How is the bar formed in a Barred-Spiral galaxy?

How is the bar formed in a Barred-Spiral galaxy?

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How does the bar in a barred spiral galaxy form? What prevents it from being spirals all the way down like water spiralling down a plughole?

"Galactic bars develop when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. The tiny elongations in the stars' orbits grow and get locked into place, forming a bar. The bar becomes even more pronounced as it collects more and more stars in elliptical orbits. Eventually, a high fraction of the stars in the galaxy's inner region join the bar. This process has been demonstrated repeatedly with computer-based simulations."


10 Barred Spiral Galaxy Facts – How They’re Different

Barred spiral galaxies are some of the most interesting galaxies to look at in our night sky. Whilst originally galaxies where just classified as spiral, nowadays we can split these again by calling them either spiral galaxies or barred spiral galaxies, with the obvious difference being the bar in the center of a barred spiral.

As you’d expect, they do have spiral arms at the edge of the galaxy, though they can be affected visually by the bar – this can also affect the motion of the stars in the galaxy too. They make up many of the untold galaxies in our night sky, so let’s learn some interesting facts about barred spiral galaxies.

NGC 1398: An utterly perfect spiral galaxy

It's nice to catch up with old friends sometimes, isn't it? If it's been a while, with the passage of time you see them better — more clearly — and your own experience may give you better insight on them as well.

Oh — now that I write that out I see that this works for humans, too. But I meant it for astronomical objects, specifically galaxies.

More Bad Astronomy

Even more specifically I meant NGC 1398, a perfect spiral galaxy about 65 million light-years away in the constellation of Fornax:

NGC 1398, a barred spiral galaxy that is, quite simply, gorgeous. Credit: ESO

Well hello old friend!

I wrote about NGC 1398 a few years ago, when the astronomer (and another old friend!) Adam Block released a gorgeous image of it. Adam used a 0.81-meter telescope in Arizona, and he does amazing work.

In this case, though, the image above was created using the monster 8-meter Very Large Telescope in Chile. The major difference is that a bigger telescope sees fainter features more easily and also produces sharper images, so you can see more detail. The camera used, the FOcal Reducer and low dispersion Spectrograph 2 (also called FORS2), is an extremely sensitive detector that can see a large area of the sky (that's the "focal reducer" part, lowering the telescope's magnification) as well as take spectra, breaking the light up from an object into individual colors to help astronomers analyze it (the "spectrograph" part).

This image is a combination of four filters to create a sort-of natural-looking shot of the galaxy. It uses a red, green, and blue filter to show more or less what the eye sees, plus an extra filter that selects out the light from warm hydrogen, to accentuate gas clouds where stars are born. That's displayed as pink, and you can see these nebulae dotting the spiral arms of the galaxy.

And what spiral arms! They're wound up so tightly that at first glance they look like rings. In fact, poking around the old science journal literature about NGC 1398, I found that for a while astronomers weren't sure just what the structure of this galaxy was some said the inner arms formed a ring, some said it's not really a ring but that the outer arms form a ring-like structure (called a pseudoring).

This image makes it clear that the inner ring is definitely just a very tightly wound set of arms, and the outer arms don't really form a either (I think the large number of spurs — short arms branching off the main ones — helped confound the astronomers looking at earlier, lower-resolution images).

NGC 1398 is a barred spiral, with that rectangular-shaped feature running across the core and ending at the inner "ring." This is a common feature in spirals — the Milky Way has a big one — and they can affect how the stars and gas move around the galactic center (I describe this in the earlier post on NGC 1398).

Bars are generally made of old stars, which tend to be red massive, luminous blue stars die young, leaving behind lower-mass redder ones. This means stars haven't formed in the bar in a long, long time. In the spiral arms, stars are constantly being formed in gas clouds, making them appear blue. You can see this in the image, but astronomers also use single-color images, sometimes in wavelengths of light our eyes cannot even detect, to see this more clearly.

For example, check out this ultraviolet image of NGC 1398 taken by the GALEX spacecraft:

NGC 1398 in the ultraviolet, taken using the orbiting GALEX observatory. Credit: NASA/JPL-Caltech

Even though GALEX uses a small telescope, look at how sharply defined the arms are! Young stars put out a lot of UV, and these tend to be confined in the arms. The inner "ring" of arms stands out as well, as does the very center of the galaxy (this happens sometimes in barred galaxies, as gas is funneled by the gravity of the stars in the bar toward the galactic center).

