A galactic outflow traced by its extended Mg II emission out to a $\sim30$ kpc radius in the Hubble Ultra Deep Field with MUSE

TL;DR

This study reports the discovery of a large Mg II emission halo around a star-forming galaxy at z=0.737, revealing insights into galactic outflows and the circumgalactic medium through deep MUSE observations and radiative transfer modeling.

Contribution

It introduces a simple outflow radiative transfer model fitted with Bayesian MCMC to interpret extended Mg II emission and constrain the outflow's kinematics and geometry.

Findings

Extended Mg II emission spans ~30 kpc from the galaxy.

Outflow velocity increases with radius, reaching ~120 km/s at 10 kpc.

Model fits better within inner regions, suggesting additional processes influence outer CGM.

Abstract

We report the discovery of a rare Mg II $\lambda$$\lambda$ 2796, 2803 doublet emission halo around a star forming galaxy with $\log (M_\star$/M$_\odot) = 10.3 \pm 0.3$ at $z=0.737$ in deep (9.94 h) VLT/MUSE data from the MUSE-HUDF mosaic. While the central region prominently displays an absorption-dominated Mg II doublet, characterized by discernible P-Cyg features, our examination reveals a remarkably extended Mg II emission, spanning approximately $\sim30$ kpc from the central galaxy. We introduce a simple outflow radiative transfer modeling scheme based on the Sobolev approximation, and we employ a Bayesian Monte Carlo Markov Chain (MCMC) fitting to find the best-fitting parameters that match our data. The model reproduces several key features of the observed Mg II halo and allows us to constrain the kinematics and geometry of the outflowing gas. Our data are consistent with a biconical wind whose velocity increases with radius, pointing nearly towards the observer, with an opening angle of $59\pm4^{\circ}$ In general, we find that our outflow model performs better in the inner regions of the galactic wind ($\lesssim 10$ kpc $\approx 6$ half-light radii), reaching a velocity of $\sim120$ km s$^{-1}$ at 10 kpc from the central galaxy. However, discrepancies between the data and the model in the outer regions suggest the possible influence of additional mechanisms, such as inflows, satellite interactions, or turbulence, which might significantly shape the circumgalactic medium (CGM) of galaxies at larger impact parameters. This analysis underscores the complexity of galactic outflows and encourages further exploration of the processes governing the dynamics of galactic winds through spatially resolved studies of the CGM.