James Webb Space Telescope

NASA’s James Webb Space Telescope is the largest and most powerful telescope ever launched to space. Its mirror is composed of 18 individual segments that have been aligned so accurately, that they effectively work as a single giant (21.6-foot, or 6.5-meter) reflector. The process of adjusting each of these separately functioning hexagonal mirror segments requires constant oversight from a dedicated team of engineers and optics scientists. We invite Dr. Marcio B. Meléndez, principal astronomical optics scientist for Webb at the Space Telescope Science Institute, to tell us more about the challenges of aligning the telescope after launch, and what is required to keep it that way during scientific operations.

“Soon after the successful launch and deployment of Webb, an intricate process of aligning its large golden mirrors began. It took nearly three months to go from the initial deployments of 18 individual unfocused segments that had just flown to space, to a completely aligned system bounded only by the optical design.

“Though the precise alignment of the telescope was completed in early 2022 during commissioning, it does not stay that way naturally due to various factors such as temperature variations and so-called ‘tilt’ events, so a lifelong maintenance program is required. The wavefront sensing team responsible for keeping Webb’s mirrors in order has been monitoring, investigating, trending, and occasionally moving its primary mirror segments during science operations. These activities are carried out from Webb’s Mission Operations Center, located at the Space Telescope Science Institute in Baltimore.

“This telescope monitoring program consists of a series of observations that use special optical sensing equipment inside the Near Infrared Camera instrument (NIRCam), with a set of lenses that intentionally defocus the images of stars by a known amount. These defocused star images contain measurable features that enable the team to derive the alignment of the telescope, using a process called phase retrieval to determine what we call the ‘wavefront error.’ The telescope monitoring observations are currently scheduled every other day, interspersed among Webb’s science observations, with short runtimes of about 20 minutes. All telescope monitoring observations are publicly available via the MAST archive.  Observatory users and other interested investigators can also view and model the optical quality using specialized tools.

“The maintenance program also takes a ‘selfie’ using a specialized ‘pupil imaging’ lens, designed to take images of the mirror segments and not of the sky, four times a year. These pupil images are used to assess the health of the primary mirrors. During each observation the team measures Webb’s pointing stability or ‘jitter,’ which has remained six times better than design requirements. The Fine Guidance Sensor is used to command a small onboard steerable mirror to lock onto a target, while moving in orbit, without deviating more than the thickness of a human hair, seen at a distance of seven miles (11 kilometers).

“The overall optical performance of the telescope is far better than the design requirement, meaning the observations are even more sensitive to faint objects, and more discerning of fine features than was expected. The optical requirement for Webb was set to 150 nanometers of wavefront error, coming from a combination of uncorrectable surface figure imperfections and correctable telescope misalignments. The current uncorrectable errors are very low, at about 65 nanometers The telescope alignment program aims to achieve and maintain this, and when the observed misalignments accumulate above predetermined criteria, the primary mirror segments are commanded and the system is realigned.

“Each segment from the primary mirror can be repositioned in six ‘degrees of freedom,’ meaning six different types of movement. A segment’s curved surface can also be changed somewhat to adjust its focal length. The Webb telescope mirrors maintain passive alignment through stable support from the backplane structure. As Webb points to different locations in the sky, the heat absorbed from the Sun changes, causing small (0.1 kelvins) temperature changes on the support structure that drive small physical movements. These tiny displacements cause mirror misalignments. This distortion is very small and accounts for only a few nanometers of change in the wavefront. In addition to this, there are sudden offsets to the structure that we call tilt events. These distinct jumps do not reverse themselves, and our current understanding of these events is that they are associated with small but abrupt releases of energy that was stored in the mirror support structure.

“The telescope mirror control updates were required to be less frequent than every two weeks. When a telescope misalignment is observed, the telescope team makes a correction within 48 hours following a well-coordinated procedure between different flight systems. During this time, we create a set of mirror movements intended to re-align the segments. These movements are transformed into commands that are then uploaded and executed. After applying these corrective moves, a new set of observations is taken to confirm the alignment of the telescope. Since the beginning of science operations, we have applied over 25 corrective moves. A time series of all wavefront measurements and the corresponding segment offsets is shown in Figure 3. On Oct. 3, a mirror correction was performed, after a record of 186 days since the previous mirror control update.

“With the rigorous overall maintenance program of measurement and control, the wavefront team ensures Webb’s optical performance is at the highest possible level to uncover the hidden mysteries of the universe.”

About the author:  Dr. Marcio B. Meléndez is a principal astronomical optics scientist at the Space Telescope Science Institute. He is a member of the wavefront sensing team in the telescope branch at STScI.

The fact that Webb’s mirror alignment has required fewer corrections than anticipated not only provides more observation time to conduct Webb science, but also offers important takeaways for future missions. Fewer adjustments indicates better than expected telescope stability, which will be a crucial consideration for missions like NASA’s future Habitable Worlds Observatory. The Habitable Worlds Observatory will be the first space telescope designed to search for life as we know it on Earth-sized planets around nearby Sun-like stars, while exploring many broader transformative astrophysics questions that will reveal secrets of the universe.

 

 

 

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

Measuring the Hubble constant, the rate at which the universe is expanding, is an active area of research among astronomers around the world who analyze data from both ground- and space- based observatories. NASA’s James Webb Space Telescope has already contributed to this ongoing discussion. Earlier this year, astronomers used Webb data containing Cepheid variables and Type Ia supernovae, reliable distance markers to measure the universe’s expansion rate, to confirm NASA’s Hubble Space Telescope’s previous measurements.

Now, researchers are using an independent method of measurement to further improve the precision of the Hubble constant — gravitationally lensed supernovae. Brenda Frye from the University of Arizona, and a team of many researchers from different institutions around the world, are leading this effort after Webb’s discovery of three points of light in the direction of a distant and densely populated cluster of galaxies. We invite Dr. Frye to tell us more about what the team has nicknamed Supernova H0pe and how gravitational lensing effects are providing insights into the Hubble constant:

“It all started with one question by the team: ‘What are those three dots that weren’t there before? Could that be a supernova?’ The points of light, not visible in 2015 Hubble imaging of the same cluster, were obvious when the images of PLCK G165.7+67.0 arrived on Earth from Webb’s Guaranteed Time Observations of the Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) ‘Clusters’ program. The team notes the question was the first to pop to mind for good reason: ‘The field of G165 was selected for this program due to its high rate of star formation of more than 300 solar masses per year, an attribute that correlates with higher supernova rates.’

