Spiral galaxies with unusual faint features.

Here we present examples of gas-rich galaxies without obvious interactions or companions that nonetheless show evidence of interaction. A nearby example is NGC 253, in the Sculptor group. The extent of the optical halo of this galaxy is much greater than that of the neutral hydrogen envelope detected by Koribalski et al. (1995), and it also extends well beyond the fields where Hawthorn has detected diffuse H-alpha emission (unpublished). Fig. 4 is the result of combining the photographically amplified derivatives of five deep IIIa-J (ie. blue-green) plates taken with the UK Schmidt Telescope. The image has a limiting surface brightness of about 28 mag/arcsec and shows that the low surface brightness envelope is devoid of fine detail and is unusual in not being strongly truncated, as is the case for most spirals. However, it also shows a distinct, asymmetrical extension in the southern half of the galaxy. This is difficult understand, since NGC 253 is not obviously interacting with other members of the Sculptor group, and no other structures in the disk of the galaxy, or in the HI velocity profile, suggest any kind of external disturbance.

Preliminary calculations by Hawthorn (unpublished) show that neither line emission nor scattered light can account for the faint light in the 400-530nm passband recorded here, so the most likely explanation is that we are seeing an extended, asymmetrical stellar halo, perhaps distorted by the infall of a companion galaxy which has long since been absorbed. However, it should be noted that the southern part of the galaxy, where the anomalous extension is most obvious, is also the region with the strongest polarisation at 1.4 GHz, and the 0.33 GHz diagram also shows a southern spur. Since NGC 253 is seen almost edge-on, the anomalous extension could be a line-of-sight loop or arc effectively detached from the main body of the galaxy, either the result of an ancient encounter, or, perhaps the debris from the incomplete disruption or absorption of a Magellanic Cloud-like companion.

M83 (NGC 5236) is a spectacular face-on spiral galaxy. Colour photographs show it to have clumpy, well-defined spiral arms, evidently rich in young blue stars. The galaxy is remarkably symmetrical and has no obvious companions or evidence of interaction. However, deep images (Fig. 5) reveal an extensive but very faint arcuate structure to the north west of the galaxy, clearly centred on it, but completely disconnected from it. The arc has a maximum projected distance of about 55 kpc from the nucleus of M83 and is of a similar length, though this is very difficult to determine precisely. The structure was first noted some years ago by Cannon (private communication). It is apparently devoid of the fine detail typical of the extensive but faint Galactic reflection nebulosity which is detectable in the Centaurus-Hydra direction and is seen on many deep plates taken on different centres, so is certainly real.

The galaxy also has a very extensive HI halo with 80 percent of the detected HI beyond the Holmberg radius, much more than is normal in such galaxies (Huchtmeier and Bohnenstengel 1981). A distinct ridge in the HI velocity profiles in seen in their data, at a position that corresponds to the optical loop, but extending well beyond it. If M83 were strongly inclined to the line of sight, it may well be that the faint loop would appear as an extension to one end of the galaxy, rather like that in NGC 253.

The well-known Sombrero galaxy, M104 (NGC 4594) shows no signs of interaction even on the deepest optical images, nor is there anything unusual in the HI distribution or velocity structure, and though the HI mass is unusually small, there is no evidence of large radial velocities (Bajaja et al. 1984). However, M104 shows an unexpected faint feature at a considerable projected distance to the SSW of the galaxy (position angle about 200 degrees). This takes the form of a faint, loop (Fig. 6) which appears to be associated with M104, but, like that in M83, seemingly detached from it. The loop is at a projected distance of about 20 arc min, extending 100 kpc from the nucleus if the distance to M104 is taken as 18.6 Mpc. The loop is sharp-edged and quite distinctive, but in the image reproduced as Fig. 6 there is also a strong suggestion of a more diffuse faint structure of similar extent to the NNE of the galaxy, diametrically opposed to the loop at position angle 20 degrees.

There is no previous mention of this feature in the literature, and there is no evidence of these structures in the contours of M104 reproduced by Burkhead (1979). However, Burkhead's contours, which extend to 28 mag/arcsec, were red-light exposures aimed at tracing the extent of the red bulge of M104, while the image reproduced in Fig. 6 was made on IIIa-J plates which would tend to emphasise blue features.

Our final example of a spiral galaxy with unexpected faint features is NGC 2855, an SA(r) galaxy remarkable for its ordinariness. It appears in many lists of galaxies in the literature but is not singled out as in any way notable. However, combined multiple exposures reveal an enormous, faint, almost complete loop or perhaps a disk seen in projection. Like most of the faint features noted here, it has a surface brightness fainter than 28 mag/arcsec. At this stage, that is all that can be said about it, but the enormous extent of this feature makes NGC 2855 worthy of further study.

© Copyright Astronomical Society of Australia 1997