APOD: Facing NGC 1232 (2024 Apr 18)

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APOD: Facing NGC 1232 (2024 Apr 18)

Post by APOD Robot » Thu Apr 18, 2024 4:06 am

Image Facing NGC 1232

Explanation: From our vantage point in the Milky Way Galaxy, we see NGC 1232 face-on. Nearly 200,000 light-years across, the big, beautiful spiral galaxy is located some 47 million light-years away in the flowing southern constellation of Eridanus. This sharp, multi-color, telescopic image of NGC 1232 includes remarkable details of the distant island universe. From the core outward, the galaxy's colors change from the yellowish light of old stars in the center to young blue star clusters and reddish star forming regions along the grand, sweeping spiral arms. NGC 1232's apparent, small, barred-spiral companion galaxy is cataloged as NGC 1232A. Distance estimates place it much farther though, around 300 million light-years away, and unlikely to be interacting with NGC 1232. Of course, the prominent bright star with the spiky appearance is much closer than NGC 1232 and lies well within our own Milky Way.

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Re: APOD: Facing NGC 1232 (2024 Apr 18)

Post by Ann » Thu Apr 18, 2024 7:07 am

Yes, I like it! :D


What I particularly like about Neil Corke's portrait of NGC 1232 is that he shows us the pink emission nebulas in the arms. These are invisible in the ESO APOD. I also like the inclusion of a foreground star, HD 19764 of spectral class K1 III. I find its color perfect for its spectral class. Also note how, on the "top arm", the top end of it continues to the left, but without the young blue stars.

It is actually quite common that galaxies have features of old stars that reach beyond the bright blue regions of star formation. Note in this brilliant photo by R. Jay GaBany how starbursting NGC 4449 is enveloped in a halo of old stars (and there is an elongated structure of old stars near it):

NGC 4449 R Jay GaBany.png
NGC 4449. Credit: R. Jay GaBany.

The most mysterious aspect of NGC 1232 is its apparent companion galaxy, NGC 1232A, seen immediately to the lower left of NGC 1232 proper. It's hard to accept that NGC 1232A is a large background galaxy, located some 300 million light-years away, compared with 47 million light-years away for NGC 1232, because the two galaxies look so similar in shape and stellar types and only different in size. But when you think of it, it makes sense that NGC 1232A is a background object. Let's look at some galaxies that are either interacting or located at very different distances from one another:


So the Heron galaxy and M101/NGC 5474 are examples of large galaxies interacting with a much smaller galaxy. The smaller galaxies are very visibly affected by the interaction.

NGC 7320 and the background ring galaxy inside Hoag's Object are examples of foreground or background galaxies that are not interacting.

If we look at NGC 1232A again, we see no obvious signs of interaction. Unfortunately, there exist no really sharp pictures of it.


NGC 1232A does look somewhat disturbed, but some galaxies do, for no immediately obvious reason. Consider NGC 1313, which lives in splendid isolation but is still acting up.


Also let's compare NGC 1232 itself with M101. Remember that M101 is indeed interacting with NGC 5474.


In my opinion, M101 looks considerably more disturbed than NGC 1232. If NGC 1232A had indeed been a satellite of NGC 1232, the shape of the larger galaxy would probably have been disturbed. So I think we can agree that NGC 1232A is indeed a background object!

Ann
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Re: APOD: Facing NGC 1232 (2024 Apr 18)

Post by Randall_Rathbun » Fri Apr 19, 2024 6:04 pm

Something which perplexes me is that when I type in my brower's search engine that websites come up with different data for NGC1232 distance and size. Most webpages say 60 million light years away, 1 site said 61 million, and only APOD said 47 million light years. There was repetition of the size of 200,000 light years, but one reputable astronomy observatory said the size was 100,000 light years similar to the Milky Way. So how do we sort out all these conflicting facts?

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Re: APOD: Facing NGC 1232 (2024 Apr 18)

Post by Ann » Fri Apr 19, 2024 7:05 pm

Randall_Rathbun wrote: Fri Apr 19, 2024 6:04 pm Something which perplexes me is that when I type in my brower's search engine that websites come up with different data for NGC1232 distance and size. Most webpages say 60 million light years away, 1 site said 61 million, and only APOD said 47 million light years. There was repetition of the size of 200,000 light years, but one reputable astronomy observatory said the size was 100,000 light years similar to the Milky Way. So how do we sort out all these conflicting facts?
There are several methods to measure the distances to nearby galaxies. The first one is using a special kind of variable stars, called Cepheid variables.