“Initial analyses confirmed that these dots corresponded to an exploding star, one with rare qualities. First, it’s a Type Ia supernova, an explosion of a white dwarf star. This type of supernova is generally called a ‘standard candle,’ meaning that the supernova had a known intrinsic brightness. Second, it is gravitationally lensed.

“Gravitational lensing is important to this experiment. The lens, consisting of a cluster of galaxies that is situated between the supernova and us, bends the supernova’s light into multiple images. This is similar to how a trifold vanity mirror presents three different images of a person sitting in front of it. In the Webb image, this was demonstrated right before our eyes in that the middle image was flipped relative to the other two images, a ‘lensing’ effect predicted by theory.

“To achieve three images, the light traveled along three different paths. Since each path had a different length, and light traveled at the same speed, the supernova was imaged in this Webb observation at three different times during its explosion. In the trifold mirror analogy, a time-delay ensued in which the right-hand mirror depicted a person lifting a comb, the left-hand mirror showed hair being combed, and the middle mirror displayed the person putting down the comb.

“Trifold supernova images are special: The time delays, supernova distance, and gravitational lensing properties yield a value for the Hubble constant or H0 (pronounced H-naught). The supernova was named SN H0pe since it gives astronomers hope to better understand the universe’s changing expansion rate.

“In an effort to explore SN H0pe further, the PEARLS-Clusters team wrote a Webb Director’s Discretionary Time (DDT) proposal that was evaluated by science experts in dual-anonymous review and recommended by the Webb Science Policies Group for DDT observations. In parallel, data were acquired at the MMT, a 6.5-meter telescope on Mt. Hopkins, and the Large Binocular Telescope on Mt. Graham, both in Arizona. In analyzing both observations, our team was able to confirm that SN H0pe is anchored to a background galaxy, well behind the cluster, that existed 3.5 billion years after the big bang.

“SN H0pe is one of the most distant Type Ia supernovae observed to date. A different team member made another time delay measurement by analyzing the evolution of its light dispersed into its constituent colors or ‘spectrum’ from Webb, confirming the Type Ia nature of SN H0pe.

“Seven subgroups contributed lens models describing the 2D matter distribution of the galaxy cluster. Since the Type Ia supernova is a standard candle, each lens model was ‘graded’ by its ability to predict the time delays and supernova brightnesses relative to the true measured values.

“To prevent biases, the results were blinded from these independent groups and revealed to each other on the announced day and time of a ‘live unblinding.’ The team reports the value for the Hubble constant as 75.4 kilometers per second per megaparsec, plus 8.1 or minus 5.5. [One parsec is equivalent to 3.26 light-years distance.] This is only the second measurement of the Hubble constant by this method, and the first time using a standard candle. The PEARLS program lead investigator remarked, ‘This is one of the great Webb discoveries, and is leading to a better understanding of this fundamental parameter of our universe.’

“Our team’s results are impactful: The Hubble constant value matches other measurements in the local universe, and is somewhat in tension with values obtained when the universe was young. Webb observations in Cycle 3 will improve on the uncertainties, allowing more sensitive constraints on H0.”

About This Article

Authors

• • • Brenda Frye is a professor in the Department of Astronomy at the University of Arizona and astronomer at the Steward Observatory. She is co-lead of the PEARLS-Clusters branch of the Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) team.
    • Several members of the research team also contributed to this piece: Seth Cohen (Arizona State University), Patrick Kamieneski (Arizona State University), Mari Polletta (INAF Instituto di Astrofisica Spaziale e Fisica Cosmica), Justin Pierel (Space Telescope Science Institute), Wenlei Chen (University of Oklahoma), Massimo Pascale (University of California at Berkeley), Rogier Windhorst (Arizona State University, PEARLS Principal Investigator)

Related Links

• Published science paper: JWST Spectroscopy of SN H0pe: Classification and Time Delays of a Triply-imaged Type Ia Supernova at z = 1.78
• Published science paper: JWST Photometric Time-Delay and Magnification Measurements for the Triply-Imaged Type Ia “Supernova H0pe” at z = 1.78
• Published science paper: The JWST Discovery of the Triply-imaged Type Ia “Supernova H0pe” and Observations of the Galaxy Cluster PLCK G165.7+67.0′
• Published science paper: Spectroscopy of the supernova H0pe host galaxy at redshift 1.78
• Published science paper: Birds of a Feather: Resolving Stellar Mass Assembly With JWST/NIRCam in a Pair of Kindred z=2 Dusty Star-forming Galaxies Lensed by the PLCK G165.7+67.0 Cluster
• Accepted science paper: SN H0pe: The First Measurement of H0 from a Multiply-Imaged Type Ia Supernova, Discovered by JWST
• The full description and list of team members for Guaranteed Time Observation programs #1176 and #2738

 

Exoplanets are common in our galaxy, and some even orbit in the so-called habitable zone of their star. NASA’s James Webb Space Telescope has been busy observing a few of these small, potentially habitable planets, and astronomers are now hard at work analyzing Webb data. We invite Drs. Knicole Colón and Christopher Stark, two Webb project scientists at NASA’s Goddard Space Flight Center, to tell us more about the challenges in studying these other worlds.

“A potentially habitable planet is often defined as a planet similar in size to Earth that orbits in the ‘habitable zone’ of its star, a location where the planet could have a temperature where liquid water could exist on its surface. We currently know of around 30 planets that may be small, rocky planets like Earth and that orbit in the habitable zone. However, there is no guarantee that a planet that orbits in the habitable zone actually is habitable (it could support life), let alone inhabited (it currently supports life). At the time of writing, there is only one known habitable and inhabited planet – Earth!

