Starship Asterisk*

mihondo2016 wrote:Can gravitational lenses create 'normal' (non-smeared) images? If so, would there be some other property that would identify it as being lensed?

No. A normal lens has zero power in the center, and its power increases towards the edges. That's just the opposite of a gravitational lens, which is why you can't produce a normal image. (Think of the glass analog of a gravitational lens as looking like the base of a wine glass.)

Or could we be over-counting the number of objects due to lensing effects that we are not aware of?

I don't think there are any hidden or uncertain lens effects to worry about. But the math required to work backwards from the lensed image isn't simple, and generally doesn't produce a closed solution. So there's a statistical element to the reconstruction, and it's possible to get it wrong. Where there is more than one background galaxy, as in today's image, separating them is usually possible simply because they aren't going to have identical redshifts and other spectral properties.

Chris Peterson wrote:Digging through these FITS files is not for the faint of heart, however!

But once you know how, oh what a world awaits...

Chris Peterson wrote:

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.

Thanks. I was surprised at how blurry, dark and seemingly non-informative the raw images are.

Ann

Ann wrote:Thanks. I was surprised at how blurry, dark and seemingly non-informative the raw images they are.

Haha, you should see it with a linear stretch. The browser version of the FITS viewer has a generic stretch applied to make things considerably less dark already.

It's a fascinating image .

Trying to understand the concept of warped spacetime does cause my brain to struggle. I even have flights of fancy that it might be wrapped spacetime where light wraps round objects and causes the lensing!

There is a star to the top left and one about in the far right middle that have very obvious diffraction spikes. I am curious why their diffraction spikes colours differ so much. I assume the left one is a red star and the right one is a blue one but the very marked differences in their spikes colours still seems a bit odd.

DavidLeodis wrote:There is a star to the top left and one about in the far right middle that have very obvious diffraction spikes. I am curious why their diffraction spikes colours differ so much. I assume the left one is a red star and the right one is a blue one but the very marked differences in their spikes colours still seems a bit odd.

The colors of the spikes differ because the colors of the stars do.

Chris Peterson wrote:

DavidLeodis wrote:There is a star to the top left and one about in the far right middle that have very obvious diffraction spikes. I am curious why their diffraction spikes colours differ so much. I assume the left one is a red star and the right one is a blue one but the very marked differences in their spikes colours still seems a bit odd.

The colors of the spikes differ because the colors of the stars do.

Thanks for your help Chris .

As the colours in the diffraction spikes seem to alternate, such as for example red to green to red etc in the top left star, then it is not a continuous spectrum. As spectrums of stars are extremely useful in providing information I wonder how much information can be obtained from studying diffraction spikes colours in images?

DavidLeodis wrote:

Chris Peterson wrote: The colors of the spikes differ because the colors of the stars do.

Thanks for your help Chris :).

As the colours in the diffraction spikes seem to alternate, such as for example red to green to red etc in the top left star, then it is not a continuous spectrum. As spectrums of stars are extremely useful in providing information I wonder how much information can be obtained from studying diffraction spikes colours in images?

Diffraction is wavelength dependent, which is why we see different colors at different distances from the star. I think I've heard of people trying to get useful information from the spikes, but really, the resolution is probably too low to derive much.

Thanks for that interesting information Chris .

The diffraction spikes (along with the rest of the point spread function) probably tell more about the instrument doing the imaging than the objects themselves. Hubble's infrared PSF is a dead giveaway, though. Anyone who has spent time looking at Hubble data can probably tell an infrared dataset without knowing beforehand what band they're looking at.

At 11 o'clock is a smallish elliptical, likely belonging to Abell S1063. Above it is what looks like an arc. Could that be a lensed background galaxy?

I must say all over again that I just love this image.

Ann

geckzilla wrote:The diffraction spikes (along with the rest of the point spread function) probably tell more about the instrument doing the imaging than the objects themselves. Hubble's infrared PSF is a dead giveaway, though. Anyone who has spent time looking at Hubble data can probably tell an infrared dataset without knowing beforehand what band they're looking at.

Thanks for that geckzilla .

Seeing how the arcs of lensed galaxies always seem to be bluish I am wondering if other colours could result? Also, if it is possible to obtain a spectrum of the arcs of lensed galaxies would they have differing spectrums or all be the same?

With interference recently I've been getting some lovely spectral effects on my television . Unwatchable, but pretty!

DavidLeodis wrote:Seeing how the arcs of lensed galaxies always seem to be bluish I am wondering if other colours could result? Also, if it is possible to obtain a spectrum of the arcs of lensed galaxies would they have differing spectrums or all be the same?

