CN and HNC line emission in IR luminous galaxies

Abstract

We have observed HNC 1-0, CN 1-0 & 2-1 line emission in a sample of 13 IR luminous (LIRGs, LIR > 1011 L⊙) starburst and Seyfert galaxies. HNC 1-0 is detected in 9, CN 1-0 is detected in 10 and CN 2-1 in 7 of the galaxies and all are new detections. We also report the first detection of HC3N (10-9) emission in Arp 220. The excitation of HNC and CN emission requires densities n > 104 cm-3. We compare their intensities to that of the usual high density tracer HCN. The I(HCN)/I(HNC) 1-0 and I(HCN)/I(CN) 1-0 line intensity ratios vary significantly, from 0.5 to ≳6, among the galaxies. This implies that the actual properties of the dense gas is varying among galaxies who otherwise have similar I(CO)/I(HCN) line intensity ratios. We suggest that the HNC emission is not a reliable tracer of cold (10 K) gas at the center of LIR galaxies, as it often is in the disk of the Milky Way. Instead, the HNC abundance may remain substantial, despite high gas temperatures, because the emission is emerging from regions where the HCN and HNC formation and destruction processes are dominated by ion-neutral reactions which are not strongly dependent on kinetic temperature. We find five galaxies (Mrk 231, NGC 7469, NGC 7130, IC 694 and NGC 2623) where the I(HCN)/I(HNC) intensity ratio is close to unity. Four are classified as active galaxies and one as a starburst. In other active galaxies, however, the I(HCN)/I(HNC) is >4. The CN emission is on average a factor of two fainter than the HCN for the luminous IR galaxies, but the variation is large and there seems to be a trend of reduced relative CN luminosity with increasing IR luminosity. This trend is discussed in terms of other PDR tracers such as the [C II] 158 μm line emission. One object, NGC 3690, has a CN luminosity twice that of HCN and its ISM is thus strongly affected by UV radiation. We discuss the I(HCN)/I(HNC) and I(HCN)/I(CN) line ratios as indicators of starburst evolution. However, faint HNC emission is expected both in a shock dominated ISM as well as for a cloud ensemble dominated by dense warm gas in the very early stages of a starburst. Additional information will help resolve the dichotomy.