JWST’s hunt for most distant galaxies gets double boost

Many obstacles must be overcome in seeking the Universe’s first galaxies.

Over the course of 50 days, with a total of over 2 million seconds of total observing time (the equivalent of 23 complete days), the Hubble eXtreme Deep Field (XDF) was constructed from a portion of the prior Hubble Ultra Deep Field image. Combining light from ultraviolet through visible light and out to Hubble’s near-infrared limit, the XDF represented humanity’s deepest view of the cosmos: a record that stood until it was broken by JWST. In the red box, where no galaxies are seen by Hubble, the JWST’s JADES survey revealed the most distant galaxy to date: JADES-GS-z13-0. Extrapolating beyond what we see to what we know and expect must exist, we infer a total of ~2 sextillion stars within the observable Universe.

These incredible cosmic distances necessitate immense light-gathering power.

Shown during an inspection in the clean room in Greenbelt, Maryland in late 2021, NASA’s James Webb Space Telescope was photographed at the moment of completion. Only weeks later, it would successfully launch and deploy, leading to an unprecedented set of advances in astronomy. From mirrors to instruments, it was kept cleaner, from start to finish, than any observatory ever.

Large-aperture telescopes and long observing times are required.

The JWST, now fully operational, has seven times the light-gathering power of Hubble, but will be able to see much farther into the infrared portion of the spectrum, revealing those galaxies existing even earlier than what Hubble could ever see, owing to its longer-wavelength capabilities and much lower operating temperatures. Galaxy populations seen prior to the epoch of reionization should abundantly be discovered, and Hubble’s old cosmic distance record has already been broken.

The expanding Universe dramatically shifts the emitted galactic light toward redder wavelengths.

This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. The expanding Universe allows for galaxies up to 15 billion light-years beyond our present cosmic horizon to eventually become visible, even while fewer and fewer galaxies become reachable.

Telescopes optimized for infrared and longer-wavelength views are mandatory.

Preliminary total system throughput for each NIRCam filter, including contributions from the JWST Optical Telescope Element (OTE), NIRCam optical train, dichroics, filters, and detector quantum efficiency (QE). Throughput refers to photon-to-electron conversion efficiency. By using a series of JWST filters extending to much longer wavelengths than Hubble’s limit (between 1.6 and 2.0 microns), JWST can reveal details that are completely invisible to Hubble. The more filters that are leveraged in a single image, the greater the amount of details and features that can be revealed.

JWST, even with its incredible NIRCam instrument, only identifies ultra-distant galaxy candidates.

This annotated, rotated image of the JADES survey, the JWST Advanced Deep Extragalactic Survey, shows off the new cosmic record-holder for most distant galaxy: JADES-GS-z13-0, whose light comes to us from a redshift of z=13.2 and a time when the Universe was only 320 million years old. This galaxy appears about twice as large, in terms of angular diameter, as it would appear if it were half the distance away: a counterintuitive consequence of our expanding Universe.

Only spectroscopic follow-up can confirm these galactic distances.

The exposures in different photometric bands (top) of candidate galaxy HD2, along with two possible spectral fits (curves) to the data points (red). Note how although a high redshift (z = 12) solution is favored over a low redshift (z = 3.5) interpretation, both are possible, and the unambiguous signature from spectroscopy is not available. In the absence of spectroscopy, this can only be labeled an ultra-distant candidate, not a robust detection.

JWST can conduct spectroscopic measurements with its NIRSpec instrument.

NIRSpec is a spectrograph rather than imager but can take images, such as the 1.1 micron image shown here, for calibrations and target acquisition. The dark regions visible in parts of the NIRSpec data are due to structures of its microshutter array, which has several hundred thousand controllable shutters that can be opened or shut to select which light is sent into the spectrograph. Only a selection of targets within the same field of view, however, can have their spectrum taken at once.

Through emission lines and/or the key “Lyman break” feature, JWST has confirmed many record-breakers.

This illustration shows the spectrum from the most distant galaxy identified in JWST’s first deep-field image, along with the spectral lines that correspond to various elements and ions. The spectrum showcases the power of spectroscopy to reveal an incontrovertible distance and redshift for this object, and these techniques are being used to identify the most distant galaxies detectable by JWST.

