It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Thursday, April 03, 2025
SPACE/COSMOS
Galaxies die earlier than expected
An international team led by UNIGE shows that red and dead galaxies can be found only 700 million years after the Big Bang, indicating that galaxies stop forming stars earlier than predicted by models.
Three spectra taken by the JWST/NIRSpec superimposed on an image taken by the JWST/NIRCam, two instruments on board the James Webb Space Telescope. The record galaxy is shown in the middle. It appears in red in the image and its spectrum decreases towards the left (short wavelengths). For comparison, the spectra at the top and bottom, in blue and violet, show typical star-forming galaxies at a similar time in cosmic history.
For a long time, scientists thought that only actively star-forming galaxies should be observed in the very early Universe. The James Webb space telescope now reveals that galaxies stopped forming stars earlier than expected. A recent discovery by an international team, led by astronomers from the University of Geneva (UNIGE), deepens the tension between theoretical models of cosmic evolution and actual observations. Among hundreds of spectra obtained with the Webb program RUBIES, the team has found a record-breaking galaxy that had already stopped forming stars during an epoch where galaxies are normally growing very rapidly. This study is published in the Astrophysical Journal.
In the early Universe, a typical galaxy accretes gas from the surrounding intergalactic medium and turns that gas into stars. This process increases its mass, leading to even more efficient gas accretion and accelerated star formation. However, galaxies do not grow indefinitely, due to a process astronomers refer to as ‘‘quenching’’.
In the local Universe, about half of the observed galaxies have stopped forming stars — they have quenched and ceased growing. Astronomers refer to them as quiescent, quenched, or ‘‘red and dead’’ galaxies. They appear red because they no longer contain young, bright blue stars — only older, smaller red stars remain.
A particularly high fraction of quiescent galaxies is found among massive galaxies, which are often observed to have elliptical morphologies. It normally takes a long time to form such red and dead galaxies because they must first build up a large number of stars before the star-formation process finally shuts down. It is still a major puzzle what actually causes quenching in galaxies. “Finding the first examples of massive quiescent galaxies (MQGs) in the early Universe is critical as it sheds light on their possible formation mechanisms”, says Pascal Oesch, associate professor in the Department of Astronomy at the UNIGE Facutly of Science and co-author of the paper. The hunt for such systems has thus been a major goal of astronomers for years.
Observations at odds with theoretical expectations
With advancing technology, particularly near-infrared spectroscopy, astronomers have confirmed massive quiescent galaxies (MQGs) at increasingly earlier cosmic times. Their inferred abundance has been challenging to reconcile with theoretical models of galaxy formation, which predict that such systems should take longer to form. With the James Webb space telescope (JWST), this tension has been pushed to a redshift of 5 (1.2 billion years after the Big Bang), where several MQGs have been confirmed in recent years. The new study led by UNIGE reveals that these galaxies formed even earlier and more rapidly than previously thought.
In JWST Cycle 2, the wide-area program RUBIES (the Red Unknowns: Bright Infrared Extragalactic Survey), one of the largest European-led programs for extragalactic research using the NIRSpec instrument, has obtained spectroscopic observations of several thousand galaxies, including hundreds of newly discovered sources from early JWST imaging data.
A record holder “dead” galaxy
Among these novel spectra, scientists identified the most distant MQG found to date, with a spectroscopic redshift of 7.29, just ~700 million years after the Big Bang. The NIRSpec/PRISM spectrum reveals a surprisingly old stellar population in such a young Universe. Detailed modelling of the spectrum and imaging data shows that the galaxy formed a stellar mass of more than 10 billion (10¹⁰) solar masses within the first 600 million years after the Big Bang, before rapidly ceasing star formation, thus confirming its quiescent nature.
“The discovery of this galaxy, named RUBIES-UDS-QG-z7, implies that massive quiescent galaxies in the first billion years of the Universe are more than 100 times more abundant than predicted by any model to date”, says Andrea Weibel, PhD student in the Department of Astronomy at the UNIGE Faculty of science and first author of the paper. This, in turn, suggests that key factors in theoretical models (e.g., the effects of stellar winds, and the strength of outflows powered by star formation and massive black holes) may need to be revisited. Galaxies died much earlier than these models can predict.
Insights into the cores of giant galaxies
Finally, the small physical size of RUBIES-UDS-QG-z7, measured at just ~650 light-years, implies a high stellar mass density comparable to the highest central densities observed in quiescent galaxies at slightly lower redshifts (z ~2–5). These galaxies are likely to evolve into the cores of the oldest and most massive elliptical galaxies in the local Universe. “The discovery of RUBIES-UDS-QG-z7 provides the first strong evidence that the centers of some nearby massive ellipticals may have already been in place since the first few hundred million years of the Universe”, concludes Anna de Graaff, principal investigator of the RUBIES program, postdoctoral researcher at the Max Planck Institute for Astronomy in Heidelberg and second author of the paper.
The approaching asteroid was observed using the 2.5-meter Nordic Optical Telescope (NOT) in La Palma, Canary Islands. The image in the top right corner shows the asteroid detected by the telescope in the centre of the green circle. The other bright spots are background stars, smeared due to the movement of the telescope.
