Abstract
We report of five compact, extremely young (<10Myr) and blue (\beta<-2.5,
Fłambda=łambda^\beta) objects observed with VLT/MUSE at z=3.1169, 3.235, plus
three additional objects at z=6.145. These sources are magnified by the Hubble
Frontier Field galaxy clusters MACS~J0416 and AS1063. Their intrinsic half
light radii (Re) are in the range between 14 to 140pc. We estimate de-lensed
stellar masses ~1-20x10^6 Msun and magnitudes m_UV=28.8-31.4 (i.e.,
-17<M_UV<-15). Multiple images of these systems are widely separated in the sky
(up to 50 arcsec) and individually magnified by factors 3-40. Remarkably, the
inferred physical properties of two objects are similar to those expected in
some globular cluster formation scenarios, representing the best candidate
proto-globular clusters (proto-GC) discovered so far. Interestingly, one of
them at z=3.1169, with de-lensed Re~43pc and available rest-frame optical
spectroscopy, shows a velocity dispersion \sigma_v~20 km\s, implying a
dynamical mass dominated by the stellar mass. Another object at z=6.145, with
de-lensed M_UV ~ -15.3 (m_UV~31.4), shows a stellar mass and a star-formation
rate surface density compatible with the values expected in the most popular GC
formation scenarios. An additional star-forming region at z=6.145, with
de-lensed m_UV~32 and stellar mass and star formation rate estimates of 0.5 x
10^6 Musn and 0.06 Msun/yr, is also identified. These objects currently
represent the faintest spectroscopically confirmed star-forming systems at z>3,
elusive even in the deepest blank fields. We consider the possibility that
proto-GCs might contribute to the ionization budget of the universe and augment
the Lya visibility during reionization. This work underlines the crucial role
of JWST in characterizing the rest-frame optical and near-infrared properties
of such low-luminosity high-z objects.
Description
[1612.01526] Paving the way for the JWST: witnessing globular cluster formation at z>3
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