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I know Hubble's galaxy classification scheme and the bifurcation from elliptical galaxies into two types of spiral ones. I am also aware of different theories on how spiral galaxies form e.g. through collision with another one. I was always wondering in how much any kind of galaxy morphological classification actually relates with the dynamics of a galaxy. The following paragraph of the Wikipedia article seem to support my uncomfort:
To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy. Nonetheless, in June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating.
Differently put: Would a lone elliptical galaxy eventually become a spiral one? And if so, how to define the transition point? For me, it does not appear obvious how to set a threshold after which density fluctuations within an elliptical galaxy deserve to be called spiral arms. More technical: Which confidence band around the average radial density distribution of a given galaxy has to be breached in order for the galaxy to be called spiral?
I think there are a few misconceptions floating around here. The Hubble Sequence is not a sequence in time. Hubble did not mean to imply that galaxies flow from one side to the other in the sequence (He may have thought it was a possibility though). It is just meant for classification. As it turns out a small fraction of galaxies have changed class, such as an occasional major merger of two spirals, orbiting the right way, can form an elliptical. But, the vast majority of ellipticals (most at the cores of clusters) formed that way very early on. A few spirals may have been converted to S0 galaxies by gas stripping as they fell into clusters. But, most spirals started out as spirals, and probably so for most S0s. The angular momentum per gram, a conserved quantity, is quite different for ellipticals, S0s and spirals.
I do agree that a given spiral galaxy can sometimes have a bar and sometimes not.
To start, if I'm reading your question correctly you've got the general galaxy evolution model backwards, at least as far as morphology goes. At a very high level the picture goes like this: most if not all (large) galaxies form as spirals, then at varying points in their lives they merge with other large galaxies (either ellipticals or other spirals) and the product becomes an elliptical galaxy. This happens faster and more often in dense environments like galaxy clusters, and takes longer to occur for more isolated galaxies like the Milky Way (which will merge with Andromeda in a few billion years).
Question on isolated spirals
If what you were asking about is isolated spirals, well no I don't think an isolated spiral would ever turn into an elliptical galaxy by itself (at least not on life-age-of-the-Universe timescales). It may eventually start to resemble an elliptical galaxy (in that its stellar halo may grow) depending on how much it gets harassed by other small dwarf galaxies. No galaxy is truly isolated, there is always a hierarchy of smaller galaxies nearby and they gravitationally interact with the host galaxy. For example the Milky Way has the Sagittarius dwarf galaxy, and the Large and Small Magellanic clouds which will merge with us soon (it's basically happening now for Sagittarius, and the Magellanic clouds will be a few billion years). None of these events will be impactful enough to turn the Milky Way into a true elliptical, that will only occur with it merges with Andromeda.
Question on morphology versus internal dynamics
To specifically answer your other question on how morphology relates to the internal dynamics of a spiral galaxy: it most definitely does, but the relationship is complicated. When you see a spiral galaxy with a central bar, or spiral arms, these features are being generated by dynamical instabilities within the galaxy. This could happen thanks to a number of causes, which may operate from within the galaxy itself, or from outside the galaxy. One of the most common means of exciting a bar or spiral arms in a disk galaxy is by interactions with smaller dwarf satellite galaxies that orbit around the larger spiral galaxy and gravitationally perturb it. If this part of the answer sounds like a cop out, well it sort of is. Exactly how disk perturbations like bars and spiral arms are started, how they work, what the heck they do to the galaxy, these are all truly bleeding edge research topics that astronomers are working on right now.
Hope that helps!
Hubble Looks at Stunning Spiral Galaxy: NGC 5037
This Hubble image shows NGC 5037, a spiral galaxy some 91 million light-years away in the constellation of Virgo. Image credit: NASA / ESA / Hubble / D. Rosario / L. Shatz.
NGC 5037 is a faint spiral galaxy discovered by the German-born British astronomer William Herschel on December 31, 1785.
Also known as IRAS 13123-1619 and LEDA 46078, the galaxy is located 91 million light-years away in the constellation of Virgo.
