APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

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APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by APOD Robot » Fri Feb 24, 2017 5:05 am

Image NGC 3621: Far Beyond the Local Group

Explanation: Far beyond the local group of galaxies lies NGC 3621, some 22 million light-years away. Found in the multi-headed southern constellation Hydra, the winding spiral arms of this gorgeous island universe are loaded with luminous blue star clusters, pinkish starforming regions, and dark dust lanes. Still, for astronomers NGC 3621 has not been just another pretty face-on spiral galaxy. Some of its brighter stars have been used as standard candles to establish important estimates of extragalactic distances and the scale of the Universe. This beautiful image of NGC 3621, is a composite of space- and ground-based telescope data. It traces the loose spiral arms far from the galaxy's brighter central regions for some 100,000 light-years. Spiky foreground stars in our own Milky Way Galaxy and even more distant background galaxies are scattered across the colorful skyscape.

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Ann
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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by Ann » Fri Feb 24, 2017 11:46 am

Nice image! :D I'm always glad to see RGB-images of galaxies. And I'm always glad to see pictures processed by Robert Gendler and Roberto Colombari!
NGC 3621, Hubble data
NGC 7793. Photo:
Josef Pöpsel.



















The galaxy itself, NGC 3621, is interesting. I really apologize for posting a picture of NGC 3621 that is not today's APOD. I was unable to find the Gendler/Colombari processed version of this picture that was small enough to post here.

The reason why I wanted to post pictures of NGC 3621 and NGC 7793 side by side is that both these galaxies are classified as Hubble class Scd galaxies - well, NGC 3621 is an SBcd galaxy, a galaxy with a bar. As you can see, both galaxies have "messy" arms, where the spiral pattern is broken up by regions of star formation popping up everywhere.

Scd galaxies, by the way, are basically just a step away from irregular galaxies. As a quick reminder, here you can see an SBa (or SB0-a) galaxy, NGC 1291, by Josef Pöpsel, here is an SBb galaxy, M58 by Adam Block, here is an Sc galaxy, M74 by Leonardo Orazi, and here you can see a 1.1 MB picture of an Sd galaxy, NGC 2552. (Sorry about the large size of the Sd galaxy, but it was the only picture of an Sd galaxy that I could find!)

My point is that an Scd galaxy is just "one step ahead" of pretty irregular galaxies, and since NGC 3621 is classified as an SBcd galaxy, I would normally expect it to have a lot of star formation and be quite blue. I would also expect the galaxy to be rather faint. Interestingly, NGC 3621 is not extremely blue: B-V = 0.620, U-B = −0.080, which is blue but not remarkable. Also, and every bit as interesting, NGC 3621 is quite bright. My software, Guide, says that the luminosity of NGC 3621 is 19 billion times the Sun, or 0.8 times the Milky Way. But Guide estimates that the distance to NGC 3621 is only 17 million light-years, and the new, improved estimates say that the distance is 22 million light-years. So it seems to me that NGC 3621 is pretty similar in luminosity to the Milky Way.

Interesting!

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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by rcolombari » Fri Feb 24, 2017 11:55 am

Thanks so much, Ann.

Bests,
Roberto

heehaw

Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by heehaw » Fri Feb 24, 2017 1:22 pm

Nice that those two bright foreground stars do not happen to lie right on this galaxy! Is there any bright galaxy that IS "spoiled" by a bright star in our own galaxy just happening to lie directly on our line of sight to the distant galaxy?

tomatoherd

Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by tomatoherd » Fri Feb 24, 2017 1:45 pm

What i love in this image is this: at about 12 o'clock above the central galaxy, on its upper arm, there are a couple foreground, Milky Way stars, some blue stars of NGC 3621, and some red-shifted distant background galaxies, all within arc-minutes or less of each other. "Close", far, and very far....all lined up.

