APOD: Arp 286: Trio in Virgo (2022 Jun 10)

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APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by APOD Robot » Fri Jun 10, 2022 4:05 am

Image Arp 286: Trio in Virgo

Explanation: This colorful telescopic field of view features a trio of interacting galaxies almost 90 million light-years away, toward the constellation Virgo. On the right two spiky, foreground Milky Way stars echo the extragalactic hues, a reminder that stars in our own galaxy are like those in distant island universes. With sweeping spiral arms and obscuring dust lanes, the dominant member of the trio, NGC 5566, is enormous, about 150,000 light-years across. Just above it lies smaller, bluish NGC 5569. Near center a third galaxy, NGC 5560, is apparently stretched and distorted by its interaction with massive NGC 5566. The trio is also included in Halton Arp's 1966 Atlas of Peculiar Galaxies as Arp 286. Of course, such cosmic interactions are now appreciated as part of the evolution of galaxies.

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by Ann » Fri Jun 10, 2022 6:02 am

Another portrait of the galactic trio of Arp 286 was the APOD on August 13, 2010, and it's interesting to compare the two APODs.


North is up in Nicolas Rolland's image, but not in Stephen Leshin's.

In Nicolas Rolland's image, you can clearly see a bridge of stars between elongated galaxy NGC 5560 and large galaxy NGC 5566. There is even a tidal "flow" of stars from the "lower" (southern) part of the disk of NGC 5566 towards NGC 5560. I can't make out the bridge of stars between NGC 5566 and NGC 5560 in Stephen Leshin's image.

Note the great color difference between these two images. In Nicolas Rolland's image, all three galaxies are more or less the same color, and the main difference is that the small round galaxy, NGC 5569, displays a few diffuse flecks of blue in its arm system, and it is not so brightly orange in its center.

But take a look at Stephen Leshin's image! NGC 5569 is cornflower-blue all over and polka-dotted with little white spots of star clusters in its arms. Elongated galaxy NGC 5560 has an orange center and elongated blue arms, and at least one arm is well populated with star clusters. Large ringed and two-armed galaxy NGC 5566 is almost all yellow, but it does display a string of pearls of clusters in its ring and its two long arms. Note in Stephen Leshin's image an obvious bright pink dot near the "top" of the "top arm" of NGC 5566. This pink dot is particularly obvious in the larger version of the picture.

The pink dot is a bright emission nebula, where massive stars are being born. So even in this overwhelmingly yellow galaxy there is room for bright young stars.

Clearly the colors in Stephen Leshin's image are very saturated, but they do bring out interesting facts about the three galaxies. I checked the color indexes for NGC 5566, the large galaxy, and NGC 5560, the elongated galaxy. The overall color of NGC 5566 is as yellow as that of many elliptical galaxies, but there is still star formation there, as we have seen. NGC 5560 is noticeably bluer than its large neighbor, and in particular, it is more ultraviolet. The third galaxy, NGC 5569, is too small and faint to have had its colors measured.

It is interesting that large galaxies are often quite yellow, even if they are still forming stars. Andromeda is even yellower and even less ultraviolet than NGC 5566.


And yet, even Andromeda is still forming stars.

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by MikeA » Fri Jun 10, 2022 6:47 am

I understand the spiky stars are in the Milky Way, but what about the others? Some look a bit hazy which I suppose are more distant galaxies.

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by Ann » Fri Jun 10, 2022 9:03 am

MikeA wrote: Fri Jun 10, 2022 6:47 am I understand the spiky stars are in the Milky Way, but what about the others? Some look a bit hazy which I suppose are more distant galaxies.
This is a part of the background sky (at upper left) in the APOD. I have identified some of the dots as stars (S) and others as galaxies (G).

Background stars galaxies in APOD 10 June 2022.png

Most of the dots in the APOD are stars. Very faint dots are more likely to be galaxies.

Ann
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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by MikeA » Fri Jun 10, 2022 9:46 am

Thanks Ann.

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by kybergreg » Fri Jun 10, 2022 9:52 am

Hello,
after clicking the image I get this error:

Not Found
The requested URL /apod/image/2206/Arp286-202203-CDK24-FLIPL9000-LRGB_NicolasROLLAND_signature_LD2048.jpg was not found on this server.

Looks like a possible typo in the link, but it is only me getting this error then the problem is somewhere else.

JIRKA

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by gmPhil » Fri Jun 10, 2022 10:03 am

@JIRKA - it's not only you.

