APOD: Messier 81 (2019 Apr 17)

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APOD: Messier 81 (2019 Apr 17)

Post by APOD Robot » Wed Apr 17, 2019 4:08 am

Image Messier 81

Explanation: One of the brightest galaxies in planet Earth's sky is similar in size to our Milky Way Galaxy: big, beautiful Messier 81. Also known as NGC 3031 or Bode's galaxy for its 18th century discoverer, this grand spiral can be found toward the northern constellation of Ursa Major, the Great Bear. The detailed telescopic view reveals M81's bright yellow nucleus, blue spiral arms, pink starforming regions, and sweeping cosmic dust lanes. Some dust lanes actually run through the galactic disk (left of center), contrary to other prominent spiral features though. The errant dust lanes may be the lingering result of a close encounter between M81 and its smaller companion galaxy, M82. Scrutiny of variable stars in M81 has yielded one of the best determined distances for an external galaxy -- 11.8 million light-years.

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Ann
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Re: APOD: Messier 81 (2019 Apr 17)

Post by Ann » Wed Apr 17, 2019 5:50 am

M81, M82 and tiny Holmberg IX, as well as galactic cirrus from the Milky Way.
Photo: Andre van den Hoeven.
Ah, good old M81. It is a red galaxy, with a huge yellow bulge. It is also a grand design galaxy, with two elegant and symmetrical spiral arms. Interactions with neighboring galaxies M82 and NGC 3077 might have enhanced the spiral pattern. We might notice, too, that M81, like the Andromeda galaxy, is unbarred.















An interesting object in today's APOD is dwarf satellite galaxy Holberg IX. I'm not too happy with the picture at right, because of the filters used for the image, a wideband green filter and a wideband near infrared filter. That's not so good for a galaxy like Holmberg IX, which is dominated by hot ultraviolet stars.

But the caption is better:
Spacetelescope.com wrote:

Of the more than 20,000 stars that can be resolved in this Hubble image, only about 10% are considered to be old stars with ages of billions of years. The rest are thought to be young stars with ages of only 10 - 200 million years. Due to the Advanced Camera for Surveys' resolution in this image, astronomers have noted that the old and the young stars have distinct spatial distributions which might be related to their origin.
I recommend that you take a look at this 1 MB picture of Holmberg IX from Large Binocular Telescope Observatory. You can really see the difference in distribution between the young stars (at upper right) and the old stars (at lower left). Also note an object at upper center left, a faint circular nebula with what looks like a pair of blue stars inside it. You can actually see the nebula in today's APOD, where it looks like a faint, faint pink object at bottom center, seemingly detached from the blue "body" of the rest of the tiny galaxy.
Neutral hydrogen connecting M81, M82, NGC 3077 and dwarf galaxies Ho IX and
the Garland. Source:
https://www.researchgate.net/figure/A-h ... _231079790
The rejuvenation event of Holmberg IX likely happened 200 million years ago:
Simulations predict that the triplet M81, M82, and nearby NGC 3077 had a close passage 200-300 million years ago. This close encounter may have triggered the newer star formation that has occurred in Holmberg IX.
So Holmberg IX is totally dominated by the light of blue stars, which is why Holmberg IX looks so blue in today's APOD. Note that it looks bluer than the spiral arms of M81. Note, however, that no new star formation is taking place in this tiny dwarf galaxy. The gas that created all the star formation in Holmberg IX has been used up or dispersed. The faint pink nebula that can be just made out in the APOD might possibly be star formation, but my guess is that it is more likely the remnant of an outburst of a pair of massive stars in Holmberg IX. Something like Eta Carina (image by NASA, ESA, and J. Hester), but on a much, much more modest scale.

Ann
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Re: APOD: Messier 81 (2019 Apr 17)

Post by Boomer12k » Wed Apr 17, 2019 7:36 am

Great image...love the colors and clarity...

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orin stepanek
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Re: APOD: Messier 81 (2019 Apr 17)

Post by orin stepanek » Wed Apr 17, 2019 10:25 am

What can I say: very nice! :thumb_up: :thumb_up: Similar to the MW!
Orin

Smile today; tomorrow's another day!

Yahchanan

Re: APOD: Messier 81 (2019 Apr 17)

Post by Yahchanan » Wed Apr 17, 2019 7:59 pm

Hi,

> Some dust lanes actually run through the galactic disk (left of center),

Has that been verified? Maybe those dust lanes are in front of the galaxy, being part of the MW's interstellar flux?

~Yahchanan

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Ann
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Re: APOD: Messier 81 (2019 Apr 17)

Post by Ann » Thu Apr 18, 2019 4:20 am

Yahchanan wrote:
Wed Apr 17, 2019 7:59 pm
Hi,

> Some dust lanes actually run through the galactic disk (left of center),

Has that been verified? Maybe those dust lanes are in front of the galaxy, being part of the MW's interstellar flux?

