Letter to the Editor

Gravitational torques in spiral galaxies: Gas accretion as a driving mechanism of galactic evolution

¹ School of Computational and Applied Mathematics, University Witwatersrand, Private Bag 3, WITS 2050, South Africa
² Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
³ École Normale Supérieure, 45 rue d'Ulm, 75005 Paris, France
⁴ Instituto Nacional de Astrofísica, Optica y Electrónica, Calle Luis Enrique Erro 1, 72840 Tonantzintla, Puebla, México
⁵ Dept. of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, Alabama 35487, USA

Corresponding author: D. L. Block, block@cam.wits.ac.za

Received: 8 July 2002
Accepted: 18 September 2002

Abstract

The distribution of gravitational torques and bar strengths in the local Universe is derived from a detailed study of 163 galaxies observed in the near-infrared. The results are compared with numerical models for spiral galaxy evolution. It is found that the observed distribution of torques can be accounted for only with external accretion of gas onto spiral disks. Accretion is responsible for bar renewal – after the dissolution of primordial bars – as well as the maintenance of spiral structures. Models of isolated, non-accreting galaxies are ruled out. Moderate accretion rates do not explain the observational results: it is shown that galactic disks should double their mass in less than the Hubble time. The best fit is obtained if spiral galaxies are open systems, still forming today by continuous gas accretion, doubling their mass every 10 billion years.