APOD: The Leo Trio (2024 Mar 21)

[APOD Robot]

The Leo Trio

Explanation: This popular group leaps into the early evening sky around the March equinox and the northern hemisphere spring. Famous as the Leo Triplet, the three magnificent galaxies found in the prominent constellation Leo gather here in one astronomical field of view. Crowd pleasers when imaged with even modest telescopes, they can be introduced individually as NGC 3628 (left), M66 (bottom right), and M65 (top). All three are large spiral galaxies but tend to look dissimilar, because their galactic disks are tilted at different angles to our line of sight. NGC 3628, also known as the Hamburger Galaxy, is temptingly seen edge-on, with obscuring dust lanes cutting across its puffy galactic plane. The disks of M66 and M65 are both inclined enough to show off their spiral structure. Gravitational interactions between galaxies in the group have left telltale signs, including the tidal tails and warped, inflated disk of NGC 3628 and the drawn out spiral arms of M66. This gorgeous view of the region spans over 1 degree (two full moons) on the sky in a frame that covers over half a million light-years at the trio's estimated distance of 30 million light-years.

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[Ann]

At this time of year, when spring is in the air or at least in the sky in the northern hemisphere, it is of course very suitable to greet the new season with the most iconic galaxies of spring, the Leo Triplet.

The Leo Trio. Image Credit & Copyright: Steve Cannistra

Note the puffed-up halo of M66, and he strange halo of NGC 3628 (fattest at the edges!), versus the serene, lens-shaped outline of M65. It looks as if M66 and NGC 3628 have recently interacted and disturbed each other's gravitational wells, whereas M65 has been sitting in the stands, with slicked-down hair and well-manicured hands, calmly watching the hue and cry and the sound and the fury of its galactic neighbors.

Indeed, according to Wikipedia, M66 and NGC 3628 may have interacted 800 million years ago. M65 may also have interacted, but "much less strongly".

An image showcasing the color differences between M65 and M66 is this picture by Bill Snyder:

M65 (top left) and M66. Credit: Bill Snyder.

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Note the red dust lanes and blue star clusters of M66. Newborn hot blue stars ionize hydrogen in their vicinity and make it glow red, so both blue and red colors in RGB+Hα portraits of galaxies are signs of star formation. Note, however, that M65 lacks both blue and red colors:

M65. Credit: ESA/Hubble & NASA.

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Yes, you can see little blue clusters and individual blue stars in this rather old Hubble picture of M65, but these blue stars are so few and faint that it takes Hubble to see them.

But guess what? In 2013 there was a supernova in M65, and it was a core-collapse supernova too, a supernova from a massive Betelguese type of star! Fancy that!

Supernova SN 2013am in M65. Credit: Tyler Allred at Cloudy NIghts.

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Super-bright supernova SN 2006gy was found in NGC 1260, which is an even redder and deader galaxy than M65. Credit: X-ray: NASA / CXC, Nathan Smith, Weidong Li (UC Berkeley) et al.; IR: PAIRITEL/Lick/UC Berkeley/J.Bloom, C.Hansen

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So both SN 2013am in M65 and SN 2006gy in NGC 1260 have shown us that apparently red and dead galaxies, apparently devoid of star formation, can sometimes still host stars so massive that they will go supernova. If you ask me, however, it takes at least a substantial dust lane to create such a massive core-collapse star, and both M65 and NGC 1260 have them.

NGC 1260, with a small but substantial and supernova-spawning dust lane near its core. Credit: Hubble Space Telescope/Fabian RRRR

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I don't expect to see a core-collapse supernova in dust lane-less M87, for example, or even in the thin and shredded dust lanes of NGC 1316.


NGC 3628, the Hamburger Galaxy, certainly doesn't lack a substantial dust lane. A link in today's caption takes us to a gorgeous portrait of NGC 3628. Also take a look at an amazing picture of the 300,000 light-year-long tidal tail of NGC 3628!

