Lower mass normalization of the stellar initial mass function for dense massive early-type galaxies atz~ 1.4

Abstract

Aims. This paper aims at understanding whether the normalization of the stellar initial mass function (IMF) of massive galaxies varies with cosmic time and/or with mean stellar mass density Σ = M /(2πR 2 e ). Methods. We have tackled this question by taking advantage of a spectroscopic sample of 18 dense (Σ > 2500 M pc −2 ) massive early-type galaxies (ETGs) that we collected at 1.2 < ∼ z < ∼ 1.6. Each galaxy in the sample was selected in order to have available: i) a high-resolution deep HST-F160W image to visually classify it as an ETG; ii) an accurate velocity dispersion estimate; iii) stellar mass derived through the fit of multiband photometry; and iv) structural parameters (i.e. effective radius R e and Sersic index n) derived in the F160W-band. We have constrained the mass-normalization of the IMF of dense high-z ETGs by comparing the true stellar masses of the ETGs in the sample (M true ) derived through virial theorem, hence IMF independent, with those inferred through the fit of the photometry which assume a reference IMF (M ref ). Adopting the virial estimator as proxy of the true stellar mass, we have implicitly assumed that these systems have zero dark matter. However, recent dynamical analysis of massive local ETGs have shown that the dark matter fraction within R e in dense ETGs is negligible (<5−10%) and simulations of dissipationless mergers of spheroidal galaxies have shown that this fraction decreases going back with time. Accurate dynamical models of local ETGs performed by the ATLAS 3D team have shown that the virial estimator is prone to underestimating or overestimating the total masses. We have considered this, and based on the results of ATLAS 3D we have shown that for dense ETGs the mean value of total masses derived through the virial estimator with a non-homologous virial coefficient and Sersic-R e are perfectly in agreement with the mean value of those derived through more sophisticated dynamical models, although, of course, the estimates show higher uncertainties. Results. Tracing the variation of the parameter Γ = M true /M ref with velocity dispersion σ e , we have found that, on average, dense ETGs at z = 1.4 follow the same IMF-σ e trend of typical local ETGs, but with a lower mass-normalization. The observed lower normalization could be evidence of i) an evolution of the IMF with time or ii) a correlation with Σ. To discriminate between the two possibilities, we have compared the IMF-σ e trend that we have found for high-z dense ETGs with that of local ETGs with similar mean stellar mass density and velocity dispersion and we have found that the IMF of massive dense ETGs does not depend on redshift. The similarity between the IMF-σ e trends observed both in dense high-z and low-z ETGs over 9 Gyr of evolution and their lower massnormalization with respect to the mean value of local ETGs suggests that, independently of formation redshift, the physical conditions which characterized the formation of a dense spheroid on average lead to a mass spectrum of newly formed stars ...

The Fundamental Plane of cluster spheroidal galaxies at z ∼ 1.3: evidence for mass-dependent evolution

“…9. We note that the variation holds both for low-density and highdensity galaxies, in agreement with what previously found by Gargiulo et al (2015) for field spheroids at high redshift. For comparison, the M dyn /M * -σ relation of a sample of local spheroids extracted from the SPIDER sample (gray symbols; La Barbera et al 2010a) is also shown.…”

“…We notice that the relation between M dyn and M * dyn , as well the trend of M * dyn /M * -σ, i.e. the variation of IMF on σ and its possible evolution is out of the scope of the present work and it is treated in detail in other papers (e.g., Cappellari et al 2012Cappellari et al , 2013Tortora et al 2013;Ferreras et al 2013;Gargiulo et al 2015;La Barbera et al 2015;Lyubenova et al 2016, and references therein).…”

“…La , or mass density (at constant dark matter (DM) fraction, e.g. Gargiulo et al 2015) or mass of galaxies (see, the Introduction in De Masi et al 2019, for a concise review). Some of these studies consider the IMF normalization ratio M * dyn /M * , often called Γ, as a comparison between stellar mass M * , obtained for a reference IMF (e.g.…”

Implications of Galaxy Buildup for Putative IMF Variations in Massive Galaxies

“…Comparing dynamical to simple stellar population (SSP) masses, Shetty & Cappellari (2014 find a Salpeter IMF, rather than the "universal" Chabrier, for their sample of massive (>10 11 M e ) ETGs atz 0.8. For a sample of ETGs atz 1.4 with both dynamical and photometric mass estimates, Gargiulo et al (2015a) report an IMF-σ relation consistent with the trends observed atz 0. They further posit that the IMF of dense (>2500 M e pc −2 ) ETGs is independent of redshift over the past ∼9 Gyr.…”

“…Based on studies of ETG populations atz 1 andz 1.4, observations beyond the local universe are beginning to suggest that the IMF-σ relation remains roughly constant over the past 8Gyr (Martín-Navarro et al 2015b;Gargiulo et al 2015a). Contrary to these studies, Sonnenfeld et al (2015) find that the overall log(a IMF ) at fixed velocity dispersion decreases from thez 0 value out toz 0.8.…”