Astronomy

What is the movement of stars within the nucleus of a barred spiral galaxy

What is the movement of stars within the nucleus of a barred spiral galaxy


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I'm looking for visualizations that show the movement of stars in barred galaxies but not having much luck. I've found many that simulate spiral galaxy arms in general, but none that include a bar or a nucleus.

I'm trying to word my question in a way that is not dependent on my own assumptions, but some of my assumptions are:

  1. the area that includes the bar is the nucleus.
  2. the nucleus and the bulge are not necessarily the same thing. (Bars within the nucleus is a result of gravity waves. Bulge is an artifact of how the galaxy was formed.)
  3. the edge boundary of the nucleus is a gravitational "tipping point".
  4. stars are not crossing this boundary (ie: they either belong to the nucleus or to the arms, not both).

What is the movement of stars within the nucleus of a barred spiral galaxy, and how do their galactic orbits differ from stars outside the nucleus?


Think of what happens since the beginning. At first, at very large scale, you have a region of space a little more dense than what is around it, and it starts attracting more and more gas, forming a cloud. Gas falls in, in a spiral movement, giving an overall spin to the thing.

In the meantime, stars form, most of them have a movement that follows the one of the initial gas cloud. However, gravity does its work and bring more and more stars to the core. They gather little by little in the "galactic plane" - I just learned this is due to angular momentum conservation - could be similar thing that makes planets a little flatter at the poles.

I don't understand exactly how the arms and the bar form, but the thing is you have this globally revolving stars population, and more and more of them coming towards the galactic center under the effect of gravity ; here is explained for our own galaxy, within 100 parsec from the Galactic center there is a stellar density about a 100 times higher as the one experienced in the vicinity of our sun)

The stars revolving far from the center will keep the overall rotating around the center movement because they stay roughly unperturbed by each other - everything can change there if a merger with a bigger or similar-sized galaxy, and it can become an elliptical and erratically moving structure in this case.

On the other hand, a densely packed core means there are more and more chances for the stars to do close fly-by and change there course erratically. This also happens in star clusters. This explain why in the center, there is no clear overall movement as the one you observe on the galactic arms.


It's a little hard to know how to answer your question, partly because you're using "nucleus" in a confusing fashion. So let me try answering what might be your general question, which is "how do stars move in the region spanned by the bar?"

("Nucleus" is generally used to refer to the inner few hundred parsecs -- or even, these days, just the inner ten parsecs or so. Since bars extend to anywhere from several hundred parsecs to as much as ten thousand parsecs in radius, they are usually well outside the "nucleus". [I am deliberately ignoring so-called "nuclear bars" -- which are generally only a few hundred parsecs in size -- in order to keep things simple ;-)])

Stars in a disk galaxy generally all orbit in the same direction and in the same plane. Some of the orbits are close to circular, while others are more elongated. These elongated orbits precess -- that is, the axis of the apparent ellipse rotates with time, so the star traces out a rosette. In a barred galaxy, there are a lot of stars which are on elongated orbits which are aligned, and which precess at the same rate. Thus, they maintain a common oval region that is dense with stars (the "bar") which rotates as a whole at a more-or-less constant speed (even as the stars making up the bar are doing their own individual orbits at different speeds). The combined gravity of all these stars in an oval configuration is what keeps the whole thing synchronized and consistent, although there are some stars on more chaotic orbits.

There is a kind of magic radius called "corotation". This is where an imaginary star in a circular orbit would go once round the center of the galaxy in the same time that the bar as a whole make one complete turn. Stars inside corotation go once around faster than the bar turns, while stars outside this radius would go once round slower than the bar. Stars orbiting inside the corotation radius can form part of the bar (and generally have to be part of the bar if they're not on some very chaotic orbit). Stars outside corotation cannot be part of the bar; their orbits will actually tend to be elongated perpendicular to the bar, though as you get further away from the bar, its gravitational influence becomes less and less important, so that stars further out can have orbits that are pretty much circular.

There is good evidence -- both theoretical and observational -- that bars cannot extend further out than the corotation radius, and that bars in fact tend to extend about 3/4 or 4/5 of the way to corotation (with the stars further out but still inside corotation probably having very chaotic orbits).


Sculpted in Sculptor: An Incredibly Beautiful Barred Spiral Galaxy

First discovered in 1798 by German-English astronomer William Hershel, NGC 613 is a galaxy that lies in the southern constellation of Sculptor 67 million light-years away.

Featured here in a new image from the NASA/ESA Hubble Space Telescope, NGC 613 is a lovely example of a barred spiral galaxy. It is easily distinguishable as such because of its well defined central bar and long arms, which spiral loosely around its nucleus. As revealed by surveys, about two thirds of spiral galaxies, including our own Milky Way galaxy, contain a bar.

