CNO Abundances in Metal-Rich Environments: Infrared Space Observatory Spectroscopy of Ionized Gas in M51

Abstract

We present Infrared Space Observatory (ISO) Long Wavelength Spectrograph observations centered on the H II region CCM 10 in the spiral galaxy M51. We detect several emission lines in this spectrum, including [O I] 63 and 145 μm, [C II] 158 μm, [O III] 88 μm, and [N II] 122 μm, with a significant upper limit on [N III] 57 μm. We use these measurements to estimate abundances for C, N, and O in the interstellar medium in M51. We compare our [O III] 88 μm flux with the flux for [O III] λ5007 obtained from narrowband imaging. The derived 5007Å/88μm ratio yields an electron temperature T_e=5300+/-300 K. This temperature agrees with estimates of T[O III] based on photoionization models of CCM 10 with logO/H=-3.2, an abundance that is about a factor of 2 smaller than earlier results for this object derived from photoionization modeling of the visible spectrum. A possible cause for the discrepancy is that the older photoionization models based on CLOUDY predict significantly larger optical emission-line strengths than the current version of CLOUDY at the same metallicity; models with the more recent version are more consistent with the observed spectra of M51 H II region with reduced O/H. Assuming N/O=N⁺²/O⁺², the upper limit for [NIII]/[OIII] yields logN/O<-0.5, which is consistent with the trend of N/O versus O/H seen in other spiral galaxies. C/O is estimated from the [CII]/[OI] ratio using photodissociation region models constrained by published CO line ratios, [CI]/CO, and [OI]63μm/145μm. With these various constraints on n_e and G₀, the observed [CII]/[OI] intensity ratio is in good agreeement with that predicted by photodissociation region models with approximately solar abundances. We infer from this that C/O in M51 is consistent with the solar neighborhood value.