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Silicate Emission in the Spitzer IRS Spectrum of FSC 10214+4724

We present the first mid-infrared (MIR) spectrum of the z = 2.2856 ultraluminous infrared galaxy FSC 10214+4724, obtained with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The spectrum spans a rest-wavelength range of 2.3-11.5 μm, covering a number of key diagnostic emission and absorption features. The most prominent feature in the IRS spectrum is the silicate emission at rest-frame ~10 μm. We also detect an unresolved emission line at a rest wavelength of 7.65 μm that we identify with [Ne VI], and a slightly resolved feature at 5.6 μm identified as a blend of [Mg VII] and [Mg V]. There are no strong PAH emission features in the FSC 10214+4724 spectrum. We place a limit of 0.1 μm on the equivalent width of 6.2 μm PAH emission but see no evidence of a corresponding 7.7 μm feature. Semiempirical fits to the spectral energy distribution suggest that ~45% of the bolometric luminosity arises from cold (~50 K) dust, half arises from warm (190 K) dust, and the remainder, ~5%, originates from hot (~640 K) dust. The hot dust is required to fit the blue end of the steep MIR spectrum. The combination of a red continuum, strong silicate emission, little or no PAH emission, and no silicate absorption makes FSC 10214+4724 unlike most other ULIRGs or AGNs observed thus far with the IRS. These apparently contradictory properties may be explained by an AGN that is highly magnified by the lens, masking a (dominant) overlying starburst with unusually weak PAH emission.