“The potentially habitable worlds Webb is observing are all transiting exoplanets, meaning their orbits are nearly edge-on so that they pass in front of their host stars. Webb takes advantage of this orientation to perform transmission spectroscopy when the planet passes in front of its star. This orientation allows us to examine the starlight filtered through the atmospheres of planets to learn about their chemical compositions. However, the amount of starlight blocked by the thin atmosphere of a small rocky planet is tiny, typically much smaller than 0.02%. Simply detecting an atmosphere around these small worlds is very challenging. Identifying the presence of water vapor, which may bolster the possibility of habitability, is even harder. Searching for biosignatures (biologically produced gases) is extraordinarily difficult, but also an exciting endeavor.

“There are currently only a handful of small, potentially habitable worlds that are considered accessible to atmospheric characterization with Webb, which includes the planets LHS 1140 b and TRAPPIST-1 e.

“Some recent theoretical work exploring the detectability of gaseous molecules in the atmosphere of the super-Earth-size planet LHS 1140 b highlights several challenges in searching for biosignatures. The work notes approximately 10-50 transits of the planet around its host star, equivalent to 40-200 hours of observing time with Webb, would be needed to attempt a detection of potential biosignatures, such as ammonia, phosphine, chloromethane, and nitrous oxide, in the best-case scenario of a clear, cloud-free atmosphere.

“Given that Webb cannot view the LHS 1140 system year-round because of the system’s location on the sky, it would take multiple years if not close to a decade to collect 50 transit observations of LHS 1140 b. Searching for biosignatures may require even more than 50 transit observations if the planet atmosphere is cloudy. Most small exoplanets are known to have clouds or hazes that dampen or obscure the signal being searched for. The atmospheric signals of these biosignature gases also tend to overlap with other expected atmospheric signals (e.g. due to gaseous methane or carbon dioxide), so distinguishing between the various signals is another challenge.

“A potential avenue in the search for biosignatures is in the study of Hycean planets, which are a theoretical class of super-Earth-size planets with a relatively thin hydrogen-rich atmosphere and a substantial liquid water ocean. The super-Earth K2-18 b is a candidate for a potentially habitable Hycean planet based on current data from Webb and other observatories. Recently published work used NIRSpec and NIRISS to detect methane and carbon dioxide in the atmosphere of K2-18 b, but not water. This means the suggestion that K2-18 b is a Hycean world with a liquid water ocean remains based on theoretical models, with no direct observational evidence yet. The authors of the work also hinted at the possible presence of the potential biosignature dimethyl sulfide in the atmosphere of K2-18 b, but the potential dimethyl sulfide signal is too weak for a conclusive detection in the current data. The concept and study of the class of Hycean planets is very new, such that alternative interpretations to the liquid water ocean scenario (and therefore to the potential for a habitable environment) are still being explored. Upcoming Webb observations with the NIRSpec and MIRI instruments should shed further light on the nature of the potential Hycean planet K2-18 b and on the possible presence of dimethyl sulfide in its atmosphere.

“One other confounding factor that makes Webb’s study of small, potentially habitable worlds challenging is that the host stars can exhibit signs of water vapor, too.This was explored in recent Webb observations of the rocky exoplanet known as GJ 486 b. We therefore have the added challenge of determining whether water vapor detected by Webb is actually from a planet’s atmosphere and not from its star.

“The detection of biosignatures in the atmospheres of small, potentially habitable transiting planets that orbit cool stars is an extremely challenging endeavor, typically requiring ideal conditions (e.g., cloud-free atmospheres) or assuming early Earth environments (i.e., different than modern Earth as we know it), the detection of signals significantly smaller than 200 parts per million, a well-behaved star without significant water vapor in star spots, and a significant amount of telescope time to reach sufficient signal-to-noise. It is also important to keep in mind that detection of a single biosignature by any means does not constitute discovery of life. Discovery of life on an exoplanet will likely require a large set of unambiguously detected biosignatures, data from multiple missions and observatories, and extensive atmospheric modeling efforts, a process likely taking years.

“The power of Webb is that it has the sensitivity to detect and begin to characterize the atmospheres of a handful of the most promising potentially habitable planets orbiting cool stars. Webb particularly has the ability to detect a range of molecules important for life, like water vapor, methane, and carbon dioxide. Our goal is to learn as much as we can about worlds that may be potentially habitable, even if we cannot definitively identify habitable signatures with Webb. Webb observations, combined with exoplanet studies by NASA’s upcoming Nancy Grace Roman Space Telescope, will ultimately lay the foundation for the future Habitable Worlds Observatory, which will be NASA’s first mission purpose-built to directly image and search for chemical traces caused by life on Earth-like planets around Sun-like stars.”

About the Authors:

Knicole Colón is an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and serves as the James Webb Space Telescope deputy project scientist for Exoplanet Science.

Christopher Stark is an astrophysicist in the Exoplanets and Stellar Astrophysics Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and serves as the James Webb Space Telescope deputy observatory project scientist.

 

 

 

 

 

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

Over the last two years, scientists have used NASA’s James Webb Space Telescope (also called Webb or JWST) to explore what astronomers refer to as Cosmic Dawn –  the period in the first few hundred million years after the big bang where the first galaxies were born. These galaxies provide vital insight into the ways in which the gas, stars, and black holes were changing when the universe was very young. In October 2023 and January 2024, an international team of astronomers used Webb to observe galaxies as part of the JWST Advanced Deep Extragalactic Survey (JADES) program. Using Webb’s NIRSpec (Near-Infrared Spectrograph), they obtained a spectrum of a record-breaking galaxy observed only two hundred and ninety million years after the big bang. This corresponds to a redshift of about 14, which is a measure of how much a galaxy’s light is stretched by the expansion of the universe. We invited Stefano Carniani from Scuola Normale Superiore in Pisa, Italy, and Kevin Hainline from the University of Arizona in Tucson, Arizona, to tell us more about how this source was found and what its unique properties tell us about galaxy formation.

