Diffuse UV light associated with the Spiderweb Galaxy: evidence for in-situ star formation outside galaxies

TL;DR

This study reveals diffuse UV light around a high-redshift radio galaxy, indicating widespread in-situ star formation in the galaxy's halo, which is a different mode from the typical clump-like star formation in the early universe.

Contribution

It provides evidence for a diffuse mode of star formation in the halos of massive high-redshift galaxies, expanding understanding of galaxy growth mechanisms.

Findings

Diffuse UV intergalactic light is likely from young stars in a halo.

Approximately 44% of star formation occurs in diffuse mode within 70kpc.

Diffuse star formation may be a significant component in galaxy evolution at high redshift.

Abstract

We present detailed images of diffuse UV intergalactic light (IGL), situated in a 60kpc halo that surrounds the radio galaxy MRC 1138-262 at z=2. We discuss the nature of the IGL and rule out faint cluster galaxies, nebular continuum emission, synchrotron, inverse Compton emission and scattering of galactic stellar light as possible sources of the IGL. Dust scattered quasar light is an unlikely possibility that cannot be ruled out entirely. We conclude that the source of the IGL is most likely to be a young stellar population distributed in a halo encompassing the radio and satellite galaxies, undergoing star formation at a rate greater than 57 Msun/yr. Within 70kpc of the radio core, approximately 44% of the star formation that is traced by UV light occurs in this diffuse mode. The average UV colour of the IGL is bluer than the average galaxy colour, and there is a trend for the IGL to become bluer with increasing radius from the radio galaxy. Both the galaxies and the IGL show a UV colour--surface brightness relation which can be obtained by variations in either stellar population age or extinction. These observations show a different, but potentially important mode of star formation, that is diffuse in nature. Star formation, as traced by UV light, occurs in two modes in the high redshift universe: one in the usual Lyman break galaxy clump-like mode on kpc scales, and the other in a diffuse mode over a large region surrounding massive growing galaxies. Such a mode of star formation can easily be missed by high angular resolution observations that are well suited for detecting high surface brightness compact galaxies. Extrapolating from these results, it is possible that a significant amount of star formation occurs in large extended regions within the halos of the most massive galaxies forming at high redshift.