Glimpsing the Stars that ended the ‘cosmic dark ages’

Researchers have observed overlapping bubbles of hydrogen ionised by the earliest stars in the universe in the collection of galaxies EGS 77.

This rendition shows ionized bubbles formed by three galaxies in galaxy cluster EGS77.

Credit: V. Tilvi et al./National Science Foundation’s Optical-Infrared Astronomy Research Laboratory/KPNO/AURA

Several overlapping bubbles of hydrogen gas ionised by the earliest stars in the universe have been observed by astronomers using the Mayall telescope at Kitt Peak National Observatory. The bubbles date back to 680 million years into the universe’s history, just 5% of the Universe’s current age of 13.8 billion years. Thus, the observation represents the earliest direct evidence from the epoch known as the cosmic dark ages — in which the first stars formed and began reionising hydrogen gas dispersed through the cosmos.

The team identified a trio of galaxies referred to as EGS 77 — most distant group of galaxies ever imaged and thus seen further back in time than any other object glimpsed by astronomers — it was announced at a press conference held during the 235th meeting of the American Astronomical Society (AAS) in Honolulu, Hawaii.

Far enough back, in fact, to see the stars within the galaxy as they were just after the ‘cosmic dark ages’ ended— a period when the universe was filled with nothing more complex than neutral hydrogen, thus no stars or galaxies. At the point in history, these galaxies are imaged the stars are taking part in a process of ‘reionising’ this neutral hydrogen — the immense energy they emit stripping away electrons from the gas.

This process led to photons being able to travel freely through the universe, akin to a ‘cosmic fog’ lifting and allowing the first glimpses of stars and galaxies to be seen across the vastness of space. Thus, concluding the cosmic dark ages that began 500,000 years into the Universe’s life and ended with the formation of the first stars.

“The young universe was filled with hydrogen atoms, which so attenuate ultraviolet light that they block our view of early galaxies,” explains James Rhoads at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “EGS 77 is the first galaxy group caught in the act of clearing out this cosmic fog.”

Tema member Vithal Tilvi who discusses the findings in a paper published in the Astrophysical Journal, elaborates: “EGS 77 has formed a large bubble that allows its light to travel to Earth without much attenuation.

“Eventually, bubbles like these grew around all galaxies and filled intergalactic space, reionizing the universe and clearing the way for light to travel across the cosmos.”

A long time ago in galaxies far, far away…

EGS77 was discovered as part of the Cosmic Deep And Wide Narrowband (Cosmic DAWN) survey, for which Rhoads serves as principal investigator. The team imaged a small area in the constellation Boötes using a custom-built filter on the National Optical Astronomy Observatory’s Extremely Wide-Field InfraRed Imager (NEWFIRM) attached to the 4-meter Mayall telescope at Kitt Peak National Observatory near Tucson, Arizona.

his illustration of the EGS77 galaxy group shows the galaxies surrounded by overlapping bubbles of ionized hydrogen. By transforming light-quenching hydrogen atoms to ionized gas, ultraviolet starlight is thought to have formed such bubbles throughout the early universe, gradually transitioning it from opaque to completely transparent. Background: This composite of archival Hubble Space Telescope visible and near-infrared images includes the three galaxies of EGS77 (green circles). (Credits: NASA, ESA and V. Tilvi (ASU))

“These results show the value of maintaining instruments at our national observatories that are powerful and can flexibly adapt to pursue new scientific questions, questions that may not have been in mind when an instrument was originally built,” adds Ron Probst, a DAWN team member who also helped to develop NEWFIRM.

After the galaxies had been spotted and identified, the team were able to calculate their distances from Earth, and in the process, their ages. This confirmation was made by using spectra captured by the MOSFIRE spectrograph at the Keck I telescope at the W. M. Keck Observatory on Maunakea in Hawaii. The three galaxies all show strong emission lines corresponding to hydrogen (in what is referred to as the Lyman α) at a high redshift (z=7.7) which told them they were viewing these galaxies as they were when the universe was just 680 million years old.

That fact that the astronomers were able to spot these Lyman α lines at all is due to the fact that at 2.2 million light-years across the overlapping bubbles of ionised hydrogen are so large that photons are redshifted before they reach the boundary, thus allowing them to escape unscathed.

“While this is the first galaxy group identified as being responsible for cosmic reionization, future NASA missions will tell us much more,” explains co-author Sangeeta Malhotra at Goddard. “The upcoming James Webb Space Telescope is sensitive to Lyman alpha emission from even fainter galaxies at these distances and may find more galaxies within EGS 77.”

Although future missions promise to reveal much more information about EGS 77, spotting more reionisation bubbles could be a challenge.

“We expected that reionisation bubbles from this era in cosmic history would be rare and hard to find,” says Sangeeta Malhotra, a collaborator at NASA GSFC, “so confirmation of this transition is important.”

It is hoped that NASA’s planned Wide-Field Infrared Survey Telescope (WFIRST) could spot further examples from this transition period, thus teaching us more than ever before about the dawn of the universe.