Assessing the connection between galactic conformity and assembly-type bias

¹ Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile
² CONICET, Instituto de Astronomía Teórica y Experimental (IATE), Laprida 854, Córdoba X5000BGR, Argentina
³ Universidad Nacional de Córdoba (UNC), Observatorio Astronómico de Córdoba (OAC), Laprida 854, Córdoba X5000BGR, Argentina

^⋆ Corresponding author: ivan.lacerna@uda.cl

Received: 28 April 2025
Accepted: 7 September 2025

Abstract

Context. Galaxies across the Universe display a conformity in the fraction of quenched galaxies out to large distances, which is much greater around quenched central galaxies than it is for star-forming ones. On the other hand, simulations have shown that the clustering of halos and the galaxies within them depends on secondary properties beyond the halo mass. This phenomenon is referred to as the assembly bias.

Aims. Our aim is to study whether samples that show galactic conformity also display an assembly bias. We also aim to see whether the amplitudes of these two effects are correlated.

Methods. We used synthetic galaxies at z = 0 from the semi-analytical model SAG run on the MultiDark Planck 2 (MDPL2) cosmological simulation. We measured both the conformity and galaxy assembly bias for different samples of central galaxies at a fixed host halo mass. We focused on central galaxies hosted by low-mass halos of 10^11.6 ≤ M_h/h⁻¹ M_⊙ < 10^11.8 because this is a mass range where a strong assembly bias has been reported. The samples of central galaxies were separated according to their specific star formation rates and stellar ages.

Results. We find that the level of conformity exhibited by our different samples is correlated with their measured level of assembly bias. We also find that removing galaxies around massive halos diminishes the conformity signal and lowers the amount of assembly bias.

Context. The high correlation in the amplitude of conformity and assembly bias for different samples, both with and without removing galaxies near massive halos, clearly indicates the strong relationship between both phenomena.