APOD: Rubin's Galaxy (2021 Sep 18)

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APOD: Rubin's Galaxy (2021 Sep 18)

Post by APOD Robot » Sat Sep 18, 2021 4:06 am

Image Rubin's Galaxy

Explanation: In this Hubble Space Telescope image the bright, spiky stars lie in the foreground toward the heroic northern constellation Perseus and well within our own Milky Way galaxy. In sharp focus beyond is UGC 2885, a giant spiral galaxy about 232 million light-years distant. Some 800,000 light-years across compared to the Milky Way's diameter of 100,000 light-years or so, it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. Part of an investigation to understand how galaxies can grow to such enormous sizes, UGC 2885 was also part of An Interesting Voyage and astronomer Vera Rubin's pioneering study of the rotation of spiral galaxies. Her work was the first to convincingly demonstrate the dominating presence of dark matter in our universe.

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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Ann » Sat Sep 18, 2021 7:31 am

How did Rubin's Galaxy, UGC 2885, grow so large? Professor A. Mateur will explain! Let's begin by considering the famous Whirlpool galaxy, M51.

M51 inner arms annotated NASA ESA.png
Inner (original?) arms of M51 (marked as white) versus
the full size of the spiral arms. NASA/ESA.

I read someplace (and just because you read it doesn't make it true) that the original spiral arms of M51 were much shorter than the ones we see today, and that the arms were "drawn out" by the interactions with neighboring galaxy NGC 5195. But the impressive size of the optical spiral arms is nothing compared with the enormous tidal "radio arm" that has been drawn out of M51 as a consequence of its interaction with galaxy NGC 5195.

It seems obvious that galactic interactions can enhance the spiral structure of a galaxy. Another example of this is M100, whose outer diffuse arm seems to be connected with a small satellite galaxy, NGC 4323.

But interaction can affect spiral and elliptical galaxies quite differently:

Interacting galaxies IC 1181 at left and NGC 6050 at right in the Hercules cluster.png
Interacting galaxies IC 1178/1181 and NGC 6050 A and B in the Hercules Cluster.
Photo: ESO.

Look at the interacting pairs IC 1178/1181 at left and NGC 6050 A and B at right in the Hercules cluster. IC 1178/1181 is a pair of elliptical (or possibly lenticular) galaxies with no spiral arms or dust structures. Their interaction has spread old yellow stars in two huge clouds around them. But NGC 6050 A and B is a tight, small, well-formed pair of starforming galaxies with spiral arms.

To get a truly, truly large and well-formed spiral galaxy, we need just the right conditions.

The galaxy has to grow large perhaps through collisions.

The collisions must all enhance the spiral structure, not destroy it.

The interlopers should probably be completely absorbed by the large galaxy. We may in fact be talking about very large reservoirs of hydrogen surrounding the galaxy and raining down on it as discrete clouds.

I think, although I'm not sure, that the central black hole in the major galaxy should not grow too massive. Because if it is very massive, I believe it tends to prevent the galaxy's major feature from growing too extensive. So the collisions should feed the disk more than the core of the galaxy.

Once the spiral galaxy has grown extremely large, all major collisions must cease, so as not to disrupt the spiral structure of the galaxy. This is probably best achieved if the galaxy is located in a void.

Galactic voids tend to grow over time. If UGC 2885 originally had several neighbors and managed to absorb very many of them like a vacuum cleaner, and the space around the galaxy actually grew pretty empty because it was located in a growing void, then that might just possibly explain the galaxy's humongous size.

