LIGHTS. Limitations of Far-Infrared Tracers for Galactic Cirrus Mitigation in the Era of Ultra-Deep Imaging

Current and forthcoming deep optical imaging surveys are reaching surface brightness levels fainter than 30 mag/arcsec squared, where Galactic cirrus emission becomes a significant contaminant. This poses a major challenge for the study of extremely low-surface-brightness extragalactic structures. We investigate the relationship between far-infrared emission, as traced by the Herschel 250 μ m band, and optical surface brightness in the Sloan g- and r-bands. We find that, for Galactic cirrus with μ g 27 mag/arcsec squared, a linear relation is consistent with the data, assuming optically thin dust, with optical cirrus emission amounting to about 0.1% of the far-infrared flux. Using Herschel far-infrared data to trace Galactic cirrus enables its identification over wide areas, but at the cost of significantly reducing spatial resolution. This limitation introduces source confusion and restricts the effective surface brightness depth that can be robustly probed in optical data. We show that, when cirrus emission is modeled using Herschel 250 μ m data, the limiting surface brightness is μ V about 28 mag/arcsec squared, substantially shallower than the nominal depth of surveys such as LIGHTS or the 10-year coadd LSST (μ V about 30.5 mag/arcsec squared). These results highlight a fundamental limitation in the use of currently available far-infrared data for cirrus mitigation in ultra-deep optical imaging, and emphasize the need for higher-resolution cirrus tracers to fully exploit the depth of upcoming surveys.