APOD: N11: Star Clouds of the LMC (2022 Apr 12)

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APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by APOD Robot » Tue Apr 12, 2022 4:05 am

Image N11: Star Clouds of the LMC

Explanation: Massive stars, abrasive winds, mountains of dust, and energetic light sculpt one of the largest and most picturesque regions of star formation in the Local Group of Galaxies. Known as N11, the region is visible on the upper right of many images of its home galaxy, the Milky Way neighbor known as the Large Magellanic Cloud (LMC). The featured image was taken for scientific purposes by the Hubble Space Telescope and reprocessed for artistry. Although the section imaged above is known as NGC 1763, the entire N11 emission nebula is second in LMC size only to the Tarantula Nebula. Compact globules of dark dust housing emerging young stars are also visible around the image. A recent study of variable stars in the LMC with Hubble has helped to recalibrate the distance scale of the observable universe, but resulted in a slightly different scale than found using the pervasive cosmic microwave background.

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Ann » Tue Apr 12, 2022 5:38 am

Okay, let's find out where we are in today's APOD!

LMC with Tarantula and N11 Compex Eckhard Slawik.png
The Large Magellanic Cloud with the Tarantula Nebula and N11 Complex.
Photo: Eckhard Slawik.

As you can see in Eckhard Slawik's image at left, the N11 complex is located way up to the northwest, quite some distance away from the bright bar of the Large Magellanic Cloud. Many pictures of LMC actually miss the N11 complex, because it is located so far out on the outskirts of the galaxy.

The N11 complex consists of several nebulas with their own NGC designations. Tony Tanner's image on the right appears to show us the correct annotation (although I think that the name "NGC 1760" may possibly refer to the entire N11 complex). I hope I'm allowed to use Tony Tanner's image. If not, go to this page and scroll down to the next to last image. And why not read the interesting info there, too?

The APOD caption says that today's APOD shows us NGC 1763. That's not correct, in my opinion. If you go to the Wikipedia page for NGC 1763, you get two images which supposedly both show us NGC 1763 (and scroll down to make sure that you see the second image, too). But they clearly don't show the same nebula.

N11 Complex Alex Woronow.png
The N11 complex with NGC 1763 and NGC 1769.
Photo: Alex Woronow.

Take a look at Alex Woronow's fine picture of the N11 complex, and look at the nebula designated NGC 1769. If you click on the image to see the full size of it, you can clearly see the distinctive dust lane that is so prominent in today's APOD. (Mark Hanson and Martin Pugh have also produced a superb image of the N11 complex, and you may want to check that one out too.)

I'd say that today's APOD shows us NGC 1769, not NGC 1763. Of course, it's possible that the NGC designations of the two nebulas have been mixed up. But in any case, the Wikipedia page I linked to shows us one picture of NGC 1763 and one picture of NGC 1769. Which is which? Take your pick.

Ann
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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by VictorBorun » Tue Apr 12, 2022 6:57 am

Do I get it right?
When we see some short-living blue giants in a red glowing bubble of α Hydrogen, what we see is a place of the gravitational collapse of a compressed area in cold transparent media of Hydrogen + dopes.
Here this media belongs to the Large Magellanic Cloud and is [ur"https://academic.oup.com/mnras/article/ ... 51/3573852"]orbiting Milky Way at 378 km/s[/url] and spins around the core of the LMC at 70 km/s or less, but the space around Milky Way is so empty that this gas remains cold and transparent until a gravitational compression starts at one place or other?

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by VictorBorun » Tue Apr 12, 2022 6:58 am

VictorBorun wrote: Tue Apr 12, 2022 6:57 am Do I get it right?
When we see some short-living blue giants in a red glowing bubble of α Hydrogen, what we see is a place of the gravitational collapse of a compressed area in cold transparent media of Hydrogen + dopes.
Here this media belongs to the Large Magellanic Cloud and is orbiting Milky Way at 378 km/s and spins around the core of the LMC at 70 km/s or less, but the space around Milky Way is so empty that this gas remains cold and transparent until a gravitational compression starts at one place or other?

