Stellar mass is not the best predictor of galaxy metallicity

The gravitational potential–metallicity relation ΦZR

¹ Dpto. de Física Teórica y del Cosmos, Facultad de Ciencias (Edificio Mecenas), Universidad de Granada, 18071 Granada, Spain
e-mail: lsanchezm@ugr.es
² Instituto Carlos I de Física Teórica y computacional, Universidad de Granada, 18071 Granada, Spain
³ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
⁵ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106 Ensenada, 22800 BC, México

Received: 20 April 2023
Accepted: 13 November 2023

Abstract

Context. Interpreting the scaling relations followed by galaxies is a fundamental tool for assessing how well we understand galaxy formation and evolution. Several scaling relations involving the galaxy metallicity have been discovered through the years, the foremost of which is the scaling with stellar mass. This so-called mass–metallicity relation is thought to be fundamental and has been subject to many studies in the literature.

Aims. We study the dependence of the gas-phase metallicity on many different galaxy properties to assess which of them determines the metallicity of a galaxy.

Methods. We applied a random forest regressor algorithm on a sample of more than 3000 nearby galaxies from the SDSS-IV MaNGA survey. Using this machine-learning technique, we explored the effect of 148 parameters on the global oxygen abundance as an indicator of the gas metallicity.

Results. M_⋆/R_e, as a proxy for the baryonic gravitational potential of the galaxy, is found to be the primary factor determining the average gas-phase metallicity of the galaxy (Z_g). It outweighs stellar mass. A subsequent analysis provides the strongest dependence of Z_g on M_⋆/R_e^0.6. We argue that this parameter traces the total gravitational potential, and the exponent α ≃ 0.6 accounts for the inclusion of the dark matter component.

Conclusions. Our results reveal the importance of the relation between the total gravitational potential of the galaxy and the gas metallicity. This relation is tighter and likely more primordial than the widely known mass–metallicity relation.