The field around the z=6.31 quasar SDSS J1030+0524

Project Rationale

The existence of supermassive black holes (SMBHs) with M_BH = 10⁹ M_☉ powering luminous quasars at z~6 and beyond, represents a persistent challenge for extragalactic astronomy. Theory strongly argues that these objects must have formed within the most massive dark matter halos (M_halo ~ 10^12-13 M_☉) situated in the most overdense regions of the early Universe, where large reservoirs of gas are available for an efficient BH fueling.
Within these environments, high accretion rates can be triggered and sustained by both frequent mergers of proto-galaxies and possibly by the steady flows of cold gas from which proto-galaxies can form. Such early large scale structures (LSSs), whose cores would eventually evolve into local massive galaxy clusters, are expected to be traced by significant galaxy overdensities, that may extend up to scales as large as ~10 physical Mpc (pMpc) from the quasar.
The total number and spatial distribution of galaxies in these overdensities, however, depend on the many competing physical processes that promote or prevent galaxy formation. For instance, both supernovae explosions in some of these galaxies and the release of energy from the QSO itself in the form of radiation and gas outflows, may heat and expel gas from the dark matter halos in the LSS, hampering further star formation and galaxy assembly. This "negative feedback" may in fact complicate the detection of these early structures, especially within a few hundreds kpc from the QSO.
Current observations lag behind theoretical modeling, and there is still no direct measurement of the environment around early SMBHs. The attempts to directly measure these overdensities have started since the discovery of high-z QSOs, mainly by selecting Lyman Break Galaxy (LBG) and Lyman Alpha Emitter (LAE) candidates at the QSO redshift. Yet, all these efforts did not produce conclusive results.

LBT program

In 2012 we started an optical imaging campaign to search for galaxy overdensities around four luminous QSOs at z~6 with the Large Binocular Camera at the Large Binocular Telecope. The large field of view of the LBC (~25x25 arcmin²) allowed us to search for z~6 LBG candidates on scales as large as 4 projected Mpc from the QSOs. When combining the four fields, we found significant evidence for large-scale overdensities of LBG candidates around early quasars (Morselli et al. 2014).

The field around SDSS J1030+0524, a z=6.31 QSO powered by a billion solar mass black hole, was found to be the most overdense, reinforcing the results of previous small-scale studies with ACS/HST. In 2016 we obtained deep-and-wide near-IR imaging of the J1030 field with WIRCAM at the CFHT. Using these new observations plus archival Spitzer/IRAC data, we pushed the selection of z~6 galaxy candidates to fainter fluxes, measured photometric redshifts, and improved the rejection of contaminants, reinforcing the large scale overdensity estimate (Balmaverde et al. 2017).

LSS at z≃6.3 around the central luminous QSO: "the Web of the Giant"

Using three of the largest telescopes around the world - namely, the Large Binocular Telescope (LBT), the ESO Very Large Telescope (VLT), and the W.M. Keck Observatory Telescope - we were able to confirm the presence of a Large Scale Structure (LSS) around SDSS J1030+0524. The structure is populated by at least six galaxies, whose redshifts were determined through multi-object spectroscopic observations lasting up to eight hours. The redshifts of the six galaxies cover the range between 6.219–6.355 and, assuming that the peculiar velocities are negligible, this range corresponds to radial separations of ±5 physical Mpc from the QSO. (Mignoli et al. 2020)

The result is significant at more than 3.5σ and the level of the galaxy overdensity is at least 1.5–2 within the large volume sampled (∼780 physical Mpc³). This is the first spectroscopic identification of a galaxy overdensity around a supermassive black hole in the first billion years of the Universe. This finding helps us better understand how supermassive black holes formed and grew to their enormous sizes. It supports the theory that the most distant and massive black holes form and quickly grow within massive (>10¹²M☉) dark matter halos in large web-like structures which contain plenty of gas to fuel them. (ESO Press Release #2016)

Chandra Large Program

The field around SDSS J1030+0524 features the best multi-band coverage among all known z~6 quasars. Besides the data already mentioned above, the quasar is covered by HST/WFC3 data. The field has been also observed by Spitzer IRAC (30x30 arcmin²) and MIPS (~12x12 arcmin²), and by the VLA at 1.4GHz (~ 30 arcmin diameter). Furthermore, it is part of the Multiwavelength Chile-Yale survey (MUSYC), that provides additional imaging in UBVRIzJHK.

