The heartbeat of stellar halos: Insights from the stellar halo mass-metallicity relation

TL;DR

This study addresses when stellar halos settle onto the present-day stellar halo mass–metallicity relation (MZhR) by tracking halo assembly from $z\approx3.5$ to $z=0$ using CIELO cosmological zoom-in simulations. It introduces halo cardiograms to quantify the buildup of mass and metallicity and defines a stability time $t_{\rm stable}$ as the first lookback when median metallicity remains within $\pm0.1$ dex of its $z=0$ value. The results show that the MZhR is already established by $z\approx3.5$ with a slope similar to today and only weak evolution to $z=0$, with metallicity increasing by about $0.21$ dex for a fixed halo mass. A key finding is a near one-to-one relation between $t_{\rm stable}$ and the merger time of the first main contributors ($t^{\rm SHMC1}_{\rm merger}$), yielding a scatter of about $2.2$ Gyr and enabling $t_{\rm stable}$ to act as a proxy for the last major merger; observational proxies $t_{90}$ and the age–metallicity relation AMR provide avenues to test these predictions in real galaxies, including the Milky Way and Andromeda.

Abstract

This work investigates the presence and evolution of the MZhR from redshift z=3.5 to z=0, and identifies when galaxies settle on the present-day MZhR. We used central galaxies with log10(Mgal/Msun)=[9,11] from CIELO simulations. We identified stellar halos, from z=3.5 to z=0, using the AM-E method, focusing on the region between the 1.5 optical radius and the virial radius. We presented halo cardiograms, a novel approach to studying the assembly history of stellar halos. Using them, we defined a stability time (tst) as the first time that the median halo metallicity does not change more than \pm 0.1 dex with respect to its value at z=0. CIELO stellar halos reproduce the present-day observed MZhR. At z=3.5, stellar halos already define an MZhR whose slope is similar to the slope at z=0. For a fixed stellar halo mass, the metallicity increases ~0.21 dex from z=3.5 to z=0, reflecting the progressive chemical enrichment provided by the accretion of satellites with diverse masses and different levels of enrichment. When the first stellar halo main contributor (SHMC1) provides a mass fraction at least 20% higher than the remaining contributors, the stellar halo metallicity is set once SHMC1 is fully disrupted (tmerger). This yields a clear correlation between tst and tmerger, with a scatter of 2.2 Gyr driven by the relative importance of the second and third main contributing satellites. We provide two observational tracers for tst: t90 and a stability time from the age-metallicity relation. Our results suggest that estimating tst could serve as a proxy for dating the moment at which the stellar halo reaches the present-day MZhR, as well as for dating the last major merger that builds them. Combined with an estimation of the merger time of the main contributing satellite, it can provide insights into the relative importance of the second and third contributing satellites. (abridged)

Figures (9)

• Figure 1: The mass-metallicity relationship for stellar halos at $z=0$. Observational data from GHOSTS HST survey Harmsen_2017Gozman_2023 are represented by stars. Estimates for the accreted stellar halo mass and metallicity measured at 30 kpc are displayed for the Auriga galaxies Monachesi_2019, the semi-analytic L-Galaxies Murphy_2022 and the CIELO galaxies (blue squares). In addition, we show the CIELO stellar halos measured between $1.5\rm r_{\rm opt}$ and $\rm r_{\rm vir}$ (orange circles), an aperture that encompasses all stellar populations populations (in-situ, endo-debris and ex-situ stars).
• Figure 2: Top panels: Stellar halo mass and metallicity at different redshifts (indicated in each panel). The black line and gray region show the MZhR at $z=0$ and its 1$\sigma$ dispersion. The solid colored lines represent a fit with fixed slope from $z=0$, minimizing the y-intercept at that redshift. The dashed colored line is a linear fit at each time. Bottom panels: Difference in metallicity with respect to $z=0$, $\Delta \rm [Fe/H]=[Fe/H]_z-[Fe/H]_{z=0}$, as a function of the stellar halo mass at the same redshift, $\rm M_{\star,{halo}}(z)$. Negative $\Delta$ values indicate lower metallicities relative to the final values at $z=0$. Only stellar halos with more than 500 stellar particles are displayed according our numerical resolution criteria.
• Figure 3: Stellar halo mass and metallicity of four representative galaxies, color-coded by lookback time. The CIELO MZhR at $z=0$ and 1$\sigma$ dispersion are shown as a black and shaded gray region, respectively. CIELO galaxy ID and stellar halo mass are displayed in the upper-right corner of each panel.
• Figure 4: Variations in metallicity (left column) and stellar halo mass (right column) relative to their values at $z=0$ as a function of lookback time. The arrows indicate the main stellar contributors: SHMC1 (red) and SHMC2 (light blue) at infall (solid) and merger (dotted) times. The pink shaded region marks differences within $\pm$0.1 dex from $z=0$ of the stellar mass or metallicity. The pink square represent the stability time defined in Sec.\ref{['sec:heartbeat_sh']}.
• Figure 5: Variations in stellar halo mass (upper panel) and metallicity (lower panel) as a function of lookback time, offset by the merger time of their SHMC1. The purple shaded region represents $\pm1$ standard deviation (16th and 84th percentiles) of the total sample. The pink shaded region indicates a $\pm$0.1 dex variation in mass (upper panel) or metallicity (lower panel) relative to $z=0$.