Starship Asterisk*

by **Ann** » Sun Apr 12, 2015 5:06 am

This is going to be my last post here, I promise.

Geck, you just helped me put my finger on what is so strange about the bar of NGC 2903, and what it is that makes it different from the bar of your fictional galaxy. The galaxy you drew has a bar whose bar endings bend up or down in the same directions as the nearby spiral arms. The bar end on the left bends upwards, just like the nearby spiral arms, and the bar end on the right bends downwards, like the nearby spiral arms.

That is what the bar of NGC 2903 doesn't do. Its "left" bar end (in the APOD) really does bend upwards, just like the adjacent spiral arms. But its "right" bar end doesn't. It sticks out, straight and yellow, beyond the spiral arms, and then just abruptly ends.

That is what makes the bar of NGC 2903 unique, as far as I know.

Ann

P.S. Nice song, bystander.

by **bystander** » Sat Apr 11, 2015 9:57 pm

geckzilla wrote:...
This is a fictional galaxy.

Which galaxy is it? Is it a galaxy far, far away?

Click to play embedded YouTube video.

by **geckzilla** » Sat Apr 11, 2015 8:43 pm

I don't just mean dust lanes associated with bars. I drew a picture to help illustrate what I mean. Here, the orange represents the bar portion which has a clearly straight line emanating from the nucleus and bends somewhat abruptly into arms. Meanwhile, other structures of the arms such as younger stars and dust lanes do not follow the bar pattern, represented here with blue and black lines. This is a fictional galaxy.

by **Ann** » Sat Apr 11, 2015 8:29 pm

Thanks, Geck, interesting.

There are all sorts of dust lanes associated with bars. There are, for example, some interesting longitudinal dust lanes in the elongated bar of NGC 2523 - I tried to find a good picture of this galaxy, but I found nothing on the net that was better than James D Wray's old UBV image from 1979. Indeed, all sorts of dust lanes are associated with bars - this large image by Adam Block shows one example, NGC 1530. Another good example (and large picture, unfortunately) is NGC 1097 by Robert Gendler and ESO.

But what makes the bar of NGC 2903 unique to me is that the bar is so long, so yellow, so different in color from the spiral arms, and the fact that a sharply defined yellow piece of it sticks out on the other side of what appears to be a spiral arm. I haven't seen that anywhere else, in any other galaxy.

I'm a stickler for color, as you know, Geck. In NGC 2903 I see something that is glaringly apparent in RGB as well as in UBV images. It may not be obvious at all in infrared images or in general black and white images, but that doesn't mean that the feature isn't there.

After all, you wouldn't accept it if I said that an infrared feature isn't important just because it can't be seen in RGB.

Ann

by **geckzilla** » Sat Apr 11, 2015 8:07 pm

Sorry if I missed your point. Not sure why you'd snap back at me for it.

by **rstevenson** » Sat Apr 11, 2015 7:56 pm

geckzilla wrote:You might find a more appropriate analogue in NGC 1672. http://apod.nasa.gov/apod/ap070418.html

No, I chose NGC 1300 because it had such a clear and simple structure, making obvious the dust crossing the bar, thus allowing me to make my point. Whereas in visible light at least, NGC 1672 appears not to have a bar at all, though I know there is one, made more obvious in IR. So, allow me to make my points the way I want to, and you make yours as you wish.

Rob

by **geckzilla** » Sat Apr 11, 2015 7:49 pm

I'm just looking at them with Aladin. Useful software. Here are screenshots. The brightnesses between the two galaxies might not be comparable. I'm not really sure how these all sky maps deal with the large range of bright objects across the sky. Anyway, the point is to look at the structures and and not the brightness. What I see is two bars whose structures are not necessarily representative of the entire structure of the galaxy. You've got dust lanes crossing in front of both seemingly unaffected by whatever process created the bar. It's just easier to see in NGC 2903. NGC 1672 has its own peculiar aspects. I'm not really sure how you can judge NGC 3187 as being more similar since it is edge on. You note the shape of the arms in 1672 as being a little different from 2903 but you can't even see the shape of the arms in 3187. It's just an edge-on blobby mess.

by **Ann** » Sat Apr 11, 2015 7:22 pm

Geck wrote:
I already posted a link to NGC 1672 just before your post which has a similarly nebulous bar which is quite easily noticed in infrared observations but much more easily missed in other wavelengths.

