The mass-metallicity relation of bulges

TL;DR

The paper investigates whether bulges possess a distinct stellar mass–metallicity relation, $MZ_{ m{*}}R$, and how bulge assembly channels imprint this relation. Using 44 central galaxies from the CIELO chemo-dynamical simulations, bulge and disk components are separated with an AM-E decomposition, and bulge stars are traced to birth channels (BB in-situ, DB, accreted) to build the bulge $MZ_{ m{*}}R$. Bulges show a clear $MZ_{ m{*}}R$ that is more metal-rich than disks by about $0.40$ dex and offset from the galaxy $MZ_{ m{*}}R$ by roughly $0.15$ dex, with accreted and DB populations driving the scatter via their lower enrichment. The dispersion correlates with ex-situ fractions and disk migration, with most accreted material coming from two to three major satellites, indicating that bulge assembly history is encoded in the chemical relation. Overall, the bulge $MZ_{ m{*}}R$ acts as a fossil record of assembly, linking chemical enrichment to the relative importance of in-situ, accreted, and secular processes, in agreement with MaNGA trends.

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 morphology and chemistry could be produced. Aim. We aim at exploring the existence of a stellar mass-metallicity relation of bulges, MZ*R, and analyze the possible imprint of characteristics features by accretion and migration of stars, which could store information on their assembly histories. Methods. We use 44 central galaxies from the CIELO cosmological simulations. Their stellar masses are within the range of [10^7.6, 10^10.6] Msun. We decomposed the galaxy into bulge and disk using the circularity and binding energies. We track the stellar populations in bulges back in time to their birth location, classifying them as bulge-born in-situ, and disk-born stars 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 contribute by two or three satellites at most. In some bulges, we also find up to a 32% of stars that migrated from the disk due to secular evolution, with a median of 10%. Regardless of the formation histories, we found 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 fraction of accreted and migrated stars tend to be less metal-rich (abridged).

Figures (10)

• Figure 1: Mass fraction of stellar populations contributing to the bulges. Stars formed in-situ (red dots and shaded regions), accreted stars (violet dots and shaded regions) and stars formed in the disk component and subsequently incorporated into the bulge through secular evolution (yellow dots and shaded regions) are shown. The median values (dots and lines) and the 16$^{\rm th}$-84$^{\rm th}$ percentiles (shaded regions) of each of the different populations are depicted.
• Figure 2: Cumulative contribution of satellites to the accreted stellar populations according to the number of major contributors. The cumulative tracks are color-coded by the accreted fraction of the corresponding bulges. As a reference, lines at 80% and 3 contributors are depicted. The median trend of the contributions is included for reference (solid black line).
• Figure 3: The MZ$_{\mathrm{*}}$R of the bulges (red squares) and disks (yellow circles) of the CIELO galaxies, defined by the median [Fe/H]. For comparison, the CIELO galaxy MZ$_{\mathrm{*}}$R is also included (light blue line). Additionally, observational trends reported by 2024:Jegatheesan for the bulges (red shaded regions) and disks (yellow shaded regions) are also included. Shaded regions are determined by the $16^{\rm th}$-$84^{\rm th}$ percentiles.
• Figure 4: [Fe/H] for the bulge (squares) and disk (circles) components as a function of the galaxy stellar mass. The solid lines denote the median trends for both components(orange and yellow, respectively). The plane is color-coded by bulge-total-ratio (left panel) and median age of the corresponding components (right panel). The disk components with an excess of [Fe/H], respect to the disk MZ$_{\mathrm{*}}$R, are highlighted. To quantify third dependences of the bulge, Partial Correlation Coefficients analysis 2020:Bluck angles are included.
• Figure 5: Offset between the metallicity of a given stellar population and the metallicity predicted by the bulge MZ$_{\mathrm{*}}$R for the corresponding bulge mass as $\rm [Fe/H] -[Fe/H]_{\rm MZ_*R}$. We considered accreted (purple), disk born (gold) and bulge born (orange) stellar populations. Left panel: metallicity offset as a function of the stellar mass of the bulge (left panel) and the median age of the stellar population itself (right panel). The lines correspond to the density countours at 20%, 40%, 60% and 80% of each distribution.