HVC’s Favouring Star Forming Processes in galaxy discs: a basic calculation and some recent HI observations

¹ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain ;
e-mail: valeria@iac.es, eca@iac.es, jeb@iac.es
² University of Hertfordshire, Hatfield, UK ;
e-mail: E.Brinks@herts.ac.uk
³ Consejo Superior de Investigaciones Científicas, Spain

Abstract

Indirect evidence from detailed chemical evolution studies shows that infall of low metallicity gas appears to have proceeded at a rather constant rate during the lifetime of the Galactic disk. We explore two possible implications: (a) the effect of an infalling high velocity cloud (HVC) on interstellar cloud stability leading to enhanced star formation, and (b) the presence of these HVC’s around neighbouring galaxies, which would generalize this aspect of the evolution of galaxies in groups. (a) Using a simple analytical model we show how the overpressure produced in the galactic plane by HVC infall reduces the effective Jeans mass of the existing clouds, thus tending to enhance the SFR. (b) Using HI data from the THINGS survey we detect HI “fluff” around 4 galaxies of a total sample of 32 observed. Projecting some of the largest Galactic HVC’s to the distances of the THINGS galaxies we produce a useful estimate of the fraction of the HVC masses that we can detect at these distances, finding a value of order 10%, and go on to make tentative estimates about infall rates for nearby galaxies.