APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

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APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by APOD Robot » Thu Feb 23, 2023 5:05 am

Image Arp 78: Peculiar Galaxy in Aries

Explanation: Peculiar spiral galaxy Arp 78 is found within the boundaries of the head strong constellation Aries. Some 100 million light-years beyond the stars and nebulae of our Milky Way galaxy, the island universe is an enormous 200,000 light-years across. Also known as NGC 772, it sports a prominent, outer spiral arm in this detailed cosmic portrait. Tracking along sweeping dust lanes and lined with young blue star clusters, Arp 78's overdeveloped spiral arm is pumped-up by galactic-scale gravitational tides. Interactions with its brightest companion galaxy, the more compact NGC 770 seen above and right of the larger spiral, are likely responsible. Embedded in faint star streams revealed in the deep telescopic exposure, NGC 770's fuzzy, elliptical appearance contrasts nicely with spiky foreground Milky Way stars in matching yellowish hues.

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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by Ann » Thu Feb 23, 2023 6:34 am

APOD 23 February 2023 annotated.png

To me it looks as if NGC 772 is interacting with three of its satellite galaxies, LEDA 212884, PGC 7509 and NGC 770, which all appear to be located rather close to one another one one side of large galaxy NGC 772. The combined pull of these satellite galaxies may have "gravitationally grabbed hold of NGC 772's spiral arms" and twisted them out of shape.

In particular, to me it looks like the bright blue arm had been tugged at as if it was a puppet on a string.


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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by johnnydeep » Thu Feb 23, 2023 3:45 pm

Ok, so how many arms does this spiral have, and which one is the "pumped" up one due to interactions with the companion galaxy? The Noirlab image didn't help to clear it up for me, since the two parallel arms prominently shown there - one blue, one dusty - don't seem to be the same one(s) Ann talks about in her post:

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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by Ann » Thu Feb 23, 2023 5:17 pm

johnnydeep wrote: Thu Feb 23, 2023 3:45 pm Ok, so how many arms does this spiral have, and which one is the "pumped" up one due to interactions with the companion galaxy? The Noirlab image didn't help to clear it up for me, since the two parallel arms prominently shown there - one blue, one dusty - don't seem to be the same one(s) Ann talks about in her post:

The blue arm and the dusty arm are not the same. They are two different arms.

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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by johnnydeep » Thu Feb 23, 2023 6:48 pm

Ann wrote: Thu Feb 23, 2023 5:17 pm
johnnydeep wrote: Thu Feb 23, 2023 3:45 pm Ok, so how many arms does this spiral have, and which one is the "pumped" up one due to interactions with the companion galaxy? The Noirlab image didn't help to clear it up for me, since the two parallel arms prominently shown there - one blue, one dusty - don't seem to be the same one(s) Ann talks about in her post:

The blue arm and the dusty arm are not the same. They are two different arms.

Ann
Right. But is either of them the "pumped up one"?
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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by Ann » Thu Feb 23, 2023 8:09 pm

johnnydeep wrote: Thu Feb 23, 2023 6:48 pm
Ann wrote: Thu Feb 23, 2023 5:17 pm
johnnydeep wrote: Thu Feb 23, 2023 3:45 pm Ok, so how many arms does this spiral have, and which one is the "pumped" up one due to interactions with the companion galaxy? The Noirlab image didn't help to clear it up for me, since the two parallel arms prominently shown there - one blue, one dusty - don't seem to be the same one(s) Ann talks about in her post:

The blue arm and the dusty arm are not the same. They are two different arms.

Ann
Right. But is either of them the "pumped up one"?
No idea. All I can say is that NGC 772 has two "parallel arms", "following the same path" and "bending equally".

Spiral galaxies don't normally look like that.

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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by johnnydeep » Fri Feb 24, 2023 2:24 pm

Ann wrote: Thu Feb 23, 2023 8:09 pm
johnnydeep wrote: Thu Feb 23, 2023 6:48 pm
Ann wrote: Thu Feb 23, 2023 5:17 pm

The blue arm and the dusty arm are not the same. They are two different arms.

Ann
Right. But is either of them the "pumped up one"?
No idea. All I can say is that NGC 772 has two "parallel arms", "following the same path" and "bending equally".

Spiral galaxies don't normally look like that.

Ann
By "pumped up" I'm just using the wording from the text and the pic I posted above. It's paired with "overdeveloped". Neither of the two parallel arms look that way to me despite the statements that they were thusly affected by interaction with the companion galaxies on the other side of the galaxy. So, I remain unsure what is really being referred to here.
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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by bystander » Fri Feb 24, 2023 3:54 pm

Ann wrote: Thu Feb 23, 2023 8:09 pm
No idea. All I can say is that NGC 772 has two "parallel arms", "following the same path" and "bending equally".

Spiral galaxies don't normally look like that.

Ann
There may be as many as three spiral arms. There is an outer arm visible in the APOD that is not as evident in the Gemini North image.
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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by Chris Peterson » Fri Feb 24, 2023 4:25 pm

johnnydeep wrote: Fri Feb 24, 2023 2:24 pm
Ann wrote: Thu Feb 23, 2023 8:09 pm
johnnydeep wrote: Thu Feb 23, 2023 6:48 pm

Right. But is either of them the "pumped up one"?
No idea. All I can say is that NGC 772 has two "parallel arms", "following the same path" and "bending equally".

