Discovery of nine Ly alpha emitters at redshift z similar to 3.1 using narrowband imaging and VLT spectroscopy

Narrowband imaging surveys aimed at detecting the faint emission from the 5 007 Angstrom [O III] line of intracluster planetary nebulae in Virgo also p robe high-redshift z similar to 3.1 Ly alpha emitters. Here we report on th e spectroscopic identification of nine Lya emitters at 3.13 with fluxes bet ween 2 x 10(-17) and 2 x 10(-16) ergs cm(-2) s(-1) obtained with the FORS s pectrograph at Unit 1 of the ESO Very Large Telescope (VLT UT1). The spectr a of these high-redshift objects show a narrow, isolated Lya emission with very faint (frequently undetected) continuum, indicating a large equivalent width. No other features are visible in our spectra. Our Ly alpha emitters are quite similar to those found by Hu, Cowie, and colleagues in 1998. For a flat universe with H-0 = 70 km s(-1) Mpc(-1) and q(0) = 0.5 (Omega(A) = 0), the Ly alpha luminosity of the brightest source is 1.7 x 10(9) L-circle dot, and the comoving space density of the Ly alpha emitters in the search ed volume is 5 x 10(-3) Mpc(-3) Using simple population synthesis models, o n the assumption that these sources are regions of star formation, we concl ude that the nebulae are nearly optically thick and must have a very low du st content in order to explain the high observed Ly alpha equivalent widths . For the cosmological and star formation parameters we adopted, the total stellar mass produced would seem to correspond to the formation of rather s mall galaxies, some of which are perhaps destined to merge. However, one of our sources might become a serious candidate for a protogiant spheroidal g alaxy if we assumed continuous star formation, a low mass cutoff of 0.1 M-c ircle dot in the initial mass function (IMF), and a flat accelerating unive rse with Omega(0), = 0.2 and Omega(Lambda) = 0.8. The implied star formatio n density in our sampled comoving volume is probably somewhat smaller than, but of the same order of magnitude as, the star formation density at z sim ilar to 3 derived by other authors from Lyman break galaxy surveys. This re sult agrees with the expectation that the Ly alpha emitters are a low-metal licity (or low-dust) tail in a distribution of star-forming regions at high redshifts. Finally, the Ly alpha emitters may contribute as many H-ionizin g photons as QSOs at z similar to 3. They are therefore potentially signifi cant for the ionization budget of the early universe.