A group of American astronomers led by Vasily Kokorev from the University of Texas has obtained new data confirming one of the leading hypotheses about the nature of the so-called “Little Red Dots” discovered by the James Webb Space Telescope in 2022. Spectral analysis showed that these objects are likely supermassive black holes surrounded by a dense shell of partially ionized gas. Study published in the magazine The Astrophysical Journal.
Scientists initially studied the galaxy cluster Abell S1063 in search of Population III stars, the first stars to form in the Universe. However, during observations, the “little red dot” GLIMPSE-17775, located far beyond this cluster, accidentally came into the telescope’s field of view.
The successful observation of GLIMPSE-17775 was facilitated by the so-called gravitational lensing effect – a phenomenon in which the path of light coming from a more distant source is deformed by the gravity of a nearby massive object. Thus, the Abell S1063 cluster intensified the light of a distant “red dot” located behind it. It was thanks to this that astronomers were able to study GLIMPSE-17775 in detail: gravitational lensing actually turned 30 hours of observation into 80 hours due to the curvature of the fabric of space-time.
The James Webb Telescope recorded more than 40 spectral lines emanating from the object. Scientists observed GLIMPSE-17775 as it appeared about 1.8 billion years after the Big Bang. The combination of the high infrared sensitivity of the telescope and the natural cosmic “magnifying glass” made it possible to obtain the most detailed spectrum of the “little red dot” in the entire history of studying such objects.
“When we first saw this spectrum, it was like a puzzle with pieces scattered all over the floor. We began to lift each fragment, measure the spectral lines and put them together into a single mosaic. At first, some elements seemed meaningless, but then several parts came together, and we realized that we really had something important,” Kokorev told the National Aeronautics and Space Administration (NASA).
Analysis of the data showed that GLIMPSE-17775 is likely a fast-growing supermassive black hole hidden inside a dense cocoon of gas. This cocoon absorbs a significant portion of the radiation emanating from the vicinity of the black hole, forming the characteristic features of the observed spectrum.
One of the key clues in favor of this model was that the spectral lines of hydrogen, oxygen and helium do not correspond to the simple picture of a rotating gas cloud. Instead, they fit well with a model that takes into account the effect of electron scattering—the broadening of spectral lines due to the interaction of light with electrons. This effect is considered to be a characteristic property of a dense multilayer gas shell that surrounds an extremely powerful radiation source.
The researchers were also particularly interested in the 16 lines of iron in the spectrum, which they called “iron forest.” To form such a set of lines, an exceptionally strong source of high-energy radiation is required, for example, a black hole that actively absorbs matter from the outside.
The results explain another feature of the “little red dots.” Most of these objects are known to emit weak X-rays. If they are truly surrounded by dense gas cocoons, then the radiation is simply absorbed by this gas and does not reach telescopes.
“In the future, I really want to understand more about what exactly powers the central engines of the little red dots. Although we currently believe that these are black holes, there are other interesting hypotheses. Perhaps in a year or two we will finally get a final answer to the question of what actually is the source of energy for these objects,” Kokorev concluded.
