by Paul Gilster | Jul 26, 2024 | Deep Sky Astronomy & Telescopes |
Put two massive galaxy clusters into collision and you have an astronomical laboratory for the study of dark matter, that much discussed and controversial form of matter that does not interact with light or a magnetic field. We learn about it through its gravitational effects on normal matter. In new work out of Caltech, two such clusters, each of them containing thousands of galaxies, are analyzed as they move through each other. Using data from observations going back decades, the analysis reveals dark and normal matter velocities decoupling as a result of the collision.
Collisions on galactic terms have profound effects on the vast stores of gas that lie between individual galaxies, causing the gas to become roiled by the ongoing passage. Counter-intuitively, though, the galaxies themselves are scarcely affected simply because of the distances between them, and for that matter between the individual stars that make up each.
We need to keep an eye on work like this because according to the paper in the Astrophysical Journal, so little of the matter in these largest structures in the universe is in the form we understand. That’s a telling comment on how much work we have ahead if we are to make sense of the structure of a cosmos we would like to explore. In fact, the authors make the case that only 15 percent of the mass in the clusters under study is normal matter, most of it in the form of hot gas but also locked up in stars and planets. That would make 85 percent of the cluster mass dark matter.
The clusters in question are tagged with the collective name MACS J0018.5+1626. All matter, including dark matter, interacts through gravity, while normal matter is also responsive to electromagnetism. That means that normal matter slows down in these clusters as the gas between the individual galaxies becomes turbulent and superheated, while the dark matter within the clusters moves ahead in the absence of electromagnetic effects. Lead author Emily Silich (a Caltech grad student working with principal investigator Jack Sayers) likens the effect to that of a collision between dump trucks carrying sand. “The dark matter is like the sand and flies ahead.”
Image: This artist’s concept shows what happened when two massive clusters of galaxies, collectively known as MACS J0018.5+1626, collided: The dark matter in the galaxy clusters (blue) sailed ahead of the associated clouds of hot gas, or normal matter (orange). Both dark matter and normal matter feel the pull of gravity, but only the normal matter experiences additional effects like shocks and turbulence that slow it down during collisions. Credit: W.M. Keck Observatory/Adam Makarenko.
Some years back we looked at the two colliding galaxy clusters known collectively as the Bullet Cluster (see A Gravitational Explanation for Dark Matter). There, the behavior of the component materials of the clusters has been analyzed in the study of dark matter, but the clusters are seen from Earth with a spatial separation. In the case of MACS J0018.5, the clusters are oriented such that one is moving toward us, the other away. These challenging observations made it possible to analyze the velocity differential between dark and normal matter for the first time in a cluster collision.
Caltech’s Sayers explains:
“With the Bullet Cluster, it’s like we are sitting in a grandstand watching a car race and are able to capture beautiful snapshots of the cars moving from left to right on the straightway. In our case, it’s more like we are on the straightway with a radar gun, standing in front of a car as it comes at us and are able to obtain its speed.”
I’m reminded of my previous post on Chris Lintott’s book, where the astrophysicist takes note of the role of surprise in astronomy. In this case, the scientists used the kinetic Sunyaev-Zel’dovich effect (SZE), a distortion of the cosmic microwave background spectrum caused by scattering of photons off high-energy electrons, to measure the speed of the normal matter in the clusters. With the two clusters moving in opposite directions as viewed from Earth, untangling the effects took Silich to data from NASA’s Chandra X-ray Observatory (another reminder of why Chandra’s abilities, in this case to measure extreme temperatures of interstellar gas, are invaluable).
Adds Sayers:
“We had this complete oddball with velocities in opposite directions, and at first we thought it could be a problem with our data. Even our colleagues who simulate galaxy clusters didn’t know what was going on. And then Emily got involved and untangled everything.”
Nice work! The analysis tapped many Earth- and space-based facilities. Data from the Caltech Submillimeter Observatory (CSO), now being relocated from Maunakea to Chile, go back fully twenty years. The European Space Agency’s Herschel and Planck observatories, along with the Atacama Submillimeter Telescope Experiment in Chile, were critical to the analysis, and data from the Hubble Space Telescope were used to map the dark matter through gravitational lensing. With the clusters moving through each other at 3000 kilometers per second – one percent of the speed of light – collisions like these are in Silich’s words “the most energetic phenomena since the Big Bang.”
Dark matter explains many phenomena including galaxy rotation curves, which imply more mass than we can see, and gravitational lensing has been used to show that visible mass is insufficient to explain the lensing effect. But we still don’t know what this stuff is, assuming it is real and not a demonstration of our need to refine General Relativity through theories like Modified Newtonian Dynamics (MOND). What we need is direct detection of dark matter particles, an ongoing effort whose resolution will shape our understanding of galactic structure and conceivably point to new physics.
