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

Abstract

We present Spitzer IRS spectra (R ∼ 600, 10 -38 µm) of 38 positions in the Galactic Center (GC), 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-ofsight 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, and H 2 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 the Arches Cluster ionizing the Arched Filaments 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 Cluster Wolf-Rayet stars. On the other hand, the H 2 line ratios can be explained with multi-component models of warm molecular gas in photodissociation regions without the need for H 2 production in shocks.

The Galactic center: not an active galactic nucleus

“…Although high-resolution spectra from Spitzer/IRS have a relatively low spectral resolution (R ≡ Δλ/λ = 600), radial velocities can be obtained with a precision of a few tens of kilometers per second for strong emission lines (e.g., Simpson et al 2007). We computed radial velocities in the local standard of rest (LSR), after correcting for the spacecraft motion: the correction terms (keyword VLSRCORR provided by the SSC) for 2008 May and October observing runs were +38.9 km s −1 and −18.5 km s −1 , respectively.…”

“…The first approach is based on the method developed in Simpson et al (2007), who determined the flux ratio at 10 μm and 14 μm (F 14 /F 10 ) and used it to infer the optical depth of the 9.7 μm silicate absorption feature (τ 9.7 ) near the Radio Arc Bubble. Following the Simpson et al prescription, we estimated mean fluxes at 10.00 μm λ 10.48 μm and 13.50 μm λ 14.30 μm from SH spectra after a 3σ rejection.…”

“…As shown by vertical gray strips in Figure 2, these two continuum wavelength ranges are almost free of PAHs features and ionic emission lines. Then we estimated τ 9.7 for each line of sight, by using a simple linear relationship between the 9.7 μm silicate optical depth and the continuum flux ratio, τ 9.7 = (ln (F 14 /F 10 ) − 0.809)/0.560, which was inferred from tabulated values in Simpson et al (2007). The top panel in Figure 4 shows the spatial distribution of τ 9.7 derived in this way for all lines of sight in our program.…”

“…Figure 5 compares τ 9.7 derived using the Simpson et al (2007) method to the visual extinction (A V ) from Schultheis et al (2009). Figure 5 shows a large scatter between these two independent extinction estimates.…”

“…A more recent survey in the mid-IR was carried out by Simpson et al (2007) using the Infrared Spectrograph (IRS; Houck et al 2004) on board the Spitzer Space Telescope (Werner et al 2004), which has a higher sensitivity than ISO at 10 μm-38 μm. The Simpson et al survey consists of highresolution spectra of 38 positions along a narrow 24 long strip at a Galactic longitude l ≈ +0.…”