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It’s difficult to determine the shape of our galaxy. So difficult that only in the last century did we learn that the Milky Way is just one galaxy among billions. So it’s not surprising that despite all our modern telescopes and spacecraft we are still mapping the shape of our galaxy. And one of the more interesting discoveries is that the Milky Way is warped. One explanation for this is that our galaxy has undergone collisions, but a new study argues that it’s caused by dark matter.

The first evidence that the Milky Way isn’t a purely flat disk came from the Gaia Spacecraft. it has mapped the positions and motions of more than a billion stars, and from this we have some idea of our galaxies outer structure. For one thing, the Milky Way seems to extend outward more than we’ve thought, and the edge seems to have a rippled structure to it. Further analysis also shows a warp to the outer edge of the galactic disk. The general consensus is that these are caused by ancient galactic collisions, such as the collision with the Sagittarius dwarf galaxy about 6 billion years ago. This new study argues instead that warped galaxies such as ours are caused by a tilted halo of dark matter.

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The galaxy ESO 510-G13 has a clearly warped shape. Credit: NASA and The Hubble Heritage Team (STScI/AURA)

We know that most galaxies, including our own, are surrounded by a massive halo of dark matter. Most of the mass in a galaxy is contained within this halo, so halos can affect the structure of galaxies over time. If the halo is tilted relative to the plane of a galaxy, the gravitational effects of the halo could skew the galactic disk. The question is whether this is significant or common enough to cause the structure we see in the Milky Way.

The authors begin by looking at data from the TNG50 run of the IllustrisTNG simulations. These are supercomputer simulations of galactic evolution that include cosmological evolution and dark matter as well as detailed magnetohydrodynamic interactions. From this data, the authors show that the inner region of the dark matter halo can be significantly tilted relative to the galactic plane and that these can be caused both by galactic collisions and near misses between galaxies. The orientation of the halos can persist for billions of years, which is plenty long enough to induce a galactic warp.

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The simulated evolution of a warped galaxy. Credit: Han, et al

The team then looked at an archetypal galaxy within the TNG50 simulation, having a similar size and age to the Milky Way, including a titled dark matter halo. Based on 6 billion years of simulated evolution, the final result showed noted similarities between the observed Milky Way and the model galaxy.

Astronomers have observed other galaxies with a warped plane, and there is some evidence that as many as half of spiral galaxies in the Universe are warped to some degree. This suggests the influence of long-term and common gravitational interactions between a galaxy and its twisted halo. Of course, this is just one study, so further observations and simulations will be needed to understand the details of how many galaxies such as the Milky Way evolve.

Reference: Han, Jiwon Jesse, et al. “Tilted Dark Halos are Common, Long-Lived, and can Warp Galactic Disks.” arXiv preprint arXiv:2309.07208 (2023).

Reference: Poggio, Eloisa, et al. “Evidence of a dynamically evolving Galactic warp
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Gaia is Now Finding Planets. Could it Find Another Earth?

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September 26, 2023

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Giant Planet Detection

The ESA launched Gaia in 2013 with one overarching goal: to map more than one billion stars in the Milky Way. Its vast collection of data is frequently used in published research. Gaia is an ambitious mission, though it seldom makes headlines on its own.

But that could change.

Gaia relies on astrometry for much of its work, and astrometry is the measurement of the position, distance, and motions of stars. It’s so sensitive that it can sometimes detect the slight wobble a planet imparts to its much more massive star. Gaia detected its first two transiting exoplanets in 2021, and it’s expected to find thousands of Jupiter-size exoplanets beyond our Solar System.

But new research takes it even further. It shows that Gaia should be able to detect Earth-like planets up to 30 light-years away.

The new paper is “The Possibility of Detecting our Solar System Through Astrometry,” and is available on the pre-press site arxiv.org. It has a single author: Dong-Hong Wu from the Department of Physics, Anhui Normal University, Wuhu, Anhui, China.

Astronomers find most exoplanets with the transit method. A spacecraft like TESS monitors a section of the sky and looks at many stars at once. When a planet passes between us and one of the stars, it’s called a transit. It creates a dip in starlight that TESS’s sensitive instruments can detect. When TESS detects multiple, predictable dips, it signifies a planet.

But that’s not the only way to detect them. Astrometry can do it too, and that’s Gaia’s way.

