Mid-infrared spectroscopy of two lensed star-forming galaxies

We present low-resolution, rest-frame ∼5-12 μm Spitzer/IRS spectra of two lensed z ∼ 2 UV-bright star-forming galaxies, SDSS J120602.09+514229.5 and SDSS J090122.37+181432.3. Using the magnification boost from lensing, we are able to study the physical properties of these objects in greater detail than is possible for unlensed systems. In both targets, we detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2, 7.7, and 11.3 μm, indicating the presence of vigorous star formation. For J1206, we find a steeplyrising continuum and significant [S iv] emission, suggesting that a moderately hard radiation field is powering continuum emission from small dust grains. The strength of the [S iv] emission also implies a sub-solar metallicity of ∼0.5Z⊙, confirming published rest-frame optical measurements. In J0901, the PAH lines have large rest-frame equivalent widths (>1 μm) and the continuum rises slowly with wavelength, suggesting that any active galactic nucleus (AGN) contribution to LIR is insignificant, in contrast to the implications of optical emission-line diagnostics. Using [Oiii] line flux as a proxy for AGN strength, we estimate that the AGN in J0901 provides only a small fraction of its mid-infrared continuum flux. By combining the detection of [Ar ii] with an upper limit on [Ar iii] emission, we infer a metallicity of ≲1.3Z⊙. This work highlights the importance of combining rest-frame optical and mid-IR spectroscopy in order to understand the detailed properties of star-forming galaxies at high redshift.