1 Introduction

This is our second paper on the imaging of the neutral hydrogen component as a tracer of environmental effects on galaxies in the Coma cluster. Global properties of Coma related with its dynamical state, as derived from our VLA 21 cm line survey, have been discussed in a previous paper (Bravo-Alfaro et al. 2000, hereafter referred to as Paper I). In the present work we give a complete catalogue of H I maps, channel maps, and velocity fields (for cases with sufficient resolution) for 19 detected galaxies. We compare the detailed H I distribution and kinematics with model predictions for the fate of H I in galaxies in a cluster environment. We pay particular attention to a class of so called starburst (SB) and post starburst (PSB) galaxies identified by Caldwell et al. (1993, C93 throughout this paper). For these galaxies we derive star formation rates (SFR) from the radio continuum emission and discuss the implications of current gas content and star formation rate on the evolution of the galaxies.

Several physical mechanisms have been proposed to play a major role affecting the evolution of galaxies in clusters; these processes can be classified according to three types: (a) interactions between the ICM and the interstellar medium (Gunn & Gott 1972; Nulsen 1982; Kenney & Young 1989), (b) interactions with the cluster gravitational field (e.g. Bekki 1999), and (c) galaxy-galaxy interactions (see Icke 1985; Lavery & Henry 1994; Moore et al. 1996, 1999). In many cases more than one of these processes will be at work, but which mechanism dominates under specific physical conditions and how it depends on the galaxy trajectory through the cluster is still a matter of debate. For instance Solanes et al. (2000) conclude from an observational study of eighteen clusters that ICM-ISM interactions are most important to explain the observed H I deficiency. In contrast, Moore et al. (1999) suggest in a numerical study that, in combination with ram-pressure stripping, galaxy harassment may convert disk galaxies into S0s. In spite of this debate, the fact that spirals in the central region of clusters like Virgo show smaller H I than optical disks (Warmels 1988; Cayatte et al. 1990, 1994), is unquestionably associated with the interaction with the ICM. These studies demonstrate the importance of detailed H I imaging of individual galaxies in clusters with a range of ICM conditions to assess what gas removal processes are at work.

Another fundamental process linked with the evolution of galaxies in clusters is the triggering and stopping of bursts of star formation (van den Bergh et al. 1990). As the ISM is the raw material from which stars form, the evolution of H I deficient galaxies and their star formation properties are undoubtedly affected by the interaction with the environment. From statistical analyses on large scales the SFR is known to decrease with increasing density (Dressler et al. 1999), whereas at smaller scales the ICM may produce significant enhancements to the star formation activity in individual galaxies, as shown in clusters like A1367 (Gavazzi et al. 1995) and Coma (Bothun & Dressler 1986). Recent work supports different scenarios: (1) On the theoretical side, Bekki (1999) concludes that the tidal gravitational field of a group-cluster merger may trigger a burst of star formation, accounting for the PSB population in clusters like Coma, while Vollmer et al. (2001b) suggest that local enhancement of star formation could be due to re-accretion of gas clouds after a ram pressure stripping event. (2) Observationally, Dressler et al. (1999) find a trend in the position of SB and PSB galaxies around the cluster, making the hot gas environment the best explanation for this phenomenon (see also Poggianti et al. 1999). From a study of spectroscopically selected post starburst galaxies, which turned out to be mostly located in the field, Zabludoff et al. (1996) conclude that in those more isolated galaxies the post starburst phenomenon is most likely caused by galaxy interactions and mergers.

In this paper we give the atlas of our VLA H I imaging of spirals in Coma, showing that the scenario depicted by the neutral hydrogen constitutes strong evidence in favor of ICM-ISM interaction to explain the H I morphologies and the star formation histories in the cluster. We also present results on the radio continuum emission obtained as a byproduct of the spectral line observations. These results are used to derive the star formation rate in the so called post starburst galaxies to explore the possibility that these are in fact dust enshrouded starbursts as has recently been suggested by Smail et al. (1999) for the intermediate redshift cluster CL0939+4713 (see also Miller & Owen 2001).

We organize the paper as follows. In Sect. 2 we review the observations. In Sect. 3 we describe H I observational results of individual detected galaxies, in Sect. 4 we discuss the role of ram pressure affecting galaxies in Coma, and compare H I observations with numerical simulations of ram pressure stripping by Abadi et al. (1999) and Vollmer et al. (2001b). In Sect. 5 we discuss the PSB galaxies; we compare our H I mass limits and current star formation rates based on the continuum with those of the more vigorously star forming galaxies and discuss the implications. The results are summarized in Sect. 6. In Appendix A, available in electronic form, we give the complete catalogue of H I channel maps. Throughout the paper we assume a distance of 70 Mpc to the Coma cluster (with H₀=100 kms^-1Mpc^-1), where an angular size of 1 arcmin corresponds to a linear size of ${\sim}$20 kpc.