The paper is Silich et al. 2024. “ICM-SHOX. I. Methodology Overview and Discovery of a Gas–Dark Matter Velocity Decoupling in the MACS J0018.5+1626 Merger,” Astrophysical Journal 968 (2): 74. Full text.
by Paul Gilster | Jun 12, 2024 | Deep Sky Astronomy & Telescopes |
Our recent discussions of X-ray beaming to propel interstellar lightsails seem a good segue into Don Wilkins’ thoughts on the Chandra mission. Chandra, of course, is not a deep space probe but an observatory, and a revolutionary one at that, with the capability of working at the X-ray wavelengths that allow us to explore supernovae remnants, pulsars and black holes, as well as making observations that advance our investigation of dark matter and dark energy. This great instrument swims into focus today because it faces a funding challenge that may result in its shutdown. It’s a good time, then, to take a look at what Chandra has given us since launch, and to consider its significance as efforts to save the mission continue. We should get behind this effort. Let’s save Chandra.
by Don Wilkins
On July 23, 1999, the Chandra X-ray Observatory deployed from Space Shuttle Columbia. Chandra along with the Hubble Space Telescope, Spitzer Space Telescope (decommissioned when its liquid helium ran out) and Compton Gamma Ray Observatory (de-orbited after gyroscope failure), composed NASA’s fleet of ‘Great Observatories,’ so labeled because of their coverage from gamma rays through X-rays, visible light and into the infrared, spanning the electromagnetic spectrum. This scientific fleet expanded – and continues to expand – our understanding of the cosmos.
A two-stage Inertial Upper Stage booster put the observatory into an elliptical orbit. Apogee is approximately 139,000 kilometers (86,500 miles) — more than a third of the distance to the Moon – with perigee at 16,000 kilometers (9,942 miles) and an orbital period of 64 hours and 18 minutes. The numbers are significant: The consequence of this orbit is that about 85 percent of the time, Chandra orbits above the charged particle belts surrounding the Earth, allowing periods of uninterrupted observing time up to 55 hours.
Figure 1. The Chandra Observatory monitors X-rays, a critical and unique window into the hottest and most energetic places in the Universe. Credit: NASA.
A Huge Return on Investment
Chandra’s achievements include:
- The first image of the compact object, possibly a neutron star or black hole, at the center of the supernova remnant Cassiopeia A.
- The detection of jets from and a ring around the Crab Nebula.
- The detection of X-ray emitting loops, rings and filaments encircling Messier 87’s supermassive black hole.
- The discovery, by high school students, of a neutron star in the supernova remnant IC 443.
- The detection of X-ray emissions from Sagittarius A*, the black hole at the center of the Milky Way. Chandra observed a Sagittarius A* X-ray flare 400 times brighter than normal, and found a large halo of hot gas surrounding the Milky Way.
- The X-ray observation of a supernova, SN 1987A, shock wave.
- The observation of streams of high-energy particles light years in length flowing from neutron stars.
- Observations of planets within the Solar System found X-rays associated with Uranus that may not be the reflection of solar X-rays. Chandra also found that Jupiter’s X-ray emissions occur at its poles, and it detected X-ray emissions from Pluto.
- Found a dip in X-ray intensity as the planet HD 189733b transited its parent star. This is the first time a transit of an exoplanet was observed using X-rays.
- Detected the shadow of a small galaxy as it is cannibalized by a larger one.
- Found a mid-mass black hole, between stellar-sized black holes and galactic core black holes, in M82.
- Associated X-rays with the gamma ray burst GRB 91216 (Beethoven Burst). Chandra observations also associated X-rays with a gravitational source (Figure 2).
- Produced controversial data suggesting RX J1856.5-3754 and 3C58 are quark stars.
- Measured the Hubble constant as 76.9 km/s per megaparsec (a megaparsec is equal to 3.26 million light years).
- Found strong evidence dark matter exists by observing super cluster collisions. Observations placed limits on dark matter self-interaction cross-sections.
- Combined with the outputs of other telescopes yielded insights into astronomical phenomena, Figure 3.
Building Chandra was a significant technical challenge. Mirror alignment accuracy, from one end of the mirror assembly to the other, a distance of 2.7 meters or 9 feet, is accurate to 1.3 micrometers (50 millionths of an inch). X-rays are focused using complex geometrical shapes which must be precisely formed and kept ultraclean. Even today, no X-ray telescope provides the sharpness of image that Chandra does.
Figure 2 Chandra made the first X-ray detection of a gravitational wave source. The inset shows both the Chandra non-detection, or upper limit, of X-rays from GW170817 and its subsequent detection. The main panel is the Hubble image of NGC 4993. Credit: NASA.
Figure 3. The image is a combination of the outputs of three telescopes. Webb highlights infrared emission from dust; Chandra the X-ray emissions and Hubble shows stars in the field. Credit: NASA/CXC/SAO
Chandra’s Demise?
This list of discoveries may be ending. Although it is estimated Chandra has another decade of operation, proposed budget cuts eliminate the $74 million necessary for Chandra operation, maintenance and analysis of data. Unless the funding is restored by way of a budget line item directing NASA to spend the funds on Chandra, the agency’s X-ray observation is stopped for years until a new telescope can be brought on line. The skills of highly talented scientists and engineers working on Chandra will be dissipated.
The problem is significant. NASA’s own NuStar instrument (Nuclear Spectroscopic Telescope Array) and the joint NASA/JAXA XRISM effort (X-Ray Imaging and Spectroscopy Mission) have their virtues, as does the European Space Agency’s XMM-Newton (X-ray Multi-Mirror Mission). But Chandra’s eyes are sharper, with imagery 10 times crisper than XMM-Newton’s, 30 times sharper than NuStar, and fully 150 times sharper than XRISM.