Indeed, as I looked through Wray's The Color Atlas of Galaxies I, too, noted NGC 1672, of course. I just didn't think it was really similar to NGC 2903. To me, NGC 2903 seems to have a disk that is relatively "flat", but the spirals arms or dust features crossing the bar seem to be out of plane. I don't think NGC 1672 looks like that, but instead it seems to be more like NGC 3187, where the arms and the bar are located in entirely different planes, quite unlike NGC 2903.

So to me, M108 is the galaxy that most closely resembles NGC 2903, where the disk seems to be relatively "flat", but some dust features are put of plane. However, NGC 1672 may be more interesting and more similar to NGC 2903 than it appears to be to me. Do you have a good picture of the bar of NGC 1672, Geck? And do those infrared images show spiral features - not just minor dust features - that are crossing and partly hiding the bar? Note that dust features that seem to radiate outwards from the bar are quite common in barred galaxies, but they don't hide the bar.

Ann

by **geckzilla** » Sat Apr 11, 2015 6:34 pm

Ann wrote:
geckzilla wrote:It's a little baffling to me how the dust and star forming regions completely ignore the bar and appear as a continuous spiral overlapping the bar. It is something I have never noticed before but seems to be typical for bars so this is a normal example rather than an exception.
I don't think the bar of NGC 2903 is typical. I think it is weird.

You will forgive me for once again referring to James D Wray's The Color Atlas of Galaxies. In that atlas, Wray very often compared galaxies with one another.

. . .

But Wray could see no counterpart to the bar of NGC 2903. I really think that the bar of NGC 2903 is highly unusual. However, I think there just might be a counterpart, and that might be M108, NGC 3556.

Perhaps if Wray looked again with modern computer equipment and easy access to search through vast amounts of data, he would be more successful. Bars come in a few different styles and I have no problem finding images of galaxies with characteristics similar to NGC 2903. I already posted a link to NGC 1672 just before your post which has a similarly nebulous bar which is quite easily noticed in infrared observations but much more easily missed in other wavelengths. I agree that 2903 is at least a little weird, but it's not weird enough for me to say that there is no other bar similar to it. Really, the more I looked at NGC 2903, the less weird it seemed. The really weird objects tend to get even weirder upon further inspection.

by **Ann** » Sat Apr 11, 2015 6:00 pm

geckzilla wrote:It's a little baffling to me how the dust and star forming regions completely ignore the bar and appear as a continuous spiral overlapping the bar. It is something I have never noticed before but seems to be typical for bars so this is a normal example rather than an exception.

I don't think the bar of NGC 2903 is typical. I think it is weird.

You will forgive me for once again referring to James D Wray's The Color Atlas of Galaxies. In that atlas, Wray very often compared galaxies with one another. So, for example, Wray wrote about NGC 2841:

The broad outer disk is similar to that of NGC 2775, although long dust lanes are more in evidence here.

He wrote about NGC 2959:

Compare with NGC 2787 and NGC 936.

So Wray often asked his readers to understand a galaxy better by looking at a picture of another galaxy that is similar to it. (That, by the way, is why I often like to point out other galaxies that are similar to this or that galaxy portrait of this or that APOD - I try to take after James D Wray.)

But Wray could see no counterpart to the bar of NGC 2903. I really think that the bar of NGC 2903 is highly unusual. However, I think there just might be a counterpart, and that might be M108, NGC 3556. According to Wikipedia, M108 is a barred galaxy, and photographs show it to have a disturbed disk with out-of-plane dust features.