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Ann » Tue Apr 12, 2022 9:58 am

VictorBorun wrote: Tue Apr 12, 2022 6:58 am
VictorBorun wrote: Tue Apr 12, 2022 6:57 am Do I get it right?
When we see some short-living blue giants in a red glowing bubble of α Hydrogen, what we see is a place of the gravitational collapse of a compressed area in cold transparent media of Hydrogen + dopes.
Here this media belongs to the Large Magellanic Cloud and is orbiting Milky Way at 378 km/s and spins around the core of the LMC at 70 km/s or less, but the space around Milky Way is so empty that this gas remains cold and transparent until a gravitational compression starts at one place or other?
Not sure exactly what you mean, Victor.


The emission nebula IC 5146, better known as the Cocoon Nebula, is an example of an emission nebula, a red-glowing cloud of ionized hydrogen surrounding a small cluster and its dominant blue (but dust-reddened) star BD +46 3474. I don't know if the gas here is particularly "cold", not compared with its surroundings.

The red hydrogen gas is not collapsing. However, the fact that there is a cluster here at all is proof that there used to be a thick cloud of gas and dust where we now see the nebula and the cluster. The cluster was born when the gas and dust of this cloud contracted. We now see the remnants of that cloud, which itself was a part of a long cosmic dust lane.

Gas and dust has to contract in order to initiate star formation. And once a star has formed, its core will eventually grow more and more compact under the influence of gravity at the same time as the star is running out of fuel to sustain itself. The core of stars like the Sun will contract to the size of about the Earth as the Sun turns into a white dwarf. Stars of more than 8 solar masses normally turn into neutron stars the size of a large city like New York. Only the cores of the most massive stars turn into black holes.

The strong stellar wind of hot massive stars blow away the red nebulas. That is why we see no nebulosity around the massive circa 12 million year old Double Cluster in Perseus. So the red hydrogen gas is actually being scattered away by the stars that make them glow red in the first place.

Click to play embedded YouTube video.
As for runaway stars, take a look at the Youtube video of a simulation of the formation of a globular star cluster. At 0.32, a star is being slingshotted out of the cluster. What happened? Well, the star was a member of a binary that came to close to another star, a single or a binary star. One component of the binary was captured by the other star, while the other component of the binary got a huge kick in the opposite direction, out of the cluster.

It takes a major kick indeed for a star to get kicked out of the Milky Way altogether, but if a binary star gets too close to, say, the central black hole of the Milky Way, then it can happen.

It is possible that a supernova can also send a star flying out of its galaxy. It is much more likely that this will happen to stars in the Large Magellanic Cloud than to stars in the Milky Way, because the gravity of the LMC is so much lower than the gravity of the Milky Way. And in any case, there are lots and lots of pairs of very massive LMC stars that can interact with one another so that one component is sent flying.

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Ann » Tue Apr 12, 2022 10:12 am

I knew it! Today's APOD does not show NGC 1763, but NGC 1769!

Take a look at the Simbad entries on these nebulas (or, as Simbad calls them, associations of stars).

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by orin stepanek » Tue Apr 12, 2022 12:28 pm

N11_HubbleLake_1600.jpg
maybe moor stars being born than dying?
cat-in-thug-costume-jpg.jpg
Hippy Kitty; or biker? :mrgreen:
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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by VictorBorun » Tue Apr 12, 2022 12:34 pm

Ann wrote: Tue Apr 12, 2022 9:58 am
VictorBorun wrote: Tue Apr 12, 2022 6:57 am Here this media belongs to the Large Magellanic Cloud and is orbiting Milky Way at 378 km/s and spins around the core of the LMC at 70 km/s or less, but the space around Milky Way is so empty that this gas remains cold and transparent until a gravitational compression starts at one place or other?
Not sure exactly what you mean, Victor.
You partly answered my questions, thanks.
What remains is this:

how come the interstellar Hydrogen+dopes remains invisible except briefly when illuminated by new-born blue giants —
even while it is moving at 378 km/s with the Large Magellanic Cloud along the orbit around Milky Way?

