Star complexes and stellar populations in NGC 6822

Comparison with the Magellanic Clouds

¹ Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens, 15783 Athens, Greece e-mail: ankaramp@phys.uoa.gr
² Institute for Astronomy and Astrophysics, National Observatory of Athens, PO Box 20048, 11810 Athens, Greece
³ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain

Received: 3 October 2008
Accepted: 27 January 2009

Abstract

Aims. The star complexes (large scale star forming regions) of NGC 6822 were traced and mapped and their size distribution was compared with the size distribution of star complexes in the Magellanic Clouds (MCs). The spatial distributions of different age stellar populations were compared with each other.

Methods. The star complexes of NGC 6822 were determined by using the isopleths, based on star counts, of the young stars of the galaxy, using a statistical cutoff limit in density. In order to map them and determine their geometric properties, an ellipse was fitted to each distinct region satisfying this minimum limit. The Kolmogorov-Smirnov statistical test was used to study possible patterns in their size distribution. Isopleths were also used to study the stellar populations of NGC 6822.

Results. The star complexes of NGC 6822 were detected and a list of their positions and sizes was produced. Indications of hierarchical star formation, in terms of spatial distribution, time evolution and preferable sizes were found in NGC 6822 and the MCs. The spatial distribution of the various age stellar populations has indicated traces of an interaction in NGC 6822, dated before 350 ± 50 Myr.