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The 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 IIλλ2796, 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 Ly α 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. [ABSTRACT FROM AUTHOR]

Emission signatures from galactic winds provide an opportunity to directly map the outflowing gas, but this is traditionally challenging because of the low surface brightness. Using very deep observations (27 h) of the Hubble Deep Field South with the Multi Unit Spectroscopic Explorer (MUSE) instrument, we identify signatures of an outflow in both emission and absorption from a spatially resolved galaxy at z = 1.29 with a stellar mass M* = 8 × 109M*, star formation rate SFR = 77+40-25 M* yr-1, and star formation rate surface brightness ΣSFR = 1.6M☉ kpc-2 within the [OII] λλ3727,3729 half-light radius R1/2, [OII] = 2.76 ± 0.17 kpc. From a component of the strong resonant Mg II and Fe II absorptions at --350 km s-1, we infer a mass outflow rate that is comparable to the star formation rate. We detect non-resonant Fe II* emission, at λ2365, λ2396, λ2612, and λ2626, at 1.2--2.4--1.5--2.7 × 10-18 erg s-1 cm-2 respectively. The flux ratios are consistent with the expectations for optically thick gas. By combining the four non-resonant Fe II* emission lines, we spatially map the Fe II* emission from an individual galaxy for the first time. The Fe II* emission has an elliptical morphology that is roughly aligned with the galaxy minor kinematic axis, and its integrated half-light radius, R1/2, Fe II * =4.1 ± 0.4 kpc, is 70% larger than the stellar continuum (R1/2,☉ ≃2.34 ± 0.17) or the [OII] nebular line. Moreover, the Fe II* emission shows a blue wing extending up to --400 km s-1, which is more pronounced along the galaxy minor kinematic axis and reveals a C-shaped pattern in a p--v diagram along that axis. These features are consistent with a bi-conical outflow. [ABSTRACT FROM AUTHOR]

In merger-driven models of massive galaxy evolution, the luminous quasar phase is expected to be accompanied by vigorous star formation in quasar host galaxies. In this paper, we use high column density damped Lyα (DLA) systems along quasar sight lines as natural coronagraphs to directly study the far-UV (FUV) radiation from the host galaxies of luminous background quasars. We have stacked the spectra of ∼2000 DLA systems (NH I > 1020.6 cm–2) with a median absorption redshift 〈 z〉 = 2.6 selected from quasars observed in the SDSS-III Baryon Oscillation Spectroscopic Survey. We detect residual flux in the dark troughs of the composite DLA spectra. The level of this residual flux significantly exceeds systematic errors in the Sloan Digital Sky Survey fiber sky subtraction; furthermore, the residual flux is strongly correlated with the continuum luminosity of the background quasar, while uncorrelated with DLA column density or metallicity. We conclude that the flux could be associated with the average FUV radiation from the background quasar host galaxies (with medium redshift 〈 z〉 = 3.1) that is not blocked by the intervening DLA. Assuming that all of the detected flux originates from quasar hosts, for the highest quasar luminosity bin (〈 L〉 = 2.5 × 1013L☼), the host galaxy has an FUV intensity of 1.5 ± 0.2 × 1040 erg s–1 Å–1; this corresponds to an unobscured UV star formation rate of 9 M☼ yr–1. [ABSTRACT FROM AUTHOR]