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Ann » Tue Apr 12, 2022 1:24 pm

VictorBorun wrote: Tue Apr 12, 2022 12:34 pm
Ann wrote: Tue Apr 12, 2022 9:58 am
VictorBorun wrote: Tue Apr 12, 2022 6:57 am Here this media belongs to the Large Magellanic Cloud and is orbiting Milky Way at 378 km/s and spins around the core of the LMC at 70 km/s or less, but the space around Milky Way is so empty that this gas remains cold and transparent until a gravitational compression starts at one place or other?
Not sure exactly what you mean, Victor.
You partly answered my questions, thanks.
What remains is this:

how come the interstellar Hydrogen+dopes remains invisible except briefly when illuminated by new-born blue giants —
even while it is moving at 378 km/s with the Large Magellanic Cloud along the orbit around Milky Way?
Don't know what you mean by "Hydrogen + dopes".

Anyway. The intergalactic hydrogen gas is very, very thin. It may be ionized, but if so, it will be way too thin - the actual hydrogen ions would be too few and far between - to produce light that we would normally capture on photographs. Indeed, I think it would take very long exposures through hydrogen alpha filters, and then very, very careful processing, to bring out that very very faint Hα light that I think you are talking about.

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Chris Peterson » Tue Apr 12, 2022 1:44 pm

VictorBorun wrote: Tue Apr 12, 2022 12:34 pm
Ann wrote: Tue Apr 12, 2022 9:58 am
VictorBorun wrote: Tue Apr 12, 2022 6:57 am Here this media belongs to the Large Magellanic Cloud and is orbiting Milky Way at 378 km/s and spins around the core of the LMC at 70 km/s or less, but the space around Milky Way is so empty that this gas remains cold and transparent until a gravitational compression starts at one place or other?
Not sure exactly what you mean, Victor.
You partly answered my questions, thanks.
What remains is this:

how come the interstellar Hydrogen+dopes remains invisible except briefly when illuminated by new-born blue giants —
even while it is moving at 378 km/s with the Large Magellanic Cloud along the orbit around Milky Way?
Interstellar hydrogen is either neutral or ionized, depending on its temperature. There are many mechanisms which can both heat or cool these atoms. We detect ionized hydrogen all over. If I just aim my camera at some random part of the sky and use an H-alpha filter, I'm likely to see ionized hydrogen clouds and wisps. Neutral hydrogen is seen with radio images made at 21 cm. In other words, interstellar hydrogen is not "invisible", whether neutral or ionized. The density of interstellar hydrogen also varies widely, and around stellar collections the gravitational fields are much higher (primarily from the free hydrogen, not the stars) and the gas densities are also higher, so emissions are brighter.
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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by De58te » Tue Apr 12, 2022 2:46 pm

Pardon me if I add my two cents, but the ESA's "zoom video into LHA 120-N11" located under the "visible on the upper right" link clearly zooms into NGC 1769 which I recognize thanks to Ann's Tony Tanner illustrated photo. By the way, I was surprised that as we approached the LMC I was surprised that it faded off the edge of the video instead of zooming in to it as I suspected. Apparently the N11 complex is way off to the side.

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by VictorBorun » Tue Apr 12, 2022 4:51 pm

Chris Peterson wrote: Tue Apr 12, 2022 1:44 pm We detect ionized hydrogen all over.
I don't get it.
We can see a red glowing bubble where the hydrogen is excited by young blue giants of NGC 1769 inside the bubble.
But we see that the glow ends further away from that star cluster.
Suppose the hydrogen is still there, just not ionized.
So the Large Magellanic Cloud with all its hydrogen orbits Milky Way at 380 km/s and the hydrogen is not excited by ram pressure.
How so?

