APOD: A Blue Bridge of Stars between... (2014 Jul 15)

[APOD Robot]

A Blue Bridge of Stars between Cluster Galaxies

Explanation: Why is there a blue bridge of stars across the center of this galaxy cluster? First and foremost the cluster, designated SDSS J1531+3414, contains many large yellow elliptical galaxies. The cluster's center, as pictured above by the Hubble Space Telescope, is surrounded by many unusual, thin, and curving blue filaments that are actually galaxies far in the distance whose images have become magnified and elongated by the gravitational lens effect of the massive cluster. More unusual, however, is a squiggly blue filament near the two large elliptical galaxies at the cluster center. Close inspection of the filament indicates that it is most likely a bridge created by tidal effects between the two merging central elliptical galaxies rather than a background galaxy with an image distorted by gravitational lensing. The knots in the bridge are condensation regions that glow blue from the light of massive young stars. The central cluster region will likely undergo continued study as its uniqueness makes it an interesting laboratory of star formation.

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[geckzilla]

Definitely a weird one. It's difficult to interpret with all the lensing distortions and because it's so small.

[Ann]

If those blue squiggles are young stars at the same distance as the two central ellipticals, then that is a tremendous amount of star formation. All those lensed background galaxies suggest that the yellow ellipticals are really massive and presumably large in size.

Ann

[metamorphmuses]

I can't wait until the James Webb Space Telescope points at this and takes a nice sharp view of it.

As to the phenomenon itself, on a gut level I feel like it might have something special to say about Dark Matter, too. I can't help but try to picture how the Dark Matter halos of the galaxies are merging and molding the "bridge" of stars. Or maybe not, but when I look at the image and the lensing, I feel like the invisible mass there is beckoning at us.

[Markus Schwarz]

As to the phenomenon itself, on a gut level I feel like it might have something special to say about Dark Matter, too. I can't help but try to picture how the Dark Matter halos of the galaxies are merging and molding the "bridge" of stars. Or maybe not, but when I look at the image and the lensing, I feel like the invisible mass there is beckoning at us.

From what I understood from this paper the special feature here is that the central "blue band" is a region of star formation. It is NOT an image of a background galaxy distorted by gravitational lensing due to dark matter. Of course, the effect of dark matter was included to prove that the star forming region belongs to the center of the cluster.

[Douglas]

Many of the lensing artifacts are not obeying the simple concentrics the mind is attempting to impose upon this field so it may be there's several lenses superimposing upon each other. Just the sheer number of lensings that defy those concentrics and a number at or near perpendicular?

Only higher resolution will tell.

2 galaxies to the right of the very red galaxy, itself to the right of the center "squiggle", also have a non-concentric "squiggle bridge".

[rstevenson]

Most gas around and within galaxies is too hot to coalesce into stars, and it can't do so until it cools sufficiently. So one of the great puzzles here is how so much gas can have cooled enough to allow star formation to occur. The necessary clumping is relatively easy to imagine, with so much gravity around to push and pull the gas. It's the cooling that is hard to figure out. But there are a lot of good minds going to work on the problem. Scientists love puzzles.

Rob

[Chris Peterson]

If those blue squiggles are young stars at the same distance as the two central ellipticals, then that is a tremendous amount of star formation. All those lensed background galaxies suggest that the yellow ellipticals are really massive and presumably large in size.

At z=1.335, what we're seeing as blue here was actually emitted in the near UV, so even hotter than we'd assume if the activity were closer.

Also, most lensing comes from a galaxy's dark matter. The galaxies can be very massive but still not particularly large.

[Beyond]

Massive=Dense?

[Chris Peterson]

Massive=Dense?

Not necessarily. However, lensing suggests high density (although in the case of galaxies, density is always very low in terms of the sort of things we have everyday experience with).

[Boomer12k]

Always interesting the shapes and warps of Gravitational Lensing....


I spy with my little eye...something that starts with "G".....

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[Boomer12k]

I wonder what WE look like from over there.... do they see a similar effect from us?

:---[===] *

[Chris Peterson]

I wonder what WE look like from over there.... do they see a similar effect from us?

No. Our galaxy is too close to the lensing galaxy to create a similar effect. It's not a symmetrical geometry, with the lensing galaxies centered between us and the lensed galaxies.

