by Ann » Fri Apr 02, 2021 7:01 am
NGC 3521: Galaxy in a Bubble.
Image Credit & Copyright: Acquisition - Eric Benson, Processing - Dietmar Hager
By far the most interesting thing about NGC 3521 is the thick "smoke" of old yellow stars that seems to arise from the galaxy's center and billow outward to envelope the disk as well. Note the little "curl" of yellowish haze at the "top end" of the disk. (And note the very blue background galaxies seen through the halo of NGC 3521 at bottom right.)
Which brings us to the question of more or less visible galactic halos and/or outer arms. Because galaxies do have halos and other low-surface brightness mostly outer features.
NGC 4214 is a good example of a (dwarf) galaxy that looks blue in most amateur images because of its brilliant young clusters and intense star formation. But much of the galaxy is dominated by of faint old yellow stars.
In the galactic pair of NGC 4618/4625, NGC 4618 has the brighter starforming disk and a relatively small yellow "border" (this is probably the thick disk of NGC 4618). NGC 4625, by contrast, has a large set of outer spiral arms which become bright only in ultraviolet light.
NASA/JPL-Caltech/Carnegie Observatories/DSS wrote about NGC 4625:
As the image demonstrates, the lengthy spiral arms are nearly invisible when viewed in optical light while bright in ultraviolet. This is because they are bustling with hot, newborn stars that radiate primarily ultraviolet light.
The youthful arms are also very long, stretching out to a distance four times the size of the galaxy's core. They are part of the largest ultraviolet galactic disk discovered so far.
Located 31 million light-years away in the constellation Canes Venatici, NGC 4625 is the closest galaxy ever seen with such a young halo of arms.
But there is certainly at least one galaxy with a larger (but much fainter!) set of outer spiral arms, namely giant low surface brightness galaxy Malin 1.
Giant low surface brightness galaxy Malin 1.
Credit: Credit: CFHT, SDSS, Pan-STARRS, Giuseppe Donatiello.
Because of its enormous set of extremely low surface brightness arms, the size of Malin 1's disk is some 650,000 light-years in diameter. As for well-known spiral galaxy M101, few of us are used to seeing its extensive set of invisible "ghost arms".
Ann
[float=left][attachment=1]NGC3521-LRGB-1024c[1].jpg[/attachment][c][size=80][color=#0040FF]NGC 3521: Galaxy in a Bubble.
Image Credit & Copyright: Acquisition - Eric Benson, Processing - Dietmar Hager[/color][/size][/c][/float]
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By far the most interesting thing about NGC 3521 is the thick "smoke" of old yellow stars that seems to arise from the galaxy's center and billow outward to envelope the disk as well. Note the little "curl" of yellowish haze at the "top end" of the disk. (And note the very blue background galaxies seen through the halo of NGC 3521 at bottom right.)
Which brings us to the question of more or less visible galactic halos and/or outer arms. Because galaxies do have halos and other low-surface brightness mostly outer features.
[float=left][img3="Amateur image of NGC 4214. Photo: Jeffjnet (http://jeffjastro.com)"]https://upload.wikimedia.org/wikipedia/commons/1/11/NGC4214_JeffJohnson.jpg[/img3][/float][float=right][img3="A Hubble image of NGC 4214. We can see that NGC 4214 has a reddish spheroidal 'main body' with huge and brilliant pockets of star formation. Note the very large population of old red stars in the non-starforming parts of the galaxy. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration Acknowledgment: R. O’Connell (University of Virginia) and the WFC3 Scientific Oversight Committee"]https://upload.wikimedia.org/wikipedia/commons/thumb/4/49/NGC_4214.jpg/1024px-NGC_4214.jpg[/img3][/float]
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NGC 4214 is a good example of a (dwarf) galaxy that looks blue in most amateur images because of its brilliant young clusters and intense star formation. But much of the galaxy is dominated by of faint old yellow stars.
[float=left][img3="Galactic pair NGC 4618 (left) and NGC 4625 (right). NGC 4618 has a visible 'soft' non-starforming outer disk, while NGC 4625 lacks such a feature. Very faint spiral arms are just visible. Photo: JW Inman(?)." ]http://www.jwinman.com/starcharts/images/NGC%204625.jpg[/img3][/float][float=right][img3="NGC 4625 (upper left) and NGC 4618 (lower right) in ultraviolet and visible light. Far ultraviolet light is shown as blue, near ultraviolet light as green, and visible light as red. Credit: NASA/JPL-Caltech/Carnegie Observatories/DSS."]https://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/NGC_4625_and_NGC_4618.jpg/600px-NGC_4625_and_NGC_4618.jpg[/img3][/float]
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In the galactic pair of NGC 4618/4625, NGC 4618 has the brighter starforming disk and a relatively small yellow "border" (this is probably the thick disk of NGC 4618). NGC 4625, by contrast, has a large set of outer spiral arms which become bright only in ultraviolet light.
[quote][url=http://www.galex.caltech.edu/media/glx2005-04r_img01.html]NASA/JPL-Caltech/Carnegie Observatories/DSS[/url] wrote about NGC 4625:
As the image demonstrates, the lengthy spiral arms are nearly invisible when viewed in optical light while bright in ultraviolet. This is because they are bustling with hot, newborn stars that radiate primarily ultraviolet light.
The youthful arms are also very long, stretching out to a distance four times the size of the galaxy's core. They are part of the largest ultraviolet galactic disk discovered so far.
Located 31 million light-years away in the constellation Canes Venatici, NGC 4625 is the closest galaxy ever seen with such a young halo of arms.[/quote]
But there is certainly at least one galaxy with a larger (but much fainter!) set of outer spiral arms, namely giant low surface brightness galaxy Malin 1.
[float=left][attachment=0]Malin 1 Guiseppe Donatiello.png[/attachment][c][size=80][color=#0040FF]Giant low surface brightness galaxy Malin 1.
Credit: Credit: CFHT, SDSS, Pan-STARRS, Giuseppe Donatiello.[/color][/size][/c][/float] [float=right][img3="Dim outer 'ghost arms' of giant spiral galaxy M101. (Credit: Roberto Abraham)."]https://images.ctfassets.net/cnu0m8re1exe/4J9210PywbtrbQiI5QvDkX/3efd187de3f4244f79165160b423df87/Dim_Galaxy.jpg?w=650[/img3][/float]
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Because of its enormous set of extremely low surface brightness arms, the size of Malin 1's disk is some 650,000 light-years in diameter. As for well-known spiral galaxy M101, few of us are used to seeing its extensive set of invisible "ghost arms".
Ann