Reaching new heights: James Webb Space Telescope discovers record-breaking ancient galaxy

Recently, astronomers announced the discovery of a galaxy with a redshift of 13, when the universe was about 330 million years old. Previously, the record was held by a galaxy with a redshift of about 11, when the universe was about 420 million years old. Webb has pushed the farthest galaxy seen by humans back to the birth of the universe by about 100 million years. This is just an easy start for Webb. We can expect that Webb will make greater breakthroughs in the near future and lead humans to crack the mysteries of the very early universe.

Written by Wang Shanqin

On July 12, 2022, the first batch of images obtained by the James Webb Space Telescope (JWST or "Webb") were officially released in a highly anticipated event. These photos shocked not only professionals in the astronomical community, but also many people outside the circle.

Image: In 2016, the assembled primary mirror of Webb and the folded secondary mirror and bracket. 丨Image source: NASA

However, before one melon is over, another one arises: on July 20, a collaborative team led by astronomers from Harvard University announced that they had discovered a record-breaking galaxy in the images taken by Webb: it was formed about 330 million years after the Big Bang, making it the oldest galaxy discovered so far .

How to determine the redshift of a celestial body?

Redshift is one of the most critical bases for measuring the distance and age of celestial bodies. Due to the movement of the celestial body itself or the expansion of the universe itself, the light waves emitted by the celestial body will change. If the light wave becomes longer, it is redshift; if the light wave becomes shorter, it is negative redshift, that is, blueshift.

The name was chosen this way because of the limitations of the times: more than 100 years ago, the wavelengths that astronomers could observe were basically limited to visible light, and in visible light, red light has the longest wavelength, and blue-violet light has the shortest wavelength. Therefore, the movement of other colors of visible light toward the red end is called redshift. As the range of observed wavelengths expands, astronomers have long observed the phenomenon of red light moving toward infrared. However, according to convention, such movement is still called "redshift" rather than "infraredshift". We just need to remember that "redshift" generally refers to the lengthening of wavelengths.

The light emitted by celestial bodies contains radiation emitted by atoms of many elements. These radiations are caused by the transition of electrons in atoms, and they all have fixed wavelengths. When part of the light emitted by celestial bodies is on its way to the earth, the radiation of certain bands is absorbed by its own atmosphere or interstellar medium, and the intensity becomes weaker, showing up as absorption lines.

If the wavelength of an absorption line of an element in the measured spectrum of a celestial body is different from the wavelength measured in the laboratory, it means that it has been redshifted or blueshifted. Subtracting the two and dividing by the wavelength measured in the laboratory is the value of the redshift or blueshift.

Figure: Schematic diagram of the red shift of the absorption line (dark line in the figure). The arrow indicates the movement of the spectrum line. 丨Source: Georg Wiora

For example, when the electrons of hydrogen atoms jump from the 2nd, 3rd, 4th, 5th, and 6th orbits to the 1st orbit (ground state), the wavelengths of the radiation emitted are 121.57 nanometers, 102.57 nanometers, 97.254 nanometers, 94.974 nanometers, and 93.780 nanometers, respectively. These are the first few of the famous "Lyman line series". These lines are also called Lyman α line, Lyman β line, Lyman γ line, etc. If we observe that the wavelength of a celestial body's Lyman α line is 1215.7 nanometers, then we can subtract 1215.7 from the wavelength 121.57 measured in the laboratory, and then divide by 121.57. The number 9 obtained is the redshift value.

When radiation from some distant galaxies passes through numerous hydrogen-rich intergalactic molecular clouds, the Lyman-alpha lines (and other Lyman lines) contained in them are severely absorbed by the hydrogen in the molecular clouds, causing their brightness to drop drastically, resulting in the brightness of radiation with wavelengths equal to and shorter than the Lyman-alpha line being much lower than that at other wavelengths. Such galaxies are called " Lyman-break galaxies " (LBGs).

