APOD: Ultraviolet Rings of M31 (2015 Jul 24)

[APOD Robot]

Ultraviolet Rings of M31

Explanation: A mere 2.5 million light-years away the Andromeda Galaxy, also known as M31, really is just next door as large galaxies go. So close and spanning some 260,000 light-years, it took 11 different image fields from the Galaxy Evolution Explorer (GALEX) satellite's telescope to produce this gorgeous portrait of the spiral galaxy in ultraviolet light. While its spiral arms stand out in visible light images of Andromeda, the arms look more like rings in the GALEX ultraviolet view, a view dominated by the energetic light from hot, young, massive stars. As sites of intense star formation, the rings have been interpreted as evidence Andromeda collided with its smaller neighboring elliptical galaxy M32 more than 200 million years ago. The large Andromeda galaxy and our own Milky Way are the most massive members of the local galaxy group.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Ann]

APOD Robot wrote:

While its spiral arms stand out in visible light images of Andromeda

I don't agree. Few spiral galaxies have arms that really stand out, and the arms (or rings) of M31 are downright faint in visible light. But in ultraviolet light the hot stars of Andromeda stand out, and the arms (or rings) really dominate.

As sites of intense star formation

There is no intense star formation in Andromeda.

the rings have been interpreted as evidence Andromeda collided with its smaller neighboring elliptical galaxy M32 more than 200 million years ago

Click to view full size image

Yes, this is really fascinating. I can't help being reminded of the Cartwheel galaxy, which has a brilliant ultraviolet outer ring and one or possibly two much smaller inner rings. The Cartwheel galaxy is shaped the way it is because it has collided with a satellite galaxy, and the stars in the Cartwheel ripple like the concentric waves of a pond. I find it downright fascinating to think that our big sister galaxy might be a milder version of the mighty Cartwheel bulls-eye!

[Pianosorplanets]

That's actually the hottest image of M31 I've seen from GALEX. Up until now, my impression was that the UV output was significantly less than today's image would make it appear. This is more what I'm used to seeing:



I suppose that the exposure time on today's image was longer but is it more revealing? Seems to me this colder image is more demonstrative of what's going on in there. The longer exposure seems to cloud the issue.

[Ann]

That's actually the hottest image of M31 I've seen from GALEX. Up until now, my impression was that the UV output was significantly less than today's image would make it appear. This is more what I'm used to seeing:



I suppose that the exposure time on today's image was longer but is it more revealing? Seems to me this colder image is more demonstrative of what's going on in there. The longer exposure seems to cloud the issue.

A good thing about the picture you posted is that it shows that even in ultraviolet light, the center of Andromeda is brighter than its ring-arms.

But galaxy pictures are very often presented in such a way that the light from the nucleus and bulge is suppressed in order to bring out the light from the arms.

Ann

[chuckster]

Andromeda and the Milky Way are on a collision course, and I've heard that at least 11 other galaxies are getting closer to us as well, though perhaps not on collision courses themselves. I've also read that Andromeda is actually huge in our sky, but it is dim because of "low surface brightness". Does that mean dust is hiding it, in optical wavelengths, from human eyes ?

[Ann]

Andromeda and the Milky Way are on a collision course, and I've heard that at least 11 other galaxies are getting closer to us as well, though perhaps not on collision courses themselves. I've also read that Andromeda is actually huge in our sky, but it is dim because of "low surface brightness". Does that mean dust is hiding it, in optical wavelengths, from human eyes ?

There is not much dust between us and M31, because we don't see it through the disk of the Milky Way. Actually the surface brightness of the inner bulge of M31 is pretty high. That is because of the high concentration of old yellow stars and red giants there. But in the outer bulge the concentration of stars are lower, and the spiral-ring features are dim. There are comparatively few hot young stars in M31, and mostly they are far apart.

