Emission line objects in NGC 6822. New Planetary Nebula candidates

Abstract

Images obtained with the CTIO 4-m telescope and the MOSAIC-2 wide field camera in [O III] 5007 and Hαon-band and off-band filters are analyzed to search for emission lines objects in the galaxy NGC 6822. In particular we search for PN candidates. In addition, data of a sub-sample of objects obtained with ESO VLT-FORS2 are used to calibrate the MOSAIC imaging. A large number of line emission regions were detected, for which we measured instrumental magnitudes in all the filters. Based on some criteria to distinguish between PNe and compact HII regions, we found 26 PN candidates, increasing the known sample in 8 objects. Also we detected a number of compact HII regions and 20 stellar objects emitting in Halpha. For all the objects we present coordinates, instrumental magnitudes and nebular [O III] and Halpha+[N II] fluxes. The observed PNLF for the PN [O III] 5007 magnitudes and the cumulative PNLF were calculated. We confirm that the PNLF presents a dip similar to the one detected for the SMC at 2.5 mag down the maximum. The cumulative PNLF returns a value M^\star_{5007}=-3.71^{+0.21}_{-0.42} for the peak absolute magnitude of the PNLF which is faint compared to the value expected for galaxies with metallicity similar to the one of NGC 6822 but similar within uncertainties. From our best fit to the observed PNLF we obtained a rough distance modulus m-M = 23.64 ^{+0.23}_{-0.43} mag, which agrees within uncertainties with recent values reported in the literature. The number of PN in the brightest 0.5 mag normalized to the galactic bolometric luminosity, alpha_{0.5} was estimated to be (3.8^{+0.90}_{-0.71}) E-9, which is similar to the values derived for galaxies with recent star formation and small galaxies (M_B fainter than - 18 mag).