We present the results of a study investigating the dust attenuation law at z \simeq 5 , based on synthetic spectral energy distributions ( SEDs ) calculated for a sample of N=498 galaxies drawn from the First Billion Years ( FiBY ) simulation project .
The simulated galaxies at z \simeq 5 , which have M _ { 1500 } \leq - 18.0 and 7.5 \leqlog ( M / M _ { \odot } ) \leq 10.2 , display a mass-dependent \alpha -enhancement , with a median value of [ \alpha / Fe ] _ { z = 5 } \leavevmode \nobreak \simeq \leavevmode \nobreak 4 % \leavevmode \nobreak \times \leavevmode \nobreak [ \alpha / Fe ] _ { Z _ { \odot } } .
The median Fe/H ratio of the simulated galaxies is 0.14 \pm 0.05 which produces steep intrinsic UV continuum slopes ; \langle \beta _ { i } \rangle = -2.4 \pm 0.05 .
Using a set of simple dust attenuation models , in which the wavelength-dependent attenuation is assumed to be of the form A ( \lambda ) \propto \lambda ^ { n } , we explore the parameter values which best reproduce the observed z = 5 luminosity function ( LF ) and colour-magnitude relation ( CMR ) .
We find that a simple model in which the absolute UV attenuation is a linearly increasing function of log stellar mass ( A _ { 1500 } = 0.5 \times { log ( M / M _ { \odot } ) } -3.3 ) , and the dust attenuation slope ( n ) is within the range -0.7 \leq n \leq - 0.3 , can successfully reproduce the LF and CMR over a wide range of stellar population synthesis model ( SPS ) assumptions , including the effects of massive binaries .
This range of attenuation curves is consistent with a power-law fit to the Calzetti attenuation law in the UV ( n = -0.55 ) .
In contrast , curves as steep as the Small Magellanic Cloud ( SMC ) extinction curve ( n = -1.24 ) are formally ruled out .
Finally , we show that our models are consistent with recent 1.3mm ALMA observations of the Hubble Ultra Deep Field ( HUDF ) , and predict the form of the z \simeq 5 IRX - \beta relation .