Tracing the rise of supermassive black holes: A panchromatic search for faint, unobscured quasars at z > 6 with COSMOS-Web and other surveys (2401.11826v2)

Abstract: We report the identification of 64 new candidates of compact galaxies, potentially hosting faint quasars with bolometric luminosities of $L_\mathrm{bol} = 10^{43}$--10$^{46}$ erg s$^{-1}$, residing in the reionization epoch within the redshift range of $6 \lesssim z \lesssim 8$. These candidates were selected by harnessing the rich multiband datasets provided by the emerging JWST-driven extragalactic surveys, focusing on COSMOS-Web, as well as JADES, UNCOVER, CEERS, and PRIMER. Our search strategy includes two stages: applying stringent photometric cuts to catalog-level data and detailed spectral energy distribution fitting. These techniques effectively isolate the quasar candidates while mitigating contamination from low-redshift interlopers, such as brown dwarfs and nearby galaxies. The selected candidates indicate physical traits compatible with low-luminosity active galactic nuclei, likely hosting $\approx10^5$--$10^7~M_\odot$ supermassive black holes (SMBHs) living in galaxies with stellar masses of $\approx10^8$--$10^{10}~M_\odot$. The SMBHs selected in this study, on average, exhibit an elevated mass compared to their hosts, with the mass ratio distribution slightly higher than those of galaxies in the local Universe. As with other high-$z$ studies, this is at least in part due to the selection method for these quasars. An extensive Monte Carlo analysis provides compelling evidence that heavy black hole seeds from the direct collapse scenario appear to be the preferred pathway to mature this specific subset of SMBHs by $z\approx7$. This work underscores the significance of further spectroscopic observations, as the quasar candidates presented here offer exceptional opportunities to delve into the nature of the earliest galaxies and SMBHs that formed during cosmic infancy.

• The paper demonstrates a two-stage selection process using stringent photometric cuts and detailed SED fitting to isolate high-z quasar candidates.
• The study identifies 64 candidates with SMBH masses of ~10^5–10^7 M☉, showing elevated mass ratios relative to their host galaxies.
• The findings support the direct collapse scenario for heavy black hole seeds and emphasize JWST's role in unveiling early SMBH evolution.

Overview of the Panchromatic Search for Faint, Unobscured Quasars at High Redshift

The paper "Tracing the Rise of Supermassive Black Holes: A Panchromatic Search for Faint, Unobscured Quasars at $z \gtrsim 6$ with COSMOS-Web and Other Surveys" presents a systematic approach to identifying faint quasars in the early universe, aiming to understand the formation and evolution of supermassive black holes (SMBHs) during the epoch of reionization. The authors employ a panchromatic dataset derived from several JWST-driven surveys, notably COSMOS-Web, as well as JADES, UNCOVER, CEERS, and PRIMER, to achieve their scientific objectives.

Methodology and Candidate Selection

The paper outlines a two-stage selection process for quasar candidates. In the first stage, stringent photometric cuts are applied to catalog-level data to isolate potential high-redshift quasars while excluding low-redshift interlopers. The authors emphasize the importance of accurate photometric redshift estimations to enhance the reliability of their selections. The second stage involves detailed spectral energy distribution (SED) fitting to further discriminate quasar candidates from contaminants like brown dwarfs and nearby galaxies. This approach effectively mitigates contamination and confirms the quasar nature of the candidates.

Key Findings

The research identifies 64 new candidates of compact galaxies that potentially host faint quasars at $6 \lesssim z \lesssim 8$, with bolometric luminosities ranging from $L_\mathrm{bol} = 10^{43}$–$10^{46}$ erg s$^{-1}$. These candidates indicate physical characteristics compatible with low-luminosity active galactic nuclei (AGNs), likely containing SMBHs with masses of $\approx 10^5$–$10^7$ $M_\odot$, residing in galaxies with stellar masses $\approx 10^8$–$10^{10}$ $M_\odot$.

A significant finding is that the SMBHs of these candidates exhibit elevated mass relative to their host galaxies, with a mass ratio distribution slightly higher than those observed in the local universe. This characteristic, partially influenced by the selection method, suggests these sources belong to an upper envelope of SMBH mass distributions at high redshift.

Implications for Black Hole Seed Formation Scenarios

The paper provides compelling evidence favoring the direct collapse scenario for heavy black hole seeds, suggesting that such seeds are the preferred pathway to the rapid growth of SMBHs observed in this high-redshift quasar sample. Many of the selected candidates might have originated from seeds with masses around $\sim 10^5$ $M_\odot$, assuming thin disk accretion with an average Eddington ratio of $f_\mathrm{Edd}=0.6\pm0.3$ and a radiative efficiency of $\epsilon = 0.2\pm0.1$. This finding supports the hypothesis that massive seed black holes played a crucial role in early SMBH evolution, providing key insights into their initial mass distribution and growth mechanisms.

Future Directions

The paper underscores the necessity for future spectroscopic observations to validate these quasar candidates and further probe their properties, such as gas-phase metallicity and host galaxy dynamics. The ability of JWST to explore the early universe with unprecedented depth and resolution opens exciting possibilities for understanding the intricate connections between SMBHs and their host galaxies at epochs close to cosmic dawn.

In conclusion, this paper advances the understanding of early SMBH growth and provides a valuable foundation for future investigations into the nature and origins of the first quasars, shedding light on their pivotal role in shaping the early universe.