A possible successor is in the planning stages. With 100 times Chandra’s resolution, the Lynx X-ray Observatory could study individual stars more than 16,000 light-years away, or 12.5 times the range of Chandra, using an X-ray Mirror Assembly that is the most powerful X-ray optic ever conceived. Even so, it is probable three decades or more would elapse before the new observatory would orbit and it might well involve significant cost overruns. ESA plans the Athena mission (Advanced Telescope for High ENergy Astrophysics), a large-aperture grazing-incidence X-ray telescope, but Athena would not come online until the late 2030s. In the meantime, we have a healthy Chandra.
Although we are dealing with a 25 year old spacecraft, its efficiency is both high and stable, and enough fuel remains for another decade of operations, while the cost of operations has remained stable for decades. Considered as a bridge to Lynx, Chandra is at the mercy of Congress largely through the FY25 appropriations process. The particulars of how to support this great observatory are available on the Save Chandra website, which is the voice of a grassroots effort within the astronomical community.
Saving Chandra would avert what some are describing as “an extinction-level-event for X-ray astronomy in the US.” An open community letter with 87 pages of signatures recently submitted to NASA Science Leadership strongly supports this statement.
by Paul Gilster | Mar 20, 2024 | Deep Sky Astronomy & Telescopes |
Don’t you love the way the cosmos keeps us from getting too comfortable with our ideas? The Hubble Constant (H0), which tells us about the rate of expansion of the universe, is still a hot issue because observations from both the Hubble Space Telescope and JWST don’t tally with what the European Space Agency’s Planck mission concluded from its data on the Cosmic Microwave Background (CMB).
How exactly do we fine tune the standard model of cosmology to make sense of this? The so-called Hubble Tension is hardly the only issue raised by the acquisition of new and better data, although it may be the biggest. All kinds of questions linger about what dark energy is, not to mention dark matter. Of course, challenging observations are hardly limited to cosmology. Dialing down to the stellar level, new work has emerged challenging the way white dwarf stars evolve. Contrary to all expectation, some white dwarfs seem to stop cooling, and can indeed live to a satisfying old age.
A white dwarf is what is left after a star goes through its red giant phase and sheds its outer layers. After all, most stars (over 95 percent) don’t have the mass to become a neutron star or black hole. A star like the Sun will one day enlarge, then contract, casting off its outer layers to form a planetary nebula and leaving a white dwarf behind. Planets in our inner system would likely be engulfed in the red giant phase, but get out around the asteroid belt and the chances of survival are high, with subsequent outward migration in the white dwarf period because of the star losing mass.
Sirius B is a white dwarf 8.6 light years away, and there are eight of the objects among the 100 nearest star systems. They should be cooling down because fusion no longer occurs, with the dense plasma in the star’s interior freezing so that the star solidifies from the center out. The cooling process itself can take billions of years, which is why I find these objects so appealing. They’re another example of an exotic place where planets can orbit and conceivably produce some kind of life, and recent studies have uncovered that many of them show signs of atmospheric ‘pollution,’ meaning they’ve ingested materials near them. As many as 50 percent show metals in their spectra.
Image: This is an artist’s impression of a white dwarf (burned-out) star accreting rocky debris left behind by the star’s surviving planetary system. It was observed by Hubble in the Hyades star cluster. At lower right, an asteroid can be seen falling toward a Saturn-like disk of dust that is encircling the dead star. Infalling asteroids pollute the white dwarf’s atmosphere with silicon. These dead stars are located 150 light-years from Earth in a relatively young star cluster, Hyades, in the constellation Taurus. The star cluster is only 625 million years old. The white dwarfs are being polluted by asteroid-like debris falling onto them. Credit: NASA, ESA, and G. Bacon (STScI).
Finding a debris disk around a white dwarf is one thing, but planets are another matter. Only a few have so far been identified. There have been white dwarf surveys to look for surviving gas giants in such systems but their numbers are low. What we have found, though, is intriguing. WD 1856+534 b, for instance, orbits in a remarkably tight orbit and raises further questions about orbital evolution. A planetesimal designated SDSS J1228+1040 b likewise appears in a tight orbit within a white dwarf debris disk.
Because the contrast between star and planet in a white dwarf system could be as low as 1:200, according to a new paper, JWST gives us significant capabilities for imaging planets around white dwarfs. Susan Mullally (Space Telescope Science Institute) is lead author of the paper, which appears in The Astrophysical Journal Letters. The researchers describe two giant planet candidates orbiting the white dwarfs WD 1202−232 and WD 2105−82. The projected separations are 11.47 and 34.6 AU respectively, and the best take on mass for each is in the range of 1 to 7 Jupiter masses. “If confirmed,” note the authors, “using common proper motion, these giant planets will represent the first directly imaged planets that are similar in age, mass, and orbital separation as the giant planets in our own solar system.”
So we’re getting there as we slowly build our exoplanet catalog around such stars. Even so, the questions seem to be multiplying because of the fact mentioned above: There is a population of white dwarfs along the so-called Q Branch of the Hertzsprung–Russell (H–R) diagram that is maintaining a constant luminosity over billions of years. This would mean there must be a source of energy inhibiting the cooling process.
