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Massive Neutrinos Promote the Size Growth of Early-Type Galaxies

The effect of massive neutrinos on the evolution of the early type galaxies (ETGs) in size ($R_{e}$) and stellar mass ($M_{\star}$) is explored by tracing the merging history of galaxy progenitors with the help of the robust semi-analytic prescriptions. We show that as the presence of massive neutrinos plays a role of enhancing the mean merger rate per halo, the high-$z$ progenitors of a descendant galaxy with fixed mass evolves much more rapidly in size for a $Λ$MDM ($Λ$CDM + massive neutrinos) model than for the $Λ$CDM case. The mass-normalized size evolution of the progenitor galaxies, $R_{e}[M_{\star}/(10^{11}M_{\odot})]^{-0.57}\propto (1+z)^{-β}$, is found to be quite steep with the power-law index of $β\sim 1.5$ when the neutrino mass fraction is $f_ν=0.05$, while it is $β\sim 1$ when $f_ν=0$. It is concluded that if the presence and role of massive neutrinos are properly taken into account, it may explain away the anomalous compactness of the high-$z$ ETGs compared with the local ellipticals with similar stellar masses.