The origin of the gas disk in NGC 5266

The two orthogonal gas distributions in NGC 5266 have probably settled into two stable planes: this is characteristic of a triaxial galaxy with a stationary potential (Heiligman & Schwarzschild, 1979). However, although most of the HI lies in these two planes, there are some hints in the data that the two disks might be part of a single warped structure. If so, NGC 5266 would resemble some of the classical warps such as NGC 3718 (Schwarz, 1985) or NGC 660 (Gotterman & Mahon 1990; Arnaboldi & Galletta 1993), though with the important difference that relatively little gas is found at intermediate radii. Possibly this means that the warp is `breaking up' into two disks. Perhaps, the warp is due to slow figure rotation which would have to be retrograde, as suggested by Varnas et al. 1987.

A puzzling feature of NGC 5266 is the origin of the huge amount of HI. It is generally believed that the HI in elliptical galaxies has an external origin (e.g. Knapp et al. 1985). If NGC 5266 is the result of a merger, it seems likely that two gas-rich spiral galaxies must have been involved in order to provide the large HI mass that we observe. Furthermore, a large amount of neutral gas must have survived the collision without forming stars. NGC 5266 shows, indeed, a ratio of the total hydrogen mass to the total (blue) luminosity () which is a typical value for spiral galaxies but much higher than seen in most ellipticals.

The numerical simulations for a merger of two spirals presented by Hibbard & Mihos 1995 for the case of NGC 7252, show that a relatively old merger (t > 1.2 Gyr) can have a similar gas morphology to that observed in NGC 5266, i.e.\ part of the HI is settling in the galaxy while the tidal arms are also still present. Thus, it seems likely that NGC 5266 is a similar system to NGC 7252, but at a later stage in its evolution.

If this is the case, NGC 5266 would have already passed the period of high activity and rapid star formation that is believed to result from the merger of two spirals. This is supported also by its IR characteristics. The very low rate of current star formation in this object (despite the large mass of HI) could be due to the low surface density of HI gas, which cannot support star formation (Kennicutt, 1989).

© Copyright Astronomical Society of Australia 1997