Photometric Survey of the Polar ring galaxy NGC 6822

Abstract

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. 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. 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. We map the reddening over the whole area. We establish that the stellar outer structure of NGC 6822 is elliptical in shape, with $ε=0.36$ and a major-axis PA = 65$^\circ$, 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. 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.