The mass-metallicity relation of bulges

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile. Av. Vicuña Mackenna 4860, Santiago, Chile.
² Centro de Astro-Ingeniería, Pontificia Universidad Católica de Chile. Av. Vicuña Mackenna 4860, Santiago, Chile
³ Instituto de Astronomía y Física del Espacio, CONICET-UBA, Casilla de Correos 67, Suc. 28, 1428 Buenos Aires, Argentina

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 September 2025
Accepted: 9 November 2025

Abstract

Context. Bulges, located at the central regions of galaxies, are complex structures, expected to be shaped by the physical processes involved in the assembly history of their host galaxy, such as gravitational collapse, mergers, interactions, and bars. As a consequence, a variety of bulges with distinct morphologies and chemistry could be produced.

Aim. We aim to explore the existence of a stellar mass-metallicity relation of bulges, MZ_*R, and analyze the possible imprint of distinctive features by accretion and migration of stars, which could store information in their assembly histories.

Methods. We used 44 central galaxies from the CIELO cosmological simulations. Their stellar masses are within the range of ∼[10^7.6, 10^10.6] M_⊙. We decomposed the galaxy into bulge and disk using the circularity and binding energies. We tracked the stellar populations in bulges back in time to their birth locations, classifying them as bulge- and disk-born, in-situ, and accreted.

Results. We find that most of the stars in our bulges are formed in-situ, but 33% of our bulges show a non-negligible contribution of stellar accretion from satellites, which could add to about 35% of the population. The accreted material is generally contributed by two or three satellites at most. In some bulges, we also find up to 32% of stars that migrated from the disk due to secular evolution, with a median of 10%. Regardless of the formation histories, we find a clear MZ_*R for bulges, which is more enriched by about 0.4 dex than the corresponding relation of the disk components, and about 0.15 dex more enriched than the galaxy MZ_*R. We find evidence that the dispersion in the bulge MZ_*R is influenced by both stellar accretion from satellites and migration from the disk, such that, at a fixed bulge mass, bulges with higher fractions of accreted and migrated stars tend to be less metal-rich. Therefore, we find a MZ_*R for bulges, which is consistent with an increase in metallicity with increasing mass, while its dispersion stores information on the contribution from different formation channels.