Although the James Webb Space Telescope has detected ancient galaxies, we have yet to see anything from the first 250 million years of the Universe.
In mid-2022, there was discussion about who would break the cosmic distance record. In 2016, a galaxy GN-z11 set a definitive record: it came to us when the Universe was only 407 million years old. But a few years later, some claimed that a new galaxy, HD1, was even more distant, pushing HD1 to the top of Wikipedia’s list of most distant astronomical objects and even the Guinness Book of Records. Harvard University also defined it.
However, neither “record holder” would last after the launch of the James Webb. As soon as the first deep-field photo was released, nearly 100 candidates emerged for what could be a record-breaking galaxy in that particular area of space. In December 2022, a series of new galaxies were announced, with the most distant, JADES-GS-z13-0, coming from when the Universe was just 320 million years old – 2.3% of its current age. Then, on May 30, 2024, a new cosmic record surpassed all the others: JADES-GS-z14-0, whose light was emitted when the Universe was only 285 million years old: 2.1% of its current age.
But why haven’t we found anything even older? Let’s try to understand it.
The difference between candidate galaxy and confirmed galaxy
Let’s start with the difference between a candidate galaxy and a confirmed galaxy. When you observe an object in the Universe, and this applies to any object, you must consider the trade-off between the amount of time you can devote to a particular observation and the amount of scientific knowledge you can gain from the observations you are conducting. In astronomy, the goal is to collect quantities of light large enough to be able to precisely identify what a single object is and what its properties are.
Photometry
This means that, first, we need to find these objects and identify them as interesting. This normally happens through the photometria. , we are doing it now with the James Webb and will continue to do it with any ground-based and space-based telescopes we have available in the future. On the other hand, a telescope is a gigantic “bucket of light”, thanks to which we open a huge eye on the Universe and collect all the light that enters for the entire time we observe. But we don’t want to collect all the light spanning all the different wavelengths at once. We want to be sensitive to light of different wavelengths and energies.
The way we do this is by positioning what we call filter photometric on our tools. Instead of collecting all the light emitted by objects in our field of vision, we only collect light that falls within a specific wavelength range: each individual filter allows light within a certain range of colors (and wavelengths) to pass through it, where it reaches the instruments, while filtering out all other wavelengths of light.
What properties must a cosmic object have to be defined as “interesting”
By then observing the same field of view with multiple different photometric filters over a period of time, we can begin to see what that region of space looks like, as well as any objects bright enough to appear in that region. In astronomical terms, if we’re hunting distant galaxies, we don’t want to simply add up that light and assign it a color. This may be the right method for the human brain, but not for identifying record-breaking galaxies. Instead, we want to look for a certain class of signal from an object that exhibits these properties:
- In short wavelengths of light it is completely invisible and does not emit any detectable light.
- Therefore, once a certain wavelength threshold is exceeded, it not only becomes visible, but also constantly bright.
The reason is simple. Galaxies are giant collections of stars, gas, dust, plasma, and (typically) a central supermassive black hole. They emit light over a wide variety of wavelengths, but the strongest light signal comes from stars. The greater the population of young, hot, massive, newly formed stars, the bluer the light from a galaxy. In particular, a large amount of visible and ultraviolet light will be emitted, with the brightest objects emitting a huge amount of ultraviolet light. However, from a very distant galaxy, that light will be , or stretched to longer wavelengths, as it travels through the Universe before reaching the eyes of our telescope.
The expansion of the Universe and spectroscopy
If you want to observe photons coming to you from a distant galaxy, however, you can’t simply look at those wavelengths and expect to see an emission signal. The expansion of the Universe must be taken into account. But this is only enough to identify something as an ultra-distant candidate galaxy. To make sure that our estimate of the redshift photometric is accurate, i.e. that the object is really at the distance we assign to it, we need spectroscopic confirmation. Spectroscopy involves:
- gather a large amount of light around a particular object (although it is possible to do this on more than one object at the same time),
- breaking that light down into its individual component wavelengths and recording them all,
- and revealing the signature of individual atoms, ions and (sometimes) molecules that emit and/or absorb that object’s light.
Compared to photometry, spectroscopy is expensive: requires much longer observation periods. But unlike photometry, where there are multiple possible explanations for an object’s distance, spectroscopy can break the ambiguity by showing which sets of lines and which wavelengths correspond to the object in question.
Because the James Webb hasn’t (yet) broken the cosmic distance record
To date, there are still at least a dozen known candidate galaxies that could break the current cosmic distance record if spectroscopic data were available. However, with only redshift photometric data to rely on, none of these should be relied upon. It is also possible that when the James Webb makes long-term observations, even fainter and more distant candidate photometric galaxies will be revealed, and that these could surpass all current cosmic records. It’s important to remember that we’re only two years into the life of the space telescope, with about two decades of observations still ahead of us. With each new galaxy and each new record, we are gaining data-driven insights into aspects of the early Universe that, just two years ago, were just a matter of theoretical speculation.
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