# Galaxy Groups within 3500 km s$^{-1}$

## TL;DR

This paper analyzes galaxy groups within 3500 km/s to empirically constrain the halo mass function, revealing a notable deviation from theoretical predictions at around 10^12 solar masses.

## Contribution

It provides observational constraints on the halo mass function using detailed galaxy group properties, highlighting a significant deviation from models at low masses.

## Key findings

- Halo counts drop by a factor of ~3 below Sheth-Tormen predictions at 10^12 M_sun.
- Detailed scaling relations enable group definitions over a wide mass range.
- Constructed the halo mass function from local galaxy group data.

## Abstract

A study of the group properties of galaxies in our immediate neighborhood provides a singular opportunity to observationally constrain the halo mass function, a fundamental characterization of galaxy formation. Detailed studies of individual groups have provided the coefficients of scaling relations between a proxy for the virial radius, velocity dispersion, and mass that usefully allows groups to be defined over the range $10^{10} - 10^{15}$ $M_\odot$. At a second hierarchical level, associations are defined as regions around collapsed halos extending to the zero velocity surface at the decoupling from cosmic expansion. The most remarkable result of the study emerges from the construction of the halo mass function from the sample. At $\sim10^{12}$ $M_\odot$ there is a jog from the expectation Sheth-Tormen function, such that halo counts drop by a factor $\sim 3$ in all lower mass bins.