Halfway to the Peak: The JWST MIRI 5.6 μm Number Counts and Source Population

Abstract We present an analysis of eight JWST Mid-Infrared Instrument (MIRI) 5.6 μ m images with 5 σ depths of ≈0.1 μ Jy. We detect 2854 sources within our combined area of 18.4 arcmin 2 . We compute the MIRI 5.6 μ m number counts, including an analysis of the field-to-field variation. Compared to earlier published MIRI 5.6 μ m counts, our counts have a more pronounced knee, at roughly 2 μ Jy. The location and amplitude of the counts at the knee are consistent with the Cowley et al. model predictions, although these models tend to overpredict the counts below the knee. In areas of overlap, 84% of the MIRI sources have a counterpart in the COSMOS2020 catalog. These MIRI sources have redshifts that are mostly in the z ∼ 0.5–2, with a tail out to z ∼ 5. They are predominantly moderate to low stellar masses (10 8 –10 10 M ⊙ ) main-sequence star-forming galaxies, suggesting that with ≈2 hr exposures, MIRI can reach well below M * at cosmic noon and reach higher mass systems out to z ∼ 5. Nearly 70% of the COSMOS2020 sources in areas of overlap now have a data point at 5.6 μ m (rest-frame near-IR at cosmic noon), which allows for more accurate stellar population parameter estimates. Finally, we discovered 31 MIRI-bright sources not present in COSMOS2020. A crossmatch with IRAC channel 1 suggests that 10%–20% of these are likely lower mass ( M * ≈ 10 9 M ⊙ ), z ∼ 1 dusty galaxies. The rest (80%–90%) are consistent with more massive but still very dusty galaxies at z > 3.