Now check out a near-infrared (just outside the color range of the human eye) image taken using the Spitzer Space Telescope:

NGC 1398 seen in the near-infrared by Spitzer Space Telescope. Credit: NASA/JPL-Caltech/Juan Carlos Munoz/Judy Schmidt

See how the bar is suddenly much brighter relative to the arms? The bar is old stars, remember, which are redder, so they show up well in this wavelength. The arms are there, but fainter, since there aren't as many older red stars there compared to young blue ones.

But this gets more complicated (or, as scientists like to call it, "fun"): when you look out to even longer infrared wavelengths the arms become bright again. Check out this image from WISE, the Wide-field Infrared Survey Explorer, which I rotated to more or less match the VLT image:

NGC 1398 in the far-infrared, taken by WISE. Credit: NASA/JPL-Caltech/UCLA

Spiral arms have lots of dust in them: tiny grains of rocky material or long-chain carbon molecules. These block visible light (check out the original VLT image above again the dust is what creates the lovely filigree patterns in the arms), but if it's warmed by nearby stars it glows in the infrared wavelengths seen by WISE. In those images, the carbon molecules are seen as green (remember, this is light we can't see, so the images taken by WISE are assembled and assigned colors to help us understand them warm carbon molecules glow at a wavelength of 12 microns [about 20 times the reddest light humans can see], which is displayed as green). The red blobs are dust clouds heated by stars being born, and you can compare them to how they look in other wavelengths in the images above, too.

Galaxies are immense structures, and immensely complex, too. They really are like old friends to astronomers we've observed them for a long time, but they still constantly surprise us. Using bigger telescopes to investigate them is important, of course, but perhaps even more important is using telescopes that see other light than our eyes can. Only then do the different features stand out, isolating them, so that we can study them and figure out what's what.

I've always loved this idea it's easy to fall into the trap of thinking that what you are seeing is what you get. You have to look at things in different ways to truly appreciate them, to understand them.

It's a good lesson, and in this case it's literally written across the sky.

A barred galaxy's massive molecular inflow

The barred spiral galaxy NGC1300 as seen by Hubble. Astronomers think that galactic bars help funnel material into the nuclear regions of galaxies where they help trigger star formation and feed the supermassive black hole. The nuclear region is heavily obscured in the optical, but infrared and submillimeter wavelengths can penetrate the dust. Analyses of new infrared spectra of water vapor and other gases have now confirmed and quantified these processes in the barred spiral ESO320-G030. Credit: NASA, ESA, and the Hubble Heritage Team STScI/AURA

Large amounts of gas are sometimes funneled to a galaxy's nuclear regions, with profound consequences. The gas triggers starburst activity and can also feed the supermassive black hole, converting it into an active galactic nucleus (AGN) indeed the supermassive black holes in AGN are thought to gain most of their mass in these accretion events. Eventually, outward pressure from supernovae, shocks, and/or AGN activity terminate the inflow. Galaxy mergers are thought to be one mechanism capable of triggering these massive inflows by disrupting the medium. A less dramatic cause may result from gas flows induced by a combination of galactic rotation and the gravitational instabilities generated by galactic bars, the elongated central structures (composed of stars) found in numerous spiral galaxies including the Milky Way.

What happens to infalling gas when it encounters a nuclear region is poorly understood because the very high obscuration around galactic nuclei makes optical observations challenging. Astronomers have therefore been relying on data from far-infrared and submillimeter wavelength observations which can penetrate the dust, although longer wavelength imaging typically lacks the high spatial resolution needed. Infrared spectroscopy has been one of the premier ways to overcome both difficulties because the radiation not only penetrates the dust, the strengths and shapes of spectral lines can be modeled to infer even small dimensions as well as temperatures, densities, and other characteristics of the emitting regions.

CfA astronomers Eduardo Gonzalez-Alfonso, Matt Ashby, and Howard Smith led a team that modeled infrared spectra of water vapor from the nuclear region of the ultraluminous galaxy ESO320-G030, about 160 million light-years away, a galaxy that emits about one hundred times as much energy as the Milky Way. The data were obtained with the Herschel Space Observatory and the ALMA submillimeter facility. This galaxy shows no signs of having been in a merger, nor does it show any signs of AGN activity, but it does have a clear and complex central bar structure and infalling gas that was previously discovered through infrared spectroscopy.

