The complex puzzle known as little red dots has become more complete since their initial discovery by NASA’s James Webb Space Telescope in 2022. Now a particular little red dot’s spectrum is helping connect many of the pieces.
A team of astronomers led by
Vasily Kokorev at the University of Texas at Austin identified the lucky dot in
question: GLIMPSE-17775. By carefully analyzing the dot’s spectrum captured by
Webb — the deepest spectrum to
date of a little red dot — the research team has identified multiple lines of
evidence, all of which support the interpretation that GLIMPSE-17775 is a
supermassive black hole enveloped in a dense cocoon of partially ionized gas,
a model referred to as the BH* (black hole star) scenario. A paper describing
the results was published today in The Astrophysical Journal.
“I think part of the scientific
community is converging on a singular picture — that little red dots can be
explained by black hole star models. But none of the previous little red dots
have all of the pieces of evidence in the same place,” said Kokorev, lead
author of the study. “With GLIMPSE-17775 we can test these models because of
how deep and amazing this source’s spectrum is.”
Image: Abell
S1063 with Pullout of GLIMPSE-17775 (NIRCam Image)
While the primary purpose of NASA’s James Webb Space
Telescope’s observations of galaxy cluster Abell S1063 was to look for a
certain population of stars, scientists obtained a detailed spectrum of
GLIMPSE-17775 from the dataset. This little red dot is located behind Abell
S1063.
Image: NASA, ESA, CSA, Vasily Kokorev (UT Austin);
Image Processing: Alyssa Pagan (STScI)
Connecting puzzle pieces
Soon after Webb first began science
operations, it discovered a new, mysterious type of object in the very early
universe – abundant red objects that emerged about 600 million years after the
big bang. Scientists have explored multiple explanations for these little red dots, including the black hole star scenario.
A set of fortunate circumstances
brought about this new, elaborate spectrum of a little red dot. The little red
dot that would come to be known as GLIMPSE-17775 was fortunately included in
Webb’s imaging and spectroscopy efforts for a project that sought to look
for Population III stars and faint galaxies in galaxy cluster Abell S1063. This little red dot
is more distant than the galaxy cluster and magnified by gravitational lensing. (GLIMPSE-17775 has a cosmological redshift of 3.5, meaning it existed about 1.8 billion
years after the big bang.)
While Webb provided a 30-hour
spectrum of the little red dot, the effect of gravitational lensing made it
equivalent to 80 hours of telescope time. This combination of Webb’s infrared
sensitivity and nature’s own “magnifying glass” amplified the amount of detail
that could be gleaned from GLIMPSE-17775. The result was more than 40 spectral lines from this small, red source, which is the most detailed little red
dot spectrum to date.
“When we saw the spectrum for the
first time, it was like having all the pieces of a puzzle scattered on the
floor,” said Kokorev. “We picked up each piece of the puzzle, measured the
lines, and started combining the different pieces into a mosaic. Maybe a few
pieces looked like nothing at first, but then a couple of them came together,
and we realized that there was something there.”
The spectroscopic data collected by
Webb contains multiple lines of evidence that support the interpretation that
little red dot GLIMPSE-17775 is a black hole star: a rapidly accreting, or
growing, black hole enveloped in a dense gas cocoon, which is reprocessing the
light emitted from near the black hole and producing the features seen in the
spectrum.
Image:
Evidence of a 'Black Hole Star'
NASA’s James Webb Space Telescope captured the deepest
spectrum to date of a little red dot. More than 40 spectral
lines have
been discerned from the data, many of which independently support the theory
that GLIMPSE-17775 is a black hole enshrouded by a hot, dense gas cocoon.
Illustration: NASA, ESA, CSA,
Vasily Kokorev (UT Austin); Designer: Leah Hustak (STScI)
Lines of evidence
Among the 40-plus lines that the
team detected in GLIMPSE-17775’s spectrum were various independent indicators
that all align with the BH* scenario. For example, the team found that many of
the spectral lines, such as hydrogen, oxygen, and helium, do not fit a simple
model of a rotating gas cloud. Instead, the best fit model includes a
broadening effect known as electron scattering, a telltale sign that a dense,
layered gas cocoon is enshrouding this source.
The strength and ratios of certain
lines to each other, most notably the 16 iron lines that compose what the team
has dubbed an “iron forest” and certain oxygen lines, require a high-energy
source to produce them, like a rapidly accreting black hole. Additionally,
astronomers noted the fluorescence and absorption of helium in the spectrum,
both of which individually suggest that there is a dense medium enveloping a
powerful source.
The BH* scenario not only fits
GLIMPSE-17775; it also accounts for why most little red dots are faint in
X-rays, since any such emission is likely absorbed by the dense gas cocoon.
One missing element of the
GLIMPSE-17775 puzzle piece is the part of the spectrum that would reveal what’s
known as a Balmer break, or a strong dip in the emitted light that’s a
signature characteristic of little red dots. To build a more comprehensive understanding
of this little red dot, the team incorporated ancillary data from two observing
programs that used NASA’s Hubble Space Telescope: the Frontier Fields and
BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) programs.
The Webb and Hubble data together
help explain why the Balmer break is weaker than what typically is found in
other little red dots: A giant host galaxy is surrounding GLIMPSE-17775.
Although a little red dot’s host galaxy is not something that has been usually
seen at such scale before, it isn’t inconsistent with the dense gas cocoon
model. The black hole star model of little red dots attributes excess blue
light to stars in the host galaxy.
When Webb first discovered little
red dots, some researchers thought these objects had “broken cosmology,” unsure
how galaxies could have grown so big so quickly in the early universe to
account for all this light coming from their stars. However, the team believes
the GLIMPSE-17775 puzzle piece fits nicely in the existing framework of the
universe’s evolutionary history, because black hole masses don’t need to be as
high in order to explain the broad emission lines.
“Everything fits, nothing is
broken, and I think that makes the puzzle that is our universe even better,”
said Kokorev. “Looking ahead, I’m eager to dive deeper and learn about what is
powering the central engines of little red dots. While we think it’s a black
hole, there are some other interesting theories being proposed, which is
exciting. Maybe in a year or two, we’ll have the final answer to what powers
these sources.”
The James Webb Space Telescope is
the world’s premier space science observatory. Webb is solving mysteries in our
solar system, looking beyond to distant worlds around other stars, and probing
the mysterious structures and origins of our universe and our place in it. Webb
is an international program led by NASA with its partners, ESA (European Space
Agency) and CSA (Canadian Space Agency).
To learn more about Webb, visit: https://science.nasa.gov/webb
Source: NASA Webb Finds Strongest Evidence Yet for ‘Black Hole Stars’ - NASA Science
