SPT0311-58 JWST, ALMA, and HST images

The extinction-corrected Pa{alpha} lower limits to the star formation rates are 383 and 230M_{sun}_/yr for the E and W galaxies, respectively. This represents 50 of the SFRs derived from the [CII]158um line and infrared light for the E galaxy and as low as 6% for the W galaxy. The specific star formation (sSFR) in the stellar clumps ranges from 25 to 59Gyr^-1^ assuming a star formation with a 50-100Myr constant rate.

This sSFR is 3 to 10 times larger than the values measured in galaxies of similar mass at redshifts 6 to 8: SPT0311-58 thus clearly stands out as a starburst system when compared with typical massive star-forming galaxies at similar high redshifts. The MIRI observations reveal a clumpy stellar structure, with each clump having 3 to 5 x10^9^M_{sun}_ mass in stars, leading to a total stellar mass of 2.0 and 1.5 x10^10M_{sun}_ for the E and W galaxies, respectively.

The overall gas mass fraction is M_gas_/M*~3, similar to that of z~4.5-6 star-forming galaxies, suggesting a flattening of the gas mass fraction in massive starbursts up to redshift 7. The kinematics of the ionised gas in the E galaxy agrees with the known [CII] gas kinematics, indicating a physical association between the ionised gas and cold dust/gas clumps. The situation in the W galaxy is more complex as it appears to be a velocity offset of about +700km/s in the Pa{alpha} relative to the [CII] emitting gas.

The nature of this offset, and its reality, is not fully established and requires further investigation. The observed properties of SPT0311-58 such as the clumpy distribution at sub(kpc) scales and the very high average extinction are similar to those observed in low- and intermediate-z luminous (E galaxy) and ultraluminous (W galaxy) infrared galaxies, even though SPT0311-58 is observed only ~800Myr after the Big Bang.

Massive, heavily obscured, clumpy starburst systems like SPT0311-58 likely represent the early phases in the formation of massive high-redshift bulge/spheroids and luminous quasars. MIRI and JWST will be able to explore for the first time the rest-frame near-infrared stellar and ionised gas structure of these galaxies, even during the Epoch of Reionization as demonstrated by this study.