The present observations demonstrate that the amount of gas in the bar region is very small. It is likely that any available gas has been transported inwards as a result of shocks at the leading side of the bar (Athanassoula 1992). Since the gas density is so low this implies that the bar must be rather long lived in order to have had enough time to transport all the gas to the centre. In addition this means that the galaxy, at least the central region, must be undisturbed which is substantiated by the very regular optical image of the galaxy.
Even though NGC 3992 exhibits some specific features in its rotation
curve, a decomposition cannot give a tight constraint on the
contribution of the disc to the total rotation. As noted above,
the determined maximum disc M/L ratio is rather large compared to
other galaxies while one might have expected the opposite. For such
a problem there is always a way out: put NGC 3992 at a larger distance
and so behind the UMa cluster. To make the M/L ratio
equal to that of
the highest values of the other but similar galaxies of the cluster,
the distance to NGC 3992 has to be increased from 18.6 to
24 Mpc. To make the M/L ratio equal to the average
value of the other HSB galaxies, the distance needs to be
28 Mpc. That distance would still be compatible with the
position of NGC 3992 and the position of its companions on the
Tully-Fisher relation (see Paper II).
In a recent numerical study of normal galaxies, including
gas and star formation (Bottema, in preparation) it is
demonstrated that for discs with an average thickness the bar
instability sets in for
.
On the other hand for a relatively thin disc, which
implies a disc with a low stellar velocity dispersion, the bar
instability already occurs at lower
values.
Therefore, if NGC 3992 would have a less massive disc the
formation of the galaxy must have been rather specific.
Although we now enter the realm of speculation, one might
for example imagine a slowly and gently forming disc with modest gas content.
Star formation is low and disc heating by the few molecular clouds
proceeds slowly. In such a cold stellar disc, even if it is not maximal
a bar can be generated and in the absence of substantial gas accretion
can remain for a time comparable to the lifetime of the galaxy.
To further investigate the matter of bar existence and formation one should resort to numerical calculations. As mentioned above the main parameters governing the bar instability of a disc are the ratio of dark to luminous matter and the disc thickness. Unfortunately matters are more complicated because the past history of a galaxy will be of influence on its present morphology.
Not only for NGC 3992 but also for galaxies in general the main question which remains unanswered is: what is the precise ratio of dark to luminous matter? With the advent of large telescopes it now becomes feasible to do more detailed and more extensive observations to determine the stellar velocity dispersions of galactic discs. In that way the results for the sample of Bottema (1993) should be checked and extended.
Because of the large amount of observational material it was decided to split up the description of the NGC 3992 group in two parts. This part (Paper I) deals with the main galaxy and focuses on its barred nature and mass distribution. In Paper II the observations of the three small companions are described. Velocity fields and rotation curves are derived and rotation curve decompositions have been made. For all the four galaxies of the group an analysis is presented of colours, M/L ratios, and position of the galaxies in the TF relation of the Ursa Major cluster.
Finally a compilation of the main conclusions of this paper:
Acknowledgements
The observations presented in this paper were obtained with the Westerbork Synthesis Radio Telescope (WSRT) which is operated by the Netherlands Foundation for Research in Astronomy (NFRA). We thank R. Sancisi, J. Gerssen, and I. Garcia-Ruiz for insightful discussions. R.B. thanks the Kapteyn Institute for hospitality and support.
Copyright ESO 2002