“The instruments on Webb were designed to find and understand the earliest galaxies, and in the first year of observations as part of the JWST Advanced Deep Extragalactic Survey (JADES), we found many hundreds of candidate galaxies from the first 650 million years after the big bang. In early 2023, we discovered a galaxy in our data that had strong evidence of being above a redshift of 14, which was very exciting, but there were some properties of the source that made us wary. The source was surprisingly bright, which we wouldn’t expect for such a distant galaxy, and it was very close to another galaxy such that the two appeared to be part of one larger object. When we observed the source again in October 2023 as part of the JADES Origins Field, new imaging data obtained with Webb’s narrower NIRCam (Near-Infrared Camera) filters pointed even more toward the high-redshift hypothesis. We knew we needed a spectrum, as whatever we would learn would be of immense scientific importance, either as a new milestone in Webb’s investigation of the early universe or as a confounding oddball of a middle-aged galaxy.

“In January 2024, NIRSpec observed this galaxy, JADES-GS-z14-0, for almost ten hours, and when the spectrum was first processed, there was unambiguous evidence that the galaxy was indeed at a redshift of 14.32, shattering the previous most-distant galaxy record (z = 13.2 of JADES-GS-z13-0). Seeing this spectrum was incredibly exciting for the whole team, given the mystery surrounding the source. This discovery was not just a new distance record for our team; the most important aspect of JADES-GS-z14-0 was that at this distance, we know that this galaxy must be intrinsically very luminous. From the images, the source is found to be over 1,600-light years across, proving that the light we see is coming mostly from young stars and not from emission near a growing supermassive black hole. This much starlight implies that the galaxy is several hundreds of millions of times the mass of the Sun! This raises the question: How can nature make such a bright, massive, and large galaxy in less than 300 million years?

“The data reveal other important aspects of this astonishing galaxy. We see that the color of the galaxy is not as blue as it could be, indicating that some of the light is reddened by dust, even at these very early times. JADES researcher Jake Helton of Steward Observatory and the University of Arizona also identified that JADES-GS-z14-0 was detected at longer wavelengths with Webb’s MIRI (Mid-Infrared Instrument), a remarkable achievement considering its distance. The MIRI observation covers wavelengths of light that were emitted in the visible-light range, which are redshifted out of reach for Webb’s near-infrared instruments. Jake’s analysis indicates that the brightness of the source implied by the MIRI observation is above what would be extrapolated from the measurements by the other Webb instruments, indicating the presence of strong ionized gas emission in the galaxy in the form of bright emission lines from hydrogen and oxygen. The presence of oxygen so early in the life of this galaxy is a surprise and suggests that multiple generations of very massive stars had already lived their lives before we observed the galaxy.

“All of these observations, together, tell us that JADES-GS-z14-0 is not like the types of galaxies that have been predicted by theoretical models and computer simulations to exist in the very early universe. Given the observed brightness of the source, we can forecast how it might grow over cosmic time, and so far we have not found any suitable analogs from the hundreds of other galaxies we’ve observed at high redshift in our survey. Given the relatively small region of the sky that we searched to find JADES-GS-z14-0, its discovery has profound implications for the predicted number of bright galaxies we see in the early universe, as discussed in another concurrent JADES study (Robertson et al., recently accepted). It is likely that astronomers will find many such luminous galaxies, possibly at even earlier times, over the next decade with Webb. We’re thrilled to see the extraordinary diversity of galaxies that existed at Cosmic Dawn!”

These spectroscopic observations were taken as part of Guaranteed Time Observations (GTO) program 1287, and the MIRI ones as part of GTO program 1180.

About the authors:
Stefano Carniani is an assistant professor at Scuola Normale Superiore in Italy. He is also a member of the Webb/NIRSpec GTO team and studies galaxy and black hole evolution across cosmic time.

Kevin Hainline is an associate research professor at the Steward Observatory, University of Arizona. He is also a member of the Webb/NIRCam Science team, and is using data from the JADES GTO survey to explore the evolution of galaxies and active galactic nuclei at the highest redshifts.

News Media Contacts:

Alise Fisher
Headquarters, Washington
202-358-2546
alise.m.fisher@nasa.gov

Laura Betz
Goddard Space Flight Center, Greenbelt, Md.
301-286-9030
laura.e.betz@nasa.gov

Christine Pulliam
Space Telescope Science Institute, Baltimore, Md.
410-338-4366
cpulliam@stsci.edu

NASA’s James Webb Space Telescope is delivering on its promise to explore the farthest reaches of the universe, looking back to a time when galaxies were just beginning to form. Scientists have been eagerly waiting to use this complex observatory to understand details that have been out of reach. We invite Alan Dressler, astronomer emeritus at the Carnegie Institute for Science Observatories, to describe his journey of scientific discovery from the early days of NASA’s Hubble Space Telescope to the exciting, new era of astronomy with the James Webb Space Telescope.

“In 1993 I was asked by AURA – the Associated Universities of Research in Astronomy – to lead a committee of 20 astronomers in identifying the important questions astronomers might answer in the next 20 years, given the astonishing and surprisingly deep view of the universe from the Hubble Space Telescope (HST). Astronomers had expected Hubble might be able to ‘look back in time’ to when the universe was half its present age of 13.8 billion years, or around 7 billion years ago. This by itself was an exciting prospect, but Hubble did far better, discovering that light from distant galaxies was brighter – and more intense – than expected. Our HST & Beyond committee realized that Hubble could take us to the threshold of the actual birth of galaxies, within the billion years after the big bang. This was a ‘once-in-a-species’ opportunity to witness our own cosmic origins.

“Without galaxies to host generations of stars and accumulate the heavy chemical elements that are essential ingredients in planets – and life – our universe would have fizzled. However, to actually see galaxy births would require something beyond Hubble: a bigger telescope, with extreme sensitivity to infrared light. The HST & Beyond committee advised NASA to build such a telescope – an extraordinary challenge, because only a super-cold telescope placed far from Earth could see the faint glow of these cosmic cradles. It took 25 years and nearly 20,000 smart, dedicated people, but the James Webb Space Telescope has done it! Since the start of science observations in July 2022, different programs use the telescope to take pictures of galaxies in the throes of birth, more than 13 billion years ago. And there has been a great surprise! Galaxies were born in explosive bursts of star formation – unlike anything that we have ever seen.