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by orin stepanek » Fri Jun 10, 2022 11:38 am

Arp286-202203-CDK24-FLIPL9000-LRGB_NicolasROLLAND_signature_LD1024.jpg
Chomp time;still it's going to take awhile to accomplish! :mrgreen: It is a
beautiful trio!
arp87web_c800.jpg
ARP87 A lot of ARP's in the ARP catalog! :D
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Smile today; tomorrow's another day!

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by he-sk » Fri Jun 10, 2022 12:12 pm

kybergreg wrote: Fri Jun 10, 2022 9:52 am Hello,
after clicking the image I get this error:

Not Found
The requested URL /apod/image/2206/Arp286-202203-CDK24-FLIPL9000-LRGB_NicolasROLLAND_signature_LD2048.jpg was not found on this server.

Looks like a possible typo in the link, but it is only me getting this error then the problem is somewhere else.

JIRKA
This link works: https://apod.nasa.gov/apod/image/2206/A ... ure_LD.jpg

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by RJN » Fri Jun 10, 2022 1:43 pm

kybergreg wrote: Fri Jun 10, 2022 9:52 am Hello,
after clicking the image I get this error:

Not Found
The requested URL /apod/image/2206/Arp286-202203-CDK24-FLIPL9000-LRGB_NicolasROLLAND_signature_LD2048.jpg was not found on this server.

Looks like a possible typo in the link, but it is only me getting this error then the problem is somewhere else.

JIRKA
Thanks! Fixed it. - RJN

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by beryllium732 » Fri Jun 10, 2022 6:41 pm

The bluish NGC 5569 is it because it has moore O-type stars and more hydrogen gas clouds?

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by Ann » Sat Jun 11, 2022 7:09 am

beryllium732 wrote: Fri Jun 10, 2022 6:41 pm The bluish NGC 5569 is it because it has moore O-type stars and more hydrogen gas clouds?

That's a good question, and I should have tried to answer it sooner.

The overall population of NGC 5569 is certainly different from its big bright neighbor, NGC 5566.


We can be sure that a large bright yellow galaxy like NGC 5566 contains huge numbers of yellow stars. We know that there must be so many of them because yellow stars are on average quite faint, so it takes huge numbers of them to produce bright yellow light. Therefore, if a galaxy is both bright and yellow, we know that it is very star-rich and very massive.

NGC 5569 is faint and bluish (even though it is not nearly as cornflower-blue as it appears to be in Stephen Leshin's image, because no galaxies are). And because NGC 5569 is faint and bluish, we know that it is relatively poor in stars overall as galaxies go. We also know that, relative to bright yellow NGC 5566, small bluish NGC 5569 is particularly poor in yellow stars. Even so, NGC 5569 most definitely contains more yellow than blue stars!

How can that be? How can a bluish galaxy contain more yellow than blue stars? The answer is that on average, blue stars are (much) brighter than yellow stars.


Take a look at the above diagram of all 449 stars within 49 light-years of the Sun. The stars are ordered in such a way that the brighter stars are at top of the diagram and the fainter ones at bottom, and the bluer stars are at left and the redder ones at right. As you can see, most of the stars fall on a diagonal line running from lower right to upper left. The stars on this line are main sequence stars like the Sun, which means that they shine by fusing hydrogen to helium in their cores.

The reason why some main sequence stars are blue and bright and very many main sequence stars are red and faint is because the blue stars are massive, and the red stars are low in mass. Massive stars fuse their hydrogen at a furious rate and become very hot and blue, whereas the low-mass stars burn their hydrogen painfully slowly and stay comparatively very cool and red.

A few stars in the diagram are located off the main sequence to the upper right of it. These stars have run out of hydrogen in their cores and support themselves by other means of fusion, and they are typically red or yellow and relatively bright.

A few blue stars are seen to the lower left. They are white dwarfs, and they are extremely faint.

As you can see, there are huge numbers of small red stars. The small red stars vastly outnumber all other stars, but they provide very little light, because they are so faint.

The brightest stars, on average, are the blue stars, the ones at upper left. Note that there are very few of them. As you can see, too, the bluest stars in the diagram are not very blue. They are made to look very blue in the diagram, but the blue dots mostly represent A-type stars. There are certainly no O-type stars within 49 light-years of the Sun, and there are no early-type B-type stars, either. At best there might be a B9-type star within 49 light-years of the Sun, but I can't think of what star that would be.


Blue stars are rare for the following reasons:

1) There are always more low-mass (faint non-blue) than high-mass (bright blue) stars born in every burst of star formation.

NGC 602.png
Bright star cluster in NGC 602 in the Small Magellanic Cloud.
Note the smallish number of bright stars and the larger number of fainter stars.


