Starship Asterisk*

Posted: **Fri Jul 22, 2016 4:07 am**

Galaxy Cluster Abell S1063 and Beyond

Explanation: Some 4 billion light-years away, galaxies of massive Abell S1063 cluster near the center of this sharp Hubble Space Telescope snapshot. But the fainter bluish arcs are magnified images of galaxies that lie far beyond Abell S1063. About twice as distant, their otherwise undetected light is magnified and distorted by the cluster's largely unseen gravitational mass, approximately 100 trillion times the mass of the Sun. Providing a tantalizing glimpse of galaxies in the early universe, the effect is known as gravitational lensing. A consequence of warped spacetime it was first predicted by Einstein a century ago. The Hubble image is part of the Frontier Fields program to explore the Final Frontier.

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Posted: **Fri Jul 22, 2016 4:10 am**

http://asterisk.apod.com/viewtopic.php?t=36183

Posted: **Fri Jul 22, 2016 4:23 am**

So...um.... why are they blue again? Ann?

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Posted: **Fri Jul 22, 2016 4:39 am**

Boomer12k wrote:So...um.... why are they blue again? Ann?

This image isn't close to being "natural" color. It was collected through seven different broadband filters, five of those in the near IR. The blue channel is a mix of two filters, one covering purple and blue, the other covering the entire visible spectrum longer than that. So basically, the blue represents visible light, and red and green map to IR up to about 1.7 microns.

Posted: **Fri Jul 22, 2016 5:32 am**

Boomer12k wrote:So...um.... why are they blue again? Ann?

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NGC 4449, a starbursting
galaxy in the nearby universe.
Photo: R. Jay GaBany.

Ultraviolet light + sufficent redshift = blue light.

Basically the blue arcs are blue because they are stretched and redshifted images of galaxies which are dominated by hot ultraviolet stars. These galaxies haven't had time to build up large yellow populations, and they haven't had time to create copious amounts of dust, either. Their light output peaks at ultraviolet wavelengths.

: Click to view full size image

Source: https://www.youtube.com/watch?v=WWFuo-ToW5w

But since the galaxies are so far away and their light has traveled so far to reach us, a good deal of their ultraviolet light has been redshifted by the expansion of the universe into longer-wave visible blue and violet light.

Look at the image at right. A spectral line that was originally located exactly at 400 nm in the violet part of the visible spectrum has been redshifted to a wavelength of 530 nm in the green part of the spectrum. Please note that it isn't just the spectral line that has been redshifted, but all the violet light that was originally emitted by the galaxy has been redshifted into the green part of the spectrum. The violet light that is visible in the redshifted spectrum was originally ultraviolet light.

: Click to view full size image

Spectral energy distribution
of blue and red dwarf galaxies in the Coma Cluster.
Source: http://www.aanda.org/articles/aa/full_h ... 28-09.html

At left you can see three types of dwarf galaxies in the Coma Cluster. The important point is that the blue dwarf galaxies (middle row) are dominated by shortwave light, whereas the red galaxies (bottom row) are dominated by longwave light. Distant shortwave galaxies may look blue because of redshift.

The gargantuan elliptical galaxy in the foreground of today's APOD is likely made up almost exclusively of old yellow stars. This galaxy produces very little ultraviolet light in the first place, so there is precious little ultraviolet light to be redshifted into the blue part of the spectrum. This galaxy therefore looks very yellow - indeed, it is so far away that the light it emits is redshifted too, making the galaxy look a lot more orange than it would have been if it had been as nearby as the Virgo Cluster ellipticals.

I think today's APOD is gorgeous. There is so much going on in it. The background galaxies are splendid. There is such a menagerie of them! It's fun to try to classify them by visual inspection, and it's fun to guess if one or more of the background galaxies have been lensed into two or more images.

Of course the foreground elliptical looks like a real monster. I's a galaxy cluster, you say? It looks like one huge slightly (American) football-shaped monstrosity. I wonder how much mass it contains. And I wonder how massive its central black hole might be, and how massive it could become in the future. Gravitational waves out of merging black holes in Abell S1063, anyone?