In 2015, a 500ks Chandra Large Program was granted to our group with the aim of obtaining the best X-ray spectrum of a z>6 quasar to date and investigate the presence of moderate-luminosity AGN within the candidate large scale structure around the quasar. The observations, performed in 2017 with the ACIS-I array, revealed an unexpectedly high variability level for such a luminous quasar: its X-ray spectrum was significantly harder and fainter than what was observed in the past, suggesting large changes in the quasar accretion rate or obscuration level, or both. The results on the quasar properties have been published in Nanni et al. 2018.

The Chandra survey in the J1030 field is currently the deepest X-ray look at at z=6 quasar and the 5th deepest X-ray survey in general after the 7Ms CDFS, the 2Ms CDFN, the 800ks AEGIS survey and the 400ks survey in the SSA22 field. It reaches a 0.5-2 keV flux limit of 6x10^-17 erg cm² s^-1 in the central sq. arcmin. Its excellent multi-band coverage guarantees prompt identification of X-ray sources. The catalog of 256 X-ray sources detected in the field has been published in Nanni et al. 2020. For a subsample of candidate obscured AGN, we derived a redshift estimate based on X-ray spectral analysis. The results are published in Peca et al. 2021.
Because of its equatorial location, SDSS J1030 is in fact an ideal field to exploit current and future ground based instrumentation on both hemispheres, and an extended spectroscopic campaign with LBT was performed to compute a redshift for a large fraction of the Chandra J1030 population. Overall, we measured a spectroscopic redshift for 123 sources (51% of the sample); we then took advantage of the excellent multiwavelength coverage of the J1030 field to compute a photometric redshift through fitting of the spectral energy distribution (SED) for the remaining sources. The results of the spectro-photometric campaign are reported in Marchesi et al. 2021.
More details, as well as the high-level scientific data products derived from the Chandra observations, can be found at the dedicated page.

JVLA deep observation

VLA 1.4 GHz observations of the central part of the field have been published by Petric et al. 2003 and re-analysed by us. The QSO itself was not detected down to a sensitivity of ~20 μJy rms (constraining the 1.4 GHz rest-frame luminosity to < 5x10²⁵ W Hz⁻¹ at 3σ). Such observations revealed an FR II radio galaxy at the center of the field, that we later discovered to be associated with a proto-cluster at z=1.69.
New radio continuum JVLA imaging was obtained at L-band (1-2 GHz) in 2018. The L-band primary beam response (FWHM∼ 30 arcmin) nicely matches the entire region of interest, and in particular entirely encompasses the Chandra field of view. These observations reach a 1σ sensitivity of ∼2.5 μJy at the center of the field, making it of the deepest radio-survey to date. We extracted a catalogue of 1489 radio-sources down to a flux density of ∼12.5 μJy (5σ), for which we presented source counts, finding good agreement with recent measurements and models. We detected for the first time the QSO in the radio band, and derived an optical radio-loudness R = 0.62 ± 0.12, that makes it the most radio-quiet AGN at z ≳ 6 discovered so far. Thanks to the sinergy between the deep JVLA and Chandra observations, we investigated the relation between radio and X-ray luminosity of 243 X-ray selected sources in a wide redshift range (0 ≲ z ≲ 3), for which Marchesi et al. 2021 provided spectrocopic and/or photometric redshifts and classification. We found significant correlations between the radio and X-ray luminosities of different source populations and interpreted them as signatures of different accretion processes. These results are presented D'Amato et al. 2022.

