After many years of waiting, the James Webb Space Telescope went to space on December 25th, 2021. In the six months that followed, the observatory unfurled its Sun shield, deployed its primary and secondary mirrors, aligned its mirror segments, and flew to its current position at the Earth-Sun lagrange 2 point. The first images presented the most detailed views of the Universe. NASA released an image of the most distant galaxy ever observed.

According to a new study by an international team of scientists, the JWST will allow them to measure the mass of early galaxies. The team obtained mass estimates from some distant galaxies that were many times more accurate than before using data from the Near-Infrared Camera. Their findings show how Webb will change our understanding of how the universe began.

The research team consisted of people from Italy, Australia, Thailand, and the ASTRO 3D collaboration.

One of the most important physical properties for understanding galaxy formation and evolution is stellar mass. The total amount of stars in a galaxy is measured by the amount of gas and dust being converted into new stars. It is the most direct way of tracing a universe. Astronomers can study how the universe evolved by looking at the oldest galaxies.

Accurately measuring the early galaxies has been a problem for astronomy. Calculating the star's mass on a source-by- source basis is not usually the best method for estimating the total mass of stars. Hubble studies of the most distant galaxies were limited to the UV spectrum.

By the time the light reaches us, it is redshifted. Due to the expansion of the universe, the light's wavelength is shortened and shifted towards the red end of the spectrum. At a distance of more than 13.46 light-years, the light will be shifted to the part of the spectrum that is visible in the IR. Santini sent an email to Universe Today.

“The bulk of the stars in galaxies, those that mostly contribute to its stellar mass, emit at optical-near infrared (NIR) wavelengths… [B]y the time the light takes to travels from a distant galaxy to our telescopes, the light emitted by its stars is no more in the optical regime. E.g., for a z=7 galaxy, the light originally emitted at 0.6 micron, reaches our telescope with a wavelength of 4.8 micron. The higher the redshift (i.e. the more distant the galaxy), the stronger is this effect.”

“This implies that we need infrared detectors to measure galaxy stellar masses (the light emitted by the bulk of their stars is out of reach of the Hubble Space Telescope). The only IR telescope we had before the advent of JWST was Spitzer Space Telescope, dismissed a few years ago. However, its 85 cm mirror was not comparable with the 6.5 m mirror of JWST. Most of the distant galaxies were out of reach of Spitzer too: due to its limited sensitivity and angular resolution, they were not detected (or affected by high levels of noise) on its images.

A spectral diagram comparing emitted light from an object to the observed redshifted light. As the Universe expands, it stretches light into lower frequencies or towards the red portion of the spectrum. Credit: NASA/ESA/C. Christian/Z. Levay (STScI)

There are a lot of dust-rich red galaxies that are not visible to the naked eye. Estimates of the stellar mass density of the early Universe could be off by up to six. Santini said that the JWST would open a new window into studying the oldest and faintest galaxies in the Universe. Santini said that the first-ever precision measurement of galactic mass out to the farthest distances would be enabled by the use of the telescope.

“Due to all these limitations in measuring the stellar mass, a commonly used approach before the launch of JWST was to convert the UV light (which is easily measured by HST) into a stellar mass estimate by assuming an average mass-to-UV light ratio. The mass-light relation was calibrated with the few and uncertain measurements we had, and it was representative only of those galaxy populations that were more easily observed (young, dust-free galaxies). Stellar mass measurements were therefore prone to large uncertainties (both when directly measured, and even more when inferred from the UV light).”

Santini and his international team of researchers relied on images obtained by NIRCAM on June 28th and 29th, 2022, as part of their first set of observations. They measured the stellar mass of 21 distant galaxies by probing their UV emission and red shifted-optical light. Santini said that this allowed them to avoid the large extrapolations and uncertainties of previous surveys and increased the accuracy of their mass measurement by a factor of 5 to 10.

The M/L ratio is far from ideal with a single average value. It spans about two orders of magnitude. This finding shows that the population of early galaxies was mostly heterogeneous, with many different physical conditions.

The first image taken by the James Webb Space Telescope. Credit: NASA, ESA, CSA, and STScI

These results are part of a growing collection of studies showing how important the mission will be. Astronomers will greatly benefit from the ability to offer more tightly-constrained estimates of stellar mass in the universe. Santini said, "Agreed."

“The major implication is that previous results regarding the mass growth process in galaxies could be affected by significant systematics. In our work we assess, for example, the level of systematic uncertainty affecting the cosmic stellar mass density. The latter describes the global growth of galaxies in the Universe as a function of time. Its assessment at early epochs is subject to large variance from one work to another. We found that the systematic uncertainty resulting from the assumption of a standard mass-to-light can be as high as a factor of a few, definitely too large compared to the level of precision we aim to reach, and it could at least partly explain the mismatch in the results of the literature.”

The clearest and most detailed images of the universe have already led to new discoveries. It shows how it can assist in the characterization of exoplanet atmospheres and determine if they are actually Habitable. It will play a vital role in determining the characteristics of the earliest galaxies in the Universe, how they have evolved, and possibly the role that Dark Matter and Dark Energy play.

ArXiv is further reading.