Paving the way for the JWST: witnessing globular cluster formation at z > 3

Abstract

We report on five compact, extremely young (<10Myr) and blue (\\beta_UV<-2.5,\nF_\\lambda =\\lambda^\\beta) objects observed with VLT/MUSE at redshift 3.1169,\n3.235, in addition to three objects at z=6.145. These sources are magnified by\nthe Hubble Frontier Field galaxy clusters MACS~J0416 and AS1063. Their\nde-lensed half light radii (Re) are between 16 to 140pc, the stellar masses are\n~1-20 X 10^6 Msun, the magnitudes are m_uv=28.8 - 31.4 (-17<Muv<-15) and\nspecific star formation rates can be as large as ~800Gyr^-1. Multiple images of\nthese systems are widely separated in the sky (up to 50'') and individually\nmagnified by factors 3-40. Remarkably, the inferred physical properties of two\nobjects are similar to those expected in some globular cluster formation\nscenarios, representing the best candidate proto-globular clusters (proto-GC)\ndiscovered so far. Rest-frame optical high dispersion spectroscopy of one of\nthem at z=3.1169 yields a velocity dispersion \\sigma_v~20km/s, implying a\ndynamical mass dominated by the stellar mass. Another object at z=6.145, with\nde-lensed Muv ~ -15.3 (m_uv ~ 31.4), shows a stellar mass and a star-formation\nrate surface density consistent with the values expected from popular GC\nformation scenarios. An additional star-forming region at z=6.145, with\nde-lensed m_uv ~ 32, a stellar mass of 0.5 X 10^6 Msun and a star formation\nrate of 0.06 Msun/yr is also identified. These objects currently represent the\nfaintest spectroscopically confirmed star-forming systems at z>3, elusive even\nin the deepest blank fields. We discuss how proto-GCs might contribute to the\nionization budget of the universe and augment Lya visibility during\nreionization. This work underlines the crucial role of JWST in characterizing\nthe rest-frame optical and near-infrared properties of such low-luminosity\nhigh-z objects.\n