2017A&A...608A...4M

The C III] λλ1907, 1909 emission doublet has been proposed as an alternative to Lyman-α in redshift confirmations of galaxies at z≥6 since it is not attenuated by the largely neutral intergalactic medium at these redshifts and is believed to be strong in the young, vigorously star-forming galaxies present at these early cosmic times. We present a statistical sample of 17 C III]-emitting galaxies beyond z∼1.5 using ∼30h deep VLT/MUSE integral field spectroscopy covering 2 square arcminutes in the Hubble Deep Field South (HDFS) and Ultra Deep Field (UDF), achieving C III] sensitivities of ∼2x10^–17erg/s/cm² in the HDFS and ∼7x10^–18erg/s/cm² in the UDF. The rest-frame equivalent widths range from 2 to 19Å. These 17 galaxies represent ∼3% of the total sample of galaxies found between 1.5≤z≤4. They also show elevated star formation rates, lower dust attenuation, and younger mass-weighted ages than the general population of galaxies at the same redshifts. Combined with deep slitless grism spectroscopy from the HST/WFC3 in the UDF, we can tie the rest-frame ultraviolet CIII] emission to rest-frame optical emission lines, namely [OIII] λ5007, finding a strong correlation between the two. Down to the flux limits that we observe (∼1x10^–18erg/s/cm² with the grism data in the UDF), all objects with a rest-frame [OIII] λλ4959, 5007 equivalent width in excess of 250Å, the so-called extreme emission line galaxies, have detections of CIII] in our MUSE data. More detailed studies of the CIII]-emitting population at these intermediate redshifts will be crucial to understand the physical conditions in galaxies at early cosmic times and to determine the utility of CIII] as a redshift tracer.