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

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
e-mail: s.i.ferrero@astro.uio.no
² Department of Physics & Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
³ Instituto de Astronomía Teórica y Experimental, CONICET-UNC, Laprida 854, X5000BGR Córdoba, Argentina
⁴ Observatorio Astronómico de Cárdoba, Universidad Nacional de Córdoba, Laprida 854, X5000BGR Córdoba, Argentina
⁵ Consejo de Investigaciones Científicas y Técnicas de la República Argentina, Avda. Rivadavia 1917, C1033AAJ CABA, Argentina

Received: 2 November 2020
Accepted: 25 February 2021

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.