Ann

[Animal of Stone]

Always good to greet the neighbors. When i see this image it always reminds me of deep space. Who knew we were so close

[Chris Peterson]

I suppose that the exposure time on today's image was longer but is it more revealing?

We are a visually oriented species, so it is natural that we tend to read too much into images. But most astronomical images are not about clearly understanding any absolute brightness, rather about understanding structure. So in most images, we deliberately set the white and black points to the extremes of the structure we're interested in, to maximize contrast and let us examine the most detail. We do this regardless of things like exposure time (increasing exposure time doesn't produce a brighter image, it produces an image with lower noise). You can't compare two images and say one suggests "hotter" or "colder" or anything like that, because intensity in an image is determined by processing, and so is relative intensity between different regions (because given the wide dynamic range present, a linear brightness scale is almost useless). The raw data can be examined mathematically to elucidate details about absolute intensity.

Today's image is about visualizing the location and structure of active regions. And it succeeds in that purpose.

[Chris Peterson]

I've also read that Andromeda is actually huge in our sky, but it is dim because of "low surface brightness". Does that mean dust is hiding it, in optical wavelengths, from human eyes ?

It doesn't have particularly low surface brightness. It's a fairly typical spiral galaxy, quite like our own, and has a similar surface brightness. That is, visually, M31 looks very much like the Milky Way in brightness. A patch of light somewhat larger than the Moon that resembles an isolated patch of Milky Way glow. This is true for many galaxies, but most are too small for our eyes to clearly see without some magnification. Our acuity is low for dim objects, so they need to be large in order to be seen.

[JuanAustin]

Speaking of arms and rings, have there ever been any artist illustrations of what Andromeda would look like from a face-on view like ours has been illustrated from a face-on perspective?

[Jim Leff]

It doesn't look as if M31 is starting to warp due to its proximity to us. I'm assuming Milky Way, too, remains undisturbed for now. How close do the two need to come before gravitation starts to cause malformation (to an extent that might be noticeable in an APOD photo from someone's telescope in a third galaxy)?

[ta152h0]

wonder if this is what sparked Einstein to think about time- space warp knowing the far side light takes 260000 years longer to get here ??

[neufer]

wonder if this is what sparked Einstein to think about time- space warp knowing the far side light takes 260000 years longer to get here ??

• Einstein came up with Special Relativity in 1905 & General Relativity in 1915.

<<In 1920, the Great Debate between Harlow Shapley and Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant Doppler shift. In 1922 Ernst Öpik presented a method to estimate the distance of M31 using the measured velocities of its stars. His result put the Andromeda Nebula far outside our Galaxy at a distance of about 450,000 parsecs (1,500,000 ly). Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of M31.>>

[Chris Peterson]

wonder if this is what sparked Einstein to think about time- space warp knowing the far side light takes 260000 years longer to get here ??

When Einstein started thinking about these things we had no clear idea of the size of galaxies or their actual distances from us.

[neufer]

[size=150][color=#0000FF]The Flanderomeda Nedula[/color][/size]

---

Always good to greet the neighbors.
When i see this image it always reminds me of deep space.
Who knew we were so close

Speaking of arms and rings, have there ever been any artist illustrations of what Andromeda would look like from a face-on view like ours has been illustrated from a face-on perspective?

[Visual_Astronomer]

I've also read that Andromeda is actually huge in our sky, but it is dim because of "low surface brightness". Does that mean dust is hiding it, in optical wavelengths, from human eyes ?

It doesn't have particularly low surface brightness. It's a fairly typical spiral galaxy, quite like our own, and has a similar surface brightness. That is, visually, M31 looks very much like the Milky Way in brightness. A patch of light somewhat larger than the Moon that resembles an isolated patch of Milky Way glow. This is true for many galaxies, but most are too small for our eyes to clearly see without some magnification. Our acuity is low for dim objects, so they need to be large in order to be seen.

As Chris said, the limitation is the eye. With the naked eye under dark skies you can see a small fuzzy spot where the brightest central core is. But the actual angular size is about 6 full moons wide.

