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Astron. Astrophys. 356, 127-133 (2000)

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4. Summary and conclusions

We investigated the presence and significance of a tidal extension of the brightness profile of M 92. The main results of our study are:

  1. The presence of an extra-tidal profile extending out to [FORMULA] from the cluster center, at a significance level of [FORMULA] out to [FORMULA]. We found no strong evidence for preferential direction of elongation of the profile. This may imply that we are detecting the extra-tidal halo of evaporating stars, which will later form a tidal stream. Moreover, the tidal tail might be compressed along the line of sight -see, for instance, Fig. 18 of G95. In fact, G95 point out that tidal tails extend over enormous distances ahead and behind the cluster orbit, and the volume density is subject to the open-orbit analogous of Kepler's third law: near apogalacticon, stars in the tidal tail undergo differential slowing-down, so that the tail converges upon the cluster. Actually, most models (e.g., Murali & Dubinski 1999) predict that the extra-tidal material should continue to follow the cluster orbit and thus take the shape of an elongated tail, or a stream. The stream has been already revealed in dwarf spheroidal galaxies of the local group (Mateo et al. 1998), but whether the stream can also be visible in significantly smaller objects like globular clusters is currently a moot point.

  2. By constructing the surface density map and performing a Gaussian smoothing, the low-frequency features are enhanced over the background. We find some marginal evidence for a possible elongation in the extra-tidal extention based on a visual inspection of this map. This elongation may be aligned in a direction perdendicular to the Galactic center, although we already know that the significance of this result is low; additional observations will be required to settle the issue. A similar displacement is described in Fig. 3 of Johnston (1998).

Finally, we want to stress the power of the DPOSS material in conducting this kind of programs, either by using the standard output catalogs, as they come out from the processing pipeline, or the specific re-analysis of the digitized plate scans. In the future we will extend this study to most of the globular clusters present on the DPOSS plates.

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000