JWST Glimpses the Cosmic Dawn of the Universe

The James Webb Space Telescope (JWST) continues to push the boundaries of astronomy and cosmology, the very job it was created for. First conceived during the 1990s, and with development commencing about a decade later, the purpose of this next-generation telescope is to pick up where Spitzer and the venerable Hubble Space Telescope (HST) left off – examining the infrared Universe and looking farther back in time than ever before. One of the chief objectives of Webb is to observe high-redshift (high-Z) galaxies that formed during Cosmic Dawn.

This period refers to the Epoch of Reionization, where the first galaxies emitted large amounts of ultraviolet (UV) photons that ionized the neutral hydrogen that made up the intergalactic medium (IGM), causing the Universe to become transparent. The best way to measure the level of star formation is the H-alpha emission line, which is visible in the mid-infrared spectrum for galaxies with high redshifts. Using data from the Mid-Infrared Instrument (MIRI), an international team of researchers was able to resolve the H-alpha line and observe galaxies with redshift values higher than seven (z>7) for the first time.

The international team was made up of researchers from the Kapteyn Astronomical Institute, the Center of Astrobiology (CSIC-CAS), the Cosmic Dawn Center (DAWN), the National Space Institute (DTU-Space), the UK Astronomy Technology Centre, the Niels Bohr Institute’s Dark Cosmology Centre (DARK), the Max-Planck-Institute for Astronomy (MPIA), the Centre for Extragalactic Astronomy, Dublin Institute for Advanced Studies (DIAS), ETH Zurich, the European Space Agency (ESA), the Space Telescope Science Institute (STScI), and multiple universities. The paper that describes their findings is being reviewed for publication in The Astrophysical Journal.

The research was led by Pierluigi Rinaldi, a Ph.D. student at the Kapteyn Astronomical Institute dedicated to studying high-redshift galaxies and galaxy formation and evolution. As he and his colleagues explained in their paper, studying high-redshift galaxies during the Epoch of Reionization has always been a major challenge due to a lack of suitable facilities. In previous studies, astronomers have relied on another spectral line known as Lyman-alpha, where neutral hydrogen is hit by a photon and is boosted to the next energy level.

However, this line is either very faint or not present in galaxies dated to Reionization Epoch because this spectral line was absorbed by neutral hydrogen in the IGM. Hence why astronomers have been looking for the H-alpha (H-a) emission line, which appears in the red part of the spectrum when an electron moves between the second and third orbit. The H-a line is not affected by the opacity of the intergalactic medium and thus allows astronomers to study star formation in these early galaxies. As Rinaldi told Universe Today via direct message, this is now possible using Webb‘s instruments:

“In order to observe the Ha emission line at z ~ 7, which corresponds to approximately 700 million years after the Big Bang, we need instruments capable of observing wavelengths around 5.6 microns (mid-infrared wavelengths). Until now, aside from Spitzer, there have been limited opportunities to achieve this. For example, the Hubble Space Telescope lacked this capability. Spitzer could help us, but the spatial resolution of this space telescope was far to be good for this purpose. However, the JWST now enables us to accomplish this task. Specifically, the MIRI instrument (provides us with the unique opportunity to search for H-alpha emitters at very high redshifts.”

Riguili and his colleagues first searched for indications of emission lines by analyzing the more sensitive images acquired by Webb’s Near-Infrared Camera (NIRCam). Then they analyzed ultra-deep images of these preselected galaxies at longer wavelengths with MIRI, which were acquired during a Guaranteed Time Observation (GTO) period awarded to the ESA. The researchers also used ancillary data from the Hubble eXtreme Deep Field (XDF), which was previously the deepest image of the Universe ever made.

Based on their analysis, they found several very bright sources (5.6 microns), indicating that a very prominent H-alpha emission line was present in observed galaxies. As Rinaldi explained:

“To confirm that we were indeed observing the H-a emission, we employed photometric techniques that allowed us to isolate galaxies displaying an ‘excess’ in their photometry. Assuming that these galaxies were at the correct distance (i.e., redshift), we derived the H-a emission fluxes. By conducting follow-up spectroscopic observations, we were able to verify the redshift of our sources. Therefore, our initial estimates regarding their redshift were accurate, thanks to the excellent wavelength coverage available in the eXtreme Hubble Deep Field.”

The astronomers hope to follow up by observing these same galaxies using Webb’s suite of spectrometers to learn more about the shape of the galaxies’ emission lines. This data could reveal more about the gas flux within these early galaxies and the dynamics of star formation. But at the moment, the team hopes to pursue these exciting findings and see what implications the first-ever detection of galaxies in the Cosmic Dawn will have.

“Our current focus is to comprehend the role of these H-a emitters within the context of the Epoch of Reionization, which refers to the moment when the first galaxies started to appear and reionize the Universe,” said Rinaldi. “We are actively working on this project right now and hope to publish our exciting results very soon!”

Further Reading: Astronomie.nl, The Astrophysical Journal