Discovery of a galaxy overdensity around a powerful, heavily obscured FRII radio galaxy atz= 1.7: star formation promoted by large-scale AGN feedback?

Abstract

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...

Enhanced X-Ray Emission from the Most Radio-powerful Quasar in the Universe’s First Billion Years

“…Paliya et al (2020) reported detecting seven blazars at 3.1  z  4.7 with extended emission seen by Chandra, and Napier et al (2020) detected two extended structures on opposite angles of a z = 4.26 radio galaxy. While Nanni et al (2018) identified a potential extended component associated with a z = 6.31 quasar, further analysis by Gilli et al (2019) found that the reported emission is associated with a foreground structure. Finally, Fabian et al (2014) reported two potential structures around a z = 7.1 quasar, but were unable to rule out this being caused by source confusion in the XMM-Newton observations.…”

Web of the giant: Spectroscopic confirmation of a large-scale structure around the z = 6.31 quasar SDSS J1030+0524

“…The MUSE integral field spectrograph has a 1 arcmin 2 FOV and a spatial sampling of 0.2 × 0.2 arcsec 2 , covering the wavelength range 4750−9350 Å with a spectral bin of 1.25 Å pixel −1 . The data reduction and analysis of the archival 6.4 h MUSE data are presented in Gilli et al (2019). In brief, we measured a redshift for 102 objects (16 of them at z > 4) in the square arcmin region around the central QSO, a region that did not include any of the LBG candidates selected by Balmaverde et al (2017).…”

The MURALES survey

“…Positive feedback, that is, star formation triggered by jets or by the expansion of radio lobes, has been reported in a few individual cases at low redshifts: young stars have been detected in the filaments along the jet of Centaurus A (Mould et al 2000;Rejkuba et al 2002;Crockett et al 2012;Neff et al 2015;Santoro et al 2015;Salomé et al 2016), in an object along the path of the jet in NGC 541 (the Minkowski object, Croft et al 2006), and at the termination of the jet of NGC 5643 (Cresci et al 2015). Conversely, positive feedback appears to be important in high-redshift radio galaxies (see Miley & De Breuck 2008; O'Dea & Saikia 2021 for a review and, e.g., Steinbring 2014Steinbring , 2011Gilli et al 2019 for recent results). The analysis of the far-infrared spectral energy distribution of a complete sample of z>1 3CR sources indicates that ∼40% of them are undergoing episodes of star formation with rates of hundreds of solar masses per year (Podigachoski et al 2015).…”