Ann
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Tom Fleming

Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Tom Fleming » Sat Sep 18, 2021 10:32 am

To quote today Sep 18th ...it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. So a population of 100 billion for our MW. Modern convention puts the actual number around 3-400 billion as I'm given to understand. Is this just a rounding off for a bit of dramatic presentation?
Thanks

heehaw

Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by heehaw » Sat Sep 18, 2021 10:44 am

Tom Fleming wrote: Sat Sep 18, 2021 10:32 am To quote today Sep 18th ...it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. So a population of 100 billion for our MW. Modern convention puts the actual number around 3-400 billion as I'm given to understand. Is this just a rounding off for a bit of dramatic presentation?
Thanks
Here's more on this fascinating galaxy: https://en.wikipedia.org/wiki/UGC_2885

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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Ann » Sat Sep 18, 2021 11:02 am

Tom Fleming wrote: Sat Sep 18, 2021 10:32 am To quote today Sep 18th ...it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. So a population of 100 billion for our MW. Modern convention puts the actual number around 3-400 billion as I'm given to understand. Is this just a rounding off for a bit of dramatic presentation?
Thanks
The way I understand it - and I haven't tried to look it up, mind you - is that the Milky Way has a huge halo that really pushes the number of stars belonging to our galaxy to whole new levels.

I think a figure of 100 billion stars is way too low for our galaxy.

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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by orin stepanek » Sat Sep 18, 2021 12:58 pm

RubinsGalaxy_hst1024.jpg
Does extended Galaxy refer to it's large size?
BTW very nice galaxy: Rubin!! 8-)
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by johnnydeep » Sat Sep 18, 2021 3:51 pm

Ann wrote: Sat Sep 18, 2021 11:02 am
Tom Fleming wrote: Sat Sep 18, 2021 10:32 am To quote today Sep 18th ...it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. So a population of 100 billion for our MW. Modern convention puts the actual number around 3-400 billion as I'm given to understand. Is this just a rounding off for a bit of dramatic presentation?
Thanks
The way I understand it - and I haven't tried to look it up, mind you - is that the Milky Way has a huge halo that really pushes the number of stars belonging to our galaxy to whole new levels.

I think a figure of 100 billion stars is way too low for our galaxy.

Ann
Also, the Wikipedia article for UGC 2885 cites its size as "only" 463000 ly, not 800000 ly, and that it could have grown this large, not via mergers, but the slow and steady accumulation of intergalactic gas. From https://en.wikipedia.org/wiki/UGC_2885:
UGC 2885 is classified as a field galaxy—a class of galaxies found in remote, under-dense and “vacant” sections of space, far from other major galaxies. NASA has reported that the theorized main source for disk growth for UGC 2885 came from the accretion of intergalactic hydrogen gas, rather than through the repeated process of galactic collision, as most galaxies are thought to grow.

The lack of interaction is evident from the near-perfect structure of the spiral arms and disk, lack of tidal tails, and modest rate of star formation—approximately 0.5 solar masses/year.

Additionally, despite being originally classified as an unbarred spiral galaxy, new Hubble images clearly show the presence of a small bar cutting across the ring structure of the core. This is peculiar, as most bars are thought to form through minor gravitational perturbations brought on by satellite and neighboring galaxies, which is something this galaxy lacks. This galaxy highlights that bars are able to form in spiral galaxies without the influence of another galaxy—this indicates that other forces, such as interactions between stars, gas and dust, as well as the gravitational influence of dark matter, might play a role in their development.
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DL MARTIN

Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by DL MARTIN » Sat Sep 18, 2021 4:06 pm

The image of Rubin's Galaxy is 232 million years old. Might dark matter be associated with the intervening time lapse?

Judy

Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Judy » Sat Sep 18, 2021 4:50 pm

Great picture! and thank you so much for the callout to Vera Rubin! And for the link to An Interesting Voyage! What a wonderful read.

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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Chris Peterson » Sat Sep 18, 2021 5:05 pm

DL MARTIN wrote: Sat Sep 18, 2021 4:06 pm The image of Rubin's Galaxy is 232 million years old. Might dark matter be associated with the intervening time lapse?
The image of this galaxy is a few days old.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Eclectic Man » Sat Sep 18, 2021 5:09 pm

APOD Robot wrote: Sat Sep 18, 2021 4:06 am Image Rubin's Galaxy

Explanation: In this Hubble Space Telescope image the bright, spiky stars lie in the foreground toward the heroic northern constellation Perseus and well within our own Milky Way galaxy. ....
ASIDE - NOT RELATED TO ASTRONOMY

The explanation refers to the 'heroic northern constellation Perseus'. In Natalie Haynes' excellent book on women in the Greek Myths 'Pandora's Jar' (ISBN 978-1-5098-7311-1) she points out that the myth of Perseus usually has him being accompanied and actively helped by both Athena and Hermes when he does his great deeds, and that Medusa is asleep when Perseus decapitates her. Somewhat less heroic than the Harry Hamlin film 'Clash of the Titans'.

