A lack of LAEs within 5Mpc of a luminous quasar in an overdensity at z=6.9: potential evidence of quasar negative feedback at protocluster scales

Abstract: High-redshift quasars are thought to live in the densest regions of space which should be made evident by an overdensity of galaxies around them. However, campaigns to identify these overdensities through the search of Lyman Break Galaxies (LBGs) and Lyman $\alpha$ emitters (LAEs) have had mixed results. These may be explained by either the small field of view of some of the experiments, the broad redshift ranges targeted by LBG searches, and by the inherent large uncertainty of quasar redshifts estimated from UV emission lines, which makes it difficult to place the Ly-$\alpha$ emission line within a narrowband filter. Here we present a three square degree search ($\sim 1000$ pMpc) for LAEs around the $z=6.9$ quasar VIKJ2348-3054 using the Dark Energy CAMera (DECam), housed on the 4m Blanco telescope, finding 38 LAEs. The systemic redshift of VIK J2348--3054 is known from ALMA [CII] observations and place the Ly-$\alpha$ emission line of companions within the NB964 narrowband of DECam. This is the largest field of view LAE search around a $z>6$ quasar conducted to date. We find that this field is $\sim$ 10 times more overdense when compared to the Chandra Deep-Field South, observed previously with the same instrumental setup as well as several combined blank fields. This is strong evidence that VIKJ2348-3054 resides in an overdensity of LAEs over several Mpc. Surprisingly, we find a lack of LAEs within 5 physical Mpc of the quasar and take this to most likely be evidence of the quasar suppressing star formation in its immediate vicinity. This result highlights the importance of performing overdensity searches over large areas to properly assess the density of those regions of the Universe.

• The paper identifies a significant LAE overdensity around quasar VIK J2348--3054, yet finds a surprising void within 5 Mpc.
• The paper employs DECam imaging to detect 38 LAEs over 1000 pMpc², comparing these findings with data from the CDFS.
• The study suggests intense quasar radiation may photoevaporate hydrogen, suppressing star formation and signaling negative feedback at protocluster scales.

Analysis of the Absence of LAEs in Proximity to a High-Z Quasar: Indications of Quasar-Induced Negative Feedback at Protocluster Scales

This paper investigates the intriguing absence of Lyman Alpha Emitters (LAEs) near a luminous quasar, VIK J2348--3054, situated at the redshift z = 6.9. Utilizing the Dark Energy Camera (DECam) on the 4-meter Blanco telescope, the study constitutes the largest field of view survey for LAEs around a high-redshift quasar, covering an impressive area of approximately 1000 pMpc².

Key Findings

1. LAE Detection Around VIK J2348--3054: The survey identified 38 LAEs, significantly indicating that VIK J2348--3054 resides in an overdense region compared to the averaged field surveys such as the Chandra Deep-Field South (CDFS).
2. Noteworthy Void: Contrary to expectations, the research highlights a notable lack of LAEs within 5 physical Mpc of the quasar, suggesting that the quasar might be exerting a suppressive influence on star formation in its immediate vicinity.
3. Overdensity Evidence: The study determines a density field around the quasar that is about ten times higher than in the CDFS and several combined blank fields observed with similar setups, thus underscoring strong evidence of an overdensity.

Implications and Speculations

• Negative Feedback Mechanism: The unexpected scarcity of LAEs proximal to the quasar supports the hypothesis of negative feedback effects, wherein intense ionizing radiation from the quasar photoevaporates hydrogen, thereby inhibiting gas condensation and subsequent star formation.
• Significance of Accurate Measurements: This research underpins the necessity for reliable redshift estimations using precise systemic measures such as those from ALMA [CII] observations, compared to those previously reliant on UV emission lines.
• Future Observations: The paper posits that future exploration should range over larger areas and utilize systemic redshift measurements to better capture the complexities in the environments surrounding high-redshift quasars.
• Environmental Density Studies: In a larger context, this study suggests that high-redshift quasars are potentially nestled in dense galaxy regions, a hypothesis necessitating further observational evidence, particularly concerning LAE suppression, to confirm the nature and extent of quasar impacts.

This study serves as a significant step in understanding the interplay between high-redshift quasars and their cosmic environments, opening up pathways for further investigation into the mechanisms driving protocluster formation and evolution. The discussion of possible negative feedback offers compelling insight into the dynamic processes shaping the early Universe’s structure and evolution.