Explanation: Many spiral galaxies have bars across their centers. Even our own Milky Way Galaxy is thought to have a modest central bar. Prominently barred spiral galaxy NGC 1672, featuredhere, was captured in spectacular detail in an image taken by the orbiting Hubble Space Telescope. Visible are dark filamentary dust lanes, young clusters of bright blue stars, red emission nebulas of glowing hydrogen gas, a long bright bar of stars across the center, and a bright active nucleus that likely houses a supermassive black hole. Light takes about 60 million years to reach us from NGC 1672, which spans about 75,000 light years across. NGC 1672, which appears toward the constellation of the Dolphinfish (Dorado), is being studied to find out how a spiral bar contributes to star formation in a galaxy's central regions.
I saw today's APOD in the Recent Submissions thread recently, and it's a splendid image!!!
It's great to see Hubble take three-color pictures. Or maybe the color data comes from another telescope?
The central part of NGC 1672 is fascinating. There is a moderately large, very white churning spiral there. Either this spiral is so bright that it's overexposed, or else its color may be indicative of a mixed population, with a young blue starburst overlaying an old population of yellow stars.
And the entire galaxy looks incredible, with its strongly twisted shape. This is such a beautiful and dynamic image!
Ann wrote:It's great to see Hubble take three-color pictures. Or maybe the color data comes from another telescope?
If all of the original data was used (the "featured here" link goes to the data, but not the image as processed) it was produced from 4 filters- a broad blue one, a narrower green one, a very narrow red one, and a broad near IR one. With these filters it's possible to approximate what might be considered a "true color" image, but it will definitely show anomalies that make it look quite different from what you'd get with a conventional RGB filter set.
Ann wrote:It's great to see Hubble take three-color pictures. Or maybe the color data comes from another telescope?
If all of the original data was used (the "featured here" link goes to the data, but not the image as processed) it was produced from 4 filters- a broad blue one, a narrower green one, a very narrow red one, and a broad near IR one. With these filters it's possible to approximate what might be considered a "true color" image, but it will definitely show anomalies that make it look quite different from what you'd get with a conventional RGB filter set.
That might explain the unusual appearance of the nuclear region.
Ann wrote:It's great to see Hubble take three-color pictures. Or maybe the color data comes from another telescope?
If all of the original data was used (the "featured here" link goes to the data, but not the image as processed) it was produced from 4 filters- a broad blue one, a narrower green one, a very narrow red one, and a broad near IR one. With these filters it's possible to approximate what might be considered a "true color" image, but it will definitely show anomalies that make it look quite different from what you'd get with a conventional RGB filter set.
Mainly just the dust looking redder because of the near IR. Near IR is a really good substitute for red, though.
Ann wrote:That might explain the unusual appearance of the nuclear region.
It's a kind of HDR processing. The middle was typically bright but has been processed to match the overall contrast of the image.
The first thing that came to my mind when looking at this image was, "What is going on in the gravity well at the center of that galaxy?" Then I read the last sentence which indicates it may be the other way around. The study of circumnuclear discs, like that of our galaxy, seems to offer much towards our understanding of the variety of shapes of galaxies and the role of their active nuclei.
Definitely an interesting galaxy, but it looks more like a distorted spiral to me than a true barred spiral like is seen in the 1/9/2016 APOD.
I continue to wonder if barred spirals are not the late stage of a galaxy merger. The start is two galaxies intially interacting as the approach and get distorted. The second stage is a bridge forming between the cores of the galaxies. If the galaxies are get close enough and are going slow enough with respect to each other, the bridge takes in a large amount of mass from the two colliding galaxies and becomes like a new galactic bulge. At this point, the bridge is becoming the bar and the original galaxies become the spiral arms of a new larger spiral galaxy. Then the bar contracts and the spiral arms get wrapped tighter and tighter as time goes on. Given that, the branches in the spiral arms are relics of the sprial arms in the original galaxies.
If that theory holds, NGC 1672 is from a slow-approach and semi-glancing collision which has moved a lot of its original angular momentum into the bar. It is also not a recent collision as the bar is already contracting. The original galaxies must have also had a lot of gas and dust and this is also contributing to the appearance of the galaxy and its high rate of star formation (which is a result of the collision and merger).
Ann wrote:It's great to see Hubble take three-color pictures. Or maybe the color data comes from another telescope?
If all of the original data was used (the "featured here" link goes to the data, but not the image as processed) it was produced from 4 filters- a broad blue one, a narrower green one, a very narrow red one, and a broad near IR one. With these filters it's possible to approximate what might be considered a "true color" image, but it will definitely show anomalies that make it look quite different from what you'd get with a conventional RGB filter set.
Mainly just the dust looking redder because of the near IR. Near IR is a really good substitute for red, though.
The red is also composed of narrowband hydrogen and nitrogen emissions. Those would not show up nearly so well through a broadband red filter, so this approach shows a lot more structure than we'd normally see with RGB.
Guest wrote:For the life of me I cannot see the bar
Same here. I got my BS in Physics, and as part of the fun, took a 100-level astronomy course. Identifying barred spiral galaxies was nearly impossible for me, too. They need to have a mouse-over to indicate what, exactly, the bar is supposed to be here. That's the one exam question I missed. Then, I went on to grad school and had the unfortunate pleasure of being a TA for the same 100-level astronomy course. I never had to explain the difference to anyone, so I just let it go.