Ann

Posted: **Fri Jul 22, 2016 5:34 am**

Chris Peterson wrote:
Boomer12k wrote:So...um.... why are they blue again? Ann?
This image isn't close to being "natural" color. It was collected through seven different broadband filters, five of those in the near IR. The blue channel is a mix of two filters, one covering purple and blue, the other covering the entire visible spectrum longer than that. So basically, the blue represents visible light, and red and green map to IR up to about 1.7 microns.

Taking redshift into account, it comes close in some places. The funny thing there is something of a focal plane for visible light when looking across such vast distances...

Posted: **Fri Jul 22, 2016 6:21 am**

That lensed galaxy or galaxies appear to be blue-shifted. Better watch out, they're coming our way!!

Posted: **Fri Jul 22, 2016 6:40 am**

stormin651 wrote:That lensed galaxy or galaxies appear to be blue-shifted. Better watch out, they're coming our way!!

They are all cosmologically redshifted, not to be confused with Doppler redshifting (or blueshifting).

Posted: **Fri Jul 22, 2016 9:10 am**

When I was a summer student at Kitt Peak National Observatory, in 1965 or so, I vividly remember Kitt Peak astronomer Roger Lynds showing everyone a photo Art Hoag had taken showing a "nebula" - an extended thingy - that looked like no nebula anyone had seen before: a smooth arc! No one suggested the right answer, gravitational lensing, even though the Barnothys (husband and wife) had given endless AAS talks on the possibility.

Posted: **Fri Jul 22, 2016 11:55 am**

For gravity to be considered as a "weak" force, it sure has a profound affect on how we see the Universe...

Posted: **Fri Jul 22, 2016 12:39 pm**

I now have 23 Abells in my photo collection, thanks to the wonderful postings here. Three of them, as this one, include an 'S' in their designations, while the other 20 do not. What does the 'S' mean? The Wikipedia article about the Abell catalogue doesn't mention the 'S' and therefore seems to suggest it's not part of the standard Abell numbering system.

Rob

Posted: **Fri Jul 22, 2016 1:07 pm**

"For gravity to be considered as a "weak" force, it sure has a profound affect on how we see the Universe..."
Yup! Because F=Mm/r^2 where m is your mass while M is that of THE ENTIRE EARTH - yet even so, you are not just a smear on the surface of the Earth! Your leg muscles are adequate to overcome the gravitational attraction of The Entire Earth. Superman? Nope, super weak force! It is a profound mystery why the four forces (strong=nuclear, weak=beta decay, electromagnetic, gravitational) have such a vast range in strengths. And if any one of them were a factor of 2 different in strength, life could not exist on Earth. Hmmm! The only non-religious explanations are 1) pure chance, and 2) the Multiverse - go far enough away (beyond the visible Universe) and the strengths are randomly different, and no life is possible. Wow would I like to know what the answer is!

Posted: **Fri Jul 22, 2016 2:02 pm**

rstevenson wrote:I now have 23 Abells in my photo collection, thanks to the wonderful postings here. Three of them, as this one, include an 'S' in their designations, while the other 20 do not. What does the 'S' mean? The Wikipedia article about the Abell catalogue doesn't mention the 'S' and therefore seems to suggest it's not part of the standard Abell numbering system.

The Abell catalog (which is a deprecated name... these objects should be referred to as ACO for Abell-Corwin-Olowin) is constructed from data from two major surveys, one of the northern sky and one of the southern. Objects from the first are numbered ACO 0001 - ACO 4075. Objects from the second are numbered ACO S 0001 - ACO S 1174. The 1989 paper which defines the catalog is available here.

Posted: **Fri Jul 22, 2016 2:06 pm**

heehaw wrote:"For gravity to be considered as a "weak" force, it sure has a profound affect on how we see the Universe..."
Yup! Because F=Mm/r^2 where m is your mass while M is that of THE ENTIRE EARTH - yet even so, you are not just a smear on the surface of the Earth! Your leg muscles are adequate to overcome the gravitational attraction of The Entire Earth. Superman? Nope, super weak force! It is a profound mystery why the four forces (strong=nuclear, weak=beta decay, electromagnetic, gravitational) have such a vast range in strengths. And if any one of them were a factor of 2 different in strength, life could not exist on Earth. Hmmm! The only non-religious explanations are 1) pure chance, and 2) the Multiverse - go far enough away (beyond the visible Universe) and the strengths are randomly different, and no life is possible. Wow would I like to know what the answer is!

