Pieter Barthel

Emission mechanisms in composite galaxies

High resolution radio and X-ray observations of a subsample of composite LINER/H II galaxies with arcsec-scale radio cores were obtained. These observations yield a 12% detection rate of low luminosity AGN among the entire composite sample in the Palomar survey. Four of these low luminosity AGN also possess sub-parsec jet-like radio extensions. While canonical advection-dominated accretion flows may be repsonsible for the radio and X-ray emission in submillijansky cores like NGC 7331, they do not appear consistent with the radio emission in these milliarcsecond-scale radio cores; the latter sources are more likely to have an energetically important contribution from a radio-emitting jet.

GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies

We present here a new spectral energy distribution (SED) fitting approach that we adopt to select radio-excess sources amongst distant star-forming galaxies in the GOODS-Herschel (North) field and to reveal the presence of hidden, highly obscured AGN. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 um detections using some of the deepest Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to date, we have robustly identified a sample of 51 radio-excess AGN (~1300 deg^-2) out to redshift z~3. These radio-excess AGN have a significantly lower far-IR/radio ratio (q<1.68) than the typical relation observed for star-forming galaxies (q~2.2). We find that ~45% of these radio-excess sources have a dominant AGN component in the mid-IR band, while for the remainders the excess radio emission is the only indicator of AGN activity. The fraction of radio-excess AGN increases with X-ray luminosity reaching ~60% at Lx~10^44-10^45 erg/s, making these sources an important part of the total AGN population. However, almost half (24/51) of these radio-excess AGN are not detected in the deep Chandra X-ray data, suggesting that some of these sources might be heavily obscured. We also find that the specific star formation rates (sSFRs) of the radio-excess AGN are on average lower that those observed for X-ray selected AGN hosts, indicating that our sources are forming stars more slowly than typical AGN hosts, and possibly their star formation is progressively quenching.

Optical-radio alignment in compact steep-spectrum radio sources

We discuss Hubble Space Telescope (HST) WFPC2 observations through the broad red filter F702W of 30 3CR sources from the Compact Steep Spectrum (CSS) radio source sample, and present 11 new HST/WFPC2 images through linear ramp filters (LRF), isolating either rest-frame CO nl 3727 Angstrom or [O III] 5007 Angstrom radiation. In nearly all the CSS galaxies, we find high surface brightness emission that is aligned with the radio axis. The strong nuclear PSF prevents us from detecting such aligned light at similar levels in most of the CSS quasars. However, a comparison between CSS galaxies and quasars with PSF signatures removed reveals no inconsistency with the viewing angle unification scheme. The alignment effect in CSS sources is not a strong function of redshift, and is seen over the entire redshift range of the sample (0.2 less than or similar to z less than or similar to 1.5). Our analysis of the LRF images and complementary KPNO spectroscopy reveals that the aligned light is predominantly emission-line gas. These observations demonstrate the existence of dense gas in the host galaxies of CSS sources, strongly interacting with the expanding radio sources. Assuming such a shock interaction, cooling-time arguments suggest that lobe expansion speeds of greater than or similar to 1000 km s(-1) and ambient densities of similar to 1 cm(-3) are consistent with the observed offset between the leading edge of the radio lobe and the optical line emission.

Further clues to the nature of composite LINER/H II galaxies

We have analyzed new, archival and published high resolution radio and X-ray observations of a sample of composite LINER/H  galaxies known to exhibit AGN-like properties. Five of the 16 AGN candidates have milliarcsecond-scale detections and are found to display a compact, flat spectrum, high brightness temperature radio core, four of which also exhibit extended radio emission. Five of the eight AGN candidates with available high resolution X-ray observations were found to possess a hard X-ray nuclear source, two of which have no milliarcsecond radio detection. The combined high resolution radio and X-ray data yield a 50% detection rate of low luminosity AGN among the AGN candidates, which translates into a 12% detection rate for the entire composite LINER/H  sample. In the sources where the AGN has been unambiguously detected, the ionizing power of the AGN is not sufficient to generate the observed emission lines, unless the hard X-rays are heavily obscured. We attempt to apply a canonical advection-dominated accretion flow (ADAF) and jet model to the sample sources in order to explain the observed radio and X-ray emission. While ADAFs may be responsible for the observed emission in submillijansky radio cores like NGC 7331, they do not appear consistent with the radio emission observed in the milliarcsecond-scale radio detected cores; the latter sources are more likely to have an energetically important contribution from a radio-emitting jet.