I recommend this book for anyone interested in either Greek Myths or the history of the depictions of women and men in 'heroic' literature. Other women included in the book are Penelope, Medea, Eurydice, Helen, Helen's sister Clytemnestra, the Amazons and Buffy the Vampire Slayer! (Not kidding).

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Moldy Mary's Hunt for P. rubens

Post by neufer » Sat Sep 18, 2021 6:04 pm

Eclectic Man wrote: Sat Sep 18, 2021 5:09 pm
The explanation refers to the 'heroic northern constellation Perseus'. In Natalie Haynes' excellent book on women in the Greek Myths 'Pandora's Jar' (ISBN 978-1-5098-7311-1) she points out that the myth of Perseus usually has him being accompanied and actively helped by both Athena and Hermes when he does his great deeds, and that Medusa is asleep when Perseus decapitates her. Somewhat less heroic than the Harry Hamlin film 'Clash of the Titans'. I recommend this book for anyone interested in either Greek Myths or the history of the depictions of women and men in 'heroic' literature.
https://mcdreeamiemusings.com/blog/2019/8/11/1013vvme5498w77bglwoh5ck4exowx wrote:
Mouldy Mary and the Cantaloupe
History of Medicine : January 24, 2020

<<It’s a well known story and example of medical serendipity. Alexander Fleming (1881-1955) a Scottish microbiologist who returned to his laboratory following his summer holiday and found his growth plates of Staphylococcal bacteria had been contaminated with mould. Wherever the mould was growing the bacterial cells had been killed. When it came to penicillin Fleming’s discovery was only the beginning. And the penicillin still in use today owes much to an unsung hero called Mary and a mouldy cantaloupe... >>
https://www.scientificamerican.com/article/illinois-announces-official-state-microbe/ wrote:
Illinois Announces Official State Microbe
Scientific American: History of Medicine : January 24, 2020

<<[Andrew] Moyer's 1944 publication on P. rubens mentions [Mary] Hunt only in the paper's acknowledgments, and the press referred to her as “Moldy Mary.” P. rubens could better tolerate a new fermentation process that let it quickly produce hundreds of times more penicillin than previously studied strains, which let the Allies massively scale up antibiotic production. The same strain is still used to manufacture penicillin today, Price says. “To think—a moldy cantaloupe from a fruit market in Peoria revolutionized the medical field for millions of people,” says State Representative Stephanie Kifowit, who co-sponsored the Illinois House version of the P. rubens bill.>>
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by johnnydeep » Sat Sep 18, 2021 7:04 pm

orin stepanek wrote: Sat Sep 18, 2021 12:58 pm RubinsGalaxy_hst1024.jpg

Does extended Galaxy refer to it's large size?
...
It seems so. From the link https://ui.adsabs.harvard.edu/abs/2017h ... H/abstract
UGC 2885 was discoverd to be the most extended disk galaxy [250 kpc diameter] by Vera Rubin in the 1980's. We ask for HST observations of UGC 2885 as it is close enough to resolve the GC population with HST but it is a substantially more extended disk than any studied before.
BTW, this 2017 reference at least agrees with the 800 kly diameter figure in the APOD text (250 kpc * 3.26 = 815 kly). The Wikipedia article quotes a diameter of only 463 kly based on a reference to several sources but I can't find that value actually confirmed in any of them. Personally, I like the larger value since it increases the pure awesomeness of Rubin's Galaxy!
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Chris Peterson » Sat Sep 18, 2021 7:24 pm

johnnydeep wrote: Sat Sep 18, 2021 7:04 pm
orin stepanek wrote: Sat Sep 18, 2021 12:58 pm RubinsGalaxy_hst1024.jpg

