APOD: Nearby Spiral Galaxy NGC 4945 (2015 May 28)

[APOD Robot]

Nearby Spiral Galaxy NGC 4945

Explanation: Large spiral galaxy NGC 4945 is seen edge-on near the center of this cosmic galaxy portrait. In fact, NGC 4945 is almost the size of our own Milky Way Galaxy. Its own dusty disk, young blue star clusters, and pink star forming regions standout in the sharp, colorful telescopic image. About 13 million light-years distant toward the expansive southern constellation Centaurus, NGC 4945 is only about six times farther away than Andromeda, the nearest large spiral galaxy to the Milky Way. Though the galaxy's central region is largely hidden from view for optical telescopes, X-ray and infrared observations indicate significant high energy emission and star formation in the core of NGC 4945. Its obscured but active nucleus qualifies the gorgeous island universe as a Seyfert galaxy and home to a central supermassive black hole.

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[Ann]

This is another one of those sideways dusty starforming/starbursting galaxies. Other well-known examples are of course M82 and NGC 253. They are very interesting galaxies, but I can't help thinking it's a pity that they are so dusty, because you can't see their blue stars very well.

But this is a nice image!

Ann

[Boomer12k]

....and Green Clovers.... (it sounded like a cereal commercial...)

And it is a bit husky too... and seems to have more than its fair share of dust...

Got out tonight and got a first shot of the season of The Moon, and a couple of craters...
:---[===] *

[starsurfer]

This is another one of those sideways dusty starforming/starbursting galaxies. Other well-known examples are of course M82 and NGC 253. They are very interesting galaxies, but I can't help thinking it's a pity that they are so dusty, because you can't see their blue stars very well.

But this is a nice image!

Ann

Isn't there a link between the amount of dust in a galaxy and star formation rate? Obviously other factors trigger star formation but an abundance of the raw material required is a plus.

[Ann]

This is another one of those sideways dusty starforming/starbursting galaxies. Other well-known examples are of course M82 and NGC 253. They are very interesting galaxies, but I can't help thinking it's a pity that they are so dusty, because you can't see their blue stars very well.

But this is a nice image!

Ann

Isn't there a link between the amount of dust in a galaxy and star formation rate? Obviously other factors trigger star formation but an abundance of the raw material required is a plus.

Dust helps star formation, but star formation creates dust, too.

Ann

[Chris Peterson]

Isn't there a link between the amount of dust in a galaxy and star formation rate? Obviously other factors trigger star formation but an abundance of the raw material required is a plus.

The raw material is primarily hydrogen. The ratio of dust to hydrogen in even the dustiest galaxies or nebulas is extremely small. But dust appears to play a critical role in stimulating the gravitational collapse necessary to start the star forming process. All you need is hydrogen to make stars, but dust increases the formation rate.

[Craine]

Is it me, or is this galaxy slightly distorted?
It's like the tips are curving upward on the right and downward on the left. It is slightly easier to see on a negative, but still very faint.

If so, is there another galaxy nearby causing this?

[Ron-Astro Pharmacist]

This is an elemental question but hydrogen's spectrum is often discussed in this forum. Do the amounts the various hydrogen isotopes differ amongst the galaxies? Deuterium abundance appears to have some relevance in the ongoing understanding of our universe but "is" what causes its differences often used to compare galaxies in the study of cosmology? Seems like dust absorption of deuterium has a profound change on the number of molecules per million and may be a good tool for comparison if measurable by spectral analysis.

There are probably better methods available but deuterium abundance an interesting topic by itself.

[captainwiggins48]

We are viewing a galaxy that is 13 million light years away, in other words the image is 13 million years old. My question, what exists of the galaxy today? Shouldn't our remarks on it's appearance be in past tense, and our observations reflect the time differential? For instance, just how accurate is 13 million year old information that has also traveled 13 million light years?

[Craine]

We are viewing a galaxy that is 13 million light years away, in other words the image is 13 million years old. My question, what exists of the galaxy today? Shouldn't our remarks on it's appearance be in past tense, and our observations reflect the time differential? For instance, just how accurate is 13 million year old information that has also traveled 13 million light years?

