Formation of Ultra Massive Galaxies
Massive-galaxy Candidates
Recent James Webb Space Telescope (JWST) observations by team of Astronomers from the UK, and around the world, 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 submillimetre surveys. These JWST candidates have been interpreted as challenging the ΛCDM cosmology but, so far, these studies have mostly relied on only rest-frame ultraviolet data and have lacked spectroscopic confirmation of their redshifts. Here is a report of a systematic study of 36 massive dust-obscured galaxies with spectroscopic redshifts between 5 and 9 from the JWST FRESCO survey. No tension is found with the Λ cold dark-matter model in the sample. However, three ultra-massive galaxies require an exceptional fraction of 50 per cent of baryons converted into stars—two to three times higher than the most efficient galaxies at later epochs. The contribution from an active galactic nucleus is unlikely because of their extended emission. Ultra-massive galaxies account for as much as 17 per cent of the total cosmic star-formation-rate density at redshifts between about five and six.
Extremely massive galaxies contribute significantly to the total cosmic SFRD in the early Universe. This has been derived in the redshift data given by the Hα sample (z = 4.9 – 6.6). Data from three ultra-massive galaxies, shows that the SFR density reaches 2.4 x 10-3 solar masses per year cubic megaparsec at a redshift of z ≈ 5.8. Compared with the total cosmic SFRD at z ≈ 5.8, it is found that ultra-massive galaxies with efficient baryon-to-star conversion (ϵ > 0.2) alone can account for as much as 17% of the cosmic SFRD at z ≈ 5.8. This finding suggests a substantial proportion of extremely efficient star formation in the early Universe.
Fig. 4
The black line is the computation of the total cosmic SFRD, ψ, corrected for dust attenuation, as a function of redshift. At z ≳ 4, the curve is primarily based on ultraviolet-selected galaxies. The red star corresponds to the SFRD from the spectroscopically confirmed ultra-massive DSFGs (S1, S2 and S3) from the FRESCO survey with ϵ > 0.2. The error bar on the y axis corresponds to the 1σ uncertainty in ψ, and the error bar on the x axis represents the redshift coverage of our Hα sample (z = 4.9–6.6).
Acronyms
ΛCDM – Lambda Cold Dark Matter.
FRESCO – First Reionization Epoch Spectroscopically Complete Observations.
FRW – Friedmann-Robertson-Walker model.
SFRD (ψ) – Star Formation Rate Density.
SMGs – Submillimeter galaxies (SMGs) are a class of ultraluminous infrared galaxies (ULIRGs).
ULIRGs – Galaxies with infrared luminosity greater than 1012 solar luminosities.
Glossary
Submillimetre Survey – Studies of submillimeter galaxies (SMGs) that use submillimeter telescopes to explore the distant universe.
Spectroscopic Redshift (z) – According to the FRW model, the redshift of a distant galaxy is primarily caused by the expansion of space, and is not due to Doppler shift.
Cosmological Constant (Λ) – A property of space that is causing the expansion of the Universe to accelerate.
Reionization – The event that caused most of the neutral hydrogen in the Universe to become ionized.
Epoch of Reionization – The time at which reionization occurred which is thought to have been when the Universe was less than 10% of its current age.
Baryon to Star conversion (ε) – In the local Universe, the efficiency for converting baryonic gas into stars is very low. In dark matter halos where
Baryon – Baryons are heavy subatomic particles that are made up of three quarks. Both protons and neutrons, as well as other particles, are baryons.
References
Xiao, M., Oesch, P.A., Elbaz, D. et al. (2024) Accelerated formation of ultra-massive galaxies in the first billion years. Nature 635, 311–315. https://doi.org/10.1038/s41586-024-08094-5
Jones, M.H., Lambourne R.J.A., Serjeant, S. (2015) An Introduction to Galaxies and Cosmology. The Open University, Cambridge University Press, Cambridge.
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