Tammy Plotner wrote about M108:

Located about 45 million light years away from Earth and running away from us at 772 kilometers per second, this disturbed looking galaxy is rich in dark dust, star forming regions and a supershell.
...
“Since this galaxy is isolated, the supershells are unlikely to have been created through impacting external clouds, yet the required input energy is also greater than that available from the observed internal star formation rate. Thus it would appear that some form of energy enhancement (such as magnetic fields) must also be important in creating these features. The supershells are so dominant that they distort the outer major axis.

Perhaps, if we had a more face-on view of M108, its bar might look a bit broken by the out-of-plane supershell.

Personally I don't think that NGC 2903 looks like it might be the product of a recent merger. Its outer features are far too "settled" and regular for that. Also M108 may or may not - probably not - be the product of a merger. But galaxies may have upheavals for other, more mysterious reasons. It's anybody's guess why isolated NGC 1313 is having the star formation chaos tantrum that it, in fact, does have.

So I don't think the weird bar of NGC 2903 is the product of a merger, although something strange has clearly occurred inside this galaxy.

Ann

by **geckzilla** » Sat Apr 11, 2015 12:54 pm

You might find a more appropriate analogue in NGC 1672. http://apod.nasa.gov/apod/ap070418.html

by **rstevenson** » Sat Apr 11, 2015 12:41 pm

It might help to take a look at NGC 1300, since it's aligned so we can see its bar clearly from above/below...

We can see that the bar is a clearly delineated structure within the galaxy, but that it also has overlying filaments of gas and dust, with lots of star formation occurring in nearby spiral arms.

But NGC 1300 has a much simpler structure than NGC 2903, with much more room between the spiral arms and the bar. From this POV, it's tempting to picture it as a single thin layer. But like all galaxies, it's not that thin. There's lots of vertical depth in a galaxy, and lots of gas and dust can slide over and under the arms and the central bar.

Probably NGC 2903 has been disrupted fairly recently by a galaxy merger, stirring things up and setting off the large amount of star formation we see. With lots of data and some excellent modeling, we might be able to deduce the approximate mass and the direction of passage of the disrupting galaxy.

Rob

by **geckzilla** » Sat Apr 11, 2015 9:47 am

It's a little baffling to me how the dust and star forming regions completely ignore the bar and appear as a continuous spiral overlapping the bar. It is something I have never noticed before but seems to be typical for bars so this is a normal example rather than an exception.

by **Ann** » Sat Apr 11, 2015 4:21 am

Dad is watching wrote:I took this image and converted it to 8-bit grey scale to see if I could get a better look at the 'bar'. What I found was that the left half of the bar virtually disappeared and the hi-lites seemed to align with the inner arms of the spiral. The right side of the 'bar' seems to curve toward the bottom of the image as it approaches the central region and hi-lites of the 'bar' seemed to follow along the arms as well. Neither one seems to bisect the galaxy center and the 'bar' itself seems to lack any real structure. Are we looking at an optical illusion cause by the angle we are viewing it at, and by lighting and shadow?

The bar is not an illusion. A gray scale may not show it, but a color picture will. Most bars are old and yellow, and the bar of NGC 2903 is bright, old and yellow. A color picture will clearly show that the stellar population in the bar is distinctly different from the stellar populations in the spiral arms.

Ann

by **Dad is watching** » Sat Apr 11, 2015 12:51 am

I took this image and converted it to 8-bit grey scale to see if I could get a better look at the 'bar'. What I found was that the left half of the bar virtually disappeared and the hi-lites seemed to align with the inner arms of the spiral. The right side of the 'bar' seems to curve toward the bottom of the image as it approaches the central region and hi-lites of the 'bar' seemed to follow along the arms as well. Neither one seems to bisect the galaxy center and the 'bar' itself seems to lack any real structure. Are we looking at an optical illusion cause by the angle we are viewing it at, and by lighting and shadow?

by **Boomer12k** » Fri Apr 10, 2015 11:37 pm

AND....GREAT WORK TONY!!!!!!!!!!!!

He helped me with some of my pictures.