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Chris Peterson » Tue Apr 12, 2022 6:45 pm

VictorBorun wrote: Tue Apr 12, 2022 4:51 pm
Chris Peterson wrote: Tue Apr 12, 2022 1:44 pm We detect ionized hydrogen all over.
I don't get it.
We can see a red glowing bubble where the hydrogen is excited by young blue giants of NGC 1769 inside the bubble.
But we see that the glow ends further away from that star cluster.
Suppose the hydrogen is still there, just not ionized.
So the Large Magellanic Cloud with all its hydrogen orbits Milky Way at 380 km/s and the hydrogen is not excited by ram pressure.
How so?
Why do you assume that the end of the glow in the image is the end of the ionization? There is a great deal of ionized gas in the interstellar medium. It's just so tenuous that the intensity is low. At the exposure time of this image, the apparent edge of ionization is likely just where the density is so low that the intensity is below the noise level.
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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by AVAO » Tue Apr 12, 2022 9:18 pm

Ann wrote: Tue Apr 12, 2022 10:12 am I knew it! Today's APOD does not show NGC 1763, but NGC 1769!

Take a look at the Simbad entries on these nebulas (or, as Simbad calls them, associations of stars).

Ann
That's right, Ann!

The Wikipedia entry https://en.wikipedia.org/wiki/NGC_1763 is incorrect and should be corrected.

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Shadow » Tue Apr 12, 2022 9:54 pm

I see a tardigrade and a dolphin.

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by AVAO » Tue Apr 12, 2022 10:22 pm

AVAO wrote: Tue Apr 12, 2022 9:18 pm
Ann wrote: Tue Apr 12, 2022 10:12 am I knew it! Today's APOD does not show NGC 1763, but NGC 1769!

Take a look at the Simbad entries on these nebulas (or, as Simbad calls them, associations of stars).

Ann
That's right, Ann!

The Wikipedia entry https://en.wikipedia.org/wiki/NGC_1763 is incorrect and should be corrected.
LHA 120-N 11B is the same then NGC1763
LHA 120-N 11C is the same then NGC1769

The esa pages are a bit confusing or contradictory here:
https://esahubble.org/projects/fits_lib ... e_ngc_1763 Wrong!
https://esahubble.org/images/heic1301a/
https://www.esa.int/ESA_Multimedia/Imag ... anic_Cloud
https://www.esa.int/ESA_Multimedia/Imag ... ion_region

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by papajohn@optusnet.com.au » Tue Apr 12, 2022 11:15 pm

There be a dragon in that image.
The dust clouds in that image remind me of a weedy sea dragon.
Regards John

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Ann » Wed Apr 13, 2022 10:01 am

AVAO wrote: Tue Apr 12, 2022 9:18 pm
Ann wrote: Tue Apr 12, 2022 10:12 am I knew it! Today's APOD does not show NGC 1763, but NGC 1769!

Take a look at the Simbad entries on these nebulas (or, as Simbad calls them, associations of stars).

Ann
That's right, Ann!

The Wikipedia entry https://en.wikipedia.org/wiki/NGC_1763 is incorrect and should be corrected.
A bit of correction has taken place (and I'd like to think someone did it because of me identifying the nebula in the APOD as NGC 1769, but I can't be sure):
Wikipedia file wrote:

NGC 1769 Attention: This image shows NGC 1769 not NGC 1763!
But the main Wikipedia page (a stub) still shows a thumbnail of NGC 1769 and calls it NGC 1763.

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Ann » Wed Apr 13, 2022 10:36 am

Got to say a few more things about NGC 1763 and NGC 1769.

Steve Gottlieb wrote about NGC 1763:

The showpiece is certainly NGC 1763, which sits near the center of a stunning field of emission nebulae and clusters...

NGC 1763 is a very bright, very large irregular nebula, shaped like a kidney-bean or a fetus...

Overall the surface brightness is very high, though uneven, and much fainter haze and filaments flow out from the Bean in most directions...

Involved with NGC 1763 is a large cluster catalogued as OB-association LH 10 (the youngest cluster in the LMC-N11 complex), with roughly two dozen resolved stars. This cluster includes a number of 12-13 mag stars (several of which are massive O3-type stars)...

