Accelerated Formation of Ultra-Massive Galaxies in the First Billion Years

Abstract

Recent JWST observations have revealed an unexpected abundance of massive galaxy candidates in the early Universe, extending further in redshift and to lower luminosity than what had previously been found by sub-millimeter surveys. These JWST candidates have been interpreted as challenging the $\Lambda$CDM cosmology, but, so far, they have mostly relied only on rest-frame ultraviolet data and lacked spectroscopic confirmation of their redshifts. Here we report a systematic study of 36 massive dust-obscured galaxies with spectroscopic redshifts between $z_{\mathrm{spec}}=5-9$ from the JWST FRESCO survey. We find no tension with the $\Lambda$CDM model in our sample. However, three ultra-massive galaxies (log$M_{\star }/M_{\odot }$ $\gtrsim 11.0$) require an exceptional fraction of 50% of baryons converted into stars -- two to three times higher than even the most efficient galaxies at later epochs. The contribution from an active nucleus is unlikely because of their extended emission. Ultra-massive galaxies account for as much as 17% of the total cosmic star formation rate density at $z\sim 5-6$.

Nature in press. Updated to the accepted version. 24 pages, 4 main figures, 7 supplementary figures, 3 supplementary tables

Cited by in corpus (6)

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• Unveiling the hidden universe with JWST: The contribution of dust-obscured galaxies to the stellar mass function at $z\sim 3-8$
• Variation of the low-mass end of the stellar initial mass function with redshift and metallicity