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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by MarkBour » Sat Feb 25, 2017 9:08 pm

Ann wrote:Nice image! ... processed by Robert Gendler and Roberto Colombari!
Yes. I don't know enough about processing to say anything deeper, but this image displays a stunning richness and balance of color. I think it's really beautiful.
tomatoherd wrote:What i love in this image is this: at about 12 o'clock above the central galaxy, on its upper arm, there are a couple foreground, Milky Way stars, some blue stars of NGC 3621, and some red-shifted distant background galaxies, all within arc-minutes or less of each other. "Close", far, and very far....all lined up.
That's indeed a particularly nice alignment ... which feeds in to heehaw's comment.
heehaw wrote:Nice that those two bright foreground stars do not happen to lie right on this galaxy! Is there any bright galaxy that IS "spoiled" by a bright star in our own galaxy just happening to lie directly on our line of sight to the distant galaxy?
I like this question ... some thoughts this is inducing:
  • Zsolt Frei may be the person to ask ... https://arxiv.org/abs/astro-ph/9606040
  • At this point, concerning distant galaxies, I'm assuming we don't yet have any knowledge that would say "this portion of sky is richer in distant-but-observable galaxies than any other", do we? Perhaps one of the sky surveys is giving us an idea, but I assume that would only be able to be determined with deep field images all over the place, right?
  • So, assuming there are distant galaxies most everywhere you look, then behind any globular or open cluster, there ought to be plenty of galaxies that are going to be difficult to image, and in fact I would wager there are galaxies that are completely eclipsed by stars. That would be a matter for statistics; except for stars that would actually move from our parallax, we're not going to know about such occurrences any time soon, barring some super-clever astronomer finding a technique to detect such a thing.
    Two edge-case thoughts ...
  • Speaking of stars we can parallax, I assume you're leaving Sol out of the question. :D Because of its proximity, it is huge in the sky and is always blocking lots of them, but at other dates is out of the way. But I think It is implicit in your question that Sol should not count.
  • Trivially, you could say that every image of the Milky way is spoiled by local stars :D
    Perhaps a valid answer ...
  • Speaking of local, there are lots of Milky Way stars in the super-hi-res Hubble image of Andromeda.
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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by Ann » Sun Feb 26, 2017 5:32 am

MarkBour wrote:
heehaw wrote:Nice that those two bright foreground stars do not happen to lie right on this galaxy! Is there any bright galaxy that IS "spoiled" by a bright star in our own galaxy just happening to lie directly on our line of sight to the distant galaxy?
I like this question ... some thoughts this is inducing:
  • Zsolt Frei may be the person to ask ... https://arxiv.org/abs/astro-ph/9606040
  • At this point, concerning distant galaxies, I'm assuming we don't yet have any knowledge that would say "this portion of sky is richer in distant-but-observable galaxies than any other", do we? Perhaps one of the sky surveys is giving us an idea, but I assume that would only be able to be determined with deep field images all over the place, right?
Image
The dipole anisotropy of the cosmic microwave background.
Source: https://map.gsfc.nasa.gov/universe/bb_cosmo_fluct.html
I'm just guessing, mind you, but perhaps the cosmic microwave background (CMB) can give us some clues as to the "galactic density" in various parts of the sky. The picture at left shows a remarkable anisotropy (yin-yang pattern) of the CMB in the sky. But this has nothing to do with the number of galaxies in the sky, but instead reflects "the motion of the Sun relative to the rest frame of the cosmic microwave background." Nothing strange there. The dipole anisotropy can be removed, which leaves the CMB background looking like the image at right:
[c]The CMB after the dipole anistropy had been removed. The Milky Way shows off prominently in red.
Source: https://map.gsfc.nasa.gov/universe/bb_cosmo_fluct.html[/c]
https://map.gsfc.nasa.gov/universe/bb_cosmo_fluct.html wrote:
(The image) shows the microwave sky after the dipole anisotropy has been subtracted from the map. This removal eliminates most of the fluctuations in the map: the ones that remain are thirty times smaller. On this map, the hot regions, shown in red, are 0.0002 Kelvin hotter than the cold regions, shown in blue.
The bright red horizontal band is of course the Milky Way. For comparison, you may check out this superb visible-light portrait of the Milky Way by Nick Risinger.
Clearly the Milky Way has to be removed from the CMB images in order to get all-sky CMB pictures. How do you do that? I don't know.

