# A finer view of the conditional galaxy luminosity function and magnitude-gap statistics

## TL;DR

This study refines the understanding of galaxy luminosity functions and magnitude-gap statistics in galaxy groups, revealing complex dependencies on group properties and challenging simple models.

## Contribution

It introduces a two-component CLF model that better explains observed gap statistics and highlights limitations of using magnitude gaps for cosmological studies.

## Key findings

- Best-fit CLFs have steeper satellite high ends at low masses.
- Observed gap statistics are incompatible with single Schechter functions.
- Higher gap trends diminish when considering group mass instead of velocity dispersion.

## Abstract

The gap between first and second ranked galaxy magnitudes in groups is often considered a tracer of their merger histories, which in turn may affect galaxy properties, and also serves to test galaxy luminosity functions (LFs). We remeasure the conditional luminosity function (CLF) of the Main Galaxy Sample of the SDSS in an appropriately cleaned subsample of groups from the Yang catalog. We find that, at low group masses, our best-fit CLF have steeper satellite high ends, yet higher ratios of characteristic satellite to central luminosities in comparison with the CLF of Yang et al. (2008). The observed fractions of groups with large and small magnitude gaps as well as the Tremaine & Richstone (1977) statistics, are not compatible with either a single Schechter LF or with a Schechter-like satellite plus lognormal central LF. These gap statistics, which naturally depend on the size of the subsamples, and also on the maximum projected radius, $R_{\rm max}$, for defining the 2nd brightest galaxy, can only be reproduced with two-component CLFs if we allow small gap groups to preferentially have two central galaxies, as expected when groups merge. Finally, we find that the trend of higher gap for higher group velocity dispersion, $\sigma_{\rm v}$, at given richness, discovered by Hearin et al. (2013), is strongly reduced when we consider $\sigma_{\rm v}$ in bins of richness, and virtually disappears when we use group mass instead of $\sigma_{\rm v}$. This limits the applicability of gaps in refining cosmographic studies based on cluster counts.