Spitzer IRS Observations of the Galactic Center: Shocked Gas in the Radio Arc Bubble

Abstract

We present Spitzer IRS spectra (R ? 600, 10Y38 ?m) of 38 positions in the Galactic center, all at the same Galactic longitude and spanning?0.3? in latitude. Our positions include the Arches Cluster, the Arched Filaments, regions near the Quintuplet Cluster, the ‘‘Bubble’’ lying along the same line of sight as the molecular cloud G0.11?0.11, and the diffuse interstellar gas along the line of sight at higher Galactic latitudes. From measurements of the [O iv], [Ne ii], [Ne iii], [Si ii], [S iii], [S iv], [Fe ii], [Fe iii], and H2 S(0), S(1), and S(2) lines, we determine the gas excitation and ionic abundance ratios. The Ne/H and S/H abundance ratios are ?1.6 times that of the Orion Nebula. The main source of excitation is photoionization,with theArches Cluster ionizing theArchedFilaments and the Quintuplet Cluster ionizing the gas nearby and at lower Galactic latitudes including the far side of the Bubble. In addition, strong shocks ionize gas to O+3 and destroy dust grains, releasing iron into the gas phase (Fe/H ? 1:3 ; 10?6 in the Arched Filaments and Fe/H ? 8:8 ; 10?6 in the Bubble). The shock effects are particularly noticeable in the center of the Bubble, but O+3 is present in all positions.We suggest that the shocks are due to the winds from the Quintuplet ClusterWolf-Rayet stars. On the other hand, the H2 line ratios can be explained with multicomponent models of warm molecular gas in photodissociation regions without the need for H2 production in shocks.