“Yet it is possible to see the delicate structures of gas and dust within the galaxy in extraordinary detail,” Hubble astronomers said.
NGC 5037 is a member of a small group of galaxies called the NGC 5044 group.
It also belongs to the Virgo Cluster, a massive collection of roughly 2,000 galaxies.
NGC 5037 hosts an active galactic nucleus (AGN) heavily obscured by dust.
In 2017, a team of astronomers from Japan detected water maser emission toward the AGN using the Nobeyama 45-m telescope.
The new image of NGC 5037 is made up of observations from Hubble’s Wide Field Camera 3 (WFC3) in the near-infrared and optical parts of the spectrum.
Two filters were used to sample various wavelengths. The color results from assigning different hues to each monochromatic image associated with an individual filter.
“WFC3 was installed on Hubble by astronauts in 2009, during servicing mission 4, which was Hubble’s fifth and final servicing mission,” the astronomers said.
“Servicing mission 4 was intended to prolong Hubble’s life for another five years.”
Image & Video
ALMA image of the galaxy BRI 1335-0417 at 12.4 billion years ago. ALMA detected emissions from carbon ions in the galaxy. Spiral arms are visible on both sides of the compact, bright area in the galaxy center. Credit: ALMA (ESO/NAOJ/NRAO), T. Tsukui & S. Iguchi
Study Provides New Insights into Origin of Spiral Arms in Disk Galaxies
The origin and fate of the spiral arms in disk galaxies have been debated by astrophysicists for decades, with two theories predominating. One holds that the arms come and go over time. A second and widely held theory is that the material that makes up the arms – stars, gas and dust – is affected by differences in gravity and jams up, like cars at rush hour, sustaining the arms for long periods.
The new findings, accepted for publication in the Astrophysical Journal (arXiv.org version), fall somewhere in between the two theories and suggest that the arms arise in the first place as a result of the influence of giant molecular clouds – star forming regions or nurseries common in galaxies. Introduced into the simulation, the clouds act as ‘perturbers’ and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely.
“We show for the first time that stellar spiral arms are not transient features, as claimed for several decades,” explained first study author Dr Elena D’Onghia of the University of Wisconsin-Madison.
“The spiral arms are self-perpetuating, persistent, and surprisingly long lived,” added co-author Dr Mark Vogelsberger from Harvard-Smithsonian Center for Astrophysics.
Dr D’Onghia said: “past theory held the arms would go away with the perturbations removed, but we see that the arms self-perpetuate, even when the perturbations are removed. It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there.”
The study modeled stand-alone disk galaxies, those not influenced by another nearby galaxy or object. Some recent studies have explored the likelihood that spiral galaxies with a close neighbor (a nearby dwarf galaxy, for example) get their arms as gravity from the satellite galaxy pulls on the disk of its neighbor.
“The new simulations can be used to reinterpret observational data, looking at both the high-density molecular clouds as well as gravitationally induced ‘holes’ in space as the mechanisms that drive the formation of the characteristic arms of spiral galaxies.”
Bibliographic information: Elena D’Onghia et al. Self-Perpetuating Spiral Arms in Disk Galaxies. ApJ, accepted for publication arXiv: 1204.0513
The Leo Triplet, also called the M66 group, is a small group of spiral galaxies in the constellation Leo. The Leo Triplet is composed of two Messier objects, M65(top) and M66(bottom), as well as NGC 3628(left), which is also called the Hamburger galaxy due to its shape. All three are large spiral galaxies but tend to look different because their galactic disks are tilted at different angles to our line of sight. NGC 3628, is temptingly seen edge-on, with obscuring dust lanes cutting across its puffy galactic plane. The disks of M66 and M65 are both inclined enough to show off their spiral structure. Gravitational interactions between galaxies in the group have left telltale signs, including the 300,000 light-years long tidal tail and warped, inflated disk of NGC 3628 and the drawn-out spiral arms of M66. The three large spiral galaxies can be seen in a single field of view that covers over half a million light-years at a distance of 35 million light-years from Earth.
