Oxygen Abundances of HII Regions in the Polar Ring of NGC 2685

Previous abstract Next abstract

Session 48 -- Spiral, Dwarf and Irregular Galaxies
Display presentation, Wednesday, 1, 1994, 9:20-6:30

[48.04] Oxygen Abundances of HII Regions in the Polar Ring of NGC 2685

P.B. Eskridge (SAO), R.W. Pogge (OSU)

A small fraction of S0 galaxies are surrounded by delicate rings of gas, dust, and stars nearly perpendicular to the plane of the galaxy disk. As these polar rings are typically quite blue, HI rich, and show signs of dust and H$\alpha$ emission (see Whitmore et al., 1990 AJ 100 1489), they are generally believed to be debris from the accretion of a dwarf Irregular (dI) galaxy by the S0. While a great deal of work has been devoted to understanding the dynamics of polar rings (e.g., Steiman-Cameron \& Durisen 1988 ApJ 325 26; Sackett \& Sparke 1990 ApJ 361 408), very little attention has been paid to the properties of the rings themselves. The prevalence of H$\alpha$ emission from polar rings convinced us that it would be possible to determine the abundances of the ring material from nebular spectrophotometry and thus test the assertion that the ring material is accreted from a metal-poor dI galaxy. We present the results of this work for the prototype polar-ring galaxy NGC 2685. Long-slit spectrophotometry of a number of HII regions in the polar ring of NGC 2685 were obtained with the 2.7m telescope at the McDonald Observatory, and with the MMT. Spectral coverage encompasses all relevant diagnostic lines from [O\,II] 3727\AA~to [S\,II] 6731\AA~at sufficient spectral resolution to clearly resolve all close line-pairs. Because [O\,III] 4363\AA~is not visible in our spectra, we have had to use the semi-empirical calibration of Edmunds and Pagel (1984 MNRAS 211 507) to derive oxygen abundances. Although oxygen abundances based on this method are potentially uncertain by up to a factor of two, the abundances we determine are $\sim$Solar for the HII regions in the polar ring of NGC 2685; much higher than those found for any dI galaxy. If this result holds for all polar rings, it will provide a strong constraint for models of the formation and evolution of these systems. We are continuing this research by obtaining H$\alpha$ imaging and subsequent spectrophotometric data for a sample of kinematically confirmed polar-ring galaxies.

Wednesday program listing