APOD: HI4PI: The Hydrogen Sky (2016 Oct 24)

[APOD Robot]

HI4PI: The Hydrogen Sky

Explanation: Where are the Milky Way's gas clouds and where are they going? To help answer this question, a new highest-resolution map of the sky in the universe's most abundant gas -- hydrogen -- has been completed and recently released, along with its underlying data. Featured above, the all-sky map of hydrogen's 21-cm emission shows abundance with brightness and speed with color. Low radial speeds toward us artificially colored blue and low radial speeds away colored green. The band across the middle is the plane of our Milky Way Galaxy, while the bright spots on the lower right are the neighboring Magellanic Clouds. The HI4PI map collects data from over one million observations with the northern Eiffelsberg 100-Meter Radio Telescope in Germany and the southern Parkes 64-Meter Radio Telescope in Australia, also known as "The Dish". The details of the map not only better inform humanity about star formation and interstellar gas in our Milky Way galaxy, but also how much light this local gas is likely to absorb when observing the outside universe. Many details on the map are not yet well understood.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Ann]

Interesting map. Our neighboring galaxies look more intriguing than our own, however.

I guess that "wet shaggy orange dog" at lower right is the Magellanic Clouds. The Large (?) Magellanic Cloud would be the "head" of the dog, connected to the "torso" (the Small Cloud?) to its lower left by a thin "neck". From the "torso" grows four "legs", and the dog has a stubby, white "tail". The dog is generally very shaggy, with a log of wild fur growing this way and that. Also the dog is wet, because it has left its trail to its lower left.

There is a lot of hydrogen here, unsurprisingly!

The narrow purple disks at lower left would be M31 and M33, I suppose. Unlike the Magellanic Clouds, Andromeda and Triangulum are not the least bit shaggy. Their purple color suggests that they are approaching us fast, while the orange color of the Magellanic dog suggests it is on the run from us.

Ann

[Alcove72]

"Amateur asternaut" would be above my rank! Could someone indicate which direction our Milky Way is rotating in this view? It would be tempting to guess that we are rotating toward the blue side; but I cannot readily confirm this. IF we are rotating toward the green side, that would seem very curious.

[bystander]

http://asterisk.apod.com/viewtopic.php?t=36487

[Boomer12k]

It looks like the Magellanic Clouds were ONE once....like a dwarf galaxy that got torn and pulled apart....

:---[===] *

[Boomer12k]

How can anything be "coming at" or "going away" if we all rotate in the same direction at the same basic speed of rotation? Which galaxies have been touted as doing.

:---[===] *

[Ann]

"Amateur asternaut" would be above my rank! Could someone indicate which direction our Milky Way is rotating in this view? It would be tempting to guess that we are rotating toward the blue side; but I cannot readily confirm this. IF we are rotating toward the green side, that would seem very curious.

Good question, which should be answered by someone more mathematically adept than I am.

However, I guess we should start in the middle. The little black dot in the middle of this map might just be the center of the Milky Way. From the location of this black dot, the blue disk to the right of it appears to be approaching, and the green disk to the left of it appears to be receding.

But what about the broad bright disk fragments outside the inner disk, whose rotation appears to be inverted?

Consider this picture of a torus. The Milky Way is not a torus, but to us, it looks like a flat band across the sky. But from any vantage point, we can only see a part of it. At any one time, we can't see all the of Milky Way that is observable from the Earth. At any given time of the year, only parts of the Milky Way are visible from any part of the Earth.

I think today's APOD can be thought of as a torus or a circle that has been "cut it up at the back" and spread out to allow us to see all of the Milky Way that is ever observable from the Earth. This makes the Milky way look elongated. The map has given the Milky Way a shape that it doesn't have, just so that we can "see it".

Today's map obviously doesn't show us the part of the Milky Way that is permanently hidden from us. Nevertheless and in some way, the image of the Milky Way has been "cut up and spread out", so that its apparent rotation curve has been inverted.

At least that is what I think.

Ann

[French Patrick]

What is the pale green spiral structure in the middle up of the picture ?

Patrick

[neufer]

How can anything be "coming at" or "going away" if we all rotate in the same direction at the same basic speed of rotation? Which galaxies have been touted as doing.

• All politics most gas motions are local.

---

Our solar journey through space is carrying us through a cluster of very low density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long. These clouds are identified by their motions, indicated in this graphic with blue arrows.

[leon.l7027@gmail.com]

Similar to Alcove 2, and Ann: If as noted the blue is coming toward us, and the green is going away from us, that has to mean that the inner spiral arm(s?) of the Milky Way are (from above) rotating clockwise, and the larger outer arm(s?) are rotating counterclockwise. If this has been presented before, I have missed it. Glad to have it now, and the additional comments and graphics of Ann and neufer.

[Chris Peterson]

Similar to Alcove 2, and Ann: If as noted the blue is coming toward us, and the green is going away from us, that has to mean that the inner spiral arm(s?) of the Milky Way are (from above) rotating clockwise, and the larger outer arm(s?) are rotating counterclockwise. If this has been presented before, I have missed it. Glad to have it now, and the additional comments and graphics of Ann and neufer.

No, it doesn't necessarily suggest anything at all about the motion of what we're looking at. It's an indicator of our own relative motion within the galaxy. As you'd expect, half the sky is redshifted and half is blueshifted.

[Peter Smith]

Similar to Alcove 2, and Ann: If as noted the blue is coming toward us, and the green is going away from us, that has to mean that the inner spiral arm(s?) of the Milky Way are (from above) rotating clockwise, and the larger outer arm(s?) are rotating counterclockwise. If this has been presented before, I have missed it. Glad to have it now, and the additional comments and graphics of Ann and neufer.