NGC 3628. Credit: Martin Pugh.

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Tidal tail of NGC 3628. Credit: Mark Hanson.

Martin Pugh's image does show us little red and pink emission nebulas and hints of little blue star clusters. At least there is more star formation in this galaxy than in M65.


And now I'm going to use my third and last attachment to show you a gorgeous wide angle portrait of the Leo Trio and its surroundings, including bright blue star theta Leonis.

The Leo Trio and surroundings. Credit:
Rogelio Bernal Andreo.

Okay, I'm off. Bye bye!

Ann

[JimB]

Also take a look at an amazing picture of the 300,000 light-year-long tidal tail of NGC 3628!


Tidal tail of NGC 3628. Credit: Mark Hanson.

Thanks for highlighting this Ann. It took me a while to realise that the tidal tail of NGC 3628 is not really visible on the APOD, but extends a long way off the bottom edge of the frame.

Why does the tail extend in that direction when the interacting galaxy M66 is nowhere near there? Do we know the computed path of the previous interaction?

[Chris Peterson]

At this time of year, when spring is in the air or at least in the sky in the northern hemisphere, it is of course very suitable to greet the new season with the most iconic galaxies of spring, the Leo Triplet.

While extreme saturation may serve to emphasize certain features, and therefore has some scientific value, I don't find it particularly aesthetic. I think that something closer to what our eyes would see given greater sensitivity would be much less colorful.
_

[Chris Peterson]

Also take a look at an amazing picture of the 300,000 light-year-long tidal tail of NGC 3628!


Tidal tail of NGC 3628. Credit: Mark Hanson.

Thanks for highlighting this Ann. It took me a while to realise that the tidal tail of NGC 3628 is not really visible on the APOD, but extends a long way off the bottom edge of the frame.

Why does the tail extend in that direction when the interacting galaxy M66 is nowhere near there? Do we know the computed path of the previous interaction?

Of course, these bodies are in complex orbits around each other. Determining those orbital paths is difficult, because we can only directly measure the radial components of their velocities. But the presence of the tidal structure probably allows for experimenting with different orbits using numerical simulation to see what could reasonably produce the observed state of things. I don't know if somebody has done this for this galaxy group, but they've done it for others, and given the prominence of the Leo triplet, I'd be surprised if it hasn't been tried here, as well.

[Ann]

At this time of year, when spring is in the air or at least in the sky in the northern hemisphere, it is of course very suitable to greet the new season with the most iconic galaxies of spring, the Leo Triplet.

While extreme saturation may serve to emphasize certain features, and therefore has some scientific value, I don't find it particularly aesthetic. I think that something closer to what our eyes would see given greater sensitivity would be much less colorful.
_
NGC3628.jpg
M66.jpg

Well, Chris, aesthetics is in the eye of the beholder.

Ann

[Chris Peterson]

At this time of year, when spring is in the air or at least in the sky in the northern hemisphere, it is of course very suitable to greet the new season with the most iconic galaxies of spring, the Leo Triplet.

While extreme saturation may serve to emphasize certain features, and therefore has some scientific value, I don't find it particularly aesthetic. I think that something closer to what our eyes would see given greater sensitivity would be much less colorful.
_
NGC3628.jpg
M66.jpg

Well, Chris, aesthetics is in the eye of the beholder.

Ann

Absolutely. Although there is some justification for arguing that some color treatments are more "accurate" than others in terms of representing something as it would appear visually simply given higher sensitivity.

[johnnydeep]

Ok, what's the prominent, yet unmentioned thing in the upper left quadrant? Is it just a local globular cluster?

And an aside: to the posters of these APODs: I really wish that instead of having to laboriously refer back and forth between the image and text descriptions such as "upper right" or "lower left" for the objects being highlighted, that ALL APOD images made use of mouse-over alternate images that were annotated! It would make things so much clearer for everyone.

[Chris Peterson]

Ok, what's the prominent, yet unmentioned thing in the upper left quadrant? Is it just a local globular cluster?

