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Rethinking the Universe: Astronomers Disturbed by the Unexpected Scale
of James Webb’s Galaxies
TOPICS:AstronomyAstrophysicsJames Webb Space TelescopeNiels Bohr Institute
By NIELS BOHR INSTITUTE MAY 18, 2023

James Webb Space Telescope Galaxy Cluster SMACS 0723
James Webb in front of the galaxy cluster SMACS 0723, the first field of
galaxies to be revealed from the telescope. Early results from the James
Webb Space Telescope have revealed surprisingly large early galaxies
that challenge current cosmic models. Credit: NASA, ESA, CSA, STScI

The first results from the James Webb Space Telescope have hinted at
galaxies so early and so massive that they are in tension with our
understanding of the formation of structure in the Universe. Various
explanations have been proposed that may alleviate this tension. But now
a new study from the Cosmic Dawn Center suggests an effect that has
never before been studied at such early epochs, indicating that the
galaxies may be even more massive.

If you have been following the first results from the James Webb Space
Telescope, you have probably heard about the paramount issue with the
observations of the earliest galaxies:

They are too big.
From a few days after the release of the first images, and repeatedly
through the coming months, new reports of ever-more distant galaxies
appeared. Disturbingly, several of the galaxies seemed to be “too massive.”

From our currently accepted concordance model of the structure and
evolution of the Universe, the so-called ΛCDM model, they simply
shouldn’t have had the time to form so many stars.

Although ΛCDM is not a holy indestructible grail, there are many reasons
to wait before claiming a paradigm shift: The measured epochs at which
we see the galaxies could be underestimated.

Their stellar masses could be overestimated. Or we could just have been
lucky and somehow discovered the most massive of the galaxies at that time.

A closer look
But now Clara Giménez Arteaga, Ph.D. student at the Cosmic Dawn Center,
proposes an effect that could further increase the tension:

In essence, a galaxy’s stellar mass is estimated by measuring the amount
of light emitted by the galaxy, and calculating how many stars are
needed to emit this amount. The usual approach is to consider the
combined light from the whole galaxy.

However, taking a closer look at a sample of five galaxies, observed
with James Webb, Giménez Arteaga found that if the galaxy is regarded
not as one big blob of stars, but as an entity build up of multiple
clumps, a different picture emerges.

Galaxy Cluster SMACS 0723 Surroundings
Galaxy cluster SMACS This image of galaxy cluster SMACS 0723 and its
surroundings was the first image released from the James Webb Space
Telescope in July 2023. The five zoom-ins are each roughly 19,000
lightyears across, and show galaxies seen some 13 billion years back in
time. Careful analysis of these galaxies reveals that if we cannot
resolve a galaxy, we may severely underestimate the total mass of its
stars. Image credit: NASA, ESA, CSA, STScI / Giménez-Arteaga et al.
(2023), Peter Laursen (Cosmic Dawn Center).

“We used the standard procedure to calculate stellar masses from the
images that James Webb has taken, but on a pixel-by-pixel basis rather
than looking at the whole galaxy,” describes Giménez Arteaga.

“In principle, one might expect the results to be the same: Adding the
light from all pixels and finding the total stellar mass, versus
calculating the mass of each pixel and adding all individual stellar
masses. But they’re not.”

In fact, the inferred stellar masses now turned out to be up to ten
times larger.

The figure below shows the five galaxies with their stellar masses
determined by both ways. If the two different approaches agreed, all
galaxies would lie along the slanted line named “The same.” But they all
lie above this line.

Outshined
So what is the reason that the stellar masses turn out to be so much larger?

Giménez Arteaga explains: “Stellar populations are a mixture of small
and faint stars on one hand, and bright, massive stars on the other
hand. If we just look at the combined light, the bright stars will tend
to completely outshine the faint stars, leaving them unnoticed. Our
analysis shows that bright, star-forming clumps may dominate the total
light, but the bulk of the mass is found in smaller stars.”

Stellar mass is one of the main properties used to characterize a
galaxy, and Giménez-Arteaga’s result highlights the importance of being
able to resolve the galaxies.

But for the most distant and faint ones, this is not always possible.
The effect has been studied before, but only at much later epochs in the
history of the Universe.

The next step is therefore to look for signatures that does not require
the high resolution, and which correlate with the “true” stellar mass.

“Other studies at much later epochs have also found this discrepancy. If
we can determine how common and severe the effect is at earlier epochs,
and quantify it, we will be closer to inferring robust stellar masses of
distant galaxies, which is one of the main current challenges of
studying galaxies in the early Universe,” concludes Clara Giménez Arteaga.

The study has just been published in the Astrophysical Journal.

Reference: “Spatially Resolved Properties of Galaxies at 5 < z < 9 in
the SMACS 0723 JWST ERO Field” by Clara Giménez-Arteaga, Pascal A.
Oesch, Gabriel B. Brammer, Francesco Valentino, Charlotte A. Mason,
Andrea Weibel, Laia Barrufet, Seiji Fujimoto, Kasper E. Heintz, Erica J.
Nelson, Victoria B. Strait, Katherine A. Suess and Justus Gibson, 16 May
2023, Astrophysical Journal.
DOI: 10.3847/1538-4357/acc5ea

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