APOD: M81 and Arp s Loop (2010 Dec 09)

[APOD Robot]

M81 and Arp s Loop

Explanation: One of the brightest galaxies in planet Earth's sky and similar in size to the Milky Way, big, beautiful spiral M81 lies 11.8 million light-years away in the northern constellation Ursa Major. This deep image of the region reveals details in the bright yellow core, but at the same time follows fainter features along the galaxy's gorgeous blue spiral arms and sweeping dust lanes. It also follows the expansive, arcing feature, known as Arp's loop, that seems to rise from the galaxy's disk at the right. Studied in the 1960s, Arp's loop has been thought to be a tidal tail, material pulled out of M81 by gravitational interaction with its large neighboring galaxy M82. But a recent investigation demonstrates that much of Arp's loop likely lies within our own galaxy. The loop's colors in visible and infrared light match the colors of pervasive clouds of dust, relatively unexplored galactic cirrus only a few hundred light-years above the plane of the Milky Way. Along with the Milky Way's stars, the dust clouds lie in the foreground of this remarkable view. M81's dwarf companion galaxy, Holmberg IX, can be seen just above and left of the large spiral. On the sky, this image spans about 0.5 degrees, about the size of the Full Moon.

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[keerthi2kiran]

I see a curved vertical line of (dust?) spanning the whole length of the galaxy just to the right of the center of the galaxy.
Is it really a dust lane? or processing artifact? (or an alien highway )

[jgabany]

Hello keerthi2kiran:

The thin vertical lines you have noticed to the right of the picture's center is dust from Arp's loop, back lit by the far more distant galaxy. This portion of the loop becomes visually washed out as it passes in front of the bright spiral star system so all we can see are areas where the dust is dense enough to absorb the light and prevent it from passing through. As mentioned in the APOD description, Arp's loop is located near the outskirts of the Milky Way and positioned, strictly by chance, along our line of sight to M81 so that it appears partially overlaid.

[keerthi2kiran]

Hii Jay ,
Thanks a lot for the reply. Now it makes sense.
Regards,
Keerthi.

[BWSherwood]

The Deep view really makes clear that the 'dust lane' is part of the dust cloud in our own galaxy obsuring M81, as a part of Arp's Loop that passes between us and M81. The earlier photo linked in today's writeup doesn't show Arp's loop at all so it appears as a dust lane in M81. Amazing what 'point of view' can give us: totally erroneous conclusions. Once we can see a fuller picture we can get a fuller understanding, expanding our POV.

[Jim Leff]

Here's what's always confused me about galaxy photographs. Since we're not viewing M81 flat-on, the light from the back of the galaxy is from an entirely different time as the light from the front. So why does it still look so symmetrical in its spiral? Shouldn't it be all bunched up and weird, reflecting the (vastly) different distance/time?

[orin stepanek]

This galaxy has great symmetry

Click to view full size image

[Chris Peterson]

Here's what's always confused me about galaxy photographs. Since we're not viewing M81 flat-on, the light from the back of the galaxy is from an entirely different time as the light from the front. So why does it still look so symmetrical in its spiral? Shouldn't it be all bunched up and weird, reflecting the (vastly) different distance/time?

Because it isn't a "vastly different distance/time". Large galaxies are on the order of 100,000 ly across, and 1000 ly thick. But galaxies take more than 100 million years to rotate, so the structure simply doesn't move significantly during the short time-of-light delay between the closest and the most distant parts.

[VictorB]

Half way between the outer edge of M81 and the bottom of the photo is a collection of 4 blue stars that looks like a Einstein Cross????

Victor B

[Jim Leff]

Here's what's always confused me about galaxy photographs. Since we're not viewing M81 flat-on, the light from the back of the galaxy is from an entirely different time as the light from the front. So why does it still look so symmetrical in its spiral? Shouldn't it be all bunched up and weird, reflecting the (vastly) different distance/time?

Because it isn't a "vastly different distance/time". Large galaxies are on the order of 100,000 ly across, and 1000 ly thick. But galaxies take more than 100 million years to rotate, so the structure simply doesn't move significantly during the short time-of-light delay between the closest and the most distant parts.

And no other axes of movement would meaningfully impact one side of the galaxy versus the other?

[Chris Peterson]

And no other axes of movement would meaningfully impact one side of the galaxy versus the other?

No. Structurally, a spiral galaxy simply doesn't change much in a few tens of thousands of years, which is about the largest time difference we're likely to observe from one part to another.

[emc]

Today’s APOD is another beauty.

So galactic change brings new meaning to the phrase, “Wait for it… wait for it”
which prompted a web search for star motion, which lead to this cool video from the Hubble website regarding Omega Centauri.

Click to play embedded YouTube video.

[Sam]

Here's what's always confused me about galaxy photographs. Since we're not viewing M81 flat-on, the light from the back of the galaxy is from an entirely different time as the light from the front. So why does it still look so symmetrical in its spiral? Shouldn't it be all bunched up and weird, reflecting the (vastly) different distance/time?

Because it isn't a "vastly different distance/time". Large galaxies are on the order of 100,000 ly across, and 1000 ly thick. But galaxies take more than 100 million years to rotate, so the structure simply doesn't move significantly during the short time-of-light delay between the closest and the most distant parts.

So even for a scientist modeling galactic structure, the distortion is insignificant?

