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

Abstract

The detection of two z ∼ 13 galaxy candidates (Harikane et al. 2021b) 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 − 25× 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 ∼ 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 ∼ 10 8 M accreting at or slightly above the Eddington rate (f Edd ∼ 1.0). This black hole mass at z ∼ 13 would require challenging, but not implausible, growth parameters, in line with what required for z ∼ 7.5 quasars detected thus far. If spectroscopically confirmed, these two sources will represent a remarkable laboratory to study the Universe at previously inaccessible redshifts.

Modelling the star-formation activity and ionizing properties of high-redshift galaxies

“…As observations with the JWST have already started to reveal tantalizing insights into the complex nature of star formation in the early Universe, a self-consistent treatment of the properties of the pristine Population III stars and how they transition into the metal-enriched Population JCAP07(2024)078 II stars in semi-analytical models like ours will be extremely beneficial for unambiguously understanding their impact on observables like the UV luminosity functions at z > 12 [166], the 21-cm signal from Cosmic Dawn [167] and the chemical enrichment of the IGM [168]. Furthermore, some studies have also invoked possibilities like contributions from non-stellar sources like an accreting supermassive black hole (AGN) [49,169] and/or decrement of dust attenuation at z > 10 [72, 170-172] for explaining the recent JWST observations. It would be interesting to include the effects of dust obscuration and AGN activity in our model and revisit the present analysis.…”

Massive black holes at high redshifts from superconducting cosmic strings

“…However, observations of high redshift quasars suggest that supermassive black holes are already in place at early times. The most robust high redshift black hole observation to date has a mass of 1.6 × 10 9 M ⊙ at z = 7.642 [1], although there have recently been observations of two galaxy candidates [93] at z ∼ 13, which may be interpreted as quasars hosting black holes of order M BH ≈ 10 8 M ⊙ [94]. Conservatively, a population of "light" black hole seeds (initial mass M BH ∼ 10 2 M ⊙ ) will form through the collapse of Pop III stars.…”

Lyman-$α$ Emitting Galaxies (LAEs) at Cosmic Dawn: Implications and Predictions

“…This does not mean, of course, that there can be no star-forming LBGs at substantially higher redshifts (see e.g Pacucci et al 2022;Harikane et al 2022),. but simply that they cannot be seen as LAEs.…”