High-redshift quasars host galaxies: is there a stellar mass crisis?

TL;DR

This study uses a semi-analytical model to explore high-redshift quasar host galaxies, revealing a potential stellar mass crisis due to overestimated masses compared to observations, and discusses possible explanations.

Contribution

It demonstrates that standard formation scenarios predict stellar masses larger than observed, highlighting uncertainties in dynamical mass measurements at high redshift.

Findings

Predicted stellar masses are 3-30 times higher than observational limits.

Approximately 40% of progenitors are classified as Sub Millimeter Galaxies during transition.

Uncertainties in dynamical mass measurements may explain the mass discrepancy.

Abstract

We investigate the evolutionary properties of a sample of quasars at 5<z<6.4 using the semi-analytical hierarchical model GAMETE/QSOdust. We find that the observed properties of these quasars are well reproduced by a common formation scenario in which stars form according to a standard IMF, via quiescent star formation and efficient merger-driven bursts, while the central BH grows via gas accretion and BH-BH mergers. Eventually, a strong AGN driven wind starts to clear up the ISM of dust and gas, damping the star formation and un-obscuring the line of sight toward the QSO. In this scenario, all the QSOs hosts have final stellar masses in the range $(4-6)\times 10^{11} M_{sun}$, a factor 3-30 larger than the upper limits allowed by the observations. We discuss alternative scenarios to alleviate this apparent tension: the most likely explanation resides in the large uncertainties that still affect dynamical mass measurements in these high-z galaxies. In addition, during the transition between the starburst-dominated and the active QSO phase, we predict that about 40% of the progenitor galaxies can be classified as Sub Millimeter Galaxies, although their number rapidly decreases with redshift.