Emission line objects in NGC 6822. New planetary nebula candidates ^*,**

Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 70264, Méx. D. F., 04510 México e-mail: [lhernand;miriam]@astroscu.unam.mx

Received: 17 June 2008
Accepted: 6 November 2008

Abstract

Aims. Images obtained with the CTIO 4-m telescope and the MOSAIC-2 wide field camera in [O iii] 5007 and Hα on-band and off-band filters are analyzed to search for emission line objects in the dwarf galaxy NGC 6822. In particular we search for planetary nebula (PN) candidates. In addition, imaging and spectroscopy of a sub-sample of objects obtained with the ESO VLT and FORS 2 spectrograph are used to calibrate the MOSAIC imaging.

Methods. In the continuum-subtracted images, a large number of line emission regions were detected, for which we measured instrumental magnitudes in all the filters. The [O iii] 5007 and Hα + [N ii] magnitudes were calibrated with the spectroscopy.

Results. Based upon some criteria to distinguish between PNe and compact HII regions, we have found 26 PN candidates, increasing the known sample by 8 objects. Also we detected a number of compact HII regions and about 20 stellar-like objects emitting in Hα. For all the objects we present coordinates, instrumental magnitudes and nebular [O iii] and Hα + [N ii] fluxes. The observed luminosity function for the PN [O iii] 5007 magnitudes (PNLF) and the cumulative PNLF were calculated. We confirm that the PNLF presents a dip similar to the one detected for the Small Magellanic Cloud at 2.5 mag below the maximum. The cumulative PNLF returns a value for the peak absolute magnitude of the PNLF which is faint (but within uncertainties) compared to the value expected for galaxies with a metallicity similar to that of NGC 6822. From our best fit to the observed PNLF we obtained a rough estimate of the distance modulus  mag, which agrees within uncertainties with recent values from Cepheid stars reported in the literature. Also the number of PNe in the brightest 0.5 mag, normalized to the galactic bolometric luminosity, was estimated to be  E-9. This number is similar to the values derived for galaxies with recent star formation and small galaxies (M_B fainter than -18 mag) and larger than the values for early-type galaxies.