Extreme gas pressures in the galactic bulge

MOLECULAR gas in the innermost regions of the Galactic Centre must be in a very different state from the interstellar medium in the solar neighbourhood. Diffuse CS emission is seen only at the Galactic Centre^1,2, the emissivity of CO is sixty times higher there than near the Sun³, and linewidths of individual molecular features are typically five to ten times broader than elsewhere in the Galaxy. We propose that all these characteristics, for which there has so far been no coherent explanation, reflect the existence of high gas pressure - about 5 x 10⁶ K cm^-3, or two and a half orders of magnitude higher than that in the solar neighbourhood - in the inner 500 parsecs of the Galaxy. This high pressure can be inferred from the presence of hot X-ray-emitting gas, which is also known to be present in the bulges of many other galaxies^4,5. The molecular gas in these bulges should thus resemble that in the centre of our own Galaxy.