Here we’re dealing with data from the Gaia satellite, cited in 2019 to announce the discovery of this population of white dwarfs. White dwarf cooling is thought to involve crystallization of core materials into a solid phase. Sihao Cheng (Institute for Advanced Study, Princeton) and colleagues point to this transition in the 2019 paper:
As a liquid-to-solid phase transition in the white dwarf core, crystallization releases energy through latent heat (e.g., van Horn 1968) and phase separation (e.g., Garcia-Berro et al. 1988; Segretain et al. 1994; Isern et al. 1997), which can indeed create a cooling delay. However, the observed pile-up on the Q branch is higher and narrower than expected from the standard crystallization model (Tremblay et al. 2019, Figure 4), suggesting that there exists a cooling anomaly, i.e., an extra cooling delay in addition to crystallization.
Explaining this phenomenon is the task of a just published paper in Nature from Antoine Bédard (University of Warwick), with Cheng as a co-author. Its hypothesis is that freezing of the interior into a solid state does not produce the expected result, the solidification of the star from the inside out. Instead, crystals formed upon freezing displace heavier liquids downward, a mechanism that releases gravitational energy. This, then, would be the source of the persistent star’s energy, and would constitute, according to Bédard, “a whole new astrophysical phenomenon.” This delay in cooling could mean that we are underestimating the age of some dwarfs by billions of years.
Cheng sees the question of stellar age as central, even if we’re not sure why some white dwarfs take this path and others do not:
“One fascinating aspect of this discovery is that the physics involved is similar to something we observe in daily life: the frozen crystals within the white dwarf star float instead of sink. We might compare their behavior to ice cubes floating in water. Our work will necessitate updates to astronomy textbooks. We hope that it will also prompt astronomers to reassess the methods employed to calculate the age of stellar populations.”
It’s always satisfying to think that our textbooks will need to be updated on a regular basis, for the pace of discovery is accelerating. In this case, something as fundamental as stellar age is up for grabs. Indeed, as the Bédard and Cheng paper notes, this “population of freezing white dwarfs maintains a constant luminosity for a duration comparable with the age of the universe.” These white dwarfs, at least, fall entirely out of the category of ‘dead stars’ and force a healthy re-thinking of our assumptions.
The Mullally paper is “JWST Directly Images Giant Planet Candidates Around Two Metal-polluted White Dwarf Stars,” Astrophysical Journal Letters 962 (15 February 2020), L32 (full text). The 2019 paper discussing the cooling issues in white dwarfs is Cheng et al., “A Cooling Anomaly of High-mass White Dwarfs,” The Astrophysical Journal Vol. 886, No. 2 (25 November 2019), 100 (full text). The new paper on white dwarf cooling is Bédard et al., “Buoyant crystals halt the cooling of white dwarf stars,” Nature 627 (06 March 2024), pp. 286-288 (abstract).
by Paul Gilster | Mar 8, 2024 | Deep Sky Astronomy & Telescopes |
If individual star systems show us a wide variety of formation scenarios – and we just examined recent ESO work on circumstellar disks in different star-forming regions – the variety in galaxy evolution is even more spectacular. I’m reminded here of an unusual find when my uncle Roland died unexpectedly and I became his executor. Clearing out his house preparatory to sale, I discovered a series of astronomy photographs that he had blown up to huge scale. An image of M31, the great spiral of Andromeda, was fully six feet long and gorgeously mounted. I remembered nights as a child when he would visit from Florida and point out celestial objects for me to observe with my 3-inch reflector. M31, he told me then, was considerably wider than the Moon in the sky.
When I checked, I found that Andromeda had an angular size of 3 degrees, as opposed to about half a degree for the Moon. Even so, our spectacular sister galaxy is actually a difficult catch, with only its brighter central region visible to the naked eye, and even there tricky to find depending on local conditions of light pollution. Here I chuckle, remembering that I inherited from my uncle his eight-inch Celestron. The bane of his life was his backyard neighbor, who would power up huge outdoor security lights at the most inappropriate times. No luck seeing M31 under those conditions!
Image: M31 through a small telescope, with the Moon’s size shown for reference. Credit: Caradon Observatory.
Timothy Ferris produced a spectacular book called, simply, Galaxies, published by Random House in 1988. If you’re a deep sky devotee, it’s worth seeking out in a used book store, as it’s a coffee-table volume with spectacular photography. The following passage captures some of the grandeur verbally, though it’s from Ferris’ equally valuable Seeing in the Dark (Simon & Schuster, 2002):
The very concept of space is inadequate for dealing with galaxies; one must invoke time as well. The Andromeda galaxy is steeply inclined to our line of sight, only fifteen degrees from edge-on. Since the visible part of its disk is roughly one hundred thousand light years in diameter, the starlight reaching our eyes from its more distant side is about one hundred thousand years older than the light we simultaneously see coming from the near side. When the starlight from the far side of Andromeda started its journey, Homo habilis, the first true humans, did not yet exist. By the time the near-side light started out, they did. So within that single field of view lies a swath of time that brackets our ancestors’ origins – and that, like the incomplete dates in a biographical sketch of a living person (1944-?), inevitably raises the question of our destiny as a species. When the light leaving Andromeda tonight reaches Earth, 2.25 million years from now, who will be here to observe it? We think of Einstein’s spacetime as an abstraction, but to observe a galaxy is to sense its physical reality.
