APOD: Messier 81 (2021 Mar 12)

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APOD: Messier 81 (2021 Mar 12)

Post by APOD Robot » Fri Mar 12, 2021 5:07 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 sharp, detailed telescopic view reveals M81's bright yellow nucleus, blue spiral arms, pinkish 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 the nearby galaxy M82 lurking outside of this frame. M81's faint, dwarf irregular satellite galaxy, Holmberg IX, can be seen just below the large spiral. Scrutiny of variable stars in M81 has yielded a well-determined distance for an external galaxy -- 11.8 million light-years.

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Re: APOD: Messier 81 (2021 Mar 12)

Post by Ann » Fri Mar 12, 2021 6:14 am

I'm glad to see a picture by Wissam Ayoub become an APOD! 😀 Wissam has recently started posting pictures at Starship Asterisk*, and he has posted some really fine images here! 😀



















The most interesting aspect of Wissam Ayoub's image, in my opinion, is the little blue satellite galaxy, Holmberg IX (bottom). Undoubtedly this galaxy has formed as a result of the interaction between M81 and its two relatively large neighbors, M82 and NGC 3077.

If you check out the neutral hydrogen radio data picture at right by de Blok et al, you can see how neutral hydrogen flows between these three galaxies. Obviously this gas has accumulated in certain places and helped give rise to Holmberg IX. In the picture by de Blok et al, Holmberg IX is located to the left of M81 (where the gas, shown in blue, is "thicker").



There is another obvious blue gaseous "concentration" in the de Blok et al image, and it is located above the position of Holmberg IX in the picture by de Blok et al. That other concentration is (a part of) Arp's Loop. Bernard Miller has taken a fine picture of M81 with both Holmberg IX and Arp's Loop.

In Bernard Miller's image, Holmberg IX is located above M81 and Arp's Loop is to the right of Holmberg IX. As you can see in Bernard Miller's image, the gas in Holmberg IX has given birth to a good number of young stars, but there are not many young stars in Arp's Loop. But there are indeed a few young stars there.

If anyone is interested in reading about dwarf galaxies in the M81 Group, I recommend the paper Stellar Population and Structural Properties of Dwarf Galaxies and Young Stellar Systems in the M81 Group by Sakurako Okamoto et al.

Ann
Last edited by Ann on Fri Mar 12, 2021 6:37 pm, edited 1 time in total.
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Re: APOD: Messier 81 (2021 Mar 12)

Post by Knight of Clear Skies » Fri Mar 12, 2021 6:39 am

Thanks Ann, I'd never seen the radio image of the M81 group before, very revealing.

Don't suppose you know of any neutral hydrogen images of nearby molecular clouds, such as Orion, Taurus, Perseus and Cygnus X?
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Re: APOD: Messier 81 (2021 Mar 12)

Post by Ann » Fri Mar 12, 2021 10:17 am

Knight of Clear Skies wrote:
Fri Mar 12, 2021 6:39 am
Thanks Ann, I'd never seen the radio image of the M81 group before, very revealing.

Don't suppose you know of any neutral hydrogen images of nearby molecular clouds, such as Orion, Taurus, Perseus and Cygnus X?
What I found as I googled was this:

Faculty.ung.edu wrote about the picture:

The HI distribution of the Milky Way was determined by using a technique in which the radial velocity of HI clouds is determined by radio spectroscopy of the 21 cm radio emission line of neutral hydrogen. These radial velocities combined with the known rotation curve of the Milky Way allow the positions of these HI clouds to be plotted relative to the Sun. This technique fails within a small angle directly towards and away from the galactic center where all the clouds would have essentially the same radial velocity of about zero and so their distances from the Sun can't be determined. (Note the absence of data in the plot within a small angle towards the center of the galaxy in the left diagram. Some features in this illustration such as the central ring, nearby small spiral structures, and the spiral structure in the Solar neighborhood have been filled in using other techniques.)
Is this a simulation of the "face-on" appearance of the Milky Way?

You tell me.

Ann
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Re: APOD: Messier 81 (2021 Mar 12)

Post by orin stepanek » Fri Mar 12, 2021 12:20 pm

BodesGalaxyM81_1024.jpg

Very pretty; and it's blue, Ann!

