Astron. Astrophys. 349, 411-423 (1999)

Astron. Astrophys. 349, 411-423 (1999)

1. Introduction

A galaxy cluster is an ideal laboratory for studying the influence of the galaxy's environment on its appearance and/or evolution. There are three types of interactions which can provide the mechanisms for changing the galaxy's properties, i.e. morphology, luminosity and gas content as they enter the cluster.

galaxy-galaxy gravitational interactions
galaxy-cluster gravitational interactions due to the cluster potential
galaxy-cluster interactions due to an individual galaxy's motion in the hot plasma of the cluster (ram pressure stripping).

In the case of galaxy-galaxy interactions one can observe important distortions of the stellar and gas content (see e.g. Barnes & Hernquist 1996, Combes 1997, Moore et al. 1996, Olson & Kwan 1990) as the tidal forces act on both components of the galaxy. The influence of the cluster potential can cause distortions of the stellar and gas content and its velocity field only if the galaxy passes near enough the cluster centre (Valluri 1993). However, ram pressure stripping (Gunn & Gott 1972) is only acting on the gas content. This provides a tool to discriminate between these kinds of interactions (Combes et al. 1988).

Spiral galaxies located near the cluster centre show very different gas characteristics in a cluster than in the field. They are very deficient in HI (Chamaraux et al. 1980, Bothun et al. 1982, Giovanelli & Haynes 1985, Gavazzi 1987, 1989) and their HI disk sizes are also considerably reduced (van Gorkom & Kotanyi 1985, Warmels 1988, Cayatte et al. 1990, 1994). Concerning the stellar content, their intrinsic colour indices are not significantly different from field galaxies of the same morphological type (Gavazzi et al. 1991, Gavazzi et al. 1998). There is a special class of galaxies with a very low arm inter-arm contrast defined as anemics by van den Bergh (1976). But despite the HI deficiency, cluster galaxies do not show a reduced CO content (Kenney & Young 1986, Casoli et al. 1991, Boselli et al. 1997a) nor a reduced infrared luminosity (Bicay & Giovanelli 1987).

A promising way to study the galaxy's interaction with its environment is to look at details of the emission of the interstellar gas and its velocity distribution. As the atomic gas is located at large galactic radii and is relatively diffuse, it is the most sensible tracer for perturbations which are induced by forces exterior to the galaxy.

The most common distortion of the outer HI content of spiral galaxies in general are warps. They can be described by a collection of spinning, concentric rings whose angular moment vector is more and more inclined with increasing radius (Rogstad et al. 1974). Thus, the un-tilted warp is a completely symmetric feature. Warps are a common feature in spiral galaxies (see e.g. Sancisi 1976, Bosma 1978, and Briggs 1990) although their origin is still not clear (see e.g. Binney 1992, Jiang & Binney 1999).

It can be interpreted as a discrete bending mode in flattened halos (Lynden-Bell 1965, Sparke 1984, Sparke & Casertano 1988). However, if the proper dynamics of the halo are taken into account, the warp fades away rapidly (Nelson & Tremaine 1995, Binney et al. 1998).
According to cosmology, galactic halos should be disturbed (Ryden & Gunn 1987, Ryden 1988, Quinn & Binney 1992). They are not in dynamical equilibrium and could so generate a warped disc.
Infalling galaxies could tidally induce or form a warp or a polar ring (Jiang & Binney 1999).
Non-gravitational forces as ram pressure or magnetic pressure could be responsible for a warp.

Binney (1992) points out, that ram pressure stripping should cause an axisymmetric response which has the form of a rim.

In this paper, we present and discuss a new detailed HI map of the anemic galaxy NGC 4548 which is located at [FORMULA] from the centre of the Virgo cluster looking at the kinematics in detail. The HI map is compared to an H line map and an optical B image. CO pointings with velocity information are added to the cube in order to give more information about the kinematics at small galactic radii. A rotation curve is fitted and compared to the ones obtained by Guhathakurta et al. (1988) and Rubin et al. (1999). The local perturbation observed is discussed in the framework of warps.

Online publication: September 2, 1999
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