Ultramassive dense early-type galaxies: Velocity dispersions and number density evolution sincez= 1.6

Abstract

Aims. We investigate the stellar mass assembly history of ultramassive (M 10 11 M ) dense (Σ = M /2πR 2 e > 2500 M pc −2 ) early-type galaxies (ETGs, elliptical and spheroidal galaxies) selected on basis of visual classification over the last 9 Gyr. Methods. We traced the evolution of the comoving number density ρ of ultramassive dense ETGs and compared their structural (effective radius R e and stellar mass M ) and dynamical (velocity dispersion σ e ) parameters over the redshift range 0 < z < 1.6. We derived the number density ρ at 1.6 < z < 1 from the MUNICS and GOODS-South surveys, while we took advantage of the COSMOS spectroscopic survey to probe the intermediate redshift range [0.2−1.0]. We derived the number density of ultramassive dense local ETGs from the SDSS sample taking all of the selection bias affecting the spectroscopic sample into account. To compare the dynamical and structural parameters, we collected a sample of 11 ultramassive dense ETGs at 1.2 < z < 1.6 for which velocity dispersion measurements are available. For four of these ETGs (plus one at z = 1.91), we present previously unpublished estimates of velocity dispersion, based on optical VLT-FORS2 spectra. We probe the intermediate redshift range (0.2 < ∼ z < ∼ 0.9) and the local Universe with different ETGs samples. Results. We find that the comoving number density of ultramassive dense ETGs evolves with z as ρ(z) ∝ (1 + z) 0.3 ± 0.8 implying a decrease of ∼25% of the population of ultramassive dense ETGs since z = 1.6. By comparing the structural and dynamical properties of high-z ultramassive dense ETGs over the range 0 < ∼ z < 1.6 in the [R e , M , σ e ] plane, we find that all of the ETGs of the high-z sample have counterparts with similar properties in the local Universe. This implies either that the majority (∼70%) of ultramassive dense ETGs already completed the assembly and shaping at z = 1.4, or that, if a significant portion of dense ETGs evolves in size, new ultramassive dense ETGs must form at z < 1.5 to maintain their number density at almost constant. The difficulty in identify good progenitors for these new dense ETGs at z < ∼ 1.5 and the stellar populations properties of local ultramassive dense ETGs point towards the first hypothesis. In this case, the ultramassive dense galaxies missing in the local Universe could have joined, in the last 9 Gyr, the so colled non-dense ETGs population through minor mergers, thus contributing to mean size growth. In any case, the comparison between their number density and the number density of the whole population of ultramassive ETGs relegates their contribution to the mean size evolution to a secondary process.

The VIMOS Public Extragalactic Redshift Survey (VIPERS)

“…Damjanov et al (2015) found an almost constant number density over the redshift range 0.2< z <0.8 for colour selected passive compact galaxies with M > 8×10 10 M ⊙ . Gargiulo et al (2016) found ρ(z) ∝ (1 + z) 0.3±0.8 for a sample of morphologically selected elliptical dense galaxies (Σ 2500 M ⊙ pc −2 ) with M >10 11 M ⊙ in the redshift range 0 < z < 1.6, consistent with our results. Cassata et al (2013) found a very mild decrease of compact galaxies (1σ below the local SMR), and a mild increase in the number density of normal galaxies (consistent at 1σ with the local SMR) from z ∼ 1 to z ∼ 0.5, in qualitative agreement with our results.…”

“…paper_I mass relation [SMR]), and in the rest-frame waveband used to derive the effective radius, make it difficult to compare results from different studies (or at different cosmic epochs, see e.g. Gargiulo et al 2016). Evidence shows that in conjunction with an increase in the mean radial size of PGs by a factor of ∼ 4 from z ∼ 1.5-2 to z = 0, the number of PGs per unit volume increases by a factor ∼ 10 (e.g.…”

COMPACT E+A GALAXIES AS A PROGENITOR OF MASSIVE COMPACT QUIESCENT GALAXIES AT 0.2 < z < 0.8

“…One explanation for this reported precipitous decline in the number density is that quiescent compact galaxies somehow grow at intermediate redshifts. However, several studies examining dense regions in the local universe (Valentinuzzi et al 2010;Poggianti et al 2013) and quiescent galaxies at intermediate redshifts (Carollo et al 2013;Damjanov et al 2014Damjanov et al , 2015Gargiulo et al 2016) challenge these initial measurements of a rapid decline in the abundance of compact galaxy population at late times. These studies find that the number density of compact galaxies at z < 1 is nearly constant.…”

Cluster and field elliptical galaxies atz~ 1.3

“…However, this is not the case. Indeed, the number density of ultramassive dense elliptical galaxies in the field seems to be constant over the redshift range 0.2 < z < 1.5 (Gargiulo et al 2016). This suggests that both the populations of high-mass elliptical galaxies, in cluster and in the field, are mostly formed at that redshift.…”

“…Actually, the definition of Σ R e here and commonly adopted does not take into account the presence of color gradients observed in most of the early-type galaxies at high redshift (e.g., Gargiulo et al 2011Gargiulo et al , 2012Guo et al 2011;Chan et al 2016, Ciocca et al 2016. Color gradients imply that the M/L is not radially constant and that, consequently, the stellar mass profile does not follow exactly the surface brightness profile.…”