A new study by an international team of scientists has found that the James Webb Space Telescope (JWST) uses the near-infrared camera (NIRCam) observation data, which will enable astronomers to more accurately grasp the data of early galaxies. More information on behalf of the JWST will advance our understanding of the growth and evolution of the universe’s oldest galaxies.
Stellar mass is one of the most important physical properties for understanding galaxy formation and evolution. Because the total amount of stars in the galaxy will increase as new stars are formed in the galaxy, it will be the most direct way to track the growth of the galaxy. Observing the oldest galaxies in the universe (over 13 billion light-years) helps to understand how galaxies evolve.
Astronomers have been facing difficulties in the past trying to accurately observe these ancient galaxies. Usually astronomers measure the mass-to-light ratio (M/L) of galaxies, and use the light produced by the galaxy to estimate the total mass of stars, rather than calculating the mass of each star. The Hubble Space Telescope (HST) study of the most distant galaxy (GN-z11, which formed regarding 13.5 billion years ago) is limited to the ultraviolet (UV) spectrum.
Light from distant, ancient galaxies experiences a significant redshift when it reaches Earth. As light travels through space and time, due to the expansion of the universe, light waves will lengthen and move toward the red end of the spectrum. For galaxies with a redshift (z) of 7 or higher (13.46 light-years away or more), most of the light will be shifted to the infrared part of the spectrum. For example, in a galaxy Z=7, the light initially emitted at a wavelength of 0.6 microns eventually reaches the Earth telescope at a wavelength of 4.8 microns. The higher the redshift value (the farther the galaxy is), the stronger the effect.
This means that infrared light telescopes are needed to measure the mass of galaxies, because HST cannot observe the light of most distant galaxies. Before JWST started work, the infrared light telescope used by scientists was Spitzer Space Telescope (SST), but SST will be released in 2020. Retired on January 30, 2008. However, the 85-cm primary mirror of the SST is still not comparable to the 6.5-meter primary mirror of the JWST. Due to the limited sensitivity and angular resolution of the SST, most distant galaxies cannot be detected by the SST.
▲ A schematic diagram of the spectrum comparing the light emitted by an object with the observed red-shifted light. As the universe expands, it stretches light into the lower frequency or red part of the spectrum. (Source: NASA / ESA / C. Christian / Z. Levay (STScI))
In addition, previous observations may have missed many red galaxies, which are rich in dust (shielding light) and have extremely faint ultraviolet spectra. Scientists believe that estimates of the stellar mass density of the early universe may differ by as much as six times, and the JWST infrared instrument suite and high sensitivity will open new windows for future studies of the universe’s oldest and faintest galaxies.
A method commonly used to observe stellar mass before JWST launches is to convert ultraviolet light (HST measurements) into a stellar mass estimate by assuming an average mass-to-ultraviolet light ratio. The mass-light ratio relationship is calibrated with a small number of uncertain measurements and can only represent members of the more easily observed galaxies (young, dust-free galaxies). Previous measurements of stellar mass are prone to large errors.
Through the observation data of the JWST near-infrared camera, scientists can observe the quality of ultraviolet light and red-shifted visible light (redshift range of 6.7~12.3) of 21 galaxies, which can avoid the uncertainty of a large number of previous assumptions and improve the quality measurement accuracy by 5~ 10 times.
Using JWST’s NIRCam bluest band to measure stellar ultraviolet light and compare the mass light, it was found that the mass-to-light ratio cannot be represented by a single average, and the values span regarding two orders of magnitude. From a physical point of view, this finds that the species representing early galaxies are very heterogeneous, with galaxies exhibiting a wide variety of physical properties.
JWST’s excellent observation ability and stricter ability to evaluate the mass of stars in galaxies are beneficial for scientists to study cosmology in the widest range. Scientists believe that prior knowledge of the process of galaxy mass growth may be influenced by important systematics, assessing the extent of systematic uncertainty that affects the mass density of stars in the universe. Recently, the cosmic galaxy growth has been described as a function of time, and early estimates of patterns vary widely from case to case. Scientists have found that the assumption of a standard mass-to-light ratio can lead to system uncertainties that can be several times larger than what is required for accurate observations.Research published inarXiv。
To date, JWST has captured the clearest and most detailed images of the universe and demonstrated optical capabilities that have resulted in many new discoveries. It plays an important role in how the universe’s ancient galaxies evolve, and the role that dark matter and dark energy play.
(This article is written by Taipei Planetarium Reprinted with permission; source of the first image:Flickr/NASA′s James Webb Space Telescope CC BY 2.0）
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