by Ann » Tue Dec 26, 2017 5:22 pm
BDanielMayfield wrote:Ann wrote: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
Chris Peterson wrote: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
[quote="BDanielMayfield"][quote="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 [i]4000[/i] 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 [color=#FF0000]it seems certain[/color] 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[/quote]
[quote="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.[/quote]
[size=110][b][color=#0040FF]Ann, what made you so certain that the two galaxies were near each other[/color], [color=#FF00FF]and do you remain as convinced after reading Chris' comment?[/color][/size][/b][/quote]
[b][size=110][color=#0040FF]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.[/color]
[color=#FF00FF]2) I'm less sure of my judgment now. I have a lot of confidence in Chris.[/color][/size][/b]
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, [url=http://www.caelumobservatory.com/mlsc/m61s.jpg]here portrayed by Adam Block[/url].
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