And an aside: to the posters of these APODs: I really wish that instead of having to laboriously refer back and forth between the image and text descriptions such as "upper right" or "lower left" for the objects being highlighted, that ALL APOD images made use of mouse-over alternate images that were annotated! It would make things so much clearer for everyone.

A star.
_

[johnnydeep]

Ok, what's the prominent, yet unmentioned thing in the upper left quadrant? Is it just a local globular cluster?

And an aside: to the posters of these APODs: I really wish that instead of having to laboriously refer back and forth between the image and text descriptions such as "upper right" or "lower left" for the objects being highlighted, that ALL APOD images made use of mouse-over alternate images that were annotated! It would make things so much clearer for everyone.

A star.
_
new_image_Annotated1.jpg

Thanks. I guess it looks a little too perfect to be a GC. It's particulate appearance threw me off I supposed.

And why does the Tycho catalog (all those TYCxxx.yyy designators), have that name? I can't find it explained anywhere.

[Ann]

Ok, what's the prominent, yet unmentioned thing in the upper left quadrant? Is it just a local globular cluster?

And an aside: to the posters of these APODs: I really wish that instead of having to laboriously refer back and forth between the image and text descriptions such as "upper right" or "lower left" for the objects being highlighted, that ALL APOD images made use of mouse-over alternate images that were annotated! It would make things so much clearer for everyone.

That's a star, HD 98388. It's a star of spectral class F8V, ~2.5 times brighter than the Sun. It is located ~150 light-years away from us, which is a distance very similar to the distance to the Hyades (which is itself the nearest cluster to us).

More importantly, this star could not be a globular cluster, or at least not a globular cluster at the same distance from us as the Leo Trio.

Why could this star not be a globular cluster? It's because it is way, way too bright to be a globular cluster at the same distance from us as the Leo Trio. Technically it could - in view of the relative blurriness of the picture - be a foreground Milky Way globular cluster seen in the same part of the sky as the Leo Trio. But if that had been the case, it would have made an appearance every time the Leo Trio was photographed, and this globular would have been so famous that you wouldn't have had to ask if it was a globular cluster, because the APOD caption would have told you that it was!

But let's return to the question of the brightness of globulars versus the brightness of the galaxies that host them. There is a truly fascinating case of a puny little dwarf galaxy, the Fornax dwarf galaxy, which is so faint that it is almost see-through, that sports no less than five globular clusters. Three of these globulars are as bright as typical Milky Way globulars. How weird!

The Fornax dwarf galaxy and four of its globulars.
Credit: ESO/Digitized Sky Survey 2

But consider this picture of the Large Magellanic Cloud and one of its globulars:

A globular cluster in the Large Magellanic Cloud. Credit: HLA/Fabian RR/ESO/VMC Survey/Astronomie.nl [CC BY-SA 3.0]

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Disregard the inset for a moment and concentrate on the galaxy. Can you even see the globular? Naah, me either. And the LMC isn't even a tremendously bright galaxy.


Unlike its larger "sibling", the other Magellanic Cloud, the SMC, really does seem to team up with a bright and brilliant globular cluster, 47 Tuc:

The Small Magellanic Cloud and globular cluster 47 Tuc (right). Credit: Paul Lloyd

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It looks impressive, doesn't it? But 47 Tuc doesn't belong to the SMC. It belongs to the Milky Way, so it is much closer to us than the SMC. The fact that this large bright cluster and the small faint galaxy are seen so close together in the sky is pure coincidence.


Take a look at giant elliptical galaxy M87 and some of its globulars:

M87 itself is the large fuzzy object covering most of the image. Hundreds of little point-like objects are M87 globulars. Note that the largest and brightest of the small bright objects are background galaxies. Credit: NOIRLab/Gemini Observatory/AURA/NSF

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And that's why the star HD 98388 couldn't be a globular at the distance of the Leo Trio, because it is way, way too bright.

