by Ann » Sun Jan 24, 2021 7:36 am
VictorBorun wrote: ↑Sun Jan 24, 2021 6:09 am
trying to imagine a 3d, it's easy to see dusty outskirts as wide tyre around a thin plate wheel.
The part near the core is more puzzling. There surely is a large ghostly shine bulge through which we see the far side of the arms' ends.
But what are the dusty lanes seen very close to the core due to the backlight of the core?
They look like lying strictly in the plane.
How come the dust lanes obey the flattening viscosity of the gas disk and the stellar population of bulge does not?
Is dust more glued into near-core thinning gas then the stars?
I'm not absolutely sure what you're asking here, Victor, mostly because I don't speak Math. But bulges are typically very different from dusty disks.
An aspect that is interesting to me is the way the dust lanes on one side of the bulge look dark and stand out sharply, while the dust lanes on the other side of the bulge are almost invisible. This is very obvious in the picture of NGC 2841, and it can be seen in the picture of M81, too.
My guess, for what it's worth, is that the dust on one side of the bulge of a galaxy that is tilted to our line of sight is blocking light from the bulge behind it, so that it becomes a "dark nebula".
The dust on the other side of the bulge (the side that is farther away from us) is actually weakly lit up by light from the bulge, which is typically brighter than the disk outside it. This makes the dust on the side that is farther away from us "disappear".
There are two more pictures of NGC 2841 that I want to show you.
The image on the left is an SDSS picture of NGC 2841. I like SDSS images because I find them "honest". They normally use the same filters,
g-r-i. The "g" filter picks out light from blue stars, the "r" filter reacts to normal red light and to H-alpha emission nebulas, and the "i" filter is sensitive to, mostly, near infrared light.
This means that in SDSS images, clusters of young stars will look blue, emission nebulas will look green, and old stars will look yellow.
Now look at the SDSS picture of NGC 2841. As you can see, it looks "overwhelmingly yellow".
Conclusion? There
are young blue stars and emission nebulas in NGC 2841. Yes, there are. But the contribution they make to the overall light from NGC 2841 is vanishingly small.
Now take a look at the NASA/GALEX/Wikisky picture of NGC 2841, which shows the galaxy in ultraviolet light emitted by hot stars. As you can see, there are indeed some hot stars in NGC 2841.
What I find most interesting about the ultraviolet image (apart from the fact that it reveals outer relatively ultraviolet arms or rings of NGC 2841) is that it seems ro reveal remnants of an "inner ring" around the bulge of NGC 2841. Can you see it? There are two dark parallel lines on both sides of the bulge of NGC 2841. I think they might be remnants of a ring.
Ann
[quote=VictorBorun post_id=310151 time=1611468552 user_id=145500]
trying to imagine a 3d, it's easy to see dusty outskirts as wide tyre around a thin plate wheel.
The part near the core is more puzzling. There surely is a large ghostly shine bulge through which we see the far side of the arms' ends.
But what are the dusty lanes seen very close to the core due to the backlight of the core?
They look like lying strictly in the plane.
How come the dust lanes obey the flattening viscosity of the gas disk and the stellar population of bulge does not?
Is dust more glued into near-core thinning gas then the stars?
[/quote]
[float=left][img3="NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration; Acknowledgment: M. Crockett and S. Kaviraj (Oxford University, UK), R. O'Connell (University of Virginia), B. Whitmore (STScI), and the WFC3 Scientific Oversight Committee."]https://www.nasa.gov/sites/default/files/styles/full_width_feature/public/images/517541main_p1106ay_full.jpg[/img3][/float][float=right][img3="Flocculent galaxy NGC 2775. Note the sharp 'dividing line' between the dusty disk and the bulge. NASA / ESA / Hubble / J. Lee / PHANGS-HST Team / Judy Schmidt, www.geckzilla.com."]https://upload.wikimedia.org/wikipedia/commons/thumb/8/86/NASA-Galaxy-NGC-2775-20200702.jpg/1024px-NASA-Galaxy-NGC-2775-20200702.jpg[/img3][/float]
[float=right][img3="Nearby grand design galaxy M81, which is tilted to our line of sight. NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: A. Zezas and J. Huchra (Harvard-Smithsonian Center for Astrophysics)."]https://cdn.spacetelescope.org/archives/images/screen/heic0710a.jpg[/img3][/float]
I'm not absolutely sure what you're asking here, Victor, mostly because I don't speak Math. But bulges are typically very different from dusty disks.
An aspect that is interesting to me is the way the dust lanes on one side of the bulge look dark and stand out sharply, while the dust lanes on the other side of the bulge are almost invisible. This is very obvious in the picture of NGC 2841, and it can be seen in the picture of M81, too.
My guess, for what it's worth, is that the dust on one side of the bulge of a galaxy that is tilted to our line of sight is blocking light from the bulge behind it, so that it becomes a "dark nebula".
The dust on the other side of the bulge (the side that is farther away from us) is actually weakly lit up by light from the bulge, which is typically brighter than the disk outside it. This makes the dust on the side that is farther away from us "disappear".
There are two more pictures of NGC 2841 that I want to show you.
[float=left][img3="NGC 2841 through Sloan Digital Sky Survey g-r-i filters. Robert Lupton and the SDSS Consortium."]https://upload.wikimedia.org/wikipedia/commons/f/fb/NGC_2841_SDSS.jpg[/img3][/float][float=right][img3="Ultraviolet image of NGC 2841. Photo: NASA/GALEX/WikiSky."]https://upload.wikimedia.org/wikipedia/commons/5/53/NGC_2841_GALEX_WikiSky.jpg[/img3][/float]
The image on the left is an SDSS picture of NGC 2841. I like SDSS images because I find them "honest". They normally use the same filters, [url=https://www.asahi-spectra.com/opticalfilters/img/fgr_ugrizband.jpg]g-r-i[/url]. The "g" filter picks out light from blue stars, the "r" filter reacts to normal red light and to H-alpha emission nebulas, and the "i" filter is sensitive to, mostly, near infrared light.
This means that in SDSS images, clusters of young stars will look blue, emission nebulas will look green, and old stars will look yellow.
Now look at the SDSS picture of NGC 2841. As you can see, it looks "overwhelmingly yellow".
Conclusion? There [b][i]are[/i][/b] young blue stars and emission nebulas in NGC 2841. Yes, there are. But the contribution they make to the overall light from NGC 2841 is vanishingly small.
[float=left][img3="M94, a ring galaxy. Note the 'gap' between an inner and an outer ring. Photo: jhayes_tucson, https://www.cloudynights.com/topic/500480-m94-the-cats-eye-galaxy-with-a-c14-edge/."]https://www.cloudynights.com/uploads/monthly_05_2015/post-211853-0-78760500-1431921224_thumb.jpg[/img3][/float]
Now take a look at the NASA/GALEX/Wikisky picture of NGC 2841, which shows the galaxy in ultraviolet light emitted by hot stars. As you can see, there are indeed some hot stars in NGC 2841.
What I find most interesting about the ultraviolet image (apart from the fact that it reveals outer relatively ultraviolet arms or rings of NGC 2841) is that it seems ro reveal remnants of an "inner ring" around the bulge of NGC 2841. Can you see it? There are two dark parallel lines on both sides of the bulge of NGC 2841. I think they might be remnants of a ring.
Ann