Abstract
We contrast predictions for the high-redshift galaxy population and
reionization history between Cold Dark Matter (CDM) and an alternative DM model
based on the recently developed ETHOS framework (Effective Theory of Structure
Formation; Cyr-Racine et al. 2016, Vogelsberger et al. 2016). We focus on an
ETHOS model that alleviates the small-scale CDM challenges within the Local
Group, and perform the currently highest resolution hydrodynamical volume
$\sim$ (36 Mpc)$^3$ simulations within ETHOS and CDM combined with the
IllustrisTNG galaxy formation model (gas cell mass $\sim10^5M_ødot$, gas
softening $\sim$ 180 pc) to quantify the abundance of galaxies at high redshift
and their impact on reionisation. While current observations of high-redshift
luminosity functions cannot differentiate between ETHOS and CDM, deep JWST
surveys of strongly-lensed, inherently faint galaxies could potentially detect
or constrain a primordial cutoff in the power spectrum. We find that ETHOS
galaxies have higher ultraviolet (UV) luminosities than their CDM counterparts
and a faster build up of the fainter end of the UV luminosity function; a
distinct behaviour offering a promising avenue to identify a primordial power
spectrum cutoff. This effect, however, makes the optical depth to reionisation
less sensitive to this cutoff, such that the ETHOS model differs from the CDM
$\tau$ value by only 10% and is consistent with Planck limits, as long as the
effective escape fraction is in the 0.1 -- 0.5 range. We conclude that high
redshift observations in the JWST era have the potential to probe non-CDM
models that offer attractive solutions to the Local Group CDM problems.
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