Discovery of a galaxy overdensity around a powerful, heavily obscured
  FRII radio galaxy at z=1.7: star formation promoted by large-scale AGN
  feedback?
  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 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 of the identified overdensity members are blue, compact
galaxies that are actively forming stars at rates of $\sim$8-60 $M_{\odot}$
yr$^{-1}$. Based on a 500ks Chandra ACIS-I observation we found that the FRII
nucleus hosts a luminous QSO ($L_{2-10keV}=1.3\times10^{44}$ erg s$^{-1}$,
intrinsic and rest-frame) that is obscured by Compton-thick absorption
($N_H=1.5\pm0.6\times 10^{24}$ cm$^{-2}$). Our Chandra observation, the deepest
so far for a distant FRII within a galaxy overdensity, revealed significant
diffuse X-ray emission within the region covered by the overdensity. In
particular, X-ray emission extending for $\sim$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. The probability of observing by chance four out of the six
$z=1.7$ sources 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 positive
AGN feedback on cosmological scales. [shortened version]