Nevertheless, if we assume that astronomers know what they are doing when they remove the signal from the Milky Way (but how certain of their facts can they be?), we are left with a map looking like the one at left. As you can see, it isn't perfectly smooth. It is clear that the CMB is not the same temperature everywhere. And since it is generally assumed that tiny variations in the cosmic microwave background eventually led to the formation of galaxies, galaxy clusters and voids, it seems reasonable that the galaxy count down to the appearance of the earliest galaxies in the universe probably isn't the same in absolutely every direction.



Well away from the Milky Way signal on the sky, there is a mysterious cold spot in the CMB.
Wikipedia wrote:
The CMB Cold Spot or WMAP Cold Spot is a region of the sky seen in microwaves that has been found to be unusually large and cold relative to the expected properties of the cosmic microwave background radiation (CMB). The "cold spot" is approximately 70 µK colder than the average CMB temperature (approximately 2.7 K), whereas the root mean square of typical temperature variations is only 18 µK.[1][note 1] At some points, the "cold spot" deviates 140 µK colder than the average CMB temperature.
...
A 2015 study shows the presence of a supervoid that has a radius of 1.8 billion light years and is centered at 3 billion light-years of our galaxy in the direction of the Cold Spot, likely being associated with it.[4] This would make it the largest void detected, and one of the largest structures known.[17][18]

It seems likely that in the region of the CMB cold spot, the galaxy count would be lower than in most other parts of the sky. Similarly, in particularly "hot" spots, the galaxy count would likely be higher.
Simulation of the structure of the universe. Millenium Simulation, MPA Garching,
V. Springel, S. White et al.
The universe isn't smooth. Instead it is made up of filaments of galaxies and galaxy clusters, and voids between them. If you look in the direction of a bright filament, the galaxy count should definitely be higher than in the direction of a dark void. However, the distribution of filaments and voids should be fairly the same all over the sky. It's like looking at a checked shirt: It does not look the same in every spot, but the overall pattern is the same.
MarkBour wrote:So, assuming there are distant galaxies most everywhere you look, then behind any globular or open cluster, there ought to be plenty of galaxies that are going to be difficult to image, and in fact I would wager there are galaxies that are completely eclipsed by stars. That would be a matter for statistics; except for stars that would actually move from our parallax, we're not going to know about such occurrences any time soon, barring some super-clever astronomer finding a technique to detect such a thing.
I'm just saying, You can't be Sirius! Globular cluster hidden in glare of brightest star. :wink:

Indeed, there ought to be lots and lots of galaxies that haven't been detected because they are hidden behind behind all kinds of stuff in the Milky Way. I guess that all we can do is to make educated guesses as to how many galaxies there are in the parts of the sky that are located behind the Milky Way. We should assume that the region hidden behind the Milky Way is no different from other parts of the sky, but of course we can't be absolutely, totally certain. Just almost totally certain.

By the way, did you know that the earliest galaxy hunters (in the early 20th century and possibly the late 19th century) used to call the region of the Milky Way "the zone of avoidance"? Well, because galaxies avoided that zone! Fancy that!

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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by neufer » Sun Feb 26, 2017 8:21 pm

Ann wrote:
...there ought to be lots and lots of galaxies that haven't been detected because they are hidden behind behind all kinds of stuff in the Milky Way. I guess that all we can do is to make educated guesses as to how many galaxies there are in the parts of the sky that are located behind the Milky Way. We should assume that the region hidden behind the Milky Way is no different from other parts of the sky, but of course we can't be absolutely, totally certain. Just almost totally certain. By the way, did you know that the earliest galaxy hunters (in the early 20th century and possibly the late 19th century) used to call the region of the Milky Way "the zone of avoidance"? Well, because galaxies avoided that zone! Fancy that!
  • The primary reason that galaxies are hiding behind the Milky Way
    is that they are simply too embarrassed :oops: about their names:
https://en.wikipedia.org/wiki/Dwingeloo_1 wrote: <<Dwingeloo 1 is a barred spiral galaxy about 10 million light-years away from the Earth, in the constellation Cassiopeia. It lies in the Zone of Avoidance and is heavily obscured by the Milky Way. The size and mass of Dwingeloo 1 are comparable to those of Triangulum Galaxy. Dwingeloo 1 has two smaller satellite galaxies — Dwingeloo 2 and MB 3 — and is a member of the IC 342/Maffei Group of galaxies.

