[Paper Review] Explaining the scatter in the galaxy mass-metallicity relation with gas flows

Maria L. van Loon, Peter Mitchell+1 more|arXiv (Cornell University)|Jan 1, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy59 references24 citations

TL;DR

This study uses the EAGLE cosmological hydrodynamical simulation to show that gas inflows, outflows, and star formation rates—particularly gas fraction and specific inflow rate—explain the scatter in the galaxy mass-metallicity relation (MZR) at low to intermediate masses (M⋆/M⊙ ≲ 10¹⁰). The gas fraction is the strongest predictor of MZR scatter, with inflow and outflow rates also contributing independently, while black hole feedback dominates at high masses, and galaxies remain above or below the MZR for several gigayears once displaced.

ABSTRACT

The physical origin of the scatter in the relation between galaxy stellar mass and the metallicity of the interstellar medium, i.e. the mass-metallicity relation (MZR), reflects the relative importance of key processes in galaxy evolution. The EAGLE cosmological hydrodynamical simulation is used to investigate the correlations between the residuals of the MZR and the residuals of the relations between stellar mass and, respectively, specific inflow, outflow, and star formation rate as well as the gas fraction for central galaxies. At low redshift, all these residuals are found to be anticorrelated with the residuals of the MZR for M<SUB>⋆</SUB>/M<SUB>⊙</SUB> ≲ 10<SUP>10</SUP>. The correlations between the residuals of the MZR and the residuals of the other relations with mass are interrelated, but we find that gas fraction, specific inflow rate, and specific outflow rate all have at least some independent influence on the scatter of the MZR. We find that, while for M<SUB>⋆</SUB>/M<SUB>⊙</SUB> &gt; 10<SUP>10.4</SUP> the specific mass of the nuclear black hole is most important, for M<SUB>⋆</SUB>/M<SUB>⊙</SUB> ≲ 10<SUP>10.3</SUP> gas fraction and specific inflow rate are the variables that correlate most strongly with the MZR scatter. The time-scales involved in the residual correlations and the time that galaxies stay above the MZR are revealed to be a few Gyr. However, most galaxies that are below the MZR at z = 0 have been below the MZR throughout their lifetimes.

Motivation & Objective

• To understand the physical origin of the scatter in the galaxy mass-metallicity relation (MZR), which reflects the interplay of key galaxy evolution processes.
• To investigate how residual correlations between MZR and variables like specific inflow, outflow, star formation rate, and gas fraction influence the observed scatter.
• To determine the relative importance of inflow, outflow, and star formation processes in shaping metallicity deviations from the MZR.
• To assess the role of black hole feedback in high-mass galaxies and the timescales over which galaxies remain above or below the MZR.

Proposed method

• The study uses the EAGLE cosmological hydrodynamical simulation (Ref-L100N1504) to analyze central galaxies at z = 0 with stellar masses M⋆/M⊙ ≲ 10¹¹.
• Residuals of the MZR are computed by comparing observed metallicity to the median MZR at fixed stellar mass.
• Residuals of other relations (e.g., stellar mass vs. specific inflow rate, outflow rate, sSFR, gas fraction) are calculated and cross-correlated with MZR residuals.
• Partial correlation analysis is applied to isolate independent contributions of each variable to the MZR scatter.
• Galaxy tracks are traced backward in time to determine how long galaxies have remained above or below the MZR.
• Timescale analysis is performed by averaging fluxes over longer intervals to distinguish short- and long-term fluctuations.

Experimental results

Research questions

• RQ1 What drives the scatter in the galaxy mass-metallicity relation (MZR) at low to intermediate stellar masses?
• RQ2 How do gas inflows, outflows, and star formation rates correlate with MZR residuals, and what is their relative importance?
• RQ3 To what extent are the correlations between MZR residuals and inflow/outflow/sSFR/gas fraction independent or coupled?
• RQ4 What are the characteristic timescales over which MZR deviations persist, and how do they relate to inflow/outflow fluctuations?
• RQ5 How does the role of black hole feedback in shaping MZR scatter vary with stellar mass?

Key findings

• At M⋆/M⊙ ≲ 10¹⁰, the gas fraction and specific inflow rate show the strongest anti-correlation with MZR residuals, indicating they are primary drivers of scatter.
• The gas fraction reduces the residual MZR scatter the most (from ≈0.13 dex to ≈0.07 dex), followed by specific inflow rate, while outflow and sSFR have smaller but non-negligible independent contributions.
• For M⋆/M⊙ > 10¹⁰.⁴, specific black hole mass becomes the dominant factor, with anti-correlation between residual black hole mass and MZR residuals.
• Galaxies below the MZR at z = 0 have typically remained below it for ≈12 Gyr, while those above have stayed above for ≳6 Gyr, indicating long-term deviations.
• Longer timescale averaging strengthens the anti-correlation between MZR residuals and specific inflow/outflow rates, indicating that fluctuations over several Gyr drive the scatter.
• The residual correlation between MZR and sSFR is not driven by short-term fluctuations, as it remains unchanged when averaged over longer timescales.