2) Blue stars are short-lived, and that goes especially for O-type stars. Bright cluster NGC 3293 was undoubtedly born with O-type stars. Now these stars have used up the hydrogen in their cores and evolved into giants. Most of the brightest stars are still blue, but one has turned into a cool red supergiant.


3) The most massive blue stars eventually disappear completely by exploding as supernovas.


4) After the brightest blue stars have exploded as supernovas, the less massive blue stars gradually turn yellow and red and eventually become faint white dwarfs.


In rich open cluster M11, the O-type stars have died long ago, and most of the B-type stars are gone, too. Possibly a few stars of spectral type B8 or B9 remain.

This is my point. For a galaxy to look bluish, it must have formed rather a lot of stars quite recently. Because that is the only time when there will be enough blue stars for the galaxy to look blue.

However: If there are a lot of O-type stars in a galaxy, there will also be several large bright nebulas.


Take a look at the nebulas in Orion:


There are at least five O-type stars in Orion: Theta 1-C Orionis, which powers the Orion nebula, Iota Orionis, just south of the Orion Nebula, Sigma Orionis, which provides the red background for the Horsehead Nebula, Altnitak of Orion's Belt, which powers the Flame Nebula, and Meissa, Lambda Orionis, which ionizes the Lambda Orionis nebula.

O-type stars and nebulas almost always go together. Not absolutely always, because it happens that a nebula disperses before the O-type star or stars inside it all evolve off the main sequence and become cooler. But as a rule, there are nebulas where there are O-type stars. Therefore, if we don't see obvious nebulas in a galaxy, there are probably not a lot of O-type stars there, either.

Let's look for pink emission nebulas - starforming nebulas, where bright blue stars are born - in Stephen Leshin's APOD from 2010:

Pink nebulas in APOD 13 August 2010 Stephen Leshin.png

As you can see, there are a few pink nebulas in large yellow galaxy NGC 5566, and there is at least one bright pink nebula in NGC 5560. But no pink nebulas can be seen in NGC 5569.

There is another thing that is present in NGC 5566 and NGC 5560, and that is obvious dark dust lanes. Stars are typically born from dusty gas clouds, where the dust helps the gas to cool down and contract, so that it can start forming stars. We see no obvious dust clouds in NGC 5569.

Conclusion:

There are undoubtedly good numbers of "lesser blue stars" of spectral classes A and late B in NGC 5569. There are also quite a lot of star clusters in NGC 5569, because we can see them. They look like white dots in Stephen Leshin's image. But there is not a lot of ongoing star formation, so there is not a lot of O-type stars in NGC 5569.


I would like to add that, yes, I do think that there are nebulas and O-type stars in NGC 5569. In fact, I'm sure of it. The presence of so many obvious young clusters in NGC 5560 virtually guarantees that star formation is still going on there, and that an O-star or two are still being born. It's just that the nebulas are too small and faint to show up in Stephen Leshin's image.

Ann
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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by AVAO » Sun Jun 12, 2022 10:16 am

Ann wrote: Sat Jun 11, 2022 7:09 am Conclusion:

There are undoubtedly good numbers of "lesser blue stars" of spectral classes A and late B in NGC 5569. There are also quite a lot of star clusters in NGC 5569, because we can see them. They look like white dots in Stephen Leshin's image. But there is not a lot of ongoing star formation, so there is not a lot of O-type stars in NGC 5569.

I would like to add that, yes, I do think that there are nebulas and O-type stars in NGC 5569. In fact, I'm sure of it. The presence of so many obvious young clusters in NGC 5560 virtually guarantees that star formation is still going on there, and that an O-star or two are still being born. It's just that the nebulas are too small and faint to show up in Stephen Leshin's image.

Ann
Thanks Ann for this incredibly informative and lengthy post before!
(I can't contribute that much on the subject, since I'm not an expert like you.)

But what really amazes me is that in so many galaxy families, the youngest child "shines in blue".
This could be an interesting research topic...

AVAO

Image
The Seyfert's Sextet (APOD 2013 December 10)

Image
Stephan's Quintet (APOD 2012 February 25)

Image
Robert's Quartet (ESO: https://www.eso.org/public/images/eso0535a/)

... and just for fun a deep dive to the core of NGC 5566:

Image
Original data source: ESO

Image
https://live.staticflickr.com/65535/521 ... 1700_h.jpg
Original data source: NASA HST Jac Berne (flickr)

Image
[url]https://live.staticflickr.com/65535/521 ... da73_h.jpg
Original data source: NASA HST Jac Berne (flickr)

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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by Ann » Sun Jun 12, 2022 5:52 pm

Thanks, AVAO, I'm very glad you appreciate it! :D

And thanks for posting that closeup of the core of NGC 5566. That's amazing! Does it have two cores? Two supermassive black holes? I could find no information of that on the internet, but it sure looks like there are two hungry monsters at the center of NGC 5566!