Does extended Galaxy refer to it's large size?
...
It seems so. From the link https://ui.adsabs.harvard.edu/abs/2017h ... H/abstract
UGC 2885 was discoverd to be the most extended disk galaxy [250 kpc diameter] by Vera Rubin in the 1980's. We ask for HST observations of UGC 2885 as it is close enough to resolve the GC population with HST but it is a substantially more extended disk than any studied before.
BTW, this 2017 reference at least agrees with the 800 kly diameter figure in the APOD text (250 kpc * 3.26 = 815 kly). The Wikipedia article quotes a diameter of only 463 kly based on a reference to several sources but I can't find that value actually confirmed in any of them. Personally, I like the larger value since it increases the pure awesomeness of Rubin's Galaxy!
Between uncertainties in distance and the lack of any well defined edge, I'm skeptical of the claimed diameters of any galaxies. I'm not sure it's even that meaningful of a concept in many cases.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by johnnydeep » Sat Sep 18, 2021 7:34 pm

Chris Peterson wrote: Sat Sep 18, 2021 7:24 pm
johnnydeep wrote: Sat Sep 18, 2021 7:04 pm
orin stepanek wrote: Sat Sep 18, 2021 12:58 pm RubinsGalaxy_hst1024.jpg

Does extended Galaxy refer to it's large size?
...
It seems so. From the link https://ui.adsabs.harvard.edu/abs/2017h ... H/abstract
UGC 2885 was discoverd to be the most extended disk galaxy [250 kpc diameter] by Vera Rubin in the 1980's. We ask for HST observations of UGC 2885 as it is close enough to resolve the GC population with HST but it is a substantially more extended disk than any studied before.
BTW, this 2017 reference at least agrees with the 800 kly diameter figure in the APOD text (250 kpc * 3.26 = 815 kly). The Wikipedia article quotes a diameter of only 463 kly based on a reference to several sources but I can't find that value actually confirmed in any of them. Personally, I like the larger value since it increases the pure awesomeness of Rubin's Galaxy!
Between uncertainties in distance and the lack of any well defined edge, I'm skeptical of the claimed diameters of any galaxies. I'm not sure it's even that meaningful of a concept in many cases.
True. The longer the photographic exposure, the more the dimmer star stuff starts appearing at the outer edges of any galaxy, thereby increasing its apparent size. I think we need a definition of diameter something like: the minimal extent within which 95% (say) of the mass is concentrated.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Chris Peterson » Sat Sep 18, 2021 8:00 pm

johnnydeep wrote: Sat Sep 18, 2021 7:34 pm
Chris Peterson wrote: Sat Sep 18, 2021 7:24 pm
johnnydeep wrote: Sat Sep 18, 2021 7:04 pm

It seems so. From the link https://ui.adsabs.harvard.edu/abs/2017h ... H/abstract



BTW, this 2017 reference at least agrees with the 800 kly diameter figure in the APOD text (250 kpc * 3.26 = 815 kly). The Wikipedia article quotes a diameter of only 463 kly based on a reference to several sources but I can't find that value actually confirmed in any of them. Personally, I like the larger value since it increases the pure awesomeness of Rubin's Galaxy!
Between uncertainties in distance and the lack of any well defined edge, I'm skeptical of the claimed diameters of any galaxies. I'm not sure it's even that meaningful of a concept in many cases.
True. The longer the photographic exposure, the more the dimmer star stuff starts appearing at the outer edges of any galaxy, thereby increasing its apparent size. I think we need a definition of diameter something like: the minimal extent within which 95% (say) of the mass is concentrated.
It's analogous to measuring beam diameters in optics and radio technology. Beams don't typically have sharp edges. And there are well defined criteria for assigning diameters, like 1/e² or full width at half maximum (FWHM), the latter being very common for measuring stellar image diameters.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Ann » Sat Sep 18, 2021 8:31 pm


Take a look at the above image of galaxy NGC 4565 in red light, 6660 Å (666 nm).

It sure looks as if the bright optical disk of this galaxy has an edge, albeit a diffuse one.