The Dwingeloo 1 galaxy was discovered in 1994 by the Dwingeloo Obscured Galaxy Survey (DOGS), which searched for neutral hydrogen (HI) radio emissions at the wavelength of 21 cm from objects in the Zone of Avoidance. In this zone gas and dust in the disk of the Milky Way galaxy block the light from the galaxies lying behind it. The galaxy was, however, first noted as an unremarkable feature on Palomar Sky Survey plates earlier in the same year, but was not recognized as such. It was also independently discovered a few weeks later by another team of astronomers working with Effelsberg 100-m Radio Telescope.

After the discovery, Dwingeloo 1 was classified as a barred spiral galaxy. The distance to it was found to be approximately 3 Megaparsecs. In its overall size and mass, the galaxy is comparable to Triangulum Galaxy. Dwingeloo 1 was eventually named after the 25m radio telescope in the Netherlands that was used in the DOGS survey and first detected it.

Dwingeloo 1 is a highly obscured galaxy, which makes distance determination a difficult problem. The initial estimate, made soon after the discovery and based on the Tully–Fisher relation, was about 3 Mpc. Later, this value was slightly increased to 3.5–4 Mpc. In 1999 another estimate was published, claiming a distance of more than 5 Mpc. It was based on the infrared Tully–Fisher relation. As of 2011, the distance to Dwingeloo 1 is thought to be approximately 3 Mpc, based on its likely membership in the IC 342/Maffei group.

As a barred spiral galaxy, Dwingeloo 1 has a central bar and two distinct spiral arms beginning from the ends of the bar at nearly right angle and wound counterclockwise. The length of the arms is up to 180°. The disk of the galaxy is inclined with respect to the observer, with the inclination angle being 50°. The galaxy recedes from the Milky Way at a speed of about 256 km/s. The visible radius of Dwingeloo 1 is approximately 4.2', which at the distance of 3 Mpc corresponds to about 4 kpc. The neutral hydrogen is detected as far as 6 kpc (7.5') from the center. The total mass of the galaxy within the latter radius is estimated at 31 billion Solar masses. The total mass of the galaxy is about 1/4 that of the Milky Way, out to the measured distance of 6 kpc.

The distribution of the neutral hydrogen in Dwingeloo 1 is typical one for barred spiral galaxies—it is rather flat with a minimum in the center or along the bar. The total mass of the neutral hydrogen is estimated at 370–450 million Solar masses. Dwingeloo 1 is a molecular gas-poor galaxy. The total mass of the molecular hydrogen does not exceed 10% of that of neutral hydrogen. Optical observations detected around 15 H II regions situated mainly along the spiral arms.>>
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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by MarkBour » Mon Feb 27, 2017 12:29 am

Ann wrote: ... an introduction to the CMB ... including the cold spot (supervoid) and the zone of
avoidance
Thanks, Ann, that was a really fine tutorial!
neufer wrote:Dwingeloo 1 and friends ...
I agree, that's a silly-sounding name, at least to Americans.
But thanks for pointing out that galaxy. I have to like any barred-spiral galaxy for which the spiral arms come right out of the ends of the bar.
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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by rstevenson » Mon Feb 27, 2017 3:07 pm

MarkBour wrote:...
neufer wrote:Dwingeloo 1 and friends ...
I agree, that's a silly-sounding name, at least to Americans. ...
From Wikipedia...
Dwingeloo is a town halfway between Meppel and Assen in the Dutch province of Drenthe. It is a part of the municipality of Westerveld.

The town is known internationally because of the radio telescope of the Dwingeloo Radio Observatory (which at the time of its completion in 1956 was the largest radio telescope in the world), located on the edge of the Dwingeloo Heath, 3 km south of the village.
Not very silly after all, not even (I hope) to Americans.

