LBT-MODS spectroscopy of high-redshift candidates in the Chandra J1030 field. A newly discovered z ∼ 2.8 large-scale structure

We present the results of a spectroscopic campaign with the Multi-Object Double Spectrograph (MODS) instrument mounted on the Large Binocular Telescope (LBT), aimed at obtaining a spectroscopic redshift for seven Chandra J1030 sources with a photometric redshift ≥2.7 and optical magnitude r_AB = [24.5 − 26.5]. We obtained a spectroscopic redshift for five out of seven targets: all of them have z_spec ≥ 2.5, thus probing the reliability of the Chandra J1030 photometric redshifts. The spectroscopic campaign led to the serendipitous discovery of a z ∼ 2.78 large-scale structure (LSS) in the J1030 field: the structure contains four X-ray sources (three of which were targeted in the LBT-MODS campaign) and two non-X-ray-detected galaxies for which a VLT-MUSE spectrum was already available. We also find 26 galaxies with a photometric redshift in the range z_phot = [2.68 − 2.88], which we consider candidate LSS members. The X-ray members of the LSS are hosted in galaxies that are significantly more massive (log(M_*/M_⊙) = [10.0 − 11.1]) than both those hosting the two MUSE-detected sources (log(M_*/M_⊙) < 10) and those belonging to the photometric sample (median stellar mass log(M_*/M_⊙) = 10.0). Both observations and simulations show that massive galaxies, and particularly objects with log(M_*/M_⊙) > 10, are among the best tracers of LSSs and filaments in the cosmic web. Consequently, our result can explain why X-ray-detected active galactic nuclei have also been shown to be efficient tracers of LSS.