There are other colors in there now, but the blue is dominating them. It might be possible to differentiate them if one ignores the foreground and balances the colors on just the background galaxies. I can't say I've ever tried doing that.

DavidLeodis wrote:
Seeing how the arcs of lensed galaxies always seem to be bluish I am wondering if other colours could result? Also, if it is possible to obtain a spectrum of the arcs of lensed galaxies would they have differing spectrums or all be the same?

Not all the arcs of lensed galaxies look blue. At right is an example of a lensed galaxy that looks reddish. But you can still see some bluish lensed background galaxies in the picture.

The reason why so many background galaxies look bluish is, as far as I can understand, that these very distant galaxies hail back to an era where starforming, ultraviolet-bright galaxies were the norm (not that elliptical galaxies were absent at that time, but they were much rarer than today). The foreground galaxies acting as lenses, by contrast, are almost always massive elliptical galaxies made up almost exclusively (apart from dark matter and the like) of old yellow stars. Even when you take redshift into account, the background galaxies will typically be rich in shortwave light, while the foreground ellipticals will be lacking in it. In other words, the lensed background galaxies will be bluer that the foreground ellipticals, and most pictures will enhance the color difference and show the background galaxies as quite blue.

In some cases, and for whatever reasons, the lensed galaxies are not all that blue after all. That reddish lensed galaxy in the image here may be quite dusty.

Ann

Yes, they are all definitely very blue compared to foreground objects. I have doubts that much more can be done with them. Part of the reason is that most of them are no more than maybe 5 pixels wide. They're just dots until they get smeared by the lens.

geckzilla wrote:

DavidLeodis wrote:Seeing how the arcs of lensed galaxies always seem to be bluish I am wondering if other colours could result? Also, if it is possible to obtain a spectrum of the arcs of lensed galaxies would they have differing spectrums or all be the same?

There are other colors in there now, but the blue is dominating them. It might be possible to differentiate them if one ignores the foreground and balances the colors on just the background galaxies. I can't say I've ever tried doing that.

I tried combining the 606 and 435 frames into a single pseudo natural color image, and the arcs appeared much less blue. They're putting out quite a lot of longer wavelength light, as well. But I didn't try to color balance the image using a reference star, or do any photometric tricks on the raw data, so the balance could still be way off.

To really understand the color of these distant galaxies we need to look at IR spectra for them, and then undo the cosmological redshift.

The color of a lensed galaxy depends, obviously, at least partly on the filters used and the way the colors detected by the filters are mapped.

Nevertheless, if an ultraviolet-bright galaxy is sufficiently distant, the ultraviolet light it emits will be redshifted all the way into the infrared part of the spectrum. The light that reaches us from such a galaxy will be extremely faint, and it will be redder than a moderately nearby (say, 4 billion light-years or so) elliptical galaxy.

Consider a galaxy that is extremely distant, intrinsically ultraviolet-rich but cosmologically redshifted entirely into the infrared part of the spectrum. If such a galaxy were to be lensed by a moderately nearby massive elliptical galaxy, the extremely distant galaxy might look like an extremely faint reddish arc. It would be redder than the foreground elliptical.

What I'm trying to say is that the lensing of distant galaxies into elongated arcs by foreground massive elliptical galaxies will not automatically create lensed arcs that are bluer than the foreground ellipticals.

EDIT: This tiny hugely redshifted galaxy has been lensed, although not into the shape of an arc.

https://www.spacetelescope.org/news/heic1217/ wrote:

Along the way, 8 billion years into its journey, the galaxy’s light took a detour along multiple paths around the massive galaxy cluster MACS J0647.7+7015. Due to the gravitational lensing, the team observed three magnified images of MACS0647-JD with Hubble. The cluster’s gravity boosted the light from the faraway galaxy, making the images appear far brighter than they otherwise would, although they still appear as tiny dots in Hubble’s portrait.

Ann

Hi, I'm new to this forum and this is my first posting. I know this posting is a bit late, but hopefully someone is still paying attention.
Unfortunately I can't post the picture I wanted to so I will try to describe it in words.
Just below the center of the central bright spot there are two small, nearly identical objects that look like galaxies. One is the first object directly below and slightly to the right, the other is slightly above and to the left of the first. I clipped these out and mirrored one and rotated both until they lined up. Nearly a perfect match. Looks like there are two separate focal points such that one is behind its focal point and the other is in front of it. Are these objects actually galaxies? There are both blue areas and yellow areas in the object. If the blue started out as ultra violet, what did the yellow start out as? Are the blue areas star forming regions for some of the first generation stars? - Thanks, DonJ

Yes, Don, they are actually galaxies. That particular example you pointed out is probably repeated at least a few other times in the image, too. A popular analogy for this is to take a wine glass and break the top off and look through the bottom and observe how the image you receive through the lens is distorted. An article for a different Frontier Fields cluster has images showing exactly which galaxies are duplicate images of one another:
https://frontierfields.org/2014/07/09/s ... ds-images/

I don't have specific answers about the light from individual galaxies, but yes, the light from this image all started out very blue and became gradually reddened by cosmological redshifting. What wavelength it started out as depends on how distant they are and they're not all at the same distance, but the different wavelengths aren't separated by much more than a couple hundred or so nanometers.

