Analysis of the Intrinsic Mid-infrared L band to Visible-Near-infrared Flux Ratios in Spectral Synthesis Models of Composite Stellar Populations

We analyze the intrinsic flux ratios of various visible-near-infrared filters with respect to 3.5 μm for simple and composite stellar populations (CSPs), and their dependence on age, metallicity, and star formation history (SFH). UV/optical light from stars is reddened and attenuated by dust, where different sightlines across a galaxy suffer varying amounts of extinction. Tamura et al. (2009) developed an approximate method to correct for dust extinction on a pixel-by-pixel basis, dubbed the "" method, by comparing the observed flux ratio to an empirical estimate of the intrinsic ratio of visible and ∼3.5 μm data. Through extensive modeling, we aim to validate the "" method for various filters spanning the visible through near-infrared wavelength range, for a wide variety of simple and CSPs. Combining Starburst99 and BC03 models, we built spectral energy distributions (SEDs) of simple (SSP) and composite (CSP) stellar populations for various realistic SFHs, while taking metallicity evolution into account. We convolve various 0.44-1.65 μm filter throughput curves with each model SED to obtain intrinsic flux ratios . When unconstrained in redshift, the total allowed range of is 0.6-4.7, or almost a factor of eight. At known redshifts, and in particular at low redshifts (z ≲ 0.01), is predicted to span a narrow range of 0.6-1.9, especially for early-type galaxies (0.6-0.7), and is consistent with observed values. The method can therefore serve as a first-order dust-correction method for large galaxy surveys that combine JWST (rest-frame 3.5 μm) and HST (rest-frame visible-near-IR) data.