by Ann » Sun Nov 20, 2022 7:18 am
johnnydeep wrote: ↑Sat Nov 19, 2022 7:53 pm
Thanks for that article. And from the next paragraph:
We present N-body simulations of such an encounter that reproduce the essential features of NGC 1097’s jets: A long and narrow “X”-shaped morphology centered near the spiral’s nucleus, right-angle bends, and no discernible dwarf galaxy remnant. A series of jetlike distributions are formed, with the earliest appearing ~1.4 Gyr after impact. Well-defined X shapes form only when the more massive galaxy has a strong disk component.
Ram-pressure stripping of the dwarf’s interstellar medium would be expected to occur while passing through NGC 1097’s disk, accounting for the jets’ lack of H I and H II.
The remnants’ (B-V) color would still agree with observations even after ~3 Gyr of passive evolution, provided the cannibalized dwarf was low-metallicity and dominated by young stars at impact.”
This is how ram pressure works:
Galaxies lose gas when they move fast through a medium. The galaxy in the picture above is entering a cluster of galaxies, which is full of million-degree intracluster gas. As ESO 137-001 plunges through this hot intergalactic medium, drawn there by the gravitational pull of the cluster, the hapless captured galaxy is losing its gas. Enormously long filaments of gas are shown in purple in the image, whereas newly formed stars are shown in blue, much closer to the galactic disk. The galaxy itself is losing its gas and its ability to form new stars.
The small galaxy that left the X-formed stellar streams centered on the core of NGC 1097 would have lost its gas and its ability to form new stars as it plunged through the disk of NGC 1097. My own math-idiot guess is that it might have plunged through the disk of NGC 1097 twice.
The Milky Way has star streams, too:
Star streams in the Milky Way. Palomar 5 is at lower left. Credit: SDSS/Ana Bonaca.
Palomar 5 star stream is the densest star stream in the Milky Way. Its origin is the globular cluster Palomar 5.
As to why the star streams of NGC 1097 would remain mostly unchanged in color and distinguishable from stars in NGC 1097 and its companion after 3 billion years, this would happen if the shredded galaxy was sufficiently metal-poor and thus different from NGC 1097 and its visible companion.
Let's look at some color-magnitude diagrams of clusters of different ages and metallicities:
M67 is an old metal-rich cluster. Metal-poor clusters evolve slightly differently than metal-rich ones as they age:
Very metal-poor stars go though a "blue phase" during their evolution. They spend some time on the "horizontal branch", where they are much smaller and hotter than they are during their red giant phase. Metal-rich stars do go through a slightly hotter phase during their evolution, too, but in their case, the temperature and color change is so small that it doesn't affect their color very much. They remain cooler, larger and redder than the Sun.
Very metal-poor clusters can stay at the same overall color for billions of years, as their stars move in and out of the red giant branch and the (blue) horizontal branch.
So, to summarize:
A dwarf galaxy may have plunged through the disk of NGC 1097, probably impacting the large galaxy quite close to its center. The dwarf galaxy lost its gas during this crossing and lost its ability to form new stars. The galaxy's existing stars would spread out in long streams. If this small galaxy was metal-poor, its stars would be different and distinguishable from the stars of the visible companion of NGC 1097, which is most likely more metal-rich than the shredded dwarf.
Note that the visible companion lacks gas and dust, too. It, too, may have lost its gas during its interactions with its large bully of a neighbor.
I have looked at other images of the small companion galaxy, NGC 1097A, and haven't found much evidence of it having shells. But we do see at least two shells in the APOD.
Possible shell galaxy NGC 1097A.
David Malin explained the shells of NGC 3293 as remnants of the galaxy's history of expanding and contracting, possibly as a consequence of small-scale collisions, which may have left stars behind, similar to how waves lapping on a shore may leave little bits of debris behind.
NGC 1097 has probably rocked the boat of NGC 1097A, too.
Click to play embedded YouTube video.
