We report the discovery of a galaxy overdensity around a Compton-thick Fanaroff-Riley type II (FRII) radio galaxy at z=1.7 in the deep multiband survey around the z=6.3 quasi-stellar object (QSO) SDSS J1030+0524. Based on a 6hr VLT/MUSE and on a 4hr LBT/LUCI observation, we identify at least eight galaxy members in this structure with spectroscopic redshift z=1.687-1.699, including the FRII galaxy at z=1.699. Most members are distributed within 400 kpc from the FRII core. Nonetheless, the whole structure is likely much more extended, as one of the members was serendipitously found at ∼800 kpc projected separation. The classic radio structure of the FRII itself extends for ∼ 600 kpc across the sky. Most of the identified overdensity members are blue, compact galaxies that are actively forming stars at rates of ∼8-60 M yr −1 . For the brightest of them, a half-light radius of 2.2±0.8 kpc at 8000Å rest-frame was determined based on adaptive optics-assisted observations with LBT/SOUL in the Ks band. We do not observe any strong galaxy morphological segregation or concentration around the FRII core. This suggests that the structure is far from being virialized and likely constitutes the progenitor of a local massive galaxy group or cluster caught in its main assembly phase. Based on a 500ks Chandra ACIS-I observation, we found that the FRII nucleus hosts a luminous QSO (L 2−10keV = 1.3 × 10 44 erg s −1 , intrinsic and rest-frame) that is obscured by Compton-thick absorption (N H = 1.5±0.6×10 24 cm −2 ). Under standard bolometric corrections, the total measured radiative power (L rad ∼ 4 × 10 45 erg s −1 ) is similar to the jet kinetic power that we estimated from radio observations at 150MHz (P kin = 6.3 × 10 45 erg s −1 ), in agreement with what is observed in powerful jetted AGN. Our Chandra observation is the deepest so far for a distant FRII within a galaxy overdensity. It revealed significant diffuse X-ray emission within the region that is covered by the overdensity. In particular, X-ray emission extending for ∼240 kpc is found around the eastern lobe of the FRII. Four out of the six MUSE star-forming galaxies in the overdensity are distributed in an arc-like shape at the edge of this diffuse X-ray emission. These objects are concentrated within 200 kpc in the plane of the sky and within 450 kpc in radial separation. Three of them are even more concentrated and fall within 60 kpc in both transverse and radial distance. The probability of observing four out of the six z = 1.7 sources by chance at the edge of the diffuse emission is negligible. In addition, these four galaxies have the highest specific star formation rates of the MUSE galaxies in the overdensity and lie above the main sequence of field galaxies of equal stellar mass at z=1.7. We propose that the diffuse X-rays originate from an expanding bubble of gas that is shock heated by the FRII jet, and that star formation is promoted by the compression of the cold interstellar medium of the galaxies around the bubble, which may be remarkable evidence of pos...

Aims. The standard active galactic nuclei (AGN)-galaxy co-evolutionary scenario predicts a phase of deeply "buried" supermassive black hole growth coexisting with a starburst (SB) before feedback phenomena deplete the cold molecular gas reservoir of the galaxy and an optically luminous quasar (QSO) is revealed ("SB-QSO evolutionary sequence"). The aim of this work is to measure the cold gas reservoir of three highly obscured QSOs to test if their gas fraction is similar to that of submillimeter galaxies (SMGs), as expected by some models, and place these measurements in the context of the SB-QSO framework. Methods. We target CO(1-0) transition in BzK4892, a Compton Thick (CT) QSO at z=2.6, CO(1-0) in BzK8608 and CO(2-1) in CDF153, two highly obscured (N H ≈ 6 × 10 23 cm −2 ) QSOs at z = 2.5 and z = 1.5, respectively. For all these targets, we place 3σ upper limits on the CO lines, with L CO < (1.5 ÷ 2.8) × 10 10 K km/s pc 2 . We also compare the molecular gas conditions of our targets with those of other systems at z > 1, considering normal star forming galaxies and SMGs, unobscured and obscured AGN from the literature. For the AGN samples, we provide an updated and (almost) complete collection of targets with CO follow-up at z > 1. Results. BzK4892 displays a high star formation efficiency (SFE = L IR /L CO > 410 L /(K km s −1 pc 2 )) and a gas fraction f gas = M gas /(M star + M gas ) < 10%. Less stringent constraints are derived for the other two targets ( f gas < 0.5 and SFE > 10 L /(K km s −1 pc 2 )). From the comparison with literature data, we found that, on average, i) obscured AGN at z > 1 are associated with higher SFE and lower f gas with respect to normal star forming galaxies and SMGs; ii) mildly and highly obscured active galaxies have comparable gas fractions; iii) the SFE of CT and obscured AGN are similar to those of unobscured AGN.Conclusions. Within the SB-QSO framework, these findings could be consistent with a scenario where feedback can impact the host galaxy already from the early phases of the SB-QSO evolutionary sequence.