Support for a unified model of radio galaxies and quasars from isotropic [O II] emission

UNIFIED models1–3 of radio-loud quasars and powerful radio galaxies suggest that they are intrinsically similar objects observed from different angles. This can be tested by comparing the isotropically emitted radiation from the spatially extended nebulae surrounding the nuclei; the unified models predict that the intensities of these emissions should be comparable for the two classes of object. But when this prediction was examined4 for the [O III] 5,007-Å emission line, it was found that quasar [O III] luminosities significantly exceed those of otherwise similar radio galaxies. We have measured the spatially integrated [O II] 3,727-Å emission-line luminosities for a number of quasars and radio galaxies taken from the 3C catalogue. Supplementing our data with values from the literature, we find no systematic difference in the [O II] luminosities. We argue that this emission is indeed isotropic, and that our results are consistent with the unification hypothesis; the [O III] line, on the other hand, may still have a significant component from the nuclear region, and thus be subject to pronounced anisotropic obscuration.

Rapidly growing black holes and host galaxies in the distant Universe from the Herschel Radio Galaxy Evolution Project

We present results from a comprehensive survey of 70 radio galaxies at redshifts 1 10(12) L-circle dot) or hyper-luminous (L-tot(IR) > 10(13) L-circle dot) infrared galaxies. We fit the infrared SEDs with a set of empirical templates which represent dust heated by a variety of starbursts (SB) and by an active galactic nucleus (AGN). We find that the SEDs of radio galaxies require the dust to be heated by both AGN and SB, but the luminosities of these two components are not strongly correlated. Assuming empirical relations and simple physical assumptions, we calculate the star formation rate (SFR), the black hole mass accretion rate ((M) over dot(BH)), and the black hole mass (M-BH) for each radio galaxy. We find that the host galaxies and their black holes are growing extremely rapidly, having SFR approximate to 100-5000 M-circle dot yr(-1) and. (M) over dot(BH) approximate to 1-100 M(circle dot)yr(-1). The mean specific SFRs (sSFR) of radio galaxies at z > 2 : 5 are higher than the sSFR of typical star forming galaxies over the same redshift range, but are similar or perhaps lower than the galaxy population for radio galaxies at z < 2.5. By comparing the sSFR and the specific. (M) over dot(BH) (s(M) over dot(BH)), we conclude that black holes in radio loud AGN are already, or soon will be, overly massive compared to their host galaxies in terms of expectations from the local M-BH-M-Gal relation. In order to catch up with the black hole, the galaxies require about an order of magnitude more time to grow in mass at the observed SFRs compared to the time the black hole is actively accreting. However, during the current cycle of activity, we argue that this catching up is likely to be difficult because of the short gas depletion times. Finally, we speculate on how the host galaxies might grow sufficiently in stellar mass to ultimately fall onto the local MBH-MGal relation.

RAPID COEVAL BLACK HOLE AND HOST GALAXY GROWTH IN MRC 1138-262: THE HUNGRY SPIDER

We present a detailed study of the infrared spectral energy distribution of the high-redshift radio galaxy MRC1138-26 at z = 2.156, also known as the Spiderweb Galaxy. By combining photometry from Spitzer, Herschel and LABOCA we fit the rest-frame 5 300µm emission using a two component, starburst and active galactic nucleus (AGN), model. The total infrared (8 1000µm) luminosity of this galaxy is (1.97±0.28)×10 13 L⊙ with (1.17±0.27) and (0.79±0.09)×10 13 L⊙ due to the AGN and starburst components respectively. The high derived AGN accretion rate of � 20% Eddington, and the measured star formation rate (SFR) of 1390±150M⊙yr −1 , suggest that this massive system is in a special phase of rapid central black hole and host galaxy growth, likely caused by a gas rich merger in a dense environment. The accretion rate is sufficient to power both the jets and the previously observed large outflow. The high SFR and strong outflow suggest this galaxy could potentially exhaust its fuel for stellar growth in a few tens of Myr, although the likely merger of the radio galaxy with nearby satellites suggest bursts of star formation may recur again on time scales of several hundreds of Myr. The age of the radio lobes implies the jet started after the current burst of star formation, and therefore we are possibly witnessing the transition from a merger-induced starburst phase to a radio-loud AGN phase. We also note tentative evidence for [CII]158µm emission. This paper marks the first results from the Herschel Galaxy Evolution Project (Project HeRG ´ E), a systematic study of the evolutionary state of 71 high redshift, 1 < z < 5.2, radio galaxies. Subject headings: galaxies: active, formation, high redshift, individual (MRC 1138-262)