I think it's like those 3-d hidden picture things and we're the folks with only one functional eye. Or it's a big joke astronomers play on people 'not in the club.'
I agree that NGC 1672 isn't a very typical barred spiral galaxy. The most obvious of the type show a sharp kink at both ends of the bar, like this one...
But NGC 1672 has a kink at only one end, and even that is arguable. Is it a kink, or is it just a place where a pink star-forming region closer to the central bulge happens to bump up against the next spiral out -- from our point of view?
Guest wrote:For the life of me I cannot see the bar
Same here. I got my BS in Physics, and as part of the fun, took a 100-level astronomy course. Identifying barred spiral galaxies was nearly impossible for me, too. They need to have a mouse-over to indicate what, exactly, the bar is supposed to be here. That's the one exam question I missed. Then, I went on to grad school and had the unfortunate pleasure of being a TA for the same 100-level astronomy course. I never had to explain the difference to anyone, so I just let it go.
I think it's like those 3-d hidden picture things and we're the folks with only one functional eye. Or it's a big joke astronomers play on people 'not in the club.'
Interesting. I've certainly seen barred spirals where the bar is extremely subtle. But with this galaxy- both in this image and others- it fairly leaps out at me. I don't see how it could be missed.
Guest wrote:For the life of me I cannot see the bar
Same here. I got my BS in Physics, and as part of the fun, took a 100-level astronomy course. Identifying barred spiral galaxies was nearly impossible for me, too. They need to have a mouse-over to indicate what, exactly, the bar is supposed to be here. That's the one exam question I missed. Then, I went on to grad school and had the unfortunate pleasure of being a TA for the same 100-level astronomy course. I never had to explain the difference to anyone, so I just let it go.
I think it's like those 3-d hidden picture things and we're the folks with only one functional eye. Or it's a big joke astronomers play on people 'not in the club.'
M88, an unbarred galaxy. Jim Quinn/Adam Block/NOAO/AURA/NSF.
The bar of NGC 1672 can be described as the thick, curving, elongated dust lane that begins on either side of the nuclear region and winds out all the way through the yellow, elongated bulge of the galaxy. The (starforming) spiral arms seem to start at the ends of the bar.
M88, by contrast, has no bulge. The arms start winding out from deep in the center of the galaxy.
Consider two other galaxies, one barred and one unbarred :
Note the strong bar of NGC 1365, with the obvious, slightly curving dust lane running right through it. The spiral arms of NGC 1365 begin at either end of the bar. The rather weak central dust lanes running through the very spherical yellow bulge of NGC 1566 don't seem to affect the shape or location of the spiral arms at all.
Finally, many barred galaxies also have rings. The galaxy at left, NGC 6792, actually has a small bright bar inside its inner bright ring. Outside the inner bright ring there is a bar, and you can see the characteristic twin dust lanes winding their way through the yellow bar and bulge (here the bulge looks more like an oval). Outside that large bar there is a faint, quite non-circular bluish ring. So we have a galaxy with a bar within a ring within a bar within a ring.
But not all barred galaxies have rings, and not all galaxies with rings have bars.
Hmm. A bar is, to me, a long, straight, uniform sort of thick thing, in the starry context, either a dark or bright stripe. I cannot see anything resembling this in any of the images! What am I missing. An alternative definition of a bar, perhaps? I think we should be told ...
chosulman wrote:Hmm. A bar is, to me, a long, straight, uniform sort of thick thing, in the starry context, either a dark or bright stripe. I cannot see anything resembling this in any of the images! What am I missing. An alternative definition of a bar, perhaps? I think we should be told ...
You have to ignore the dust/dark parts. What you're looking for is the bright glowing part crossing the nucleus.
chosulman wrote:Hmm. A bar is, to me, a long, straight, uniform sort of thick thing, in the starry context, either a dark or bright stripe. I cannot see anything resembling this in any of the images! What am I missing. An alternative definition of a bar, perhaps? I think we should be told ...
It is now clear to me that I am also one of those people who cannot easily detect the bars in barred spiral galaxies. I was looking for a much smaller bar near the core and would never have considered the bar to be anywhere near as big as what Chris has indicated with his green polygon.
The more I look at galaxy images, the less I know. I'm suddenly starting to understand why deep sky photography gets such varied responses from the peanut gallery (pass the nuts, please) ... our minds' eyes are all seeing different stuff.
Ann wrote:I think the bar that Chris has indicated is too long, particularly at upper right.
That is entirely possible. Certainly there is some ambiguity with respect to where the "kink" occurs, and the central obscuring dust doesn't help with that. Not that I was trying to be terribly precise; it just seemed as if a few people were having difficulty figuring out where the bar was at all. Hope I helped with that.
Ann wrote:I think the bar that Chris has indicated is too long, particularly at upper right.
That is entirely possible. Certainly there is some ambiguity with respect to where the "kink" occurs, and the central obscuring dust doesn't help with that. Not that I was trying to be terribly precise; it just seemed as if a few people were having difficulty figuring out where the bar was at all. Hope I helped with that.
It can help somewhat to view the galaxy in infrared. Less dust to obfuscate the view. Here's a Spitzer view at 3.6 microns (IRAC1). Almost no dust, here. The bar itself actually seems bowed northward. The eastern half is definitely ambiguous, but I'd put the ends of the bar at the slightly brighter sections that typically punctuate bars. The circumnuclear ring is also very apparent in this image!
NGC 1672: Barred Spiral Galaxy from Hubble