[MarkBour]

I agree with metamorphmuses. My gut also tells me that the study of this location may bring a lot of new insights.

My first question is about the central part of the image. Are there two galaxies that are almost in line from our viewpoint, are they not just in line, but merging, or, what seems much more likely at first glance, is this just a lens-doubled image of a single galaxy?

My second question is about the edge-on galaxy a little above those (spiral, or elliptical, I can't say). Why doesn't it look distorted, or show multiple images? Is it between us and the lens?

[geckzilla]

My second question is about the edge-on galaxy a little above those (spiral, or elliptical, I can't say). Why doesn't it look distorted, or show multiple images? Is it between us and the lens?

Has to be either part of the lens or in front of the lens.

[ddonnelly]

Those large lensed circles around the central galaxy are BLUE. If they are from 'way more distant galaxies they should be RED, eh?

Second issue: There are at least four more galaxy images in this frame with those little blue spots near the center. Not all of them are double (merging) galaxies. Merging may have nothing to do with their origin.

[starsurfer]

There's a few connected galaxy systems in the nearby universe. One of the most well known is Keenan's System, an image by Stephen Leshin, another is Arp 295. One galaxy cluster that contains tidal bridge systems is the Hercules Galaxy Cluster, they can be seen in this image by Tony Hallas.

[Ann]

There's a few connected galaxy systems in the nearby universe. One of the most well known is Keenan's System, an image by Stephen Leshin, another is Arp 295. One galaxy cluster that contains tidal bridge systems is the Hercules Galaxy Cluster, they can be seen in this image by Tony Hallas.

Those are interesting and noteworthy examples of merging galaxies connected by bridges, starsurfer. However, they are different from the remarkable galaxies and bridge in today's APOD. "Your" galaxies are all spiral galaxies, and the bridges between them show few or no signs of star formation.

A merging pair of galaxies with a bridge that resembles the one in today's APOD is Arp 194. But again, the merging galaxies are spirals, not ellipticals.

However, note the complex merging process that has produced Arp 194. Note that the galaxy on the left in the picture contains two nuclei, and is therefore itself a product of a merger. There is what can be described as a mild sort of blue bridge (really a starforming spiral arm) between the two nuclei of that galaxy. But the brilliant starforming bridge between the two main galaxies is of course the most interesting feature of Arp 194.

Ann

[Chris Peterson]

Those large lensed circles around the central galaxy are BLUE. If they are from 'way more distant galaxies they should be RED, eh?

Keep in mind that "redshifted" doesn't necessarily mean "red". And also, that this image is made up of data imaged in four moderately wide photometric bands ranging from IR to near UV, which have been mixed into the RGB channels in a somewhat complex fashion... so the relationship between actual color and color in the image isn't readily apparent.

[geckzilla]

Very interesting article from the Frontier Fields blog regarding gravitational lensing. I love this kind of analysis which details exactly which galaxy is a repeat image of another:
http://frontierfields.org/2014/07/09/se ... ds-images/

[Beyond]

That's easy for you to say! I can't tell at all.

[geckzilla]

My first question is about the central part of the image. Are there two galaxies that are almost in line from our viewpoint, are they not just in line, but merging, or, what seems much more likely at first glance, is this just a lens-doubled image of a single galaxy?

Perusing this paper, one discovers that some astronomers have made some relevant deductions.

4.1. Origin of the ultraviolet emission

We consider three scenarios for the physical nature and origin of the clumpy NUV continuum emission.

1. It is a gravitationally lensed image (or a set of images) from a higher redshift background galaxy (or multiple galaxies).
2. It is a chance superposition along the line of sight, arising instead from a projection effect due to unrelated foreground sources.
3. It arises from ongoing or very recent star formation taking place within the stellar envelope of the interacting elliptical galaxies.

Scenarios (1) and (2) are entirely ruled out by spectroscopic evidence. [Hereafter it gets quite technical. Check the paper for details.]

Also discussed in the paper is evidence that the two elliptical galaxies are indeed in close proximity to one another and a conclusion that they must be undergoing a merge.

