New CO observations and simulations of the NGC 4438/NGC 4435 system

Abstract

Abstract. NGC 4438 is a highly perturbed spiral with a stellar tidal tail and extraplanar molecular gas, now very HI deficient, crossing the center of the Virgo cluster at high speed. Various authors have attributed the perturbed appearance to the ram pressure of the intracluster medium, the tidal interaction with NGC 4435, and an ISM−ISM collision between the ISM of NGC 4438 and NGC 4435. We present new CO observations covering virtually all of NGC 4438 and the center of NGC 4435 and detailed simulations including all of the above effects. For the first time CO is detected in NGC 4435. In NGC 4438 we find double line profiles at distances up to 40 to the west and south-west and redshifted lines with respect to galactic rotation in the south of the center. The lack of gas to the North and East coupled with the large gaseous extent to the West and the redshifted and double line profiles can only be reproduced with a ram pressure wind. NGC 4438 is most probably on its first passage through the cluster center and has been stripped of its HI only over the past 100 Myr. While an ISM−ISM collision between NGC 4435 and NGC 4438 may occur, the effect is not significant compared to ram pressure and tidal forces, not surprising for the passage of an S0 galaxy 5−10 kpc from the center of NGC 4438. We also detect narrow CO lines, in the absence of detected HI, in the northern tidal arm some 15 kpc from the center of NGC 4438. This can be understood from the simulations assuming a few percent of the gas is too dense to experience the ram pressure wind. NGC 4438 has changed greatly over the past 100 Myr due to its plunge through the center of the Virgo cluster and the interaction with the S0 NGC 4435. The ram pressure wind has a strong effect which is increased by the interaction with NGC 4435, bringing gas further from NGC 4438 where the ram pressure strips it away.

Ram pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438

“…The tidal arms of this highly inclined galaxy are evidence of its tidal interaction ∼100 Myr ago with NGC 4435 (Combes et al 1988;Vollmer et al 2005) or M 86 . The disk of interstellar matter is severely truncated and extraplanar gas is found to the west of the galaxy center.…”

“…4). In the other regions, the absolute of the velocity difference do not Vollmer et al (2005). The arrow indicates a region of unusually large velocity differences.…”

“…The disk of interstellar matter is severely truncated and extraplanar gas is found to the west of the galaxy center. This extraplanar gas consists of all ISM phases: (i) molecular hydrogen observed in CO (Combes et al 1988;Vollmer et al 2005); (ii) atomic hydrogen (Cayatte et al 1990;Hota et al 2007); (iii) warm ionized gas observed in Hα (Kenney et al 1995;Chemin et al 2005); (iv) hot ionized gas observed in Xrays Machacek et al 2004); and (v) magnetic fields and cosmic ray electrons observed in the radio continuum Hota et al 2007). While it is agreed that the distortion of the stellar content of NGC 4438 is due to a tidal encounter, different mechanisms were put forward to explain the displacement of all ISM phases: (i) the tidal interaction which extracted the molecular gas from the center and left it to the west of the galaxy (Combes et al 1988); (ii) a collision between the ISM of the two galaxies (Kenney et al 1995); and (iii) ram pressure stripping due to the rapid motion of NGC 4438 through the hot intracluster medium (ICM, Chincarini & de Souza 1985).…”

“…While it is agreed that the distortion of the stellar content of NGC 4438 is due to a tidal encounter, different mechanisms were put forward to explain the displacement of all ISM phases: (i) the tidal interaction which extracted the molecular gas from the center and left it to the west of the galaxy (Combes et al 1988); (ii) a collision between the ISM of the two galaxies (Kenney et al 1995); and (iii) ram pressure stripping due to the rapid motion of NGC 4438 through the hot intracluster medium (ICM, Chincarini & de Souza 1985). Vollmer et al (2005) combined new CO(1-0) observations with detailed numerical simulations. This lead to the following interaction scenario for the NGC 4438/NGC 4435 system: NGC 4435 passed through the disk of NGC 4438 ∼100 Myr ago at a radial distance of ∼5−10 kpc.…”

Herschel-SPIRE observations of the disturbed galaxy NGC 4438

“…However, while Combes et al (1988) and Vollmer et al (2005) invoked a high-velocity (∼800 km s −1 ) gravitational interaction with the companion early-type galaxy NGC 4435, Kenney et al (2008) have revealed the presence of a series of Hα+[NII] filaments connecting NGC 4438 to the giant elliptical M 86 (see Fig. 1).…”