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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Chris Peterson » Sat Sep 18, 2021 8:40 pm

Ann wrote: Sat Sep 18, 2021 8:31 pm

Take a look at the above image of galaxy NGC 4565 in red light, 6660 Å (666 nm).

It sure looks as if the bright optical disk of this galaxy has an edge, albeit a diffuse one.

Ann
Which is what a criterion like I referenced above would allow a number to be assigned to. But in fact, some or most galaxies do have a large halo of gravitationally bound stars around them, with a much lower density than the more visible galaxy. So how does that get measured and quantified? It's kind of like our own Solar System. At its extreme, it may extend two light years- meaning that there are probably gravitationally bound objects orbiting the Sun at that distance. Where do we draw the line, though, in defining its size?
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by orin stepanek » Sat Sep 18, 2021 9:34 pm

johnnydeep wrote: Sat Sep 18, 2021 7:04 pm
orin stepanek wrote: Sat Sep 18, 2021 12:58 pm RubinsGalaxy_hst1024.jpg

Does extended Galaxy refer to it's large size?
...
It seems so. From the link https://ui.adsabs.harvard.edu/abs/2017h ... H/abstract
UGC 2885 was discoverd to be the most extended disk galaxy [250 kpc diameter] by Vera Rubin in the 1980's. We ask for HST observations of UGC 2885 as it is close enough to resolve the GC population with HST but it is a substantially more extended disk than any studied before.
BTW, this 2017 reference at least agrees with the 800 kly diameter figure in the APOD text (250 kpc * 3.26 = 815 kly). The Wikipedia article quotes a diameter of only 463 kly based on a reference to several sources but I can't find that value actually confirmed in any of them. Personally, I like the larger value since it increases the pure awesomeness of Rubin's Galaxy!
plus one! +1 8-)
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Ann » Sun Sep 19, 2021 3:56 am

Chris Peterson wrote: Sat Sep 18, 2021 8:40 pm
Ann wrote: Sat Sep 18, 2021 8:31 pm

Take a look at the above image of galaxy NGC 4565 in red light, 6660 Å (666 nm).

It sure looks as if the bright optical disk of this galaxy has an edge, albeit a diffuse one.

Ann
Which is what a criterion like I referenced above would allow a number to be assigned to. But in fact, some or most galaxies do have a large halo of gravitationally bound stars around them, with a much lower density than the more visible galaxy. So how does that get measured and quantified? It's kind of like our own Solar System. At its extreme, it may extend two light years- meaning that there are probably gravitationally bound objects orbiting the Sun at that distance. Where do we draw the line, though, in defining its size?
I don't question for a moment that many or most galaxies have a surrounding halo, but I do think that there is a difference between the populations in the visible disk and the stellar populations in the halo.


There should be a way to talk about the (approximate) size of a galaxy's (reasonably visible) disk (or reasonably visible elliptical body), while allowing for the possibility that the galaxy may possess a huge halo that boosts both its size and its total luminosity by at least a magnitude.

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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by alter-ego » Sun Sep 19, 2021 4:12 am

Chris Peterson wrote: Sat Sep 18, 2021 8:00 pm
johnnydeep wrote: Sat Sep 18, 2021 7:34 pm
Chris Peterson wrote: Sat Sep 18, 2021 7:24 pm

Between uncertainties in distance and the lack of any well defined edge, I'm skeptical of the claimed diameters of any galaxies. I'm not sure it's even that meaningful of a concept in many cases.
True. The longer the photographic exposure, the more the dimmer star stuff starts appearing at the outer edges of any galaxy, thereby increasing its apparent size. I think we need a definition of diameter something like: the minimal extent within which 95% (say) of the mass is concentrated.
It's analogous to measuring beam diameters in optics and radio technology. Beams don't typically have sharp edges. And there are well defined criteria for assigning diameters, like 1/e² or full width at half maximum (FWHM), the latter being very common for measuring stellar image diameters.
I think an image or link should be referenced for to back up that 800-kly size. For example, maybe it has to do with a measured globular cluster distribution. I don't like the fact the stated size is ~2.5x larger than the size of the presented Hubble image. At least state the visible size in the APOD image (~320,000ly), then state the extended size and what measurements it's based on, with the appropriate links. Even lacking a formal diameter definition, I'd be satisfied with any image, or data plot that could accurately show a very faint fuzz-ball surrounding the galaxy 2.5x the displayed visible size. Also, in order to claim a relative size, the same data must exist in some form for the MW.
Yeah, I'm not happy with the APOD description here. To me, it's misleading considering the presented image.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by VictorBorun » Sun Sep 19, 2021 7:59 am

