APOD: Grand Spiral Galaxy NGC 1232 (2017 Dec 26)

[APOD Robot]

Grand Spiral Galaxy NGC 1232

Explanation: Galaxies are fascinating not only for what is visible, but for what is invisible. Grand spiral galaxy NGC 1232, captured in detail by one of the Very Large Telescopes, is a good example. The visible is dominated by millions of bright stars and dark dust, caught up in a gravitational swirl of spiral arms revolving about the center. Open clusters containing bright blue stars can be seen sprinkled along these spiral arms, while dark lanes of dense interstellar dust can be seen sprinkled between them. Less visible, but detectable, are billions of dim normal stars and vast tracts of interstellar gas, together wielding such high mass that they dominate the dynamics of the inner galaxy. Leading theories indicate that even greater amounts of matter are invisible, in a form we don't yet know. This pervasive dark matter is postulated, in part, to explain the motions of the visible matter in the outer regions of galaxies.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Ann]

NGC 1232 is a large, bright, elegant and relatively red spiral galaxy of Hubble type Sc or SBc. The galaxy has a relatively large and bright old yellow population.

A fascinating aspect of this galaxy is that it has a small companion, NGC 1232A, seen at 7 o'clock in today's APOD. The yellow bar of the small companion is clearly visible.

The radial velocity of NGC 1232 is around 1600 kilometers per second, which suggests that its distance from us is around 59 million light-years, or so my software says anyway. Optical observations agree with measurements of the radial velocity derived from other parameters, such as neutral hydrogen, the movement of the Local Group and the 3K background.

Amazingly, however, the radial velocity of 1232A as derived from optical observations appears to be around 4000 kilometers per second, which would put NGC 1232A far in the background of NGC 1232 proper! But non-optical measurements of the distance to the small barred spiral suggests that it is located right next to its big "sibling".

To me it seems certain that the two unequally sized galaxies are indeed next-door neighbors. A small companion like NGC 1232A would be the perfect "tool" for winding up NGC 1232 and giving it its elegant shape.

Ann

[marially3@yahoo.com]

What are the orange dots seen in pairs at 1, 4, 7 and 9 o'clock?

[rstevenson]

What are the orange dots seen in pairs at 1, 4, 7 and 9 o'clock?

I suspect they are bright star-forming regions. Their 'pairness' would be coincidental. We humans see patterns whether they're meaningful or not.

Rob

[Ann]

What are the orange dots seen in pairs at 1, 4, 7 and 9 o'clock?

I suspect they are bright star-forming regions. Their 'pairness' would be coincidental. We humans see patterns whether they're meaningful or not.

Rob

I can't spot anything that looks like orange pairs to me.

But there are orange dots scattered all over the face of the galaxy. In my experience, such orange dots are either background galaxies, foreground stars or bright red giants in the galaxy itself.

I don't think the orange dots are regions of star formation. The filters used for this image are an ultraviolet filter centered at 360 nm, a blue one centered at 420 nm, and an orange-red one at 600 nm. I don't see how such filters would make regions of star formation look like bright orange point sources.

I would go for bright red giants here. There is a bright foreground star at 11 o'clock, and there are two obvious elongated background galaxies, one at 2 o'clock and one at 10 o'clock. There are a few other galaxy candidates, in particular a weird-looking bright object close to 12 o'clock.

But there are also a number of widely scattered bright orangish point sources, and it wouldn't surprise me entirely if they are very bright red giants in NGC 1232. But I may be wrong.

Ann

[Chris Peterson]

A fascinating aspect of this galaxy is that it has a small companion, NGC 1232A, seen at 7 o'clock in today's APOD. The yellow bar of the small companion is clearly visible.

The radial velocity of NGC 1232 is around 1600 kilometers per second, which suggests that its distance from us is around 59 million light-years, or so my software says anyway. Optical observations agree with measurements of the radial velocity derived from other parameters, such as neutral hydrogen, the movement of the Local Group and the 3K background.

Amazingly, however, the radial velocity of 1232A as derived from optical observations appears to be around 4000 kilometers per second, which would put NGC 1232A far in the background of NGC 1232 proper! But non-optical measurements of the distance to the small barred spiral suggests that it is located right next to its big "sibling".

The only reliable and unambiguous optical measurement of distance comes from the redshift, and that is very different for the two galaxies. It seems extremely unlikely that they are anywhere near each other. There's no way to reconcile a z=0.022161 galaxy as a satellite of a z=0.005611 galaxy (that's over 6000 km/s vs. 1600 km/s). I'd value of the redshift distance over that of a distance computed by using the magnitude in a theoretical calculation.

[BDanielMayfield]

A fascinating aspect of this galaxy is that it has a small companion, NGC 1232A, seen at 7 o'clock in today's APOD. The yellow bar of the small companion is clearly visible.

The radial velocity of NGC 1232 is around 1600 kilometers per second, which suggests that its distance from us is around 59 million light-years, or so my software says anyway. Optical observations agree with measurements of the radial velocity derived from other parameters, such as neutral hydrogen, the movement of the Local Group and the 3K background.

Amazingly, however, the radial velocity of 1232A as derived from optical observations appears to be around 4000 kilometers per second, which would put NGC 1232A far in the background of NGC 1232 proper! But non-optical measurements of the distance to the small barred spiral suggests that it is located right next to its big "sibling".

