Galaxy and Mass Assembly (GAMA): Accurate number densities and environments of massive ultra-compact galaxies at 0.02 < z < 0.3

Abstract

Context. Massive ultra-compact galaxies (MUGs) are common at z = 2−3, but very rare in the nearby Universe. Simulations predict that the few surviving MUGs should reside in galaxy clusters, whose large relative velocities prevent them from merging, thus maintaining their original properties (namely stellar populations, masses, sizes and dynamical state). Aims. Our goal is to obtain a complete census of the MUG population at 0.02 &lt; z &lt; 0.3, determining the number density, population properties and environment. Methods. We have taken advantage of the high-completeness, large-area spectroscopic GAMA survey, complementing it with deeper imaging from the KiDS and VIKING surveys. We find a set of 22 bona-fide MUGs, defined as having high stellar mass (&gt; 8 × 1010 M⊙) and compact size (Re &lt; 2 kpc). An additional set of seven lower-mass objects (6 × 1010 &lt; M⋆/M⊙ &lt; 8 × 1010) are also potential candidates according to typical mass uncertainties. Results. The comoving number density of MUGs at low redshift (z &lt; 0.3) is constrained at (1.0 ± 0.4)×10−6 Mpc−3, consistent with galaxy evolution models. However, we find a mixed distribution of old and young galaxies, with a quarter of the sample representing (old) relics. MUGs have a predominantly early or swollen disk morphology (Sérsic index 1 &lt; n &lt; 2.5) with high stellar surface densities (⟨Σe⟩∼1010 M⊙ Kpc−2). Interestingly, a large fraction feature close companions – at least in projection – suggesting that many (but not all) reside in the central regions of groups. Halo masses show these galaxies inhabit average-mass groups. Conclusions. As MUGs are found to be almost equally distributed among environments of different masses, their relative fraction is higher in more massive overdensities, matching the expectations that some of these galaxies fell in these regions at early times. However, there must be another channel leading some of these galaxies to an abnormally low merger history because our sample shows a number of objects that do not inhabit particularly dense environments.

A Duality in the Origin of Bulges and Spheroidal Galaxies

“…Considering the rarity of high-z red nuggets surviving in the local Universe (Trujillo et al 2009;Tortora et al 2016;Charbonnier et al 2017;Buitrago et al 2018), our results have important implication for the evolution of this population. The population of red nuggets, formed at high redshift through rapid compaction (Damjanov et al 2009;Dekel et al 2009), could possibly evolve in today's early type galaxies or settle in the center of a disk galaxy.…”

The stellar halos of ETGs in the IllustrisTNG simulations

“…A fraction of these compact red early galaxies formed at high z may survive intact to present times as relic galaxies (Buitrago et al 2018;Scognamiglio et al 2020), but these are extremely rare objects and, to date, only a handful have been spectroscopically confirmed (Trujillo et al 2014;Ferré-Mateu et al 2017;Spiniello et al 2021). However, the compact red progenitor galaxies are also expected to be found in the cores of present-day massive ETGs (e.g., Hopkins et al 2009;Bezanson et al 2009).…”

Structure and morphology of relic galaxies in the Local Universe

“…In addition, a two-sample Kolmogorov-Smirnov test at significance level of 0.05 shows that the Sérsic indices of the control and candidate samples probably come from different distributions, suggesting that the sample of relic candidates could be a population of compact objects in the Local Universe with structure that could resemble confirmed relic properties, such as disk-like morphology if n 2.5 Buitrago et al (2018).…”