2000A&A...356..695R

We present ISO observations of the S(0), S(1), S(2), and S(3) rotational lines of molecular hydrogen from two molecular clouds near the Galactic Center (GC). We have also measured continuum dust emission at infrared wavelengths with ISO and the rotational radio lines J=1-0 of ¹³CO and C¹⁸O and J=2-1 of C¹⁸O with the IRAM-30m telescope. Using the dust continuum spectra and the CO lines we derive a total visual extinction of ∼15-20 magnitudes toward these GC clouds. After correcting the H₂ data for extinction, the gas temperatures are ∼250K and the column densities of warm gas are ∼2x10²¹cm^–2. This is the first direct measure of the H₂ column densities of the warm component; with this, we estimate an NH₃ abundance in the warm gas of ∼2x10^–7. The column density of warm gas is, at least, a factor of 100 larger than the corresponding column densities derived from the warm dust. The observed ortho-to-para ratio (OTPR) is ∼1, clearly below the local thermodynamical equilibrium (LTE) OTPR for gas at 250K of ∼3. Low velocity shocks (∼10km/s) are the most likely explanation for the column densities of warm gas and dust and the non-LTE H₂ OTPR.