After observing a Lyman break galaxy , the wavelength of the break is measured and compared with the wavelength of the Lyman alpha line in the laboratory (121.57 nanometers), and its redshift can be calculated. The actual operation is of course more complicated: through model fitting, the theoretical energy spectrum is obtained, and the specific wavelength of the Lyman break is determined, and then its redshift is calculated.

Past champion: GN-z11

Previously, the oldest galaxy astronomers had discovered in Hubble images was GN-z11. The G in this number stands for the Great Observatories Origins Deep Survey (GOODS), which is a multi-band observation conducted by the Hubble Space Telescope (hereinafter referred to as "Hubble") in conjunction with some space X-ray telescopes and ground-based telescopes. GOODS observes two specific areas of the sky, the south and the north, represented by S and N respectively. So GN represents the northern area observed by this project.

Figure: A magnified image of GNz-11 observed in the northern region of GOODS (inset). This is a composite of visible and near-infrared data obtained by Hubble's ACS and WFC3. 丨Image source: NASA, ESA, and P. Oesch (Yale University)

In 2016, a team led by astronomer Pascal Oesch of Yale University combined images from Hubble and Spitzer Space Telescope observations and used the grating spectrograph of Hubble's Wide Field Camera 3 (WFC3) to obtain the spectrum of this galaxy located in the GN observation area. They found that the radiation flux ratio on both sides of the break wavelength of the galaxy's energy spectrum is less than 0.32. **[1]** Therefore, it is a Lyman break galaxy.

Oesch et al. used the Lyman-break galaxy template to fit its energy spectrum (see the figure below), and determined that the wavelength of the "Lyman break" is about 1.47 microns (1470 nanometers), and its redshift is about 11.09. Because its redshift is about 11, it has "z11" in its number.

Figure: The energy spectrum fitting of GNz-11 shows that its redshift is about 11.09. The dark red line is the theoretical energy spectrum fitted using the "Lyman Break Galaxy" (LBG) template. The downward arrow indicates the upper limit of the observation. The quality of the fitting of the other two models is much lower, which excludes the possibility that this galaxy is a low redshift galaxy. 丨Image source: Reference [1]

Calculating the age of galaxies based on redshift depends on some cosmological parameters. Assuming the current Hubble constant is 69.6, the proportion of matter in the universe is 0.286, and the proportion of dark energy is 0.714, the age of our universe is 137.21 years (see: https://www.astro.ucla.edu/~wright/CosmoCalc.html).

In such a universe, the universe in which the galaxy with a redshift of 11 is located is 419 million years old, and its "lifespan" is at least 13.302 billion years.

New record holder: GLASS-z13

GN-z11 held the title for only a few years before it was replaced by a more distant galaxy discovered by Webb. The record-breaking galaxy was named GLASS-z13.

Figure: False color image of GLASS-z13. Source: Naidu et al, P. Oesch, T. Treu, GLASS-JWST, NASA/CSA/ESA/STScI

GLASS in GLASS-z13 is the abbreviation of Grism Lens Amplified Survey from Space. The GLASS project observed 10 galaxy clusters and 10 empty areas near them. One of the galaxy clusters is Abell 2744, which is also one of the six galaxy clusters photographed by the famous Hubble Frontier Fields (HFF) that year.

Figure: The image of the region of the sky where Abell 2744 is located (left) and the "parallel region" near this region obtained by the Hubble Frontier Field Project. 丨Image source: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI) (left); Reference [2] (right)

Webb used the Near Infrared Camera (NIRCam) to take images of galaxies in and around the sky where Abell 2744 is located, and used the Near-Infrared Imager and Slitless Spectrograph (NIRISS) and the Near-Infrared Spectrograph (NIRSpec) to obtain spectra of celestial bodies. These data obtained by Webb were released as part of the early release science (ERS) data. The project was therefore called "GLASS-JWST-ERS".