“I had hoped that I might share in this epic journey – so long in coming – and fortunately, I have. The team that built Webb’s NIRCam (Near-Infrared Camera), the telescope’s primary camera, has taken extremely deep pictures in nine infrared colors with its Guaranteed Time Observations program called JWST Advanced Deep Extragalactic Survey (JADES). JADES surveyed the southern portion of the Great Observatories Origins Deep Survey (GOODS), a region of the sky used to perform deep astronomical surveys. This GOODS-South field is wide enough to capture about 1,000 galaxies of the universe’s first billion years. This was the time when rapid expansion of the universe had cooled it enough for gravity to pull together hydrogen and helium gas, enabling the birth of the first stars. This first generation will be very hard to detect (and may be undetectable – even with Webb), but we are seeing the second-generation stars that did the ‘major lifting’ of chemical enrichment.

“Galaxies ‘grow’ by turning the (mostly hydrogen) gas into generations of stars – adding stellar mass. Today’s galaxies do this slowly – not much gas is left. However, astronomers suspected that, in the gas-rich early universe, growth would be rapid and dynamic, and that has turned out to be quite an understatement. As a population of stars ages, their combined light turns from blue to red: Astronomers have long struggled to distinguish a relatively young galaxy from a much older one, because a population of stars that is 2 billion years old is already just as ‘red’ as another galaxy that is 10 billion years old. This is like judging the age of people only from their height. But here is where things turn to our favor: The observatory is showing us galaxies from the first billion years – extremely young, where the light comes from stars about twice as massive, and much bluer, than our Sun. These so-called A-stars are fantastic ‘cosmic clocks,’ with lifetimes between 100 million years and 1 billion years. Over a billion years it’s easy to tell the youngest from the oldest. We use them to decode the histories of star formation of the first galaxies.

“These NIRCam pictures record the infrared colors of the combined light of these young stars.  To understand the astrophysics, astronomers prefer to spread starlight into a highly detailed  ‘rainbow,’ called a spectrum, with hundreds, or thousands, of colors in it. But the first galaxies are too faint for that, so instead we use a ‘simple rainbow’ of seven colors obtained from NIRCam images taken with different filters to analyze the age and history of each galaxy.  Astronomers call this ‘simple rainbow’ the ‘spectral energy distribution or SED.   

“Over several years l developed a computer code – SEDz* – that analyzes seven-color SEDs to extract a galaxy’s history of star formation. The figure below shows how the distinct SEDs of young stellar populations are used to measure star formation histories. The top left panel shows the SED (the data) of a faint galaxy and the model that SEDz* produces by combining the light of stellar populations that, added together, describe the galaxy’s history of star formation. The SEDz* solution includes measuring the redshift of the galaxy (the ‘when’) – the red-ward shift of light due to the expansion of the universe (why we look in the infrared to see these first galaxies). SEDz* solves for the combination of populations of stars – how much mass at what age? – best matches the data. For every well-observed galaxy, only one combination of stellar population ages – one star formation history – ‘works.’ In some cases, comparatively few stars – too few to detect – are born in the roughly 500 million years in-between, so we describe these as bursts of rapid and intense star formation – starbursts.

“The astonishing thing is that, together, such bursts are found for more than 70% of the roughly 900 galaxies in the sample. About half of all histories in this study are single bursts – meaning that all the stars combined, having 100 million up to 1 billion times the mass of our Sun, are born in a time of about 100 million years, and probably much less. There are no starbursts of this magnitude occurring in the modern universe, or even in the previous 10 billion years that led to our time. These early starbursts easily surpass any we’ve ever seen, and computer models of how galaxies were built did not predict them. The challenge for theorists will be to understand the physics that allow – actually ‘require’ – such violent events to kick things off.

“Below are two more examples, a single starburst, and an infant galaxy growing smoothly and steadily – more like what we expected for most of them. Such longer histories that spread over the first billion years are outnumbered three to one by the bursts, but each of these ‘steady builders’ accumulate considerable stellar mass, so they substantially contribute to building today’s population of galaxies, like our own Milky Way.

“The early universe of galaxies began in a fury of what might be called ‘explosive star formation,’ with only a modest fraction of galaxies growing in a slower, steadier pace. In the billion years that follow, these burst galaxies must grow about 10 times in stellar mass. That much we know. How they do it? That we surely don’t. And that’s the challenge.”

About the Author

Alan Dressler is a staff scientist emeritus at the Carnegie Institution for Science in Washington. He studies how galaxy structures and shapes change, the pace and character of star birth, and how large galaxies form from earlier, smaller systems. The findings in this blog were published in the March 27, 2024 issue of The Astrophysical Journal.

 

 

 

 

 

Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.

In November 2023, NASA’s James Webb Space Telescope observed a massive cluster of galaxies named MACS J0138.0-2155. Through an effect called gravitational lensing, first predicted by Albert Einstein, a distant galaxy named MRG-M0138 appears warped by the powerful gravity of the intervening galaxy cluster. In addition to warping and magnifying the distant galaxy, the gravitational lensing effect caused by MACS J0138 produces five different images of MRG-M0138.

In 2019, astronomers announced the surprising find that a stellar explosion, or supernova, had occurred within MRG-M0138, as seen in images from NASA’s Hubble Space Telescope taken in 2016. When another group of astronomers examined the 2023 Webb images, they were astonished to find that seven years later, the same galaxy is home to a second supernova. Justin Pierel (NASA Einstein Fellow at the Space Telescope Science Institute) and Andrew Newman (staff astronomer at the Observatories of the Carnegie Institution for Science) tell us more about this first time that two gravitationally lensed supernovae were found in the same galaxy.

“When a supernova explodes behind a gravitational lens, its light reaches Earth by several different paths. We can compare these paths to several trains that leave a station at the same time, all traveling at the same speed and bound for the same location. Each train takes a different route, and because of the differences in trip length and terrain, the trains do not arrive at their destination at the same time. Similarly, gravitationally lensed supernova images appear to astronomers over days, weeks, or even years. By measuring differences in the times that the supernova images appear, we can measure the history of the expansion rate of the universe, known as the Hubble constant, which is a major challenge in cosmology today. The catch is that these multiply-imaged supernovae are extremely rare: fewer than a dozen have been detected until now.

