Photometric survey of the polar ring galaxy NGC 6822

• Published in: Astronomy and astrophysics (Berlin) Vol. 451; no. 1; pp. 99 - 108
• Main Authors: Battinelli, P., Demers, S., Kunkel, W. E.
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.05.2006
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Red giant stars; C star; Color; stars: carbon; Local group; Carbon stars; galaxies: individual: NGC 6822; Stellar structure; galaxies: structure; Galaxy structure; Dwarf galaxies; Small Magellanic Cloud; Stellar population; Elliptic shape; Irregular galaxies; Galaxy groups; galaxies: Local Group; Ring galaxies; Radial velocity; Age; Photometry
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20054718

Context.We have previously established, from a carbon star survey, that the Local Group dwarf irregular galaxy NGC 6822 is much larger than its central bright core. Aims. Four MegaCam fields are acquired to survey a 2$^\circ\times 2^\circ$ area centred on NGC 6822 to fully determine its extent and map its stellar populations. Methods.Photometry of over one million stars is obtained in the SDSS g', r', i' to three magnitudes below the TRGB. RGB stars, selected from their magnitudes and colours, are used to map the NGC 6822 stellar distribution up to a distance of 60 arcmin. Results.We map the reddening over the whole area. We establish that the stellar outer structure of NGC 6822 is elliptical in shape, with $\epsilon=0.36$ and a major-axis PA = 65°, contrasting with the orientation of the HI disk. The density enhancement can be seen up to a semi-major axis of $36'$ making NGC 6822 as big as the Small Magellanic Cloud. We fit two exponentials to the surface density profile of the spheroid, and identify a bulge with a scale length of $3.85'$ and an outer spheroid with a scale length of $10.0'$. We find intermediate-age C stars up to ${\sim}40'$ while demonstrating that the SDSS filters are unsuitable to identify extragalactic C stars. Conclusions.NGC 6822 is a unique Local Group galaxy with shape and structure suggesting a polar ring configuration. Radial velocities of carbon stars have indeed demonstrated that there are two kinematical systems in NGC 6822.