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).
by Paul Gilster | Feb 23, 2023 | Deep Sky Astronomy & Telescopes |
When something turns up in astronomical data that contradicts long accepted theory, the way forward is to proceed with caution, keep taking data and try to resolve the tension with older models. That would of course include considering the possibilities of error somewhere in the observations. All that is obvious enough, but a new paper on JWST data on high-redshift galaxies is striking in its implications. Researchers examining this primordial era have found six galaxies, from no more than 500 to 700 million years after the Big Bang, that give the appearance of being massive.
We’re looking at light from objects 13.5 billion years old that should be anything but mature, if compact, galaxies. That’s a surprise, and it’s fascinating to see the scrutiny to which these findings have been exposed. The editors of Nature have helpfully made available a peer review file containing back and forth comments between the authors and reviewers that give a jeweler’s eye look at how intricate the taking of high-redshift measurements can be. Reading this material offers an inside look at how the scientific community tests and refines its results enroute to what may need to be a modification of previous models.
It’s the availability of that peer review file that, as much as the findings themselves, occasions this post, as it offers laymen like myself a chance to see the scientific publication process at work. That cannot be anything but salutary in an era when complicated ideas are routinely pared into often misleading news headlines.
Image: Images of six candidate massive galaxies, seen 500-700 million years after the Big Bang. One of the sources (bottom left) could contain as many stars as our present-day Milky Way, according to researchers, but it is 30 times more compact. Credit: NASA, ESA, CSA, I. Labbe (Swinburne University of Technology). Image processing: G. Brammer (Niels Bohr Institute’s Cosmic Dawn Center at the University of Copenhagen). All Rights Reserved.
One note of caution emerges in the abstract to this work: “If verified with spectroscopy, the stellar mass density in massive galaxies would be much higher than anticipated from previous studies based on rest-frame ultraviolet-selected samples.”
That’s a pointer to what seems to be needed next. Penn State’s Joel Leja modeled the light from these objects, and I like the openness to alternative explanations that he injects here:
“This is our first glimpse back this far, so it’s important that we keep an open mind about what we are seeing. While the data indicates they are likely galaxies, I think there is a real possibility that a few of these objects turn out to be obscured supermassive black holes. Regardless, the amount of mass we discovered means that the known mass in stars at this period of our universe is up to 100 times greater than we had previously thought. Even if we cut the sample in half, this is still an astounding change.”
So the question of mass looms just as large as the formation process even if these do not turn out to be galaxies. No wonder Leja says the research team has been calling the six objects ‘universe breakers.’ On the one hand, the question of mass gets into fundamental issues of cosmology and the models that have long served astronomers. If galaxies actually form at this level at such an early time in the universe, then the mechanisms of galaxy formation demand renewed scrutiny. Leja is suggesting that a spectrum be produced for each of the new objects that can confirm the accuracy of our distance measurements, and also demonstrate what these ‘galaxies’ are made up of.
The paper on this remarkable finding itself continues to evolve. It’s Labbé et al., “A population of red candidate massive galaxies ~600 Myr after the Big Bang.” Nature 22 February 2023 (abstract). Note this editorial comment from the abstract page: “We are providing an unedited version of this manuscript to give early access to its findings. Before final publication, the manuscript will undergo further editing.” I’d like to read that final edit before commenting any further.
by Paul Gilster | Oct 21, 2022 | Deep Sky Astronomy & Telescopes |
Some years back, I reminisced in these pages about reading Poul Anderson’s World Without Stars, an intriguing tale first published in 1966 about a starship in intergalactic space that was studying a civilization for whom the word ‘isolation’ must have taken on utterly new meaning. Imagine a star system tens of thousands of light years away from the Milky Way, a place where an entire galaxy is but a rather dim feature in the night sky. Poul Anderson discussed this with Analog editor John Campbell:
One point came up which may interest you. Though the galaxy would be a huge object in the sky, covering some 20? of arc, it would not be bright. In fact, I make its luminosity, as far as this planet is concerned, somewhere between 1% and 0.1% of the total sky-glow (stars, zodiacal light, and permanent aurora) on a clear moonless Earth night. Sure, there are a lot of stars there — but they’re an awfully long ways off!
For more on galactic brightness, see The Milky Way from a Distance. The Anderson tale was originally serialized as The Ancient Gods in the June and July, 1966 issues of Campbell’s magazine. Long-time readers will remember its cover, which I ran back in 2012, along with a discussion of how artist Chesley Bonestell approached the cover art, which shows the distant galaxy as far brighter than it would actually appear. Bonestell brightened it even knowing this to make the cover interesting while still suggesting just how far away the vast ‘city of stars’ actually was in the story.
Where Black Holes Roam
Intergalactic space is, I would assume, about as empty a place as could be. Yet new work out of the University of Sydney delves into just what we might find if we could see what’s out there. And it turns out that it is quite a lot. The university’s David Sweeney is lead author on a paper in Monthly Notices of the Royal Astronomical Society. The researchers discuss what they describe as the ‘galactic underworld,’ which is comprised of the compact remnants of massive stars. In other words, stars that have collapsed onto themselves and produced neutron stars and black holes.
Remember that black holes and neutron stars form from stars more than eight times the size of our Sun. If less than about 25 times the mass of the Sun, the star forms a neutron star, its tiny sphere jammed with neutrons prevented from collapsing further by neutron degeneracy pressure. Sweeney and team say that thirty percent of the black holes and neutron stars out there have been completely ejected from the galaxy. Given the age of the galaxy, over 13 billion years, a vast number of such objects must have formed, the 30 percent ejected by the ‘kick’ induced by their creation in a supernova.
