A unified scenario for the origin of spiral and elliptical galaxy structural scaling laws

Abstract

Elliptical (E) and spiral (S) galaxies follow tight, but different, scaling laws that link their stellar masses, radii, and characteristic velocities. Mass and velocity, for example, scale tightly in spirals with little dependence on galaxy radius (the ‘Tully-Fisher relation’; TFR). On the other hand, ellipticals appear to trace a 2D surface in size-mass-velocity space (the ‘Fundamental Plane’; FP). Over the years, a number of studies have attempted to understand these empirical relations, usually in terms of variations of the virial theorem for E galaxies and in terms of the scaling relations of dark matter halos for spirals. We use Lambda cold dark matter (ΛCDM) cosmological hydrodynamical simulations to show that the scaling relations of both ellipticals and spirals arise as the result of (i) a tight galaxy mass–dark halo mass relation and (ii) the self-similar mass profile of cold dark matter halos. In this interpretation, E and S galaxies of a given stellar mass inhabit halos of similar masses, and their different scaling laws result from the varying amounts of dark matter enclosed within their luminous radii. This scenario suggests a new galaxy distance indicator applicable to galaxies of all morphologies and provides simple and intuitive explanations for long-standing puzzles, such as why the TFR is independent of surface brightness, or what causes the ‘tilt’ in the FP. Our results provide strong support for the predictions of ΛCDM in the strongly non-linear regime, as well as guidance for further improvements to cosmological simulations of galaxy formation.

A new framework for understanding the evolution of early-type galaxies

“…Since the galaxy properties change with time, the slope of the FP is expected to change with redshift. This is confirmed by the numerical models of single galaxies, large-scale cosmological simulations, and observational surveys at different redshifts, among others (see Beifiori et al 2017;Rosito et al 2019a,b;Lu et al 2019;Ferrero et al 2021;de Graaff et al 2022 and references therein).…”

Velocity Dispersions of Quiescent Galaxies in IllustrisTNG

“…We also note that the anisotropy observed at σ > 250 km s −1 when comparing σ 3D and σ 1D or contrasting σ 1D measurements along different axes is unrelated to this offset. Ferrero et al (2021) also show that the M * -σ * relation of the subhalos in TNG100 and EAGLE simulation is offset from the observed relation. They derive the observed relation based on stellar velocity dispersions within effective radii of ∼300 earlytype galaxies included in ATLAS3D (Cappellari et al 2013) and SLACS (Bolton et al 2006) data.…”

The weak dependence of velocity dispersion on disc fractions, mass-to-light ratio, and redshift: implications for galaxy and black hole evolution

“…In our models, the weak dependence of velocity dispersion on 𝐵/𝑇 is just a reflection of the virial theorem -velocity dispersion depends more on the total mass within an aperture and less on how this mass is distributed in it. This may partly explain the similarity in the scaling relations of early and latetype galaxies (e.g., Bernardi et al 2011;Ferrero et al 2021), as well as the universality of the 𝑀 𝑏ℎ − 𝜎 relation (e.g., Ferrarese & Merritt 2000;Shankar et al 2016).…”

“…Velocity dispersion is also a tracer of the distribution of dark matter in the inner regions of the galaxies, via the dynamical mass 𝑀 𝑑𝑦𝑛 (< 𝑅) ∝ 𝑅 𝜎 𝑎 𝑝 (𝑅) 2 , which traces the gravitational influence of the total mass within 𝑅 on test (stellar or gas) particles. The ratio between dynamical and stellar mass within the effective radius, 𝑀 𝑑𝑦𝑛 (< 𝑅 𝑒 )/𝑀 * (< 𝑅 𝑒 ), appears to be increasing with stellar mass, 𝑀 𝑑𝑦𝑛 ∝ 𝑀 * 1+𝛼 , with 𝛼 ∼ 0.2 − 0.3 (e.g., Pahre et al 1998;Padmanabhan et al 2004;Gallazzi et al 2006;Hopkins et al 2009), which is related to the overall "tilt" of the fundamental plane (FP) of galaxies (e.g., Djorgovski & Davis 1987;Dressler et al 1987), where the FP terminology is usually mostly applied to earlier type galaxies (e.g., Bernardi et al 2020, but see also Ferrero et al 2021). Different effects could contribute to the tilt of the FP and more specifically to the slope of the 𝑀 𝑑𝑦𝑛 /𝑀 * ratio, from an increasing contribution of dark matter in larger/more massive galaxies, to the effect of stellar non-homology, radial anisotropy, and/or systematic variations of the stellar population (e.g., Ciotti et al 1996;Trujillo et al 2004;Bertin & Lombardi 2006;Cappellari et al 2006;Hyde & Bernardi 2009;Hopkins et al 2009;Cappellari et al 2013;D'Onofrio et al 2013;Zahid et al 2016;Zahid & Geller 2017;Bernardi et al 2020;Mould 2020;D'Eugenio et al 2021).…”