Letter to the Editor

Nature versus nurture: relic nature and environment of the most massive passive galaxies at z < 0.5

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
e-mail: crescenzo.tortora@inaf.it
² School of Physics and Astronomy, Sun Yat-sen University Zhuhai Campus, Daxue Road 2, 519082 Tangjia, Zhuhai, Guangdong, PR China
³ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁴ INAF – Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
⁵ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁶ Dipartimento di Fisica e Astronomia – Alma Mater Studiorum Università di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy
⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁸ INFN – Sezione di Bologna, Viale Berti-Pichat 6/2, 40127 Bologna, Italy
⁹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁰ Dipartimento di Scienze Fisiche, Università di Napoli Federico II, Compl. Univ. Monte S. Angelo, 80126 Napoli, Italy
¹¹ Instituto de Astrofísica Pontificia Universidad Católica de Chile, Avenida Vicuña Mackenna, 4860 Santiago, Chile
¹² Zentrum für Astronomie, Universitatät Heidelberg, Philosophenweg 12, 69120 Heidelberg, Germany
¹³ Institut für Theoretische Physik, Ruprecht-Karls-Universität Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany

Received: 7 May 2020
Accepted: 4 June 2020

Abstract

Relic galaxies are thought to be the progenitors of high-redshift red nuggets that for some reason missed the channels of size growth and evolved passively and undisturbed since the first star formation burst (at z >  2). These local ultracompact old galaxies are unique laboratories for studying the star formation processes at high redshift and thus the early stage of galaxy formation scenarios. Counterintuitively, theoretical and observational studies indicate that relics are more common in denser environments, where merging events predominate. To verify this scenario, we compared the number counts of a sample of ultracompact massive galaxies (UCMGS) selected within the third data release of the Kilo Degree Survey, that is, systems with sizes R_e <  1.5 kpc and stellar masses M_⋆ >  8 × 10¹⁰ M_⊙, with the number counts of galaxies with the same masses but normal sizes in field and cluster environments. Based on their optical and near-infrared colors, these UCMGS are likely to be mainly old, and hence representative of the relic population. We find that both UCMGS and normal-size galaxies are more abundant in clusters and their relative fraction depends only mildly on the global environment, with denser environments penalizing the survival of relics. Hence, UCMGS (and likely relics overall) are not special because of the environment effect on their nurture, but rather they are just a product of the stochasticity of the merging processes regardless of the global environment in which they live.