Ann
[quote=johnnydeep post_id=327266 time=1668887609 user_id=132061]
Thanks for that article. And from the next paragraph:
[quote]We present N-body simulations of such an encounter that reproduce the essential features of NGC 1097’s jets: A long and narrow “X”-shaped morphology centered near the spiral’s nucleus, right-angle bends, and no discernible dwarf galaxy remnant. A series of jetlike distributions are formed, with the earliest appearing ~1.4 Gyr after impact. Well-defined X shapes form only when the more massive galaxy has a strong disk component.
[b][size=110][color=#FF0000]Ram-pressure stripping of the dwarf’s interstellar medium would be expected to occur while passing through NGC 1097’s disk, accounting for the jets’ lack of H I and H II.[/color][/size][/b]
[b][size=110][color=#FF8000]The remnants’ (B-V) color would still agree with observations even after ~3 Gyr of passive evolution, provided the cannibalized dwarf was low-metallicity and dominated by young stars at impact.”[/color][/size][/b][/quote]
[/quote]
This is how ram pressure works:
[img3="Ram pressure stripping of gas from galaxy ESO 137-001. Credit: NASA Goddard Space Flight Center"]https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/NASA%27s_Hubble_Finds_Life_is_Too_Fast%2C_Too_Furious_for_This_Runaway_Galaxy_%2812952512944%29.jpg/1024px-NASA%27s_Hubble_Finds_Life_is_Too_Fast%2C_Too_Furious_for_This_Runaway_Galaxy_%2812952512944%29.jpg[/img3]
Galaxies lose gas when they move fast through a medium. The galaxy in the picture above is entering a cluster of galaxies, which is full of million-degree intracluster gas. As ESO 137-001 plunges through this hot intergalactic medium, drawn there by the gravitational pull of the cluster, the hapless captured galaxy is losing its gas. Enormously long filaments of gas are shown in purple in the image, whereas newly formed stars are shown in blue, much closer to the galactic disk. The galaxy itself is losing its gas and its ability to form new stars.
The small galaxy that left the X-formed stellar streams centered on the core of NGC 1097 would have lost its gas and its ability to form new stars as it plunged through the disk of NGC 1097. My own math-idiot guess is that it might have plunged through the disk of NGC 1097 twice.
The Milky Way has star streams, too:
[float=left][attachment=1]556dfd42c13c1[1].png[/attachment][c][size=85][color=#0040FF]Star streams in the Milky Way. Palomar 5 is at lower left. Credit: SDSS/Ana Bonaca.[/color][/size][/c] [/float]
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Palomar 5 star stream is the densest star stream in the Milky Way. Its origin is the globular cluster Palomar 5.
[float=left][img3="The Palomar 5 star stream. Image: SDSS/Michael Odenkirchen et al."]https://classic.sdss.org/news/releases/prf1.gif[/img3][/float][float=right][img3="Palomar 5 has crossed the disk of the Milky Way twice. The orbit (red line) of Palomar 5 in the halo of our Milky Way as reconstructed from the tidal tails and the known position, distance and radial velocity of the cluster. The image used to illustrate the Milky Way Galaxy is courtesy of the Hubble Heritage project (STScI/NASA)."]https://classic.sdss.org/news/releases/prf2.gif[/img3][/float]
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As to why the star streams of NGC 1097 would remain mostly unchanged in color and distinguishable from stars in NGC 1097 and its companion after 3 billion years, this would happen if the shredded galaxy was sufficiently metal-poor and thus different from NGC 1097 and its visible companion.
