M 87 (NGC 4486) is the largest, most massive galaxy in the Virgo cluster. Its radial velocity is km s-1 (de Vaucouleurs et al. 1991) and it is likely to be the central object of the cluster. Arp & Bertola (1969) suggested a diameter of almost 1 at the 27 B mag arcsec-2 isophote level, also an image of its faint envelope has been presented by Kormendy & Bahcall (1974). Weil et al. (1997) presented a B-band image of M 87 by photographically co-adding five IIIa-J UK Schmidt plates. An asymmetry was seen in the form of a diffuse "fan" of stellar material extending out to about 100 kpc to the SE along the projected major axis. They show that accretion of a small spheroidal galaxy into a larger potential can account for this structure.
A normal contrast image of M 87 from the co-added data array is shown in Fig. 1a, in which four neighbouring galaxies are indicated and enlarged at higher contrast. A high contrast image of the same field is also presented in Fig. 1b. The approximate dimensions of the halo in this field are , ( kpc2), to a surface brightness of approximately 28 R mag arcsec-2, which is large, but not as large as some cD haloes (e.g. A3571, Kemp & Meaburn 1991a, which is of the order of 200 kpc2), although it shows the same increase of ellipticity with radius as most cD haloes. The asymmetry of M 87 noticed by Weil et al. is independently confirmed by our image, although it is not as prominent as it is on the blue plates.
|Figure 1: a) Normal contrast images of M 87 and high contrast images of its companions NGC 4476, NGC 4478, NGC 4486b and NGC 4486a. All the small images are the same scale. The 1 scale is for the inset images and the 10 for the main image. b) A deep, high contrast images of M 87, covering the same area as the normal contrast image in Fig. 1a. The dimensions of the frame are|
The dwarf ellipticals NGC 4476, 4478, 4486a and b are all optically close to M 87 in projection (see Fig. 1a). In the present data, there is no obvious evidence for interaction between M 87 and any of these galaxies, as there are no features indicating such interaction at the outer isophote levels of these galaxies (distortions, elongations, filaments etc.). The redshifts of NGC 4476, 4478 and 4486b all indicate that they are likely members of the Virgo cluster, though NGC 4476 would have a high relative velocity of km s-1 compared with M 87. NGC 4486a appears to be a foreground object at km s-1. NGC 4486b has been classified as a prototype compact elliptical (Rood 1965) though the results of Prugniel et al. (1987) suggest that it could equally be classified as a normal elliptical, but they find NGC 4478 to have the truncated profile of a compact elliptical.
We present normal and high contrast images of the central region of the Virgo cluster, covering a field of (see Figs. 2a and b). The giant elliptical galaxies M 84 and M 86 are very prominent in this image, and there are several almost edge-on disk galaxies (NGC 4388, NGC 4402 and NGC 4425) and dwarf elliptical galaxies (NGC 4387 and NGC 4413). This field is in the vignetted region of the films and there is a noticeable variation in the background across this field at high contrast. Linear marks, which can be scratches on an individual film or satellite tracks, can be seen in the field, and there are several blemishes ("white marks") in the SE region. Each of these defects only occurs in one of the set of 13 films, but the variation in backgrounds over the set of 13 films (including the relative variations across films caused by the variations in emulsion sensitivity, see Sect. 3.6) results in median-filtering not being able to remove such features completely, and this is especially noticeable at high contrast in the vignetted areas.
|Figure 2: Normal a) high contrast b) images of the centre of the Virgo cluster of galaxies containing the galaxies M 84, M 86, NGC 4387, NGC 4388, NGC 4402, NGC 4413, NGC 4425. The dimensions of the whole image are|
M 86 (NGC 4406) is a giant elliptical galaxy (SO1(3) E3) with a high negative redshift, -227 km s-1 (Binggeli et al. 1985) compared to the average velocity of the cluster ( km s-1; Faber et al. 1989) and its velocity dispersion (762 km s-1, Rangarajan et al. 1995). Forman et al. (1979) suggest that the galaxy is on a radial orbit, passing through the centre of the cluster about every years. It is also a luminous X-ray object ( 1042 erg s-1) with most of the X-rays radiated by thermal bremsstrahlung from the interstellar medium, and has a well-studied plume of stripped material to the NW (Rangarajan et al. 1995), reaching 12 (60 kpc) from the centre. The dimensions reached by the halo of M 86 in Fig. 2 are ( kpc2) which is comparable to the dimensions of the X-ray halo. Rangarajan et al. (1995) and others have suggested that the intra-cluster medium of Virgo is ram-pressure stripping the ISM of M 86, producing the plume and other features visible at X-ray wavelengths. Nulsen & Carter (1987) reproduce a deep image of M 86 obtained by Malin, showing distortions in its outer isophotes due to regions of excess optical emission associated with features in the X-ray emission, and they suggest that stars are forming from the cooling hot gas in these regions, though Rangarajan et al. (1995) suggest that the excess optical emission is due to scattering of starlight from a high dust concentration.
