APOD: Galaxy Cluster Abell 370 and Beyond (2017 May 06)

[APOD Robot]

Galaxy Cluster Abell 370 and Beyond

Explanation: Some 4 billion light-years away, massive galaxy cluster Abell 370 only appears to be dominated by two giant elliptical galaxies and infested with faint arcs in this sharp Hubble Space Telescope snapshot. The fainter, scattered bluish arcs along with the dramatic dragon arc below and left of center are images of galaxies that lie far beyond Abell 370. About twice as distant, their otherwise undetected light is magnified and distorted by the cluster's enormous gravitational mass, dominated by unseen dark matter. 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. Far beyond the spiky foreground Milky Way star at lower right, Abell 370 is seen toward the constellation Cetus, the Sea Monster. It is the last of six galaxy clusters imaged in the recently concluded Frontier Fields project.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[bystander]

http://asterisk.apod.com/viewtopic.php?t=37145

[neufer]

Moby Dick by Herman Melville
Chapter 74: The Sperm Whale's Head

Far back on the side of the head, and low down, near the angle of either whale’s jaw, if you narrowly search, you will at last see a lashless eye, which you would fancy to be a young colt’s eye; so out of all proportion is it to the magnitude of the head.... Is it not curious, that so vast a being as the whale should see the world through so small an eye, and hear the thunder through an ear which is smaller than a hare’s? But if his eyes were broad as the lens of Herschel’s great telescope; and his ears capacious as the porches of cathedrals; would that make him any longer of sight, or sharper of hearing? Not at all.- Why then do you try to “enlarge” your mind? Subtilize it.

[Ann]

Did you know that the Hubble Space Telescope is a superhero? No?

Click to play embedded YouTube video.

Then you should check out this super-family friendly video about Hubble, the superhero space telescope! And you should give it a chance, because at 1.25 there is a brief but fascinating explanation of what that amazing "arc" in Abell 370 is really made of.

The only other comment I have about today's APOD is...WOW!!!! I can't believe it! Particularly not when you compare this version of Abell 370 with previous portraits of it. The depth and resolution of today's image is... what can I say, roll over, Deep Field and Beethoven?

Ann

[Boomer12k]

I am writing a new song...."I'm a LENS-MAN, BABY... you're gonna be amazed...."...er...something, something.... "galaxies galore..." um...."warped images of love seen through a lens..." uhhh...."Lens of my heart, lens of my soul..." ohhh...."warping our love,"...something, something...... OK, I'll keep working on it..

Really awesome image!!!!

:---[===] *

[distefanom]

But, wouldn't be possible to some super-computer, to (let's say) re-construct those "arcs" to the original galaxy shapes, in this way "recognize" which arc image is a "ghost" of the very same galaxy which is beside the group?
I think it would be interesting, since one could "image" the "shape" of the dark matter around all this?

[Chris Peterson]

But, wouldn't be possible to some super-computer, to (let's say) re-construct those "arcs" to the original galaxy shapes, in this way "recognize" which arc image is a "ghost" of the very same galaxy which is beside the group?

No. In order to do that, it would be necessary to know the exact characteristics of the lens, which we don't know because we don't know the precise matter distribution (most of which is dark matter). In actual practice, what is done is to treat the background galaxies as point sources (which is a reasonable approximation) and then use the shape of the lensing to infer the mass distribution. That is, we use the lensed image to understand the lens, not the lens to understand the background sources. (And that is reasonable, since we don't really care what the background galaxies look like... they are galaxies, and we understand their morphology and have countless thousands that we can study using ordinary images.)

[bystander]

But, wouldn't be possible to some super-computer, to (let's say) re-construct those "arcs" to the original galaxy shapes, in this way "recognize" which arc image is a "ghost" of the very same galaxy which is beside the group?

This has been done (at least once, probably more).

