Context :

Aims : To investigate the ingredients , which allow star-forming galaxies to present Ly \alpha line in emission , we studied the kinematics and gas phase metallicity of the interstellar medium .

Methods : We used multi-object near-infrared spectroscopy with Magellan /MMIRS to study nebular emission from z \simeq 2 - 3 star-forming galaxies discovered in three MUSYC fields .

Results : We detected emission lines from four active galactic nuclei and 13 high-redshift star-forming galaxies , including H \alpha lines down to a flux of ( 4 \pm 1 ) E-17 erg sec ^ { -1 } cm ^ { -2 } .
This yielded seven new redshifts .
The most common emission line detected is [ OIII ] 5007 , which is sensitive to metallicity .
We were able to measure metallicity ( Z ) for two galaxies and to set upper ( lower ) limits for another two ( two ) .
The metallicity values are consistent with 0.3 < Z / Z _ { \odot } < 1.2 , 12 + log ( O/H ) \sim 8.2-8.8 .
Comparing the Ly \alpha central wavelength with the systemic redshift , we find \Delta v _ { Ly \alpha - [ OIII ] 5007 } = 70-270 km sec ^ { -1 } .

Conclusions : High-redshift star-forming galaxies , Ly \alpha emitting ( LAE ) galaxies , and H \alpha emitters appear to be located in the low mass , high star-formation rate ( SFR ) region of the SFR versus stellar mass diagram , confirming that they are experiencing burst episodes of star formation , which are building up their stellar mass .
Their metallicities are consistent with the relation found for z \leq 2.2 galaxies in the Z versus stellar mass plane .
The measured \Delta v _ { Ly \alpha - [ OIII ] 5007 } values imply that outflows of material , driven by star formation , could be present in the z \sim 2 - 3 LAEs of our sample .
Comparing with the literature , we note that galaxies with lower metallicity than ours are also characterized by similar \Delta v _ { Ly \alpha - [ OIII ] 5007 } velocity offsets .

Strong F ( [ OIII ] 5007 ) is detected in many Ly \alpha emitters .
Therefore , we propose the F ( Ly \alpha ) /F ( [ OIII ] 5007 ) flux ratio as a tool for the study of high-redshift galaxies ; while influenced by metallicity , ionization , and Ly \alpha radiative transfer in the ISM , it may be possible to calibrate this ratio to primarily trace one of these effects .