The astronomers observed and modeled twenty spectral features of water vapor, enough diagnostic lines to model the complexity of the emitting regions. The successful results required a three-component nuclear model: a warm envelope (about 50 kelvin) about 450 light-years in radius within which is a second component, a nuclear disk about 130 light-years in radius, and finally a much warmer compact core (100 kelvin) about 40 light-years in radius. These three components alone emit nearly 70% of the galaxy's luminosity from a starburst that is making about 18 solar-masses of stars a year (the Milky Way averages about one per year). The mass inflow rate into the region is about the same as the star production—about 18 solar-masses per year. In addition to these conclusions about the nuclear region, the astronomers use their best-fit results to model successfully 17 other molecular species (besides water) seen in the far infrared spectra, including ionized molecules and carbon and nitrogen-bearing molecules. The combined results, in particular the extremely high abundance of ionized molecules, suggest the strong presence of enhanced ionizing cosmic rays and shed light on the chemistry of the complex nuclear zone.

Barred Spiral Galaxy

As its name implies, a barred spiral galaxy is a spiral galaxy with a bar through the center.

Hubble introduced the ‘tuning fork’ scheme for describing the shapes of galaxies (“morphologies” in astronomer-speak) in 1936. In this, the two arms of the fork are barred spirals (from SBa to SBc) and spirals without bars (from Sa to Sc) the S stands for spiral, B for ‘it’s got a bar’, and a/b/c for how tightly wound the spiral arms are. This was later extended to a fourth type, SBm and Sm, for irregular barred spiral galaxies which have no bulge.

In 1959, Gérard de Vaucouleurs extended the scheme to the one perhaps the most commonly used by astronomers today (though there’ve been some mods since). In this scheme spirals without bars are SA, and those which have really weak bars are SAB barred spirals remain SB. He also added a ‘d’ (SAd, SBd), and a few other things, like rings.

About half of spiral galaxies are barred examples include M58 (SBc), M61 (SABbc), the Large Magellanic Cloud (LMC, Sm), … and our own Milky Way galaxy!

The bars are mostly stars (usually), unlike spiral arms (which have lots of gas and dust besides stars). The formation and evolution of bars is an active area of research in astronomy today they seem to form from close encounters of the galaxy kind (galaxy near-collisions), funnel gas into the central bulge (where the super-massive black holes there snack on it), and are sustained by the same density waves which keep the arms alive.

Why not join the Galaxy Zoo project, and have some fun classifying spiral galaxies into whether they have bars or not (and getting to see some amazing sights too)?

Astronomers identify stars in the Milky Way’s central bar

One of the most interesting revelations in modern astronomy has been that we don’t just live in a spiral galaxy. In fact, we live in a barred spiral galaxy. Observations via radio telescopes had suggested the barred nature of our Milky Way, and in 2005, the Spitzer Space Telescope backed up those suggestions while studying the galaxy in the infrared region of the spectrum. Today (December 19, 2012), astronomers from the Sloan Digital Sky Survey III (SDSS-III), working with an international team, announced they’ve found hundreds of stars rapidly moving together in long, looping orbits around the center of the Milky Way. They think these stars are part of the Milky Way bar.

Artist’s impression of Milky Way, with central bar. Small blue dot is Earth – not to scale! Solid red arrows show high-speed stars moving away from Earth, discovered by SDSS-III. Dashed arrows show stars moving toward Earth, which are expected to be seen in a future sky survey. Credit: Jordan Raddick (Johns Hopkins) and Gail Zasowski (Ohio State / U. of Virginia). Artist’s concept by NASA/JPL-Caltech/R. Hurt.

Although various lines of evidence suggested the existence of a Milky Way bar, scientists had no way of knowing which Milky Way stars are part of the bar. To find out, the team needed to know the velocities of stars near the center of the Milky Way. That data would tell them which stars are moving as a group. And they had to observe in the infrared part of the spectrum because the center of our galaxy – which is 25,000-30,000 light-years away – is hidden from our view by dust.

To get the data they needed, they participated in a project called APOGEE (Apache Point Observatory Galactic Evolution Experiment). The project uses a custom-built high-resolution infrared spectrograph attached to the 2.5-meter Sloan Foundation Telescope in New Mexico. APOGEE is a larger survey aimed at characterizing 100,000 stars in our Milky Way Galaxy. These astronomers used data from the first few months of APOGEE observations to find out the velocities for nearly 5,000 stars near the Milky Way’s center.