You wrote:
But what really amazes me is that in so many galaxy families, the youngest child "shines in blue".
I don't think that the small blue galaxies are necessarily younger than the big yellow ones, but they have evolved quite differently.

I believe that the galaxies that later grew very big probably started out bigger than most other galaxies, even way back when galaxies first started forming. The ones that later grew so large probably formed from a slightly larger chunk of gas and dark matter than most other galaxies. Back in those days, the Universe was (comparatively) tiny and awash in gas, and those early galaxies must have slurped up huge helpings of gas that kept feeding their star formation.


According to an astrobite from 2016, star formation peaked when the Universe was 3.5 billion years old, and its star formation has been winding down ever since. I'm pretty sure I have heard that the star formation of the Universe peaked even earlier, but you get my point: For a while there, hydrogen gas was there for the taking, and galaxies could just grab it and convert the gas into stars. Until too much of the gas had been locked up inside legions of stingy little red dwarf stars that are surprisingly massive for their puny appearance, and which hoard their (somewhat) meager share of gas until the end of time - or at least, until the Universe has doubled or tripled or quadrupled or... (you get it) - its present age!

So after a while galaxies generally stopped growing by simply acquiring free-floating gas from their vicinity. Instead, they started growing by merging with other galaxies, and by cannibalizing smaller galaxies. There was a great picture posted lately that demonstrated how it works:


In this interacting pair, the big galaxy is stealing stars and gas from the small galaxy. That is always how it works, isn't it? The big guy wins over the small guy?

As you can see in the Hubble image, the smaller component of Arp 282, IC 1559, looks bluish. So how blue is it really? Not too blue, it turns out. Its color is almost exactly the same as the color of dwarf galaxy Leo 1, which is seen very close to first magnitude blue star Regulus in the sky:



IC 1559 and Leo 1 have almost identical B-V indexes, which is a measure of how blue a galaxy is. For Leo 1, the B-V is +0.72, which means that its overall color is somewhere between "neutral" and "yellowish". Leo 1 is a dwarf galaxy completely devoid of young blue stars, but its stars are a little bluer than we would expect from their spectral classes due to the fact that they are extremely metal-poor.
Wikipedia wrote:

Typical to a dwarf galaxy, the metallicity of Leo I is very low, only one percent that of the Sun. Gallart et al. (1999) deduce from Hubble Space Telescope observations that the galaxy experienced a major increase (accounting for 70% to 80% of its population) in its star formation rate between 6 Ga and 2 Ga (billion years ago). There is no significant evidence of any stars that are more than 10 Ga old. About 1 Ga ago, star formation in Leo I appears to have dropped suddenly to an almost negligible rate, roughly coinciding with its latest periastron passage of the Milky Way. Ram pressure stripping would have removed its gas, decreasing its star formation rate.
So Leo 1 lost its gas when it passed close to the Milky Way, and then it stopped forming stars! But the stars it has got are very metal-poor, which makes their color bluer than it would otherwise have been. Do note that puny little Leo 1 has a central black hole of 3 million solar masses, almost as big as Sgr A*, the central black hole of our own big bully of a galaxy! :shock:

Anyway. The B-V index of Leo 1 is +0.72, whereas the B-V index of IC 1559 is +0.74! Their overall colors are almost identical! We can tell at a glance that IC 1559 can't be very blue, because it has very few clusters. And soon it will stop forming stars altogether, because NGC 169 is stealing gas from IC 1559, which powers its own star formation.

Why, then, do we see small blue galaxies in groups where the other galaxies are much yellower and bigger? My answer is that the small blue galaxies have only quite recently entered the group. They have not yet had their gas stolen from them by the big yellow bullies, but their gas has been "stirred" in a way that favors star formation.

Let's look at Stephan's Quintet:

Wikipedia wrote:
The brightest member of the visual grouping (and the only non-member of the true group) is NGC 7320, which has extensive H II regions, identified as red blobs, where active star formation is occurring...

