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? (1909.00814v2)

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 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]

• The paper identifies a significant galaxy overdensity around a Compton-thick FRII radio galaxy, demonstrating active star formation in eight member galaxies.
• Utilizing observations from VLT/MUSE, LBT/LUCI, and Chandra, the study confirms star-forming rates of 8–60 M⊙/yr across blue, compact galaxies.
• Evidence from X-ray and radio data indicates that AGN feedback, via heated gas bubbles and pressure differences, may promote starburst activity in protocluster environments.

Analysis of Galaxy Overdensity Associated with FRII Radio Galaxy at z=1.7

The research paper presents an investigation into a remarkable galaxy overdensity surrounding a Compton-thick Fanaroff-Riley type II (FRII) radio galaxy at a redshift of z=1.7. This paper combines extensive observational data from multiple facilities, including VLT/MUSE, LBT/LUCI, Chandra X-ray Observatory, and others, to unravel details about the structure, formation activity, and the potential impact of active galactic nucleus (AGN) feedback on large scales.

Key Findings

The paper identifies eight galaxy members within the overdensity region, confirming the FRII host and several other star-forming galaxies through spectroscopic analysis. Most galaxies display high star formation rates (SFRs), indicating active star formation processes. These star-forming galaxies lie primarily in the blue and compact range, with rates of 8-60 $M_{\odot}$/yr. Galaxy distribution analyzes imply they are not confined to a virialized structure, suggesting that the structure is in a significant assembly stage, hinting at a transformation into a local massive galaxy cluster.

X-ray and Radio Observations

The deep X-ray observations from Chandra provide insights into the nucleus of the FRII galaxy, revealing a luminous quasar heavily obscured by Compton-thick absorption. The interplay between X-ray and radio data unveils a fascinating scenario where the diffuse X-ray emission seems to coincide with the positions of several star-forming overdensity members. This diffuse emission is likely a thermal phenomenon, potentially resulting from a gas bubble heated by the FRII jet, characterized by a temperature threshold of about 5 keV.

AGN Feedback Implications

A significant proposition of the paper is that the AGN feedback from the FRII radio galaxy may be promoting star formation within member galaxies of the overdensity. The pressure differences and the spatial configuration of the galaxies at the edge of the diffuse X-ray emission provide compelling evidence of AGN-induced star formation boosts. Four galaxies with the highest specific star formation rates are positioned at the boundary of this expanding hot gas, supporting the hypothesis that the propagation of radio features influences starburst activities.

Future Considerations

This research builds a strong case for AGN feedback mechanisms operating on much larger scales than traditionally considered, contributing significantly to galaxy evolution in nascent structures like protoclusters. The paper sets the stage for further targeted observations, leveraging the depth and reach of upcoming facilities such as ALMA and the enhanced radio observatories like LOFAR. Continued investigation and data collection could refine our understanding of galaxy formation and AGN impact.

Conclusion

The paper's findings contribute to a fuller understanding of galaxy cluster formation and the role of powerful radio galaxies as tracers and possibly modulators of such overdensity structures. The data suggest a compelling narrative wherein AGN feedback, often known for its suppression effects, could foster star formation under specific conditions, shifting paradigms in cosmic structure formation. Further observations and simulations will be crucial in confirming the broader impact of these mechanisms across different environments in the universe.