Forward modeling linear variable filter data: High resolution PAH and Paschen-$α$ maps from SPHEREx

Polycyclic aromatic hydrocarbon (PAH) emission is a powerful tool for the study of the interstellar medium (ISM) in both Galactic and extragalactic contexts. It can reveal ionization state, radiation field, metallicity, composition, and ISM structure as sculpted by processes like turbulence and stellar feedback. Here we use NASA's SPHEREx to produce clean, high-resolution PAH maps at spectral resolution R ~ 35 (compared to WISE at R ~ 2). Prior treatments of the SPHEREx linear variable filter data seek to convert them to more conventional band-averaged maps that can leave striping artifacts associated with spatially variable wavelength sampling, especially at emission lines. In contrast, we deliver a scalable forward-modeling method capable of generating high-quality all-sky SPHEREx emission-line maps at 8.5 arcsec FWHM angular resolution (on 6.4 arcsec pixels), while naturally producing maps without visible striping. We reformat the data into a pixel-level database, mask stellar contamination, and infill missing pixels with a conditional, data-driven, anisotropic Gaussian process. We apply our method to produce 3.3 $μ$m and 3.4 $μ$m PAH and Paschen-$α$ emission maps of a set of fields spanning a range of Galactic and extragalactic conditions including the Rosette nebula, the Corona Australis molecular cloud, M33, and M31. We demonstrate the quality and synergies of our SPHEREx 3.3 $μ$m PAH maps by comparisons with other available gas and dust tracer maps and achieve sensitivities up to σ(I3.3) = 2.7 × 10⁻⁸ erg s⁻¹ cm⁻² sr⁻¹. We estimate that a two-year extension of the SPHEREx survey, with modest modifications to the scanning strategy, would match or exceed the PAH sensitivity of WISE over the whole sky.