Galaxies that are invisible in deep optical-NIR imaging but detected at
longer wavelengths have been the focus of several recent observational studies,
with speculation that they could constitute a substantial missing population
and even dominate the cosmic star formation rate density at z≳4. The
depths now achievable with JWST at the longest wavelengths probed by HST,
coupled with the transformative resolution at longer wavelengths, are already
enabling detailed, spatially-resolved characterisation of sources that were
invisible to HST, often known as `HST-dark' galaxies. However, until now, there
has been little theoretical work to compare against. We present the first
simulation-based study of this population, using highly-resolved galaxies from
the Feedback in Realistic Environments (FIRE) project, with multi-wavelength
images along several lines of sight forward-modelled using radiative transfer.
We naturally recover a population of modelled sources that meet commonly-used
selection criteria (HAB>27mag and
HAB−F444W>2.3). These simulated HST-dark galaxies lie at high
redshifts (z=4−7), have high levels of dust attenuation (AV=2−4), and
display compact recent star formation
(R1/2,4.4μm≲1kpc). Orientation is very
important: for all but one of the 17 simulated galaxy snapshots with HST-dark
sightlines, there exist other sightlines that do not meet the criteria. This
result has important implications for comparisons between observations and
models that do not resolve the detailed star-dust geometry, such as
semi-analytic models or coarsely-resolved hydrodynamical simulations.
Critically, we demonstrate that HST-dark sources are not an unexpected or
exotic population, but a subset of high-redshift, highly-dust-attenuated
sources viewed along certain lines of sight.Comment: 12 pages, 8 figures. Accepted for publication in Ap