Discovery of Nine Lyα Emitters at Redshiftz ∼ 3.1 Using Narrowband Imaging and VLT Spectroscopy

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Narrowband imaging surveys aimed at detecting the faint emission from the 5007 A [O III] line of intracluster planetary nebulae in Virgo also probe high-redshift z ~ 3.1 Lyα emitters. Here we report on the spectroscopic identification of nine Lyα emitters at z = 3.13 with fluxes between 2 × 10-17 and 2 × 10-16 ergs cm-2 s-1 obtained with the FORS spectrograph at Unit 1 of the ESO Very Large Telescope (VLT UT1). The spectra of these high-redshift objects show a narrow, isolated Lyα emission with very faint (frequently undetected) continuum, indicating a large equivalent width. No other features are visible in our spectra. Our Lyα emitters are quite similar to those found by Hu, Cowie, and colleagues in 1998. For a flat universe with H0 = 70 km s-1 Mpc-1 and q0 = 0.5 (ΩΛ = 0), the Lyα luminosity of the brightest source is 1.7 × 109 L☉, and the comoving space density of the Lyα emitters in the searched volume is 5 × 10-3 Mpc-3. Using simple population synthesis models, on the assumption that these sources are regions of star formation, we conclude that the nebulae are nearly optically thick and must have a very low dust content in order to explain the high observed Lyα 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 small galaxies, some of which are perhaps destined to merge. However, one of our sources might become a serious candidate for a protogiant spheroidal galaxy if we assumed continuous star formation, a low mass cutoff of 0.1 M☉ in the initial mass function (IMF), and a flat accelerating universe with Ω0 = 0.2 and ΩΛ = 0.8. The implied star formation 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 ~ 3 derived by other authors from Lyman break galaxy surveys. This result agrees with the expectation that the Lyα emitters are a low-metallicity (or low-dust) tail in a distribution of star-forming regions at high redshifts. Finally, the Lyα emitters may contribute as many H-ionizing photons as QSOs at z ~ 3. They are therefore potentially significant for the ionization budget of the early universe.