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This study analyzed deep spectroscopic observations of nine massive quiescent galaxies at redshift z≈0.7 using JWST data to constrain the initial mass function (IMF) beyond the local universe. The researchers found that the most massive galaxies contain an excess of low-mass stars compared to the Milky Way's IMF, with the oldest galaxy showing the most bottom-heavy IMF. This suggests that early "impossibly massive" galaxies observed by JWST may have been approximately 4 times more massive than previously estimated, potentially increasing tension with existing galaxy formation models.
Why it matters
These findings challenge fundamental assumptions used to calculate galaxy masses across cosmic time and may require revisions to models of early galaxy formation and evolution. If early galaxies systematically had more low-mass stars than assumed, this could resolve or amplify discrepancies between observations and theoretical predictions about how quickly massive galaxies assembled in the early universe.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: James Webb Space Telescope (JWST) observations have revealed that massive galaxies formed and evolved faster than predicted by galaxy formation models, with many having already assembled a large mass in stars approximately 12 billion years ago. However, masses of distant galaxies are uncertain, as they assume a distribution of stellar birth masses (the initial mass function (IMF)) similar to that in the Milky Way. Specifically, the contribution from low-mass stars, which make up the bulk of stellar mass, is not directly observed, but inferred based on an extrapolation of the Milky Way IMF. Here, we provide robust constraints on the low-mass IMF beyond the local Universe from full-spectrum models. Using ultra-deep spectra of nine massive quiescent galaxies at redshift $z$ $approx0.7$ from the JWST Initial Mass Function of Early Red NIRSpec Objects program, extended to bluer wavelengths with deep Very Large Telescope Large Early Galaxy Astrophysics Census spectra, we find that the most massive galaxies have excess low-mass stars. Remarkably, our oldest galaxy (formation redshift $z_{rm form} > 5$) has the most bottom-heavy IMF. This galaxy may be a descendant of JWST’s ‘impossibly early’ galaxies, implying that the latter may have had similarly bottom-heavy IMFs increasing their masses by a factor of approximately $4pm1$. Our findings may thus amplify the tension with galaxy formation models.
Source: Hidden mass in early galaxies revealed by bottom-heavy initial mass functions