We can calculate the redshift using features in the galaxy's spectrum, which is an observation that spreads out the light from a target by wavelength, essentially sampling the light at very small intervals. "Redshift tells us how long the light has spent being stretched to longer wavelengths by the expansion of the universe as it travels to reach us. To determine the 'when' in the past, we use the galaxy's redshift. When light from a distant galaxy (or any object in space) reaches us, we are seeing that galaxy as it appeared in the past. "Light takes time to travel through space. We asked Micaela Bagley, a postdoctoral fellow at the University of Texas at Austin, to explain how astronomers analyze light from distant galaxies and determine "when in the universe's history" we are observing them. This goal requires that we identify samples of galaxies at different moments in the universe's history to explore how their properties evolve with time. One of the James Webb Space Telescope's science goals is to understand how galaxies in the early universe formed and evolved into much larger galaxies like our own Milky Way. The galaxy has a best-fit photometric redshift of 9 (when the universe was 550 million years old), but the probability distribution (right panel) covers the redshift range of 7–11 (when the universe was between 420 to 770 million years old.) Credit: Micaela Bagley We measure the average flux in each filter (circles) and fit these six data points with different galaxy models at a range of redshifts to determine the probability that the galaxy is at each redshift. The NIRCam filter transmissions and wavelength coverages are shown by the colored shaded regions. The light has been redshifted (or stretched out) by a factor of 10. The wavelength at which the light was emitted and observed is listed along the top and bottom, respectively. A model galaxy spectrum with a strong spectral break and several emission lines is shown in gray. An illustration of measuring a photometric redshift using six broadband imaging filters (left panel).
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