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 3.5micron for simple and composite stellar populations, and their dependence on age, metallicity and star formation history. 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 "beta_V" method, by comparing the observed flux ratio to an empirical estimate of the intrinsic ratio of visible and ~3.5micron data. Through extensive modeling, we aim to validate the "beta_V" method for various filters spanning the visible through near-infrared wavelength range, for a wide variety of simple and composite stellar populations. Combining Starburst99 and BC03 models, we built spectral energy distributions (SEDs) of simple (SSP) and composite (CSP) stellar populations for various realistic star formation histories (SFHs), while taking metallicity evolution into account. We convolve various 0.44--1.65micron filter throughput curves with each model SED to obtain intrinsic flux ratios beta_{lam,0}. When unconstrained in redshift, the total allowed range of beta_{V,0} is 0.6--4.7, or almost a factor of eight. At known redshifts, and in particular at low redshifts (z<0.01), beta_{V,0} 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 beta_V values. The beta_lam method can therefore serve as a first-order dust correction method for large galaxy surveys that combine JWST (rest-frame 3.5micron) and HST (rest-frame visible--near-IR) data.