Proto-cluster at z=1.7

The combination of deep X-ray, radio and optical data allowed the discovery of another remarkable system at the center of the J1030 field, that is a galaxy overdensity at z=1.69 surrounding a powerful FRII radio galaxy.
The structure is mostly populated by blue, star forming galaxies, and likely constitutes the progenitor of a local cluster caught in its main assembly phase. The Chandra 0.5Ms observation - the deepest so far for a distant FRII - revealed that its nucleus hosts a luminous, Compton-thick QSO, and that four spots of diffuse X-ray emission are present around it, the most significant around the Eastern lobe of the radio galaxy. The origin of the whole X-ray emission is not entirely clear, as the four spots have apparently different spectral shapes. Most likely, the main spot is produced by an expanding bubble of hot gas (T > 5 keV) shock-heated and inflated by the FRII jet. Significant contribution from Inverse Compton scattering of CMB photons by the relativistic electrons in the lobe (IC-CMB) cannot be ruled out though. Four proto-cluster members lie at the sharp boundary of the main spot of diffuse X-rays and their transverse and radial separations are consistent with the bubble size (∼250 kpc diameter). The star formation rates per unit stellar mass of these four galaxies are the highest of all proto-cluster members and 2-5 times higher than the average of field galaxies with equal stellar mass and redshift. This argues for remarkable positive AGN feedback on large scales, in which the expanding hot gas bubble compresses the galaxies’ inter-stellar medium promoting star formation.
This is the first time that an accreting supermassive black hole is seen to exert positive feedback on multiple galaxies at hundreds of kpc distance. If confirmed, this mechanism will represent a new and important process of large scale structure formation.

The above results have been published in Gilli et al. 2019, and have been the subject of a NASA/CXC press release.

We observed the system at 3mm with ALMA and at 150MHz with LOFAR. ALMA observations probe the molecular gas content of the structure, including its galaxy members, whereas LOFAR observations probe populations of old electrons that are invisible at GHz frequencies and may fill the observed X-ray bubble. Three new, gas-rich (M_H2 = 1.5-4.8 x 10¹⁰ M_☉) protocluster members were discovered by ALMA through their CO(2-1) emission. A large, ~ 2 x 10¹¹ M_☉ reservoir of molecular gas, again traced by CO(2-1), was also revealed around the FRII host galaxy. By combining ALMA with previous results, we found that the overall structure will likely evolve into a >10¹⁴ M_☉ galaxy cluster by z=0, and that the FRII host is the likely progenitor of the Brightest Cluster Galaxy (D'Amato et al. 2020). We performed a spectral energy distribution (SED) analysis in the optical/infrared bands of the FRII host galaxy (D'Amato et al. 2021), finding that it features a vigorous star formation rate (SFR) of ∼570 M_☉/yr and a stellar mass of M_* ∼ 3.7 × 10¹¹ M_☉. The corresponding specific SFR = 1.5 ± 0.5 Gyr^-1 is high, and classifies this object as a starburst galaxy that will deplete its molecular gas reservoir in ∼ 3.5 × 10⁸ yr. The reduction and analysis of LOFAR data are ongoing.

Thanks to the deep JVLA observations we unveiled extended diffuse radio emission associated with the lobes of the FRII galaxy (D'Amato et al. 2022), which is not detected in the previous VLA observations. The emission reveals a complex morphology in the FRII galaxy lobes that, coupled with the X-ray diffuse emission, may point towards an interaction between the radio-jets and the external medium. Despite the JVLA observations do not feature a polarization calibrator, we developed a custom calibration procedure that allowed us to investigate the polarized emission of the FRII galaxy (D'Amato et al. 2021). We found that the polarization fraction increases in the regions in which the total intensity shows a bending morphology, and that the magnetic field orientation follows the direction of the bendings. We interpret these features as further evidence of an interaction between the FRII and the intracluster medium.

We further followed up the FRII radio galaxy with LOFAR at 144 MHz. Overall, the low-frequency observations show a morphology consistent with what was already observed at 1.4 GHz. However, they further reveal a large-scale eastern jet connecting the eastern lobe to the AGN nucleus and a new elongation of the Western lobe in North-West direction. The large size of the source enabled us to perform a resolved radio spectral index analysis of the source by combining LOFAR and JVLA data, which is a very unique opportunity for a source at this high redshift. This revealed a tentative unexpected flattening of the radio spectral index at the edge of the backflow in the western lobe, which might be indicating plasma compression. The spatial coincidence between this region and the thermal X-ray bubble C suggests a causal connection between the two. The X-ray bubble C is indeed consistent with a thermal origin and is found to be significantly overpressurised with respect to the ambient medium, suggesting that it will tend to expand and release its energy into the surroundings, contributing to the overall intracluster medium heating. In contrast to previous estimates for the bright X-ray component A, we find that inverse Compton scattering between the radio-emitting plasma of the eastern lobe and cosmic microwave background photons can account for a large fraction (∼45%-80%) of its total 0.5-7 keV measured flux.