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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by MarkBour » Tue Feb 28, 2017 4:43 pm

rstevenson wrote:
MarkBour wrote:...
neufer wrote:Dwingeloo 1 and friends ...
I agree, that's a silly-sounding name, at least to Americans. ...
... Not very silly after all, not even (I hope) to Americans.
Rob
Sorry, Rob. I think many people must actually relish silly names in the States. Ding Dong and Looneyville (both in Texas) come to mind. http://www.dumb.com/funny_city_names.php

Dwingeloo might sound even more humorous to our British friends, with the meaning of "loo" in that country (and as "dwingle" can mean to linger, loiter, or tarry).
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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by Ann » Tue Feb 28, 2017 4:52 pm

The village of Fucking, Austria.
Photo: AOL Travel.
Not only galaxies have embarrassing names.

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APOD 2017 : NGC 3621

Post by dempsey24 » Wed Mar 01, 2017 12:52 pm

Approximately 3/4 of the way down towards the left-hand side, there is a faint, vaguely North America, shape which appears to have a mirror image slightly to its right - what is this?

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Re: APOD 2017 : NGC 3621

Post by Ann » Wed Mar 01, 2017 2:16 pm

You should probably have posted here instead.

I think I can see your "North America".

The region appears to be dominated by blue but not tremendously bright young stars, probably comparable to Sirius. Sirius, which looks so bright in our skies, is a modest star as blue stars go. At a distance of only 8.6 light-years - extremely nearby as Milky Way stars go - it radiates about 22 solar luminosities in visible light.

Extremely hot bright blue stars can easily be thousands of times as bright as the Sun. But such stars burn their fuel at a furious rate and explode as supernovas in a few million years.
[c]M46 (left) and M47 (right).
Photo: Sergio Equivar.[/c]
To appreciate the difference between "very bright blue stars" and modestly bright blue stars", take a look at the picture at left of young clusters M46 and M47. M46, at left, is about 300 million years old, and all its really bright blue stars have died. It is dominated by stars like Sirius. Cluster M47, at right, is 80 million years old, and dominated by stars like the bright ones of the Pleiades, although a bit hotter and brighter still. The Pleiades is considered a bit older than M47, but a lot younger than M46.

I think that most of your "North America" region in NGC 3621 is dominated by stars like the ones in M46.

What about the "Gulf of Mexico"? How was it formed? I think it might be the remnant of an old supernova. This supernova may have blown much of the region around it free of gas, and it may have driven other gas in front of it like a snowplow, until the gas ran into some really dense gas clouds elsewhere and built up like a pile of snow. In such compressed gas, star formation may easily occur. Look at the small bright semicircle in blue and pink to the upper right of "North America" in the APOD. This is clearly a place where hot bright stars are being born.

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Re: APOD 2017 : NGC 3621

Post by BDanielMayfield » Wed Mar 01, 2017 5:30 pm

dempsey24 wrote:Approximately 3/4 of the way down towards the left-hand side, there is a faint, vaguely North America, shape which appears to have a mirror image slightly to its right - what is this?
Ann wrote:You should probably have posted here instead.
Yeah, but you kinda F___ up this thread Ann. Dempsey must not have wanted his very first post to follow that. :wink:
"Happy are the peaceable ... "

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Re: APOD: NGC 3621: Far Beyond the Local Group (2017 Feb 24)

Post by BDanielMayfield » Wed Mar 01, 2017 5:36 pm

MarkBour wrote:Ding Dong and Looneyville (both in Texas) come to mind.
Hey, I resemble that remark :!: :)
"Happy are the peaceable ... "

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Re: APOD 2017 : NGC 3621

Post by Ann » Wed Mar 01, 2017 11:56 pm

BDanielMayfield wrote:
dempsey24 wrote:Approximately 3/4 of the way down towards the left-hand side, there is a faint, vaguely North America, shape which appears to have a mirror image slightly to its right - what is this?
Ann wrote:You should probably have posted here instead.
Yeah, but you kinda F___ up this thread Ann. Dempsey must not have wanted his very first post to follow that. :wink:
Yes, ummm. Sorry. Won't post that picture again!

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