L Band to Visible–Near-IR Flux Ratios of Various Stellar
Populations
We analyze the intrinsic flux ratios of various visible--near-infrared filters
with respect to 3.5μm (L band) for simple and composite stellar
populations, and their dependence on age, metallicity and star formation
history. This study is motivated by the fact that UV/optical light from stars
is reddened and attenuated by dust via scattering and absorption, where
different sightlines across a galaxy suffer varying amounts of extinction.
Ignoring the effects of this extinction could lead to, e.g., underestimated
stellar mass, SFR, or higher inferred metallicity. Tamura et al. (2009)
developed an approximate method to correct for dust extinction on a
pixel-by-pixel basis, dubbed the ``βV'' method, by
comparing the observed flux ratio to an empirical estimate of the intrinsic
flux ratio of visible and ~3.5μm broadband data. Here, we aim to validate
and test the limits of the βV method for various filters
spanning the visible through near-infrared wavelength range. Through extensive
modeling, we test their assumptions for the intrinsic flux ratios for a wide
variety of simple and composite stellar populations. We build spectral energy
distributions (SEDs) of simple stellar populations (SSPs), by adopting
Starburst99 and BC03 models for young (<30 Myr) and old
(>100 Myr) stellar populations, respectively, and linear combinations
of these for the intermediate ages. We then construct composite stellar
population (CSP) SEDs by combining SSP SEDs for various realistic star
formation histories (SFHs), while taking metallicity evolution into account. We
convolve the filter response curves of various 0.44–1.65μm filters
commonly used for HST imaging surveys and mid-IR filters of WISE,
Spitzer/IRAC and JWST/NIRCam with each model SED to obtain the
intrinsic flux ratios (βλ,0) with respect to
those mid-IR filters. When nearly unconstrained in redshift
(0≲z≲4), the total allowed range of
βV,0 is 0.6–4.7, or almost a factor of eight, due
to the prevalence of lower metallicity and younger stellar populations at
higher redshifts. At known redshifts, and in particular at low redshifts
(z≲0.01), the βV,0 values are predicted to
span a narrow range of 0.6–1.9, especially for early-type galaxies
(0.6–0.7), and are consistent with the observed
βV,0 values. The βλ,0
method can therefore serve as a first-order dust correction method for large
galaxy surveys that combine JWST (rest-frame 3.5μm) and HST
(rest-frame visible–near-IR) data. The full suite of simple and composite
SED models constructed, and all βλ,0 values as a
function of age and metallicity are provided in digital form for public use.
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