Spiral galaxies don't normally look like that.

Ann
By "pumped up" I'm just using the wording from the text and the pic I posted above. It's paired with "overdeveloped". Neither of the two parallel arms look that way to me despite the statements that they were thusly affected by interaction with the companion galaxies on the other side of the galaxy. So, I remain unsure what is really being referred to here.
I took it as a reference to the one arm that has lots of star formation going on, in comparison with the other that does not.
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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by johnnydeep » Fri Feb 24, 2023 6:59 pm

Chris Peterson wrote: Fri Feb 24, 2023 4:25 pm
johnnydeep wrote: Fri Feb 24, 2023 2:24 pm
Ann wrote: Thu Feb 23, 2023 8:09 pm

No idea. All I can say is that NGC 772 has two "parallel arms", "following the same path" and "bending equally".

Spiral galaxies don't normally look like that.

Ann
By "pumped up" I'm just using the wording from the text and the pic I posted above. It's paired with "overdeveloped". Neither of the two parallel arms look that way to me despite the statements that they were thusly affected by interaction with the companion galaxies on the other side of the galaxy. So, I remain unsure what is really being referred to here.
I took it as a reference to the one arm that has lots of star formation going on, in comparison with the other that does not.
That makes some sense. Odd though that the other arm on that side doesn't show that same plethora of star formation. It's also odd to me that that's happening on the side opposite all the companion galaxies, though perhaps that's explained by the galaxy having rotated way from them?
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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by Ann » Fri Feb 24, 2023 9:58 pm

Chris Peterson wrote: Fri Feb 24, 2023 4:25 pm
johnnydeep wrote: Fri Feb 24, 2023 2:24 pm
Ann wrote: Thu Feb 23, 2023 8:09 pm

No idea. All I can say is that NGC 772 has two "parallel arms", "following the same path" and "bending equally".

Spiral galaxies don't normally look like that.

Ann
By "pumped up" I'm just using the wording from the text and the pic I posted above. It's paired with "overdeveloped". Neither of the two parallel arms look that way to me despite the statements that they were thusly affected by interaction with the companion galaxies on the other side of the galaxy. So, I remain unsure what is really being referred to here.
I took it as a reference to the one arm that has lots of star formation going on, in comparison with the other that does not.
I'd say that's it, Chris.

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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by orin stepanek » Fri Feb 24, 2023 10:23 pm

NGC772-L2bh-RGB-19-8aT-cC1024.jpg
JMHO but the reddish spiral looks as though it starts at the
opposite side of the galAxy but winds up following the same path
as the blue spiral! Kinda foolish on my part! :oops:
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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by Ann » Fri Feb 24, 2023 10:39 pm

johnnydeep wrote: Fri Feb 24, 2023 6:59 pm
Chris Peterson wrote: Fri Feb 24, 2023 4:25 pm
johnnydeep wrote: Fri Feb 24, 2023 2:24 pm

By "pumped up" I'm just using the wording from the text and the pic I posted above. It's paired with "overdeveloped". Neither of the two parallel arms look that way to me despite the statements that they were thusly affected by interaction with the companion galaxies on the other side of the galaxy. So, I remain unsure what is really being referred to here.
I took it as a reference to the one arm that has lots of star formation going on, in comparison with the other that does not.
That makes some sense. Odd though that the other arm on that side doesn't show that same plethora of star formation. It's also odd to me that that's happening on the side opposite all the companion galaxies, though perhaps that's explained by the galaxy having rotated way from them?
Galaxies that are forming stars "asymmetrically", so that one arm contains a lot more star formation than the other arm (or arms), are not at all unheard of. Even though NGC 772 is an unusually striking example.

Here are two examples of other galaxies producing starbursts in some parts of one arm, but not in other places:

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Re: APOD: Arp 78: Peculiar Galaxy in Aries (2023 Feb 23)

Post by johnnydeep » Sat Feb 25, 2023 2:45 pm

Ann wrote: Fri Feb 24, 2023 10:39 pm
johnnydeep wrote: Fri Feb 24, 2023 6:59 pm
Chris Peterson wrote: Fri Feb 24, 2023 4:25 pm

I took it as a reference to the one arm that has lots of star formation going on, in comparison with the other that does not.
That makes some sense. Odd though that the other arm on that side doesn't show that same plethora of star formation. It's also odd to me that that's happening on the side opposite all the companion galaxies, though perhaps that's explained by the galaxy having rotated way from them?
Galaxies that are forming stars "asymmetrically", so that one arm contains a lot more star formation than the other arm (or arms), are not at all unheard of. Even though NGC 772 is an unusually striking example.

Here are two examples of other galaxies producing starbursts in some parts of one arm, but not in other places:

Ann
Yes, but... The two arms in question in Arp 78 are quite close and look to me nearly identical except for the inner one showing frenetic star formation and the outer one showing none at all. If the inner one was "pumped up" by a companion galaxy, why not the outer one too? But perhaps the visible appearance is just an artifact of the imaging details and shows nothing meaningful about the dust/gas content of either. E.g., maybe the inner arm ended up with most of the gas and the outer one got most of the dust.
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