NGC 7320 indicates a small redshift (790 km/s) while the other four exhibit large redshifts (near 6,600 km/s). Since galactic redshift is proportional to distance, NGC 7320 is only a foreground projection and is ~39 million light-years from Earth, making it a possible member of the NGC 7331 group, versus the 210–340 million light-years of the other four.
So NGC 7320 is a possible member of the NGC 7331 group. Let's look at a picture of NGC 7320 together with NGC 7331:


NGC 7331 is at upper right in this ultraviolet image, whereas Stephan's Quintet with non-member NGC 7320 is a lower left. Note the perfect little oval of NGC 7320, evidence of the fact that the star formation here is not violent enough to disturb the overall shape of NGC 7320. It is however sufficiently close to NGC 7331 to have "felt its presence" and to have had its gas "stirred" so it was encouraged to start forming stars.

Also note the ultraviolet "wreckage" of the true members of Stephan's Quintet. The interaction of the members, as well as the actual collision between NGC 7318 A and NGC 7318 B, has thrown out enormous tendrils of gas where great numbers of huge young star clusters are being born.

Much of the gas that has been thrown out of the true members of Stephan's Quintet will fall back onto the galaxies again, but not all of it. And as the galaxies keep interacting and colliding, their gas will eventually either be "cast out" so far away that it won't fall back, or it will be swallowed by the central black holes that are undoubtedly building up inside these galaxies, or it will just generally become so turbulent as well as "thinned out" that it will no longer be able to form any new stars.

Why are large galaxies typically so yellow? That is because they have undergone burst after burst of star formation during their long history of mergers and cannibalizations of other galaxies. Each burst of star formation leaves behind very large numbers of small red and yellow stars. The bright blue stars that are formed in the same burst of star formation die quickly, but the long-lived red and yellow stars stay on and keep building up the large galaxies' hoard of small stars.

So, to summarize. Small blue galaxies are blue because they have quite recently started forming stars, and they have not undergone sufficient numbers of bursts of star formation to have built up huge numbers of long-lived red and yellow stars.

Many, and I'd say most, small galaxies are not forming stars. They are little yellow - make that pale yellow-white - conglomerates of old metal-poor stars, similar to dwarf galaxy Leo 1. But some small galaxies have received the "push" (or whatever) that they needed to start forming stars. Suddenly they start forming blue stars all over.

An example of such a galaxy is the Large Magellanic Cloud. According to a book of mine, The Galaxies of the Local Group by Sidney van den Bergh, the LMC started forming a few clusters some 11.5 billion years ago. After that, the LMC remained almost completely "quiet" until 3 billion years ago, when it suddenly "exploded" in a burst of star formation that is still going on to this day. What happened?

The most probable answer, the way I understand it, is that the Large Magellanic Cloud came sufficiently close to the Small Magellanic Cloud some 3 billion years ago to have its gas "stirred" by the encounter. And because the LMC was always bigger than the SMC, and because the SMC was gas-rich, the LMC could start stealing stars and gas from its hapless companion and further fuel its own star formation.


So the most probable history of the Magellanic Clouds goes like this: 1) they start out as isolated galaxies with low levels of star formation, 2) they drift close enough to start interacting, which triggers star formation in both of them, 3) the LMC steals gas from the SMC, which keeps star formation at high levels in the LMC, while it starts winding down in the SMC, 4) the Magellanic Clouds are captured by the Milky Way and have started interacting with it, and 5) the LMC is going to collide with the Milky Way in 2.4 billion years' time (and will almost certainly drag the SMC with it, if the Magellanic clouds have not already merged by that time).
Wikipedia wrote:

The Milky Way and the LMC are predicted to merge in approximately 2.4 billion years.
Sci News wrote:

The Large Magellanic Cloud is the brightest satellite galaxy of the Milky Way and only entered our neighborhood about 1.5 billion years ago.

Until recently astronomers thought that it would either orbit our Galaxy for many billions of years, or, since it moves so fast, escape from our Galaxy’s gravitational pull.

However, recent measurements indicate that the Large Magellanic Cloud has nearly twice as much dark matter than previously thought.

“Since it has a larger than expected mass, the Large Magellanic Cloud is rapidly losing energy and is doomed to collide with our Galaxy,” said Durham University astronomer Marius Cautun and co-authors...

“While two billion years is an extremely long time compared to a human lifetime, it is a very short time on cosmic timescales,” Dr. Cautun said.

“The destruction of the Large Magellanic Cloud, as it is devoured by the Milky Way, will wreak havoc with our Galaxy, waking up the black hole that lives at its center and turning our Galaxy into an active galactic nucleus or quasar.”

Well, scary! So what about M33, the bluest largish spiral galaxy of the Local Group? Is it a satellite of M31, the big bully of Andromeda, and will it collide with Andromeda in a few billion years time?

No, it turns out it won't.