“Within this small club, the 2016 supernova in MRG-M0138, named Requiem, stood out for several reasons. First, it was 10 billion light-years distant. Second, the supernova was likely the same type (Ia) that is used as a ‘standard candle’ to measure cosmic distances. Third, models predicted that one of the supernova images is so delayed by its path through the extreme gravity of the cluster that it will not appear to us until the mid-2030s. Unfortunately, since Requiem was not discovered until 2019, long after it had faded from view, it was not possible to gather sufficient data to measure the Hubble constant then.

“Now we have found a second gravitationally lensed supernova within the same galaxy as Requiem, which we call Supernova Encore. Encore was discovered serendipitously, and we are now actively following the ongoing supernova with a time-critical director’s discretionary program. Using these Webb images, we will measure and confirm the Hubble constant based on this multiply imaged supernova. Encore is confirmed to be a standard candle or type Ia supernova, making Encore and Requiem by far the most distant pair of standard-candle supernova ‘siblings’ ever discovered.

“Supernovae are normally unpredictable, but in this case we know when and where to look to see the final appearances of Requiem and Encore. Infrared observations around 2035 will catch their last hurrah and deliver a new and precise measurement of the Hubble constant.”

These observations were taken as part of Webb Director’s Discretionary Time program 6549.

About the Authors:

Justin Pierel is a NASA Einstein Fellow at the Space Telescope Science Institute in Baltimore, Maryland, and co-principal investigator of the Webb Director’s Discretionary Time program 6549.

Andrew Newman is a staff astronomer at the Observatories of the Carnegie Institution for Science in Pasadena, California. He is principal investigator of Webb General Observer program 2345, which discovered Supernova Encore, and co-principal investigator of the Webb Director’s Discretionary Time program 6549.

 

One of NASA’s 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. 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.

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.

“Light takes time to travel through space. When light from a distant galaxy (or any object in space) reaches us, we are seeing that galaxy as it appeared in the past. To determine the  ‘when’ in the past, we use the galaxy’s redshift.

“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. 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. We can measure the emission lines and spectral breaks (abrupt changes in the light intensity at specific wavelengths), and compare their observed wavelengths with their known emitted wavelengths.

“One of the most efficient ways to identify galaxies is through imaging, for example with the observatory’s NIRCam (Near-Infrared Camera) instrument. We take images using multiple filters to collect the object’s light in several different colors. When we measure a galaxy’s photometry, or how bright it is in an image, we’re measuring the brightness of the object averaged across the full range of wavelengths transmitted by the filter. We can observe a galaxy with NIRCam’s broadband imaging filters, but there is a lot of detailed information hidden within each single measurement for every 0.3–1.0 microns in wavelength coverage.

“Yet we can start to constrain the shape of a galaxy’s spectrum. The spectrum’s shape is affected by several properties including how many stars are forming in the galaxy, how much dust is present within it, and how much the galaxy’s light has been redshifted. We compare the measured brightness of the galaxy in each filter to the predicted brightness for a set of galaxy models spanning a range of those properties at a range of redshifts. Based on how well the models fit the data, we can determine the probability that the galaxy is at a given redshift or ‘ moment in history.’ The best-fitting redshift determined through this analysis is called the photometric redshift.

“In July 2022, teams used NIRCam images from the CEERS Survey to identify two galaxies with photometric redshifts greater than 11 (when the universe was less than 420 million years old.) Neither of these objects were detected by NASA’s Hubble Space Telescope observations in this field because they are either too faint or are detectable only at wavelengths outside of Hubble’s sensitivity. These were very exciting discoveries with the new telescope!

“However, photometric redshift of a galaxy is somewhat uncertain. For example, we may be able to determine that a spectral break is present in a filter, but not the precise wavelength of the break. While we can estimate a best-fit redshift based on modeling the photometry, the resulting probability distribution is often broad. Additionally, galaxies at different redshifts can have similar colors in broadband filters, making it difficult to distinguish their redshifts based only on photometry. For example, red, dusty galaxies at redshifts less than 5 (or when the universe was 1.1 billion years old or older) and cool stars in our own galaxy can sometimes mimic the same colors of a high-redshift galaxy. We therefore consider all galaxies that are selected based on their photometric redshifts to be high-redshift candidates until we can obtain a more precise redshift.

“We can determine a more precise redshift for a galaxy by obtaining a spectrum. As illustrated in the following figure, our calculation of the redshift probability distribution improves as we measure the photometry of a galaxy in ever finer wavelength steps. The probability distribution narrows as we move from using broadband filters for imaging (top) to a larger number of narrower filters (middle), to a spectrum (bottom). In the bottom row we can start to key off specific features like the spectral break on the far left and emission lines to obtain a redshift probability distribution that is very precise – a spectroscopic redshift.

“In February 2023, the CEERS teams followed up their high-redshift candidates with observatory’s NIRSpec (Near-Infrared Spectrograph) instrument to measure precise, spectroscopic redshifts. One candidate (Maisie’s Galaxy) has been confirmed to be at redshift 11.4 (when the universe was 390 million years old), while the second candidate was discovered to actually be at a lower redshift of 4.9 (when the universe was 1.2 billion years old.)

“Even cases where we discover that a high-redshift candidate is actually a lower redshift galaxy can be very exciting. They allow us to learn more about conditions in galaxies and the way those conditions affect their photometry, to improve our models of galaxy spectra, and to constrain galaxy evolution across all redshifts. However, they also highlight the need to obtain spectra to confirm high-redshift candidates.

About the author:

Micaela Bagley is a postdoctoral fellow at the University of Texas at Austin and a member of CEERS. They study galaxy formation and evolution in the early universe. Micaela is also responsible for processing all the NIRCam images for the CEERS team.

 

 

NASA’s James Webb Space Telescope observed the exoplanet WASP-80 b as it passed in front of and behind its host star, revealing spectra indicative of an atmosphere containing methane gas and water vapor. While water vapor has been detected in over a dozen planets to date, until recently methane – a molecule found in abundance in the atmospheres of Jupiter, Saturn, Uranus, and Neptune within our solar system – has remained elusive in the atmospheres of transiting exoplanets when studied with space-based spectroscopy. Taylor Bell from the Bay Area Environmental Research Institute (BAERI), working at NASA’s Ames Research Center in California’s Silicon Valley, and Luis Welbanks from Arizona State University tell us more about the significance of discovering methane in exoplanet atmospheres and discuss how Webb observations facilitated the identification of this long-sought-after molecule. These findings were recently published in the scientific journal Nature.

