On the Physical Origin of Galactic Conformity

TL;DR

This paper explores the physical origin of galactic conformity, linking it to large-scale tidal environments affecting halo mass accretion rates, and predicts its dependence on scale, mass, and redshift, with implications for galaxy formation models.

Contribution

It proposes that large-scale tidal environments influence halo accretion rates, explaining galactic conformity and predicting its evolution with redshift and galaxy mass.

Findings

Halo pairs at large distances share similar assembly histories.

Galactic conformity depends on stellar mass and distance, matching observations.

The conformity signal diminishes at redshift z > 1.

Abstract

Correlations between the star formation rates (SFRs) of nearby galaxies (so-called galactic conformity) have been observed for projected separations up to 4 Mpc, an effect not predicted by current semi-analytic models. We investigate correlations between the mass accretion rates (dMvir/dt) of nearby halos as a potential physical origin for this effect. We find that pairs of host halos "know about" each others' assembly histories even when their present-day separation is greater than thirty times the virial radius of either halo. These distances are far too large for direct interaction between the halos to explain the correlation in their dMvir/dt. Instead, halo pairs at these distances reside in the same large-scale tidal environment, which regulates dMvir/dt for both halos. Larger halos are less affected by external forces, which naturally gives rise to a mass dependence of the halo conformity signal. SDSS measurements of galactic conformity exhibit a qualitatively similar dependence on stellar mass, including how the signal varies with distance. Based on the expectation that halo accretion and galaxy SFR are correlated, we predict the scale-, mass- and redshift-dependence of large-scale galactic conformity, finding that the signal should drop to undetectable levels by z > 1. These predictions are testable with current surveys to z ~ 1; confirmation would establish a strong correlation between dark matter halo accretion rate and central galaxy SFR.