~Yahchanan
Yes, the "transverse" dust lanes have been discussed a lot, and the hypothesis that these dust lanes are part of the Milky Way's galactic cirrus was very popular for a long time. Now this idea is less popular.

The Interstellar Flux Nebula around the M81 group.
Photo: D Lopez/ A Rosenberg / IAC
M81. Robert Gendler / Roberto Colombari / ESA / NASA /
Hubble Space Telescope / Subaru / NAOJ























My own objection to the idea that the transverse dust lane would actually belong to the Milky Way is that the errant dust lanes are very similar in darkness, width and length to the dust lanes that definitely belong to M81. It is a bit of a stretch to think that a feature in the Milky Way might look more or less exactly like a dust lane in M81, 12 million light-years away.

Note what the APOD caption said, too:
APOD Robot wrote:
Some dust lanes actually run through the galactic disk (left of center), contrary to other prominent spiral features though. The errant dust lanes may be the lingering result of a close encounter between M81 and its smaller companion galaxy, M82.
Look at the picture at left. Can you see a thin vaguely reddish "tendril" of some sort snaking around M81 and "descending on it" in such a way that it might coincide with the transverse dust lane across the bulge of M81?

There is definitely something there. The question is if the "tendril" belongs to the M81 group itself, 12 million light-years away, or if it has been produced by our own galaxy. I believe in the former hypothesis.

But there are still those who believe that the transverse dust lanes in M81 belong to the Milky Way. See this page.

Ann
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Yahchanan

Re: APOD: Messier 81 (2019 Apr 17)

Post by Yahchanan » Mon Apr 22, 2019 8:13 pm

Hi Ann,

Yes, I do see that. It looks like the loop extends in front of the galaxy, and the densest part of it are the transverse dust lanes. There is an image from Kochanek I stretched, I don't know how to attach it to this post. I clearly see the transverse dust lanes extend up past the galaxy exactly where your loop is. It seems pretty sure the MW flux is the reason, not some stretched-out dwarf orbiting M81. I also see it in the featured image.

I see something else, too. You also linked to an image from Kochanek showing the old pop stars in Holmberg IX and the new pop. I see an error of interpretation here. A bit of processing of the image clearly reveals why the one group of stars are red. There is a bunch of MW interstellar flux in front of it, but not in front of the bluer stars. Furthermore one part of the dust cloud is red, the upper left portion. The other has a different color and brightness. This cloud is also visible in the wider-angle image. Forget about Holmberg IX being a 2-stage galaxy.

Cheers!

~Yahchanan

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Ann
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Re: APOD: Messier 81 (2019 Apr 17)

Post by Ann » Mon Apr 22, 2019 10:18 pm

Yahchanan wrote:
Mon Apr 22, 2019 8:13 pm
Hi Ann,

Yes, I do see that. It looks like the loop extends in front of the galaxy, and the densest part of it are the transverse dust lanes. There is an image from Kochanek I stretched, I don't know how to attach it to this post. I clearly see the transverse dust lanes extend up past the galaxy exactly where your loop is. It seems pretty sure the MW flux is the reason, not some stretched-out dwarf orbiting M81. I also see it in the featured image.

I see something else, too. You also linked to an image from Kochanek showing the old pop stars in Holmberg IX and the new pop. I see an error of interpretation here. A bit of processing of the image clearly reveals why the one group of stars are red. There is a bunch of MW interstellar flux in front of it, but not in front of the bluer stars. Furthermore one part of the dust cloud is red, the upper left portion. The other has a different color and brightness. This cloud is also visible in the wider-angle image. Forget about Holmberg IX being a 2-stage galaxy.

Cheers!

~Yahchanan
I'm not arguing with you regarding the cause of the transverse dust lane seen across the bulge of M81. :wink:

Dwarf galaxy I Zwicky 18.
Photo: NASA, ESA, and A. Aloisi
But I think you are wrong about Holmberg IX. Astronomers have yet to find a single galaxy that contains young stars only.

Astronomers used to think that dwarf galaxy I Zwicky 18 was only 500 million years old, which would have made it billions of years younger than all other known galaxies. But they were wrong:
Wikipedia wrote:

Studies at the Palomar Observatory some 40 years ago led astronomers to believe that the galaxy erupted with star formation billions of years after its galactic neighbors. Galaxies resembling I Zwicky 18's youthful appearance are typically found only in the early universe. Early observations with the Hubble Space Telescope suggested an age of 500 million years old for I Zwicky 18.[4]

The Hubble Space Telescope, however, later found faint, older stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies.
So if I Zwicky 18 is at least a billion years old, I think we can be quite sure that Holmberg IX is at least that old too.

Ann
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