Asteroid impact threat estimates improved for the Earth and the Moon
An international team is currently closely tracking the near-Earth asteroid 2024 YR4. The impact probability estimates for the year 2032 has been reduced from a peak of 3 percent to below 0.001 percent.
Asteroid 2024 YR4, first discovered in December 2024, triggered a first-ever notification from the International Asteroid Warning Network (IAWN) on January 29th 2025. The early calculations predicted an impact probability of 1.3% with Earth on December 22nd 2032 and estimated its size between 40 and 90 meters. These estimates gave the asteroid a Torino scale rating of risk level 3 in the range of 0-10, which meant a target meriting close attention by astronomers.
The global astronomy community quickly launched an international observing campaign, in which the researchers at the University of Helsinki have been at the forefront. Postdoctoral researchers Zuri Gray and Grigori Fedorets have been following the asteroid closely since January 30th.
They observed the asteroid using the 2.5-meter Nordic Optical Telescope (NOT) in La Palma, Canary Islands, which the Finnish astronomers have used for a long time to study asteroids close to Earth.
– We have been focusing on the asteroid’s precise position and motion, and to analyse its rotation, size and shape. This information is crucial for better predicting the asteroid’s future trajectory, and ultimately refining its impact probability, says Gray.
– Our recent observations continue to demonstrate the vital role that NOT plays in planetary defence efforts on a global level, adds Fedorets.
Lunar impact as a risk
During mid-February, 2024 YR4’s impact probability peaked at 3%. It is the highest ever probability attained by any asteroid of the size range to pose a significant threat, in the recorded history of asteroid impact risk estimates.
The observations were briefly halted due to the full moon drowning out the asteroid’s faint signal, but the researchers in Finland were the first to obtain new data following the full moon. The observations resulted in a significant drop in the impact probability, which currently stands below 0.001%.
Meanwhile, the latest data and calculations have brought out a new, although a smaller risk: there is currently a 4 % chance of the asteroid impacting the Moon instead.
– Should the asteroid impact the Moon, the Earth-Moon system could be clouded with particles detached from the Moon and the asteroid, potentially threatening the human space infrastructure and operations, explains Academy Professor Karri Muinonen.
Infrared radiation refined the size estimate
The James Webb Space Telescope observed the asteroid on March 26th. The postdoctoral researcher Eric MacLennan is a key member of the team analysing the asteroid’s infrared properties. James Webb is currently the only telescope with the capability to measure thermal emission.
– The initial diameter estimate of the asteroid was limited to visible light measurements, which depend on both the size and reflectivity of the surface. However, thermal emission at infrared wavelengths is a direct indicator of the size. Our first attempt to observe the thermal emission on March 8 was not successful, so it was a relief when the images were transmitted back from the telescope, explains MacLennan.
After hours of work, the team arrived at an estimate: there´s 95 % chance the diameter is 46 - 74 meters. This size determination will help astronomers assess the potential consequences of an impact with the Moon.
Despite the reduced risk, Earth-based and space-based observatories will continue tracking YR4 through April or May 2025, after which it will be unobservable from Earth until its next close approach in 2028.
Persistent work paid off
In February, NOT was one of the most fast-acting and accurate data providers in comparison to other similar instruments for the global planetary defence community – a testament to the dedicated long-term work for the telescope by its staff, engineers, students, and scientists from the Nordics.
– We were able to very quickly observe the asteroid and refine the estimates for it. The flexible, high-quality, medium-sized astronomical facilities like the NOT are crucial for rapid follow-up of these type of potentially hazardous asteroids, says Associate Professor Mikael Granvik
– We have unique scientific know-how at the University of Helsinki for contributing to planetary defence, encompassing astronomical observations, laboratory measurements, theoretical and numerical research, and characterising near-Earth object orbits, rotational states, shapes, and surface structure and composition, summarises Muinonen for the background of the success.
In Finland, the governmental responsibility for the risk assessment of near-Earth objects belongs to the National Land Survey, and in the near future the Space Situational Awareness Center, which is currently being established. Monitoring the risks posed by near-Earth objects is one of the key areas of responsibility for the new Space Situational Awareness Center, in addition to the possible risks posed by space weather, space debris, and satellite operations.
– The comprehensive expertise of astronomers at the University of Helsinki in asteroid research is a critical part of the expertise needed at the governmental level to assess the possible impact risks caused by the near-Earth objects for Finland similar to other countries, summarizes Senior Research Scientist and Academy Research Fellow Anne Virkki.
Original article
A. S. Rivkin, T. Mueller, E. MacLennan, B. Holler, A. Burdanov, J. de Wit, P. Pravec, M. Micheli, M. Devogele, L. Conversi, C. A. Thomas, D. Farnocchia, J. L. Dotson, L. Wheeler, H. Hammel, S. N. Milam, J. de Leon, and A. Glantzberg. JWST Observations of Potentially Hazardous Asteroid 2024 YR4. 2025 Res. Notes AAS9 70 DOI 10.3847/2515-5172/adc6f0
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