Measuring the distances to galaxies with space telescopes

One of many James Webb Area Telescope’s science objectives is to know how galaxies within the early universe shaped and developed into a lot bigger galaxies like our personal Milky Way. This purpose requires that we establish samples of galaxies at totally different moments within the universe’s historical past to discover how their properties evolve with time.

We requested Micaela Bagley, a postdoctoral fellow on the College of Texas at Austin, to clarify how astronomers analyze gentle from distant galaxies and decide “when within the universe’s historical past” we’re observing them.

“Mild takes time to journey via space. When gentle from a distant galaxy (or any object in space) reaches us, we’re seeing that galaxy because it appeared prior to now. To find out the ‘when’ prior to now, we use the galaxy’s redshift.

“Redshift tells us how lengthy the sunshine has spent being stretched to longer wavelengths by the growth of the universe because it travels to achieve us. We will calculate the redshift utilizing options within the galaxy’s spectrum, which is an commentary that spreads out the sunshine from a goal by wavelength, basically sampling the sunshine at very small intervals. We will measure the emission strains and spectral breaks (abrupt modifications within the gentle depth at particular wavelengths), and evaluate their noticed wavelengths with their recognized emitted wavelengths.

“Some of the environment friendly methods to establish galaxies is thru imaging, for instance with the observatory’s NIRCam (Close to-Infrared Digicam) instrument. We take pictures utilizing a number of filters to gather the article’s gentle in a number of totally different colours. After we measure a galaxy’s photometry, or how vivid it’s in a picture, we’re measuring the brightness of the article averaged throughout the complete vary of wavelengths transmitted by the filter. We will observe a galaxy with NIRCam’s broadband imaging filters, however there’s a variety of detailed info hidden inside every single measurement for each 0.3–1.0 microns in wavelength protection.

“But we will begin to constrain the form of a galaxy’s spectrum. The spectrum’s form is affected by a number of properties together with what number of stars are forming within the galaxy, how a lot dust is current inside it, and the way a lot the galaxy’s gentle has been redshifted. We evaluate the measured brightness of the galaxy in every filter to the expected brightness for a set of galaxy fashions spanning a spread of these properties at a spread of redshifts. Primarily based on how nicely the fashions match the information, we will decide the chance that the galaxy is at a given redshift or ‘second in historical past.’ The perfect-fitting redshift decided via this evaluation is known as the photometric redshift.

“In July 2022, groups used NIRCam pictures from the CEERS Survey to establish two galaxies with photometric redshifts better than 11 (when the universe was lower than 420 million years outdated.) Neither of those objects have been detected by NASA’s Hubble Area Telescope observations on this area as a result of they’re both too faint or are detectable solely at wavelengths exterior of Hubble’s sensitivity. These have been very thrilling discoveries with the brand new telescope.

“Nevertheless, photometric redshift of a galaxy is considerably unsure. For instance, we could possibly decide {that a} spectral break is current in a filter, however not the exact wavelength of the break. Whereas we will estimate a best-fit redshift primarily based on modeling the photometry, the ensuing chance distribution is commonly broad.

“Moreover, galaxies at totally different redshifts can have related colours in broadband filters, making it tough to tell apart their redshifts primarily based solely on photometry. For instance, pink, dusty galaxies at redshifts lower than 5 (or when the universe was 1.1 billion years outdated or older) and funky stars in our personal galaxy can generally mimic the identical colours of a high-redshift galaxy. We due to this fact think about all galaxies which are chosen primarily based on their photometric redshifts to be high-redshift candidates till we will get hold of a extra exact redshift.

“We will decide a extra exact redshift for a galaxy by acquiring a spectrum. As illustrated within the following determine, our calculation of the redshift chance distribution improves as we measure the photometry of a galaxy in ever finer wavelength steps. The chance distribution narrows as we transfer from utilizing broadband filters for imaging (high) to a bigger variety of narrower filters (center), to a spectrum (backside). Within the backside row we will begin to key off particular options just like the spectral break on the far left and emission strains to acquire a redshift chance distribution that may be very exact—a spectroscopic redshift.

“In February 2023, the CEERS groups adopted up their high-redshift candidates with observatory’s NIRSpec (Close to-Infrared Spectrograph) instrument to measure exact, spectroscopic redshifts. One candidate (Maisie’s Galaxy) has been confirmed to be at redshift 11.4 (when the universe was 390 million years outdated), whereas the second candidate was found to truly be at a decrease redshift of 4.9 (when the universe was 1.2 billion years outdated.)

“Even instances the place we uncover {that a} high-redshift candidate is definitely a decrease redshift galaxy may be very thrilling. They permit us to study extra about circumstances in galaxies and the way in which these circumstances have an effect on their photometry, to enhance our fashions of galaxy spectra, and to constrain galaxy evolution throughout all redshifts. Nevertheless, in addition they spotlight the necessity to get hold of spectra to verify high-redshift candidates.