Let's look at some color-magnitude diagrams of clusters of different ages and metallicities:
[float=left][img3="The color-magnitude diagram of the Pleiades. Stars are brighter at top and fainter at bottom, and bluer at left and redder at right. The diagonal line shows stars that are on the main sequence and fuse hydrogen to helium in their cores. The Pleiades is a young cluster, around a hundred million years, and all the stars are on or near the main sequence. The diagram shows that the brightest stars of the Pleiades are all blue. This colour-magnitude diagram for 270 stars of the Pleiades has been adopted from the data from the paper 'Investigation of the Pleiades cluster. IV. The radial structure.', Raboud, D., Mermilliod, J.-C. A&A., 329, 101 (1998)."]https://www.southastrodel.com/Fig030b004.png[/img3][/float][float=right][img3="The color-magnitude diagram for M67, which is an old open cluster with an age of between 3.60 and 3.87 billion years. The most massive stars have used up the hydrogen in their cores and evolved off the main sequence. They are now red giants. The brightest stars of M67 are red. Credit: Yagav et al. (2008)/Matthieu Castro et al."]https://www.researchgate.net/publication/299481387/figure/fig3/AS:350256257617923@1460519007278/Color-magnitude-diagram-of-the-open-cluster-M67-The-black-filled-circles-represent-our.png[/img3][/float]
[clear][/clear]
M67 is an old metal-rich cluster. Metal-poor clusters evolve slightly differently than metal-rich ones as they age:
[float=right][img3="Color-magnitude diagram of somewhat more metal-rich globular cluster NGC 6440. Note the short horizontal branch. Source: https://people.smp.uq.edu.au/HolgerBaumgardt/globular/fits/ngc6440.html"]https://people.smp.uq.edu.au/HolgerBaumgardt/globular/fits/phot/ngc6440_cmd.gif[/img3][/float][img3="Color-magnitude diagram of old metal-poor globular cluster M55. Note the 'arc' of blue stars at upper left. Credit: B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope"]https://apod.nasa.gov/apod/image/0102/m55cmd_mochejska.jpg[/img3]
Very metal-poor stars go though a "blue phase" during their evolution. They spend some time on the "horizontal branch", where they are much smaller and hotter than they are during their red giant phase. Metal-rich stars do go through a slightly hotter phase during their evolution, too, but in their case, the temperature and color change is so small that it doesn't affect their color very much. They remain cooler, larger and redder than the Sun.
Very metal-poor clusters can stay at the same overall color for billions of years, as their stars move in and out of the red giant branch and the (blue) horizontal branch.
[img3="Stars in metal-poor globular cluster M13. The blue horizontal branch stars are very noticeable here. ESA/Hubble and NASA."]https://upload.wikimedia.org/wikipedia/commons/thumb/6/60/Heart_of_M13_Hercules_Globular_Cluster.jpg/768px-Heart_of_M13_Hercules_Globular_Cluster.jpg[/img3]
So, to summarize:
A dwarf galaxy may have plunged through the disk of NGC 1097, probably impacting the large galaxy quite close to its center. The dwarf galaxy lost its gas during this crossing and lost its ability to form new stars. The galaxy's existing stars would spread out in long streams. If this small galaxy was metal-poor, its stars would be different and distinguishable from the stars of the visible companion of NGC 1097, which is most likely more metal-rich than the shredded dwarf.
Note that the visible companion lacks gas and dust, too. It, too, may have lost its gas during its interactions with its large bully of a neighbor.
I have looked at other images of the small companion galaxy, NGC 1097A, and haven't found much evidence of it having shells. But we do see at least two shells in the APOD.
[float=left][attachment=0]APOD 16 November 2022 detail satellite galaxy.png[/attachment][c][size=85][color=#0040FF]Possible shell galaxy NGC 1097A.[/color][/size][/c][/float][float=right][img3="Elliptical shell galaxy NGC 3923. Image: David Malin."]https://www.researchgate.net/profile/Lucie_Jilkova/publication/263891735/figure/fig4/AS:669296854319105@1536584213133/Type-I-shell-galaxy-NGC-3923-Courtesy-of-David-Malin-Australian-Astronomical.png[/img3][/float]
[clear][/clear]
David Malin explained the shells of NGC 3293 as remnants of the galaxy's history of expanding and contracting, possibly as a consequence of small-scale collisions, which may have left stars behind, similar to how waves lapping on a shore may leave little bits of debris behind.
NGC 1097 has probably rocked the boat of NGC 1097A, too.
[youtube]https://www.youtube.com/watch?v=B1T06UhcX0Q[/youtube]
Ann