Astrometry has an advantage over other methods. Gaia can more accurately determine an exoplanet’s orbital parameters. This doesn’t mean the other methods aren’t valuable. They obviously are. But as the paper’s author explains, “Neither the transit nor radial velocity method provides complete physical parameters of one planet, and both methods prefer to detect planets close to the central star. On the contrary, the astrometry method can provide a three-dimensional characterization of the orbit of one planet and has the advantage of detecting planets far away from the host star.” Astrometry’s advantages are clear.

If other technological planetary civs exist—and that’s a big if—then it’s not outrageous to think they have technology similar to Gaia’s. While Gaia is impressive, there are improvements on the horizon that will make astrometry even more precise. The author asks a question in his paper: If ETIs (ExtraTerrestrial Intelligences) are using advanced astrometry equal to or even surpassing Gaia’s, “…which of them could discover the planets in the solar system, even the Earth?”

Astrometrical precision is calculated in microarcseconds, and precision decreases with distance. The ESA says that Gaia can measure a star’s position within 24 microarcseconds for objects 4000 times fainter than the naked eye. That’s like measuring the thickness of a human hair from 1000 km away. But that’s not precise enough for Wu’s scenario. His work is based on even more advanced astrometry, the type we’ll likely have in the near future. “If the astrometry precision is equal to or better than ten microarcseconds, all 8,707 stars located within 30 pcs of our solar system possess the potential to detect the four giant planets within 100 years.”

This is the heart of Wu’s paper. The 30-parsec (approx. 100 light-years) region contains almost 9,000 stars, and if an ETI from one of those stars has powerful enough astrometry, then it could detect Jupiter, Saturn, Uranus, and Neptune. The only drawback is they’d have to observe our Solar System for nearly a century to make sure the signal was clear.

This figure from the research shows how long it would take for an ETI with advanced astrometry to detect our Solar System's four giant planets. 1,” the author writes. Image Credit: Wu 2023.” class=”wp-image-163369″ srcset=”https://www.universetoday.com/wp-content/uploads/2023/09/Giant-Planet-Detection.png 492w, https://www.universetoday.com/wp-content/uploads/2023/09/Giant-Planet-Detection-374×580.png 374w, https://www.universetoday.com/wp-content/uploads/2023/09/Giant-Planet-Detection-161×250.png 161w” sizes=”(max-width: 492px) 100vw, 492px” />
This figure from the research shows how long it would take for an ETI with advanced astrometry to detect our Solar System’s four giant planets. “We find that all the four giants in our solar system could be detected and well-characterized as long as they are observed for at least 90 years with SNR > 1,” the author writes. Image Credit: Wu 2023.

There are 8707 stars within 100

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Finally! Astronomers are Starting to See the First Galaxies Coming Together With JWST

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September 26, 2023

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total stellar mass1100 1024x686 1

One of the James Webb Space Telescope’s principal science goals is to observe the epoch where we think that the first galaxies were created, to understand the details of their formation, evolution, and composition. With each deep look back in time, the telescope seems to break its own record for the most distant galaxy ever seen. Science papers are now are starting to trickle in, as astronomers are finally starting to collect enough data from JWST to build a deeper understanding of the early Universe.

In a new study published in Nature Astronomy, a team of researchers in Denmark believe they have observed some of the very first, earliest galaxies with JWST. These galaxies are so old, they are likely still in the process of being formed.

Galaxy Ratios

One known standard is that the ratio between galaxies and their heavy elements has held constant in the local Universe through the last 12 billion years of history, or about 5/6 of the age of the Universe. But with JWST, astronomers are now seeing that the youngest galaxies look different. They don’t have that same ratio of stars to heavier elements because they haven’t gone through the cycles of star formation and star death yet, enriching gas clouds with metals, i.e., elements heavier than hydrogen and helium.

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This plot shows the observed galaxies in an “element-stellar mass diagram”: The farther to the right a galaxy is, the more massive it is, and the farther up, the more heavy elements it contains. The gray icons represent galaxies in the present-day Universe, while the red show the new observations of early galaxies. These ones clearly have much less heavy elements than later galaxies, but agree roughly with theoretical predictions, indicated by the blue band. Credit: Kasper Elm Heintz, Peter Laursen.

For this study, the astronomers looked at 16 galaxies, some of the earliest galaxies ever observed. Their observations revealed that the chemical abundances in these galaxies are one-fourth of that seen in galaxies that were formed later. In their paper, the astronomers wrote that “these findings suggest that galaxies at this time are still intimately connected with the intergalactic medium and subject to continuous infall of pristine gas, which effectively dilutes their metal abundances.”

As gravity gathered together the first clumps of gas,the first stars and galaxies were formed.