M 84 (NGC 4374) is a giant elliptical galaxy (E1) with an extended halo of ( kpc2). It has an total X-ray luminosity of erg s-1 detected by Fabbiano et al. (1992) within 1.5 (7.5 kpc) of its centre (i.e. the optical emission is considerably more extended than the X-ray emission). The galaxy appears to be a weak radio source (3C 272.1; Laing et al. 1983). An organised pattern of Faraday rotation is detected by Laing & Bridle (1987) at 1.4 and 4.6 GHz, implying the existence of a magneto-ionic medium in front of the radio-emitting plasma - Laing & Bridle also suggest that this medium may be responsible for the diffuse component of M 84's X-ray emission.
Despite their apparent proximity, our data offers no evidence for an interaction between M 84 and M 86. Although the outer isophotes as seen in Fig. 2b almost overlap, an isophote map shows no particular evidence of distortion or twisting of the isophotes of either galaxy caused by the other, other than that already referred to above for M 86 (which has been interpreted as due to star formation in the cooling hot gas or scattered starlight from dust, rather than due to interaction with M 84). Of course, the very high relative velocity and radial orbit of M 86 makes such an interaction unlikely. Other galaxies in the field also show no evidence of interaction either with M 84 and M 86 or with each other, except for NGC 4388 (see below). Caon et al. (1990) considered NGC 4387, a dwarf E5 box galaxy (Nieto & Bender 1989), to be embedded in a common envelope of the overlapping haloes of M 84 and M 86. However, our data suggests that the haloes of M 84 and M 86 may not overlap physically, and NGC 4387 shows no distortions or features that suggest it is physically interacting with either of them. Also, although NGC 4402 has an optical warped disk detected by Warmels (1988), the deep image (Fig. 2b) appears more normal. A deep, blue (IIIaJ) image including this area was produced by Kormendy & Bahcall (1974), showing suggestions of an extended halo around these objects, although this may be due to non-uniformities in the emulsion. In our data there appears to be a brighter background in the area surrounding M 84 and M 86, hinting at a common halo, but these galaxies lie in the vignetted area of our field, near the NW corner (Fig. 6), and so we cannot be certain about the reality of this feature.
One of the most interesting galaxies of the central Virgo area is the near edge-on Seyfert 2 Sab galaxy NGC 4388 (Binggeli et al. 1985). It has a high systematic velocity with respect to the Virgo mean - its radial velocity is 2523 km s-1 (Binggeli et al. 1993) so may be a non-member (Faber et al. 1989). However Binggeli et al. (1985) classify it as a member, while faint extensions along the major axis, principally to the west (first noted by Phillips & Malin (1982) and visible in Fig. 2b) may be evidence of tidal interaction with other cluster members. Strong, hard X-ray emission (Hanson et al. 1990) and extended, soft X-ray emission (to a radius of 4.5 kpc, Matt et al. 1994) has been detected, although its origin is still uncertain (Antonelli et al. 1997). The dimensions of the halo in Fig. 2b are ( kpc2), so the optical halo is more extensive than the X-ray emission for this spiral galaxy.
M 89, shown in Fig. 3, is an E0 elliptical galaxy with a radial velocity of km s-1 (Binggeli et al. 1985) and is a member of the Virgo cluster. Malin (1979) reported an optical jet-like feature extending 10 from the core of the galaxy, which he detected on sets of photographically amplified and co-added UKSTU IIIaJ and IIIaF plates. He also saw three shell features concentric with the galaxy. All the features were more easily seen on the blue IIIaJ plates than on the red plates. Although the nucleus of the galaxy is a compact radio source, there was no evidence of a radio feature corresponding to the jet (Heeschen 1970). However, as yet unpublished observations at 21 cm obtained with the VLA C array show an extended feature attached to the nucleus of the galaxy and aligned with the optical jet. Sramek (1975 and references therein) found the nucleus of M 89 to be variable at several radio frequencies, while Forman et al. (1985) found an unresolved nuclear point source with an X-ray luminosity of 3 1040 erg s-1.