“With a temperature about 825 kelvins (about 1,025 degrees Fahrenheit), WASP-80 b is what scientists call a “warm Jupiter,” which are planets that are similar in size and mass to the planet Jupiter in our solar system but have a temperature that’s in-between that of hot Jupiters, like the 1,450-K (2,150-F) HD 209458 b (the first transiting exoplanet discovered), and cold Jupiters, like our own which is about 125 K (235 F). WASP-80 b goes around its red dwarf star once every three days and is situated 163 light-years away from us in the constellation Aquila. Because the planet is so close to its star and both are so far away from us, we can’t see the planet directly with even the most advanced telescopes like Webb. Instead, researchers study the combined light from the star and planet using the transit method (which has been used to discover most known exoplanets), and the eclipse method.

Using the transit method, we observed the system when the planet moved in front of its star from our perspective, causing the starlight we see to dim a bit. It’s kind of like when someone passes in front of a lamp and the light dims. During this time, a thin ring of the planet’s atmosphere around the planet’s day/night boundary is lit up by the star, and at certain colors of light where the molecules in the planet’s atmosphere absorb light, the atmosphere looks thicker and blocks more starlight, causing a deeper dimming compared to other wavelengths where the atmosphere appears transparent. This method helps scientists like us understand what the planet’s atmosphere is made of by seeing which colors of light are being blocked.

Meanwhile, using the eclipse method, we observed the system as the planet passed behind its star from our perspective, causing another small dip in the total light we received. All objects emit some light, called thermal radiation, with the intensity and color of the emitted light depending on how hot the object is. Just before and after the eclipse, the planet’s hot dayside is pointed toward us, and by measuring the dip in light during the eclipse we were able to measure the infrared light emitted by the planet. For eclipse spectra, absorption by molecules in the planet’s atmosphere typically appear as a reduction in the planet’s emitted light at specific wavelengths. Also, since the planet is much smaller and colder than its host star, the depth of an eclipse is much smaller than the depth of a transit.

The initial observations we made needed to be transformed into something we call a spectrum; this is essentially a measurement showing how much light is either blocked or emitted by the planet’s atmosphere at different colors (or wavelengths) of light. Many different tools exist to transform raw observations into useful spectra, so we used two different approaches to make sure our findings were robust to different assumptions. Next, we interpreted this spectrum using two kinds of models to simulate what the atmosphere of a planet under such extreme conditions would look like. The first type of model is entirely flexible, trying millions of combinations of methane and water abundances and temperatures to find the combination that best matches our data. The second type, called ‘self-consistent models,’ also explores millions of combinations but uses our existing knowledge of physics and chemistry to determine the levels of methane and water that could be expected. Both model types reached the same conclusion: a definitive detection of methane.

To validate our findings, we used robust statistical methods to evaluate the probability of our detection being random noise. In our field, we regard the ‘gold standard’ to be something called a ‘5-sigma detection,’ meaning the odds of a detection being caused by random noise are 1 in 1.7 million. Meanwhile, we detected methane at 6.1-sigma in both the transit and eclipse spectra, which sets the odds of a spurious detection in each observation at 1 in 942 million, surpassing the 5-sigma ‘gold standard,’ and reinforcing our confidence in both detections.

With such a confident detection, not only did we find a very elusive molecule, but we can now start exploring what this chemical composition tells us about the planet’s birth, growth, and evolution. For example, by measuring the amount of methane and water in the planet, we can infer the ratio of carbon atoms to oxygen atoms. This ratio is expected to change depending on where and when planets form in their system. Thus, examining this carbon-to-oxygen ratio can offer clues as to whether the planet formed close to its star or farther away before gradually moving inward.

Another thing that has us excited about this discovery is the opportunity to finally compare planets outside of our solar system to those in it. NASA has a history of sending spacecraft to the gas giants in our solar system to measure the amount of methane and other molecules in their atmospheres. Now, by having a measurement of the same gas in an exoplanet, we can start to perform an “apples-to-apples” comparison and see if the expectations from the solar system match what we see outside of it.

Finally, as we look toward future discoveries with Webb, this result shows us that we are at the brink of more exciting findings. Additional MIRI and NIRCam observations of WASP-80 b with Webb will allow us to probe the properties of the atmosphere at different wavelengths of light. Our findings lead us to think that we will be able to observe other carbon-rich molecules such as carbon monoxide and carbon dioxide, enabling us to paint a more comprehensive picture of the conditions in this planet’s atmosphere. Additionally, as we find methane and other gases in exoplanets, we will continue to expand our knowledge about how chemistry and physics works under conditions unlike what we have on Earth, and maybe sometime soon, in other planets that remind us of what we have here at home. One thing is clear – —the journey of discovery with the James Webb Space Telescope is brimming with potential surprises.”

About the authors:

• • • Taylor Bell is a postdoctoral research scientist at the Bay Area Environmental Research Institute (BAERI), working at NASA’s Ames Research Center in California’s Silicon Valley.
    • Luis Welbanks is a NASA Hubble Fellow at Arizona State University in Tempe, Arizona.

The rate at which the universe is expanding, known as the Hubble constant, is one of the fundamental parameters for understanding the evolution and ultimate fate of the cosmos. However, a persistent difference called the “Hubble Tension” is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the big bang afterglow.

NASA’s James Webb Space Telescope provides new capabilities to scrutinize and refine some of the strongest observational evidence for this tension. Nobel Laureate Adam Riess from the Johns Hopkins University and the Space Telescope Science Institute presents his and his colleagues’ recent work using Webb observations to improve the precision of local measurements of the Hubble constant.

“Did you ever struggle to see a sign that was at the edge of your vision? What does it say? What does it mean? Even with the most powerful telescopes, the ‘signs’ astronomers want to read appear so small that we struggle too.

