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EI2GYB > ASTRO 19.10.25 15:19l 119 Lines 7196 Bytes #24 (0) @ WW
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Subj: What happened to those 'little red dots' Webb observed?
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What happened to those 'little red dots' Webb observed?
When the James Webb Space Telescope (JWST) began operations, one of its
earliest surveys was of galaxies that existed during the very early universe.
In December 2022, these observations revealed multiple objects that appeared as
"little red dots" (LRDs), fueling speculation as to what they might be. While
the current consensus is that these objects are compact, early galaxies, there
is still debate over their composition and what makes them so red. On the one
hand, there is the "stellar-only" hypothesis, which states that LRDs are red
because they are packed with stars and dust.
This means that they could be similar to "dusty galaxies" that are observed in
the universe today. On the other hand, there is the " MBH and galaxy" theory,
which posits that LRDs are early examples of active galactic nuclei (AGNs) that
exist throughout the universe in modern times. Each model has significant
implications for how these galaxies subsequently evolved to become the types of
galaxies observed more recently.
In a recent paper posted to the arXiv preprint server, an international team of
astronomers considered the different scenarios. They concluded that LRDs began
as "stellar only" galaxies that eventually formed the seeds of the supermassive
black holes (SMBHs) at the center of galaxies today.
The research was led by Andres Escala, a professor of astronomy from the
Universidad de Chile. He was joined by colleagues from the Astronomical
Computing Institute at the University of Heidelberg, Yale University, and the
Sapienza University of Rome. The arXiv paper detailing their findings, "On the
Fate of Little Red Dots," is currently under review for publication in The
Astrophysical Journal.
The discovery of LRDs has mystified astronomers, as they possess certain
characteristics reminiscent of modern-day astronomical objects, but with some
notable differences. For instance, the "stellar only" interpretation posits
that LRDs are intensely star-forming, dusty galaxies with extremely dense core
regions. This theory explains why these galaxies appear so compact and red, two
of the most distinguishing features of LRDs. However, the widths of their
hydrogen spectral line emissions (Balmer-series lines) suggest velocity
dispersions much higher than those observed in early galaxies, which would lead
to long-term instability.
In contrast, the MBH interpretation is supported by the presence of broad
Balmer emission lines, which are suggestive of massive black holes at their
centers. However, most LRDs do not appear to emit a significant amount of X-ray
radiation (which is typical for quasars), and the black holes thought to be
present are overmassive with respect to their host galaxies. Nevertheless, as
Professor Escala told Universe Today via email, these two explanations are
considered the most likely for two reasons:
"In addition to there being strong evidence, these two predominant
interpretations are considered 'less exotic' than the alternatives.
Nevertheless, LRDs are still objects that have never been observed before at
low redshift values (z). The BH and galaxy interpretation is favorable because
such objects are 'normal' in the local universe but with different 'weights.'
"In the local universe, the MBH mass is 0.1% of the galactic mass; however, for
LRDs, it would be 10% of the host galaxy (a factor of 100 larger). The
stellar-only interpretation for LRDs states that they could be just another
galaxy like any other in the universe, but with a big difference: at least 10
times smaller than the smallest galaxies previously observed (100pc vs. 1kpc)
and with a considerably larger mass."
In contrast, Escala and his colleagues began with the stellar-only
interpretation, but considered how LRDs could evolve to fit the BH and galaxy
interpretation. Their theory would unify these two interpretations by
suggesting that they represent different evolutionary stages of these early
galaxies. As Escala noted, their theory is supported by essentially the same
evidence as the BH and galaxy interpretation. This includes how few LRDs are
detectable in X-rays compared to quasars in the more recent universe:
"Within the evolutionary path proposed in our paper, the lack of X-rays
basically tells us that most LRDs are in the early stages of such evolution.
The transitory nature of LRDs (only appearing in the universe between z=8 and
4, equivalent to 10% of the age of the universe) might support that these
objects can only be observed in their early stages, evolving later to systems
(or components of them) more similar to the ones observed at lower redshifts."
In short, their theory posits that LRDs will eventually host an MBH because,
even under the stellar-only interpretation, the extreme densities of LRDs imply
that a fair fraction of their inner regions will eventually form a massive
black hole that will be over-massive with respect to the host. This
evolutionary approach to LRDs offers a more comprehensive explanation for
Webb's observations of the early universe. This could have significant
implications for our current models of cosmology and galactic evolution, which
have been challenged by the recent discovery of these highly compact and deep
red galaxies. Said Escala:
"It means that LRDs are probably the most favorable places for MBH formation.
Even if these systems are only composed of stars, our work implies that such a
stellar system cannot be stable (at least in its inner regions) and would
inevitably tend to form MBHs. Complementing this with the other proposed
scenarios, in all possibilities, LRDs will be places for ongoing or recently
formed MBHs, given that their formation is one of the most open problems in
structure formation and cosmology."
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