Dates:09.04.2021(Empty quarter, bortle 2)
Total integration time: 3.3 hours only at f5.3
Flats, darks. Sensor temp: -10°
Equipment: GSO 8" RC with TSCCD47(0.67 reducer) - ZWO 1600mm Pro Camera - ZWO LRGB filters, 8 Position filter wheel, AZEQ6 Mount (ZWO OAG with ZWO224MC cam for guiding)
Processing software: Pixinsight(Stacking, processed RGB and L separately, BN, CC, MSLT, HT, star mask, Deconvolution, CurvesT, SCNR, finally blended the L with RGB.
Spiral galaxies may consist of several distinct components:
- A flat, rotating disc of stars and interstellar matter of which spiral arms are prominent components
- A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
- A bar-shaped distribution of stars
- A near-spherical halo of stars, including many in globular clusters
- A supermassive black hole at the very center of the central bulge
- A near-spherical dark matter halo
The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.
Spiral arms Edit
Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so bright.
A bulge is a large, tightly packed group of stars. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light.
Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, which are old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller  and are composed of young, blue Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity) others simply appear as higher density centers of disks, with properties similar to disk galaxies.
Many bulges are thought to host a supermassive black hole at their centers. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence of active nuclei in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such as Messier 106.
Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies.   Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures   which typically have a maximum visibility at half the length of the in-plane bar.
The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane) in more or less conventional circular orbits around the center of the galaxy (the Galactic Center), or in a spheroidal galactic bulge around the galactic core.
However, some stars inhabit a spheroidal halo or galactic spheroid, a type of galactic halo. The orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.
Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.
The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarfs close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the center of the galaxy, these stars often display unusually high proper motion.
Oldest spiral galaxy Edit
The oldest spiral galaxy on file is BX442. At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers think the galaxy's shape is caused by the gravitational influence of a companion dwarf galaxy. Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years.  
In June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating.  
The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure.
Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies:
- star formation caused by density waves in the galactic disk of the galaxy.
- the stochastic self-propagating star formation model (SSPSF model) – star formation caused by shock waves in the interstellar medium. The shock waves are caused by the stellar winds and supernovae from recent previous star formation, leading to self-propagating and self-sustaining star formation. Spiral structure then arises from differential rotation of the galaxy's disk.
These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms.
Density wave model Edit
Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms. 
Historical theory of Lin and Shu Edit
The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964,  attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram to the right. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits. 
Star formation caused by density waves Edit
The following hypotheses exist for star formation caused by density waves:
- As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.
- As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.
- As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.
More young stars in spiral arms Edit
Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than the rest of the galaxy. As massive stars evolve far more quickly,  their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent. 
Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms.  As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm. 
Gravitationally aligned orbits Edit
Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs) that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When the theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand-design bisymmetric spirals is explained. 
The stars in spirals are distributed in thin disks radial with intensity profiles such that   
The spiral galaxies light profiles, in terms of the coordinate R / h
Before it was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as spiral nebulae. The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mt. Wilson Observatory. Beginning in 1923, Edwin Hubble   observed Cepheid variables in several spiral nebulae, including the so-called "Andromeda Nebula", proving that they are, in fact, entire galaxies outside our own. The term spiral nebula has since fallen out of use.
The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s.   Their suspicions were confirmed by Spitzer Space Telescope observations in 2005,  which showed that the Milky Way's central bar is larger than was previously suspected.
The most ancient spiral galaxy yet
The most ancient spiral galaxy found so far, called BRI 1335-0417, at an distance of 12.4 billion light-years and at a time just 1.4 billion years after the Big Bang. Spiral arms are visible on both sides of the compact, bright area in the galaxy center. Image via ALMA/ T. Tsukui & S. Iguchi.