13 million years is just a tiny fraction of the lifetime of a galaxy that is probably close to a 1000 times older. In those 13 million years it would have turned a bit more, that is all. For comparison, it is estimated our galaxy turns once every ~250 million years or so. So, little difference if any.

[Visual_Astronomer]

We are viewing a galaxy that is 13 million light years away, in other words the image is 13 million years old. My question, what exists of the galaxy today? Shouldn't our remarks on it's appearance be in past tense, and our observations reflect the time differential? For instance, just how accurate is 13 million year old information that has also traveled 13 million light years?

As Chris pointed out a few days back, because information can only propagate at the speed of light, there really can't be simultaneous events. So for all practical purposes, what a distant object looks like to you "now" is what it "is" now. You aren't looking into the past.

[geckzilla]

We are viewing a galaxy that is 13 million light years away, in other words the image is 13 million years old. My question, what exists of the galaxy today? Shouldn't our remarks on it's appearance be in past tense, and our observations reflect the time differential? For instance, just how accurate is 13 million year old information that has also traveled 13 million light years?

As Chris pointed out a few days back, because information can only propagate at the speed of light, there really can't be simultaneous events. So for all practical purposes, what a distant object looks like to you "now" is what it "is" now. You aren't looking into the past.

The confusion on this is so pervasive that I gave up on it. People lust for wonder and time traveling imagery fulfills that desire. Not that it isn't wonderful, it's just a lot more practical to say this is what object x looks like right now instead of twisting one's brain into pretzels trying to arrange everything into various epochs.

[Chris Peterson]

We are viewing a galaxy that is 13 million light years away, in other words the image is 13 million years old. My question, what exists of the galaxy today? Shouldn't our remarks on it's appearance be in past tense, and our observations reflect the time differential? For instance, just how accurate is 13 million year old information that has also traveled 13 million light years?

As Chris pointed out a few days back, because information can only propagate at the speed of light, there really can't be simultaneous events. So for all practical purposes, what a distant object looks like to you "now" is what it "is" now. You aren't looking into the past.

The confusion on this is so pervasive that I gave up on it. People lust for wonder and time traveling imagery fulfills that desire. Not that it isn't wonderful, it's just a lot more practical to say this is what object x looks like right now instead of twisting one's brain into pretzels trying to arrange everything into various epochs.

Well, I wouldn't say we aren't looking into the past. When we look over cosmological distances, we are certainly seeing things as they were when the Universe was younger. Our understanding of most cosmology depends on that observation. What is usually pointless, and to a degree meaningless, is asking was some particular object looks like "now", because from a physical standpoint, "now" is a very slippery and tricky concept.

[geckzilla]

Yes, and if Wiggins wants to get technical, they'll have to start using past tense for everything. So if I'm looking at a tree across the street, it's essentially just as not "now" as a galaxy 13 million light years away.

[Ann]

Many years ago I read a statement somewhere by a popularizer of astronomy that there is no such thing as simultaneity. The popularizer didn't elaborate, but the idea has never quite left me. Your "now" is yours only, because "now" is dependent on spacetime, on location, and only you can be exactly where you are now. Admittedly, on the Earth we are all so close to one another that our collective "now" is for all intents and purposes one and the same. But that is never true for objects that are light-years away, let alone objects that are millions or billions of light-years away. Their "now" doesn't overlap ours in any way. Speculating about their "now" is pointless.

I sometimes think about Ötzi the Iceman, found frozen and mummified after having died in the Italian Alps some 5,000 years ago. Ötzi has travelled on spaceship Earth for 5,000 years, but he was only alive for the first 45 years of his journey. But those first 45 years of his "existence", when he was alive, are unaccessible to us. Why? Because he lived in a spacetime separate from ours. The Earth was located somewhere else in the Milky Way 5,000 years ago than it is now, and the Milky Way itself was located somewhere else in relation to the other members of the Local Group and the Virgo Cluster than it is now. We can't visit the location where Ötzi was alive. His living "now" is in the Earth's past and completely separate from our "now".