The surrounding field is rich in stars between the individual objects with some locally brighter patches of nebulosity.
All right. So much for NGC 1763. It seems to be an exciting target for observers with amateur (but still powerful) telescopes. But in the Hubble image, it looks moderately uninteresting to me, because I somehow can't memorize any distinctive features in it.


Now for NGC 1763. I have to show two pictures of it side by side:

NGC 1763 NGC 1769 annotated Mark Hanson.png
NGC 1763 and NGC 1769. Image: Mark Hanson/Martin Pugh.

Let's see what Steve Gottlieb wrote about NGC 1769:
NGC 1769 is a bright, large oval nebula oriented SW-NE, roughly 3’ x 2'. At the center is a mag 11.5 star (Sk -66 41)...

Sk-66 41 was once thought to be one of the most single massive and luminous stars in the LMC but has been shown to be a very compact cluster with over a dozen components.
Take a look at the NASA/ESA/Josh Lake picture of NGC 1769 again. Note the very tight cluster at the center of the image. Through a telescope, this cluster undoubtedly looks like a single star, and the distinctive dust lane is certainly invisible.

So through a telescope NGC 1769 looks moderately boring, but at the resolution of Hubble we can see not only the signature dust lane, but the extremely tight cluster as well.

So in short, NGC 1763 is an amateur telescope nebula 🔭, but NGC 1769 is a Hubble Space Telescope nebula!

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by VictorBorun » Fri Apr 15, 2022 4:32 pm

Chris Peterson wrote: Tue Apr 12, 2022 6:45 pm
VictorBorun wrote: Tue Apr 12, 2022 4:51 pmSo the Large Magellanic Cloud with all its hydrogen orbits Milky Way at 380 km/s and the hydrogen is not excited by ram pressure.
How so?
Why do you assume that the end of the glow in the image is the end of the ionization? There is a great deal of ionized gas in the interstellar medium. It's just so tenuous that the intensity is low. At the exposure time of this image, the apparent edge of ionization is likely just where the density is so low that the intensity is below the noise level.
come to think of it there may be no ram pressure at 380 km/s after all.
Somewhere N11 ends and the LMC ends, and intergalactic low-density media begins.
But there may be a sort of Cartesian vortex ring or half-ring in that media, orbiting Milky Way together with the LMC and keeping it nice and cold except locally, around young stellar clusters

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Ann » Fri Apr 15, 2022 6:03 pm

VictorBorun wrote: Fri Apr 15, 2022 4:32 pm
Chris Peterson wrote: Tue Apr 12, 2022 6:45 pm
VictorBorun wrote: Tue Apr 12, 2022 4:51 pmSo the Large Magellanic Cloud with all its hydrogen orbits Milky Way at 380 km/s and the hydrogen is not excited by ram pressure.
How so?
Why do you assume that the end of the glow in the image is the end of the ionization? There is a great deal of ionized gas in the interstellar medium. It's just so tenuous that the intensity is low. At the exposure time of this image, the apparent edge of ionization is likely just where the density is so low that the intensity is below the noise level.
come to think of it there may be no ram pressure at 380 km/s after all.
Somewhere N11 ends and the LMC ends, and intergalactic low-density media begins.
But there may be a sort of Cartesian vortex ring or half-ring in that media, orbiting Milky Way together with the LMC and keeping it nice and cold except locally, around young stellar clusters
If you ask me, there isn't a lot of intergalactic gas in the Local Group. Large rich galaxy clusters, with giant elliptical galaxies with huge black holes that can have tremendous outbursts and eject huge amounts of gas into the intergalactic medium, is another matter.

