On Sizes, Kinematics, M/L Gradients, and Light Profiles of Massive Compact Galaxies at z~2

TL;DR

This study uses simulations to explore the formation and properties of massive compact galaxies at redshift ~2, showing major mergers can produce such systems with realistic kinematics and gradients, but with differences in light profile shapes.

Contribution

It demonstrates that high gas fraction mergers of compact progenitors can produce realistic compact quiescent galaxies, highlighting the importance of progenitor properties and merger conditions.

Findings

Major mergers can produce ~10^11 Msun, 1 kpc galaxies with observed kinematics.

Simulated remnants show negative color gradients and M/L ratio gradients.

Simulated surface brightness profiles are more cuspy than observed galaxies.

Abstract

We present a detailed analysis of the structure and resolved stellar populations of simulated merger remnants, and compare them to observations of compact quiescent galaxies at z ~ 2. We find that major merging is a viable mechanism to produce systems of ~ 10^11 Msun and ~ 1 kpc size, provided the gas fraction at the time of final coalescence is high (~ 40%), and provided that the progenitors are compact star-forming galaxies, as expected at high redshift. Their integrated spectral energy distributions and velocity dispersions are in good agreement with the observations, and their position in the (v_{maj}/sigma, ellipticity) diagram traces the upper envelope of the distribution of lower redshift early-type galaxies. The simulated merger remnants show time- and sightline-dependent M/L ratio gradients that result from a superposition of radially dependent stellar age, stellar metallicity, and extinction. The median ratio of effective radius in rest-frame V-band light to that in mass surface density is ~ 2 during the quiescent remnant phase. This is typically expressed by a negative color gradient (i.e., red core), which we expect to correlate with the integrated color of the system. Finally, the simulations differ from the observations in their surface brightness profile shape. The simulated remnants are typically best fit by high (n >> 4) Sersic indices, whereas observed quiescent galaxies at z ~ 2 tend to be less cuspy (median n ~ 2.3). Limiting early star formation in the progenitors may be required to prevent the simulated merger remnants from having extended wings.