Superdense Galaxies and the Mass-Size Relation at Low Redshift

Abstract

We search for massive and compact galaxies (superdense galaxies, hereafter SDGs) at z = 0.03 − 0.11 in the Padova-Millennium Galaxy and Group Catalogue, a spectroscopically complete sample representative of the local Universe general field population. We find that compact galaxies with radii and mass densities comparable to high-z massive and passive galaxies represent 4.4% of all galaxies with stellar masses above 3 × 10 10 M ⊙ , yielding a number density of 4.3 × 10 −4 h 3 Mpc −3 . Most of them are S0s (70%) or ellipticals (23%), are red and have intermediate-to-old stellar populations, with a median luminosity-weighted age of 5.4 Gyr and a median mass-weighted age of 9.2 Gyr. Their velocity dispersions and dynamical masses are consistent with the small radii and high stellar mass estimates. Comparing with the WINGS sample of cluster galaxies at similar redshifts, the fraction of superdense galaxies is three times smaller in the field than in clusters, and cluster SDGs are on average 4 Gyr older than field SDGs. We confirm the existence of a universal trend of smaller radii for older luminosity-weighted ages at fixed galaxy mass. As a consequence, the median mass-size relation shifts towards smaller radii for galaxies with older stars, but the effect is much more pronounced in clusters than in the field. Our results show that, on top of the well known dependence of stellar age on galaxy mass, the luminosity-weighted age of galaxies depends on galaxy compactness at fixed mass, and, for a fixed mass and radius, on environment. This effect needs to be taken into account in order not to overestimate the evolution of galaxy sizes from high-to low-z. Our results and hierarchical simulations suggest that a significant fraction of the massive compact galaxies at high-z have evolved into compact galaxies in galaxy clusters today. When stellar age and environmental effects are taken into account, the average amount of size evolution of individual galaxies between high-and low-z is mild, a factor ∼ 1.6.

Towards a census of supercompact massive galaxies in the Kilo Degree Survey

“…1, these findings seem to trouble the current hierarchical paradigm of galaxy formation, where some relic systems at z ∼ 0 are actually expected to be found. In contrast, Poggianti et al (2013a) have found 4 galaxies fulfilling our same criteria (corresponding to 1.4% of their sample galaxies with masses larger than 8 × 10 10 M⊙), and all of these galaxies are old, with massweighted ages older than 8 Gyr. These numbers translate into a very large abundance of ∼ 10 −5 Mpc −3 .…”

“…Although observational studies at intermediate-and high-z do not point to a clear picture of how environment affects galaxy sizes (see Damjanov et al 2015b and references therein), recent cosmological simulations predict a larger fraction of massive compact systems in high-than in low-density regions (Stringer et al 2015). This prediction is supported by the fact that NGC 1277 -the only well-characterized compact, massive, ETG at z ∼ 0 -resides in the core region of a nearby rich cluster of galaxies (Trujillo et al 2014; see also Valentinuzzi et al 2010;Poggianti et al 2013a). Thus, the absence of compact galaxies at z < 0.2 could be related to the smaller fraction of dense structures in the area currently mapped by KiDS.…”

“…However, we cannot exclude that galaxy environment, which might be the actual driver of the number density of compact relics at z ∼ 0 (see, e.g. Trujillo et al 2014;Poggianti et al 2013a;Valentinuzzi et al 2010), may be different for galaxies in the KiDS-DR2 and COSMOS areas -an issue that will be addressed in forthcoming extensions of the present work.…”

“…However, a fraction of these objects might survive intact till the present epoch, resulting in compact, relic systems in the nearby Universe characterized by old stellar populations. Recently there have been some efforts to search for massive compact galaxies at low redshifts (Trujillo et al 2009;Taylor et al 2010;Valentinuzzi et al 2010;Shih & Stockton 2011;Trujillo et al 2012;Poggianti et al 2013a;Poggianti et al 2013b;Damjanov et al 2013Damjanov et al , 2014Damjanov et al , 2015aSaulder et al 2015) and investigate further their dynamical and stellar population properties, as well as the role of environment on their properties (e.g. Ferré-Mateu et al 2012;Läsker et al 2013;Ferré-Mateu et al 2015;Trujillo et al 2014;Yıldırım et al 2015;Damjanov et al 2015b;Stringer et al 2015;Wellons et al 2015a).…”

Newly Quenched Galaxies as the Cause for the Apparent Evolution in Average Size of the Population

“…The evolution of the median size (i.e., the half-light radius r 1/2 ) of the population of massive (M Galaxy > 10 10 M ) quenched early-type galaxies (Q-ETGs) at given stellar mass has been widely highlighted in recent years (e.g., Daddi et al 2005;Trujillo et al 2007;McGrath et al 2008;van Dokkum et al 2008;Cassata et al 2011;Szomoru et al 2011;Barro et al 2013;Dullo & Graham 2013;Newman et al 2012;Poggianti et al 2013;Shankar et al 2013, just to cite a few). The size of the effect is quite large, with a decrease in median r 1/2 with increasing redshift ∝ (1 + z) −1 ; in coarse terms, this implies that, at a given stellar mass, the median half-light radius of Q-ETGs is about a factor of ∼2-3 smaller at z ∼ 2 than locally, corresponding to an increase of over an order of magnitude in the median mean stellar density within the half-light radius of galaxies.…”