DonJ wrote:Hi, I'm new to this forum and this is my first posting. I know this posting is a bit late, but hopefully someone is still paying attention.
Unfortunately I can't post the picture I wanted to so I will try to describe it in words.
Just below the center of the central bright spot there are two small, nearly identical objects that look like galaxies. One is the first object directly below and slightly to the right, the other is slightly above and to the left of the first. I clipped these out and mirrored one and rotated both until they lined up. Nearly a perfect match. Looks like there are two separate focal points such that one is behind its focal point and the other is in front of it.

We are definitely seeing two images of the same galaxy being magnified and mirror imaged by the foreground galaxy cluster.

Are these objects actually galaxies?

Yes, most certainly.

There are both blue areas and yellow areas in the object. If the blue started out as ultra violet, what did the yellow start out as?

The galaxy in question is a disk galaxy with a bright bulge. It is almost inconceivable that such a galaxy wouldn't have a substantial yellow population. It is also true that the galaxy is somewhat distorted and contains bright regions of star formation. I think this distant galaxy is quite comparable to many galaxies in the nearby universe, for example M83, pictured here by Warren Keller.

To put it differently: I think that the yellow areas in the lensed background galaxy are made up of predominantly yellow stars.

Are the blue areas star forming regions for some of the first generation stars? - Thanks, DonJ

No. A galaxy that contains a bright yellow bulge and spiral arms isn't forming some of the first stars in the universe. Any galaxy that was forming some of the first stars in the universe would either be blob-shaped or wildly irregular. Also all of it would be basically the same color, since such a galaxy wouldn't contain any old or intermediate stars at all.

In any case, this galaxy isn't nearly as far away as it would have to be to be forming some of the first stars in the universe.

Given how far away the lensed galaxy probably is - 7 billion light years, perhaps? - I'd say that its light has been so redshifted by the expansion of the universe that our eyes, if we could see it, would see it as quite orange. In other words, I think that the picture that we see in this APOD has been color balanced in such a way that makes it easier to see the different populations in the background galaxies. The blue channel has been enhanced.

Ann

Ann wrote:No. A galaxy that contains a bright yellow bulge

Although you do have to be careful calling it yellow. I mean, if it's far enough away then blue might be something like far or mid ultraviolet while redder colors might be closer to near ultraviolet.

geckzilla wrote:

Ann wrote:No. A galaxy that contains a bright yellow bulge

Although you do have to be careful calling it yellow. I mean, if it's far enough away then blue might be something like far or mid ultraviolet while redder colors might be closer to near ultraviolet.

I must make it clear that I'm stating what I believe, not what I know. Obviously I don't know that this galaxy contains a well-established yellow population. I just think it does. I base my belief on my very great interest in galaxies, plus the fact that I have spent a lot of time looking at galaxies and checking out their colors, general shapes, classifications and appearances.

The lensed galaxy in question looks very much like a disk galaxy to me. It very much looks like it has a bright bulge. It looks like it has spiral arms. In my experience, such galaxies have yellow centers, although there are a precious few that don't seem to follow the pattern.
The best "all-blue" example I can think of is NGC 4656, the Hockey Stick galaxy. Adam Block's picture does show a faint yellowing at the center of the galaxy, but it is indeed faint. Mark Hansen's fine picture of NGC 4656 and its blue-and-pink-and-dusty-orange neighbour NGC 4631 shows the same thing, a faint - make that faint - yellowing at the center of the Hockey Stick. So what is the B-V of NGC 4656? Its total B-V is 0.440, which is blue but not incredible. Its effective B-V is 0.325, however, which is extremely blue. And its apparent B-V, which takes the galaxy's external and internal reddening into account, is 0.217 according to Principal Galaxy Catalog. Such a low B-V value for a relatively large galaxy is almost impossible, and the absolutely only way for a galaxy to be so blue is if it has a very large blue population and a really faint yellow one. NGC 4656 seems to qualify.