Hubble Zooms in on a Magnified Galaxy
NASA | STScI | HubbleSite | 2012 Feb 02

---

---

Hubble WFC3 Image of Core of RCS2 032727-132623
and Reconstruction of RCSGA 032727-132609

This graphic shows a reconstruction (at lower left) of the brightest galaxy whose image has been distorted by the gravity of a distant galaxy cluster. The small rectangle in the center shows the location of the background galaxy on the sky if the intervening galaxy cluster were not there. The rounded outlines show distinct, distorted images of the background galaxy resulting from lensing by the mass in the cluster. The image at lower left is a reconstruction of what the lensed galaxy would look like in the absence of the cluster, based on a model of the cluster's mass distribution derived from studying the distorted galaxy images.

Illustration Credit: NASA, ESA, and Z. Levay (STScI);
Science Credit: NASA, ESA, J. Rigby (NASA GSFC), K. Sharon (Kavli Institute for Cosmological Physics, Univ of Chicago), and M. Gladders and E. Wuyts (Univ of Chicago)

Source-plane Reconstruction of the Bright Lensed Galaxy RCSGA 032727-132609 - Keren Sharon et al

• Astrophysical Journal 746(2):161 (2012 Feb 20) DOI: 10.1088/0004-637X/746/2/161
  arXiv.org > astro-ph > arXiv:1202.0539 > 02 Feb 2012

[Chris Peterson]

But, wouldn't be possible to some super-computer, to (let's say) re-construct those "arcs" to the original galaxy shapes, in this way "recognize" which arc image is a "ghost" of the very same galaxy which is beside the group?

This has been done (at least once, probably more).

Maybe not more than once. The conditions have to be just exactly right. In this case, it was possible to correlate structure between different lensed components, which allowed for a lens model to be built. Given the lens model, raytracing allowed for a possible reconstruction of the source galaxy. (I say possible, because gravitational lenses don't produce normal images, and the reconstruction is necessarily a statistical one, although in this case apparently robust.)

[Ann]

Hubble Zooms in on a Magnified Galaxy
NASA | STScI | HubbleSite | 2012 Feb 02

---

---

Hubble WFC3 Image of Core of RCS2 032727-132623
and Reconstruction of RCSGA 032727-132609

This graphic shows a reconstruction (at lower left) of the brightest galaxy whose image has been distorted by the gravity of a distant galaxy cluster. The small rectangle in the center shows the location of the background galaxy on the sky if the intervening galaxy cluster were not there. The rounded outlines show distinct, distorted images of the background galaxy resulting from lensing by the mass in the cluster. The image at lower left is a reconstruction of what the lensed galaxy would look like in the absence of the cluster, based on a model of the cluster's mass distribution derived from studying the distorted galaxy images.

Illustration Credit: NASA, ESA, and Z. Levay (STScI);
Science Credit: NASA, ESA, J. Rigby (NASA GSFC), K. Sharon (Kavli Institute for Cosmological Physics, Univ of Chicago), and M. Gladders and E. Wuyts (Univ of Chicago)

Source-plane Reconstruction of the Bright Lensed Galaxy RCSGA 032727-132609 - Keren Sharon et al

• Astrophysical Journal 746(2):161 (2012 Feb 20) DOI: 10.1088/0004-637X/746/2/161
  arXiv.org > astro-ph > arXiv:1202.0539 > 02 Feb 2012

As a color commentator, I find the colors weird. The lensed arcs of the background galaxy inside the elliptical frames are all very blue, while the galaxies of the lensing cluster are all yellow. The colors are probably due to a combination of the lensed background galaxy being richly starforming and brightly ultraviolet, and the foreground cluster being heavily dominated by old yellow stars, plus the choice of filters and color mapping for the image.

But now to the weirdness. The reconstructed galaxy inside the large rectangle isn't strikingly blue. One arm is somewhat blue, but not very blue at all. The core of the lensed galaxy appears yellow to white.

Should we take that to mean that if we could see the galaxy as it was when it emitted the light that is now being lensed and looking so brightly blue, we would see that the galaxy wasn't very blue at all? It was, if anything, really more yellow than blue?

Or have I misunderstood something?

