2017ApJ...836..171B

The James Webb Space Telescope's Medium Resolution Spectrometer (MRS), will offer nearly two orders of magnitude improvement in sensitivity and >3x improvement in spectral resolution over our previous space-based mid-IR spectrometer, the Spitzer IRS. In this paper, we make predictions for spectroscopic pointed observations and serendipitous detections with the MRS. Specifically, pointed observations of Herschel sources require only a few minutes on source integration for detections of several star-forming and active galactic nucleus lines, out to z = 3 and beyond. But the same data will also include tens of serendipitous 0 <= z <= 4 galaxies per field with infrared luminosities ranging ∼10⁶-10¹³ L_☉. In particular, for the first time and for free we will be able to explore the L_IR < 10⁹ L_☉ regime out to z ∼ 3. We estimate that with ∼ 100 such fields, statistics of these detections will be sufficient to constrain the evolution of the low-L end of the infrared luminosity function, and hence the star formation rate function. The above conclusions hold for a wide range in the potential low-L end of the IR luminosity function, and account for the PAH deficit in low-L, low-metallicity galaxies.