We calculate constraints on radiatively decaying neutrinos from the recent detection of singly ionized helium in the diffuse intergalactic medium (IGM) at $z \simeq 3.3$. We consider a model in which neutrinos predominantly decay into invisible relativistic particles with a rate $\tau^{ -1}$, and with a small branching ratio into the radiative mode. To satisfy the observation of singly ionized helium, which puts a lower bound on the number density of singly ionized helium in the IGM, we show that: for $\tau \ga 10^{ 18} \,{ \rm sec}(1 \,{ \rm eV}/m_\nu^2)$, transition moment of neutrinos $\mu_{ 12}$ is constrained to be $\le 4 \hbox{ --}8 \times 10^{ -17} \,\mu_{ \sc B}$ for $ 110 \,{ \rm eV} \la m_\nu \la 10 \,{ \rm keV}$. We compare this bound with other astrophysical and cosmological bounds on radiatively decaying neutrinos.
Categories: astro-ph