Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered two heavily dust-obscured star-forming galaxies — REBELS-12-2 and REBELS-29-2 — that formed more than 13 billion years ago.
“Various efforts have been made over the past decades to observe distant galaxies, which are characterized by electromagnetic emissions that become strongly redshifted before reaching the Earth,” said Dr. Yoshinobu Fudamoto, an astronomer at Waseda University, the National Astronomical Observatory of Japan, and the University of Geneva, and colleagues.
“So far, our knowledge of early galaxies has mostly relied on observations with the NASA/ESA Hubble Space Telescope and large ground-based telescopes, which probe their ultraviolet (UV) emission.”
“However, recently, astronomers have started to use the unique capability of ALMA to study distant galaxies at submillimeter wavelengths.”
“This could be particularly useful for studying dusty galaxies missed in the Hubble surveys due to the dust absorbing UV emission.”
“Since ALMA observes in submillimeter wavelengths, it can detect these galaxies by observing the dust emissions instead.”
As part of the ongoing ALMA large program REBELS (Reionization-Era Bright Emission Line Survey), the researchers observed 40 distant galaxies luminous in UV.
“While analyzing the observed data for two REBELS galaxies, we noticed strong emission by dust and singly ionized carbon in positions substantially offset from the initial targets,” they said.
“To our surprise, even highly sensitive equipment like Hubble couldn’t detect any UV emission from these locations.”
“To understand these mysterious signals, we investigated matters further.”
The scientists found that the unexpected emissions came from two previously unknown galaxies near the two original REBELS targets: REBELS-12 and REBELS-29.
Named REBELS-12-2 and REBELS-29-2, these galaxies are not visible in UV or visible wavelengths as they are almost completely obscured by cosmic dust.
At a redshift of 7.35, REBELS-12-2 is the most distant dust-obscured galaxy discovered so far.
“These new galaxies were missed not because they are extremely rare, but only because they are completely dust-obscured,” Dr. Fudamoto said.
“However, it is uncommon to find such dusty galaxies in the early period of the Universe (less than one billion years after the Big Bang), suggesting that the current census of early galaxy formation is most likely incomplete, and would call for deeper, blind surveys.”
“It is possible that we have been missing up to one out of every five galaxies in the early Universe so far.”
The discovery is reported in a paper in the journal Nature.
Y. Fudamoto et al. 2021. Normal, dust-obscured galaxies in the epoch of reionization. Nature 597, 489-492; doi: 10.1038/s41586-021-03846-z