Recent studies have shown that bars are more common in galaxies now than they were in the past, which gives us important clues about galaxy formation and evolution.

A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about half of all spiral galaxies. Bars generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy, where the Solar System is located, is classified as a barred spiral galaxy.


What is the movement of stars within the nucleus of a barred spiral galaxy - Astronomy

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Barred Spiral Galaxy NGC 1300

Fast Facts

Barred spiral galaxies are so named for the central bar of stars that connects the galaxy’s spiral arms. NGC 1300 also has what is known as a "grand design" disk structure in its nucleus – a spiral within a spiral.

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NGC 4631, a barred spiral galaxy in Canes Venatici

NGC 4631 (also known as the Whale Galaxy) is an edge-on, barred spiral galaxy of about 140,000 light-years across that lies some 30 million light-years away from Earth in the small northern constellation of Canes Venatici (the Hunting Dogs), while it is receding from us at approximately 606 kilometers per second. This galaxy’s slightly distorted wedge shape gives it the appearance of a whale, hence its nickname.

This galaxy has a nearby companion, named NGC 4627, a dwarf elliptical galaxy which lies just above it. Together also called Arp 281, this interacting galaxy pair was listed in the Atlas of Peculiar Galaxies as an example of a “double galaxy”. Arp’s Atlas of Peculiar Galaxies is a catalog of 338 peculiar galaxies produced by Halton Arp in 1966.

NGC 4631 is also a gravitationally bound companion of NGC 4656 (a barred spiral galaxy of some 75 thousand light-years across and about 29 million light years away from Earth), which lies about half a degree to the southeast, and could be within half a million light-years of its neighbor. Their interaction is probably responsible for the curved end of the galaxy (which is listed as NGC 4657), and its nickname, the Hockey Stick.

NGC 4631,NGC 4627,NGC 4656 and NGC 4657 are all four part of the NGC 4631 Group of galaxies. Estimates of the number of galaxies in this group range from 5 to 27, and all studies identify very different member galaxies for this group.

The Whale Galaxy is a bright, large and extended galaxy with a very bright nucleus. It contains a central starburst, which is a region of intense star formation. The strong star formation is evident in the emission from ionized hydrogen and interstellar dust heated by the stars formed in the starburst.

The most massive stars that form in star formation regions only live for a short period of time, after which they explode as supernovae. So many supernovae have exploded in the center of NGC 4631 that they are blowing gas out of the plane of the galaxy. This superwind can be seen in X-rays and in spectral line emission. The gas from this superwind has produced a giant, diffuse corona of hot, X-ray emitting gas around the whole galaxy.

Because this nearby galaxy is seen edge-on from Earth, professional astronomers observe this galaxy to better understand the gas and stars located outside the plane of the galaxy. It can be seen with a small telescope. North is up in this image.


What is the movement of stars within the nucleus of a barred spiral galaxy - Astronomy

We present new observations of the barred spiral galaxy NGC 4314 in the CO (1-0) and (2-1) lines, the 21 cm HI line, the radio continuum, the visible and the near infrared. The CO observations, made with the IRAM 30 m telescope, show abundant molecular gas in the center of this galaxy, probably in the 12" circumnuclear ring, in the middle of the strong bar. Even more remarkable is the extreme HI deficiency, as shown by our HI detection with the Arecibo telescope: in NGC 4314, the H_2_/HI mass ratio is 60-to-1! VLA maps at 2, 6 and 20 cm show several radio continuum sources in a 12" ring around the optical nucleus. No radio continuum emission was detected from the nucleus itself, the bar or the outer spiral arms. The radio continuum emission has a shallow spectral index of -0.4, and may be a mixture of free-free and synchrotron radiation. Short exposure optical images of the galaxy also show bright spots in a ring around the nucleus, coinciding with the radio continuum structures. Near-infrared photometry of the inner region suggests that the mass-to-2μm luminosity ratio within a radius of 9 arcsec (450 pc) of the nucleus is 3 M_sun_/L_sun_, as in the center of our Galaxy. However, in this region, the H_2_ mass is 3 10^8^M_sun_, a large fraction of the total mass. Presumably the bar has driven gas into the inner region where it has formed molecular clouds. These clouds produce the O stars ionizing the HII regions seen in the visible and the thermal radio continuum, and supernovae giving rise to the nonthermal radio emission.