To me it seems certain that the two unequally sized galaxies are indeed next-door neighbors. A small companion like NGC 1232A would be the perfect "tool" for winding up NGC 1232 and giving it its elegant shape.

Ann

The only reliable and unambiguous optical measurement of distance comes from the redshift, and that is very different for the two galaxies. It seems extremely unlikely that they are anywhere near each other. There's no way to reconcile a z=0.022161 galaxy as a satellite of a z=0.005611 galaxy (that's over 6000 km/s vs. 1600 km/s). I'd value of the redshift distance over that of a distance computed by using the magnitude in a theoretical calculation.

Ann, what made you so certain that the two galaxies were near each other, and do you remain as convinced after reading Chris' comment?

[neufer]

The only reliable and unambiguous optical measurement of distance comes from the redshift, and that is very different for the two galaxies. It seems extremely unlikely that they are anywhere near each other. There's no way to reconcile a z=0.022161 galaxy as a satellite of a z=0.005611 galaxy (that's over 6000 km/s vs. 1600 km/s). I'd value of the redshift distance over that of a distance computed by using the magnitude in a theoretical calculation.

<<NGC 1232A [z(~) = 0.022161] is a satellite galaxy of NGC 1232 [z(emission) = 0.005611]. It is thought to be the cause of unusual bending in the spiral arms. In 1988, NGC 1232A was estimated to be 68 million light-years away while NGC 1232 was estimated to be 65 million light-years away.>>

With an estimated separation of ~ 3 million light-years and a measured delta z of ~1/60
NGC 1232A might well have been in vicinity of NGC 1232 ~180 million years [~1 galactic rotation] ago
and, therefore, possibly been responsible for the "unusual bending in [NGC 1232's] spiral arms."

[Ann]

A fascinating aspect of this galaxy is that it has a small companion, NGC 1232A, seen at 7 o'clock in today's APOD. The yellow bar of the small companion is clearly visible.

The radial velocity of NGC 1232 is around 1600 kilometers per second, which suggests that its distance from us is around 59 million light-years, or so my software says anyway. Optical observations agree with measurements of the radial velocity derived from other parameters, such as neutral hydrogen, the movement of the Local Group and the 3K background.

Amazingly, however, the radial velocity of 1232A as derived from optical observations appears to be around 4000 kilometers per second, which would put NGC 1232A far in the background of NGC 1232 proper! But non-optical measurements of the distance to the small barred spiral suggests that it is located right next to its big "sibling".

To me it seems certain that the two unequally sized galaxies are indeed next-door neighbors. A small companion like NGC 1232A would be the perfect "tool" for winding up NGC 1232 and giving it its elegant shape.

Ann

The only reliable and unambiguous optical measurement of distance comes from the redshift, and that is very different for the two galaxies. It seems extremely unlikely that they are anywhere near each other. There's no way to reconcile a z=0.022161 galaxy as a satellite of a z=0.005611 galaxy (that's over 6000 km/s vs. 1600 km/s). I'd value of the redshift distance over that of a distance computed by using the magnitude in a theoretical calculation.

Ann, what made you so certain that the two galaxies were near each other, and do you remain as convinced after reading Chris' comment?

1) I thought the blue knots of star formation looked very similar in size in the two galaxies, suggesting that the galaxies are at the same distance from us.

2) I'm less sure of my judgment now. I have a lot of confidence in Chris.


I might add that NGC 1232A looks rather small to me, due to the relative faintness of its yellow population compared with the brilliance of its blue knots of star formation. That sort of appearance is typical of moderately small galaxies, in my opinion. The only large galaxy I can think of whose morphology is somewhat similar to NGC 1232A is M61, a Virgo galaxy with thick, starbursting blue arms and a relatively bright but rather small elongated yellow center, here portrayed by Adam Block.

Note the small companion galaxy of M61, NGC 4301, in Adam Block's picture. It has bright blue knots of star formation, but a rather faint, not very yellow and possibly elongated center.

Ann

[Chris Peterson]

The only reliable and unambiguous optical measurement of distance comes from the redshift, and that is very different for the two galaxies. It seems extremely unlikely that they are anywhere near each other. There's no way to reconcile a z=0.022161 galaxy as a satellite of a z=0.005611 galaxy (that's over 6000 km/s vs. 1600 km/s). I'd value of the redshift distance over that of a distance computed by using the magnitude in a theoretical calculation.

<<NGC 1232A [z(~) = 0.022161] is a satellite galaxy of NGC 1232 [z(emission) = 0.005611]. It is thought to be the cause of unusual bending in the spiral arms. In 1988, NGC 1232A was estimated to be 68 million light-years away while NGC 1232 was estimated to be 65 million light-years away.>>

With an estimated separation of ~ 3 million light-years and a measured delta z of ~1/60
NGC 1232A might well have been in vicinity of NGC 1232 ~180 million years [~1 galactic rotation] ago
and, therefore, possibly been responsible for the "unusual bending in [NGC 1232's] spiral arms."

Maybe. But if so, NGC 1232A is not, and was not, a "satellite galaxy" of NGC 1232, merely a coincidentally passing object that tidally disrupted the larger galaxy in passing.