We present the discovery of spectacular double X-ray tails associated with ESO 137-001 and a possibly heated X-ray tail associated with ESO 137-002, both late-type galaxies in the closest rich cluster Abell 3627. A deep Chandra observation of ESO 137-001 allows us for the first time to examine the spatial and spectral properties of such X-ray tails in detail. Besides the known bright tail that extends to ∼80 kpc from ESO 137-001, a fainter and narrower secondary tail with a similar length was surprisingly revealed, as well as some intriguing substructures in the main tail. There is little temperature variation along both tails. The widths of the secondary tail and the greater part of the main tail also remain nearly constant with the distance from the galaxy. All these results challenge the current simulations. The Chandra data also reveal 19 X-ray point sources around the X-ray tails. We identified six X-ray point sources as candidates of intracluster ultra-luminous X-ray sources with L 0.3-10 keV of up to 2.5 × 10 40 erg s −1 . Gemini spectra of intracluster H ii regions downstream of ESO 137-001 are also presented, as well as the velocity map of these H ii regions that shows the imprint of ESO 137-001's disk rotation. For the first time, we unambiguously know that active star formation can happen in the cold interstellar medium (ISM) stripped by intracluster medium (ICM) ram pressure, and it may contribute a significant amount of the intracluster light. We also report the discovery of a 40 kpc X-ray tail of another late-type galaxy in A3627, ESO 137-002. Its X-ray tail seems hot, ∼2 keV (compared to ∼0.8 keV for ESO 137-001's tails). The Hα data for ESO 137-002 are also presented. We conclude that the high-pressure environment around these two galaxies is important for their bright X-ray tails and the intracluster star formation. The soft X-ray tails can reveal a great deal of the thermal history of the stripped cold ISM in mixing with the hot ICM, which is discussed along with intracluster star formation.

Abstract. Galaxies in clusters and groups moving through the intracluster or intragroup medium (abbreviated ICM for both) are expected to lose at least a part of their interstellar medium (ISM) by the ram pressure they experience. We perform high resolution 2D hydrodynamical simulations of face-on ram pressure stripping (RPS) of disk galaxies to compile a comprehensive parameter study varying galaxy properties (mass, vertical structure of the gas disk) and covering a large range of ICM conditions, reaching from high density environments like in cluster centres to low density environments typical for cluster outskirts or groups. We find that the ICM-ISM interaction proceeds in three phases: firstly the instantaneous stripping phase, secondly the dynamic intermediate phase, thirdly the quasi-stable continuous viscous stripping phase. In the first phase (time scale 20 to 200 Myr) the outer part of the gas disk is displaced but only partially unbound. In the second phase (10 times as long as the first phase) a part of the displaced gas falls back (about 10% of the initial gas mass) despite the constant ICM wind, but most displaced gas is now unbound. In the third phase the galaxy continues to lose gas at a rate of about 1 M yr −1 by turbulent viscous stripping. We find that the stripping efficiency depends slightly on the Mach number of the flow, however, the main parameter is the ram pressure. The stripping efficiency does not depend on the vertical structure and thickness of the gas disk. We discuss uncertainties in the classic estimate of the stripping radius of Gunn & Gott (1972, ApJ, 176, 1), which compares the ram pressure to the gravitational restoring force. In addition, we adapt the estimate used by Mori & Burkert (2000, ApJ, 538, 559) for spherical galaxies, namely the comparison of the central pressure with ram pressure. We find that the latter estimate predicts the radius and mass of the gas disk remaining at the end of the second phase very well, and better than the Gunn & Gott (1972, ApJ, 176, 1) criterion. From our simulations we conclude that gas disks of galaxies in high density environments are heavily truncated or even completely stripped, but also the gas disks of galaxies in low density environments are disturbed by the flow and back-falling material, so that they should also be pre-processed.

We perform hydrodynamical simulations of minor‐merger‐induced gas sloshing and the subsequent formation of cold fronts in the Virgo cluster. Comparing to observations, we show for the first time that the sloshing scenario can reproduce the radii and the contrasts in X‐ray brightness, projected temperature and metallicity across the cold fronts quantitatively. The comparison suggests a third cold front 20 kpc north‐west of the Virgo core. We identify several new features typical for sloshing cold fronts: an alternating distribution of cool, metal‐enriched X‐ray brightness excess regions and warm brightness deficit regions of reduced metallicity; a constant or radially decreasing temperature accompanied by a plateau in metallicity inside the cold fronts; a warm rim outside the cold fronts and a large‐scale brightness asymmetry. We can trace these new features not only in Virgo, but also in other clusters exhibiting sloshing cold fronts. By comparing synthetic and real observations, we estimate that the original minor‐merger event took place about 1.5 Gyr ago when a subcluster of 1–4 × 1013 M⊙ passed the Virgo core at 100–400 kpc distance, where a smaller mass corresponds to a smaller pericentre distance, and vice versa. From our inferred merger geometry, we derive the current location of the disturbing subcluster to be about 1–2 Mpc east of the Virgo core. A possible candidate is M60. Additionally, we quantify the metal redistribution by sloshing and discuss its importance. We verify that the subcluster required to produce the observed cold fronts could be completely ram‐pressure‐stripped before reaching the Virgo centre, and discuss the conditions required for this to be achieved. Finally, we demonstrate that the bow shock of a fast galaxy passing the Virgo cluster at ∼400 kpc distance also causes sloshing and leads to very similar cold front structures. The responsible galaxy would be located about 2 Mpc north of the Virgo centre. A possible candidate is M85.

We present three‐dimensional (3D) hydrodynamical simulations of ram pressure stripping of massive disc galaxies in clusters. Studies of galaxies that move face‐on have predicted that in such a geometry the galaxy can lose a substantial amount of its interstellar medium. But only a small fraction of galaxies is moving face‐on. In this work we focus on a systematic study of the effect of the inclination angle between the direction of motion and the galaxy's rotation axis. In agreement with some previous works, we find that the inclination angle does not play a major role for the mass loss as long as the galaxy is not moving close to edge‐on (inclination angle ≲60°). We explain this behaviour by extending Gunn & Gott's estimate of the stripping radius, which is valid for face‐on geometries, to moderate inclinations. The inclination plays a role as long as the ram pressure is comparable to pressures in the galactic plane, which can span two orders of magnitude. For very strong ram pressures, the disc will be stripped completely, and for very weak ram pressures, mass loss is negligible independent of inclination. We show that in non‐edge‐on geometries the stripping proceeds remarkably similar. A major difference between different inclinations is the degree of asymmetry introduced in the remaining gas disc. We demonstrate that the tail of gas stripped from the galaxy does not necessarily point in a direction opposite to the galaxy's direction of motion. Therefore, the observation of a galaxy's gas tail may be misleading about the galaxy's direction of motion.

We present the first 3D hydrodynamical simulations of ram pressure stripping of a disc galaxy orbiting in a galaxy cluster. Along the orbit, the ram pressure that this galaxy experiences varies with time. In this paper, we focus on the evolution of the radius and mass of the remaining gas disc, and compare it with the classical analytical estimate proposed by Gunn & Gott. We find that this simple estimate works well in predicting the evolution of the radius of the remaining gas disc. Only if the ram pressure increases faster than the stripping time‐scale, the disc radius remains larger than predicted. However, orbits with such short ram pressure peaks are unlikely to occur in other than compact clusters. Unlike the radius evolution, the mass‐loss history for the galaxy is not accurately described by the analytical estimate. Generally, in the simulations the galaxy loses its gas more slowly than predicted.