Those are not the only explanations. (And multiverse theories describe different universes, not any distance outside the visible universe.) Just because the universe would look different with different values doesn't mean it wouldn't be hospitable to life. It's just that life wouldn't look anything like we think of it. Perhaps we are as we are because that's what can exist under the particular physical constants and laws where we find ourselves. That's a pretty simple solution to the mystery. Indeed, it makes the mystery go away.

Posted: **Fri Jul 22, 2016 2:34 pm**

Chris Peterson wrote:
rstevenson wrote:I now have 23 Abells in my photo collection, thanks to the wonderful postings here. Three of them, as this one, include an 'S' in their designations, while the other 20 do not. What does the 'S' mean? The Wikipedia article about the Abell catalogue doesn't mention the 'S' and therefore seems to suggest it's not part of the standard Abell numbering system.
The Abell catalog (which is a deprecated name... these objects should be referred to as ACO for Abell-Corwin-Olowin) is constructed from data from two major surveys, one of the northern sky and one of the southern. Objects from the first are numbered ACO 0001 - ACO 4075. Objects from the second are numbered ACO S 0001 - ACO S 1174. The 1989 paper which defines the catalog is available here.

Thanks Chris. I guessed at maybe S=south, but the absence of any mention of that in the Wiki article made me doubt it. Some knowledgeable person needs to update that article a bit.

Seems to me, though, that south and north are entirely local phenomena, and if something like the [formerly known as] Abell catalogue is to be updated and standardized, the S designation could just be left out.

Rob

Posted: **Fri Jul 22, 2016 2:40 pm**

rstevenson wrote:Seems to me, though, that south and north are entirely local phenomena, and if something like the [formerly known as] Abell catalogue is to be updated and standardized, the S designation could just be left out.

As I understand the reasoning, it's because of the very different instrumentation involved in the two surveys. That results in some of the parameters having difficult to compare values. So the catalog numbers the northern and southern components differently (and also treats specially those in the overlap zone).

Posted: **Fri Jul 22, 2016 2:46 pm**

Looking at today's picture I've a question about gravitational lensing.

Seeing Abell in the center, is everything around it the result of lensing and showing us what is behind it?

Are we seeing only galaxies behind it? or are some galaxies that we see in the pic simply those surrounding Abell? If we are seeing both, where do the "surrounding" near-neighborhood galaxies stop and the lensing distant galaxies begin?

Posted: **Fri Jul 22, 2016 2:53 pm**

Bald Eagle wrote:Looking at today's picture I've a question about gravitational lensing.

Seeing Abell in the center, is everything around it the result of lensing and showing us what is behind it?

Are we seeing only galaxies behind it? or are some galaxies that we see in the pic simply those surrounding Abell? If we are seeing both, where do the "surrounding" near-neighborhood galaxies stop and the lensing distant galaxies begin?

The surrounding galaxies that actually look like galaxies or fuzzy spots are either unlensed, or minimally lensed. Most of the galaxies we see aren't far enough away to show significant lensing. Most of the light from the lensed galaxies is what we see distributed in all those blue arcs. Most of what's behind is just shining though without distortion.

The very distant galaxies, which would be invisible but for the lensing, are young, and therefore active. Active galaxies are hot, and output a lot of energy at shorter wavelengths, which in this image are redshifted into the visible, and mapped to blue. So, blue arcs are lensed galaxies, orange blobs are not.

Posted: **Fri Jul 22, 2016 4:12 pm**

heehaw wrote:The only non-religious explanations are ...

Are you sure this is exhaustive? Even though I don't know as much about this matter as Chris does, I'm not going to assume that the only non-religious explanations are those that have already been proposed. This is classic argument from ignorance, as well as being a curiosity block.

Besides, the only religious explanation proposed so far is that it was designed by an intelligent being which, for some reason that is never forthcoming, required no designer itself.