With those measurements in hand, the astronomers could then see whether some of the stars are moving together in some unusual pattern.

A map of the innermost Milky Way, with circles marking the regions explored by the SDSS-III APOGEE project. Circles marked with “X” show places where the project found high-speed stars associated with the Milky Way’s bar moving away from Earth. The lighter regions marked with dots on the other side of the galaxy’s center show places where the fourth-generation Sloan Digital Sky Survey hopes to find counterpart bar stars moving toward the Earth. Illustration Credit: David Nidever (University of Michigan / University of Virginia) and the SDSS-III Collaboration.

And indeed the stars were moving in an unusual pattern. The astronomers found that a substantial fraction of stars in the inner galaxy are moving away from us quickly. In an online post, they said:

… about 10 percent of the total stars in the sample are moving at more than 200 kilometers per second (400,000 miles per hour) away from the Earth. The observed pattern of these fast stars is similar in many different parts of the inner galaxy, and is the same above and below the midplane of the galaxy — suggesting that these measurements of fast central stars are not just a statistical fluke, but really are a feature of our galaxy.

Comparing these observations with computer models of the Milky Way’s central bar, they became convinced the stars they were measuring – those seen to be fleeing from Earth – were indeed part of the bar. In fact, this is just the part of the bar rotating away from our direction in space.

That’s very cool! But these astronomers say their work is only half done.

So far, APOGEE has only observed one side of the bar, the side where the stars are moving away from the Earth. On the other side, the stars must be moving toward Earth. But unfortunately, the Sloan telescope is inconveniently placed: the other half of the Milky Way bar is visible only from Earth’s southern hemisphere. Seeing the other side of the bar is one of the motivations for a planned fourth generation of the Sloan Digital Sky Survey. Part of this successor project will implement the same techniques using a 2.5-meter telescope in Chile to observe the rest of the inner Milky Way. The new survey is set to begin in 2014.

By the way, our Earth and sun lie near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms of the Milky Way. Read more about how we know our location in the Milky Way here.

Bottom line: On December 19, 2012, an international team of astronomers – including astronomers from the Sloan Digital Sky Survey III (SDSS-III) announced that they have identified stars in the bar of our barred spiral galaxy, the Milky Way.

Barred Galaxies

How big of an amplification can we get? In some cases, the amplification can be very strong and we can get barred galaxies .

Approximately 1/3 of spiral galaxies are barred -- bars are common, but not universal.

Numerical simulations of galaxies have shown that, by themselves, rotating disks are wildly unstable , and will spontaneously form bars. Why?

At this point, stars are no longer moving on circular orbits, they are moving on highly elongated orbits along the bar -- we say they are "trapped" in the bar.

So how do you prevent this from happening? You reduce the self-gravity of the disk, so that stars don't get trapped. And you do this by having less mass in the disk and more mass in the dark matter halo. Since the halo isn't a rotating disk, it doesn't change in response to the formation of a bar, and so the overall density perturbation is much weaker. Stars no longer get trapped in a bar. So the disk:halo mass ratio is one important factor in determining if a galaxy is unstable to forming a bar.

In fact, we can use this as a constraint on dark matter . If disk galaxies didn't have dark matter halos, they'd be very unstable against bar formation, and all spirals should be very strongly barred. This is known as the Ostriker-Peebles criterion .

But for barred galaxies, once stars are trapped in the bar, it is very hard to get them out of the bar. So are bars permanent features of galaxies? Can bars be disrupted and destroyed?

Dark Matter in the Universe

II.B Large-Scale Determinations

From the stellar orbits, one knows the rate of galactic differential rotation in the vicinity of the sun, a region about 3 kpc in radius and located about 8.5 kpc from the galactic center. The measurement of galactic rotational motion can be greatly extended through the use of millimeter molecular line observations, such as OH masers, and the 21-cm line of neutral hydrogen. Most of the disk is accessible, but at the expense of a new assumption. It is assumed that the gas motions are good tracers of the gravitational field and that random motions are small and unimportant in comparison with the rotational motions. It is also assumed that the gas is coplanar with the stellar distribution and that the motion is largely circular, with no large-scale, noncircular flows being superimposed. These assumptions are not obviously violated in the Galaxy, but may be problematic in many extragalactic systems, especially barred spiral galaxies . The maximum radial velocity along lines of sight interior to the solar radius occurs at the tangent points to the orbits. This method of tangent points presumes that the differential rotation of the Galaxy produces a gradient in the radial velocity along any line of sight through the disk for gas and stars viewed from the sun and that the maximum velocity occurs at a point at which the line of sight is tangent to some orbit. The distance can be determined from knowledge of the distance of the sun from the galactic center, about 8 kpc. Using ΘHI gives a measure of the mass interior to the point r and thus the cumulative mass of the Galaxy. Recent work has extended this to include large molecular clouds, which also orbit the galactic center.