“The sign cosmologists want to read is a cosmic speed limit sign that tells us how fast the universe is expanding — a number called the Hubble constant. Our sign is written into the stars in distant galaxies. The brightnesses of certain stars in those galaxies tell us how far away they are and thus for how much time this light has been traveling to reach us, and the redshifts of the galaxies tell us how much the universe expanded over that time, hence telling us the expansion rate.

“A particular class of stars, Cepheid variables, has given us the most precise measurements of distance for over a century because these stars are extraordinarily bright: They are supergiant stars, a hundred thousand times the luminosity of the Sun. What’s more, they pulsate (that is, expand and contract in size) over a period of weeks that indicates their relative luminosity. The longer the period, the intrinsically brighter they are. They are the gold standard tool for the purpose of measuring the distances of galaxies a hundred million or more light years away, a crucial step to determine the Hubble constant. Unfortunately, stars in galaxies are crowded together in a small space from our distant vantage point and so we often lack the resolution to separate them from their line-of-sight neighbors.

“A major justification for building the Hubble Space Telescope was to solve this problem. Prior to Hubble’s 1990 launch and its subsequent Cepheid measurements, the expansion rate of the universe was so uncertain astronomers weren’t sure if the universe has been expanding for 10 billion or 20 billion years. That’s because a faster expansion rate will lead to a younger age for the universe, and a slower expansion rate will lead to an older age of the universe. Hubble has better visible-wavelength resolution than any ground-based telescope because it sits above the blurring effects of Earth’s atmosphere. As a result, it can identify individual Cepheid variables in galaxies that are more than a hundred million light-years away and measure the time interval over which they change their brightness.

“However, we also must observe the Cepheids at the near-infrared part of the spectrum to see the light which passes unscathed through intervening dust. (Dust absorbs and scatters blue optical light, making distant objects look faint and fooling us into believing they are farther away than they are). Unfortunately, Hubble’s red-light vision is not as sharp as its blue, so the Cepheid starlight we see there is blended with other stars in its field of view. We can account for the average amount of blending, statistically, the same way a doctor figures out your weight by subtracting the average weight of clothes from the scale reading, but doing so adds noise to the measurements. Some people’s clothes are heavier than others.

“However, sharp infrared vision is one of the James Webb Space Telescope’s superpowers. With its large mirror and sensitive optics, it can readily separate the Cepheid light from neighboring stars with little blending. In the first year of Webb operations with our General Observers program 1685, we collected observations of Cepheids found by Hubble at two steps along what’s known as the cosmic distance ladder. The first step involves observing Cepheids in a galaxy with a known, geometric distance that allows us to calibrate the true luminosity of Cepheids. For our program that galaxy is NGC 4258. The second step is to observe Cepheids in the host galaxies of recent Type Ia supernovae. The combination of the first two steps transfers knowledge of the distance to the supernovae to calibrate their true luminosities. Step three is to observe those supernovae far away where the expansion of the universe is apparent and can be measured by comparing the distances inferred from their brightness and the redshifts of the supernova host galaxies. This sequence of steps is known as the distance ladder.

“We recently got our first Webb measurements from steps one and two which allows us to complete the distance ladder and compare to the previous measurements with Hubble (see figure) Webb’s measurements have dramatically cut the noise in the Cepheid measurements due to the observatory’s resolution at near-infrared wavelengths. This kind of improvement is the stuff astronomers dream of!  We observed more than 320 Cepheids across the first two steps. We confirmed that the earlier Hubble Space Telescope measurements were accurate, albeit noisier.   We have also observed four more supernova hosts with Webb and we see a similar result for the whole sample.

“What the results still do not explain is why the universe appears to be expanding so fast! We can predict the expansion rate of the universe by observing its baby picture, the cosmic microwave background, and then employing our best model of how it grows up over time to tell us how fast the universe should be expanding today. The fact that the present measure of the expansion rate significantly exceeds the prediction is a now decade-long problem called “The Hubble Tension.” The most exciting possibility is that the Tension is a clue about something we are missing in our understanding of the cosmos.

“It may indicate the presence of exotic dark energy, exotic dark matter, a revision to our understanding of gravity, or the presence of a unique particle or field. The more mundane explanation would be multiple measurement errors conspiring in the same direction (astronomers have ruled out a single error by using independent steps), so that is why it is so important to redo the measurements with greater fidelity. With Webb confirming the measurements from Hubble, the Webb measurements provide the strongest evidence yet that systematic errors in Hubble’s Cepheid photometry do not play a significant role in the present Hubble Tension. As a result, the more interesting possibilities remain on the table and the mystery of the Tension deepens.”

Editor’s Note: This post highlights data from a paper that was accepted by The Astrophysical Journal.

Author:

• Adam Riess is a Bloomberg Distinguished Professor at the Johns Hopkins University, the Thomas J. Barber Professor in Space Studies at the JHU Krieger School of Arts and Sciences, a distinguished astronomer at the Space Telescope Science Institute, and a recipient of the 2011 Nobel Prize in Physics.

 On Apr. 21, 2023, the James Webb Space Telescope team shared that one of the MIRI (Mid-Infrared Instrument) observing modes, called Medium-Resolution Spectroscopy (MRS), showed a reduction in the amount of light registered by MIRI’s detectors. Initial analysis of MIRI’s imaging mode did not show a similar effect. However, as part of the team’s investigation into the issue, additional monitoring observations were taken with MIRI imaging. Combined with earlier data, these new calibrations have revealed a reduced signal for MIRI imaging at the longer wavelengths.

This change does not substantially impact MIRI’s science capabilities but will have an impact on the exposure times needed for MIRI imaging.

There is no risk to the instrument, and the effect on imaging is less than the effect in MRS. The team is investigating the cause of this issue. Regular monitoring observations are being taken to continue measuring the response, and the team is providing updated guidance to Webb’s user community to correct for this change. MIRI’s third observing mode, Low-Resolution Spectroscopy, is currently performing normally, and the investigation of MIRI’s fourth mode, Coronographic Imaging, has not yet concluded.

The Webb team has also enacted a plan for long-term monitoring, and are exploring potential mitigations. The observatory is in good health, and each of Webb’s other scientific instruments are unaffected.

For more information, visit the Space Telescope Science Institute.