Of course the Leo galaxies have globulars of their own.

NGC 3628 (crop). Credit: Martin Pugh.

If you take a good look at the picture of NGC 3628 by Martin Pugh, you can see little lights near the dust lane (but not in the dust lane) that could be globulars. Undoubtedly NGC 3528 has globulars, as do M65 and M66, too.

Ann

[johnnydeep]

Ok, what's the prominent, yet unmentioned thing in the upper left quadrant? Is it just a local globular cluster?

And an aside: to the posters of these APODs: I really wish that instead of having to laboriously refer back and forth between the image and text descriptions such as "upper right" or "lower left" for the objects being highlighted, that ALL APOD images made use of mouse-over alternate images that were annotated! It would make things so much clearer for everyone.

That's a star, HD 98388. It's a star of spectral class F8V, ~2.5 times brighter than the Sun. It is located ~150 light-years away from us, which is a distance very similar to the distance to the Hyades (which is itself the nearest cluster to us).

More importantly, this star could not be a globular cluster, or at least not a globular cluster at the same distance from us as the Leo Trio.

...

Why could this star not be a globular cluster? It's because it is way, way too bright to be a globular cluster at the same distance from us as the Leo Trio. Technically it could - in view of the relative blurriness of the picture - be a foreground Milky Way globular cluster seen in the same part of the sky as the Leo Trio. But if that had been the case, it would have made an appearance every time the Leo Trio was photographed, and this globular would have been so famous that you wouldn't have had to ask if it was a globular cluster, because the APOD caption would have told you that it was!

...

Ann

Thanks. And I did describe it as a "local globular cluster" in my query. And, yes, even I realized that it couldn't be a GC at the same distance as those galaxies. But in view of its prominence in this APOD, as well as its GC-like "fuzziness", it should have been mentioned in the text (or in an annotation!) so that the non-experts like myself might know what it is!

[Chris Peterson]

Ok, what's the prominent, yet unmentioned thing in the upper left quadrant? Is it just a local globular cluster?

And an aside: to the posters of these APODs: I really wish that instead of having to laboriously refer back and forth between the image and text descriptions such as "upper right" or "lower left" for the objects being highlighted, that ALL APOD images made use of mouse-over alternate images that were annotated! It would make things so much clearer for everyone.

A star.
_
new_image_Annotated1.jpg

Thanks. I guess it looks a little too perfect to be a GC. It's particulate appearance threw me off I supposed.

And why does the Tycho catalog (all those TYCxxx.yyy designators), have that name? I can't find it explained anywhere.

From SIMBAD:

Write: <<TYC FFFF-NNNNN-N>>

The designation of an object in the Tycho Catalogue uses the Guide Star Catalog numbering system 'FFFF-NNNNN' (a region number (FFFF) and a number within the region (NNNNN)) followed by a Tycho specific component number (N). The embedded blanks in the format are often converted to zeros.

[johnnydeep]

A star.
_
new_image_Annotated1.jpg

Thanks. I guess it looks a little too perfect to be a GC. It's particulate appearance threw me off I supposed.

And why does the Tycho catalog (all those TYCxxx.yyy designators), have that name? I can't find it explained anywhere.

From SIMBAD:

Write: <<TYC FFFF-NNNNN-N>>

The designation of an object in the Tycho Catalogue uses the Guide Star Catalog numbering system 'FFFF-NNNNN' (a region number (FFFF) and a number within the region (NNNNN)) followed by a Tycho specific component number (N). The embedded blanks in the format are often converted to zeros.

I meant why is it the "Tycho" catalog as opposed to, say, the "Peterson" catalog?!

[Astrojhuan]

Hi all,

This is my leo triplet on dust background

From Palmira, Colombia

Greetins!