johnnydeep wrote: Sat Sep 18, 2021 3:51 pm
Ann wrote: Sat Sep 18, 2021 11:02 am
Tom Fleming wrote: Sat Sep 18, 2021 10:32 am To quote today Sep 18th ...it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. So a population of 100 billion for our MW. Modern convention puts the actual number around 3-400 billion as I'm given to understand. Is this just a rounding off for a bit of dramatic presentation?
Thanks
The way I understand it - and I haven't tried to look it up, mind you - is that the Milky Way has a huge halo that really pushes the number of stars belonging to our galaxy to whole new levels.

I think a figure of 100 billion stars is way too low for our galaxy.

Ann
Also, the Wikipedia article for UGC 2885 cites its size as "only" 463000 ly, not 800000 ly, and that it could have grown this large, not via mergers, but the slow and steady accumulation of intergalactic gas. From https://en.wikipedia.org/wiki/UGC_2885:
UGC 2885 is classified as a field galaxy—a class of galaxies found in remote, under-dense and “vacant” sections of space, far from other major galaxies. NASA has reported that the theorized main source for disk growth for UGC 2885 came from the accretion of intergalactic hydrogen gas, rather than through the repeated process of galactic collision, as most galaxies are thought to grow.

The lack of interaction is evident from the near-perfect structure of the spiral arms and disk, lack of tidal tails, and modest rate of star formation—approximately 0.5 solar masses/year.

Additionally, despite being originally classified as an unbarred spiral galaxy, new Hubble images clearly show the presence of a small bar cutting across the ring structure of the core. This is peculiar, as most bars are thought to form through minor gravitational perturbations brought on by satellite and neighboring galaxies, which is something this galaxy lacks. This galaxy highlights that bars are able to form in spiral galaxies without the influence of another galaxy—this indicates that other forces, such as interactions between stars, gas and dust, as well as the gravitational influence of dark matter, might play a role in their development.
other forces sound hazy.
Then again even tidal kind of bar's origin is hazy

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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by johnnydeep » Sun Sep 19, 2021 3:48 pm

alter-ego wrote: Sun Sep 19, 2021 4:12 am
Chris Peterson wrote: Sat Sep 18, 2021 8:00 pm
johnnydeep wrote: Sat Sep 18, 2021 7:34 pm

True. The longer the photographic exposure, the more the dimmer star stuff starts appearing at the outer edges of any galaxy, thereby increasing its apparent size. I think we need a definition of diameter something like: the minimal extent within which 95% (say) of the mass is concentrated.
It's analogous to measuring beam diameters in optics and radio technology. Beams don't typically have sharp edges. And there are well defined criteria for assigning diameters, like 1/e² or full width at half maximum (FWHM), the latter being very common for measuring stellar image diameters.
I think an image or link should be referenced for to back up that 800-kly size. For example, maybe it has to do with a measured globular cluster distribution. I don't like the fact the stated size is ~2.5x larger than the size of the presented Hubble image. At least state the visible size in the APOD image (~320,000ly), then state the extended size and what measurements it's based on, with the appropriate links. Even lacking a formal diameter definition, I'd be satisfied with any image, or data plot that could accurately show a very faint fuzz-ball surrounding the galaxy 2.5x the displayed visible size. Also, in order to claim a relative size, the same data must exist in some form for the MW.
Yeah, I'm not happy with the APOD description here. To me, it's misleading considering the presented image.
Just trying to clarify some numbers. The link to the original Hubble image states the image width as 5 arc minutes and a distance of 232 Mly.

