Are the Newly-Discovered $z \sim 13$ Drop-out Sources Starburst Galaxies or Quasars?

Abstract: The detection of two $z\sim 13$ galaxy candidates has opened a new window on galaxy formation at an era only $330$ Myr after the Big Bang. Here, we investigate the physical nature of these sources: are we witnessing star forming galaxies or quasars at such early epochs? If powered by star formation, the observed ultraviolet (UV) luminosities and number densities can be jointly explained if: (i) these galaxies are extreme star-formers with star formation rates $5-24\times$ higher than those expected from extrapolations of average lower-redshift relations; (ii) the star formation efficiency increases with halo mass and is countered by increasing dust attenuation from $z \sim 10-5$; (iii) they form stars with an extremely top-heavy initial mass function. The quasar hypothesis is also plausible, with the UV luminosity produced by black holes of $\sim 10^8 \, \mathrm{M_{\odot}}$ accreting at or slightly above the Eddington rate ($f_{\rm Edd}\sim 1.0$). This black hole mass at $z\sim13$ would require very challenging, but not implausible, growth parameters. If spectroscopically confirmed, these two sources will represent a remarkable laboratory to study the Universe at previously inaccessible redshifts.

• The paper analyzes two newly-discovered $z ext{\sim} 13$ sources detected just 330 million years after the Big Bang, investigating whether they are extreme starburst galaxies or quasars powered by early supermassive black holes.
• Under the starburst galaxy hypothesis, these sources require star formation rates 5 to 24 times higher than typical, potentially explained by extreme efficiencies, evolving efficiency with dust, or a top-heavy stellar initial mass function.
• If they are quasars, the analysis indicates the supermassive black holes (${\sim}10^8$ M$_\odot$) must have grown rapidly from heavy seeds by $z ext{\sim} 13$ through high duty cycles and efficient accretion.

Interpretation of $z \sim 13$ Drop-out Sources: Starburst Galaxies or Quasars?

In the paper, Pacucci et al. analyze two galaxy candidates at a redshift of approximately $z \sim 13$, detected only 330 million years after the Big Bang, a period crucial for understanding early universe galaxy formation. These sources are investigated under two primary hypotheses: as extreme star-forming galaxies or as quasars powered by early supermassive black holes (SMBHs). This research is pivotal as it provides insights into the nature of the very early universe and the complex dynamics of early gravitational structures and stellar formations.

Star-forming Galaxy Hypothesis

Pacucci et al. explored the possibility that these $z \sim 13$ sources are extreme starburst galaxies. They compare expected properties with extrapolated data from lower-redshift sources. The paper posits that these galaxies might have significantly higher star formation rates (SFRs), exceeding typical observed values at comparable redshift levels by factors of 5 to 24. Three potential scenarios are suggested to accommodate the observed UV luminosities and densities: (i) galaxies exhibit extreme star formation efficiencies; (ii) an evolving star formation efficiency with halo mass, affected by dust attenuation from $z \sim 10$ to $z \sim 5$; (iii) stars form with a top-heavy initial mass function (IMF). These considerations are critical in evaluating early galaxy formation processes and highlight significant deviations from the expected behavior seen in lower-redshift galaxies.

Quasar Hypothesis

The alternative hypothesis considers these sources as quasars, wherein their UV luminosity is attributed to the accretion by supermassive black holes, with masses around $10^8 M_\odot$, operating at Eddington or super-Eddington efficiencies. This scenario necessitates a rapid and efficient growth of SMBHs from primordial or direct-collapse seeds to reach the observed mass by $z \sim 13$. Here, Pacucci et al. explore growth parameters such as Eddington ratios, duty cycles, and seed masses, concluding that these black holes would need to start from heavy seeds with accretion facilitated by high duty cycles and low radiative efficiencies.

Implications and Future Directions

The study's results have substantial implications. The identification of $z\sim 13$ sources contributing to the high-luminosity end of the UV luminosity function challenges our understanding of early star formation and SMBH formation timelines. Should the quasar hypothesis be correct, these sources could provide empirical constraints on theories about SMBH seed formation and their growth trajectories in the early universe.

Furthermore, the implications of either scenario reach into future observational efforts and theoretical modeling. If future spectroscopic data confirm these redshifts, such early-Universe quasars or starburst galaxies could significantly refine existing models, especially concerning the halo mass-luminosity relationship and the behaviors of primordial black holes. Additionally, the paper highlights the anticipated contributions from observatories like JWST, ELT, and NGRST, which will further probe these early epochs and aid in resolving ambiguities in the nature of early universe objects.

In conclusion, Pacucci et al. provide a comprehensive and nuanced discussion on the possible identities of $z \sim 13$ galaxy candidates. This work opens important dialogues on the extreme conditions required for star formation and black hole growth at such early cosmic timescales, pressing for technological advancements and more precise theoretical models to deepen our understanding of cosmological genesis and evolution.