:---[===] *

by **Ann** » Fri Apr 10, 2015 6:55 pm

Chris Peterson wrote:
geckzilla wrote:It actually doesn't for me. I looked at my own picture too (I think I should reprocess that image, bar aside) and it's just really hard for me to shake the interpretation that it's a streak wholly behind the dust rather than intermingling with it.
A streak that appears behind the dust isn't quite the same as a streak that appears behind the galaxy, however. It is easier to explain the former as a physical reality, and not merely an illusion.

I looked through James D Wray's The Color Atlas of Galaxies to see if I could find any sort of match to NGC 2903. No luck, however. And that is not surprising, given how interested James D Wray was in the bar of NGC 2903.

But Wray's atlas really shows a lot of galaxies with weird and warped spiral arm configurations. I'm sure Chris is right that in NGC 2903 the bar is behind an out-of-plane dust lane belonging to a spiral arm, but it is not behind the galaxy proper.

Searching elsewhere, I did find a picture of a galaxy that has some interesting similarities to NGC 2903, however. I'm sorry that the only picture I could find of this galaxy, NGC 1808, is so big - over 1,000 KB. Nevertheless, please note the brilliantly starbursting center which is stirring up the dust and the spiral arms of NGC 1808.

Ann

by **FloridaMike** » Fri Apr 10, 2015 6:08 pm

Chris Peterson wrote: ...a complex interaction between star formation and density gradients caused by gravitational resonances...

AKA , Bartenders..

by **Craine** » Fri Apr 10, 2015 5:22 pm

I found this link interesting:
http://hubblesite.org/hubble_discoverie ... lution.pdf

by **Chris Peterson** » Fri Apr 10, 2015 4:58 pm

Just Jackson wrote:So, does anyone know why / how these bars form? Last I heard, it was a mystery.

It's not fully understood, but neither is it a complete mystery. They appear to be created by a complex interaction between star formation and density gradients caused by gravitational resonances. The fact that they occur in simulations allows the mechanisms to be explored.

by **Just Jackson** » Fri Apr 10, 2015 4:47 pm

So, does anyone know why / how these bars form? Last I heard, it was a mystery.

by **Chris Peterson** » Fri Apr 10, 2015 2:25 pm

geckzilla wrote:It actually doesn't for me. I looked at my own picture too (I think I should reprocess that image, bar aside) and it's just really hard for me to shake the interpretation that it's a streak wholly behind the dust rather than intermingling with it.

A streak that appears behind the dust isn't quite the same as a streak that appears behind the galaxy, however. It is easier to explain the former as a physical reality, and not merely an illusion.

by **Ron-Astro Pharmacist** » Fri Apr 10, 2015 2:10 pm

http://en.wikipedia.org/wiki/Barred_spiral_galaxy

From above.

…Bars are thought to be temporary phenomena in the lives of spiral galaxies; the bar structures decay over time…

Could the bar in this galaxy be on the decline?

by **geckzilla** » Fri Apr 10, 2015 1:03 pm

rstevenson wrote:A couple of years ago, our very own geckzilla did a version of the center of NGC 2903 based on HST data. It gives a much different view of the bar, removing, I think, any doubt as to its being part of the galaxy.

Rob

It actually doesn't for me. I looked at my own picture too (I think I should reprocess that image, bar aside) and it's just really hard for me to shake the interpretation that it's a streak wholly behind the dust rather than intermingling with it.

by **Boomer12k** » Fri Apr 10, 2015 12:44 pm

It looks, from this image, that there are TWO bars...not connected in the center, and the spiral arms appear over the top of them...it is most interesting, like TWO galaxies at the ENDing stages of their collision, and it has settled down quit a bit, but you still have lots of star formation. It would be interesting to see any star streams, as stars look to fill the voids between the arms, in the large gaps...but it is probably just me...other shots look lots different.

:---[===] *

Starship Asterisk*

APOD: NGC 2903: A Missing Jewel in Leo (2015 Apr 10)

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