Image: An ultraviolet look at Andromeda, from NASA’s Galaxy Evolution Explorer. Credit: NASA. I owe the reminder for the Ferris quote to the blog Ten Minute Astronomy.
How much more stunning, then, to think about the galaxy recently observed by the team doing the JWST Advanced Deep Extragalactic Survey (JADES). I will mercifully shorten its designation to JADES-GS-z7-01-QU, as the authors of the paper in Nature do. This is a so-called ‘quiescent’ galaxy, meaning that for a variety of possible reasons, star formation within it has all but ceased. The authors describe it as “a compact, discy galaxy.” And as the work of the large team led by Tobias Looser (University of Cambridge) now shows, it is also the first galaxy beyond redshift z=5 to be confirmed and characterized. Indeed, the redshift calculated for this object is z=7.3. The light from this particular galaxy would have been emitted some 13 billion years ago, a ‘mere’ 700 million years after the Big Bang.
We could also look at this galaxy in terms of its ‘comoving distance.’ The latter term is used to accommodate the fact that the universe is expanding, necessary to consider here because if an object is so far away that the light from it has traveled for most of the age of the visible universe, then during that time cosmic expansion has continued. Doing the math on this is beyond my skill set, but my research indicates that at z=7.3, the comoving distance of JADES-GS-z7-01-QU should be in the range of 30 billion light years. Mathematically inclined readers might want to fine-tune that figure.
That ‘chill up the spine’ feeling of encountering deep time/distance never quite goes away. In terms of its significance, though, we can focus in on quiescence, which is a measure of how star formation ceases in a galaxy. The galaxy in question, as observed now, has stopped forming new stars (which means it did that over 13 billion years ago). The paper indicates that the quenching period occurred 10 to 20 million years ago. Star formation seems to have been fast, ending abruptly, but what we don’t know is whether this condition is permanent. Indeed, as the paper on this work points out, how star formation is regulated in galaxies is one of the key open problems in astrophysics.
The authors run through the possibilities for slowing or stopping star formation, which include gas being expelled from galaxies by supermassive black holes or rapid star formation heating the ‘circumgalactic medium,’ thereby preventing the accretion of fresh gases. Low-mass galaxies (this is one) can be affected by feedback mechanisms that deplete the medium within galactic clusters. These differing processes operate over varying timescales, making the significance of JADES-GS-z7-01-QU clear, as noted by Roberto Maiolino (University of Cambridge), a co-author on the paper:
“We’re not sure if any of those scenarios can explain what we’ve now seen with Webb. Until now, to understand the early universe, we’ve used models based on the modern universe. But now that we can see so much further back in time, and observe that the star formation was quenched so rapidly in this galaxy, models based on the modern universe may need to be revisited.”
Image: False-colour JWST image of a small fraction of the GOODS South field, with JADES-GS-z7-01-QU highlighted. Credit: JADES Collaboration.
Thus we have the first of what should become many opportunities to learn about galaxy growth and transformation in the early universe. About the mass of the Small Magellanic Cloud, which continues to form new stars, this galaxy is dead as of the time of observation, but we can’t know what occurred in the 13 billion years before JWST was turned on it. Is quenching a widespread phenomena in the early universe, but a temporary one, so that later epochs see galactic rejuvenation? The scope of future work with JWST is beginning to take shape as we examine finds like these.
The paper is Looser et al. “A recently quenched galaxy 700 million years after the Big Bang,” Nature (06 March 2024). Abstract.
by Paul Gilster | Mar 1, 2024 | Deep Sky Astronomy & Telescopes |
A kilonova at the wrong place and time would spell trouble for any lifeforms emerging on a planetary surface. Just how we found out about kilonovae and the conditions that create them, not to mention their hypothesized effects, is the subject of Don Wilkins’ latest, a look at Cold War era surveillance that wound up pushing astronomy’s frontiers. That work now causes us to ponder the formation of an ‘island of stability’ in which exists a set of superheavy element isotopes with unique properties. It also raises interesting questions about our Solar System’s history and possible exposure to a nearby event. Based at Washington University in St. Louis, Don’s interest in deep space exploration here probes the formation and structure of matter in processes we’re only beginning to unlock.
by Don Wilkins
Setting out to discover something on Earth can sometimes reveal an unexpected result from a far more interesting source. As a case in point, consider what happened in August of 1963, when Great Britain, the US and the USSR signed a nuclear test ban treaty forbidding nuclear detonations in space or the Earth’s atmosphere. For the older space nerds, this is the same treaty that ended the Orion program. Given the Soviets’ history of violating treaties, the US launched the Vela (derived from the Spanish verb “velar”, to watch) series of satellites designed to monitor compliance with the treaty within two months of the signing. What they found was a bit of a surprise.
The satellites were heavily instrumented with x-ray, gamma-ray, neutron, optical and electromagnetic pulse (EMP) detectors along with other sensors designed to monitor the space environment. The satellites operated in pairs on opposite sides of a circular 250,000 kilometers in diameter orbit, Figure 1.