Wouldn't you know that it is a recent visitor to the Local Group?
Universe Today wrote:

“The velocities we found show that M33 cannot be on a long orbit around M31,” says co-author Ekta Patel of the University of Arizona, USA. “Our models unanimously imply that M33 must be on its first infall into M31.“
Yeah. Figures.

Anway, there you have it: A bright yellow galaxy has had a long and varied history with many "partners", whose many "kisses" have given it its bright yellow shine. A small blue galaxy has just met its first "partner". And a small pale yellow galaxy... well, it may have had a partner long ago, but now it can't remember.

Ann
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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by beryllium732 » Tue Jun 14, 2022 7:58 am

Ann wrote: Sun Jun 12, 2022 5:52 pm Thanks, AVAO, I'm very glad you appreciate it! :D

And thanks for posting that closeup of the core of NGC 5566. That's amazing! Does it have two cores? Two supermassive black holes? I could find no information of that on the internet, but it sure looks like there are two hungry monsters at the center of NGC 5566!

You wrote:
But what really amazes me is that in so many galaxy families, the youngest child "shines in blue".
I don't think that the small blue galaxies are necessarily younger than the big yellow ones, but they have evolved quite differently.

I believe that the galaxies that later grew very big probably started out bigger than most other galaxies, even way back when galaxies first started forming. The ones that later grew so large probably formed from a slightly larger chunk of gas and dark matter than most other galaxies. Back in those days, the Universe was (comparatively) tiny and awash in gas, and those early galaxies must have slurped up huge helpings of gas that kept feeding their star formation.


According to an astrobite from 2016, star formation peaked when the Universe was 3.5 billion years old, and its star formation has been winding down ever since. I'm pretty sure I have heard that the star formation of the Universe peaked even earlier, but you get my point: For a while there, hydrogen gas was there for the taking, and galaxies could just grab it and convert the gas into stars. Until too much of the gas had been locked up inside legions of stingy little red dwarf stars that are surprisingly massive for their puny appearance, and which hoard their (somewhat) meager share of gas until the end of time - or at least, until the Universe has doubled or tripled or quadrupled or... (you get it) - its present age!

So after a while galaxies generally stopped growing by simply acquiring free-floating gas from their vicinity. Instead, they started growing by merging with other galaxies, and by cannibalizing smaller galaxies. There was a great picture posted lately that demonstrated how it works:


In this interacting pair, the big galaxy is stealing stars and gas from the small galaxy. That is always how it works, isn't it? The big guy wins over the small guy?

As you can see in the Hubble image, the smaller component of Arp 282, IC 1559, looks bluish. So how blue is it really? Not too blue, it turns out. Its color is almost exactly the same as the color of dwarf galaxy Leo 1, which is seen very close to first magnitude blue star Regulus in the sky:



IC 1559 and Leo 1 have almost identical B-V indexes, which is a measure of how blue a galaxy is. For Leo 1, the B-V is +0.72, which means that its overall color is somewhere between "neutral" and "yellowish". Leo 1 is a dwarf galaxy completely devoid of young blue stars, but its stars are a little bluer than we would expect from their spectral classes due to the fact that they are extremely metal-poor.
Wikipedia wrote:

Typical to a dwarf galaxy, the metallicity of Leo I is very low, only one percent that of the Sun. Gallart et al. (1999) deduce from Hubble Space Telescope observations that the galaxy experienced a major increase (accounting for 70% to 80% of its population) in its star formation rate between 6 Ga and 2 Ga (billion years ago). There is no significant evidence of any stars that are more than 10 Ga old. About 1 Ga ago, star formation in Leo I appears to have dropped suddenly to an almost negligible rate, roughly coinciding with its latest periastron passage of the Milky Way. Ram pressure stripping would have removed its gas, decreasing its star formation rate.
So Leo 1 lost its gas when it passed close to the Milky Way, and then it stopped forming stars! But the stars it has got are very metal-poor, which makes their color bluer than it would otherwise have been. Do note that puny little Leo 1 has a central black hole of 3 million solar masses, almost as big as Sgr A*, the central black hole of our own big bully of a galaxy! :shock:

Anyway. The B-V index of Leo 1 is +0.72, whereas the B-V index of IC 1559 is +0.74! Their overall colors are almost identical! We can tell at a glance that IC 1559 can't be very blue, because it has very few clusters. And soon it will stop forming stars altogether, because NGC 169 is stealing gas from IC 1559, which powers its own star formation.

Why, then, do we see small blue galaxies in groups where the other galaxies are much yellower and bigger? My answer is that the small blue galaxies have only quite recently entered the group. They have not yet had their gas stolen from them by the big yellow bullies, but their gas has been "stirred" in a way that favors star formation.

