Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...272...77b&link_type=abstract
Astronomy and Astrophysics, Vol.272, NO. 1/MAYI, P. 77, 1993
Astronomy and Astrophysics
Astrophysics
65
Scientific paper
A new search for globular clusters carried out in the inner regions of M 31 (within about 1 kpc from the nucleus, ~ 5.5 arcmin) has yielded 3 Bo-class A, 1 B, 20 C, and 20 D new candidates never included in any previous list (see Battistini et al. 1987), and has allowed us to revise the classification of all those already detected in the same area. The total sample of possible candidates ever proposed so far has then been carefully revised, leading to the definition of the "current best sample" of M 31 cluster candidates including 341 objects. By considering the numbers of globular clusters detected in M 31 and in the Galaxy over annuli at increasing galactocentric distances, a total population ratio of about 2.5 +/- 1.0 (conservative error) has been adopted, and the total number of M 31 clusters outside a radius of ~20 kpc should be ~65. In particular, 15-20 new clusters should be detected in the annulus 20-30 kpc, partially scanned by the available surveys, but the task is difficult since most of them should be fainter than V = 18 and Palomar-like. A second sample, the "adopted best sample", thought to be sufficiently uncontaminated, homogeneous, and complete (up to about 25 kpc from the M31 nucleus and down to V ~ 18) and including 298 clusters, has then been defined to study the projected cluster density profile and to address the long-standing question of the stalled missing globulars with respect to the centrally extrapolated R^1/4^-law. The analysis of the spatial distribution has shown that: i) Adopting a R^1/4^-law, the fitted effective radius of the system, R_ef_, is highly sensitive to any variation of the adopted sample but it does not differ significantly from the observed value, R_eo_ ~ 4.59 kpc if a sufficiently unbiased sample is used. However, various tests show that the proposed R^1/4^-law is not necessarily the best fitting description of the observations. ii) Assuming R_e_ = R_eo_, a best fitting procedure based on R^1/m^-laws yields m ~ 1.6 as the optimal solution. This would imply that the spatial distribution of the globular clusters in the central regions of M 31 is much flatter than the behaviour typical of the R^1/4^-law. iii) Fitting the data with R^-n^-laws turns out to be worse for any value of n, the best being n = 0.9 if equal weights are assigned to all radial bins. iv) Various tests show that the best fitting description of the observations, the quality of the fit, and the deduced parameters are significantly depending on the adopted errors and weights. v) If the validity of the R^1/4^-law is imposed outside a fixed galactocentric distance (3,2,1,kpc), a residual central flattening persists even after this new search. vi) The same procedure applied to the sample of Galactic globular clusters leads to similar results suggesting, if confirmed, that the effective radius of the visible halo formed by the stellar spheroidal component of the parent galaxy is smaller than that of the globular cluster system. However, the separation of true halo globulars from other globular clusters in M 31 is problematic and this may still affect the results.
Battistini Pier Luigi
Bònoli Fabrizio
Casavecchia M.
Ciotti Luca
Federici Luciana
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