We study the dark and luminous mass distributions, circular velocity curves (CVC), line-of-sight kinematics, and angular momenta for a sample of 42 cosmological zoom simulations of galaxies with stellar masses from 2.0 × 10 10 M ⊙ h −1 to 3.4 × 10 11 M ⊙ h −1 . Using a temporal smoothing technique, we are able to reach large radii. We find that: (i) The dark matter halo density profiles outside a few kpc follow simple power-law models, with flat dark matter CVCs for lower-mass systems, and rising CVCs for high-mass haloes. The projected stellar density distributions at large radii can be fitted by Sérsic functions with n ∼ > 10, larger than for typical early-type galaxies (ETGs). (ii) The massive systems have nearly flat total (luminous plus dark matter) CVCs at large radii, while the less massive systems have mildly decreasing CVCs. The slope of the circular velocity at large radii correlates with circular velocity itself. (iii) The dark matter fractions within the projected stellar half mass radius R e are in the range 15-30% and increase to 40-65% at 5 R e . Larger and more massive galaxies have higher dark matter fractions. The fractions and trends with mass and size are in agreement with observational estimates, even though the stellar-to-total mass ratio is ∼2-3 times higher than estimated for ETGs. (iv) The short axes of simulated galaxies and their host dark matter haloes are well aligned and their short-to-long axis ratios are correlated. (v) The stellar root mean square velocity v rms (R) profiles are slowly declining, in agreement with planetary nebulae observations in the outer haloes of most ETGs. (vi) The line-of-sight velocity fieldsv show that rotation properties at small and large radii are correlated. Most radial profiles for the cumulative specific angular momentum parameter λ(R) are nearly flat or slightly rising, with values in [0.06, 0.75] from 2 R e to 5 R e . A few cases show local maxima in |v|/σ(R). These properties agree with observations of ETGs at large radii. (vii) Stellar mass, ellipticity at large radii ǫ(5 R e ), and λ(5 R e ) are correlated: the more massive systems have less angular momentum and are rounder, as for observed ETGs. (viii) More massive galaxies with a large fraction of accreted stars have radially anisotropic velocity distributions outside R e . Tangential anisotropy is seen only for galaxies with high fraction of in-situ stars.

We study a merger of the NGC 4839 group with the Coma cluster using X-ray observations from the XMM-Newton and Chandra telescopes. X-ray data show two prominent features: (i) a long (∼600 kpc in projection) and bent tail of cool gas trailing (towards south-west) the optical center of NGC 4839, and ii) a 'sheath' region of enhanced X-ray surface brightness enveloping the group, which is due to hotter gas. While at first glance the X-ray images suggest that we are witnessing the first infall of NGC 4839 into the Coma cluster core, we argue that a post-merger scenario provides a better explanation of the observed features and illustrate this with a series of numerical simulations. In this scenario, the tail is formed when the group, initially moving to the south-west, reverses its radial velocity after crossing the apocenter, the ram pressure ceases and the ram-pressure-displaced gas falls back toward the center of the group and overshoots it. Shortly after the apocenter passage, the optical galaxy, dark matter and gaseous core move in a north-east direction, while the displaced gas continues moving to the south-west. The 'sheath' is explained as being due to interaction of the re-infalling group with its own tail of stripped gas mixed with the Coma gas. In this scenario, the shock, driven by the group before reaching the apocenter, has already detached from the group and would be located close to the famous relic to the south-west of the Coma cluster.