APOD: Shell Game in the LMC (2016 Dec 29)

[APOD Robot]

Shell Game in the LMC

Explanation: An alluring sight in southern skies, the Large Magellanic Cloud (LMC) is seen here through narrowband filters. The filters are designed to transmit only light emitted by ionized sulfur, hydrogen, and oxygen atoms. Ionized by energetic starlight, the atoms emit their characteristic light as electrons are recaptured and the atom transitions to a lower energy state. As a result, this false color image of the LMC seems covered with shell-shaped clouds of ionized gas surrounding massive, young stars. Sculpted by the strong stellar winds and ultraviolet radiation, the glowing clouds, dominated by emission from hydrogen, are known as H II (ionized hydrogen) regions. Itself composed of many overlapping shells, the Tarantula Nebula is the large star forming region at top center. A satellite of our Milky Way Galaxy, the LMC is about 15,000 light-years across and lies a mere 180,000 light-years away in the constellation Dorado.

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[Indigo_Sunrise]

The clouds really do resemble oyster shells.
Very nice image!

[Tszabeau]

I'm a bit confused. Do the "shells" reside in the LMC or are they in the Milky Way and the LMC is the backdrop?

[Chris Peterson]

I'm a bit confused. Do the "shells" reside in the LMC or are they in the Milky Way and the LMC is the backdrop?

All of this structure (and the Tarantula Nebula) is part of the LMC.

[Ann]

I'm a bit confused. Do the "shells" reside in the LMC or are they in the Milky Way and the LMC is the backdrop?

They are most certainly in the LMC. The LMC is a vigorously starforming galaxy, so it contains all sorts of glowing nebulas, bright clusters and unstable supergiant stars.

Ann

[starsurfer]

I absolutely love this image!! Even more when rotated to north up!! I love LMC nebula.

[DavidLeodis]

As various color palettes are used to produce astronomy images I will be grateful if someone could please state what colors are likely represented by each of the ionized sulfur, hydrogen and oxygen in the false color image. The explanation states the clouds are "dominated by emission from hydrogen" which might therefore be the brownish orange areas as they cover more than the bluish and other areas. Thanks in advance for any help.

[Ann]

As various color palettes are used to produce astronomy images I will be grateful if someone could please state what colors are likely represented by each of the ionized sulfur, hydrogen and oxygen in the false color image. The explanation states the clouds are "dominated by emission from hydrogen" which might therefore be the brownish orange areas as they cover more than the bluish and other areas. Thanks in advance for any help.

Well, hmmm. I apologize for answering this question as a person who doesn't much like the Hubble palette.

Basically, when we talk about "three-filter-images" shown in false or mapped color, the shortest wavelength filter image is mapped as blue, the middle wavelength filter image is mapped as green, and the longest wavelength filter image is mapped as red.

The wavelength of OIII is 500.7 nm. Hα is 656.28 nm. SII is 671.83 nm (I think). Very occasionally, I think NII is being used instead of SII, and its wavelength is 658.35 nm. In any case, you can see that the wavelengths of SII and NII (particularly NII) are very similar to Hα, but Hα is still a little shorter than either of them.

Therefore, OIII, the shortest wavelength, is mapped as blue, Hα, the middle wavelength, is mapped as green, and SII (or even NII), the longest wavelength, is mapped as red.

Click to view full size image

The Pleiades in three infrared wavelengths, 4.5 µm (mapped as blue),
8.0 µm (mapped as green) and 24.0 µm (mapped as red).
John Stauffer (Spitzer Science Center, Caltech)
NASA/JPL-Caltech/J. Stauffer (SSC/Caltech)

The same principle is used for three-filter infrared images: the shortest infrared wavelength is mapped as blue, the middle one as green and the longest one as red.

Ann

[DavidLeodis]

Thank for your help Ann, which is appreciated .

It was that the explanation made a lot about the use of narrowband filters but surprisingly did not state what colors the ionized sulfur, hydrogen, and oxygen atoms are in the image that got me wondering.

[Roland]

Question: Are the clouds of Magellan galaxy's or are they galaxy fragments? I don't see any evidence that there is a galaxy core or black hole that normal galaxy's seem to show. Therefore to me they seem more like galaxy fragments, as in the debris from a galaxy merge.

[Ann]

Question: Are the clouds of Magellan galaxy's or are they galaxy fragments? I don't see any evidence that there is a galaxy core or black hole that normal galaxy's seem to show. Therefore to me they seem more like galaxy fragments, as in the debris from a galaxy merge.

The Large Magellanic Cloud doesn't have a core. Not all galaxies do. A galaxy is a collection of baryonic matter (stars, gas, planets etc) and non-baryonic matter (dark matter) that isn't a part of a larger galaxy, and that is held together by self-gravity.

Click to view full size image

The Clouds of Magellan.
Photo: John Gleason.

Click to view full size image

The Large Magellanic Cloud.
Photo: Eckhard Slawik.

The clouds (make that nebulas) of the Large Magellanic Cloud are just parts of the galaxy as a whole. Therefore, they are not "galaxy fragments".

However, the APOD we are discussing, seen at left, doesn't show us the large Magellanic Cloud as a whole. To see what the entire galaxy looks like, including the clouds seen in the APOD, check out the picture at right. Look at the complex of pink clouds near the center of Eckhard Slawik's picture. These pink clouds are the magnificent Tarantula nebula and associated nebulosities. They are what you see in the APOD. But the APOD shows them in so called "representative colors", so they don't look pink there. Myself, I prefer RGB images, like the picture at right.

Click to view full size image

The Phoenix Dwarf Galaxy.
Copyright: CHART32, processing: Johannes Schedler

Click to view full size image

NGC 4449. Photo: Rob Gendler.

Consider the two galaxies at left and right. At left is the Phoenix Dwarf Galaxy, a very puny, lightweight collection of stars, gas and dark matter. It doesn't have an obvious core at all. But it isn't a part of another galaxy, so it is a galaxy in its own right.

At right is NGC 4449, a galaxy that is similar to the Large Magellanic Cloud, except that it contains even more bright, large emission nebulas than the LMC. NGC 4449 is churning out stars at a prodigious rate. But just as is the case with the LMC, all the bright nebulas of NGC 4449 are just parts of the galaxy as a whole.

Ann

[starsurfer]

Just like the nebulae in our galaxy are part of the Milky Way.