Astron. Astrophys. 324, 485-489 (1997)

4. Discussion

In the inner UGC 10205 we are facing two main kinematically distinct gaseous components, namely (i) and (ii). They have quite similar velocity dispersion profiles but very different velocity curves, which produce the "figure-of-eight" appearance of UGC 10205 velocity curve.

What is the real spatial distribution of these two components? Are they really cospatial or are they spatially distinct and seen superimposed on account of a projection effect?

The simultaneous presence of the two gas components at the same distance of the galaxy center raises the problem of the viscous interaction of distinct gaseous structures with different kinematical characteristics. One possibility is the gas to be distributed in collisionless cloudlets, as suggested by Cinzano & van der Marel (1994) to explain the ionized gas kinematics in the E4 NGC 2974. If this is the case, we would expect the slow-rotating component being supported by a velocity dispersion higher than that we observed.

We are left with the interpretation already given by Miller & Rubin (1995) for NGC 5907 and by Kuijken & Merrifield (1995) for NGC 5746 and NGC 5965 that the two gas components are spatially distinct and viewed superimposed along the line-of-sight due to the high inclination of the galaxy. The linear rise of the velocity curve of component (ii) up to the points of conjunction with those of component (i) is due to the so-called "rim of the wheel" effect when viewing at an enhanced ring structure. Moreover, the radial trend of line intensity derived for the two components seem to confirm this interpretation. Indeed, the intensity of component (i) is peaked in the center, while the intensity of component (ii) is almost constant as we are expecting if component (ii) is actually an outer ring.

The above gas configuration is the one expected in a barred galaxy. Indeed the bar exerts a torque on the disk gas, which is slowly drifted from the regions around the corotation radius towards the Lindblad resonances to form rings. Kuijken & Merrifield (1995) and Merrifield (1996) showed the line-of-sight velocity distribution (LOSVD) in function of the projected radius for the closed non-intersecting orbits allowed by a barred disk potential in edge-on galaxies. (Due to its collisional nature, the gas moves only onto the closed non-intersecting orbits.) They found the LOSVDs with the characteristic "figure-of-eight" variation with radius. For this reason they considered the gaps in such LOSVDs as the signature of the presence in the disk of the gas-depleted regions due to the bar. The gas components (ii) in the velocity curve of UGC 10205 is produced by the ring formed at the outer Lindblad resonance.

The third gas component observed in the inner regions of UGC 10205 is very peculiar. It is present only on the SE side and it has a velocity ranging from to if reported to the systemic velocity of the galaxy. So it is not moving around the galaxy center in circular orbits. It could be associated to the faint features embedding UGC 10205 and visible in the R -band images shown by Rubin (1987). Its kinematics can be explained if such gas moves onto an elliptical orbit, which at the projected distance of has its tangent perpendicular to the line-of-sight. The gas of component (iii) populates only a portion of this orbit. From the available data we can not infer the proper distance of component (iii) from the galaxy center.

Outside we observe single-peaked emission lines produced by disk gas in near-circular motion. For radial distances greater than that of the outer Lindblad resonance we expect the gas to be only little disturbed by the inner triaxial potential. Because of the edge-on orientation of the disk with respect to the line-of-sight, if the gas is distributed uniformly decreasing throughout the disk we would expect to observe emission lines peaked at the local circular velocity with an asymmetry towards the lower velocities. This emission feature is present in UGC 10205 in the form of very low luminous intensity. It is superimposed to a major emission that does not show any systematic deviation from the Gaussian shape. Irregularities in the velocity curve can be easily produced by the fact of the integration along the line-of-sight being the galaxy seen on edge.

The interpretation of Kuijken & Merrifield (1995) has been recently adopted by Merrifield (1996) for NGC 2683 observed by Rubin, and by Bureau & Freeman (1996) for IC 5096. It could also be applied not only to UGC 10205 but also to the case NGC 5907, extending the explanation given by Miller & Rubin (1995). Since their two spirals have peanut-shaped bulges, Merrifield and Kuijken (1995) suggested a connection between the peanut bulges, which are detectable only in edge-on galaxies, and the bars, which are easily detectable in more face-on systems. Anyway, while NGC 2683, NGC 5746, NGC 5965 and IC 5096 have boxy/peanut-shape bulges, for NGC 5907 and UGC 10205 this crucial photometric information is still not available.

The presence of the "figure-of-eight" in gas velocity curves of edge-on spirals seems to be unrelated to the morphological type in the spiral sequence since in RC3 UGC 10205 is classified Sa, NGC 2683, NGC 5746 and NGC 5965 are Sb, IC 5096 is Sbc, and NGC 5907 is Sc.

The geometrical distribution of the gas giving rise to the "figure-of-eight" velocity curves could be used in the interpretation of phenomena like the one described by Plana & Boulesteix (1996) in the S0 NGC 7332. It is an edge-on galaxy with a boxy-shaped bulge (Fisher & Illinghworth 1994). Plana & Boulesteix (1996) separated clearly two extended coplanar counterrotating components of ionized gas, which were previously detected by Fisher & Illinghworth (1994). One of these components has a linearly increasing velocity curve ("rim of the wheel" effect). If we imagine to invert the sense of rotation of this latter component, a typical "figure-of-eight" velocity curve is obtained. Therefore a gas distribution like that of the above-mentioned edge-on barred spirals but with an outer ring counterrotating with respect to the inner gas could reproduce the kinematics of NGC 7332, for which a satisfactory interpretation has not been given. Of course, theoretical modelling should justify this situation.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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