But JWST spectroscopy costs time: an asset in high astronomical demand.

The spectra obtained by JADES and the JWST NIRSpec instrument for the four most distant galaxies found thus far by the JADES survey. The Lyman break feature, robustly identified here for each of the four galaxies, determines the distance and redshift beyond a reasonable doubt, making JADES-GS-z13-0 the current cosmic record-holder for most distant galaxy.

Thankfully, two invaluable assists to JWST science exist.

The Atacama Large Millimetre/Submillimetre Array (ALMA) consists of an array of radio telescopes. The array has the light-gathering power of the sum total of the individual dishes’ collecting areas, but has the resolution of the distance separating the dishes. It can be used to identify molecular signatures such as neutral and ionized hydrogen, carbon monoxide, and other common molecules and ions found in space.

One is the ground-based millimeter/submillimeter observatory: ALMA.

In this comparison view, the Hubble data is shown in violet, while ALMA data, revealing dust and cold gas (which themselves indicate star-formation potential), is overlaid in orange. With its views out beyond the limits of infrared astronomy but sensitive to spectroscopic features, ALMA can detect some of the most distant ionized/excited elements in cosmic history.

Optimized for longer-than-infrared wavelengths, ALMA has unprecedented spectral resolution.

One of doubly-ionized oxygen’s emission features peaks at 88 microns in the rest-frame: in the far-infrared. Owing to cosmic expansion, that light was stretched until it arrives at our eyes at ~millimeter wavelengths: in the right range for ALMA to be sensitive to it.

It sees uniquely faint, long-wavelength emission lines, like from doubly-ionized oxygen.

Based on the distance confirmed by ALMA, galaxy GHZ2/GLASS-z12 is determined to be one of the brightest, most UV-rich galaxies from the early Universe, and one of the most distant ones yet discovered.

ALMA just confirmed galaxy GHZ2/GLASS-z12: the third-most distant galaxy ever.

An ultra-distant galaxy candidate within the GLASS-JWST survey volume, along with the contours that mark the detection of doubly-ionized oxygen by ALMA. The JWST and ALMA data point toward the same object with an offset of just 0.5 arc-seconds. This galaxy, now known as GLASS-z12, is presently the 5th most distant astronomical object ever discovered.

But the second assist comes from the Universe itself.

This Hubble view of galaxy cluster Abell 2744 shows evidence of being a cosmic pile-up of multiple galaxy clusters. It was the first Hubble Frontiers Field image released and showcases one of the most powerful sources of gravitational lensing in the Universe.

Foreground galaxies and clusters can gravitationally lens background objects.

This gravitational lensing map of cluster Abell 2744, constructed by the CATS team and led by the efforts of Rachael Livermore, was the best reconstruction of a lensing map of this cluster based on Hubble data: prior to the release of JWST’s view of Abell 2744.

This magnifies and distorts their light, making otherwise too-faint galaxies detectable.

This gravitational lensing model of the regions in galaxy cluster Abell 2744 was made in light of the latest JWST data, and was created in late 2022. It showcases some of the strongest gravitational lenses in the known Universe.

Pandora’s Cluster, Abell 2744, was recently imaged by JWST.

This image shows a portion of JWST’s NIRCam imager’s views of galaxy cluster Abell 2744: Pandora’s cluster. One of the most distant galaxies yet discovered, whose light comes to us from just 450 million years after the Big Bang, is highlighted in a white box, while foreground stars within our Milky Way show their bright diffraction spikes in JWST’s cameras.

Many lensed candidates clearly stand out.

This portion of JWST’s view of Abell 2744, Pandora’s Cluster, contains, as of January 2023, the second-most distant galaxy ever discovered, highlighted in the small white box here. Its light comes to us from just 350 million years after the Big Bang.

With spectroscopy forthcoming, perhaps more records will soon fall.

Although galaxies from all throughout the Universe’s history are revealed by JWST’s NIRCam instrument in this portion of the region in and around Pandora’s cluster, Abell 2744, only the galaxies for which spectroscopic follow-up has been performed can have their distances known for certain.

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