SUBMILLIMETER HCN AND HCO+ EMISSION FROM GALAXIES

We report the first detection of extragalactic HCN and HCO+ J = 4 --> 3 emission at lambda = 850 mu m. Because their critical densities (n s imilar to 10(7) cm(-3)) and excitation energies (E(u) similar to 45 K) are large, these lines trace dense, warm gas. We have detected HCN J = 4 --> 3 toward NGC 253, NGC 4945, and IC 342 and report a sensitive upper limit toward M82. We have also detected HCO+ J = 4 --> 3 in both NGC 253 and M82. The HCN J = 4 --> 3/J = 1 --> 0 intensity ratio, whi ch is larger for higher gas densities, is at least 12 times larger in NGC 253 than that in M82. Single-component gas excitation model calcul ations for NGC 253 show that, averaged over the central 300 pc, the mo lecular gas is warm (T greater than or similar to 50 K) and dense (n s imilar to 5 x 10(5) m(-3)). For M82, the lower HCN J = 4 --> 3/J = 1 - -> 0 line ratio requires an average gas density at least 10 times smal ler. In both M82 and NGC 253, the J = 4 --> 3/J = 1 --> 0 line ratios peak at the nucleus. Consequently, molecular gas is densest in the cen tral 300 pc. The submillimeter HCN and HCO+ lines reveal large differe nces in the dense gas properties, despite similar CO and far-infrared luminosities.