A team led by Rohan Naidu of the Harvard-Smithsonian Center for Astrophysics (CfA) confirmed GLASS-z13 in the data obtained by GLASS-JWST-ERS [Note 1] . It must be mentioned that Pascal Oesch, who led the team that confirmed GN-z11, was also a member of this team**[Note 2]** and ranked second in the relevant paper.

The seven small images at the top of the figure below show Webb's NIRCam observations of GLASS-z13 in seven wavelength bands. The central wavelengths of these seven wavelength bands are 0.9 microns, 1.15 microns, 1.5 microns, 2.0 microns, 2.27 microns, 3.56 microns, and 4.44 microns. In these wavelength bands, NIRCam observed the sky area where GLASS-z13 was located for 3.3, 3.3, 1.7, 1.5, 1.5, 1.7, and 6.6 hours, respectively. [3]

Figure: GLASS-z13 image taken by Webb's NIRCam with seven filters (top), flux density in each band obtained based on the data (bottom left, the magnitude/flux in different bands constitutes the energy spectrum) and the fitted redshift (bottom right). The downward arrow indicates the upper limit of the observation. The orange-red line is the theoretical energy spectrum fitted using the "Lyman Break Galaxy" (LBG) template. 丨Image source: Reference [3]

From the observation images of the 7 bands in the figure above, it can be seen intuitively that the GLASS-z13 images do not appear in the images of the 3 bands with shorter wavelengths, so only the upper limit of the brightness can be given. The images of the 4 bands with longer wavelengths show obvious galaxy images, so accurate brightness values ​​can be obtained.

By plotting the brightness or upper limit of the seven bands on the energy spectrum, we can determine the approximate location of the wavelength of the "Lyman break" of GLASS-z13, which will be between 1.5 and 2.0 microns. Naidu et al. used the "Lyman break galaxy" ("LBG" in the figure) template to fit the theoretical energy spectrum (the orange line in the figure above) and determined the specific value of the wavelength of the "Lyman break" of this galaxy.

According to the fitting graph, we can see that the flux of GLASS-z13 drops off a cliff at a wavelength slightly larger than 1.6 microns (1600 nanometers), so this wavelength is the position of the Lyman alpha line of this galaxy. If we roughly subtract the wavelength of the Lyman alpha line in the laboratory (121.57 nanometers) from 1.6 microns (1600 nanometers) and divide it by the latter, we can get its redshift of about 12. In fact, because its break wavelength is slightly larger than 1.6 microns, its final redshift is 12.4 or 13.1 (different models give slightly different values).

According to the cosmological parameters set above, the universe where the galaxy GLASS-z13 with a redshift of 13 is located is 332 million years old, and its "lifespan" is at least 13.388 billion years, which is about 86 million years older than the galaxy with a redshift of 11, nearly 100 million years. Therefore, we can say that Webb has easily pushed the farthest galaxy seen by humans back to the birth of the universe by about 100 million years . [Note 3]

According to the model fit, the mass of GLASS-z13 is very low, only about 1 billion times the mass of the sun. **[3]** For comparison, the mass of our Milky Way is about 1 trillion times the mass of the sun. Therefore, the mass of GLASS-z13 is only about one thousandth of the mass of the Milky Way.

The model also suggests that GLASS-z13 would have been about 71 million years old in the universe at that time (give or take 32 and 33 million years respectively). **[3]** The universe itself was only about 332 million years old at that time, so it would have been created about 260 million years after the universe was created.

The near-infrared radiation detected by Webb in GLASS-z13 (and other galaxies with redshifts up to about 10) was originally ultraviolet radiation emitted by these galaxies. Due to the expansion of the universe, this ultraviolet radiation was stretched to near-infrared when it reached the Earth.

