HII Region Abundances in the Polar Ring of NGC 2685

Details HII Region Abundances in the Polar Ring of NGC 2685 HII Region Abundances in the Polar Ring of NGC 2685

1997-04-11

arxiv.org/abs/astro-ph/9704104v1

Astronomy and Astrophysics

Astrophysics

20 pages, 2 PostScript figures, uses AASTeX style files, Accepted for publication in The Astrophysical Journal

Scientific paper

10.1086/304517

We have obtained optical spectrophotometry of 11 HII regions in the polar ring of NGC 2685 (the Helix galaxy), and have used these data to study the physical characteristics of the polar-ring HII regions. The HII regions have normal spectra with no suggestion of unusual density, temperature, extinction, or composition. Semi-empirical calculations yield high oxygen abundance estimates (0.8--1.1 Z-Solar) in all HII regions. This, along with the observed (B-V) color, H-alpha equivalent width, and molecular gas properties argue against the current picture in which polar rings form from tidally captured dwarf irregular galaxies, and suggests instead that the rings are long-lived, self-gravitating structures as predicted by some dynamical models. This would allow the time required for multiple generations of star formation, and for the retention of the resulting enriched ejecta for inclusion in further generations of star formation.

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