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The VIMOS Ultra Deep Survey: Ly$α$ Emission and Stellar Populations of Star-Forming Galaxies at 2<z<2.5

The aim of this paper is to investigate spectral and photometric properties of 854 faint ($i_{AB}$<~25 mag) star-forming galaxies (SFGs) at 2<z<2.5 using the VIMOS Ultra-Deep Survey (VUDS) spectroscopic data and deep multi-wavelength photometric data in three extensively studied extragalactic fields (ECDFS, VVDS, COSMOS). These SFGs were targeted for spectroscopy based on their photometric redshifts. The VUDS spectra are used to measure the UV spectral slopes ($β$) as well as Ly$α$ equivalent widths (EW). On average, the spectroscopically measured $β$ (-1.36$\pm$0.02), is comparable to the photometrically measured $β$ (-1.32$\pm$0.02), and has smaller measurement uncertainties. The positive correlation of $β$ with the Spectral Energy Distribution (SED)-based measurement of dust extinction, E$_{\rm s}$(B-V), emphasizes the importance of $β$ as an alternative dust indicator at high redshifts. To make a proper comparison, we divide these SFGs into three subgroups based on their rest-frame Ly$α$ EW: SFGs with no Ly$α$ emission (SFG$_{\rm N}$; EW$\le$0Å), SFGs with Ly$α$ emission (SFG$_{\rm L}$; EW$>$0Å), and Ly$α$ emitters (LAEs; EW$\ge$20Å). The fraction of LAEs at these redshifts is $\sim$10%, which is consistent with previous observations. We compared best-fit SED-estimated stellar parameters of the SFG$_{\rm N}$, SFG$_{\rm L}$ and LAE samples. For the luminosities probed here ($\sim$L$^*$), we find that galaxies with and without Ly$α$ in emission have small but significant differences in their SED-based properties. We find that LAEs have less dust, and lower star-formation rates (SFR) compared to non-LAEs. We also find that LAEs are less massive compared to non-LAEs, though the difference is smaller and less significant compared to the SFR and E$_{\rm s}$(B-V). [abridged]