Mapping the Galaxy Color–Star Formation Rate Relation with Manifold Learning and Infrared Image Stacking

Abstract Modern surveys present us with billions of faint galaxies for which we only have broadband images in ∼6–8 optical-to-near-infrared (NIR) filters. Galaxy star formation rates (SFRs) are difficult to estimate accurately without spectroscopic diagnostics or far-infrared (FIR) photometry, both of which are prohibitively expensive to obtain for large numbers of faint, high-redshift galaxies. Here we present the empirical relation between SFR and broadband optical-to-NIR colors learned from Spitzer MIPS and Herschel PACS/SPIRE imaging using an innovative stacking analysis that bins galaxies with similar optical-to-NIR spectral energy distributions using a self-organizing map (SOM). Stacking based on optical-to-NIR colors ensures that our FIR stacks are built from galaxies with similar intrinsic physical properties as opposed to stacking simply by stellar mass. We train a 40 × 40 SOM using 230,638 galaxies selected from the Cosmic Evolution Survey (COSMOS) field, and stack the mid-IR to FIR images from 24–500 μ m. We are able to measure the median FIR luminosities from half of the SOM cells to calibrate the SFR. In addition to investigating the common structures of optical-to-NIR properties and FIR detections labeled on the SOM, we provide calibrated SFRs for nearly half of the galaxies in the COSMOS fields down to i -band magnitude ≤25.5, and present the evolution of the galaxy main sequence for low-mass galaxies to redshift z ∼ 2.5.