1024px-View_Towards_the_Great_Attractor[1].jpg
Large rich galaxy cluster Abell 3627. Photo: ESO.
APOD Robot wrote:

Spiral galaxy ESO 137-001 hurtles through massive galaxy cluster Abell 3627 some 220 million light years away. The distant galaxy is seen in this colorful Hubble/Chandra composite image through a foreground of the Milky Way's stars toward the southern constellation Triangulum Australe. As the spiral speeds along at nearly 7 million kilometers per hour, its gas and dust are stripped away when ram pressure with the cluster's own hot, tenuous intracluster medium overcomes the galaxy's gravity. Evident in Hubble's near visible light data, bright star clusters have formed in the stripped material along the short, trailing blue streaks. Chandra's X-ray data shows off the enormous extent of the heated, stripped gas as diffuse, darker blue trails stretching over 400,000 light-years toward the bottom right. The significant loss of dust and gas will make new star formation difficult for this galaxy. A yellowish elliptical galaxy, lacking in star forming dust and gas, is just to the right of ESO 137-001 in the frame.
Falling through the intergalactic medium of a massive cluster is something else entirely than the LMC orbiting the Milky Way.

The Virgo Cluster is a rich nearby cluster where ram pressure is at work:

Image[1].jpg
NGC 4569 (M90), the largest spiral galaxy in the Virgo Cluster,
being stripped of gas by ram pressure. Image: ©2015 CFHT/Coelum
Canada-France-Hawaii Telescope wrote:

An international team led by researchers from the Laboratoire d'Astrophysique de Marseille (LAM) has used MegaCam on CFHT to observe NGC 4569, the most massive spiral galaxy in the Virgo cluster. They observed, for the first time, spectacular tails of ionized gas that extend for over 300,000 light years, five times larger than NGC 4569 itself! This observation confirms that ram pressure stripping due to the intracluster medium is depriving NGC 4569 of its gas reservoir.

The red filaments at the right of the galaxy show the ionised gas removed by ram pressure. This is about 95% of the gas reservoir of the galaxy needed to feed the formation of new stars.
We expect to see ram pressure in "busier" environments than the Local Group (not that it couldn't ever happen here, of course).

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Chris Peterson » Fri Apr 15, 2022 6:16 pm

Ann wrote: Fri Apr 15, 2022 6:03 pm If you ask me, there isn't a lot of intergalactic gas in the Local Group.
There is a lot... but the density is extremely low. But the volume of intergalactic space is also immensely greater than the volume of all the galaxies that are in that space. I believe that something like 95% of the hydrogen in the Universe exists outside of stars and galaxies.
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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by VictorBorun » Sun Apr 17, 2022 1:46 pm

Chris Peterson wrote: Fri Apr 15, 2022 6:16 pm
Ann wrote: Fri Apr 15, 2022 6:03 pm If you ask me, there isn't a lot of intergalactic gas in the Local Group.
There is a lot... but the density is extremely low. But the volume of intergalactic space is also immensely greater than the volume of all the galaxies that are in that space. I believe that something like 95% of the hydrogen in the Universe exists outside of stars and galaxies.
We are speaking here of heating/ionizing red alpha Hydrogen inside a galaxy. What is really a lot for a head wind must compare with UV from young stellar cluster N11.
It's not a one-to-one mapping of a headwind Hydrogen atom or proton to a UV photon; a headwind proton at 380 km/s can ionize many atoms inside a galaxy.

An Hα photon has a wavelength of 656.46 nm and an energy of 1.889 eV
But to generate it, an electron must go down from an n=3 to an n=2 level, and so that electron must first go up from an n=1 to an n=3 level; so an exciting quantum must be 12.1 eV.
That's a lot; a photon with that energy is 102 nm far UV born mostly at 140 thousand Kelvins. If a blue giant is just 40 thousand Kelvins, such photons are rare.

As for a headwind proton, at 380 km/s, it has 750 eV of kinetic energy and can excite 60 Hydrogen atoms inside a galaxy.

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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Ann » Sun Apr 17, 2022 5:54 pm

VictorBorun wrote: Sun Apr 17, 2022 1:46 pm
Chris Peterson wrote: Fri Apr 15, 2022 6:16 pm
Ann wrote: Fri Apr 15, 2022 6:03 pm If you ask me, there isn't a lot of intergalactic gas in the Local Group.
There is a lot... but the density is extremely low. But the volume of intergalactic space is also immensely greater than the volume of all the galaxies that are in that space. I believe that something like 95% of the hydrogen in the Universe exists outside of stars and galaxies.
We are speaking here of heating/ionizing red alpha Hydrogen inside a galaxy. What is really a lot for a head wind must compare with UV from young stellar cluster N11.
It's not a one-to-one mapping of a headwind Hydrogen atom or proton to a UV photon; a headwind proton at 380 km/s can ionize many atoms inside a galaxy.

