Galaxies undergoing ram-pressure stripping: the influence of the bulge on morphology and star formation rate

Aims. We investigate the influence of stellar bulges on the star formation and morphology of disc galaxies that suffer from ram pressure. Several tree-SPH (smoothed particle hydrodynamics) simulations have been carried out to study the dependence of the star formation rate on the mass and size of a stellar bulge. In addition, different strengths of ram pressure and different alignments of the disc with respect to the intra-cluster medium (ICM) are applied. Methods. The simulations were carried out with the combined N-body/hydrodynamic code GADGET-2 with radiative cooling and a recipe for star formation. The same galaxy with different bulge sizes was used to accomplish 31 simulations with varying inclination angles and surrounding gas densities of 10(-27) g cm(-3) and 10(-28) g cm(-3). For all the simulations a relative velocity of 1000 km s(-1) for the galaxies and an initial gas temperature for the ICM of 10(7) K were applied. Besides galaxies flying edge-on and face-on through the surrounding gas, various disc tilt angles in between were used. To allow a comparison, the galaxies with the different bulges were also evolved in isolation to contrast the star formation rates. Furthermore, the influence of different disc gas mass fractions has been investigated. Results. As claimed in previous works, when ram pressure is acting on a galaxy, the star formation rate (SFR) is enhanced and rises up to four times with increasing ICM density compared to galaxies that evolve in isolation. However, a bulge suppresses the SFR when the same ram pressure is applied. Consequently, fewer new stars are formed because the SFR can be lowered by up to 2 M-circle dot yr(-1). Furthermore, the denser the surrounding gas, the more interstellar medium (ISM) is stripped. While at an ICM density of 10(-28) g cm(-3) about 30% of the ISM is stripped, the galaxy is almost completely (more than 90%) stripped when an ICM density of 10(-27) g cm(-3) is applied. But again, a bulge prevents the stripping of the ISM and reduces the amount being stripped by up to 10%. Thereby, fewer stars are formed in the wake if the galaxy contains a bulge. The dependence of the SFR on the disc tilt angle is not very pronounced. Merely a slight trend of decreasing star formation with increasing inclination angle can be determined. Furthermore, with increasing disc tilt angles, less gas is stripped and therefore fewer stars are formed in the wake. Reducing the disc gas mass fraction results in a lower SFR when the galaxies evolve in vacuum. On the other hand, the enhancement of the SFR in case of acting ram pressure is less pronounced with increasing gas mass fraction. Moreover, the fractional amount of stripped gas does not depend on the gas mass fraction.