On a related note, which edge of M82 is closer to us, the top (in this image) or bottom?
Is there an easy way to tell from the image alone?

---------

Sam

[Chris Peterson]

So even for a scientist modeling galactic structure, the distortion is insignificant?

Yes.

[mexhunter]

Congratulations to Jay Gabany, Jordi Gallego, Tony Hallas, and all others is a great team.
Greetings
César

[jgabany]

Hello César:

Thank you for the congratulations! I very much appreciate it and will pass your congrats to the other members of the team!

[Ann]

I, too, want to extend my congratulations to the team who produced this magnificent photo.

Personally I'm very interested in M 81's small satellite galaxy, Holmberg IX. This photo offers a splendid portrait of this diminutive dwarf. The galaxy appears to be made up almost exclusively of clusters and associations of young blue stars. An extended pink object to the upper right of Holmberg IX is clearly an emission nebula, and other pink objects in or near the small galaxy may be emission nebulae, too. That is interesting, because it means that the tiny galaxy still has some gas left to make stars of. To me that suggests that Holmberg IX hasn't had many supernovae yet, because such titanic stellar explosions would certainly blow all remaining gas out of the small and gravity-weak galaxy.

However, and very interestingly, at high magnification a relatively large number of very orange, starlike objects can be seen in and around Holmberg IX. What are they? It would be fascinating if they were red supergiants or deep red carbon stars, but I find that very unlikely. The orange objects are too orange and too bright compared with the other stars of Holmberg IX to be stars. Besides, no similar population of bright red stars are visible around the outer edges of M81. No, the orange objects are almost certainly distant, and therefore highly reddened, galaxies. So small blue dwarf galaxy Holmberg IX sits superimposed (from our point of view) on a distant galaxy cluster. Fascinating!

I, too, really appreciated the view of Arp's Loop. It is particularly interesting to see how this dust feature looks brownish when seen superimposed on the bright yellow bulge of M81, but the same dust feature looks blue against the blackness of space. And indeed, there are a few suspiciously pink spots inside the blue arc, testifying to the birth of young blue stars inside it.

Ann

[Céline Richard]

Today’s APOD is another beauty.

So galactic change brings new meaning to the phrase, “Wait for it… wait for it”
which prompted a web search for star motion, which lead to this cool video from the Hubble website regarding Omega Centauri.

Click to play embedded YouTube video.

Thank you for your video

Céline

[DavidLeodis]

It's a terrific image of M81. I can though only just make out what I think may be Arp's Loop as some faint blue bits in the right half of the image. I appreciate that will likely mean I need a much better system to display such images, but I wonder if anyone else has problems in definitely making out what is Arp's Loop.

[jgabany]

Hello DavidLeodis:

The picture has a fairly wide range of contrast, so you may simply need to adjust your monitor's contrast control. My web site has a gray scale image that will help you accomplish this task- it will only take a few seconds:

http://www.cosmotography.com/images/calibrate.html

[DavidLeodis]

Hello DavidLeodis:

The picture has a fairly wide range of contrast, so you may simply need to adjust your monitor's contrast control. My web site has a gray scale image that will help you accomplish this task- it will only take a few seconds:

http://www.cosmotography.com/images/calibrate.html

Thank you R Jay for replying to my query, which is much appreciated. I can make out all the shades in your monitor calibration page so I assume it is either the colour defintion of my monitor or my eyes (or both!) that makes Arp's Loop fairly hard to discern. I can however a bit more readily discern what seems to be Arp's Loop if I invert the colours.

Astrophotographic images taken by you have been featured in a number of APODs and they are excellent. I like looking at the night sky and did so at about 17:00 on Wednesday evening, seeing a nice crescent Moon and Jupiter. My timing was bad though, as I was not looking out around 17:35 when the brilliant bolide appeared. Typical.

[Davelooking]

Can anyone explain the vertical dust lane/line that traverses this galaxy? This structure does not seem to be rotational.It seems like a underlying force is causing a wrinkle or a rift that's manifested by an uninterrupted line of dense dust and gas .

[neufer]

Can anyone explain the vertical dust lane/line that traverses this galaxy? This structure does not seem to be rotational. It seems like a underlying force is causing a wrinkle or a rift that's manifested by an uninterrupted line of dense dust and gas .

http://asterisk.apod.com/vie ... 45#p138473

[jgabany]

I can make out all the shades in your monitor calibration page so I assume it is either the colour defintion of my monitor or my eyes (or both!) that makes Arp's Loop fairly hard to discern. I can however a bit more readily discern what seems to be Arp's Loop if I invert the colours.

Hi David:

Please point your browser to my my web site when you have a moment, access the web page for this project then mouse over the image. I have added an inverted picture with a positive insert of M81 that will make it much easier for you to see the faint structure of Arp's Loop. Here's the direct link:

http://www.cosmotography.com/images/small_ngc3031.html

Astrophotographic images taken by you have been featured in a number of APODs and they are excellent. I like looking at the night sky and did so at about 17:00 on Wednesday evening, seeing a nice crescent Moon and Jupiter. My timing was bad though, as I was not looking out around 17:35 when the brilliant bolide appeared. Typical.

Thank you very much for the compliment David! I appreciate it! However, I'm sorry to learn that you missed seeing the bolide!