Taking radiation with a wavelength of 4.44 microns as an example, dividing it by (13+1) gives 0.317 microns, or 317 nanometers, which is near-ultraviolet. When the near-infrared light with a shorter wavelength is divided by the same value, it gives the ultraviolet light with an even shorter wavelength. Therefore, only galaxies with bright enough ultraviolet radiation can be detected by Webb.

As for the visible light radiation emitted by those ancient galaxies, after such a large red shift, it has become mid-infrared radiation when it reaches the earth. Webb's mid-infrared instrument (MIRI) is a powerful tool for detecting these mid-infrared radiation.

Rolling to new heights

The paper by Naidu et al. was uploaded to the preprint site arxiv on July 19, 2022, and was published on the 20th. A similar paper that was also uploaded to arxiv on July 19 and published on the 20th was published by a team led by Marco Castellano of the Italian National Institute for Astrophysics. [4]

The paper by Castellano et al. also used data released by GLASS-JWST-ERS, from which they confirmed some galaxies with redshifts between 9 and 15. Among them, the redshifts of two galaxies with redshifts exceeding 10 were coded GHZ1 and GHZ2, and their redshifts were 10.6 and 12.35 respectively.

Castellano et al. also used the "Lyman break" method to determine the wavelength of the break, and thus their redshift. Among the seven filters used by Webb's NIRCam, two shorter wavelength bands did not detect galaxies, while the other five longer wavelength images detected galaxies. Based on this, Castellano et al. used the template of Lyman break galaxies to fit the redshifts of these galaxies, as shown in the figure below.

Figure: Images of GHZ1 (left) and GHZ2 (right) taken by Webb's NIRCam with seven filters (top) and magnitude diagrams of each band obtained based on the data (bottom, the magnitude/flux of different bands constitutes the energy spectrum). The small figure below shows the redshift obtained by fitting. The downward arrow indicates the upper limit of the observation. The figure shows obvious Lyman break features. 丨Image source: Reference [4]

The redshift of GHZ2 is slightly smaller than that of GLASS-z13, so it did not cause a media sensation. It can be seen that this field has been involuted to a quite alarming degree.

We can foresee that the competition among astronomers to find higher redshift galaxies based on Webb's data will become increasingly fierce and involuted, and new distances will continue to break old records. This competition and involution is very beneficial to human understanding of the frontiers of the visible universe. We look forward to Webb making greater breakthroughs in this regard in the future, and even discovering the first generation of galaxies and the first generation of stars.

Figure: The depth of the universe that can be detected by different telescopes at different times. The pink mark below is the redshift, and the white text marks the age of the universe at the corresponding redshift, in units of 1 billion years. Webb's observation targets are galaxies and stars in the era when the redshift is 20 and the universe is only 200 million years old. 丨Image source: NASA, ESA

Notes

[Note 1] Since GLASS-z13 is located in the region of the sky where Abell 2744 is located, it is not in the region of the sky where SMACS 0723 is located, which was included in the first full-color image released by Webb.

[Note 2] Pascal Oesch is currently affiliated with the Department of Astronomy at the University of Geneva, Switzerland, and the Niels Bohr Institute at the University of Copenhagen, Denmark.

[Note 3] Although different cosmological parameters lead to different specific values ​​of the age of the universe (ranging from 13.7 billion years to 14 billion years), the ages of ancient galaxies at different redshifts vary only very slightly.

References

[1]Oesch, PA , et al. A Remarkably Luminous Galaxy at z=11.1 Measured with Hubble Space Telescope Grism Spectroscopy, 2016, ApJ, 819, 129

[2] Lotz JM, et al. The Frontier Fields: Survey Design and Initial Results, 2017, ApJ, 837, 97

[3]Naidu, RP, et al. Two Remarkably Luminous Galaxy Candidates at z ≈ 11 − 13 Revealed by JWST, 2022, arXiv:2207.09434

[4] Castellano, M., et al. Early results from GLASS-JWST. III: Galaxy candidates at z∼9-15, 2022, arXiv:2207.09436

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