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

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisbon, Portugal
e-mail: fbuitrago@oal.ul.pt
² Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8, Campo Grande, 1749-016 Lisbon, Portugal
³ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, RH5 6NT UK
e-mail: i.ferreras@ucl.ac.uk
⁴ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF UK
⁵ ICRAR, School of Physics, University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009 Australia
⁶ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ UK
⁷ Australian Astronomical Observatory, 105 Delhi Rd, North Ryde, NSW, 2113 Australia
⁸ School of Physics, University of New South Wales, NSW, 2052 Australia
⁹ Centre for Astronomy and Particle Theory, School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD UK
¹⁰ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹¹ Department of Physics and Astronomy, 102 Natural Science Building, University of Louisville, Louisville, KY, 40292 USA
¹² H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL UK
¹³ Department of Physics and Astronomy, Macquarie University, NSW, 2109 Australia
¹⁴ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Victoria, 3122 Australia

Received: 6 July 2018
Accepted: 3 September 2018

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 < z < 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 (> 8 × 10¹⁰ M_⊙) and compact size (R_e < 2 kpc). An additional set of seven lower-mass objects (6 × 10¹⁰ < M_⋆/M_⊙ < 8 × 10¹⁰) are also potential candidates according to typical mass uncertainties.

Results. The comoving number density of MUGs at low redshift (z < 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 < n < 2.5) with high stellar surface densities (⟨Σ_e⟩∼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.