Spectrophotometry of HII regions, diffuse ionized gas, and supernova remnants in M31: The transition from photoionization to shock ionization

We present results of Kitt Peak National Observatory (KPNO) 4 m optical spectroscopy of discrete emission-line nebulae and regions of diffuse ionized gas (DIG) in M31. Long-slit spectra of 16 positions in the northeast half of M31 were obtained over a 5-15 kpc range in radial distance from the center of the galaxy with special attention to the annulus of high star formation between 8 and 12 kpc. Slit positions were chosen such that several emission-line nebulae and large sections of diffuse gas could be studied without compromising the parallactic angle. The spectra have been used to confirm 16 supernova remnant (SNR) candidates from the Braun & Walterbos (1993) catalog. The slits also covered 46 H II regions that show significant differences among the various morphological types (center-brightened, diffuse, rings). Radial gradients in emission-line ratios such as [O III]/H beta and [O II]/[O III] are observed most prominently in the center-brightened H II regions. These line-ratio trends are either much weaker or completely absent in the diffuse and ring nebulae. The line-ratio gradients seen previously in M31 SNRs by Blair, Kirshner, & Chevalier in 1981 and 1982 are reproduced well by our new data. The spectra of center-brightened H II regions and SNRs confirm previous determinations of the radial abundance gradient in M31. We use diagnostic diagrams that separate photoionized gas from shock-ionized gas to compare the spectral properties of H II regions, SNRs, and DIG. This analysis strengthens the earlier claims made by Greenawalt, Walterbos, & Braun in 1997 that the DIG in the disk of M31 is photoionized by a dilute radiation field.