The effect of ram pressure on the star formation, mass distribution and morphology of galaxies

¹ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria e-mail: wolfgang.e.kapferer@uibk.ac.at
² Laboratoire Cassiopée, CNRS, UMR 6202, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received: 19 December 2008
Accepted: 14 March 2009

Abstract

Aims. We investigate the dependence of star formation and the distribution of the components of galaxies on the strength of ram pressure. Several mock observations in X-ray, Hα and HI wavelength for different ram-pressure scenarios are presented.

Methods. By applying a combined N-body/hydrodynamic description (GADGET-2) with radiative cooling and a recipe for star formation and stellar feedback 12 different ram-pressure stripping scenarios for disc galaxies were calculated. Special emphasis was put on the gas within the disc and in the surroundings. All gas particles within the computational domain having the same mass resolution. The relative velocity was varied from 100 km s^-1 to 1000 km s^-1 in different surrounding gas densities in the range from 1  10^-28 to 5  10^-27 g/cm³. The temperature of the surrounding gas was initially 1  10⁷ K.

Results. The star formation of a galaxy is enhanced by more than a magnitude in the simulation with a high ram-pressure (5  10^-11 dyn/cm²) in comparison to the same system evolving in isolation. The enhancement of the star formation depends more on the surrounding gas density than on the relative velocity. Up to 95% of all newly formed stars can be found in the wake of the galaxy out to distances of more than 350 kpc behind the stellar disc. Continuously stars fall back to the old stellar disc, building up a bulge-like structure. Young stars can be found throughout the stripped wake with surface densities locally comparable to values in the inner stellar disc. Ram-pressure stripping can shift the location of star formation from the disc into the wake on very short timescales. As the gas in a galaxy has a complex velocity pattern due to the rotation and spiral arms, the superposition of the internal velocity field and the ram pressure causes complex structures in the gaseous wake which survive dynamically up to several 100 Myr. Finally we provide simulated X-ray, Hα and HI observations to be able to compare our results with observations in these wavebands. These simulated observations show many features which depend strongly both on the strength and the duration of the external ram pressure.