An Hα photon has a wavelength of 656.46 nm and an energy of 1.889 eV
But to generate it, an electron must go down from an n=3 to an n=2 level, and so that electron must first go up from an n=1 to an n=3 level; so an exciting quantum must be 12.1 eV.
That's a lot; a photon with that energy is 102 nm far UV born mostly at 140 thousand Kelvins. If a blue giant is just 40 thousand Kelvins, such photons are rare.

As for a headwind proton, at 380 km/s, it has 750 eV of kinetic energy and can excite 60 Hydrogen atoms inside a galaxy.
Take a look at the CFHT image of NGC 4569 (M90) and its satellite galaxy again:

Image[1].jpg

I'm not going to annotate this image, but as you can see, M90 has a satellite galaxy (to its upper right). If this galaxy, IC 3538, is indeed a satellite of M90, then it will be in orbit around M90. But the red light from Hα, ionized hydrogen, is not centered on the companion galaxy. Most of the ionized hydrogen is flowing away from a part of M90 that is located quite far from IC 3538.

I can't guarantee that IC 3538 is really a satellite galaxy of M90 and thus in orbit around the Messier galaxy. IC 3538 could be just a line of sight galaxy that is not orbiting M90.

But there is no doubt that M32 and NGC 205 are satellite galaxies of the Andromeda galaxy, and that they are orbiting the Andromeda galaxy. Therefore, if there is a gaseous medium that would cause ram on one side of around M32 and NGC 205 as they orbit Andromeda, we ought to see red gas around these galaxies. But we don't.

Ionized hydrogen around M31 and satellites Rogelio Bernal Andreo.png
Ionized hydrogen around Andromeda galaxy and satellites.
Image: Rogelio Bernal Andreo.

You may object that there is very little gas in M32 and in NGC 205, so there would not really be any gas in these galaxies to be ionized in the first place. That is true of M32, but NGC 205 does in fact contain some gas. Check out this Hubble picture of the center of NGC 205. You can see the young blue stars near its center, and you can (just barely) make out a dark cloud of gas and dust near the center of the picture.

As for the Large Magellanic Cloud, there is in fact a bridge of gas and stars between the Large Magellanic Cloud and the Small Magellanic Cloud:


There appears to be a stream of gas between the Milky Way and the LMC too, but the way I understand it, it is less pronounced than the Magellanic Bridge between the LMC and the SMC.

Gas-projection-in-the-simulated-Magellanic-System-red-scale-plotted-over-an-optical[1].png

Read the caption that comes with the image here. My point is that there is normally not a lot of ram pressure and not a lot of ionized hydrogen clouds around smallish galaxies orbiting larger ones.

Ann
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Re: APOD: N11: Star Clouds of the LMC (2022 Apr 12)

Post by Chris Peterson » Sun Apr 17, 2022 6:26 pm

Ann wrote: Sun Apr 17, 2022 5:54 pm I can't guarantee that IC 3538 is really a satellite galaxy of M90 and thus in orbit around the Messier galaxy. IC 3538 could be just a line of sight galaxy that is not orbiting M90.
It's actually IC 3583. It and M90 are an interesting pair. There is some evidence of interaction, but they have very different redshifts (M90 is actually blueshifted), giving a difference in radial velocities of more than 1300 km/s. So if it's orbiting, it's doing so very fast. And there's the problem of other evidence suggesting they are more than 30 million ly apart. M90 has a Gaia parallax measurement, but IC 3583 does not.

So the bottom line is that nobody knows one way or the other if they are an interacting pair or a coincidental line of sight pair.
Chris

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