Annotated versión

Rasa8 asi2600mc
Optolong l-extreme 35x240"
Optolong l-quad 30x240"

[Ann]

I'm staying out of the Tycho catalog discussion, but let me say that, on second thought, foreground star HD 98388 couldn't have been a Milky Way globular cluster seen in the same part of the sky as the Leo Trio. Why not? It's because it's too small. I checked the angular size of M65, and its angular diameter is 8 X 1.5 arc-minutes. But the angular size of a typical Milky Way globular cluster (I picked M53) is 12.6 arc-minutes. In other words, we expect a foreground Milky Way globular to look as large or even larger than galaxies at the distance of the Leo Trio. And HD 98388 clearly doesn't!

Edit: Okay! It could have been one of those very distant globulars of the Milky Way. Our galaxy's most distant globular, Laevens 1, is located no less than 470,000 light-years from the center of the Milky Way, and its angular size as seen from the Earth is only 0.46 arcminutes. But this globular is faint and sparse and would most likely not look like a bright star in the vicinity of the Leo Trio.

Ann

[Chris Peterson]

Thanks. I guess it looks a little too perfect to be a GC. It's particulate appearance threw me off I supposed.

And why does the Tycho catalog (all those TYCxxx.yyy designators), have that name? I can't find it explained anywhere.

From SIMBAD:

Write: <<TYC FFFF-NNNNN-N>>

The designation of an object in the Tycho Catalogue uses the Guide Star Catalog numbering system 'FFFF-NNNNN' (a region number (FFFF) and a number within the region (NNNNN)) followed by a Tycho specific component number (N). The embedded blanks in the format are often converted to zeros.

I meant why is it the "Tycho" catalog as opposed to, say, the "Peterson" catalog?!

Well, Tycho Brahe was the first astronomer to create a rich, very accurate catalog of astrometric positions, so it's logical that the catalog of Hipparcos observations should honor him.

[johnnydeep]

I'm staying out of the Tycho catalog discussion, but let me say that, on second thought, foreground star HD 98388 couldn't have been a Milky Way globular cluster seen in the same part of the sky as the Leo Trio. Why not? It's because it's too small. I checked the angular size of M65, and its angular diameter is 8 X 1.5 arc-minutes. But the angular size of a typical Milky Way globular cluster (I picked M53) is 12.6 arc-minutes. In other words, we expect a foreground Milky Way globular to look as large or even larger than galaxies at the distance of the Leo Trio. And HD 98388 clearly doesn't!

Edit: Okay! It could have been one of those very distant globulars of the Milky Way. Our galaxy's most distant globular, Laevens 1, is located no less than 470,000 light-years from the center of the Milky Way, and its angular size as seen from the Earth is only 0.46 arcminutes. But this globular is faint and sparse and would most likely not look like a bright star in the vicinity of the Leo Trio.

Ann

Yes, the angular size “angle” is one I hadn’t even considered, but as you say, even so, that alone wouldn’t necessarily rule it out!

[johnnydeep]

From SIMBAD:

I meant why is it the "Tycho" catalog as opposed to, say, the "Peterson" catalog?!

Well, Tycho Brahe was the first astronomer to create a rich, very accurate catalog of astrometric positions, so it's logical that the catalog of Hipparcos observations should honor him.

Alright, that makes sense. But that just begs another question: why is that mission called Hipparcos?! (Wikipedia isn’t telling me.)

[Chris Peterson]

I meant why is it the "Tycho" catalog as opposed to, say, the "Peterson" catalog?!

Well, Tycho Brahe was the first astronomer to create a rich, very accurate catalog of astrometric positions, so it's logical that the catalog of Hipparcos observations should honor him.

Alright, that makes sense. But that just begs another question: why is that mission called Hipparcos?! (Wikipedia isn’t telling me.)

Hmm. It's telling me.

The word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and also a reference to the ancient Greek astronomer Hipparchus of Nicaea, who is noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.

[Ann]

Also take a look at an amazing picture of the 300,000 light-year-long tidal tail of NGC 3628!


Tidal tail of NGC 3628. Credit: Mark Hanson.