Here is a photo-shopped image that is a good comparison:
http://www.slate.com/blogs/bad_astronom ... e_sky.html

[Chris Peterson]

As Chris said, the limitation is the eye. With the naked eye under dark skies you can see a small fuzzy spot where the brightest central core is. But the actual angular size is about 6 full moons wide.

That's the angular size of the actual galaxy, and as revealed by images. But not visually. To the naked eye, I'd place its size as similar to the Moon, perhaps a little larger.

[Joe Stieber]

As Chris said, the limitation is the eye. With the naked eye under dark skies you can see a small fuzzy spot where the brightest central core is. But the actual angular size is about 6 full moons wide.

That's the angular size of the actual galaxy, and as revealed by images. But not visually. To the naked eye, I'd place its size as similar to the Moon, perhaps a little larger.

I was looking at M31 last night with my unaided eyes from the relatively dark (but certainly not pristine) New Jersey Pinelands. Transparency was much better than average for July after a cold front had moved through. Under these conditions, the brighter central core of M31 was roughly the size of the moon (nominally half a degree), but the fainter lengthwise extent was at least half the 2.3 degree spacing between nearby Nu and 32 Andromedae, or more than two moon diameters (perhaps three).

[Ron-Astro Pharmacist]

Why are there delineations between forms of light but not within sets of the same types? We have given names to defined ranges for the thing we call electro-magnetic radiation or can they be uniquely different from one another when created under different conditions? Is it just for convenience we call all UV photons that all fall within certain wavelengths, frequencies and energies a "UV photon" or is there some way to differentiate it between another UV photon that was created under slightly different circumstances.

http://imagine.gsfc.nasa.gov/science/to ... trum1.html

I see that light has been broken into sub-sets but are there are also sub-sets of sub-sets?

https://en.wikipedia.org/wiki/Electromagnetic_spectrum

I ran across "Maxwell's Hand". I suspect it's an attempt to give us a visual representation how of his equations confers, into three dimensions, the mathematical aspects of EMR.

So all of the photons used to collect today's APOD are alike is some unique way but different from infrared photons, et al that have been emitted from other sources.

The center of the Andromeda galaxy looks like it is emitting photons at a variety of wavelengths or are those photons coming from matter that just varies in temperature? I know I've struggled with this before but to understand an image constructed from a single type (within the range of UV photons) leaves me wondering if I'm looking at a variety of types of matter or the same type of matter under different physical conditions.

[JuanAustin]

Neufer,
i'm so happy you could have a laugh at my expense, how very mature of you and it is my hope you were able to gain even more respect by all your fellow colleagues and fans of this site. Congratulations sir.

[geckzilla]

Neufer,
i'm so happy you could have a laugh at my expense, how very mature of you and it is my hope you were able to gain even more respect by all your fellow colleagues and fans of this site. Congratulations sir.

Take it easy, Juan. He does that to all of us.

[BMAONE23]

It is just one of numerous traits of the highly intelligent lexophile. They tend to have a vast memory of fairly obscure information and the ability to draw upon it at a moment's notice for what appears to be a seemingly disassociated analogy to most people that aren't lexophiles

[Phhhht]

Neufer,
i'm so happy you could have a laugh at my expense, how very mature of you and it is my hope you were able to gain even more respect by all your fellow colleagues and fans of this site. Congratulations sir.

Take it easy, Juan. He does that to all of us.

Take it easy , Art. She does that to all of us.

[MarkBour]

Okay, that's too many knee-jerk reactions to neufer's pun-ish post. This has turned into a Flanderomeda Nedula Oblongota.

[BMAONE23]

Okay, that's too many knee-jerk reactions to neufer's pun-ish post. This has turned into a Flanderomeda Nedula Oblongota.

And, of course, when Flanderomeda eats to much cheese he develops a bad case of Flanderomeda Strain