Variation of Emission-Line Strengths across M31
Abstract
Intensities of [N ii] A6584, [0 iii]A5007, Hfl, and [S iii AA6717 and 6731 emission lines, relative to Ha, have been measured for 53 H ii regions between 3 and 24 kpc from the nucleus of M31. When these results are combined with earlier studies of the nucleus, it is shown that the [N iij/Ha line intensity ratio decreases systematically from a value of about 2 at the nucleus to about 0.2 at 15 kpc, beyond which distance there is no certain trend. A systematic increase in the [0 ii'j/Hfl ratio from 0.4 to 2 is observed in going from 3 kpc to 15 kpc. The variations between 5 kpc and 15 kpc are interpreted as a decrease in the N/H and 0/H abundance ratios by factors of 2 and 1.2, respectively, using a simplified model for the H ii regions. Near R = 5 kpc, a group of H ii regions exhibit anomalous spectra with very high excitation and strong lines, much like the nuclear spectra which we attribute to an excitation mechanism different from that of a conventional H ii region. Overall, there is a similarity with line ratios for M33 and M101.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 1972
- DOI:
- 10.1086/151684
- Bibcode:
- 1972ApJ...177...31R