I'm going to make a background of this for my PC! :D
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Re: APOD: Messier 81 (2021 Mar 12)

Post by DL MARTIN » Fri Mar 12, 2021 4:30 pm

Just for astronomical/archaeological nicety can Messier 81 also be described as 11.8 million years ago?

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Re: APOD: Messier 81 (2021 Mar 12)

Post by johnnydeep » Fri Mar 12, 2021 5:17 pm

APOD Robot wrote:
Fri Mar 12, 2021 5:07 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 sharp, detailed telescopic view reveals M81's bright yellow nucleus, blue spiral arms, pinkish 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 the nearby galaxy M82 lurking outside of this frame. M81's faint, dwarf irregular satellite galaxy, Holmberg IX, can be seen just below the large spiral. Scrutiny of variable stars in M81 has yielded a well-determined distance for an external galaxy -- 11.8 million light-years.
Why are the inner dust lanes described as "errant"? Simply because they are close to the center? And what is an "external galaxy"? Merely one that is not the Milky Way?
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Re: APOD: Messier 81 (2021 Mar 12)

Post by neufer » Fri Mar 12, 2021 5:40 pm

DL MARTIN wrote:
Fri Mar 12, 2021 4:30 pm

Just for astronomical/archaeological nicety can Messier 81 also be described as 11.8 million years ago?
Click to play embedded YouTube video.
https://www.etymonline.com/search?q=nicety wrote:
nicety (n.), late 14c., nicete, "folly, stupidity," a sense now obsolete, from Old French niceté "foolishness, childishness, simplicity," from nice "silly" (see nice). It underwent a sense evolution parallel to that of nice, arriving at "minute, subtle point" 1580s and "exactitude, accuracy" in 1650s.
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Re: APOD: Messier 81 (2021 Mar 12)

Post by Chris Peterson » Fri Mar 12, 2021 5:48 pm

DL MARTIN wrote:
Fri Mar 12, 2021 4:30 pm
Just for astronomical/archaeological nicety can Messier 81 also be described as 11.8 million years ago?
No. Only if you are ignorant.
Chris

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Re: APOD: Messier 81 (2021 Mar 12)

Post by Ann » Fri Mar 12, 2021 7:11 pm



Let's have a look at the picture of the neutral hydrogen in the M81 Group again. As you can see, there is a lot of false-color blue (i.e., neutral hydrogen) right next to galaxies M82 (top) and NGC 3077 (far left). What has this extra helping done to these two galaxies?

43214141391_40f11c7a28_b[1].jpg
Galaxy NGC 3077 with its central starburst. Credit: ESA/Hubble & NASA.

Surprise! Both M82 and NGC 3077 are undergoing central starbursts! But both are pretty "red and dead" away from their cores!

Note in NGC 3077 dust lanes that seem to be channeling gas directly into the starburst core. M82, too, has a few dust lanes that seem to lead directly to its core, fueling star formation there.

Ann
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Re: APOD: Messier 81 (2021 Mar 12)

Post by VictorBorun » Tue Mar 16, 2021 9:40 pm

I wonder if 11.8 million light-years is outside of the Local Cluster of the Milky Way + Andromeda + Triangle
Last edited by VictorBorun on Tue Mar 16, 2021 10:38 pm, edited 2 times in total.

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Re: APOD: Messier 81 (2021 Mar 12)

Post by Chris Peterson » Tue Mar 16, 2021 9:53 pm

VictorBorun wrote:
Tue Mar 16, 2021 9:40 pm
I wonder if 11.8 million light-years is outside of the Local Cluster of the Milky Way + Andromeda + Triangle
It is. Indeed, M81 is the largest member of its own small galaxy cluster which is similar to the Local Group in size and mass.
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Re: APOD: Messier 81 (2021 Mar 12)

Post by VictorBorun » Tue Mar 16, 2021 10:38 pm

Chris Peterson wrote:
Tue Mar 16, 2021 9:53 pm
VictorBorun wrote:
Tue Mar 16, 2021 9:40 pm
I wonder if 11.8 million light-years is outside of the Local Cluster of the Milky Way + Andromeda + Triangle
It is. Indeed, M81 is the largest member of its own small galaxy cluster which is similar to the Local Group in size and mass.
Is there, then, two dark halo globes 10-12 Mly in diameter touching each other, one centered around Bode's Galaxy, the other around Andromeda — Milky Way pair?