The determination of the orbits and distances of stars and gas clouds outside of the solar orbit is difficult since the tangent point method cannot be applied for extrasolar orbits, but from the study of molecular clouds and 21-cm absorption and stellar velocities of standard stars it appears that the rotation curve outside of the solar circle is still flat, or perhaps rising. The argument that this implies a considerable amount of extended, dark mass follows from an extension of the argument for the orbital velocity about a point mass, Θ 2 (r) ∼ M(r)/r. Since the observations support Θ = const, it appears that M(r) ∼ r. The scale length for the luminous matter is small, about 5 kpc, and this is substantially smaller than the radial distances where the rotation curve has been observed to still be flat (of order 15 kpc). The same behavior has been observed in external galaxies, as we will discuss shortly. Thus, there is a substantial need for the Galaxy to have a large amount of dark matter in order to account for dynamics on scales larger than 10 kpc.

As mentioned, mass measurements of the Galaxy from stellar and gas rotation curves beyond the distance of the sun from the center are subject to several serious problems, which serve as warnings for extragalactic studies. One is that there appear to be substantial departures of the distribution from planarity. That is, the outer parts of the disk appear to be warped. This means that much of the motion may not be circular and there may be sizable vertical motions, which means that the motions are not strictly indicative of the mass. For the inner galactic plane, there is some evidence of noncircular motion perhaps caused by barlike structures in the mass distribution, thus affecting the mass studies.

An extension of the Oort method for the vertical acceleration, now taken to the halo stars, can be used to measure the total mass of the Galaxy. One looks at progressively more distant objects. These give handles on several quantities. For instance, the maximum velocity of stars falling through the plane can be compared with the maximum distances to which stars are observed to be bound to the galactic system. Halo stars of known intrinsic luminosity, such as RR Lyrae variable stars, can be studied with some certainty in their intrinsic brightness. From the observed brightness, the distance can be calculated. Thus, the distance within the halo can be found. From an observation of the vertical component of the velocity, one can obtain the total mass of the galactic system lying interior to the star's orbit. The same can be done for the globular clusters, the most distant of which should be nearly radially infalling toward the galactic center. These methods give a wide range of masses, from about 4 × 10 11 M to as high as 10 12 M. to distances of 50–100 kpc.

The phenomenological solution to the problem of subluminous mass increasing in fraction of the galactic population with increasing length scale measured assumes that the mass-to-light ratio, M/L, is a function of distance from the galactic center. For low-mass stars, this number is of order 1–5 in solar units (M/L), but for the required velocity distribution in the galaxy, this must be as high as 100. There are few objects, except Jupiter-sized or smaller masses, which have this property. The reason is simple—nuclear reactions that are responsible for the luminosity of massive objects, greater than about 0.08 M, cannot be so inefficient as to produce this high value. Even the fact that the flux distribution can be redistributed in wavelength because of the effects of opacity in the atmospheres of stars of very low mass will still leave them bright enough to observe in the infrared, if not the visible, and their total (bolometric) luminosities are still high enough to provide M/L ratios that are less than about 10. Black holes, neutron stars, or cold white dwarfs would appear good candidates within the Galaxy. But limits can be set on the space density of these objects from X-ray surveys since they would accerete material from the interstellar medium and emit X rays as a result of the infall. The observed X-ray background and the known rate of production of such relics of stellar evolution are both too low to allow these objects to serve as the sole solution to the disk DM problem. Some other explanation is required.

Anne’s Picture of the Day: The Great Barred Spiral Galaxy

NGC 1365, also known as the Great Barred Spiral Galaxy, is, as you would have guessed by its nickname, a huge barred spiral galaxy. It is over 200,000 light-years across, what makes it one of the largest galaxies known, and lies about 56 million light-years from Earth in the constellation of Fornax, while it is receding from us at approximately 1636 kilometers per second. It is a major member of the Fornax cluster which consists of 58 galaxies.

