Emission line objects in NGC 6822. New planetary nebula candidates

Free access article

Issue		A&A Volume 495, Number 2, February IV 2009


Page(s)		447 - 455
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:200810412
Published online		11 December 2008

A&A 495, 447-455 (2009)
DOI: 10.1051/0004-6361:200810412

Emission line objects in NGC 6822. New planetary nebula candidates

L. Hernández-Martínez and M. Peña

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 H $\alpha$ 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 H $\alpha$ + [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 H $\alpha$ . For all the objects we present coordinates, instrumental magnitudes and nebular [O III] and H $\alpha$ + [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 $M^\star_{5007}=-3.71^{+0.21}_{-0.42}$ 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 m-M=23.64^+0.23_-0.43 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 $\alpha_{0.5} \sim (3.8^{+0.90}_{-0.71})$ E-9. This number is similar to the values derived for galaxies with recent star formation and small galaxies ( $M_{\rm B}$ fainter than -18 mag) and larger than the values for early-type galaxies.

Key words: ISM: HII regions -- ISM: planetary nebulae: general -- galaxies: individual: NGC 6822 (DDO 209)

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