[geckzilla]

This is a very straightforward processing of the merging galaxies. It's much easier to see them with just the UVIS data which are all at the same resolution. The APOD version also includes IR data which is much lower resolution. While still useful, it also results in strange, red halos which can be confusing. Something to note is that those very slight dark spots are present in both F814W and F606W, but less so in the F814W which is typical for dust, no? I'm not seeing the spiral structure that others have mentioned. It looks like a complete mess to me which one comes to expect with mergers...

Red: F814W (Slight sharpening applied)
Green: F606W (Slight sharpening applied)
Blue: F390W (No sharpening)

Color noise reduction was applied in the darker areas. This mostly affected the blue channel.
Increased size 200% (nearest neighbor interpolation)

[Ann]

Thanks, Geck, very interesting.

Click to view full size image

Image Credits: X-ray (blue): NASA/CXC/CfA/E. O'Sullivan
Optical (brown): Canada-France-Hawaii-Telescope/Coelum.

When all is said and done, maybe the closest analog to the blue bridge between the central elliptical galaxies of SDSS J1531+3414 might be the well-known star forming shock front caused by the collision between galaxies NGC 7318 A and B in Stephan's Quintet. According to my software Guide, NGC 7318A is an elliptical while NGC 7318B is a spiral galaxy.

The blue shock front in the picture you can see in my post is glowing from invisible X-rays, mapped as blue light. However, there is real star formation producing optical and ultraviolet light there too, as can be seen in this picture.

Ann

[neufer]

When all is said and done, maybe the closest analog to the blue bridge between the central elliptical galaxies of SDSS J1531+3414 might be the well-known star forming shock front caused by the collision between galaxies NGC 7318 A and B in Stephan's Quintet. According to my software Guide, NGC 7318A is an elliptical while NGC 7318B is a spiral galaxy. The blue shock front in the picture you can see in my post is glowing from invisible X-rays, mapped as blue light. However, there is real star formation producing optical and ultraviolet light there too, as can be seen in this picture.

Except for the fact that the Stephan's Quintet case is supposed to represent a shock between merging galaxies
whereas today's APOD is supposed to represent a gravitational condensation in the wake of two already merged galaxies.

[c][size=150][b]Hickson Compact Group 92: [color=#40FF00]H-alpha = green,[/color] [color=#FF0000]8-micrometre infrared light = red,[/color] [color=#0000FF]visible red light = blue[/color][/b][/size][/c]

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<<Four of the five galaxies in Stephan's Quintet form a physical association, Hickson Compact Group 92, and are involved in a cosmic dance that most likely will end with the galaxies merging. Radio observations in the early 1970s revealed a mysterious filament of emission which lies in inter-galactic space between the galaxies in the group. This same region is also detected in the faint glow of ionized atomic hydrogen seen in the visible part of the spectrum. Two space telescopes have recently provided new insight into the nature of the strange filament, which is now believed to be a giant intergalactic shock-wave (similar to a sonic boom but traveling in intergalactic gas rather than air) caused by one galaxy (NGC 7318B) falling into the center of the group at several millions of miles per hour.

As NGC 7318B collides with gas in the group, a huge shock wave bigger than the Milky Way spreads throughout the medium between the galaxies, heating some of the gas to temperatures of millions of degrees where they emit X-rays detectable with the NASA Chandra X-ray Observatory.

Perhaps even more unexpected is the discovery of very powerful molecular hydrogen signals from the shock wave, seen by the NASA Spitzer Space Telescope which detects infrared radiation. The molecular hydrogen emission, which is seen through infrared spectral analysis using the Spitzer Space Telescope is one of the most turbulent formations of molecular hydrogen ever seen, and the strongest emission originates near the center of the green area in the visible light picture discussed earlier. This phenomenon was discovered by an international team led by scientists at the California Institute of Technology and includes scientists from Australia, Germany and China. The detection of molecular hydrogen from the collision was initially unexpected because the hydrogen molecule is very fragile and is easily destroyed in shock waves of the kind expected in Stephan's Quintet. However, one solution is that when a shock front moves through a cloudy medium like the center of the group, millions of smaller shocks are produced in a turbulent layer, and this can allow molecular hydrogen to survive. Most notable is the fact that this collision can help provide a view into what happened in the postulated beginning of the universe some 14 billion years ago, since shocked molecular hydrogen is likely to be present in the early universe.>>