Thanks for highlighting this Ann. It took me a while to realise that the tidal tail of NGC 3628 is not really visible on the APOD, but extends a long way off the bottom edge of the frame.

Why does the tail extend in that direction when the interacting galaxy M66 is nowhere near there? Do we know the computed path of the previous interaction?

I've been thinking about this question, and, since I am a math idiot, I must explain it to myself using reasoning and good illustrations of similar examples. So let's consider The Mice!

The strongly interacting pair of galaxies called Teh Mice. Image credit: I don't know.

---

This annotated Hubble image points out some of what is going on. Full image credit: I don't know.

---

Wikipedia wrote about the Mice Galaxies:

NGC 4676, or the Mice Galaxies, are two spiral galaxies in the constellation Coma Berenices. About 290 million light-years distant, they have begun the process of colliding and merging. Their "mice" name refers to the long tails produced by tidal action—the relative difference between gravitational pulls on the near and far parts of each galaxy—known here as a galactic tide. It is a possibility that both galaxies, which are members of the Coma Cluster, have experienced collision, and will continue colliding until they coalesce.

Consider this carousel:

Click to view full size image

In the carousel, the individual "seats" (or whatever they are called) are fastened with chains to a sturdy central "body". When the carousel turns, the seats are flung outward, and if one seat came loose, it would fly off in the direction of the tangent.

In galaxies passing close to each other, very strong tidal forces are pulling on them. And since the mass and therefore the gravitational hold of any moderately large galaxy is always concentrated in the center, the outer parts can more easily "come loose and fly off".

If a seat came loose from a turning carousel, the seat would, as I said, fly off in the direction of the tangent. It would not keep turning in the same direction as the carousel. In the same way, stellar streams that have come loose from galaxies follow their own momentum as they move outwards.

Galaxies M66 and NGC 3628 in Leo interacted some 800 million years ago, when the tidal tail was probably pulled out of NGC 3628. M66 and NGC 3628 have since moved away from one another, and they don't "feel" each other's presence very much any more. It is very likely that the tidal tail of NGC 3628, which may have been ejected some 800 million years ago, currently doesn't feel the presence of M66 at all.

The tidal tail of NGC 3628 is currently minding its own business, and it doesn't care about M66!

Ann

[Chris Peterson]

The tidal tail of NGC 3628 is currently minding its own business, and it doesn't care about M66!

While I get where you're trying to go with this, that's completely the wrong way to think of it! The tidal tail, like the body it has been drawn from, is in orbit around M66. It "cares" about M66 in a similar way that the Moon "cares" about the Sun, despite the appearance that it is orbiting the Earth.

What's important is distinguishing tidal gravitational effects from ordinary gravitational force. These bodies are now far enough apart that tidal forces are small... the bodies are approximately point sources to each other. The tail was created when the bodies came close enough together that different parts of them experienced different forces because they saw each other as extended. Because the tidal forces are currently small, the tail feels about the same thing as the parent body. The gravitational force is large, but because it's the same for all the stars, we're not seeing much continuing distortion.

[Ann]

The tidal tail of NGC 3628 is currently minding its own business, and it doesn't care about M66!

While I get where you're trying to go with this, that's completely the wrong way to think of it! The tidal tail, like the body it has been drawn from, is in orbit around M66. It "cares" about M66 in a similar way that the Moon "cares" about the Sun, despite the appearance that it is orbiting the Earth.

Okay, Chris. I knew you'd set me straight.

Ann

[johnnydeep]

Well, Tycho Brahe was the first astronomer to create a rich, very accurate catalog of astrometric positions, so it's logical that the catalog of Hipparcos observations should honor him.

Alright, that makes sense. But that just begs another question: why is that mission called Hipparcos?! (Wikipedia isn’t telling me.)

Hmm. It's telling me.

The word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and also a reference to the ancient Greek astronomer Hipparchus of Nicaea, who is noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.

Dang, I must be blind. It's even on the first page I see now for the Wikipedia article. <sheesh>