Here's that pic, with my own annotations, and an estimated size of the galaxy as at least 4 and perhaps as much as 4.3 of the scale lines (at 1 arc minute each), or 270 Kly - 290 Kly in diameter:

UGC2885 - Rubin's Galaxy.JPG

So, assuming the distance of 232 Mly is correct, then yes, there is a HUGE, almost 2.75x discrepancy between the two sizes, either ~290 Kly, or ~800 Kly.
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johnnydeep
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by johnnydeep » Sun Sep 19, 2021 4:19 pm

Let me add one more highly questionable "source" for a 800+ Kly diameter. From http://www.poyntsource.com/Richard/ugc_2885.htm
Type = Sc Spiral Galaxy in Perseus
RA = 3h 53.5m 2.5s DEC = +35° 35' 18'' Distance ~ 96 Mpc Diameter ~ 250 kpc
m = +13.5 Apparent size = 5.5'
Mass = 2 x 1012 M¤ (Milky Way = 5 x 1011 M¤ ) Redshift = 5,800 km/sec, not unusual

The spiral galaxy UGC 2885 is the largest known spiral galaxy being some 250,000 parsecs in size (815,000 light years). It is some 10 times larger than the Milky Way ! The diagram below shows the comparison of UGC 2885's size vs. some other well known galaxies.

This makes UGS 2885 appear comically large, and almost certainly incorrect since it makes the visible disk itself appear to be over 800 Kly in diameter! PS - that seemingly second galactic nucleus is in fact the very brightest foreground star in the APOD!

It's unclear where they got the source for the size, but one size is listed in this rather old - pre Hubble - reference from 1993.

"Canzian, B. Allen, R.J., Tilanus, R.P.J., 1993, Spiral Structure of the Giant Galaxy UGC 2885: Hά Kinematics, Astrophysical Journal, 406, p. 457."

Which I found at https://ui.adsabs.harvard.edu/abs/1993A ... C/abstract, or https://www.researchgate.net/publicatio ... kinematics, in which it states a Holmberg radius:
The velocity field of the exceptionally large Sc galaxy UGC 2885 (Holmberg radius 84 h^-1^ kpc, where h = H_0_/100 km s^-1^ Mpc^-1^) has been mapped in Hα emission with the TAURUS I imaging Fabry-Perot spectrometer using the 2.5 m telescope at La Palma. The rotation curve extracted from the velocity field agrees with published data. Ripples in the velocity field around the minor axis indicate radial flows across the spiral arms. The radial flow speeds in the plane of the disk show 50-70 km s^-1^ peak-to-peak variation, suggesting that a strong density wave is present in the underlying stellar disk. Such high speeds may alternatively be a natural consequence of the open arm spiral pattern. In addition, strong density waves may naturally occur in large spiral galaxies or in spiral galaxies as massive as UGC 2885. (Its mass is over 10^12^ h^-1^ M_sun_ within the radius to which spiral arms reach, 52 h^-1^ kpc.) A strong density wave may also be necessary for the effective maintenance of the orderly, two-armed spiral pattern that is visible in the outer disk of UGC 2885, where the gas has made only about a dozen revolutions in a Hubble time.
So, the Holmberg diameter would be 168 kpc, or 547 kly. Now, what's a Holmberg radius? From http://astro.vaporia.com/start/holmbergradius.html
The Holmberg radius of a galaxy (RH or RHO) is a measure of its radius (along the semi-major axis) based upon surface brightness. It is specifically the radius to the region of the galaxy's surface where the surface brightness has an apparent magnitude of 26.5 per square arcsecond through a B filter. In other words, that size area of that amount of surface brightness would provide light equivalent to a star of such magnitude. Since surface brightness generally does not decline with distance, the radius generally indicates the same portion of the galaxy, no matter how distant.
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Ann
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Ann » Sun Sep 19, 2021 7:25 pm

Those galaxy sizes from http://www.poyntsource.com/Richard/ugc_2885.htm must be wrong. There is no way that M51 and M104 are bigger in size than the Milky Way.

Ann
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