A study of the invisible circum-galactic medium around A z=0.77 star-forming galaxy using gravitational-ARC tomography

Abstract

Gravitational arc-tomography is a novel technique that allows one to study the absorbing high-redshift circum-galactic medium (CGM) in a two-dimensional fashion by using giant gravitational arcs as background sources. Arc-tomography gives one the unique opportunity to probe this key galaxy component at several independent positions around individual galaxies. In this thesis, gravitational-arc tomography is applied to a z=0.77 absorption system towards the giant gravitational arc SGAS J1226+2152. This system is special because the arc is composed of three different segments (North, South and East of the absorbing galaxy itself, hereafter G1), probing opposite sides of G1 mostly along its minor axis, making it perfectly suited for the study of galaxy-scale outflows. With the help of VLT/MUSE and Magellan/MagE data, I detect both Mg ii (a tracer for cool gas in the CGM) in absorption and [O ii] (a tracer for ionised gas in the ISM) in emission around G1. After fitting the line profiles at several MUSE spaxels, I am able to create both equivalent width and velocity maps for the absorbing CGM. Strong Mg ii (EW > 0.3 Å) absorption is detected on the North and South segments of the arc. A third, more distant arc segment named East shows no detections to sensible limits. The non-detections put a constraint on the maximum extension of the strong Mg ii of 54 kpc in that direction. From the equivalent width distribution as a function of impact parameter, I conclude that the CGM of G1 is clumpy and anisotropic, although it shows similar scatter to the quasar statistics. This is probably due to the fact that the arc probes mostly the minor axis of G1. An emission velocity map is made using the [O ii] detections, and fitted with a model generated with the GalPaK software, a morphokinematic fitting tool. From the emission velocity model, G1 s ISM (as tracked by [O ii]) is clearly rotating. But astonishingly, part of the much extended absorption velocity map is also consistent with co-rotation of G1 s ISM. Finally, I also find three pieces of evidence for an outflow from G1: (1) the absorption shows a second velocity component when seen against G1 itself; (2) stronger equivalent width is located at higher azimuthal angles; and (3) there is a velocity gradient along the North-South direction. Altogether, this is perhaps the first time the so called baryonic cycle around galaxies is studied in so much detail around a single object at high redshifts, thanks to the great conjunction of a novel technique, a unique dataset, and a fortunate geometrical configuration.