NIHAO XXVI: Nature versus nurture, the Star Formation Main Sequence and the origin of its scatter

TL;DR

This study uses NIHAO simulations to analyze the star formation main sequence, revealing the impact of AGN feedback on its slope and linking galaxy star formation activity to dark matter halo formation history.

Contribution

It demonstrates that including AGN feedback aligns simulated SFMS slopes with observations and links galaxy SFR deviations to halo formation times.

Findings

Inclusion of AGN feedback reproduces observed SFMS slope.

Galaxy SFR deviations correlate with halo concentration.

Galaxy formation history influences position on the SFMS.

Abstract

We investigate how the NIHAO galaxies match the observed star formation main sequence (SFMS) and what the origin of its scatter is. The NIHAO galaxies reproduce the SFMS and generally agree with observations, but the slope is about unity and thus significantly larger than observed values. This is because observed galaxies at large stellar masses, although still being part of the SFMS, are already influenced by quenching. This partial suppression of star formation by AGN feedback leads to lower star formation rates and therefore to lower observed slopes. We confirm that including the effects of AGN in our galaxies leads to slopes in agreement with observations. We find the deviation of a galaxy from the SFMS is correlated with its $z=0$ dark matter halo concentration and thus with its halo formation time. This means galaxies with a higher-than-average star formation rate (SFR) form later and vice versa. We explain this apparent correlation with the SFR by re-interpreting galaxies that lie above the SFMS (higher-than-average SFR) as lying to the left of the SFMS (lower-than-average stellar mass) and vice versa. Thus later forming haloes have a lower-than-average stellar mass, this is simply because they have had less-than-average time to form stars, and vice versa. It is thus the nature, i.e. how and when these galaxies form, that sets the path of a galaxy in the SFR versus stellar mass plane.