The MUSE Hubble Ultra Deep Field Survey.: XII. Mg II emission and absorption in star-forming galaxies

International audienceThe physical origin of the near-ultraviolet Mg ii emission remains an underexplored domain, unlike more typical emission lines that are detected in the spectra of star-forming galaxies. We explore the nebular and physical properties of a sample of 381 galaxies between 0.70 < z < 2.34 drawn from the MUSE Hubble Ultra Deep Survey. The spectra of these galaxies show a wide variety of profiles of the Mg ii λλ2796, 2803 resonant doublet, from absorption to emission. We present a study on the main drivers for the detection of Mg ii emission in galaxy spectra. By exploiting photoionization models, we verified that the emission-line ratios observed in galaxies with Mg ii in emission are consistent with nebular emission from Hii regions. From a simultaneous analysis of MUSE spectra and ancillary Hubble Space Telescope information through spectral energy distribution fitting, we find that galaxies with Mg ii in emission have lower stellar masses, smaller sizes, bluer spectral slopes, and lower optical depth than those with absorption. This leads us to suggest that Mg ii emission is a potential tracer of physical conditions that are not merely related to those of the ionized gas. We show that these differences in Mg ii emission and absorption can be explained in terms of a higher dust and neutral gas content in the interstellar medium (ISM) of galaxies showing Mg ii in absorption, which confirms the extreme sensitivity of Mg ii to the presence of the neutral ISM. We conclude with an analogy between the Mg ii doublet and the Lyα line that lies in their resonant nature. Further investigations with current and future facilities, including the James Webb Space Telescope, are promising because the detection of Mg ii emission and its potential connection with Lyα could provide new insights into the ISM content in the early Universe