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Re: APOD: Messier 81 (2021 Mar 12)

Post by neufer » Tue Mar 16, 2021 10:43 pm

Chris Peterson wrote:
Tue Mar 16, 2021 9:53 pm
VictorBorun wrote:
Tue Mar 16, 2021 9:40 pm

I wonder if 11.8 million light-years is outside of the Local Cluster of the Milky Way + Andromeda + Triangle
It is. Indeed, M81 is the largest member of its own small galaxy cluster which is similar to the Local Group in size and mass.
https://en.wikipedia.org/wiki/M81_Group wrote:
<<The M81 Group is a galaxy group in the constellations Ursa Major and Camelopardalis that includes the galaxies Messier 81 and Messier 82, as well as several other galaxies with high apparent brightnesses. The approximate center of the group is located at a distance of 3.6 Mpc, making it one of the nearest groups to the Local Group. The group is estimated to have a total mass of (1.03 ± 0.17)×1012M☉. The M81 Group, the Local Group, and other nearby groups all lie within the Virgo Supercluster (i.e. the Local Supercluster).

Messier 81, Messier 82, and NGC 3077 are all strongly interacting with each other. Observations of the 21-centimeter hydrogen line indicate how the galaxies are connected. The gravitational interactions have stripped some hydrogen gas away from all three galaxies, leading to the formation of filamentary gas structures within the group. Bridges of neutral hydrogen have been shown to connect M81 with M82 and NGC 3077. Moreover, the interactions have also caused some interstellar gas to fall into the centers of Messier 82 and NGC 3077, which has led to strong starburst activity (or the formation of many stars) within the centers of these two galaxies.>>
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Re: APOD: Messier 81 (2021 Mar 12)

Post by VictorBorun » Wed Mar 17, 2021 3:22 am

The Local Group…has a total diameter of roughly 10 million ly and a total mass of the order of 2 trillion Suns.
The M81 Group … has a total mass of (1.03 ± 0.17) trillion Suns.

If the diameter of a small cluster, dark halo and all, is ~ ∛mass, then the diameter of the M81 Group is 8 million ly.

Now the Milky Way and Andromeda are 2.5 million ly apart, and from the Milky Way observation point Andromeda is close to M81 (seen in Andromeda constellation and Ursa Major from the Solar system, respectively).

So the gap between the two clusters is 11.8 Mly (of M81 distance) − 4 Mly (of M81Group radius) − 5 Mly (of Local Group radius) − 1.25 Mly (of Milky Way excentricity) = 1.55 Mly, not much.

To set the two apart for sure we should evaluate if the future stretching of space by dark energy do the separation, should not we?
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Re: APOD: Messier 81 (2021 Mar 12)

Post by Knight of Clear Skies » Wed Mar 17, 2021 7:57 am

Ann wrote:
Fri Mar 12, 2021 10:17 am
Faculty.ung.edu wrote about the picture:

The HI distribution of the Milky Way was determined by using a technique in which the radial velocity of HI clouds is determined by radio spectroscopy of the 21 cm radio emission line of neutral hydrogen. These radial velocities combined with the known rotation curve of the Milky Way allow the positions of these HI clouds to be plotted relative to the Sun.
Is this a simulation of the "face-on" appearance of the Milky Way?

You tell me.

Ann
More structure than appearance I think. I knew radio observations were used to map out the arms of the Milky Way but didn't understand how the distances were determined, useful quote thanks.
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Re: APOD: Messier 81 (2021 Mar 12)

Post by VictorBorun » Thu Mar 18, 2021 5:31 am

Last edited by VictorBorun on Thu Mar 18, 2021 10:05 am, edited 1 time in total.

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Re: APOD: Messier 81 (2021 Mar 12)

Post by Ann » Thu Mar 18, 2021 7:04 am

Thanks, Victor, that's interesting!

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