Ann

[Chris Peterson]

As a color commentator, I find the colors weird. The lensed arcs of the background galaxy inside the elliptical frames are all very blue, while the galaxies of the lensing cluster are all yellow. The colors are probably due to a combination of the lensed background galaxy being richly starforming and brightly ultraviolet, and the foreground cluster being heavily dominated by old yellow stars, plus the choice of filters and color mapping for the image.

But now to the weirdness. The reconstructed galaxy inside the large rectangle isn't strikingly blue. One arm is somewhat blue, but not very blue at all. The core of the lensed galaxy appears yellow to white.

Should we take that to mean that if we could see the galaxy as it was when it emitted the light that is now being lensed and looking so brightly blue, we would see that the galaxy wasn't very blue at all? It was, if anything, really more yellow than blue?

In the image, red is the combined light of F160W, F125W, and F098M; red thus covers 0.9-1.7 micrometers. Green is the combined light of F814W and F606W; green thus covers 460-950 nm. Blue is the light from F390W; blue thus covers 330-440 nm. However, the lensed galaxy has a redshift of z=1.7. So in terms of the emitted wavelengths, red = 630-333 nm, green = 352-170 nm, blue = 163-122 nm. So what we're seeing is UV in the green and blue channels and most of the visible spectrum mapped to red.

Make of that what you will in assessing the appearance.

[Ann]

As a color commentator, I find the colors weird. The lensed arcs of the background galaxy inside the elliptical frames are all very blue, while the galaxies of the lensing cluster are all yellow. The colors are probably due to a combination of the lensed background galaxy being richly starforming and brightly ultraviolet, and the foreground cluster being heavily dominated by old yellow stars, plus the choice of filters and color mapping for the image.

But now to the weirdness. The reconstructed galaxy inside the large rectangle isn't strikingly blue. One arm is somewhat blue, but not very blue at all. The core of the lensed galaxy appears yellow to white.

Should we take that to mean that if we could see the galaxy as it was when it emitted the light that is now being lensed and looking so brightly blue, we would see that the galaxy wasn't very blue at all? It was, if anything, really more yellow than blue?

In the image, red is the combined light of F160W, F125W, and F098M; red thus covers 0.9-1.7 micrometers. Green is the combined light of F814W and F606W; green thus covers 460-950 nm. Blue is the light from F390W; blue thus covers 330-440 nm. However, the lensed galaxy has a redshift of z=1.7. So in terms of the emitted wavelengths, red = 630-333 nm, green = 352-170 nm, blue = 163-122 nm. So what we're seeing is UV in the green and blue channels and most of the visible spectrum mapped to red.

Thanks, Chris, really.

Make of that what you will in assessing the appearance.

Click to view full size image

NGC 4449. Photo:
Robert Gendler.

My real question was, is the color mapping different in the rectangular box than in the elliptical outlines, given that the rectangular box supposedly shows a "more realistic" portrait of the galaxy?

My implied criticism is that whoever constructed the "more realistic" portrait of the galaxy perhaps wanted the galaxy to look "normal", at least in terms of color and stellar populations. My implied criticism is that this galaxy may have been quite different from most galaxies in the nearby universe, and therefore we should, perhaps, not try to give it "normal colors" or force it to appear to have "normal stellar populations".

However, it is possible that the lensed, blue-looking galaxy had a yellow population that was redshifted all the way out of the sensitivity of the filters used for this image. So yes, it is possible that a "reconstructed" portrait of the galaxy's "real appearance" should show it as having a yellow population.

My guess, for what it's worth, is that the lensed galaxy was relatively similar in its stellar populations to a starbursting present-day dwarf galaxy like NGC 4449. NGC 4449 certainly contains a yellow population of stars, but the yellow population is clearly not very bright and absolutely not the dominant component of of the galaxy.

By the way, and if you are interested, here is a splendid 1 MB portrait of the central part of NHC 4449.

