JWST view of three infant galaxies at z=8.3 and implications for reionization
arxiv(2024)
Abstract
New JWST/NIRCam wide-field slitless spectroscopy provides redshifts for two z
> 8 galaxies located behind the lensing cluster MACS J0416.1-2403. Both
galaxies are strong [O iii]λ5007 emitters. For one galaxy, "Y1", the
existing redshift z = 8.31, based on ALMA measurements of [O iii] 88 μm and
[C ii] 157.7 μm lines, is confirmed. JWST/NIRCam images resolve this galaxy
into three components of similar colors, and the whole system extends over 3.4
kpc. The other galaxy, "JD", is at z = 8.34 instead of the previously claimed z
= 9.28. It has a companion, "JD-N", at the same redshift with projected
separation 2.3 kpc. All objects are only moderately magnified and have
intrinsic MUV ranging from -19.66 to -20.85 mag. Their eight-band NIRCam
spectral energy distributions show that the galaxies are all very young with
ages ≲11 Myr and stellar masses about 108 M_⊙. These infant
galaxies are actively forming stars at rates of a few tens to a couple of
hundred M_⊙ yr^-1, but only one of them (JD) has a blue rest-frame UV
slope. This slope indicates a high Lyman-continuum photon escape fraction that
could contribute significantly to the cosmic hydrogen-reionizing background.
The other two systems have much flatter slopes largely because their dust
extinction is twice as high as JD's albeit only AV 0.90 mag. The much lower
indicated escape fractions show that even very young, actively star-forming
galaxies can have negligible contribution to reionization when they quickly
form dust throughout their bodies.
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