What is the movement of stars within the nucleus of a barred spiral galaxy - Astronomy

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Barred Spiral Galaxy NGC 1300

Fast Facts

69 million light-years away (21 megaparsecs)

This image is 5.5 arcminutes (110,000 light-years or 34,000 parsecs).

The Hubble image was created from HST data from proposal 10342: K. Noll, H. Bond, C. Christian, L. Frattare, F. Hamilton, W. Januszewski, Z. Levay, and T. Royle (STScI), and P. Knezek (WIYN).

F435W (B), F555W (V), F814W (I), F658N (H-alpha)

Blue: F435W (B)
Green: F555W (V)
Red: F814W (I) + F658N (H-alpha) [N II]

One of the largest Hubble Space Telescope images ever made of a complete galaxy is being unveiled today at the American Astronomical Society meeting in San Diego, Calif.

The Hubble telescope captured a display of starlight, glowing gas, and silhouetted dark clouds of interstellar dust in this 4-foot-by-8-foot image of the barred spiral galaxy NGC 1300. NGC 1300 is considered to be prototypical of barred spiral galaxies. Barred spirals differ from normal spiral galaxies in that the arms of the galaxy do not spiral all the way into the center, but are connected to the two ends of a straight bar of stars containing the nucleus at its center.

At Hubble's resolution, a myriad of fine details, some of which have never before been seen, is seen throughout the galaxy's arms, disk, bulge, and nucleus. Blue and red supergiant stars, star clusters, and star-forming regions are well resolved across the spiral arms, and dust lanes trace out fine structures in the disk and bar. Numerous more distant galaxies are visible in the background, and are seen even through the densest regions of NGC 1300.

In the core of the larger spiral structure of NGC 1300, the nucleus shows its own extraordinary and distinct "grand-design" spiral structure that is about 3,300 light-years (1 kiloparsec) long. Only galaxies with large-scale bars appear to have these grand-design inner disks - a spiral within a spiral. Models suggest that the gas in a bar can be funneled inwards, and then spiral into the center through the grand-design disk, where it can potentially fuel a central black hole. NGC 1300 is not known to have an active nucleus, however, indicating either that there is no black hole, or that it is not accreting matter.

The image was constructed from exposures taken in September 2004 by the Advanced Camera for Surveys onboard Hubble in four filters. Starlight and dust are seen in blue, visible, and infrared light. Bright star clusters are highlighted in red by their associated emission from glowing hydrogen gas. Due to the galaxy's large size, two adjacent pointings of the telescope were necessary to cover the extent of the spiral arms. The galaxy lies roughly 69 million light-years away (21 megaparsecs) in the direction of the constellation Eridanus.


A galaxy with two hearts

This new Hubble image shows the spiral galaxy Messier 83, otherwise known as the Southern Pinwheel Galaxy. One of the largest and closest barred spirals to us, this galaxy is dramatic and mysterious it has hosted a large number of supernova explosions, and is thought to have a double nucleus lurking at its core.

Messier 83 is not one to blend into the background. Located some 15 million light-years away in the constellation of Hydra (The Sea Serpent), it is one of the most conspicuous galaxies of its type in our skies. It is a prominent member of a group of galaxies known as the Centaurus A/M83 Group, which also counts dusty Centaurus A (heic1110) and irregular NGC 5253 (potw1248a) as members.

Spiral galaxies come in a range of types depending on their appearance and structure — for example, how tightly wound their arms are, and the characteristics of the central bulge. Messier 83 has a "bar" of stars slicing through its centre, leading to its classification as a barred spiral. The Milky Way also belongs to this category.

These bars are thought to act a bit like a funnel, channelling gas inwards towards the galaxy's centre. This gas is then used to form new stars and also to feed the galaxy's central black hole, explaining why many barred spirals — including Messier 83 — have very active and luminous central regions.

However, Messier 83's centre is mysterious and unusual the supermassive black hole at its heart is not alone. This striking spiral displays a phenomenon known as a double nucleus — a feature that has also been spotted in the Andromeda Galaxy (heic0512), the nearest spiral galaxy to us. This does not mean that Messier 83 contains two central black holes, but that its single supermassive black hole may be ringed by a lopsided disc of stars, which orbits around the black hole and creates the appearance of a dual core [1].

As well as this double nucleus, Messier 83 has hosted quite a few supernova explosions — six in total that we have observed (SN 1923A, SN 1945B, SN 1950B, SN 1957D, SN 1968L, and SN 1983N). This number is matched by only two other galaxies: Messier 61 (potw1324a) which also has six, and NGC 6946 (opo9910e), which tops the list with nine. As well as these explosions, almost 300 supernova remnants — the older leftovers from exploded stars — have been found within Messier 83, detected using the data that make up this image. These observations are being used to study the life cycle of stars. As well as these old remnants, some 3000 star clusters have been identified in Messier 83, some of which are very young at under 5 million years old.