The more luminous a Cepheid variable is, the more slowly it pulsates. The pulsations reveal the true brightness of the star, so if astronomers compare the apparent brightness of the Cepheid with its true brightness, as revealed by its pulsations, then they know how far away it is. Therefore, if astronomers find Cepheids in a galaxy and find out how they pulsate, then they can see how far away the galaxy is. I guess no one has bothered looking for Cepheids in NGC 1232.


Another method is to use the tip of the red giant branch. After stars like the Sun have exhausted their core hydrogen, they swell to become red giants. But at a certain brightness, they undergo a helium flash and become fainter again. The problem with identifying the tip of the red giant branch is that galaxies that contain large numbers of young stars, such a NGC 1232, will have red supergiant stars that are brighter than stars at the tip of the red giant branch.


A popular method is to use supernovas type Ia. The progenitors of these supernovas are white dwarfs that have accreted "too much mass" and become unstable, so that they explode. They always explode at more or less the same brightness, because the progenitor white dwarfs contain more or less the same mass.


The easiest way to determine the distance to NGC 1232 might be to search for Cepheid variables in it. It would be necessary to find several Cepheids, in case one is anomalous, but that should not present any great problem in itself. The problem is that you probably need the Hubble Space Telescope or the JWST to find enough Cepheids, and HST and/or JWST would also have to observe NHC 1232 on many occasions to actually find the stars that are pulsating. Just taking a snapshot of a galaxy will not reveal variability in any stars. And the demand for time on HST and JWST for all sorts of research might be so high that it has not been deemed sufficiently interesting to use these telescope to search for Cepheids in NGC 1232.

HST and JWST should certainly be able to resolve red giants at the distance of NGC 1232, and it might be possible to just feed the observations into a sufficiently powerful computer to have it figure out what stars are located at the tip of the red giant branch. But again, this might not be considered a good way to use HST or JWST.

To my knowledge, no supernova of any kind has been observed in NGC 1232.

Other, cruder methods may have been used to determine the distance to NGC 1232, such as redshift. The light from a galaxy gets reddened because of the expansion of the Universe. But redshift is not really reliable for galaxies as close to us as NGC 1232, where the redshift is small and the galaxy's own proper motion may either counteract or exaggerate its redshift. It is possible that the apparent size of NGC 1232 in the sky, combined with the intricacy of its arms, has been used to guess at its distance and true size. We do expect galaxies with long and well-separated arms to be generally large in size.

A real problem when it comes to determining the distance to NGC 1232 is that this galaxy has not been observed by HST or JWST at all. A picture by ESO is the best portrait we have, but it doesn't do a really good job of resolving the stars. Observations by either HST or JWST would tell us so much more about the stellar populations in NGC 1232 and allow us to make much better guesses at this galaxy's distance and size.

Ann
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Re: APOD: Facing NGC 1232 (2024 Apr 18)

Post by AVAO » Sun Apr 21, 2024 5:19 am

Ann wrote: Fri Apr 19, 2024 7:05 pm Ann
ThanX Ann

for your wonderful comments.

What always bothered me about the NASA image of NGC 4526 is the fact that it gives the impression that the supernova is taking place far out next to the actual galaxy. In reality, this was very close to the center of the galaxy, if you understand that the dark dust lanes circle very closely around the center.

Jac

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Re: APOD: Facing NGC 1232 (2024 Apr 18)

Post by johnnydeep » Sun Apr 21, 2024 12:23 pm

AVAO wrote: Sun Apr 21, 2024 5:19 am
Ann wrote: Fri Apr 19, 2024 7:05 pm Ann
ThanX Ann

for your wonderful comments.

What always bothered me about the NASA image of NGC 4526 is the fact that it gives the impression that the supernova is taking place far out next to the actual galaxy. In reality, this was very close to the center of the galaxy, if you understand that the dark dust lanes circle very closely around the center.

Jac

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Wow, that’s quite a different impression one gets depending on the field of view!!
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Re: APOD: Facing NGC 1232 (2024 Apr 18)

Post by ChrisHeinz » Tue May 07, 2024 1:06 am

So re NGC 1232, how many spiral arms? I have counted 5, 7, & 9. ????? :-O