Fredagskollokvium: Lyman- alpha haloes around high- z star-forming galaxies revealed by MUSE

The circum-galactic medium (CGM) serves as the interface between galaxies and the larger
structures within which they evolve. Composed primarily of cold hydrogen gas (also called
primordial gas), the CGM is a major fuel source for star formation as material falls onto a galaxy
from its surrounding halo. This suggests that star formation is in fact regulated by gas exchange
between a galaxy and its vicinity. Thus, studying the surrounding environment of galaxies
represents a crucial step in understanding the mechanisms governing their formation and
evolution. Unfortunately, direct observation of the CGM is often quite difficult, since these regions
are very faint. This task becomes even more challenging for galaxies in the distant Universe,
though some techniques have been developed for this purpose. The CGM can be detected
through absorption features in the spectrum of a more-distant quasar located along a galaxy’s line
of sight or statistically, by stacking many images of galaxies together, in order to increase the
overall S/N ratio of the sample. However, these methods are not ideal : both have severe
limitations and only provide partial information about the CGM.
In this talk, I report the detection of cold hydrogen gas surrounding 145 low-mass, faint and very
distant galaxies emitting Lyman-alpha (Lya) photons (forming 80% of the tested galaxy sample
used in this work). While historically, Lya emission was seen simply as a powerful tool for detecting
distant galaxies, it is now possible to use it as a tracer of cold CGM gas in the form of Lya halos.
The sample presented here represents the largest collection ever compiled of individually-detected
Lya halos around normal star-forming galaxies, observed in an epoch when the Universe was still
forming. This achievement is possible thanks to the unrivaled sensitivity of the Multi-Unit
Spectroscopic Explorer (MUSE), a next-generation instrument installed on the Very Large
Telescope (VLT). In particular, we need only 100 hours of telescope time to detect the presence of
Lya halos, a significant improvement over previous efforts.
Our results confirm the presence of large amounts of cold gas in the immediate vicinity of distant
galaxies. While such results have been predicted by theoretical models and numerical simulations,
this work provides some of the first direct observational evidence of this fact. Besides being quasi-
ubiquitous around galaxies, the observed Lya halos show a large diversity in physical properties
which is particularly remarkable for such a small region of the sky (9’ x 9’). Moreover, the 3D galaxy-
by-galaxy nature of our analysis allows us to study the direct impact of environment on galaxies, as
well as the evolution of the CGM with cosmic time. With such a large sample, we are also able to
perform a robust statistical analysis, highlighting the fact that the stellar properties of galaxies are
not systematically linked to the Lya ones. Finally, based on theoretical models, our (spectroscopic)
observations indicate the presence of expanding materials inside and/or around the galaxies.
However, the presence of galactic inflows are less constrained by the data.
Taken as a whole, this analysis represents important, new information about the CGM properties of
the relatively faint galaxies which make up the bulk of the galaxy population in the distant Universe.