Local Group Census: The Dwarf Irregular Galaxy NGC6822

Abstract

In the framework of the Local Group Census (LGC), an imaging programme to search for emission line objects in the Local Group, we present the detection of 13 new candidate planetary nebulae (PNe) and follow-up optical spectroscopic confirmation in the dwarf irregular galaxy NGC6822. This substantially increases the PNe sample of NGC6822 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 modulus of m-M = 23.32±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 NGC6822 (nor in M33) as much as was expected from their respective metallicities. In contrast, the helium is largely enhanced in all the galaxies. The oxygen abundances in NGC6822 are high (close to those of the HII regions), which supports the idea that most PNe originated from relatively massive and young progenitors.