Astrobites at APS April 2024: Looking Ahead as We Look Far Through the JWST

Welcome to the Astrobites coverage of the 2024 April APS Meeting! We will report on highlights from each day here, from APS Plenaries to DAP (Division of Astrophysics) invited sessions to graduate student research talks.  You can find coverage of other meeting seminars here: Day 1, Day 2, Day 3, and Day 4.  This bite was written as part of our partnership with the Physical Review Journals and as part of our general conference coverage.  


Prof. Joel Leja. Image Credit

The first speaker of this session was Prof. Joel Leja from Penn State University.  In his talk, titled “The Exciting First Years and the Upcoming Long Legacy of the James Webb Space Telescope (JWST),” Prof. Leja described some of the early successes and discoveries of JWST.  Work on JWST began in 1989 (also the year another big star started her life), just one year prior to the launch of the Hubble Space Telescope (HST).  At the time, JWST was referred to as “the Next Generation Space Telescope” or NGST.  JWST has a nominal mission lifetime of just 5 years, however there is enough fuel to support a 20-year mission.  So far, JWST has crushed all expectations and is on track to take data for 20 years, however Prof. Leja shared that when JWST shuts down, it’ll likely be because of micrometroid strikes to the mirror and sunshield, detector damage from charged particles, space weathering of the sunshield/insulation, or breakdown of moving parts in instrumentation.  To learn more about JWST, you can read some of our Astrobites on the history of JWST and its launch and even the controversy on its naming.  

Prof. Leja spent the rest of his talk discussing the science goals and early science discoveries of this Great Observatory.  Some of the central missions of JWST are to 1) discover how the first stars formed, 2) figure out when the first galaxies formed, 3) understand how the first black holes formed, and, in the spirit of exploration, 4) uncover what we haven’t thought to look for yet.  In the roughly two years of data collection, JWST has observed several high-redshift galaxies (often identified by being bright in the red filters and point-like), and astronomers have found some perplexing trends.  For one, these very distant galaxies are remarkably large, and we’re not really sure why, and we’re finding these “little red dots” everywhere.  We have found that at least some of these “galaxies” are actually supermassive black holes (SMBH) which are accreting, but these SMBHs are incredibly massive, maybe 10-100% the mass of its host galaxy vs. the 0.3% local average (see our Bite here on one of the most massive SMBHs).  Another recent JWST spectrum of a galaxy from when the Universe that was only ~600 Myr has shown that at least one of these galaxies likely has a huge black hole (> millions of solar masses) AND a galaxy full of old (> 100 Myr) stars!  So what’s next for Webb?  Prof. Leja shared that he thinks that, among other things, JWST will be able to characterize the rapid formation of these massive galaxies in the very early universe and maybe directly detect the stars that reionized the Universe.  Most importantly, though, Prof. Leja shared that he thinks the best is yet to come and the most extraordinary thing JWST will do is discover something beyond our wildest imaginations. 

Prof. Courtney Dressing. Image Credit

The second speaker of this session was Prof. Courtney Dressing (a former Astrobiter!) from U.C. Berkeley.  In her talk, titled “Exploring the Diversity of Planetary Systems,” Prof. Dressing spoke on the recent discoveries JWST has made of exoplanet systems.  There are two main ways you can detect an exoplanet – “direct” or “indirect.”  Direct detection is very difficult, since planets are much fainter and smaller than their host stars, so most searches for exoplanets and their planetary systems use the “indirect” method.  The three main “indirect” methods are transit light curves, radial velocity measurements, and microlensing.  In transit light curves, as the exoplanet orbits between us and the star, it will cause a “dip” in the light curve of the star as it blocks out some of the light (similar to a partial solar eclipse here on Earth).  With radial velocity measurements, the absorption lines in the spectrum of the exoplanet-star system will shift red and blue as the planet orbits around the star.  Exoplanet microlensing is explained well in this Astrobite, but basically the idea is that the exoplanet-star system lenses some star, and there’s a unique “spike” or “dip” feature in the light curve of this lensed star that is caused by the exoplanet.  For a long time, radial velocity measurements were the primary way we detected exoplanets, but transit light curves are quickly gaining popularity.

While JWST is not primarily used to measure light curves of exoplanetary systems, it is commonly used to take spectra of exoplanet atmospheres.  In fact, the very first spectrum released by JWST was a “transmission spectrum” of WASP-96b (covered in this Astrobite).  Similar to the ideology of the transit light curve method, as an exoplanet just barely starts its transit (or alternatively, has almost just left the transit), a little bit of its atmosphere is illuminated by the starlight.  Astronomers can use JWST to take a spectrum of the system at this time to learn about the properties of the exoplanet atmosphere; this spectrum is a “transmission spectrum.”  Using this method, JWST has detected water in the atmospheres of several “hot Jupiters” (exoplanets that are roughly the same mass as Jupiter but are much hotter).  Prof. Dressing noted that many transmission spectra of smaller planets are flat and featureless, however.  She shared a spectrum of a small planet, GJ 486 b (the “a” designation goes to the star in the system!), which could show evidence for water vapor, however she noted that we are not yet able to rule out starspots as the cause. 

Dr. Louise Breuval. Image Credit

The final speaker of this session was Dr. Louise Breuval from Johns Hopkins University.  In her talk, titled “The Hubble Tension and JWST,” Dr. Breuval explained one way she and a team have used JWST imaging to try to resolve the Hubble Tension.  We have a variety of Astrobites articles on this long-standing astronomical problem, but the gist of this tension is that we have two ways of measuring the expansion of the Universe (or the “Hubble Constant,” H0), and they don’t agree with each other.  The first method uses the Cosmic Microwave Background (the CMB) and our best cosmological model (ΛCDM) and gets a value of H0 = 67.4 +/- 0.5 km/s/Mpc.  The other method uses “standard candles” (e.g., cepheid variable stars and Type Ia SNe) which have expected luminosities that we can standardize our distances to.  Depending on which standard candle is used, there is some diversity in the value of H0, but all of them, using this method, are roughly H0 = 74.03 +/- 1.42 km/s/Mpc – a significantly different result than the CMB estimation!  The second method is what Dr. Breuval’s talk was focused on.

Using the second method, we can measure H0 using cepheids by measuring the period-luminosity relation.  One of the largest sources of error in estimating this relation has been the resolution limit of HST – can we be sure there are no intervening sources near our cepheids that are contributing extra light in our measured luminosities?  JWST has superior resolution to HST, so Dr. Breuval’s team (led by Prof. Adam Riess) decided to image a variety of cepheids to see if this could be the cause of the tension.  They found that crowding was not an issue and that when using the JWST measurements, the tension actually gets stronger. Their results were published in ApJ earlier this year and the paper is accessible at the link here.  The Hubble Tension remains an elusive and open question.

JWST is in its earliest stages, and even so, we have discovered so much already, and it has even provided us with questions we hadn’t thought to ask.  We are at the precipice of a revolution of our understanding of the Universe.  The recent proposal cycle for time on JWST was oversubscribed 9:1, showing that there is community need for this observatory and its resources.  It’s a stellar time indeed to be an astronomer.


Edited by: Storm Colloms, Jessie Thwaites
Featured Image Credit: NASA, ESA, CSA/Joseph DePasquale (STScI), Anton M. Koekemoer (STScI)

About Huei Sears

Huei Sears is a fifth-year PhD candidate at Northwestern University studying astrophysics! Her research is focused on long Gamma-Ray Burst host galaxies. In addition to research, she cares a lot about science communication, and is always looking for ways to make science more accessible. In her free time, she enjoys walking along the lake, listening to Taylor Swift, & watching the X-Files.

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