Let's look at Stephan's Quintet:

Wikipedia wrote:
The brightest member of the visual grouping (and the only non-member of the true group) is NGC 7320, which has extensive H II regions, identified as red blobs, where active star formation is occurring...

NGC 7320 indicates a small redshift (790 km/s) while the other four exhibit large redshifts (near 6,600 km/s). Since galactic redshift is proportional to distance, NGC 7320 is only a foreground projection and is ~39 million light-years from Earth, making it a possible member of the NGC 7331 group, versus the 210–340 million light-years of the other four.
So NGC 7320 is a possible member of the NGC 7331 group. Let's look at a picture of NGC 7320 together with NGC 7331:


NGC 7331 is at upper right in this ultraviolet image, whereas Stephan's Quintet with non-member NGC 7320 is a lower left. Note the perfect little oval of NGC 7320, evidence of the fact that the star formation here is not violent enough to disturb the overall shape of NGC 7320. It is however sufficiently close to NGC 7331 to have "felt its presence" and to have had its gas "stirred" so it was encouraged to start forming stars.

Also note the ultraviolet "wreckage" of the true members of Stephan's Quintet. The interaction of the members, as well as the actual collision between NGC 7318 A and NGC 7318 B, has thrown out enormous tendrils of gas where great numbers of huge young star clusters are being born.

Much of the gas that has been thrown out of the true members of Stephan's Quintet will fall back onto the galaxies again, but not all of it. And as the galaxies keep interacting and colliding, their gas will eventually either be "cast out" so far away that it won't fall back, or it will be swallowed by the central black holes that are undoubtedly building up inside these galaxies, or it will just generally become so turbulent as well as "thinned out" that it will no longer be able to form any new stars.

Why are large galaxies typically so yellow? That is because they have undergone burst after burst of star formation during their long history of mergers and cannibalizations of other galaxies. Each burst of star formation leaves behind very large numbers of small red and yellow stars. The bright blue stars that are formed in the same burst of star formation die quickly, but the long-lived red and yellow stars stay on and keep building up the large galaxies' hoard of small stars.

So, to summarize. Small blue galaxies are blue because they have quite recently started forming stars, and they have not undergone sufficient numbers of bursts of star formation to have built up huge numbers of long-lived red and yellow stars.

Many, and I'd say most, small galaxies are not forming stars. They are little yellow - make that pale yellow-white - conglomerates of old metal-poor stars, similar to dwarf galaxy Leo 1. But some small galaxies have received the "push" (or whatever) that they needed to start forming stars. Suddenly they start forming blue stars all over.

An example of such a galaxy is the Large Magellanic Cloud. According to a book of mine, The Galaxies of the Local Group by Sidney van den Bergh, the LMC started forming a few clusters some 11.5 billion years ago. After that, the LMC remained almost completely "quiet" until 3 billion years ago, when it suddenly "exploded" in a burst of star formation that is still going on to this day. What happened?

The most probable answer, the way I understand it, is that the Large Magellanic Cloud came sufficiently close to the Small Magellanic Cloud some 3 billion years ago to have its gas "stirred" by the encounter. And because the LMC was always bigger than the SMC, and because the SMC was gas-rich, the LMC could start stealing stars and gas from its hapless companion and further fuel its own star formation.


So the most probable history of the Magellanic Clouds goes like this: 1) they start out as isolated galaxies with low levels of star formation, 2) they drift close enough to start interacting, which triggers star formation in both of them, 3) the LMC steals gas from the SMC, which keeps star formation at high levels in the LMC, while it starts winding down in the SMC, 4) the Magellanic Clouds are captured by the Milky Way and have started interacting with it, and 5) the LMC is going to collide with the Milky Way in 2.4 billion years' time (and will almost certainly drag the SMC with it, if the Magellanic clouds have not already merged by that time).
Wikipedia wrote:

The Milky Way and the LMC are predicted to merge in approximately 2.4 billion years.
Sci News wrote:

The Large Magellanic Cloud is the brightest satellite galaxy of the Milky Way and only entered our neighborhood about 1.5 billion years ago.

Until recently astronomers thought that it would either orbit our Galaxy for many billions of years, or, since it moves so fast, escape from our Galaxy’s gravitational pull.

However, recent measurements indicate that the Large Magellanic Cloud has nearly twice as much dark matter than previously thought.

“Since it has a larger than expected mass, the Large Magellanic Cloud is rapidly losing energy and is doomed to collide with our Galaxy,” said Durham University astronomer Marius Cautun and co-authors...