|Figure 3: a) Normal contrast image of the M 89 galaxy. The dimensions of the frame are .b) High contrast presentation of the result of unsharp masking the M 89 image. The jet-like feature at the SW, the shell at SSE and the diffuse area at the NNE can be seen very clearly. The dimensions of the frame are: .c) High contrast image of the M 89 galaxy covering the same field as Fig. 3a. The diffuse area at the NW (first detected by Malin 1979) and the extended halo of can be seen. The faint straight line extended NE and SW of the SE shell is very likely to be a satellite orbit|
Our images of the jet-like feature and shells from the co-added array are shown in Figs. 3a-c. The normal contrast image (Fig. 3a) shows an elliptical galaxy apparently without unusual features. Figure 3b was produced by masking the original data (for more details see the last paragraph of the data reduction section of this paper). A number of M 89's features (first identified by Malin (1979) in his blue plates) are shown in Figs. 3b and 3c. The jet-like feature, the innermost shell at the SSE (5 from the nucleus), a semicircular patch on the opposite side of the galaxy to the jet ("condensation D" of Malin (1979), only seen on his deepest IIIaJ plates) are seen in Fig. 3b, with a much greater clarity. In the much deeper Fig. 3c one can see the diffuse area at the NW (feature C of Malin (1979) which appears as a circular patch on the R-band images), the "jet", and an extended halo of or kpc. Note that the curved "filament" seen extending to the west of the galaxy is actually a circular defect (caused by reflection of the plate holder) on one of the original films, not quite removed by the median-filtering addition (see Sect. 3.1).
Clark et al. (1987) obtained CCD images of the jet-like feature in R and V at the 1.3 m McGraw Hill telescope. They found the jet to be bluer than the rest of the galaxy by and to have a total luminosity of 0.25% of that of the galaxy. They suggested that both the jet-like feature and shells were produced by a tidal encounter with a smaller galaxy. Similar interactions are well simulated by Hernquist & Quinn (1988) and references therein.
The apparently interacting pair NGC 4435 and NGC 4438 (radial velocities km s-1 and km s-1 respectively; Binggeli et al. 1985) are featured in Arp's catalogue of peculiar galaxies (Arp 1966) as number 120. Kotanyi et al. (1983) present Einstein X-ray observations of the area of NGC 4438 showing diffuse extended emission on the west of the galaxy. Kotanyi & Ekers (1983) also saw such emission in the 1.4 GHz radio continuum. Both sets of authors proposed that the most likely explanation for this emission was that the interstellar gas is being swept out of NGC 4438 by a wind caused by the dense surroundings of M 87 (the projected distance between NGC 4438 and M 87 is 250 kpc). Kotanyi & Ekers (1983) did not believe that there was a tidal interaction between the two galaxies because of the apparent lack of distortion of NGC 4435. However, Combes et al. (1988) present a CO map of NGC 4438 showing a central component and some molecular emission from the NW of the galaxy. Molecular gas cannot be stripped from a galaxy by intracluster medium gas, arguing in favour of a tidal interaction between the two galaxies. A numerical simulation of the interaction of the two galaxies is also presented and they argue that the galaxies's tidal interaction caused the stripping of CO and star formation in the molecular clouds caused the NW X-ray and radio continuum feature. Hummel & Saikia (1991) mapped the centre of the galaxy at 1.49 and 4.86 GHz and detected a morphologically unique (amongst the spiral galaxies) shell-like central radio source. The optical nucleus has been classified as a Liner (by Stauffer 1982; Keel 1983 and Heckman et al. 1983), but may not be the real nucleus, mostly because it does not correspond in position with the radio nucleus. There are indications that star formation in the nucleus is the most likely explanation for its strange properties. Hummel & Saikia (1991) propose that around the nucleus, a bubble of plasma developed, expanded, and escaped by making a tunnel through the surrounding gas.
Keel & Wehrle (1993) found two extended optical emission-line (H and [NII]) gas filaments in the vicinity of NGC 4438. They propose different origins for the two filaments and argue that only a variety of different mechanisms (interaction with the Virgo cluster's intergalactic medium, with NGC 4435 and nuclear energy release), can explain all of NGC 4438's peculiarities. Kenney et al. (1995) also detected these and other ionized filaments in H and [NII] kpc from the nucleus and 10 kpc toward the west and southwest. They proposed that the filaments are shock-excited and as their velocities are much less than the escape velocity for the galaxy, they suggest that these filaments consist of gas disturbed by the collision and returning to the galaxy, and they delineate regions where hot gas is coming into contact with cold gas, creating layers of shocked gas at the interface. They further suggest that the features of the disturbed ISM of the system could be produced by a high-velocity ISM-ISM collision between the massive gas-rich galaxy NGC 4438 and the less massive, less gas-rich galaxy NGC 4435 (i.e. resulting from the likely tidal interaction between the two galaxies as simulated by Combes et al. 1988). NGC 4435 suffered a more severe effect from the collision. According to Kenney et al. (1995), most of its gas is captured by the much bigger companion, and they expect deeper images to show a more disturbed halo morphology for this galaxy.