Swirly and beautiful, spiral galaxies are what we often think of when someone mentions the word galaxy. Our own Milky Way is a spiral galaxy. These galaxies are pretty common in the nearby universe. But the farther back in time and distance astronomers look, the fewer spiral galaxies they see among the multitudes of galaxies in our universe. Instead, as we go out into space – and back in time – galaxies appear more irregular in shape. And thus how and when spiral galaxies formed is one of astronomy’s classic questions. And so it was with some excitement on May 20, 2021 that astronomers reported the most ancient spiral galaxy yet found.
This galaxy is labeled BRI 1335-0417. It existed only 1.4 billion years after the Big Bang, which equals a distance from us of 12.4 billion light-years. So far away – so far back in time – and yet this galaxy has clearly visible spiral arms! Clearly, this galaxy has an important contribution to make in answering questions about spiral galaxies’ origins.
The astronomers published a paper on their findings in the peer-reviewed journal Science on May 20, 2021.
What is a spiral galaxy?
Galaxies come in many different shapes and are classified by their morphology, meaning how they look. There are elliptical, spiral and strangely irregular galaxies, all with different features. Spiral galaxies consist of a central bulge of older stars, a flat rotating disk, and arms spiraling around the disk. Spiral galaxies exist primarily in the nearby universe. As you go out far in distance, back in time, the fewer spirals you see.
Takafumi Tsukui at the university SOKENDAI in Japan is the lead author of the new paper. He said in a statement:
I was excited because I had never seen such clear evidence of a rotating disk, spiral structure, and centralized mass structure in a distant galaxy in any previous literature.
The 3 most common types of galaxies. The top row show schematic illustrations, and the bottom row shows actual images of galaxies that fit each of the 3 categories. Image via A. Feild/ STScI/ Hubblesite.
Observing the most ancient spiral galaxy
The astronomers used a radio telescope, the Atacama Large Millimetre Array or ALMA telescope, to study galaxy BRI 1335-0417. This observatory – located in the Atacama Desert of northern Chile – is able to reach a high level of resolution (detail), despite the enormous distance the the galaxy. Tsukui said:
The quality of the ALMA data was so good that I was able to see so much detail that I thought it was a nearby galaxy.
Due to both the galaxy’s distance, and the early age of the universe at that distance, galaxy BRI 1335-0417 contained a lot of dust that obscures the light from it. The dust makes the galaxy structure hard to see using visible-light telescopes like Hubble. But, at radio wavelengths, astronomers can observe specific elements within the galaxy. And so they can look past the obscuring dust.
In this case, the astronomers looked at the emission from carbon ions for information.
Using the carbon ions as a tool for tracing the galaxy’s structure, the astronomers could see the spiral shape of BRI 1335-0417. They could see this structure extends about 15,000 light-years from the center of the galaxy. This is about 1/3 of the size of the Milky Way, as a comparison. But BRI 1335-0417 is about as massive as our Milky Way galaxy, including its number of stars and amount of interstellar matter. Just because you don’t see it extend farther doesn’t mean it isn’t larger. Tsuki explained:
As BRI 1335-0417 is a very distant object, we might not be able to see the true edge of the galaxy in this observation. For a galaxy that existed in the early universe, BRI 1335-0417 was giant.
How did a spiral galaxy form so early?
Simulations show that interacting galaxies can form an end-product galaxy with spiral arms. Galaxies interacted much more in the early universe, and so might explain the presence of BRI 1335-0417 so far back in time. There are more clues to that scenario as well: BRI 1335-0417 has a large supply of gas in its outskirts, for example. That’s an indication that there’s some kind of supply delivery coming in from the outside, possibly because this galaxy has been colliding with other, smaller galaxies.
The video below is a simulation that shows how many small galaxies interact to form a larger spiral galaxy.
Video ©2007 T. Takeda, S. Nukatani, T. R. Saitoh, 4D2U Project, NAOJ.
What happened next?
What happened next is the interesting question. According to conventional theory, star-forming galaxies (like BRI 1335-0417) with lots of dust in the early universe would evolve into giant ellipticals as they age. But maybe that might not happen? Maybe a galaxy like BRI 1335-0417 would remain a spiral for a much longer time? Spirals arms are of special interest to us because, as Tsukui said:
Our solar system lodges in one of the Milky Way spiral arms. Tracing the roots of spiral structure will provide us with clues as to the environment in which the solar system was born. I hope that this research will further advance our understanding of the formation history of galaxies.
