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Abstract. We present an overview of the occurrence and properties of atomic gas associated with compact radio sources at redshifts up to z = 0.85. Searches for H  21 cm absorption were made with the Westerbork Synthesis Radio Telescope at UHFhigh frequencies (725-1200 MHz). Detections were obtained for 19 of the 57 sources with usable spectra (33%). We have found a large range in line depths, from τ = 0.16 to τ ≤ 0.001. There is a substantial variety of line profiles, including Gaussians of less than 10 km s −1 , to more typically 150 km s −1 , as well as irregular and multi-peaked absorption profiles, sometimes spanning several hundred km s −1 . Assuming uniform coverage of the entire radio source, we obtain column depths of atomic gas between 1 × 10 19 and 3.3 × 10 21 (T sp /100 K)(1/ f ) cm −2 . There is evidence for significant gas motions, but in contrast to earlier results at low redshift, there are many sources in which the H  velocity is substantially negative (up to v = −1420 km s −1 ) with respect to the optical redshift, suggesting that in these sources the atomic gas, rather than falling into the centre, may be be flowing out, interacting with the jets, or rotating around the nucleus.

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AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid–to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180$\mu$m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

Context. A tight correlation exists between far-infrared and radio emission for star-forming galaxies (SFGs), which seems to hold out to high redshifts (z ≈ 2). Any excess of radio emission over that expected from star formation processes is most likely produced by an active galactic nucleus (AGN), often hidden by large amounts of dust and gas. Identifying these radio-excess sources will allow us to study a population of AGN unbiased by obscuration and thus find some of the most obscured, Compton-thick AGN, which are in large part unidentified even in the deepest X-ray and infrared (IR) surveys. Aims. 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. Methods. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 µm 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, 3σ) than the typical relation observed for star-forming galaxies (q ≈ 2.2). Results. 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 presence of an AGN is also confirmed by the detection of a compact radio core in deep VLBI 1.4 GHz observations for eight of our radio-excess sources (≈16%; ≈66% of the VLBI detected sources in this field), with the excess radio flux measured from our SED analysis agreeing, to within a factor of two, with the radio core emission measured by VLBI. We find that the fraction of radio-excess AGN increases with X-ray luminosity reaching ∼60% at L X ≈ 10 44 −10 45 erg s −1 , 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. Amongst the radio-excess AGN we can distinguish three groups of objects: i) AGN clearly identified in infrared (and often in X-rays), a fraction of which are likely to be distant Compton-thick AGN; ii) moderate luminosity AGN (L X 10 43 erg s −1 ) hosted in strong star-forming galaxies; and iii) a small fraction of low accretion-rate AGN hosted in passive (i.e. weak or no star-forming) galaxies. 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.

Chandra observations of a complete, flux-limited sample of 38 high-redshift (1 0) indicating obscuration (N H ∼ 10 22−24 cm −2 ). These properties and the correlation between obscuration and radio core-fraction are consistent with orientation-dependent obscuration as in Unification models. About half the NLRGs have soft X-ray hardness ratios and/or high [OIII] emission line to X-ray luminosity ratio suggesting obscuration by Compton thick (CT) material so that scattered nuclear or extended X-ray emission dominates (as in NGC1068). The ratios of unobscured to Compton-thin (10 22 < N H (int) < 1.5 × 10 24 cm −2 ) to CT (N H (int)> 1.5 × 10 24 cm −2 ) is 2.5:1.4:1 in this high luminosity, radio-selected sample. The obscured fraction is 0.5, higher than is typically reported for AGN at comparable luminosities from multi-wavelength surveys (0.1 − 0.3). Assuming random nuclear orientation, the unobscured half-opening angle of the disk/wind/torus structure is ∼ 60 o and the obscuring material covers 30 o of which ∼ 12 o is Compton thick. The multi-wavelength properties reveal that many NLRGs have intrinsic absorption 10 − 1000× higher than indicated by their X-ray hardness ratios, and their true L X values are ∼10-100× larger than the hardness-ratio absorption corrections would indicate.

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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 study orientation-dependent effects in the most powerful active galactic nuclei (AGN). 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.