Distribution and Kinematics of the HI in NGC 5266

We have observed NGC 5266 with four different ATCA configurations (375m and three 1.5km configurations) and we detect a total HI mass of , which agrees with the Parkes single-dish measurements made by Varnas et al. (1987).

 
Figure: Map of the total HI distribution (contours) superimposed to the optical image from the Sky Survey.

The total HI map is shown in Fig. 1 superimposed to the optical image. It is clear that the neutral hydrogen in NGC 5266 is very extended. The total extent is about 16 arcmin, i.e. about 140kpc corresponding to times the optical half-light radius on each side. The most striking feature is that the HI is elongated along p.a., i.e. nearly perpendicular to the orientation of the dust lane (p.a.). Although in most dust-lane and polar-ring galaxies where HI is been observed, it lies in the same plane as the dust lane or ring, the presence of an outer HI ring perpendicular to the inner dust lane has been observed before in a few cases (NGC 2685, Peletier & Christodoulou 1993; or in NGC 5128, Schiminovich et al. 1994). From the position-velocity map at p.a. (i.e. major axis, Fig. 3) it is possible to see that there are actually signatures of three distinct structures. The first is a central fast-rotating component associated with the dust lane. In the position-velocity plot along the minor axis (not shown here) this fast-rotating component appears to be spatially resolved with the western side of this structure redshifted compared to the systemic velocity, i.e. it rotates in the same sense as the ionized gas and CO. The second structure is the main HI component which can be traced out to 4arcmin radius. It has a quite regular velocity field implying that this component is a rotating annulus roughly perpendicular to the dust lane. Interestingly, a high-contrast optical image of NGC 5266 from D. Malin (Fig. 2) shows that there is a good correspondence between the faintest optical isophotes and the main (elliptical-shaped) neutral gas structure, implying that it has a faint optical counterpart. The third component corresponds to the two outermost arm structures that could represent either an edge-on ring or two tidal arms.

 
Figure: Total HI (contours) superimposed to a deep optical image (grey).

Thus, the data suggest that the neutral hydrogen inside 4arcmin lies in two orthogonal planes. This has been confirmed by a simple modelling of the HI\ kinematics where we have assumed that the neutral gas inside 1 arcmin is coincident with the dust lane, but that somewhere further out it shifts to a perpendicular plane. The position angle of this outer plane happens to be very close to the position angle of the stellar major axis (Caldwell 1984; Varnas et al. 1987), consistent with the idea that the outer HI distribution has settled in a symmetry plane of the stellar galaxy. The transition from one plane to the other lies just beyond the optical dust lane and is quite abrupt, though there is evidence for a small amount of gas ``connecting'' the outer region of the optical dust band and the inner part of the main HI\ disk.

If we assume the rotation velocity to be constant with radius, we find the best fit for km , which is close to the values determined optically and from CO observations. Our modeling is also telling us that the rotation velocity can not be much lower than 250 km .

The HI emission from the outermost regions (>4 arcmin) cannot be reproduced by our simple model suggesting that the outer parts might be tidal tails of neutral gas formed in a merger, and therefore very unrelaxed structures. Alternatively, they could be part of an outer, edge-on disk or ring, as some characteristics of their kinematics seem to suggest.

Assuming a constant rotation velocity of 270km and a spheroidal mass distribution with an intrinsic flattening of q= 0.6 (as indicated by the optical image of the galaxy), we can derive the mass of the galaxy. Adopting an law light profile with (Varnas et al. 1987), we find that the derived mass-to-light ratio ranges from 4 at 1arcmin radius to = 8 at 4arcmin. Comparing our derived values with that measured for the central from optical observations ( = 3.0 at 0.3, Bertola et al. 1993), we find that increases by a factor between the centre and . If we use the outermost HI to estimate the mass-to-light ratio beyond 4arcmin radius, we derive 16 within 8arcmin radius. However, as discussed earlier, it is likely that the two outer HI structures are not in equilibrium.

 
Figure 3: Position-velocity map obtained from a slice in p.a.= and width 90 arcsec. The contour levels are -0.011,0.011 to 0.08 in step of 0.006 mJy/beam. The velocity is in units of m s

© Copyright Astronomical Society of Australia 1997