Ann

[distefanom]

Maybe it's exactly what I meant. I was paying attention to the very same galaxy, for it's peculiar characteristics, like the "blue tail" and "more than one" bright dots in it. For the very same reason my attention was captured also by the almost single-line arcs nearer to the center of the image; for the bright dots over the blue arc, which appear also in the reconstructed image... MAYBE are ghost images of the same one.
An interesting feature of this galaxy is (what seems to me) those white bright dots, also near the center of the galaxy itself; which resemble me as more-than-one nucleus galaxy; expecially in the reconstructed image.
Anybody agree?

[MarkBour]

I am writing a new song...."I'm a LENS-MAN, BABY... you're gonna be amazed...."...er...something, something.... "galaxies galore..." um...."warped images of love seen through a lens..." uhhh...."Lens of my heart, lens of my soul..." ohhh...."warping our love,"...something, something...... OK, I'll keep working on it..

Really awesome image!!!!

:---[===] *

EE "Doc" Smith would have loved it.

[MarkBour]

As I look at the many (apparent) galaxies forming a broad upper arc in this image, I was thinking, Ann, that if you can get really good, detailed imagery, color comparison can be used to help determine which items in an image such as today's are actually the same galaxy. In spite of distortion, I realize that shape can be a major clue, but I would think that color analysis could be very important supporting evidence. Surely astronomers use this in the process. Although the lens changes the path, I would think that in most cases, the various ghost images would all be roughly the same path-length from source to us, so their red-shifts should not differ by much (IMHO).

[Boomer12k]

I am writing a new song...."I'm a LENS-MAN, BABY... you're gonna be amazed...."...er...something, something.... "galaxies galore..." um...."warped images of love seen through a lens..." uhhh...."Lens of my heart, lens of my soul..." ohhh...."warping our love,"...something, something...... OK, I'll keep working on it..

Really awesome image!!!!

:---[===] *

EE "Doc" Smith would have loved it.

Exactly, I loved the Lensman Series...

:---[===] *

[Ann]

Click to view full size image

Hubble WFC3 Image of Core of RCS2 032727-132623
and Reconstruction of RCSGA 032727-132609.
Image credit: NASA, ESA, and Z. Levay (STScI);
Science Credit: NASA, ESA, J. Rigby (NASA GSFC), K. Sharon (Kavli
Institute for Cosmological Physics, Univ of Chicago), and M. Gladders
and E. Wuyts (Univ of Chicago)

Click to view full size image

NGC 3310. NASA/ESA and The Hubble Heritage Team (STScI/AURA)

I still can't get over how the reconstructed image of RCSGA 032727-132609 is systematically less blue than the lensed versions of it inside elliptical frames, in the image at left. I think that perhaps the people who made the reconstruction were inspired by the iconic, but false-color, Hubble picture of extremely ultraviolet nearby spiral galaxy NGC 3310.

Because NGC 3310 really is so ultraviolet, the galaxy was photographed through one violet and two ultraviolet filters, as well as the Hubble standard 814 nm infrared filter. The purpose was to separate the ultraviolet populations by age. Therefore, in the Hubble image, only the most ultraviolet clusters look blue at all. By contrast, the center of NGC 3310 looks downright orange, a very unrealistic color.

As I said, I think that the people who reconstructed the image of RCSGA 032727-132609, the lensed blue galaxy in the core of Core of RCS2 032727-132623, were perhaps inspired by the "intentionally redshifted Hubble image of NGC 3310" and imparted a somewhat similar "intentional redshift" to RCSGA 032727-132609.

Ann

[Nitpicker]

The "parallel" frames that accompany each of the galaxy cluster images in this Hubble project, are very close to the clusters. Do they serve a purpose beyond showing 2x2 arcmin patches of the sky nearby?

[geckzilla]

The "parallel" frames that accompany each of the galaxy cluster images in this Hubble project, are very close to the clusters. Do they serve a purpose beyond showing 2x2 arcmin patches of the sky nearby?