Figure 1. A Pair of Vela Satellites Readying for Launch. Los Angeles Air Force Base, U.S. Air Force Photo.
X-ray detectors directly sense nuclear blast. Gamma-ray and neutron detector activations would confirm the nuclear event and would prompt a stiffly worded diplomatic note sent to the Soviets. Vela satellites were positioned to monitor the Earth and the far side of the Moon. The latter involved detecting gamma radiation from radioactive debris scattered by a clandestine explosion. As a result of the separation of the satellites and separation in time between sensor triggering on the satellites, the angle to the event could be determined to about one-fifth of a radian or ten degrees. Angles to a single event observed by multiple pairs of satellites could provide a more precise direction to the source.
No diplomatic note concerning illegal nuclear tests was ever sent to the Soviets. Fortunately events which triggered the detectors but were clearly not signatures of nuclear detonations were not discarded. These formed a database which eventually led to the discovery of enormous, but short-lived gamma-ray bursts (GRBs) originating in deep space. GRBs last less than three seconds (although a recent discovery lasted an astounding 200 seconds), yet they are as luminous as 100 million galaxies, the equivalent of a 1000 novae. Gamma-ray sources have temperatures of approximately 109 K degrees and are among the hottest objects ever observed. Compounding the mystery, researchers only had a line pointing to the origin of the bursts but no distance.
GRBs occur daily and are uniformly distributed across the observable Universe. Initially no counterpart of the GRBs operating in the visual spectrum could be found. Then, in 1997, Italian astronomers caught the fading light of an object which could be linked with a GRB, Figure 2.
Figure 2. Left: Arrow points at the GRB optical counterpart. Right: An IR image of the tilted box area in the left image. The optical source is gone, and only a faint image of a very distant galaxy remains. The other two bright sources on the right side are spiral galaxies. Credit: W. M. Keck Observatory / NASA.
The favored explanation for GRBs is the collision of two neutron stars or two black holes. Astronomers named the neutron star mergers kilonovae (KN). In addition to GRBs, these collisions emit high-frequency gravitational waves (GW) and are, through rapid neutron capture (the r-process) nucleosynthesis, likely production sites of heavy elements. [1] A team led by Andres Levan examined spectroscopy of GRB 230307A, a long-duration GRB associated with a kilonova merger. A 2.15 micron emission line from that analysis is associated with tellurium (atomic mass 130), and a mid-IR peak, lanthanides production. GRB nucleosynthesis creates a wide range of atomic masses including heavy elements (mass above iron). [2]
These observations and others support the hypothesis that heavy elements within the Solar System are the remnants of a kilonova.[3-4]
Figure 3 depicts the evolution of a neutron star merger over the course of millennia. The drawing on the left depicts the aftermath a few years after the merger and at dimensions below a parsec. Gamma-rays are emitted in the dynamic ejecta and the hot cocoon. The gamma-ray jet and cocoon emissions are short-lived; the afterglow they produce emits broadband frequencies for several years. The dynamic ejecta include heavy elements which decay in less than a month to produce the UV, optical and IR displays. X-ray emissions, at potentially lethal levels, result from the interaction between the jet and the interstellar medium (ISM).
Figure 3. Structures Resulting from Neutron Star Merger
On the right hand side, a powerful shock wave from the merger produces a bubble in the ISM. Potentially lethal cosmic rays result.
Initial analysis of GRBs focused on the on-axis gamma ray bursts. M.L. Perkins’ team analyzed the data to understand threats by the off-axis emissions and the relation to other cosmic threats. [5]
According to the team:
For baseline kilonova parameters, … the X-ray emission from the afterglow may be lethal out to ∼ 5 pc and the off-axis gamma-ray emission may threaten a range out to ∼ 4 pc, whereas the greatest threat comes years after the explosion, from the cosmic rays accelerated by the kilonova blast, which can be lethal out to distances up to ∼ 11 pc. … . Based on the frequency and potential damage done, the threats in order of most to least harmful are: solar flares, impactors, supernovae, on-axis GRBs, and lastly off-axis BNS mergers.
One question concerns how close to Earth a kilonova may have manifested. The presence of two isotopes, iron-60 (Fe-60) and plutonium-244 (Pu-244) found in ocean sediments deposited 3 to 4 million years ago offers clues. These isotopes are only formed in very energetic processes.
Fe-60 can, in theory, be created in a standard supernova. Pu-244 is created only in specific classes of supernovae or the merger of a neutron star with another astronomical body, the kilonova.
Figure 4. Artist’s impression of a neutron star merger. Credit: University of Warwick / Mark Garlick.
One of the problems was the ratio between the isotopes. Researchers at the Università di Trento found, with a specific debris ejection pattern and a certain tilt of the merger event, the observed ratio of iron to plutonium isotopes could be explained by a kilonova. [6] The scientists examined rare types of supernovae such as a magneto-rotational supernova or collapsar, but concluded the kilonova was the source of the isotopes.
To determine how far from Earth the kilonova occurred, the researchers calculated the different spreads for each element based on the wind speed created by the kilonova. The answer was about 150 to 200 parsecs or about 500 to 600 light years away.
Hydrogen and helium were created with the Big Bang; heavier elements were made by fusion within the interior of stars, supernovae and kilonovae. Data provided by astronomer Jennifer Johnson from Ohio State University was used to produce the periodic table depicting the origins of elements shown in Figure 5 below.
