Galaxy sizes and the galaxy–halo connection – I. The remarkable tightness of the size distributions

Abstract

The mass and structural assembly of galaxies is a matter of intense debate. Current theoretical models predict the existence of a linear relationship between galaxy size (R e ) and the host dark matter halo virial radius (R h ). By making use of semi-empirical models compared to the size distributions of central galaxies from the Sloan Digital Sky Survey, we provide robust constraints on the normalization and scatter of the R e − R h relation. We explore the parameter space of models in which the R e − R h relation is mediated by either the spin parameter or the concentration of the host halo, or a simple constant the nature of which is in principle unknown. We find that the data require extremely tight relations for both early-type and latetype galaxies (ETGs,LTGs), especially for more massive galaxies. These constraints challenge models based solely on angular momentum conservation, which predict significantly wider distributions of galaxy sizes and no trend with stellar mass, if taken at face value. We discuss physically-motivated alterations to the original models that bring the predictions into better agreement with the data. We argue that the measured tight size distributions of SDSS disk galaxies can be reproduced by semi-empirical models in which the R e − R h connection is mediated by the stellar specific angular momenta j st ar . We find that current cosmological models of galaxy formation broadly agree with our constraints for LTGs, and justify the strong link between R e and j st ar that we propose, however the tightness of the R e − R h relation found in such ab-initio theoretical models for ETGs is in tension with our semi-empirical findings.

Using angular momentum maps to detect kinematically distinct galactic components

“…),Weinzirl et al (2009),Gargiulo et al (2015),Pedrosa & Tissera (2015),Irodotou et al (2019),Obreschkow et al (2020), andZanisi et al (2020). Finally, we note that even though…”

A physically motivated definition for the size of galaxies in an era of ultradeep imaging

“…As the vast majority of works aiming to understand the connection between the galaxy size and the dark matter halo properties use R e as a size indicator (see e.g. Kravtsov 2013;Jiang et al 2019;Zanisi et al 2020), the small increase in the effective radius in this mass range can hide a potential connection between the dark and the luminous component of the galaxies. In a companion paper (C. Dalla Vecchia et al in prep.…”

A deep learning approach to test the small-scale galaxy morphology and its relationship with star formation activity in hydrodynamical simulations

“…where we have accounted for the dependence of certain morphological features from global properties in the term ( details | global ) (e.g., spiral galaxies, which have very distinctive features, also tend to be larger than spheroids as shown by a vast body of literature -see for example Shen et al 2003;Bernardi et al 2014;Lange et al 2016;Zanisi et al 2020). The log-likelihood ratio, LLR (where only the contribution of subject remains, see eq.…”

“…As an example, we take three scaling relations that have been widely studied in the literature: the − star relation (size-mass relation, e.g. Shankar et al 2010,Bernardi et al 2014, Lange et al 2015, Zanisi et al 2020 The color code is the difference between the mean LLR of each simulation and the mean LLR of SDSS at each point of the scaling relations. A brighter color indicates a better agreement with SDSS.…”