Expected number of massive galaxy relics in the present-day Universe

TL;DR

This study estimates the current abundance of untouched massive galaxy relics from high redshift using semi-analytical models, highlighting significant uncertainties in observational data and theoretical predictions.

Contribution

It provides quantitative predictions for the number densities of massive galaxy relics using three semi-analytical models based on the Millennium simulation.

Findings

Less than 2% of high-z massive galaxies remain nearly unchanged since formation.

Predicted relic number densities are approximately 1.2x10^-6 and 5.7x10^-6 Mpc^-3 for different mass retention thresholds.

Current observational data on relic candidates are highly uncertain, complicating model validation.

Abstract

The number of present-day massive galaxies that has survived untouched since their formation at high-z is an important observational constraint to the hierarchical galaxy formation models. Using three different semianalytical models based on the Millenium simulation, we quantify the expected fraction and number densities of the massive galaxies form at z>2 which have evolved in stellar mass less than 10% and 30%. We find that only a small fraction of the massive galaxies already form at z~2 have remained almost unaltered since their formation (<2% with Delta_M*/M*<0.1 and <8% with Delta_M*/M*<0.3). These fractions correspond to the following number densities of massive relics in the present-day Universe: ~1.2x10^-6 Mpc^-3 for Delta_M*/M*<0.1 and ~5.7x10^-6 Mpc^-3 for Delta_M*/M*<0.3. The observed number of relic candidates found in the nearby Universe is today pretty uncertain (with uncertainties up to a factor of ~100) preventing to establish a firm conclusion about the goodness of current theoretical expectations to predict such important number.