MIRI instrument on JWST detects H-alpha emission for the first time during the Epoch of Reionization

By | June 13, 2023

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JWST image of the Hubble eXtreme Deep Field (XDF) showing a zoom for two of the Epoch of Reionization galaxies. The brightness of the two small images in the lower left is produced by the H-alpha emission line. Credit: arXiv (2023). DOI: 10.48550/arxiv.2301.10717

An international team of astronomers led by Pierluigi Rinaldi of the University of Groningen has detected for the first time the emission of H-alpha in individual galaxies during the so-called Epoch of Reionization, or cosmic dawn. To do this, they used the deepest images taken so far by the MIRI instrument on the James Webb Space Telescope. The result was accepted for publication in The Astrophysics Journaland is currently published on arXiv prepress server.

Star-forming galaxies produce large amounts of UV photons, but during the Epoch of Reionization these photons are absorbed by the intergalactic medium. The best tracer to measure the level of star formation is the H-alpha emission line in the optical spectrum. For galaxies with a high redshift, this line of sight is shifted to longer wavelengths, in the near- and mid-infrared.

The H-alpha line has now been studied with the MIRI instrument on JWST for the first time for galaxies with a redshift greater than seven (z>7) during cosmic dawn, the period less than a billion years after the Big Bang, during which the neutral ionized hydrogen gas. The research by Rinaldi and colleagues shows for the first time that a detailed study of star formation in early galaxies is possible and can be conducted with JWST/MIRI.

Little is known about galaxies in the Epoch of Reionization. At high redshifts, previous studies have relied on the analysis of another prominent spectral line called Lyman-alpha, but this line becomes very faint or generally not present at all in galaxies at cosmic dawn because it is absorbed by the medium intergalactic, which is mostly opaque in that early period. H-alpha is unaffected by the opacity of the intergalactic medium and thus allows astronomers to study star formation in these early galaxies.

MIRI is an important tool for this research, as is one of the other four tools on Webb, NIRCam. The ultra deep MIRI image used in this study was obtained within the so-called MIRI European Guaranteed Time. The researchers also used ancillary data from HST in the Hubble eXtreme Deep Field (XDF).

The researchers first looked for star-forming galaxies that might have emission lines upon reionization, based on their analysis of the most sensitive NIRCam images. Then they analyzed the images of these preselected galaxies at longer wavelengths with MIRI. They found that many of them are very bright at 5.6 microns, indicating that a very prominent H-alpha emission line is present in these sources.

First author Pierluigi Rinaldi says: ‘This research opens up the possibility of studying early galaxies in a way that was not possible before. The great thing is that we have shown that this type of study can be done routinely with JWST/MIRI.’

Astronomers plan to spectroscopically follow the galaxies with JWST. The goal is to learn more about the shape of the emission lines of galaxies, which could tell them more about gas kinematics and the dynamics of star formation.

More information:
P. Rinaldi et al, MIDIS: Strong (Hb + [OIII]) and the z78 redshift Ha emitters unveiled with JWST/NIRCam and MIRI imaging in the Hubble eXtreme Deep Field (XDF), arXiv (2023). DOI: 10.48550/arxiv.2301.10717

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Astrophysical Journal


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