Image: A colour rendition of the visible Milky Way galaxy (top) compared with the range of the galactic underworld (bottom). Credit: Sydney University.
As you can see in the image, the galaxy’s underworld turns out to stretch well beyond the visible limits of the disk. Peter Tuthill (Sydney Institute for Astronomy) notes the challenges involved in creating this first chart of an unseen population:
“One of the problems for finding these ancient objects is that, until now, we had no idea where to look. The oldest neutron stars and black holes were created when the galaxy was younger and shaped differently, and then subjected to complex changes spanning billions of years. It has been a major task to model all of this to find them. Newly-formed neutron stars and black holes conform to today’s galaxy, so astronomers know where to look. It was like trying to find the mythical elephant’s graveyard”
The researchers used a stellar population synthesis computer code called GALAXIA, modifying it to include stars that have exhausted their nuclear fusion life cycle, leaving behind a remnant black hole or neutron star, and excluding stars below 8 solar masses. Additional custom code was then produced to capture velocity changes to the star caused by supernovae explosions (the so-called ‘natal kick’). The effects of the kick were added to each remnant’s velocity and transformed to galactocentric coordinates, with subsequent custom code showing evolution of the stars’ paths over time.
The distribution map that emerged depicts a galaxy, and thus its remnants, changing over time, so that the Milky Way’s present shape does not predict the distribution of neutron stars and black holes surrounding it. In fact, the relatively thin and flattened disk structure gives way to triple the scale height of the Milky Way we see.
Image: Point-cloud chart of the visible Milky Way galaxy (top) versus the galactic underworld. Credit: Sydney University.
As the paper notes:
The spatial distribution of compact remnants is different from that of visible stars. The remnants are more dispersed in the vertical direction with the scale height being about 3 times larger than that of the visible stars. This is mainly due to the significant velocity kicks received by the remnants at the time of their birth.
Also interesting are these two points:
The spatial distribution of BHs is more centrally concentrated as compared to the NSs due to the smaller velocity kick they receive.
For some remnants the kick is so large that their total velocity becomes greater than their escape velocity (40% of NS and 2% of BHs). We are able to estimate a Galactic mass loss in ejected compact remnants as 2.1×108M? or ?0.4% of the stellar mass of the Galaxy.
If 30 percent of the stellar remnants over the course of the galaxy’s evolution have been ejected into intergalactic space, that leaves 70 percent that still moves through the visible disk, so that neutron stars and black holes from the earliest days of the galaxy still move unattached to any nearby star through stellar neighborhoods like our own.
Black Holes and Their Neighbors in Space
In addition to these ‘free floating’ black holes, there are those in gravitational dance with nearby stars, leaving traces that are detectable. Making that point is the recent discovery of a black hole about 12 times the mass of the Sun at roughly 1650 light years from the Solar System, one that appears to be orbited by a visible star. This is “closer to the Sun than any black hole X-ray binaries with known distances…or any of the black holes identified through other techniques.”
The work, led by Sukanya Chakrabarti (University of Alabama, Huntsville), likewise highlights the role these remnants can play in the disk we see today. Says Chakrabarti:
“In some cases, like for supermassive black holes at the centers of galaxies, [black holes] can drive galaxy formation and evolution. It is not yet clear how these non-interacting black holes affect galactic dynamics in the Milky Way. If they are numerous, they may well affect the formation of our galaxy and its internal dynamics.”
Note the term ‘non-interacting,’ which the author uses to distinguish this kind of black hole from those that show an accretion disk of dust accumulating from another object. As you might imagine, interacting black holes – or the features they produce – are easier to detect at visible wavelengths.
Finding the black hole in this work involved analyzing data on almost 200,000 binary stars, as accumulated from the European Space Agency’s Gaia mission. The intent was to find objects that seemed to have a dark companion of large mass, looking for the gravitational effects of a black hole on a visible star. The most interesting sources were followed up by the Automated Planet Finder in California, Chile’s Giant Magellan Telescope and the W.M. Keck Observatory in Hawaii. Spectroscopic measurements confirmed that the binary system contains a visible star cataloged as Gaia DR3 4373465352415301632 orbiting a dark, massive object.
Image: The cross-hairs mark the location of the newly discovered black hole. Credit: Sloan Digital Sky Survey / S. Chakrabarti et al.
As to how this system of star and black hole originally formed, this interesting speculation:
Given the combination of the large mass of the dark companion and a semi-major axis of Gaia DR3 4373465352415301632 that is neither very large nor very small, the formation channel for this system is not immediately clear. However, the most natural scenario may be that the visible G star was originally the outer tertiary component orbiting a close inner binary with two massive stars.
So here we have a search for black holes bound to visible stars, with the authors estimating that perhaps a million such stars have black hole companions. That’s an early estimate for one population of black holes, but this object, in a 185-day orbit from the star, does not represent the class of black holes and neutron stars that may move through the galaxy untethered to any visible object, as found in the investigations of the Sydney team. Just how many black holes may be peppered through the several hundred billion stars of the Milky Way, and how widely spaced are they likely to be?
Finding untethered black holes, whether within or outside the galactic disk, is not work for the faint-hearted. Surely microlensing studies are our best way to proceed?
The paper is Sweeney et al., “The Galactic underworld: the spatial distribution of compact remnants,” Monthly Notices of the Royal Astronomical Society, Volume 516, Issue 4 (November 2022), pp. 4971–4979 (abstract / preprint). The black hole discovery paper is Chakrabarti et al., “A non-interacting Galactic black hole candidate in a binary system with a main-sequence star,” in process at the Astrophysical Journal (preprint).
