# The average structural evolution of massive galaxies can be reliably estimated using cumulative galaxy number densities

## TL;DR

This study demonstrates that cumulative galaxy number densities can reliably estimate the average structural evolution of massive galaxies over a broad redshift range, aligning well with observational data and simulations.

## Contribution

It shows that the cumulative number density method accurately reproduces the average stellar density profile evolution of massive galaxies in simulations and observations.

## Key findings

- The method reproduces the median stellar mass evolution from abundance matching.
- Discrepancies at high redshift are mitigated by including scatter in number densities.
- The evolution of density profiles agrees with observational data, supporting inside-out growth models.

## Abstract

Galaxy evolution can be studied observationally by linking progenitor and descendant galaxies through an evolving cumulative number density selection. This procedure can reproduce the expected evolution of the median stellar mass from abundance matching. However, models predict an increasing scatter in main progenitor masses at higher redshifts, which makes galaxy selection at the median mass unrepresentative. Consequently, there is no guarantee that the evolution of other galaxy properties deduced from this selection are reliable. Despite this concern, we show that this procedure approximately reproduces the evolution of the average stellar density profile of main progenitors of M = 10^11.5 Msun galaxies, when applied to the EAGLE hydrodynamical simulation. At z > 3.5 the aperture masses disagree by about a factor two, but this discrepancy disappears when we include the expected scatter in cumulative number densities. The evolution of the average density profile in EAGLE broadly agrees with observations from UltraVISTA and CANDELS, suggesting an inside-out growth history for these massive galaxies over 0 < z < 5. However, for z < 2 the inside-out growth trend is stronger in EAGLE. We conclude that cumulative number density matching gives reasonably accurate results when applied to the evolution of the mean density profile of massive galaxies.