We compare model results from our semi-analytic merger tree based framework for high-redshift ( z \simeq 5 - 20 ) galaxy formation against reionization indicators including the Planck electron scattering optical depth ( \tau _ { es } ) and the ionizing photon emissivity ( \dot { n } _ { ion } ) to constrain the particle mass of Warm Dark Matter ( WDM ) .
Our framework traces the Dark Matter ( DM ) and baryonic assembly of galaxies in 4 DM cosmologies : Cold Dark Matter ( CDM ) and WDM with a particle mass of m _ { x } = 2.25 , 3 and 5 keV .
It includes all the key processes of star formation , supernova feedback , the merger/accretion/ejection driven evolution of gas and stellar mass , and the effect of the ultra-violet background ( UVB ) created during reionization in photo-evaporating the gas content of galaxies in halos with M _ { h } \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 10 ^ { 9 } M _ { \odot } .
We show that current Planck \tau _ { es } values rule out m _ { x } \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 2.5 keV WDM , even in the physically unlikely scenario that all ionizing photons produced by these galaxies escape and contribute to reionization ( i.e . f _ { esc } = 1 ) .
With the largest number of UVB-suppressed galaxies , CDM faces a “ stalling ” of the reionization process with this effect decreasing with the disappearance of small-scale structure with decreasing m _ { x } .
Finally , we find the bulk of the reionization photons come from galaxies with a halo mass M _ { h } \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 10 ^ { 9 } M _ { \odot } , stellar mass M _ { * } \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 10 ^ { 7 } M _ { \odot } and UV magnitude -18 \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } M _ { UV } \lower 2.15 pt \hbox { $ % \buildrel < \over { \sim } $ } -13 in CDM .
The progressive suppression of low-mass halos with decreasing m _ { x } leads to a shift in the “ reionization ” population to larger ( halo and stellar ) masses of M _ { h } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 10 ^ { 9 } M _ { \odot } and M _ { * } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 10 ^ { 7 } M _ { \odot } for m _ { x } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 3 keV WDM , although the UV limits effectively remain unchanged .