CNO abundances in metal-rich environments: Infrared space observatory spectroscopy of ionized gas in M51

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 5007 Å/88 μm 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 log O/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 [N III]/[O III] yields log N/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 [C II]/[O I] ratio using photodissociation region models constrained by published CO line ratios, [C I]/CO, and [O I] 63 μm/145 μm. With these various constraints on n _e and G ₀, the observed [C II]/[O I] 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.