Claudia Scarlata
IPAC - Caltech
 

"HeII: Population III Stars or Cold Accretion?"
 

Stellar mass growth in galaxies can be due to either mergers or gas accretion with sequent star formation. Recent theoretical works suggest that the gas accretion modes depend on the mass of the dark matter halo, and on the galaxy redshift. Although at redshift $z>~3$  the so called "cold accretion" mode is predicted to be the dominant  mechanism for a galaxy to acquire its gas, only a few (non conclusive) observations exist. Here we present the results of our study of  an extended \lya\ nebula located in a known overdensity  region at $z\sim 2.4$.  The data include multiwavelengths photometry covering the spectral range from 0.3 to 850$\mu$m, and a deep rest frame UV spectroscopy of the nebula and nearby sources.
The rest frame UV spectrum of the nebula shows a significant detection of \heii\  emission line. The width of the line, together with the absence of other high ionization emission lines, and the  shape of the continuum, suggest that  the studied nebula is produced by cold accretion  of pristine gas into a regular starburst galaxy.