The star formation rate density (SFRD) history of the universe is well constrained up to redshift z ∼ 2. At earlier cosmic epochs, the picture has been largely inferred from UV-selected galaxies (e.g., Lyman-break galaxies; LBGs). However, the inferred star formation rates of LBGs strongly depend on the assumed dust extinction correction, which is not well constrained at high z, while observations in the radio domain are not affected by this issue. In this work we measure the SFRD from a 1.4 GHz selected sample of ∼600 galaxies in the GOODS-N field up to redshift ∼3.5. We take into account the contribution of active galactic nuclei from the infrared-radio correlation. We measure the radio luminosity function, fitted with a modified Schechter function, and derive the SFRD. The cosmic SFRD shows an increase up to z ∼ 2 and then an almost flat plateau up to z ∼ 3.5. Our SFRD is in agreement with those from other far-IR/radio surveys and a factor 2 higher than those from LBG samples. We also estimate that galaxies lacking a counterpart in the HST/WFC3 H-band (H-dark) make up ∼25% of the ϕ-integrated SFRD relative to the full sample at z ∼ 3.2, and up to 58% relative to LBG samples.

Context. Obscured active galactic nuclei (AGN) represent a significant fraction of the entire AGN population, especially at high redshift (∼70% at z=3-5). They are often characterized by the presence of large gas and dust reservoirs that are thought to sustain and possibly obscure vigorous star formation processes that make these objects shine at far-IR and submillimeter wavelengths. Studying the physical properties of obscured AGN and their host galaxies is crucial to shedding light on the early stages of a massive system lifetime. Aims. We aim to investigate the contribution of the interstellar medium (ISM) to the obscuration of quasars in a sample of distant highly star forming galaxies and to unveil their morphological and kinematics properties. Methods. We exploit Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 4 observations of the continuum (∼2.1mm) and high-J CO emission of a sample of six X-ray selected, far-IR detected galaxies hosting an obscured AGN at z spec > 2.5 in the 7 Ms Chandra Deep Field-South (CDF-S). We measured the masses and sizes of the dust and molecular gas by fitting the images, visibilities, and spectra, and we derived the gas density and column density on the basis of a uniform sphere geometry. Finally, we compared the measured column densities with those derived from the Chandra X-ray spectra. Results. We detected both the continuum and line emission for three sources for which we measured both the flux density and size. For the undetected sources, we derived an upper limit on the flux density from the root mean square (rms) of the images. We found that the detected galaxies are rich in gas and dust (molecular gas mass in the range <0.5 -2.7 × 10 10 M for α CO = 0.8 and up to ∼ 2 × 10 11 M for α CO = 6.5, and dust mass <0.9 -4.9 × 10 8 M ) and generally compact (gas major axis 2.1-3.0 kpc, dust major axis 1.4-2.7 kpc). The column densities associated with the ISM are on the order of 10 23−24 cm −2 , which is comparable with those derived from the X-ray spectra. For the detected sources we also derived dynamical masses in the range 0.8 -3.7 × 10 10 M . Conclusions. We conclude that the ISM of high redshift galaxies can substantially contribute to nuclear obscuration up to the Compton-thick (> 10 24 cm −2 ) regime. In addition, we found that all the detected sources show a velocity gradient reminding one rotating system, even though two of them show peculiar features in their morphology that can be associated with a chaotic, possibly merging, structure.