Researchers have examined the heavy element composition of a number of stars, finding that some of these elements are the product of the radioactive decay of previously unobserved elements. [7] These predecessor elements form in a theorized “island of stability” with atomic numbers centered around 126. Isotopes in this region, beyond the fleeting transuranics, are hypothesized to possess “magic numbers” of protons and neutrons that allow them lifespans of thousands or millions of years. The rapid neutron-capture process that occurs in neutron-rich environments of neutron star mergers and supernovae appears inadequate to form the elements in the island of stability. How these transuranics were produced is a mystery.
Figure 5. Origins of Elements – Courtesy NASA’s Goddard Space Flight Center.
The effects of neutron star mergers, like rain, depends on timing. In the early stages of star formation, the collisions shower the clouds of hydrogen and helium with heavy metals necessary for life. Yet after life gained its foothold, an improperly timed – and ill-placed – kilonova could severely damage or erase what a predecessor started.
References
1. B. D. Metzger, G. Martínez-Pinedo, S. Darbha, E. Quataert, A. Arcones, D. Kasen, R. Thomas, P. Nugent, I. V. Panov, N. T. Zinner, “Electromagnetic counterparts of compact object mergers powered by the radioactive decay of r-process nuclei,” Monthly Notices of the Royal Astronomical Society, Volume 406, Issue 4, August 2010, Pages 2650–2662, https://doi.org/10.1111/j.1365-2966.2010.16864.x
2. Levan, A., Gompertz, B.P., Salafia, O.S. et al. “Heavy element production in a compact object merger observed by JWST.” Nature (2023). https://doi.org/10.1038/s41586-023-06759-1
3. Bartos, I., Marka, S. “A nearby neutron-star merger explains the actinide abundances in the early Solar System.” Nature 569, 85–88 (2019). https://doi.org/10.1038/s41586-019-1113-7
4. Watson, Darach, Hansen, Camilla J., Selsing, Jonatan, et al, “Identification of strontium in the merger of two neutron stars,’ arXiv:1910.10510 [astro-ph.HE], 23 Oct 2019
5. Perkins, M.L., Ellis, John, Fields, B.D, et al, “Could a Kilonova Kill: a Threat Assessment,” arXiv:2310.11627v1, 17 October 2023.
6. Leonardo Chiesa, et al, “Did a kilonova set off in our Galactic backyard 3.5 Myr ago?,” arXiv (2023). DOI: 10.48550/arxiv.2311.17159
7. Ian U. Roederer, et al, “Element abundance patterns in stars indicate fission of nuclei heavier than uranium,” Science, 7 Dec 2023, Vol 382, Issue 6675, pp. 1177-1180, DOI: 10.1126/science.adf1341
by Paul Gilster | Feb 28, 2023 | Deep Sky Astronomy & Telescopes |
I hadn’t intended to return so quickly to the issue of high-redshift galaxies, but SPT0418-47 jibes nicely with last week’s piece on 13.5 billion year old galaxies as studied by Penn State’s Joel Leja and colleagues. In that case, the issue was the apparent maturity of these objects at such an early age in the universe.
Today’s work, reported in a paper in The Astrophysical Journal Letters, comes from a team led by Bo Peng at Cornell University. It too uses JWST data, in this case targeting a previously unseen galaxy the instrument picked out of the foreground light of galaxy SPT0418-47. In both cases, we’re seeing data that challenge conventional understanding of conditions in this remote era. This is evidence, but of what? Are we wrong about the basics of galaxy formation? Do we need to recalibrate the models we use to understand astrophysics at high-redshift?
SPT0418-47 is the galaxy JWST was being used to study, an intriguing subject in its own right. This is an infant galaxy still forming stars in the early universe, observable through the bending of its light by a foreground galaxy to form an Einstein ring. In other words, we’re seeing gravitational lensing at work here, magnifying the young galaxy’s light, out of which information can be extracted about the primordial object. And within that light, astronomers have now found a second galaxy which manifested itself in two places in the ring.
Image: This is Figure 1 from the paper. Caption: Figure 1. Left: H? pseudo-narrowband image of the SPT0418 system, averaged over the channels including the H? emission in the original spectral cube. The strongly lensed ring and the two newly discovered sources (SE-1 and SE-2) are highlighted by a red annulus and gray and black ellipses, marked as “A,” “B,” and “C,” respectively. The lensing galaxy is shown as the central bright source. The 835 ?m continuum is plotted as the thin black contours, with the levels 2, 4, 8, 16, 32 × ? where ? = 56.7 ?Jy beam ?1. Right: the spectra of the three sources integrated over the regions highlighted in the left panel, using the same color scheme. The spectrum for the ring is scaled by a factor of 0.1 for clarity. The small black bar below the H? line marks the wavelength coverage of the pseudo-narrowband image. The potentially detected lines are marked by vertical dotted lines. Credit: The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/acb59c.
ALMA (the Atacama Large Millimeter/submillimeter Array) data could do no more than hint at the background galaxy’s existence, but working with spectral data from JWST’s NIRSpec instrument, Peng discovered the new light source within the Einstein ring. The unexpected find was a galaxy being gravitationally lensed by the same foreground galaxy that had made SPT0418-47 available for study, though considerably fainter.
