Carbon stars in the outer spheroid of NGC 6822

¹ Département de Physique, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, Qc, H3C 3J7, Canada e-mail: [demers;artigau]@astro.umontreal.ca
² INAF, Osservatorio Astronomico di Roma Viale del Parco Mellini 84, 00136 Roma, Italy e-mail: battinel@oarhp1.rm.astro.it

Received: 26 April 2006
Accepted: 12 June 2006

Abstract

Context.From a 2 2° of NGC 6822 survey we have previously established that this Local Group dwarf irregular galaxy possesses a huge spheroid having more than one degree in length. This spheroid is in rotation but its rotation curve is known only within ~ from the center. It is therefore critical to identify bright stars belonging to the spheroid to characterize, as far as possible, its outer kinematics.

Aims.We use the new wide field near infrared imager CPAPIR, operated by the SMARTS consortium, to acquire J, K_s images of two areas in the outer spheroid to search for C stars.

Methods.The colour diagram of the fields allows the identification of 192 C stars candidates but a study of the FWHM of the images permits the rejection of numerous non-stellar objects with colours similar to C stars.

Results. We are left with 75 new C stars, their mean K_s magnitude and mean colour are similar to the bulk of known NGC 6822 C stars.

Conclusions.This outer spheroid survey confirms that the intermediate-age AGB stars are a major contributor to the stellar populations of the spheroid. The discovery of some 50 C stars well beyond the limit of the previously known rotation curve calls for a promising spectroscopic follow-up to a major axis distance of 40'.