The deepest X-ray view of high-redshift galaxies: constraints on low-rate black-hole accretion (1608.02614v1)

Abstract: We exploit the 7 Ms \textit{Chandra} observations in the \chandra\,Deep Field-South (\mbox{CDF-S}), the deepest X-ray survey to date, coupled with CANDELS/GOODS-S data, to measure the total X-ray emission arising from 2076 galaxies at $3.5\leq z < 6.5$. This aim is achieved by stacking the \textit{Chandra} data at the positions of optically selected galaxies, reaching effective exposure times of $\geq10^9\mathrm{s}$. We detect significant ($>3.7\sigma$) X-ray emission from massive galaxies at $z\approx4$. We also report the detection of massive galaxies at $z\approx5$ at a $99.7\%$ confidence level ($2.7\sigma$), the highest significance ever obtained for X-ray emission from galaxies at such high redshifts. No significant signal is detected from galaxies at even higher redshifts. The stacking results place constraints on the BHAD associated with the known high-redshift galaxy samples, as well as on the SFRD at high redshift, assuming a range of prescriptions for X-ray emission due to X- ray binaries. We find that the X-ray emission from our sample is likely dominated by processes related to star formation. Our results show that low-rate mass accretion onto SMBHs in individually X-ray-undetected galaxies is negligible, compared with the BHAD measured for samples of X-ray detected AGN, for cosmic SMBH mass assembly at high redshift. We also place, for the first time, constraints on the faint-end of the AGN X-ray luminosity function ($\mathrm{logL_X\sim42}$) at $z>4$, with evidence for fairly flat slopes. The implications of all of these findings are discussed in the context of the evolution of the AGN population at high redshift.

• The paper demonstrates that stacked X-ray emissions yield >3.7σ detection at z≈4 and 2.7σ evidence at z≈5, marking a breakthrough in high-redshift observations.
• The paper reveals that low-rate SMBH accretion in undetected galaxies is minimal compared to active AGN phases, underscoring the importance of high-accretion events in SMBH growth.
• The paper constrains the faint-end of the AGN X-ray luminosity function at log L_X≈42 for z>4 and indicates that star formation processes predominantly drive the observed emissions.

Deep X-ray Insights into High-Redshift Galaxies: Black Hole Accretion Constraints

This paper by Vito et al. presents a rigorous exploration of X-ray emissions from galaxies at high redshifts (z = 3.5 to 6.5) using 7 Ms Chandra observations in the Chandra Deep Field-South (CDF-S), complemented by CANDELS/GOODS-S survey data. The paper addresses astrophysical phenomena underpinning supermassive black hole (SMBH) growth and star formation processes in the early universe, leveraging stacking techniques to maximize signal detection in individually undetected sources.

Key Findings

1. Stacked X-ray Emission Detection: The paper reports significant detection (>3.7σ) of X-ray emissions from massive galaxies at z ≈ 4. It provides evidence of emissions at z ≈ 5 with a 99.7% confidence level (2.7σ), the most significant detection at this redshift to date. No signal is detected beyond z ≈ 5.5, suggesting a sensitivity threshold at these epochs.
2. Low-Rate SMBH Accretion Negligible: Vito et al. conclude that low-rate mass accretion onto SMBHs in undetected galaxies is minimal compared to active galactic nuclei (AGN) contributions that are identified in X-ray surveys. This finding suggests active periods of significant accretion are vital for cosmic SMBH mass assembly.
3. Constraints on AGN XLF Faint-End: The paper sets constraints on the faint end of the AGN X-ray luminosity function at log L_X ≈ 42 for z > 4, finding relatively flat slopes.
4. Dominance of Star-Formation Processes: Analysis indicates that the X-ray emissions are likely dominated by star-formation processes rather than AGN activity, aligning stacked emissions more closely with stellar processes than nuclear accretion.

Implications for Galaxy Evolution

The insights offered by this paper illuminate the tight coupling between star formation rate density (SFRD) and BH accretion rate density (BHAD) during cosmic history. As the SMBH and galaxy evolve together, understanding these factors at high redshift offers valuable perspectives on the conditions of early galaxy formation. The lack of detectable accretion signals beyond z ≈ 5.5 might indicate a rapid decline or episodic nature of accretion phases, amplifying the importance of AGN in earlier epochs. The stark distinction between stellar and accretion-derived emissions further emphasizes that circumstantial galactic environments could induce star formation more prominently than black hole growth at these early stages.

Speculation and Future Directions

Vito et al.'s constraints on the faint-end XLF slopes hint at ongoing conversations regarding SMBH seed models and their implication in cosmic structures. The flat slope might support models where massive seed black holes play a crucial role, contrasting with scenarios dominated by stellar remnants. Future advancements, possibly driven by proposed missions like X-Ray Surveyor, could refine sensitivity boundaries, enabling more granular exploration of low-luminosity AGN characteristics.

The significance of these findings manifests in the context of the broader narrative of SMBH evolution, enriching dialogs about the interplay between environmental factors and intrinsic galactic dynamics. The precision and depth of this analysis provide valuable foundations for theoretical models probing the nature and nurture of early universe phenomena. Future observations should aim to synergize multi-wavelength data to corroborate these results, thus enhancing the fidelity of our cosmic ancestry narrative.