One of the goals of the Webb Space Telescope was to image the oldest galaxies, giving a new window into how our universe evolved from the Big Bang into dense, hot material and its star- and structure-filled present. And, almost as soon as the data started pouring in, things looked very promising, with strong indications that we’re picking up galaxies as they appeared just a few hundred million years after the Big Bang.
But some uncertainty remains, because unusual conditions may cause a recent galaxy to have features that make it appear much older. That might be the case with a galaxy that would otherwise Identify the oldest.
On Tuesday, two papers were published that put the problem to rest, providing a full spectrum of four early galaxies and showing that they all clearly date to just a few hundred million years after the Big Bang. Imaging of the same galaxies shows that they are full of young stars that lack most of the heavy elements seen in today’s universe.
full spectrum
Determining the age of early galaxies relies on finding a specific feature in the light the galaxies emit. Early in the universe’s history, most of it was filled with hydrogen atoms, which can absorb photons when they have enough energy to move an electron into another orbit. Thus, although an early galaxy will emit broadly across the spectrum, there will be a sharp cutoff called the Lyman break where any photon with energy greater than the Lyman break will be absorbed by the hydrogen.
Nearby, the Lyman break will be in the UV part of the spectrum. But its position is red-shifted as photons travel across the expanding universe, extending the wavelength of light. Consequently, the Lyman break travels across the entire visible spectrum and ends in the infrared as we look further and, thus, earlier in the universe’s history. Finding the exact wavelength of the break can therefore tell us how far the light has traveled and how old the galaxy is.
The first papers that came out on early galaxies relied on different filters that let in different regions of the infrared spectrum—researchers looked for galaxies that appeared at longer wavelengths but disappeared at higher ones, suggesting that the Lyman break was at wavelengths. through a low-power filter.
But this is somewhat absurd, because filters cover a range of wavelengths—we can’t know for sure where the break lies within that range. And, without capturing the details of the spectrum, it’s possible that we might miss information that indicates the feature we’re seeing isn’t actually a Lyman break, allowing a nearby galaxy to masquerade as something more distant.
So, the new work uses Webb’s NIRSpec instrument to capture the full spectrum of a set of galaxies in a region of space previously imaged by Hubble. The new papers focus on four of these: two previously identified by Hubble, and two new from Web data.
Old and early
The spectrum clearly shows Lyman breaks in all four of the spectra. Galaxies range in age from 460 million years after the Big Bang to just 325 million years after the Big Bang. The latter is the youngest galaxy whose age has been confirmed by spectroscopy. (Though again, there’s some indication that we image something closer to the Big Bang.) They’re all faint enough that we didn’t have instruments that could have acquired their spectra before we even put the web into space. .
You don’t need to know all the details, just that the red vertical lines represent the position of the Lyman break on the spectrum.
Curtis-Lake, etc. Al
What do these galaxies look like? They are relatively small, with about 108 from 109 The value of the star is times the mass of the Sun. This makes them similar in size to the Small Magellanic Cloud, a dwarf galaxy in our Local Group. But they are forming stars at a rate roughly equal to the rate of the Milky Way, which is about 10 times the rate of star formation in the Small Magellanic Cloud. So, star formation is happening at a faster rate.
The stars themselves also seem very young. Based on images of galaxies, about half of the stars in them are less than 70 million years old—perhaps considerably less. There is also little trace of heavy material in the vicinity, which would have been produced by an even earlier generation of stars. At least one galaxy contains less than 10 percent of the heavy elements seen in the Sun.
All consistent with what we expect from galaxies in the early universe. And, critically, these properties are consistent with models of galaxy formation based on our current dark energy/cold dark matter model of cosmology – which you might expect, given that our models of galaxy formation were based on constraints. Understanding Cosmology.
That doesn’t mean that cosmology is off the hook, though. There are still possible earlier galaxies that need a more detailed look. There is also the matter of the frequency of galaxies. Once we perform a comprehensive survey of a region of space, it is possible that more galaxies formed than we can count immediately after the Big Bang.
Nature Astronomy2023. DOI: 10.1038/s41550-023-01918-w, 10.1038/s41550-023-01921-1 (Regarding DOI).