Malin (1993) presented a deep image of the system of the two galaxies and detected a faint tidal tail of stars, north-northwest, pointing away from the centre (see also Kenney et al. 1995, although note that this feature is also clearly visible in the image presented by Phillips & Malin 1982). Figure 4 presents a normal and a high contrast image of the two galaxies; the tidal tail detected by Malin (1993) can be easily seen extended up to (kpc) to the NNW of NGC s4435 in the very high contrast image, although there is no particular further evidence for the distortion of the halo of NGC 4435 (both galaxies however share a common halo in projection). A newly-discovered filament of similar length can be seen extending to the SW of NGC 4438 (it is located within the curve marked in Fig. 4b). This new filament is an extension of the SW tidal tail seen in the normal contrast image. There are hints of other possible radial filaments immediately to the east of this.
|Figure 4: High a) and very high b) contrast images of the interacting galaxies NGC 4438 and NGC 4435. The dimensions of the frame are|
The expectations of Kenney et al. (1995) that NGC 4435 has the more disturbed halo are not confirmed by our deep images, although the most prominent filament (to the NW) is associated with it. These galaxies lie in a film area affected by variations in the background and many film defects not removed by the median-stacking process. Nevertheless the two faint tidal tails to NW and SW are clearly visible, there are hints of others, and it is possible that more such features could be visible in a deeper image and a more detailed study of the morphology of the halo regions could be carried out.
Arp (1966) was the first to mention the apparent interaction of the IC 3481, IC 3481A & IC 3483 galaxies. This interaction is displayed in Fig. 5, which contains three images at different contrast, which display a number of remarkable newly-detected faint features. The normal contrast image, Fig. 5a, is similar to the one presented by Arp (1966), although much more extended; the interaction between IC 3481 & IC 3481A is visible, but the tail towards IC 3483 is very faint. On the higher contrast image, Fig. 5b, the tail of the interaction and its properties are clearly seen: a loop-like structure is formed with one apex apparently just touching IC 3481A, and the other apex apparently close to IC 3483. On the very high contrast image, Fig. 5c, one can see the extent of the interaction around the IC 3481A galaxy and the common halo surrounding IC 3481 and IC 3481A. The details of how the loop-like structure connects (or not) to IC3483 is hidden by the halo of the bright foreground star. R magnitudes of 12.9, 15.9 can be measured from the CCD data obtained from the JKT, for the galaxies IC 3481 and IC 3481A respectively. Only a lower limit of 16.5 mag for the newly-detected faint features can be reported as the frame of the CCD data was not extensive enough.
|Figure 5: Normal a), high b) and very high c) contrast image of the IC 3481, IC 3481A and IC 3483 galaxies. The details of the interaction (e.g. loop-shaped tail) can be seen on the high contrast image and the extent of the phenomenon on the very high contrast image. The dimensions of the frame are|
A large-scale variation in background photographic density is apparent in a high contrast image of the co-added array (see Fig. 6). Inspection of scans of individual films reveals that the morphology of this background variation changes considerably from film to film, suggesting that, for individual films at least, that it is dominated by variations in emulsion sensitivity across the film.
We further investigated this large scale variation using three IIIaF plates of the north-east area of the Virgo cluster also taken with the UKST. This second field overlaps with our Virgo SE field. The OG590 filter (described above) was used for the IIIaF plates to give results close to the standard R-band (UKSTU handbook). We co-added the scans of three IIIaF Virgo NE plates using exactly the same procedure, as discussed in Sect. 2.
The morphology of the possible background emission in Fig. 6 is such that the brightest regions are seen in the central parts of the cluster towards the north and north-western parts of the field. As this is much what we would have anticipated, it appears that we could be detecting genuine optical, intra-cluster, medium emission, in which case the co-addition with median-filtering of the thirteen film scans would not be dominated by the variations in sensitivity across individual films. However, the reality of this most extensive emission must remain uncertain until further observations are made.
|Figure 6: Very high contrast image of the whole () field covered by the Schmidt films. A large scale, marginal, variation in photographic density is apparent. This may be a real detection of the luminous, inter-galactic, medium, or simply an artefact as consequence of the variation in sensitivity over this large field|
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