tokens: in Sect. 3 are indicated as blue circles filled with gray, while values for the average stacks in Table A.1 are indicated as open blue circles. The correlation between EW(CIII]) and EW(Ly α ) appears to be very strong ( r P = 0.99, r S = 1) and hides the wide range of properties of CIII] vs. Ly α . As a comparison with the literature, we plot the observed values from the different Ly α samples of Shapley et al. (2003) (green stars, average for Ly α samples), Rigby et al. (2015) (purple diamonds), and Stark et al. (2014 , 2015) (orange squares). This behavior is somewhat expected as the radiative transfer of Ly α or CIII] photons is markedly different. As a striking example we point to object 520391642 (second from the top in Fig. 1 ), which shows strong CIII] (EW > 20), as well as CIV, HeII, or NV, but only weak Ly α emission. Our results compare well with the literature when the large spread of the relation between EW(CIII]) and EW(Ly α ), the selection, and the size of different samples is taken into account. The average properties from stacks show that our results agree well with those of Shapley et al. (2003) . VUDS galaxies span a broad range of properties, including galaxies with and without Ly α in emission, expanding from smaller and possibly biased samples for which a stronger correlation between Ly α and CIII] is reported ( Stark et al. 2014 , 2015 ; Rigby et al. 2015 ; Guaita et al. 2017 ). We find that the chance of emitting CIII] and CIV at an appreciable level (EW > 3 Å) for galaxies with EW(Ly α ) > 25 (the broadly used definition of a LAE) is 42% and 20%, respectively. For galaxies that emit both Ly α and CIII] at these levels, the median EW(CIII]) is 7.7 Å, compared to a median EW(CIV) of 3.4 Å for the average LAE emitting CIV. The overall weak correlation that is observed between CIII] and Ly α emission strength for individual galaxies is in line with findings from photoionization models (e.g., Jaskot Ravindranath 2016 ; Zackrisson et al. 2017 ; Nakajima et al. 2018 ) when a broad range of conditions such as ionizing fields, geometry, and dust opacity is taken into account. 5. Distribution of EW(CIII]) To answer the question of how frequently CIII] emission is observed in UV-selected galaxies, we seek to establish the fraction of CIII] emitters in the VUDS sample as a function of EW(CIII]). While it is straightforward to identify strong CIII] emitters, it is more difficult to separate weak emitters from noise. We therefore proceed with a statistical analysis of the distribution of CIII] emission for the 3899 VUDS galaxies with 2 z 3.8. If CIII] emission is absent and in the presence of noise, the line measurement resulting from the process presented in Sect. 2.2 could result in either a false-positive or negative EW. As CIII] is only expected in emission, we use the distribution of EW measurements that indicates absorption as an estimate of how many CIII] emission lines could be produced from noise spikes, under the
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