The cold circumgalactic medium in emission: Mg II haloes in TNG50

We outline theoretical predictions for extended emission from Mg II, tracing cool similar to 10(4)K gas in the circumgalactic medium (CGM) of star-forming galaxies in the high-resolution TNG50 cosmological magnetohydrodynamical simulation. We synthesize surface brightness maps of this strong rest-frame ultraviolet metal emission doublet (lambda lambda 2796, 2803), adopting the assumption that the resonant scattering of Mg If can be neglected and connecting to recent and upcoming observations with the Keck/KCWI, VLT/MUSE, and BlueMUSE optical integral field unit spectrographs. Studying galaxies with stellar masses 7.5 < log (M-*/M-circle dot) < 11 at redshifts z = 0.3, 0.7, 1, and 2 we find that extended Mg II haloes in emission, similar to their Ly alpha counterparts, are ubiquitous across the galaxy population. Median surface brightness profiles exceed 10(-19 )erg s(-1) cm(-2) arcsec(-2) in the central 10 s of kpc, and total halo Mg It luminosity increases with mass for star-forming galaxies, reaching 10(40) erg s(-1) for M-*similar to 10(9.5)( )M(circle dot). Mg II halo sizes increase from a few kpc to >= 20 kpc at the highest masses, and sizes are larger for haloes in denser environments. Mg If haloes are highly structured, clumpy, and asymmetric, with isophotal axial ratio increasing with galaxy mass. Similarly, the amount and distribution of Mg ft emission depends on the star formation activity of the central galaxy. Kinematically, inflowing versus outflowing gas dominates the Mg If luminosity at high and low galaxy masses, respectively, although the majority of Mg If halo emission at z similar to 0.7 traces near-equilibrium fountain flows and gas with non-negligible rotational support, rather than rapidly outflowing galactic winds.

Automated summary

The study predicts widespread extended Mg II emission haloes surrounding star-forming galaxies, with brightness exceeding 10^(-19) erg s^(-1) cm^(-2) arcsec^(-2) in the central 10 s of kpc. The size of these haloes increases with galaxy mass and environmental density, showing a structured and asymmetric distribution.