No, it doesn't necessarily suggest anything at all about the motion of what we're looking at. It's an indicator of our own relative motion within the galaxy. As you'd expect, half the sky is redshifted and half is blueshifted.

If we are simply moving through space then I would expect half the sky to be redshifted and half blueshifted but that doesn't appear to be what we are seeing. If I understand the projection correctly, the horizontal centreline represents a circle surrounding the observer. So, still facing the black dot in the centre, front left and back right appear green, while front right and back left appear blue. It's more like a quadrupole than the dipole you describe.

Is this because parts of the galaxy closer than us to the centre are orbiting faster, while parts further than us from the centre are lagging behind?

[Fred the Cat]

So this map only indicates the state of hydrogen in our galaxy with enough energy to keep its one electron happily in its grasp. Given too much energy the other forms of hydrogens' electrons "get the flock" out of there so we can watch with different tools? But to measure hydrogen with radio waves - it all depends on how you spin it?

[MarkBour]

Correct me if this is wrong.

The method of projection for today's APOD is to take what was observed in every direction and project it onto the flat page, as in the diagram at right. You could imagine plotting the all-sky observations onto a ball that fit nicely around you, then slicing the ball along the vertical great circle directly behind you, then stretching it and flattening it onto a wall in front of you. In the diagram, the two red points were touching directly behind you before the projection process. (Indeed, the entire ellipse of the diagram was the cut you made at the beginning, so it was all on a semi-circle behind you.)

Then, if that much is correct, I don't suppose the raw data in the plot can tell us anything about how far away from us any of the detected hydrogen is, only about its direction, abundance, and apparent motion toward or away from us. But if you see a whole lot of hydrogen right where we know the Large Magellanic Cloud to be, it is a reasonable guess that much of the hydrogen is right there with it, especially if that hydrogen has velocity that matches the LMC.

[Chris Peterson]

Correct me if this is wrong.

This is wrong <g>.

The map is in galactic coordinates. You're right in that it shows the entire sky, but not as you have it labeled. The center of the map is the center of the galaxy from our viewpoint (in Sagittarius). Left and right is along the plane of the Milky Way (galactic longitude, where 0° cuts through the galactic center). Up and down is above and below the galactic plane (galactic latitude, where 0° is the galactic plane). In this image, the galactic coordinate system appears to be plotted using a Mollweide projection, which expands the low latitudes (around the Milky Way, where things are most interesting) and compresses things towards the north and south galactic poles.

Your labeling would be correct for the moment when the arch of the Milky Way is lying along the horizon, so it forms a ring around you, while you're facing south towards Sagittarius. But the sky is rarely in that configuration.

[neufer]

The little black dot in the middle of this map might just be the center of the Milky Way. From the location of this black dot, the blue disk to the right of it appears to be approaching, and the green disk to the left of it appears to be receding.

• This is true. The Sun's radial velocity is negligible so that
  one is directly observing the rotation of the inner part of the Milky Way disk.

---

But what about the broad bright disk fragments outside the inner disk, whose rotation appears to be inverted?

The broad bright disk fragments (whose rotation appears to be inverted) represents the Sun's own rotational motion through the Local Interstellar Clouds (which are not rotating as fast as the Sun).

[neufer]

I guess that "wet shaggy orange dog" at lower right is the Magellanic Clouds. The Large (?) Magellanic Cloud would be the "head" of the dog, connected to the "torso" (the Small Cloud?) to its lower left by a thin "neck". From the "torso" grows four "legs", and the dog has a stubby, white "tail". The dog is generally very shaggy, with a log of wild fur growing this way and that. Also the dog is wet, because it has left its trail to its lower left.

[b][color=#0000FF]Map of Hubble observations overlaid on LAB survey, tracing the origin of the Magellanic Stream.[/color][/b]

---

<<These images show wide and close-up views of a long ribbon of gas called the Magellanic Stream, which stretches nearly halfway around the Milky Way. In the combined radio and visible-light image at the top, the gaseous stream is shown in pink. The radio observations from the Leiden/Argentine/Bonn (LAB) Survey have been combined with the Mellinger All-Sky Panorama in visible light. The Milky Way is the light blue band in the centre of the image. The brown clumps are interstellar dust clouds in our galaxy. The Magellanic Clouds, satellite galaxies of the Milky Way, are the white regions at the bottom right. The image at the bottom, taken at radio wavelengths, is a close-up map of the Magellanic Stream that also was generated from the LAB Survey. Researchers determined the chemistry of the gas filament by using Hubble's Cosmic Origins Spectrograph (COS) to measure the amount of heavy elements, such as oxygen and sulphur, at six locations (marked with an "x") along the Magellanic Stream. COS observed light from faraway quasars that passed through the stream, and detected the spectral fingerprints of these elements from the way they absorb ultraviolet light. Quasars are the brilliant cores of active galaxies. These observations show that most of the gas was stripped from the Small Magellanic Cloud about two billion years ago — but surprisingly, a second region of the stream was formed more recently from the Large Magellanic Cloud. The pink circles to the right mark the location of the Small and Large Magellanic Clouds.>>

[MarkBour]

Excepting the bright, compact parts, which are easier to understand (I guess), the main thing that I get from this APOD, is that the apparent motion of the H gas around us is roughly as indicated at right. Art, I'm having trouble putting that together with the information you've posted.