Posted: **Fri Jul 22, 2016 4:13 pm**

Some of those galaxy cores show diffraction artifacts. This is some feat of light collection!

Posted: **Fri Jul 22, 2016 8:19 pm**

Chris Peterson wrote:
The very distant galaxies, which would be invisible but for the lensing, are young, and therefore active. Active galaxies are hot, and output a lot of energy at shorter wavelengths, which in this image are redshifted into the visible, and mapped to blue. So, blue arcs are lensed galaxies, orange blobs are not.

Chris, are you saying that the redshifted light that reaches us from the lensed galaxies isn't dominated by light at around 400-500 nm? In other words, are you saying that some of the blue-looking arcs may just as well be dominated by light at around 600 nm, and they are simply mapped to blue in order to highlight their lensed nature?

This 1 MB image shows 14 identified background quasars in the direction of spiral galaxy NGC 5792. By visual inspection, basically all these background galaxies look at least slightly bluish.

In other words, I'm asking if the lensed blue-looking galaxies aren't dominated by light around 400-500 nm. I realize that the color shown in the image is also dependent of the filters that have been used to make it. So if this image had been made with, say, a visual orange filter instead of just blue and ultraviolet and infrared filters, then the blue galaxies might well have looked more orange and less blue.

I'd like you to elaborate on the "mapped blue color", though.

Ann

Posted: **Fri Jul 22, 2016 8:55 pm**

Ann wrote:
Chris Peterson wrote: The very distant galaxies, which would be invisible but for the lensing, are young, and therefore active. Active galaxies are hot, and output a lot of energy at shorter wavelengths, which in this image are redshifted into the visible, and mapped to blue. So, blue arcs are lensed galaxies, orange blobs are not.
Chris, are you saying that the redshifted light that reaches us from the lensed galaxies isn't dominated by light at around 400-500 nm? In other words, are you saying that some of the blue-looking arcs may just as well be dominated by light at around 600 nm, and they are simply mapped to blue in order to highlight their lensed nature?

I was simply answering the question of why they look blue in the image. The blue channel is the only one recording visible light, and it covers the entire visible range. From this, we can't tell if the arcs are blue or red, because both look the same in that channel. I did look at the raw images, however (F435W and F606W) and without worrying too much about scaling, the arcs appear brighter in the longer wavelengths. So they might well appear yellow or orange in a true RGB image.

Given that the distant galaxies have redshifts on the order of 3, we can work out a bit of what the image is showing:

Code: Select all

Channel  Observed      Emitted
R        1100-1700 nm  268-415 nm
G         700-1200 nm  171-293 nm
B          380-710 nm   92-173 nm

In other words, all of the visible light is shown in blue, with the red and green channels showing only infrared. However, when we apply the redshift and look at the photons that were emitted, everything is UV, with the red channel showing very near UV (almost visible at the longest wavelengths), the green channel showing short UV, and the blue channel showing pretty hard UV. The arcs show up almost the same in all channels, which suggests that the background galaxies are radiating strongly in the UV across that entire range- perhaps just a little stronger at the shortest wavelengths, but that may be an illusion of how the channels were scaled.

Posted: **Fri Jul 22, 2016 9:00 pm**

Thanks for the reply and the explanation, Chris.

You wrote:

I did look at the raw images

Do you have a link to the raw images?

Ann

Posted: **Fri Jul 22, 2016 9:11 pm**

Ann wrote:Thanks for the reply and the explanation, Chris.

You wrote:

I did look at the raw images
Do you have a link to the raw images?

Links to download all the data are here. Digging through these FITS files is not for the faint of heart, however! You might prefer to look here, since you can browse these images without downloading them and loading them up in a FITS file viewer.

Posted: **Fri Jul 22, 2016 9:16 pm**

Can gravitational lenses create 'normal' (non-smeared) images? If so, would there be some other property that would identify it as being lensed?
Or could we be over-counting the number of objects due to lensing effects that we are not aware of?

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APOD: Galaxy Cluster Abell S1063 and Beyond (2016 Jul 22)

APOD: Galaxy Cluster Abell S1063 and Beyond (2016 Jul 22)

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