The surface brightness profile of the remote cluster NGC 2419 (Research Note)

Abstract

Context. It is well known that the bright and remote Galactic globular cluster NGC 2419 has a very peculiar structure. In particular its half-light radius is significantly larger than that of ordinary globular clusters of similar luminosity, being as large as that of the brightest nuclei of dwarf elliptical galaxies. Aims. In this context it is particularly worthwhile checking the reliability of the existing surface brightness profiles for this cluster and the available estimates of its structural parameters. Methods. Combining different datasets I derive the surface brightness profile going from the cluster center out to ≃480", i.e. ≃25 core radii (r c). The profile of the innermost 21" has been obtained from aperture photometry from four different Hubble Space Telescope ACS/WFC images. Outside of this radius, the profile has been obtained from star counts. Results. The newly obtained surface brightness profile is in excellent agreement with that provided by Trager et al. (1995, AJ, 109, 218) for r ≳ 4". The new profile is best fitted by a King model having rc = 0.32' (∼5% smaller than previous estimates), central surface brightness μv(0) = 19.55, and concentration C = 1.35. Also new independent estimates of the total integrated V magnitude (Vt = 10.47±0.07) and of the half-light radius (rh = 0.96'±0.2') have been obtained. The average ellipticity in the range 5" ≤ r ≤ 120" is 〈 ∈ 〉 = 0.19 ± 0.14. If the four points of the ellipticity profile that deviate more than 2σ from the overall mean are excluded, 〈 ∈ 〉 = 0.14 ± 0.07 is obtained. Conclusions. The structure of NGC 2419 is now reliably constrained by (at least) two fully independent observational profiles that are in good agreement with one another. Also the overall agreement between structural parameters independently obtained by different authors is satisfactory. © ESO 2007.