“When we analyzed the light from 16 of these first galaxies, we saw that they had significantly less heavy elements, compared to what you’d expect from their stellar masses and the amount of new stars they produced,” said Kasper Elm Heintz, leader of the study and assistant professor at the Cosmic Dawn Center at the Niels Bohr Institute and DTU Space in Copenhagen, Denmark, in a press release.

These results, the astronomers say, are in stark contrast to the current model where galaxies evolve in a form of equilibrium throughout most of the history of the Universe, where there is a relationship between how many stars have formed and how many heavy elements have formed.

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Illustration of galaxy formation: Diffuse gas from intergalactic space plummets toward the center, sparking star formation and becoming part of the galaxy’s rotating disk. When stars die, they return their gas to the galaxy (and the intergalactic space), now enriched with heavy elements. Credit: Tumlinson et al. (2017)
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Why Build Megastructures? Just Move Planets Around to Make Habitable Worlds

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September 25, 2023

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In 1960, Freeman Dyson proposed how advanced civilizations could create megastructures that enclosed their system, allowing them to harness all of their star’s energy and multiplying the habitable space they could occupy. In 2015, the astronomical community was intrigued when the star KIC 8462852 (aka. Tabby’s Star) began to dim inexplicably. While an analysis of the star’s light curve in 2018 revealed that the dimming pattern was more characteristic of dust than a solid structure, Tabby’s Star focused attention on the concept of megastructures and their associated technosignatures.

Dyson’s ideas were proposed at a time when astronomers were unaware of the abundance of exoplanets in our galaxy. The first confirmed exoplanet was not discovered until 1992, and that number has now reached 5,514! With this in mind, a team of researchers from Bangalore, India, recently released a paper that presents an alternative to the whole megastructure concept. For advanced civilizations looking for more room to expand, taking planets within their system – or capturing free-floating planets (FFP) beyond – and transferring them into the star’s circumsolar habitable zone (HZ) is a much simpler and less destructive solution.

The research was led by Raghav Narasimha, a physics graduate student at Christ University in Bangalore, India. He was joined by Margarita Safonova and Chandra Sivaram, a Department of Science and Technology (DST) Woman Scientist and a professor of astrophysics (respectively) at the Indian Institute of Astrophysics (IIAP) in Bangalore, India. The preprint of their paper, “Making Habitable Worlds: Planets Versus Megastructures,” recently appeared online and is being reviewed for publication in Astrophysics and Space Science.

The Problem with Megastructures

The possibility of advanced civilizations building giant structures to harness the energy of their stars is time-honored, with examples going back to the early 20th century. The earliest examples include John Desmond Bernal’s “Bernal Sphere,” which he detailed in his 1929 work “The World, the Flesh & the Devil.” According to Bernal, the source of the material for building such structures “would only be in small part drawn from the Earth; for the great bulk of the structure would be made out of the substance of one or more smaller asteroids, rings of Saturn or other planetary detritus.”

Olaf Stapledon took things a step further in his science fiction novel “Star Maker,” where he described how a future advanced human civilization was “able to construct, out in space, artificial planets for permanent habitation. These great hollow globes of artificial super-metals and artificial transparent adamant, ranged in size from the earliest and smallest structures, which were no bigger than a very small asteroid, to spheres considerably larger than the Earth.” These sources may have been the source of inspiration for Dyson’s 1960 proposal paper.

In this seminal paper, “Search for Artificial Stellar Sources of Infrared Radiation,” Dyson reasoned that a civilization’s motivations for building an “artificial biosphere” (later dubbed a “Dyson Sphere” by Nikolai Kardashev) would include harnessing energy but also multiplying the amount of space they could inhabit. Beginning with the likely possibility that civilization observed at cosmic distances would be much older and more advanced than humanity, he argued that:

“[It is] a reasonable working hypothesis that their habitat will have been expanded to the limits set by Malthusian principles. We have no direct knowledge of the material conditions which these beings would encounter in their search for lebensraum… One should expect that, within a few thousand years of its entering the stage of industrial development, any intelligent species should be found occupying an artificial biosphere which completely surrounds its parent star.”

However, at the time of writing, Dyson was working with the limits of habitable space within our Solar System, which was confined to Earth. Nevertheless, the various bodies of the Solar System (particularly Jupiter and the gas giants) have a tremendous amount of material that could (in theory) be repurposed to create an artificial biosphere. Using Jupiter as an example, Dyson argued that the planet’s mass was sufficient to create a spherical shell around the Sun about 2 to 3 meters (6.5 to 10 ft) in thickness with a density of “200 grams per square centimeter.”

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