Pavel Jachym

Gas stripping in galaxy clusters: A new SPH simulation approach

Aims. The influence of a time-varying ram pressure on spiral galaxies in clusters is explored with a new simulation method based on the N-body SPH/tree code GADGET. Methods. We have adapted the code to describe the interaction of two different gas phases, the diffuse hot intracluster medium (ICM) and the denser and colder interstellar medium (ISM). Both the ICM and ISM components are introduced as SPH particles. As a galaxy arrives on a highly radial orbit from outskirts to cluster center, it crosses the ICM density peak and experiences a time-varying wind. Results. Depending on the duration and intensity of the ISM-ICM interaction, early and late type galaxies in galaxy clusters with either a large or small ICM distribution are found to show different stripping efficiencies, amounts of reaccretion of the extra-planar ISM, and final masses. We compare the numerical results with analytical approximations of different complexity and indicate the limits of the Gunn & Gott simple stripping formula. Conclusions. Our investigations emphasize the role of the galactic orbital history to the stripping amount. We discuss the contribution of ram pressure stripping to the origin of the ICM and its metallicity. We propose gas accumulations like tails, filaments, or ripples to be responsible for stripping in regions with low overall ICM occurrence.

Ram pressure stripping of tilted galaxies

Context. Ram pressure stripping of galaxies in clusters can yield gas deficient disks. Previous numerical simulations based on various approaches suggested that, except for near edge-on disk orientations, the amount of stripping depends very little on the inclination angle. Aims. Following our previous numerical and analytical study of face-on stripping, we extend the set of parameters with the disk tilt angle and explore in detail the effects of the ram pressure on the interstellar content (ISM) of tilted galaxies that orbit in various environments of clusters, with compact or extended distributions of the intra-cluster medium (ICM). We further study how results of numerical simulations could be estimated analytically. To isolate the effect of inclination, galaxies on strictly radial orbits are considered. Methods. A grid of numerical simulations with varying parameters is produced using the tree/SPH code GADGET with a modified method for calculating the ISM-ICM interaction. These SPH calculations extend the set of existing results obtained from different codes using various numerical techniques. Results. The simulations confirm the general trend of less stripping at orientations close to edge-on. The dependence on the disk tilt angle is more pronounced for compact ICM distributions, however it almost vanishes for strong ram pressure pulses. Although various hydrodynamical effects are present in the ISM-ICM interaction, the main quantitative stripping results appear to be roughly consistent with a simple scenario of momentum transfer from the encountered ICM. This behavior can also be found in previous simulations. To reproduce the numerical results we propose a fitting formula depending on the disk tilt angle and on the column density of the encountered ICM. Such a dependence is superior to that on the peak ram pressure used in previous simple estimates.

Molecular Gas Dominated 50 kpc Ram Pressure Stripped Tail of the Coma Galaxy D100

We have discovered large amounts of molecular gas, as traced by CO emission, in the ram pressure stripped gas tail of the Coma cluster galaxy D100 (GMP 2910), out to large distances of about 50 kpc. D100 has a 60 kpc long, strikingly narrow tail which is bright in X-rays and H{\alpha}. Our observations with the IRAM 30m telescope reveal in total ~ 10^9 M_sun of H_2 (assuming the standard CO-to-H_2 conversion) in several regions along the tail, thus indicating that molecular gas may dominate its mass. Along the tail we measure a smooth gradient in the radial velocity of the CO emission that is offset to lower values from the more diffuse H{\alpha} gas velocities. Such a dynamic separation of phases may be due to their differential acceleration by ram pressure. D100 is likely being stripped at a high orbital velocity >2200 km/s by (nearly) peak ram pressure. Combined effects of ICM viscosity and magnetic fields may be important for the evolution of the stripped ISM. We propose D100 has reached a continuous mode of stripping of dense gas remaining in its nuclear region. D100 is the second known case of an abundant molecular stripped-gas tail, suggesting that conditions in the ICM at the centers of galaxy clusters may be favorable for molecularization. From comparison with other galaxies, we find there is a good correlation between the CO flux and the H{\alpha} surface brightness in ram pressure stripped gas tails, over about 2 dex.

Triggered Star Formation: From Large to Small Scales

We give examples of star formation triggering. On the large scale, SF is triggered by collisions and interactions of galaxies, and by interactions of a galaxy with the environment in a cluster. On the intermediate scale, SF is triggered in barred galaxies by resonant rings. On the small scale, the star formation is self-regulated by the feedback from young, massive stars.

Abundant molecular gas and inefficient SF in intra-cluster regions of a ram pressure stripped tail

For the first time in any ram pressure stripped galaxy, we detect large amounts of cold molecular gas in the X-ray bright, and star forming tail of ESO 137-001 in the Norma cluster. We find very low star formation efficiency in the stripped gas, similar to values found in the outer spiral disks where however molecular gas is mostly undetected. The results were recently published in Jáchym et al. (2014).