Because of its proximity, the Virgo cluster serves as a prime laboratory for the study of environmental influences because of the large number of observational tools that can be used to study its member galaxies. Although not a terribly rich cluster, the Virgo environment is nonetheless conducive to the conditions in which we expect to find morphological alteration and gas sweeping. Therefore, it is important to review the evidence for gas deficiency in Virgo at this point.
To date, HI observations have been made of several hundred galaxies in and around the Virgo cluster covering a wide variety of morphologies, types, luminosities, masses and star formation histories. Several authors have examined the occurrence of HI deficiency within the cluster (e.g., Haynes and Giovanelli 1986; Hoffman et al. 1988). A substantial number of galaxies covering a wide range of luminosity and morphological type are HI deficient by more than a factor of ten. Haynes and Giovanelli noted that the zone of HI deficiency extends through the region within about three degrees of M87. Within that zone, nearly all galaxies not seen in projection seem to be effected.
Several studies have used single dish observations to investigate the relative extent of the HI and stellar distributions of Virgo galaxies (e.g., Helou et al. 1981). Hewitt et al. (1983) matched a model HI distribution with Arecibo flat feed major axis mapping observations to derive the characteristic HI sizes of large angular diameter galaxies from the CIG. Haynes and Giovanelli (1983) then compared the HI radii measured similarly for Virgo spirals with those derived by Hewitt et al. They found that galaxies within the five degree Virgo core have HI-to-optical sizes a factor of two smaller on average than galaxies further from the cluster center. At the same time, the HI sizes are reduced along with the HI masses so that the globally averaged HI surface density scaled with the HI (not optical) radius < logMHI / D2HI > remains constant. In all respects the peripheral galaxies resembled the general isolated galaxy sample. Furthermore, Haynes and Giovanelli (1986) emphasized the one-to-one correspondence between large HI deficiency and small HI-to-optical disk size.
Aperture synthesis studies of the HI emission in galaxies in the Virgo cluster have been undertaken both with the Westerbork Synthesis Radio Telescope by Warmels (1988a, b) and with the Very Large Array (VLA) (van Gorkom and Kotanyi 1985; Cayatte et al. 1989). Both studies confirm the earlier single dish findings that the HI is preferentially removed from the outer parts of the galaxy and that the zone of stripped galaxies extends to about three degrees from M87. Warmels (1986) has found not only a systematic shrinking in the HI diameter relative to the optical for the subset within the core, but also concluded that the ratio DHI / Dopt decreases fairly smoothly with distance from M87. He identified the region within three to five degrees as a transition region where only moderate reduction of DHI / Dopt is evident. This scale is comparable to the zone of HI deficiency noticed by Haynes and Giovanelli (1986).
The VLA surveys add more information about the structure within the HI distribution and its velocity field. For more discussion of the rotation curve issue, the reader is referred to Whitmore's chapter in this volume and to Guhathakurta et al. (1988). It is clearly evident from the global display of the HI distribution in Virgo spirals shown by Jaqueline van Gorkom (van Gorkom and Kotanyi 1985), that the inner spirals have shrunken HI disks. Furthermore, the higher resolution maps presented by Cayatte et al. (1989) show peculiar details of the HI distribution. Galaxies in the western half of the cluster are typical larger in HI than objects in the eastern part, and several galaxies show HI asymmetry with a sharp edge on the side of the galaxy oriented toward M87 and a more extended distribution on the opposite side. In Virgo, we have the opportunity to identify and study galaxies in a current stripping stage.
The proximity of the Virgo cluster makes it possible to examine the molecular content, as derived from the millimeter transitions of CO, in galaxies at different distances from M87. Two major surveys of CO in Virgo spirals have been contributed by Kenney and Young (1986) and by Stark et al. (1986). Galaxies that are HI deficient have systematically larger ratios of CO flux to HI flux. If the normal conversion from CO flux to H2 mass is applied, the core spirals that are deficient by a factor of ten or more in HI may be gas-poor (HI + H2) by only a factor of two to three. Some spatial mapping has been performed so that an estimate of size of the molecular cloud distribution relative to the HI and optical disks can also be made. The spatial distributions of CO in HI poor galaxies are not smaller than those of similar objects seen in the field. It appears that the peripheral diffuse HI clouds are swept away, but the high density molecular clouds remain relatively undisturbed throughout the process.
Another estimate of the dust content within galaxies is provided by the far infrared emission detected by the Infrared Astronomical Satellite (IRAS), although the precise derivation of dust mass is complicated by the mixture of dust components at different temperatures and the unknown size distribution of the grains. In order to relate the dust and gas properties, Doyon and Joseph (1989) have found that the HI deficient galaxies in Virgo have lower 60 and 100 micron fluxes and inferred temperatures that are cooler than those with normal HI content. With the caution that several factors could play critical roles in the observed emission at the IRAS wavelength bands, those authors conclude that at least half of the cool diffuse dust has been removed from typical Virgo core spirals. This cool dust, of course is the cirrus identified with the atomic hydrogen component of the interstellar medium. Indirectly, van den Bergh (1984) has explained the inclination dependence of deviations of individual galaxies from the mean blue Tully-Fisher relation in terms of a lowered dust content among Virgo spirals.
Numerous authors have pointed out peculiarities in the properties of Virgo galaxies that can be explained by morphological alteration. Peterson et al. (1979) have claimed that the optical disks of early type galaxies in Virgo are smaller than those in the field. Bosma (1985) has examined the disk diameters of Virgo members at the same surface brightness limits and finds that although some field spirals have low surface brightness extensions, no Virgo galaxies have large outer disks. Forman et al. (1979) interpret individual galaxy X-ray sources (e.g., M84, M86, NGC 4388) as ram pressure sweeping events.
There are numerous other clues as to the star formation rate and history in Virgo cluster objects that are likely to be of relevance. Stauffer (1983) has found a much higher occurrence of the anemic phenomenon among Virgo spirals than his field sample. Kennicutt (1983) notes that the average Virgo spiral of a given morphological type is redder by about 0.07 in (B - V) than a field object of the same class. Taken alternatively, for the same color, a Virgo spiral appears morphologically about half a Hubble class later than a field spiral. Kennicutt points out that, in order to account for anemia and the color shift, one must both get rid of the blue galaxies and make all the galaxies redder. Kennicutt and Kent (1983) derive a significantly reduced high mass star formation rate for Virgo core galaxies from integrated H measurements. Indeed, the HI deficiency in Virgo is seen to correlate with the (B - V) colors, the disk line emission and the (U(242 1Å) - V) colors in the sense that the HI poor galaxies are always redder (Guiderdoni and Rocca-Volmerange 1985). Even the dwarf distribution appears to have been reddened by a reduction of star formation (Gallagher and Hunter 1986). Kennicutt (1983) suggests that the reddening can be explained as a reduction in the star formation rate by a factor of two about 109 years ago.
The combination of all of this evidence leads us to believe that spirals that pass through the center of the Virgo cluster can lose as much as ninety percent of their HI mass and suffer a reduction of their star formation rate by about a factor of two. At the same time, the molecular constituent remains relatively intact. At larger distances, we are not able to study the stripping event in as much detail as in Virgo, but can use our knowledge of the Virgo cluster to ask the appropriate questions.