 15 per cent of the total K-band flux density, except in two cases, 3C 22 and 3C 41. We conclude that the K-band emission of the distant 3CR galaxies is dominated by starlight. The magnitudes, colours and location of the distant 3CR galaxies on the projected fundamental plane indicate that their stellar populations formed at high redshift and have since been evolving passively. Large characteristic radii are derived for the 3CR galaxies, indicating that they must be highly evolved dynamically, even at a redshift of 1. At radii larger than ∼35 kpc, a combined galaxy profile clearly shows an excess of emission as compared with de Vaucouleurs' law, indicating that at least some of the galaxies possess cD-type haloes. This supports other independent evidence for the hypothesis that the distant 3CR galaxies lie in moderately rich (proto-)clusters. Since the nearby FR II galaxies in the 3CR catalogue lie in more diffuse environments and do not possess cD haloes, the galactic environments of the 3CR galaxies must change with redshift. The K-z relation of the 3CR galaxies cannot, therefore, be interpreted using a standard ‘closed-box, passively evolving stellar population’ model, whereby the galaxies that host distant 3CR sources will evolve into the galaxies that host nearby 3CR FR II sources. At redshifts z ∼ 1, the absolute K magnitudes of the stellar populations of the 3CR galaxies are brighter than those of the lower radio power 6C galaxies, indicating that the 3CR galaxies contain a greater mass of stars; this is consistent with them lying towards the centres of clusters. Powerful high-redshift radio galaxies possess radio beams the kinetic power of which is close to the Eddington limiting luminosity of a central supermassive black hole. Since the mass of the black hole is likely to scale in proportion to the mass of the host galaxy, the 3CR galaxies will contain more massive central engines than the 6C galaxies, which accounts for their more powerful radio emission. At redshifts z ≲ 0.6, the beam power of the radio sources is limited by the availability of fuel for the central engine rather than by the black hole mass, and so no correlation is expected between the radio power and the mass of the host galaxy.

The discovery of a young radio galaxy at Z = 2.390 - Probing initial star formation at Z less than approximately 3.0

We present the discovery of a weak radio galaxy from the Leiden Berkeley Deep Survey at a redshift of 2.390, the faint optical and IR counterpart of the steep-spectrum, compact radio source 53W002. Its λ dependent optical continuum morphology is compact with linear size ~10-35 kpc (H_0_= 50, q_0_ = 0). In redshifted Lyα, the galaxy is somewhat more extended (&lt;~67 kpc x 40 kpc). Its radio source is 7 times smaller than, and confined by, the Lyα gas. It shows alignment with the Lyα gas and the best seeing optical continuum images. We present nine-band photometry (Lyα UBgriJHK) for the galaxy as well as surrounding objects. The source 53W002 is not variable on time scales of years, in either radio or optical. We compare its rest-frame UV continuum with IUE spectra of various nearby galaxies with relatively recent starbursts, and nearby active galactic nuclei (AGNs). The C IV/Lyα and N V/Lyα ratios suggest that 53W002 has a Seyfert 1-like AGN, and constrain the nonthermal component to be ~35% of the observed UV continuum. Several independent age estimates yield a consistent value of 0.25-0.32 Gyr: (1) its small 4000 A break or UV-visual continuum amplitude compared with nearby galaxies; (2) a best model fit to the downturn of its UV spectrum below ~2000 A; (3) limits from the lack of detected stellar absorption features; (4) its total stellar mass (from its V- and K-band luminosity) compared with its SFR [from W<SUB>lambda</SUB>(Lyα, z = 0)]. These parameters together suggest that at z = 2.390, 53W002 had processed only a few times 10^11^ M_sun_ into stars, significantly less than most powerful radio galaxies have converted into stars at z ~2-3.8. Hence, star formation in 53W002 has proceeded slower, and started at a later epoch, than in the most powerful high-redshift radio galaxies. The available data are consistent with 53W002 being a genuinely young (radio) galaxy seen at z = 2.390 during its first major starburst. It likely started forming most of its current stars at redshifts 2.5-3.0 (for H_sun_ = 50-100, q_0_ = 0.0-0.5). This suggests that (radio) galaxies do not form the bulk of their stars coevally, but start doing so over a wide range of cosmic time.