Metal-loaded outflows in sub-Milky Way galaxies in the CIELO simulations

TL;DR

This study investigates how SN feedback drives metal-loaded outflows and governs metal retention in sub-Milky Way galaxies using 15 high-resolution CIELO-P7 zoom-in simulations across $z=[0,7]$. By tracking oxygen in the ISM, CGM, and beyond the virial radius with two dynamical outflow definitions, the authors quantify the metal transport and the evolving mass-loading factors, finding that sub-MW systems expel a larger fraction of oxygen and store metals in a CGM reservoir dominated by low-temperature gas. The results show sub-MW outflows are more metal-rich ($Z_{ m out}/Z_{ m ISM} \sim 1.5$) than those in more massive galaxies ($\le 0.5$), and that the MZR for star-forming gas aligns with observations while effective yields reveal substantial metal loss, especially at low mass. They also find that the mass-loading anti-correlation with $V_c$ and $M_{*}$ strengthens with redshift, implying more efficient feedback in smaller, more merger-rich progenitors at $z\sim 2$. Overall, sub-MW galaxies can harbor a significant CGM metal reservoir and exhibit strong metal transport by SN-driven outflows, offering critical constraints for subgrid SN feedback models and the baryon-metal cycle.

Abstract

Supernova (SN) feedback-driven galactic outflows are a key physical process that contributes to the baryon cycle by regulating the star formation activity, reducing the amount of metals in low-mass galaxies and enriching the circumgalactic (CGM) and intergalactic media (IGM). We aim to understand the chemical loop of sub-Milky Way (MW) galaxies and their nearby regions. We studied 15 simulated central sub-MW galaxies (M* <= 10^10 Msun) and intermediate-mass galaxies (M* \sim 10^10 Msun) from the CIELO-P7 high-resolution simulations. We followed the evolution of the progenitor galaxies, their properties and the characteristics of the outflows within the redshift range z = [0, 7]. We used two dynamically-motivated outflow definitions, unbound outflows and expelled mass rates, to quantify the impact of SN feedback. At z \sim 0, sub-MW galaxies have a larger fraction of their current oxygen mass in the gas phase but have expelled a greater portion beyond the virial radius, compared to their higher-mass counterparts. Galaxies with M* <\sim 10^9 Msun have 10-40 per cent of their total oxygen mass within R200 in the CGM, and an equivalent to 10-60 per cent expelled into the IGM. In contrast, more massive galaxies have most of the oxygen mass locked by the stellar populations. The CGM of low-mass galaxies predominantly contains oxygen low-temperature gas, acting as a metal reservoir. We find that the outflows are more oxygen-rich for sub-MW galaxies, Zout/ZISM \sim 1.5, than for higher-mass galaxies, Zout/ZISM <= 0.5, particularly for z < 2. Mass-loading factors of eta_out \sim 0 - 6 are detected in agreement with observations (abridged).

Figures (13)

• Figure 1: Face-on and edge-on projected gas density (first two panels, respectively) and the corresponding (O/H) (two right panels) distributions for three selected galaxies of the CIELO -P7 sample: 0181, 2627 and 7805, taken as examples. Each row shows a galaxy at a key stage of its evolution where gas outflows are detected. The $\rm M_{*}$ of the galaxies at redshift $z = 0$ is shown in each plot. The streamlines depict the median velocity direction of the gas components. The two first galaxies are more rotational dominated, while the third galaxy has a complex gas structure resulting from a recent gas-rich interaction.
• Figure 2: Three galaxies as examples: evolution of star formation rate (purple shades), $\rm \dot{M}_{\rm out}$, the rate of unbound outflow for the inner shell [1.5$\rm R_{opt}$, 0.5$\rm R_{200}$) (solid, green lines), $\rm \dot{M}_{\rm ex}$, the expelled mass rates (solid, orange lines), and $\dot{M_{\rm in}}$, the inflow mass rate (dotted, cyan line) as a function of lookback time (the inset labels denote the galaxy ID). The infall time of satellites (blue arrows) entering the virial radius and the time of minor and major mergers (pink and red arrows, respectively) when corresponds are also indicated (see Fig. \ref{['fig:SFH_all']} to see the diversity of behaviours in our sample).
• Figure 3: Ratio of the unbound outflow metallicity to the ISM metallicity, $Z_{\rm out}/Z_{\rm ISM}$, for the analysed galaxy and their progenitors as a function of redshift. The dashed black line shows $Z_{\rm out}/Z_{\rm ISM} = 1$. The colour-code denotes $\rm M_{*}$ at $z = 0$.
• Figure 4: Mass-metallicity relation for unbound outflows in the first radial interval of the CGM (green circles), for the expelled mass rates (orange circles) and for the star-forming gas (purple circles) for each analysed galaxy. Solid lines are only included to facilitate the visualisation. Observations from lee2006 and simulations from christensen2018 are shown. tremonti2004 MZR scaled relation to solar values of lodders2019 is shown in black dashed line.The 16-84 percentiles (dark grey contours) and the 2.5-97.5 percentiles (light grey contours) scaled from tremonti2004 are also displayed.
• Figure 5: Effective yields as a function of baryonic mass for our galaxies (purple circles). For comparison, observational results reported by lee2006 and Garnett2002 are also included. The shaded areas denote the 16-84 percentiles (dark grey), and the light grey contour shows the 2.5-97.5 percentiles from observations of tremonti2004. Additionally, we show the analytical model fitted by these authors tremonti2004. Black stars show the location of systems which have experienced different levels of metal loss: 80%, 60% and 40%.