VLBI, MERLIN and HST observations of the giant radio galaxy 3C 236

We present VLBI and MERLIN data at 1.66 and 4.99 GHz on the central component coincident with the nucleus of the giant radio galaxy, 3C 236. The nuclear radio structure is composed of two complexes of emission which are resolved on scales from 1 milli-arcsec (mas) to 1 arcsec. Oscillations with an amplitude of similar to5 degrees can be seen in the compact radio structure. Spectral index distributions are plotted at angular resolutions of 10 and 25 mas and allow us to identify the core component in the south-east emission complex. Re-examination of the HST WFPC-2 image of 3C 236 by de Koff et al. (2000), shows that the normal to the dust disk in the nucleus is similar to 30 degrees from the plane of the sky and within 12 degrees of parallel to the overall orientation of the radio source. We suggest that the radio axis is also at an angle of similar to 30 degrees to the plane of the sky and that the north-west jet is on the approaching side. This orientation implies an overall size of 4.5 Mpc (H-0 = 75 kms(-1) Mpc(-1), q(0) = 0.5) for 3C 236. The coincidence of a dust feature and the south-east compact jet, within the astrometric errors, leads us to suggest that the dust may be in the form of a cloud encountered by the jet in the first similar to 400 pc of its journey out from the nucleus. One-sided emission at 5 GHz on 1 mas scales would suggest that the jets are ejected initially at less than or equal to 35 degrees to the line of sight, but this is difficult to reconcile with the obvious orientation stability of the jet system as a whole. Free-free absorption of the counter-jet may be an alternative explanation for the one-sideness. At the resolution of WSRT data at 327 MHz, the jet to the south-east is apparently continuous over a distance of 2.5 Mpc, making this the largest jet known in the universe. It is likely, however, that activity in the nucleus of 3C 236 is episodic but with a shorter duty cycle than in the double-double sources studied by Schoenmakers et al. (2000) and Kaiser et al. (2000).We present VLBI and MERLIN data at 1.66 and 4.99 GHz on the central component coincident with the nucleus of the giant radio galaxy, 3C236. The nuclear radio structure is composed of two complexes of emission which are resolved on scales from 1 milli-arcsec (mas) to 1 arcsec. Oscillations with an amplitude of about 5 degrees can be seen in the compact radio structure. Spectral index distributions are plotted at angular resolutions of 10 and 25 mas and allow us to identify the core component in the south-east emission complex. Re-examination of the HST WFPC-2 image of 3C236 by de Koff et al. (2000), shows that the normal to the dust disk in the nucleus is ~30 degrees from the plane of the sky and within 12 degrees of parallel to the overall orientation of the radio source. We suggest that the radio axis is also at an angle of ~30 degrees to the plane of the sky and that the north-west jet is on the approaching side. This orientation implies an overall size of 4.5 Mpc (H_o = 75 km/s/Mpc, q_o = 0.5) for 3C236. The coincidence of a dust feature and the south-east compact jet, within the astrometric errors, leads us to suggest that the dust may be in the form of a cloud encountered by the jet in the first ~400 pc of its journey out from the nucleus. One-sided emission at 5 GHz on 1 mas scales would suggest that the jets are ejected initially at<= 35 degrees to the line of sight, but this is difficult to reconcile with the obvious orientation stability of the jet system as a whole. Free-free absorption of the counter-jet may be an alternative explanation for the one-sideness.

Clues to quasar broad-line region geometry and kinematics

We present evidence that the high-velocity C IV λ1549 emission-line gas of radio-loud quasars may originate in a disklike configuration, in close proximity to the accretion disk often assumed to emit the low-ionization lines. For a sample of 36 radio-loud z ≈ 2 quasars, we find the 20%-30% peak widths to show significant inverse correlations with the fractional radio core-flux density R, which is the radio axis inclination indicator. Highly inclined systems have broader line wings, consistent with a high-velocity field perpendicular to the radio axis. By contrast, the narrow line core shows no such relation with R, so the lowest velocity C IV-emitting gas has an inclination-independent velocity field. We propose that this low-velocity gas is located at higher disk altitudes than the high-velocity gas. A planar origin of the high-velocity C IV emission is consistent with the current results and with an accretion disk wind emitting the broad lines. A spherical distribution of randomly orbiting broad-line clouds and a polar high-ionization outflow are ruled out.

Mid-infrared Spectroscopy of High-redshift 3CRR Sources

Using the Spitzer Space Telescope, we have obtained rest-frame 9-16 μm spectra of 11 quasars and 9 radio galaxies from the 3CRR catalog at redshifts 1.0 < z < 1.4. This complete flux-limited 178 MHz selected sample is unbiased with respect to orientation and therefore suited to studying orientation-dependent effects in the most powerful active galactic nuclei (AGNs). The mean radio-galaxy spectrum shows a clear silicate absorption feature (τ_(9.7μm) = 1.1) whereas the mean quasar spectrum shows silicates in emission. The mean radio-galaxy spectrum matches a dust-absorbed mean quasar spectrum in both shape and overall flux level. The data for individual objects conform to these results. The trend of the silicate depth to increase with decreasing core fraction of the radio source further supports that for this sample orientation is the main driver for the difference between radio galaxies and quasars, as predicted by AGN unification. However, comparing our high-z sample with lower redshift 3CRR objects reveals that the absorption of the high-z radio galaxy MIR continuum is lower than expected from a scaled-up version of lower luminosity sources, and we discuss some effects that may explain these trends.