“While two billion years is an extremely long time compared to a human lifetime, it is a very short time on cosmic timescales,” Dr. Cautun said.

“The destruction of the Large Magellanic Cloud, as it is devoured by the Milky Way, will wreak havoc with our Galaxy, waking up the black hole that lives at its center and turning our Galaxy into an active galactic nucleus or quasar.”

Well, scary! So what about M33, the bluest largish spiral galaxy of the Local Group? Is it a satellite of M31, the big bully of Andromeda, and will it collide with Andromeda in a few billion years time?

No, it turns out it won't.


Wouldn't you know that it is a recent visitor to the Local Group?
Universe Today wrote:

“The velocities we found show that M33 cannot be on a long orbit around M31,” says co-author Ekta Patel of the University of Arizona, USA. “Our models unanimously imply that M33 must be on its first infall into M31.“
Yeah. Figures.

Anway, there you have it: A bright yellow galaxy has had a long and varied history with many "partners", whose many "kisses" have given it its bright yellow shine. A small blue galaxy has just met its first "partner". And a small pale yellow galaxy... well, it may have had a partner long ago, but now it can't remember.

Ann
A fantastic summary Ann! I've learned so much! Can't thank you enough!

Anyway regarding the Triangulum galaxy or M33. How come it still has so many pink bright bright hydrogen clouds left? It's late to the party so to say? And since it's yellow bulge is absent haven't had the violent mergers like the Milky Way and M31?

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Ann
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Re: APOD: Arp 286: Trio in Virgo (2022 Jun 10)

Post by Ann » Tue Jun 14, 2022 8:52 am

beryllium732 wrote: Tue Jun 14, 2022 7:58 am
A fantastic summary Ann! I've learned so much! Can't thank you enough!

Anyway regarding the Triangulum galaxy or M33. How come it still has so many pink bright bright hydrogen clouds left? It's late to the party so to say? And since it's yellow bulge is absent haven't had the violent mergers like the Milky Way and M31?
Thank you so much, I'm very glad you appreciate it! :D

As for M33, yes, it is indeed late to the party! Note what Ekta Patel said in that article in Universe Today:
Universe Today wrote:

“The velocities we found show that M33 cannot be on a long orbit around M31,” says co-author Ekta Patel of the University of Arizona, USA. “Our models unanimously imply that M33 must be on its first infall into M31.“
M33 is on its first infall into M31. This means that it can't have been close to M31 before (and not to the Milky Way either, for that matter). It has recently been affected by M31, because it's probably close enough to "feel its presence", and its gas reservoir has been stirred in such a way that it has started forming quite a lot of stars.

In order to understand how that can work, take a look at a pretty extreme case, a dwarf galaxy called NGC 5253:

In the first picture, you can see the entire galaxy of NGC 5253. You probably shouldn't pay too much attention to the bluish color of the outer disk of NGC 5253, because that may be a consequence of the filters used for the image (and this isn't a "true-color", RGB image). (Actually though, if you go to the Wikipedia page for NGC 5253 and click on the image and then look at the highest magnification of it, you will see an incredible number of faint reddish stars in the outer disk of NGC 5253.)

Instead, I want you to focus on the smooth appearance of the outer disk of NGC 5253. There are no clusters here at all, which suggests that there has been little or no star formation in the outer parts of NGC 5253 for, probably, at least a billion years.

Note however an obvious dark tendril that leads directly into the center of NGC 5253. This appears to be a huge river or flow of gas just pumping into the heart of NGC 5253! (I'm exaggerating a little, so bear with me.) In the second picture, you can see big gas clouds in red seen against the central blue star field of NGC 5253.

And there is a ginormous cluster of stars formed in the brightest of those gas clouds in the center of NGC 5253!
Alchetron, the free encyclopedia, wrote:

NGC 5352 contains a giant dust cloud, hiding a cluster of more than one million stars, among them up to 7,000 O stars. The super star cluster is 3 million years old and has a total luminosity of more than one billion suns. The amount of dust surrounding the stars is extraordinary — approximately 15,000 times the mass of our sun in elements such as carbon and oxygen.
My point is that NGC 5253 wasn't forming stars for a long time. But then suddenly, a few million years ago (which is a short time on cosmic scales), a broad thick stream of gas and dust flowed into the center of NGC 5253 and set off an enormous star burst there. Where did the gas and dust come from? Perhaps NGC 5253 picked it up from from a passing clump of dark matter that was carrying a big gas cloud along. Or perhaps the gas was always there, inside or around NGC 5253, but for one reason or another, the gas got "stirred" and and started moving into the center of NGC 5253 where it could turn into huge numbers of new stars.

Ann
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