Local Group Census: The Dwarf Irregular Galaxy NGC6822

Abstract

In the framework of the Local Group Census (LGC), an imaging pro- gramme to search for emission line objects in the Local Group, we present the detec- tion of 13 new candidate planetary nebulae (PNe) and follow-up optical spectroscopic confirmation in the dwarf irregular galaxy NGC 6822. This substantially increases the PNe sample of NGC 6822 to 17. Our sample of PNe is complete within 3.5 mag below the brightest PN. Using the PN luminosity function (PNLF), we find a distance mod- ulus of m − M =2 3.32 ± 0.45. The spectroscopic data help to discriminate between the different emission line objects (PNe, SNR, HII regions). The spectra with the highest S/N ratio allow us to determine abundances for 6 PNe and 4 HII regions. For the first time, we can compare the PN abundances of helium, nitrogen and oxygen in many LG galaxies. We find that the nitrogen is not enhanced in NGC 6822 (nor in M 33) as much as was expected from their respective metallicities. In contrast, the helium is largely enhanced in all the galaxies. The oxygen abundances in NGC 6822 are high (close to those of the H II regions), which supports the idea that most PNe originated from relatively massive and young progenitors.