Distant galaxies are even more forever out of our reach. They don't even travel on spaceship Earth, so we can't stumble on them in the ground or in our mountains.

Ann

[JackAubrey]

If I understand correctly, A Seyfert galaxy is distinguished from a "normal" galaxy because its central black hole has actively in-falling matter; or rather densely in-falling matter. Can/do Seyfert galaxies turn on and off depending on fluctuations in the density of the accretion disc? Could the Milky Way have been an active Seyfert galaxy at some point in its development, or maybe become one in a couple hundred million years?

[Ann]

Certainly galactic nuclei can turn "on" and "off". Just consider Hanny's Voorwerp, which is thought to be a huge gas cloud ionized by a jet from a nearby galaxy that used to be highly active, but is not active any longer. The jet from the galaxy is gone too, but Hanny's Voorwerp hasn't noticed yet.

Ann

[NGC3314]

Yeah, what Ann said. On top of that, the level of ionization in certain parts of the Magellanic Stream (cold hydrogen pulled from the Magellanic Clouds), and the so-called Fermi Bubbles seen in X-rays and gamma rays, fit with the idea that the Milky Way's nucleus underwent an active episode something like a million years ago. This has become an active area of investigation, with simulations now reaching the time resolution to show what processes contribute to this kind of "clicker".

[spaceaman]

This is a very good clear picture and it boggles the mind to think about it. One has to wonder, however, if it is possible to make such a picture as this, why do we never see clear close up pictures of the Moon, especially the back side. I would think that the entire surface of the moon would have been photographed and charted by now.

[geckzilla]

This is a very good clear picture and it boggles the mind to think about it. One has to wonder, however, if it is possible to make such a picture as this, why do we never see clear close up pictures of the Moon, especially the back side. I would think that the entire surface of the moon would have been photographed and charted by now.

You want to check out the LRO. It's mapped most of the surface of the moon. Some parts of the poles remain in constant shadow and have not been mapped. There are some interesting features on the far side, but since humans never see that face from Earth and the lack of mare make it more of a land of fractal craters (seriously, it's hard to tell how close you are looking because the craters look about the same at every zoom level) it receives a lot less love than our familiar side. Nonetheless, if you wished to land on the far side, you should be able find adequate resources to pick out a good spot.

[Chris Peterson]

This is a very good clear picture and it boggles the mind to think about it. One has to wonder, however, if it is possible to make such a picture as this, why do we never see clear close up pictures of the Moon, especially the back side. I would think that the entire surface of the moon would have been photographed and charted by now.

You want to check out the LRO.

For example. There are no missing parts. The entire Moon was mapped by LRO, including the polar regions. Nearside and farside, at extremely high resolution.

[geckzilla]

There are no missing parts. The entire Moon was mapped by LRO, including the polar regions.

Hmm, I really think you are wrong here, but I could have read some outdated information. For instance, if you zoom into the center of this, you'll easily spot some small holes.
http://lroc.sese.asu.edu/images/gigapan

of course, that doesn't necessarily include the most up-to-date mosaic. I have yet to find any sources definitively stating that it is either 100% or 99% or any percent done. There's holes in the poles of every lunar map I've looked at.

[Chris Peterson]

There are no missing parts. The entire Moon was mapped by LRO, including the polar regions.

Hmm, I really think you are wrong here, but I could have read some outdated information. For instance, if you zoom into the center of this, you'll easily spot some small holes.
http://lroc.sese.asu.edu/images/gigapan

of course, that doesn't necessarily include the most up-to-date mosaic. I have yet to find any sources definitively stating that it is either 100% or 99% or any percent done. There's holes in the poles of every lunar map I've looked at. :?

The poles have the best coverage, since LRO is in a polar orbit.

I didn't suggest that LRO has returned data at the highest possible resolution for every bit of the lunar surface. What I said is that it has mapped the entire surface at high resolution, with its imager and with its laser altimeter. I'd make the same case for Earth. The resolution of coverage is certainly not uniform, but I'd hardly say there are any unmapped regions.