Bottom line: Astronomers were surprised to discover spiral arms in a galaxy located in the very early universe. This makes the galaxy, BRI 1335-0417, the most ancient spiral galaxy found so far and provides clues to how and when spiral galaxies formed.
Puzzle of how spiral galaxies get their arms comes into focus
As the shapes of galaxies go, the spiral disk &mdash with its characteristic pinwheel profile &mdash is by far the most pedestrian.
This image and the video animation below show a simulation of arm formation in spiral galaxies. The simulation was performed by UW–Madison astrophysicist Elena D&rsquoOnghia, who led new research in the area along with Harvard-Smithsonian Center for Astrophysics colleagues Mark Vogelsberger and Lars Hernquist. The visualizations were created by Thiago Ize and Chris Johnson of the University of Utah&rsquos Scientific Computing and Imaging Institute.
Our own Milky Way, astronomers believe, is a spiral. Our solar system and Earth reside somewhere near one of its filamentous, swept-back arms. And nearly 70 percent of the galaxies closest to the Milky Way are spirals, suggesting they have taken the most ordinary of galactic forms in a universe with billions of galaxies.
But despite their common morphology, how galaxies like ours get and maintain their characteristic arms has proved to be an enduring puzzle in astrophysics. How do the arms of spiral galaxies arise? Do they change or come and go over time?
The answers to these and other questions are now coming into focus as researchers capitalize on powerful new computer simulations to follow the motions of as many as 100 million &ldquostellar particles&rdquo as gravity and other astrophysical forces sculpt them into familiar galactic shapes. Writing April 1 in The Astrophysical Journal, a team of researchers from the University of Wisconsin–Madison and Harvard-Smithsonian Center for Astrophysics report simulations that seem to resolve longstanding questions about the origin and life history of spiral arms in disk galaxies.
&ldquoWe show for the first time that stellar spiral arms are not transient features, as claimed for several decades,&rdquo says UW–Madison astrophysicist Elena D&rsquoOnghia, who led the new research along with Harvard-Smithsonian Center for Astrophysics colleagues Mark Vogelsberger and Lars Hernquist. &ldquoThey are self-perpetuating, persistent and surprisingly long lived.&rdquo
The origin and fate of the emblematic spiral arms in disk galaxies have been debated by astrophysicists for decades, with two theories predominating: One holds that the arms come and go over time. A second and widely held theory is that the material that makes up the arms &ndash stars, gas and dust &ndash is affected by differences in gravity and jams up, like cars at rush hour, sustaining the arms for long periods.
The new results fall somewhere in between the two theories and suggest that the arms arise in the first place as a result of the influence of giant molecular clouds, star forming regions or nurseries common in galaxies. Introduced into the simulation, the clouds, says D&rsquoOnghia, a UW–Madison professor of astronomy, act as &ldquoperturbers&rdquo and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely.
&ldquoWe find they are forming spiral arms,&rdquo explains D&rsquoOnghia. &ldquoPast theory held the arms would go away with the perturbations removed, but we see that (once formed) the arms self-perpetuate, even when the perturbations are removed. It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there.&rdquo
The new study modeled stand-alone disk galaxies, those not influenced by another nearby galaxy or object. Some recent studies have explored the likelihood that spiral galaxies with a close neighbor &mdash a nearby dwarf galaxy, for example &mdash get their arms as gravity from the satellite galaxy pulls on the disk of its neighbor.
According to Vogelsberger and Hernquist, the new simulations can be used to reinterpret observational data, looking at both the high-density molecular clouds as well as gravitationally induced holes in space as the mechanisms that drive the formation of the characteristic arms of spiral galaxies.
Some fast radio bursts come from the spiral arms of other galaxies
A Hubble Space Telescope image (left) of a galaxy known to host a ‘fast radio burst’ helps ID where in the galaxy the blast originated (oval). After image processing (right), the burst’s origin appears centered on one of the galaxy’s spiral arms.