The galaxy has a straight east-west bar and two very prominent outer spiral arms that extend in a wide curve north and south from the ends of the bar and form an almost ring like Z-shaped halo. Closer to the oval-shaped center is a second spiral structure and the young luminous hot stars, born out of the interstellar clouds, give these arms a prominent appearance and a blue color. The whole galaxy is laced with delicate dust lanes.

This image reveals very clearly the glow from vast numbers of stars in both the bar and the spiral arms. The huge bar disturbs the shape of the gravitational field of the galaxy and this leads to regions where gas is compressed and star formation is triggered.

Many huge young star clusters trace out the main spiral arms and each contains hundreds or thousands of bright young stars that are less than ten million years old. NGC 1365 is too remote for single stars to be seen in this image and most of the tiny clumps visible in the picture are really star clusters. Over the whole galaxy, stars are forming at a rate of about three times the mass of our Sun per year.

While the bar of the galaxy consists mainly of older stars, many new stars are born in stellar nurseries of gas and dust in the inner spiral close to the nucleus. The bar also funnels gas and dust gravitationally into the very center of the galaxy, where a supermassive black hole is hidden among myriads of bright new stars.

This black hole has a mass 2 million times that of our Sun and is spinning almost as fast as Einstein’s theory of gravity will allow. Supermassive black holes are surrounded by pancake-like accretion disks, formed as their gravity pulls matter inward. The bright center of NGC 1365 is thought to be due to huge amounts of superhot gas ejected from such an accretion disk circling this central black hole.

The bar rotates clockwise with velocities in the nucleus of 2000 kilometers per second resulting in one rotation in 350 million years.

Supernovae 1957C, 1983V, 2001du and 2012fr (discovered on 27 Oct 2012) were observed in this galaxy.

Dark matter is slowing the spin of the Milky Way’s galactic bar

The spin of the Milky Way’s galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation, according to a new study by researchers at University College London (UCL) and the University of Oxford.

For 30 years, astrophysicists have predicted such a slowdown, but this is the first time it has been measured.

The researchers say it gives a new type of insight into the nature of dark matter, which acts like a counterweight slowing the spin.

In the study, published in the Monthly Notices of the Royal Astronomical Society, researchers analysed Gaia space telescope observations of a large group of stars, the Hercules stream, which are in resonance with the bar – that is, they revolve around the galaxy at the same rate as the bar’s spin.

These stars are gravitationally trapped by the spinning bar. The same phenomenon occurs with Jupiter’s Trojan and Greek asteroids, which orbit Jupiter’s Lagrange points (ahead and behind Jupiter). If the bar’s spin slows down, these stars would be expected to move further out in the galaxy, keeping their orbital period matched to that of the bar’s spin.

The researchers found that the stars in the stream carry a chemical fingerprint – they are richer in heavier elements (called metals in astronomy), proving that they have travelled away from the galactic centre, where stars and star-forming gas are about 10 times as rich in metals compared to the outer galaxy.

Using this data, the team inferred that the bar – made up of billions of stars and trillions of solar masses – had slowed down its spin by at least 24% since it first formed.

Co-author Dr Ralph Schoenrich (UCL Physics & Astronomy) said: “Astrophysicists have long suspected that the spinning bar at the centre of our galaxy is slowing down, but we have found the first evidence of this happening.

“The counterweight slowing this spin must be dark matter. Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter.

“Our research provides a new type of measurement of dark matter – not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar’s spin.”

Co-author and PhD student Rimpei Chiba, of the University of Oxford, said: “Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar.

“Our finding also poses a major problem for alternative gravity theories – as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.”

The Milky Way, like other galaxies, is thought to be embedded in a ‘halo’ of dark matter that extends well beyond its visible edge.

Dark matter is invisible and its nature is unknown, but its existence is inferred from galaxies behaving as if they were shrouded in significantly more mass than we can see. There is thought to be about five times as much dark matter in the Universe as ordinary, visible matter.

Alternative gravity theories such as modified Newtonian dynamics reject the idea of dark matter, instead seeking to explain discrepancies by tweaking Einstein’s theory of general relativity.

The Milky Way is a barred spiral galaxy, with a thick bar of stars in the middle and spiral arms extending through the disc outside the bar. The bar rotates in the same direction as the galaxy.

Watch the video: How Do Sand Bars Form? (May 2022).