This mosaic image uses observations taken by Hubble's Wide Field Camera 3. It shows the galaxy in full, with dark dust lanes, fiery red patches of gas, and bright blue patches of recent star formation speckled across the spiralling arms. Although it looks sprawling, Messier 83 is just under half of the size of the Milky Way.

This new image is being released today, 9 January 2014, at the 223rd meeting of the American Astronomical Society in Washington, DC, USA.

Notes

[1] This central region is a very bizarre place. Neither of the two components making up the double nucleus are actually aligned with the galaxies kinematic centre — the region inferred to be the central part of Messier 83 from the motions of the stars within the galaxy. The "second nucleus" is not seen directly, but is detected by studying how mass within the galaxy is distributed.

Notes for editors

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

More information

Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgement: William Blair (Johns Hopkins University)


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.

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.

Reference: "A Proto-Pseudobulge in ESO 320-G030 Fed by a Massive Molecular Inflow Driven by a Nuclear Bar," Eduardo González-Alfonso, Miguel Pereira-Santaella, Jacqueline Fischer, Santiago García-Burillo, Chentao Yang, Almudena Alonso-Herrero, Luis Colina, Matthew L. N. Ashby, Howard A. Smith, Fernando Rico-Villas, Jesús Martín-Pintado, Sara Cazzoli, and Kenneth P. Stewart, Astronomy & Astrophysics , 645, 49, 2021.


Messier 61, a barred spiral galaxy in Virgo

Messier 61 (also known as NGC 4303) is a very bright barred spiral galaxy of some 100,000 light-years across, located only about 52.5 million light-years away from Earth in the constellation of Virgo (the Virgin), while it is receding from us at about 1566 kilometers per second.

This grand design galaxy spiral (a spiral galaxy with prominent and well-defined spiral arms that extend clearly around the galaxy) is one of the largest members of the Virgo Cluster, a cluster of approximately 1300 (and possibly up to 2000) galaxies held together by gravity. This cluster forms the central region of the Virgo Supercluster (or Local Supercluster), an even bigger gathering of galaxies.

Messier 61 is classified as a starburst galaxy and has an Active Galactic Nucleus. The energy source of an Active Galactic Nucleus is believed to originate in mass accretion by a supermassive black hole within the nucleus of the galaxy. Messier 61 probably has a supermassive black hole with a mass around 5 million times that of our Sun on its center.

In this image, its spiral arms can be seen in stunning detail, swirling inwards to the very center of the galaxy, where they form a smaller, intensely bright spiral. In the outer regions, these vast arms are sprinkled with bright blue regions where new stars are being formed from hot, dense clouds of gas. The high star formation across Messier 61’s disk is perhaps due to interactions with her satellite galaxies NGC 4292 and NGC 4303B.

The intensely bright spiral is a nuclear starburst ring some 730 light years from the nucleus, formed by several massive star-forming regions which contain massive hot stars with an age range between 5 and 25 million years old. The starburst ring may be associated with a second bar much smaller than the main one of this galaxy.

Six supernovae have been observed in this galaxy: SN 2008in SN 2006ov SN 1999gn SN 1964F SN 1961I SN 1926A. All have been found to be of Type II, except for SN 1964F, which was a Type I supernova. This total places Messier 61 in the top handful of galaxies alongside Messier 83, also with six, and NGC 6946, with a total of nine observed supernovae.

This image was acquired from Moorook Obervatory, Australia (an affiliation of New Mexico Skies), the image data were acquired by Ryan Hannahoe and the image was processed by Robert Gendler.


Hubble’s barred and booming spiral galaxy

This image, captured by the NASA/ESA Hubble Space Telescope’s Wide Field Camera 3 (WFC3), shows a galaxy named UGC 6093. As can be easily seen, UGC 6093 is something known as a barred spiral galaxy — it has beautiful arms that swirl outwards from a bar slicing through the galaxy’s center. It is classified as an active galaxy, which means that it hosts an active galactic nucleus, or AGN: a compact region at a galaxy’s center within which material is dragged towards a supermassive black hole. As this black hole devours the surrounding matter it emits intense radiation, causing it to shine brightly.

But UGC 6093 is more exotic still. The galaxy essentially acts as a giant astronomical laser that also spews out light at microwave, not visible, wavelengths — this type of object is dubbed a megamaser (maser being the term for a microwave laser). Megamasers such as UGC 6093 can be some 100 million times brighter than masers found in galaxies like the Milky Way.

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