One of the goals of the Webb Space Telescope was to image early galaxies. This provided a new window into how the universe evolved between dense, hot matter from the Big Bang and the present, filled with stars and structure. And as soon as the data started pouring in, there were strong indications that galaxies had emerged just a few hundred million years after the Big Bang, and it looked very promising.
However, some uncertainty remained, as unusual conditions could cause a much younger galaxy to have much older-looking features. This may be the case for galaxies.
On Tuesday, two papers solving the problem were published, providing full spectra of four early galaxies, all clearly showing that they were only a few hundred million years after the Big Bang. Images of the same galaxy show that they are full of young stars that lack most of the heavier elements found in the universe today.
full spectrum
Understanding the age of early galaxies relies on finding specific characteristics of the light they emit. Early in the history of the universe, most of it was filled with hydrogen atoms. A hydrogen atom can absorb a photon when it gains enough energy to move the electron to another orbital. Thus, early galaxies radiate broadly across the spectrum, but experience a sharp cutoff at what is called a Lyman break. Higher energy photons are absorbed by hydrogen.
Nearby, a Lyman break exists in the UV portion of the spectrum. But as a photon travels across the expanding universe, it redshifts its position and stretches the wavelength of light. As a result, the Lyman break traverses the entire visible spectrum, looking farther into the infrared, and into the early history of the universe. Therefore, finding the exact wavelength of the break allows us to know exactly how far light traveled and how old the galaxy is.
The first papers on early galaxies relied on different filters to accept different regions of the infrared spectrum. Researchers looked for galaxies that existed at longer wavelengths and disappeared at higher wavelengths. Pass through a low energy filter.
However, this is a bit imprecise, as filters cover a range of wavelengths. We cannot know for sure where within that range the breaks lie. It is possible, and the Lyman break could disguise a closer galaxy as being much more distant.
So the new study uses Webb’s NIRSpec instrument to capture the full spectrum of a series of galaxies in the region of the universe previously imaged by Hubble. A new paper focuses on four of these. Two previously identified by Hubble and his two new from Webb data.
old and early
The spectra clearly show Lyman breaks in all four spectra. The age of the galaxy varies from 460 million years after the Big Bang to just 325 million years after the Big Bang. The latter is the youngest galaxy to date whose age has been confirmed by spectroscopy. (Again, there are some indications that they’ve imaged something even closer to the Big Bang.) They’re all so faint that no instrument has even been able to get a spectrum of the web before it’s in space. was. .
You don’t have to know all the details. The vertical red line simply marks the position of the Lyman break in the spectrum.
Curtis Lake et al.
What do these galaxies look like? They’re relatively small, about 10.8 up to 109 Twice the mass of the sun, equivalent to the mass of a star. As such, they are similar in size to our nearby group of dwarf galaxies, the Small Magellanic Cloud. But they are forming stars at about the same speed as the Milky Way galaxy. This is about ten times the rate of star formation in the Small Magellanic Cloud. Therefore, star formation is progressing rapidly.
The star itself also appears to be very young. Based on images of the galaxy, about half of the stars in those galaxies are less than 70 million years old, probably much younger than her. It also has few heavy elements that would have been produced by older generations of stars. At least one galaxy contains less than 10% of the heavier elements found in the Sun.
All of this is consistent with what we would expect from the earliest galaxies in the universe. And, importantly, these properties are consistent with models of galaxy formation based on the current dark energy/cold dark matter model of cosmology. understanding cosmology.
But that doesn’t mean cosmology is off the hook. There may still be early galaxies that need closer investigation. There is also the issue of galactic frequencies. A more extensive survey of certain regions of the universe may reveal that the number of galaxies that formed shortly after the Big Bang is higher than we can explain.
Natural Astronomy, 2023. DOIs: 10.1038/s41550-023-01918-w, 10.1038/s41550-023-01921-1 (About DOIs).