They are there for astronomers to "do a science" on if they can think of something. Cosmologists can never have enough samples to characterize the Universe, for instance. All Hubble time is hyper optimized to make maximum use of the telescope, so we can't have one camera idling for hours at a time while another is doing the primary observation. So you could say that the purpose is to make use of Hubble.

[Nitpicker]

Sure, no sense being idle. I just thought it odd that each cluster was presented as a diptych in the "fast facts". Took me a while to figure out what it was showing.

[K1NS]

I know very little about the actual business of using astronomical telescopes.

Would I be able to walk up to a giant telescope somewhere on Earth, put my eye up to it, and see an extraordinary image like this? Or is this a very faint image that must be laboriously processed to come into view?

[Chris Peterson]

I know very little about the actual business of using astronomical telescopes.

Would I be able to walk up to a giant telescope somewhere on Earth, put my eye up to it, and see an extraordinary image like this? Or is this a very faint image that must be laboriously processed to come into view?

No, you would see nothing like this in a telescope (and most large telescopes don't even have eyepieces). Telescopes don't make dim objects any brighter, just bigger. If they're too dim to see with your eye (as almost all astronomical objects are), the telescope won't help.

What makes images like this possible isn't so much the telescope (although it's still required!) but the camera. A camera is able to integrate the light it collects. The longer the exposure, the more photons you record and the brighter the image gets. It doesn't necessarily require significant processing at all (although such processing is typically applied to reduce artifacts, adjust color, and generally make the image look good to our eyes).

[Nitpicker]

Chris is right that giant, professional telescopes don't have eyepieces. They have camera sensors, which can (in theory) display a live video feed to a computer monitor, in lieu of an eyepiece. But a dim object like this would still show a very dim image this way on the monitor (if not entirely black). You need to record long exposures to see anything resembling this APOD.

However, it is not quite right to say that telescopes don't make dim objects any brighter. They do, but not necessarily bright enough to see them "live" in the normal way that our eyes work. The larger the aperture of the telescope, the brighter the image (reducing required exposure times). The longer the focal length of the telescope, the larger the image (which also reduces the brightness of the image for the same aperture).

...

I jumped straight into astronomy as a hobby a few years ago and I bought a small, motorised, 6 inch aperture telescope and a collection of eyepieces. I used it to see a lot of bright stuff in the sky. But it wasn't until I started attaching cameras to the telescope, that I was able to observe things much dimmer (though not as dim as Abell 370). I barely use my eyepieces for visual observation now (unless showing things to others). Even for bright objects, I see the same or more detail through the use of a camera and a computer monitor (significantly more, following a very rudimentary level of image processing).

[Chris Peterson]

However, it is not quite right to say that telescopes don't make dim objects any brighter. They do, but not necessarily bright enough to see them "live" in the normal way that our eyes work. The larger the aperture of the telescope, the brighter the image (reducing required exposure times).

To be clear, I'm talking about using a telescope visually. Technically, a telescope used with a camera isn't even a telescope! But visually, a telescope can never make an extended object brighter than we see it with the naked eye. Only larger. In some cases, a larger object with the same surface brightness is more easily seen, since more retinal cells are involved. But the surface brightness never exceeds the naked eye view. And often it is less. Aperture doesn't matter in this regard, except to the extent that it defines the maximum magnification you can use before the surface brightness decreases from what we see without the telescope.

Photographically, increasing the aperture increases the number of photons collected, and therefore decreases the exposure time required for any given S/N.

[Nitpicker]

I was going to mention that a telescope used with a camera isn't technically a telescope, but most people seem to have moved past that point, as all the giant professional ones are called telescopes. It hardly seems worth mentioning anymore.

Back to telescopes with eyepieces ... Assuming a perfect instrument, I would consider the ratio of the area of the telescope aperture, to the area of the eyepiece exit pupil (especially if well matched to the area of the observer's pupil) to be a good measure of how much brighter objects appear in the telescope, compared with the unaided eye. Simply a measure of the increase in the number of photons per unit time, reaching the retina. So, Chris, where have I gone wrong?