What about the U-B of NGC 4656? Unfortunately, only the effective U-B value is given, but that is 0.320. That's a large positive value! It suggests that NGC 4656 contains very few O-type stars at all! And what about its far infrared magnitude? NGC 4656 is fainter in far infrared than in blue light, suggesting there are low levels of starforming material in this galaxy. The Hockey Stick appears to be a dust poor, extremely blue galaxy that is hardly ultraviolet at all, one that is dominated by A- and F-type stars, with a few (late) B-type stars and a number of evolved red giants thrown in for good measure. NGC 4656 really looks like a single generation galaxy, one that formed all its stars some 300 million years ago or so. In reality, it must contain some really old yellow stars too, and it certainly does in its bulge. (And the blade of the Hockey Stick is clearly quite ultraviolet.)

What I'm saying is that all-blue disk galaxies are extremely rare. Looking through Adam Block's galaxy images, I found another all-blue candidate: NGC 5595, at top. This galaxy is blue both when it comes to its B-V and U-B values, but it's not incredibly blue. Another picture of the same galaxy pair, again with NGC 5595 at top, is rather weirdly color balanced, but at least it shows that the bulge and the arms of NGC 5595 are much the same color. But given its B-V index of 0.530, there can be no doubt that this galaxy too has an old population.

So all-blue disk galaxies are extremely rare, and therefore I doubt that the lensed galaxy in the APOD is one. The lensed galaxy is clearly old enough to have an old yellow population. It doesn't have to have one, but I think it does.

I think that a third image of the same galaxy can be seen at 10 o'clock, right next to a smallish elliptical galaxy.

Ann

Geck: Given that the two images I pointed out are on oposite sides of their respective focal points, close to the distorting galaxy cluster, and relatively un-smeared, does that mean the Galaxy is close to the focal point? Also, given the two separate views, can one create a stereogram and get some incliation of relative distance of stars within the Galaxy? Or are they simply too far away to get significantly different perspectives in the two views?

Ann: It is unfortunate that the image is modfied, hiding the true red-shift. I was expecting to hear that the blue regions started out as ultraviolet and the yellow-ish areas started out as blue. Thanks for the detailed responses and links!

DonJ

DonJ wrote:Geck: Given that the two images I pointed out are on oposite sides of their respective focal points, close to the distorting galaxy cluster, and relatively un-smeared, does that mean the Galaxy is close to the focal point? Also, given the two separate views, can one create a stereogram and get some incliation of relative distance of stars within the Galaxy? Or are they simply too far away to get significantly different perspectives in the two views?

Ann: It is unfortunate that the image is modfied, hiding the true red-shift. I was expecting to hear that the blue regions started out as ultraviolet and the yellow-ish areas started out as blue. Thanks for the detailed responses and links!

DonJ

The only way to find out the true redshift of this galaxy is to get a good spectrum of it and identify some spectral lines. Then you can see exactly how redshifted it is, and then you can at least have a good guess at whether or not this galaxy contains a true yellow population.

In the picture a left you can see galaxies at greater and greater distances from us. The galaxy at top left is only moderately redshifted. You can see that the double line of calcium, which is in the violet part in the spectrum of the Sun, has been shifted slightly to the right for the galaxy at top. As you move down, the galaxies are farther and farther away, and the double line of calcium is moving farther and farther to the right, indicating a greater and greater redshift and distance. The images of the galaxies in the picture, as well as their redshifts, were chosen by Edwin Hubble himself to illustrate Hubble's Law.

The good thing about the double line of calcium is that it is easily identifiable. But many other lines can be used as well.

The question is whether it is at all possible to get a good spectrum of the lensed galaxy in the APOD, given how faint and small it likely is and how crowded and spectrally "contaminated" the entire region is.

But assuming we did get a good spectrum of it, it would also be necessary to photograph the galaxy through many different filters to see how bright it is in different wavelengths. For example, if the galaxy is 7 billion light-years away, and if it originally emitted a lot of ultraviolet light at, say, 200 nm, what color would that light be if it was redshifted by the amount that the universe has "stretched" during the last 7 billion years? On the other hand, if the galaxy also emitted a lot of yellow light at, say, 600 nm, what color would that light be after 7 billion years of cosmological redshift?

Ann

And by the way... I must amend what I said about the Hockey Stick galaxy not being ultraviolet. If you go to this page, you will find a small version of a large poster showing 196 galaxies, arranged according to how much far ultraviolet they emit compared to how much near ultraviolet they emit. The galaxies farthest to the left are the ones richest in far ultraviolet versus near ultraviolet light, and the ones in the rightmost column are the ones poorest in it. Also the galaxies at top are just generally bright in far ultraviolet light, and the ones at bottom are faint. The galaxy at top left is, wouldn't you know it, NGC 4656, the Hockey Stick galaxy. Indeed, it is brightly ultraviolet.

Ann