Astron. Astrophys. 332, 71-76 (1998)

4. Conclusions

In order to better interpret Fig. 7, we calculated the curve [mean b,l] to quantify the elevation of the Local Arm over the mean plane. This is depicted in Fig. 8.

Fig. 8. Mean latitude for the profile of star counts after subtracting the exponential component of the disc. The Local Arm begins at l= .

As the projected configuration of the arm is reasonably well known from Becker & Fenkart (1970) we have also plotted [mean z,l] in Fig. 9. The projected configuration of the arm is a logarithmic spiral with parameters taken from Wainscoat et al. (1992). Furthermore, longitude can be translated into real distance S, which is exactly the length along the arc, taking as the origin a point in the arm where , about 3000 pc away. Fig. 10 shows the curve [mean z,S ]. The arm is elevated about 70 pc and this elevation decreases when the arm comes closer to the Sun, where the elevation practically vanishes. The other part of the arm lies in the direction around and cannot be observed in the northern hemisphere.

Fig. 9. Mean height of the Local Arm in galactic longitude.

Fig. 10. Mean height of the Local Arm related to the real distance: the length along the arm.

We have also calculated the width of the arm, and Fig. 11 shows the [width,S ] curve. It must be interpreted with caution: as scans are limited in latitude () the arm width and the arm mean position are underestimated. We have seen that the angular width of the arm becomes very large close to the Sun. But when angles are translated to real widths, we see that the arm is thinner as the Sun is approached. At S =0 the width is larger than 200 pc.

Fig. 11. Mean width of the Local Arm in relation to the distance along the arm.

In Fig. 7 a sharp peak is observed around . The identification of this feature is not easy but it is clear that it corresponds to a real peak in the number of stars found in that direction, and not to an error in the reduction processes. Such a peak is not observed in the DIRBE data. Possible explanations should take into account the patchy nature of the absorbing clouds, and the different resolution of DIRBE and our data, but we have not found a relation with any other known feature of the galaxy, nor with detectable especial features in CO or 21 cm maps.

At a galactic latitude around , a large and unexpected deviation of the arm with respect to the mean plane prevents us to obtain reasonable values of the mean position of the Local Arm, so that the values in our Fig. 8 are highly underestimated. The Arm is to a great extent outside the small observation window of . Clearly, future observations should be carried out for in order to assure better observing conditions and better quantitative results. Nevertheless, even if incomplete, our map in Fig. 8 constitutes a first observation at this wavelength. This fact together with the substraction of the foreground sources which has been carried out, makes our observations an important source of information to study the closest known spiral arm.

There is a very noticeable agreement between our [mean z,l] curve and the similar one obtained by Kolesnick & Vedenicheva (1978). This agreement is mainly due to the fact that we are actually observing young stars. Near infrared surveys are usually used to trace the distribution of old stars, but due to the proximity of the Local Arm, what we are mainly observing are OB stars with . Nevertheless observations at different wavelengths must be obtained even if they confirm the optical description. Our map, on the other hand, is uncontaminated by other sources, i.e. by the exponential component of the disc, warp included. This is important, as the positions of the arm and the north warp maximum are found in the same direction.

Fig. 12 represents the geometrical configuration of the Local Arm, when viewed from approximately the galactic centre.

Fig. 12. Geometrical configuration of the Local Arm viewed from the galactic centre.

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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