NASA, ESA, Alexandra Mannings/University of California Santa Cruz, Wen-fai Fong/Northwestern University, Alyssa Pagan/STScI
Five brief, bright blasts of radio waves from deep space now have precise addresses.
The fast radio bursts, or FRBs, come from the spiral arms of their host galaxies, researchers report in a study to appear in the Astrophysical Journal. The proximity of the FRBs to sites of star formation bolsters the case for run-of-the-mill young stars as the origin of these elusive, energetic eruptions.
“This is the first such population study of its kind and provides a unique piece to the puzzle of FRB origins,” says Wen-fai Fong, an astronomer at Northwestern University in Evanston, Ill.
FRBs typically last a few milliseconds and are never seen again. Because the bursts are so brief, it’s difficult to nail down their precise origins on the sky. Although astronomers have detected about 1,000 FRBs since the first was reported in 2007, only 15 or so have been traced to a specific galaxy.
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The first burst to be traced to its source came from a small, blobby dwarf galaxy with a lot of active star formation (SN: 1/4/17). That FRB sends off repeated blasts from a single source, which is an unusual feature, and helped astronomers localize its host galaxy.
“After that, a lot of people thought, well, maybe all FRB hosts are like this,” says astronomer Alexandra Mannings of the University of California, Santa Cruz. But then a second repeating burst was tracked back to a spiral galaxy like the Milky Way (SN: 1/6/20). And a one-off burst was localized to a massive disk-shaped galaxy, also the size of the Milky Way. Others followed.
Mannings, Fong and colleagues thought they could learn more about the FRBs’ sources by localizing their origins even more precisely. Different parts of spiral galaxies tend to host different types of stars. The bright spiral arms tend to mark sites where new stars are being born, while the older and dimmer stars have had time to drift away from the arms into the rest of the galaxy. So figuring out which galactic neighborhoods FRBs call home can reveal a lot about what kind of objects they come from.
Using the Hubble Space Telescope, the researchers took high-resolution images of eight galaxies that were already known to host FRBs, then overlaid the FRBs’ positions onto the images. The five FRBs that came from clearly defined spiral galaxies all lay on or close to the galaxies’ spiral arms, which had not been visible in images from ground-based telescopes. The other three host galaxies had inconclusive shapes, Fong says.
The FRB locales have a fair amount of star formation, but they’re not the brightest and most active parts of their galaxies, Fong says. That suggests FRBs originate with ordinary young stars — not the youngest, most massive stars that occupy the brightest knots in the spiral arms, but not the oldest and dimmest stars that have drifted away from their homes, either.
That finding is consistent with the idea that FRBs come from highly magnetized stellar corpses called magnetars, Mannings says (SN: 6/4/20). There are a couple of ways to produce magnetars from ordinary stars. There’s the slow way, which involves waiting billions of years for a pair of neutron stars to collide (SN: 12/1/20). Or there’s the fast way, which follows the death of a single massive star. It seems like FRBs might come from an in-between process, like the death of a not-so-massive star, Mannings says.
“The fact that FRBs are found to be pretty close to, if not on, the spiral arm, near to these star forming regions, that can give us a better idea of what the timeline is like for the progenitor,” whatever created the FRB, Mannings says. “And if it is a magnetar, it lets us know that it’s not through the delayed channel, like a neutron star merger.”
The finding doesn’t entirely solve the mystery of where FRBs come from, says astrophysicist Emily Petroff of the University of Amsterdam, who was not involved in the new work. But it does help to get a broader picture of their host galaxies.
“FRBs keep throwing a lot of surprises at us, in terms of what they look like, where they’re found, how they repeat,” Petroff says. “This is maybe providing more evidence that FRBs are more related to just sort of general neutron stars.” The next step, of course, is to find more FRBs.
Questions or comments on this article? E-mail us at [email protected]
A version of this article appears in the June 19, 2021 issue of Science News.