What stands out here is the analysis of the chemical composition of the new galaxy’s light, which shows strong emission lines from hydrogen, nitrogen and sulfur atoms whose redshift showed the object to be about 10 percent of the age of the universe. The new galaxy, dubbed SPT0418-SE, appears to be close enough to SPT0418-47 that the two galaxies will interact with each other, making the duo a case study for galactic mergers. All of which is helpful, but here again we run into a fascinating problem. The newly discovered galaxy shows levels of metallicity comparable to our Sun.
It’s a conundrum. The Sun drew on earlier stellar generations to build up elements heavier than helium and hydrogen, and the Sun is roughly 4.6 billion years old. Amit Vishwas (Cornell Center for Astrophysics and Planetary Sciences) is second author on the paper:
“We are seeing the leftovers of at least a couple of generations of stars having lived and died within the first billion years of the universe’s existence, which is not what we typically see. We speculate that the process of forming stars in these galaxies must have been very efficient and started very early in the universe, particularly to explain the measured abundance of nitrogen relative to oxygen, as this ratio is a reliable measure of how many generations of stars have lived and died.”
But let’s turn back a minute, for we’re looking at two early galaxies, and it’s intriguing that SPT0418-47, the first of these, shows its own anomalies. Data from ALMA allow astronomers to see that although 12 billion years old, this object has a more mature structure than would be expected. No spiral arms are apparent, but a rotating disk and bulge are found, with stars packed tightly around the galactic center. Simona Vegetti (Max Planck Institute for Astrophysics), co-author on the 2020 paper on SPT0418-47 (citation below), had this to say three years ago:
“What we found was quite puzzling; despite forming stars at a high rate, and therefore being the site of highly energetic processes, SPT0418-47 is the most well-ordered galaxy disc ever observed in the early Universe. This result is quite unexpected and has important implications for how we think galaxies evolve.”
Image: Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, have revealed an extremely distant and therefore very young galaxy that looks surprisingly like our Milky Way. The galaxy is so far away its light has taken more than 12 billion years to reach us: we see it as it was when the Universe was just 1.4 billion years old. It is also surprisingly unchaotic, contradicting theories that all galaxies in the early Universe were turbulent and unstable. This unexpected discovery challenges our understanding of how galaxies form, giving new insights into the past of our Universe. Credit: Rizzo et al./European Southern Observatory.
So the new galaxy, SPT0418-SE, adds to earlier evidence that the early universe was considerably less chaotic than we once thought. The new paper summarizes the issue with reference to the unexpectedly strong emission lines found in the data:
This spectroscopic study of a z > 4 galaxy opens up many questions, including the spatial arrangement and stellar/gas/metallicity distribution of the companion; the merging hypothesis of SPT0418-47; the dark-matter halo of the system; the overdensity of this potentially crowded field; reconciling the relatively high chemical abundances with the short formation time and the moderate stellar mass for the whole system; and interpreting the small [N ii] 122 and 205 ?m luminosities in the context of either a soft radiation field and/or a high N/O.
But again that note of high-redshift caution that I mentioned last week:
We attempt to reconcile the high metallicity in this system by invoking early onset of star formation with continuous high star-forming efficiency or by suggesting that optical strong line diagnostics need revision at high redshift. We suggest that SPT0418-47 resides in a massive dark-matter halo with yet-to-be-discovered neighbors.
Clearly scientists will be looking hard at how high-redshift targets are interpreted even as they continue to hypothesize about astrophysical mechanisms and star formation efficiency to explain seemingly mature objects at this early era. The game is afoot, as Sherlock Holmes used to say, and we’re a long way from reaching firm conclusions. The data are going to start coming fast and furious as we keep mining JWST and using ALMA to examine the universe in this early stage, as witness the image below, which I found just this morning. It shows us another remarkable object.
Image: The radio telescope array ALMA has pin-pointed the exact cosmic age of a distant JWST-identified galaxy, GHZ2/GLASS-z12, at 367 million years after the Big Bang. ALMA’s deep spectroscopic observations revealed a spectral emission line associated with ionized Oxygen near the galaxy, which has been shifted in its observed frequency due to the expansion of the Universe since the line was emitted. This observation confirms that the JWST is able to look out to record distances, and heralds a leap in our ability to understand the formation of the earliest galaxies in the Universe. Credit: NASA / ESA / CSA / T. Treu, UCLA / NAOJ / T. Bakx, Nagoya U.
The paper is Bo Peng et al., “Discovery of a Dusty, Chemically Mature Companion to a z ? 4 Starburst Galaxy in JWST ERS Data,” The Astrophysical Journal Letters 944 No. 2 L36 (17 February 2023). Full text. The paper on SPT0418-47 is Rizzo et al., “A dynamically cold disk galaxy in the early Universe,” Nature 584 (12 August 2020), pp. 201–204. Abstract. The GHZ2/GLASS-z12 paper is Bakx et al., “Deep ALMA redshift search of a z 12 GLASS-JWST galaxy candidate,” Monthly Notices of the Royal Astronomical Society Volume 519, Issue (4 March 2023), pp. 5076–5085 (abstract).
