Spherical collapse in quintessence models with zero speed of sound

TL;DR

This paper investigates how quintessence with zero sound speed influences spherical collapse and structure formation, revealing its significant role in modifying the mass function and halo composition at low redshift.

Contribution

It provides an exact analysis of spherical collapse with zero sound speed quintessence and quantifies its impact on the dark matter and total mass functions.

Findings

Quintessence follows dark matter during collapse due to negligible pressure gradients.

The total halo mass includes a quintessence component proportional to (1+w) Omega_Q / Omega_m.

The effect on the mass function is most significant at low redshift.

Abstract

We study the spherical collapse model in the presence of quintessence with negligible speed of sound. This case is particularly motivated for w<-1 as it is required by stability. As pressure gradients are negligible, quintessence follows dark matter during the collapse. The spherical overdensity behaves as a separate closed FLRW universe, so that its evolution can be studied exactly. We derive the critical overdensity for collapse and we use the extended Press-Schechter theory to study how the clustering of quintessence affects the dark matter mass function. The effect is dominated by the modification of the linear dark matter growth function. A larger effect occurs on the total mass function, which includes the quintessence overdensities. Indeed, here quintessence constitutes a third component of virialized objects, together with baryons and dark matter, and contributes to the total halo mass by a fraction ~ (1+w) Omega_Q / Omega_m. This gives a distinctive modification of the total mass function at low redshift.