Metallicity ceiling in quasars from recycled stellar winds

Harvard-Smithsonian Center for Astrophysics (CfA), Harvard University, 60 Garden Street, Cambridge, MA 02138, USA
e-mail: shelley.cheng@cfa.harvard.edu

Received: 3 April 2023
Accepted: 2 August 2023

Abstract

Context. Optically luminous quasars are metal rich across all redshifts. Surprisingly, there is no significant trend in the broad line region (BLR) metallicity with different star formation rates (SFR), and the average N V/C IV metallicity does not appear to exceed 9.5 Z_⊙. Combined, these observations may suggest a metallicity ceiling.

Aims. Here, we conduct an exploratory study on scenarios relating to the evolution of embedded stars that may lead to a metallicity ceiling in quasar disks.

Methods. We developed a simple model that starts with gas in a “closed box,” which is enriched by cycles of stellar evolution until eventually newly formed stars may undergo significant mass loss before they reach the supernova stage and further enrichment is halted. Using the MESA code, we created a grid over a parameter space of masses (> 8 M_⊙) and metallicities (1 − 10 Z_⊙), and located portions of the parameter space where mass loss via winds occurs on a timescale shorter than the lifetime of the stars.

Results. Based upon reasonable assumptions about stellar winds, we found that sufficiently massive (8 − 22 M_⊙) and metal-rich (∼9 Z_⊙) stars lose significant mass via winds and may no longer evolve to the supernovae stage, thereby failing to enrich and increase the metallicity of their surroundings. This suggests that a metallicity ceiling is the final state of a closed-box system of gas and stars.