# The fundamental plane of evolving red nuggets

## TL;DR

This study investigates the mass structure of compact early-type galaxies at redshift 0.6, revealing their dark and stellar mass distributions, and providing evidence for inside-out growth through minor mergers.

## Contribution

It introduces the fundamental plane analysis of high-redshift compact galaxies, highlighting their non-homologous structure and the impact of IMF assumptions on dark halo inferences.

## Key findings

- The fundamental plane is tilted towards the virial plane due to compactness.
- Intrinsic scatter of the FP is similar to local galaxies.
- Dark halo profiles suggest inside-out growth via minor mergers.

## Abstract

We present an exploration of the mass structure of a sample of 12 strongly lensed massive, compact early-type galaxies at redshifts $z\sim0.6$ to provide further possible evidence for their inside-out growth. We obtain new ESI/Keck spectroscopy and infer the kinematics of both lens and source galaxies, and combine these with existing photometry to construct (a) the fundamental plane (FP) of the source galaxies and (b) physical models for their dark and luminous mass structure. We find their FP to be tilted towards the virial plane relative to the local FP, and attribute this to their unusual compactness, which causes their kinematics to be totally dominated by the stellar mass as opposed to their dark matter; that their FP is nevertheless still inconsistent with the virial plane implies that both the stellar and dark structure of early-type galaxies is non-homologous. We also find the intrinsic scatter of their FP to be comparable to the local value, indicating that variations in the stellar mass structure outweight variations in the dark halo in the central regions of early-type galaxies. Finally, we show that inference on the dark halo structure -- and, in turn, the underlying physics -- is sensitive to assumptions about the stellar initial mass function (IMF), but that physically-motivated assumptions about the IMF imply haloes with sub-NFW inner density slopes, and may present further evidence for the inside-out growth of compact early-type galaxies via minor mergers and accretion.