New Image Reveals Thousands of Galaxies in Abell 315


In an image akin to the Hubble Deep Field, ESO’s La Silla Observatory in Chile stared at a patch of sky about as big as a full Moon and observed thousands of distant galaxies. The Wide Field Imager on ESO’s 2.2 meter telescope zeroed in on a large group of galaxies that are part of the massive galaxy cluster known as Abell 315. But there’s more in this image—including relatively close asteroids that show up as blue, green or red trails, which lie in the main asteroid belt, located between the orbits of Mars and Jupiter. Also, invisible dark matter is revealed in this image through its gravitational effects, noticeably visible on this galaxy cluster.

Of course, not all the galaxies seen here are the same distance from us. Some are relatively close, as it is possible to distinguish their spiral arms or elliptical halos if you zoom in on this larger image, especially in the upper part of the image. The more distant galaxies appear just like faint of blobs — their light has traveled through the Universe for eight billion years or more before reaching Earth.

The concentration of about a hundred yellowish galaxies is the Abell 315 galaxy cluster. The cluster is located in the constellation of Cetus (the Whale).

The galaxies in these clusters contribute to only ten percent of the mass, with hot gas in between galaxies accounting for another ten percent. The remaining 80 percent is made of dark matter that lies in between the galaxies.

We know the dark matter is there because of its effects: the enormous mass of a galaxy cluster acts on the light from galaxies behind the cluster like a cosmic magnifying glass, bending the trajectory of the light and thus making the galaxies appear slightly distorted. By observing and analyzing the twisted shapes of these background galaxies, astronomers can infer the total mass of the cluster responsible for the distortion, even when this mass is mostly invisible. However, this effect is usually tiny, and it is necessary to measure it over a huge number of galaxies to obtain significant results. In the case of Abell 315, the shapes of almost 10,000 faint galaxies in this image were studied in order to estimate the total mass of the cluster, which amounts to over a hundred thousand billion times the mass of our Sun.

For more information see the ESO release and additional images.

14 Replies to “New Image Reveals Thousands of Galaxies in Abell 315”

  1. Only 100 trillion solar masses? It seems unlikely that 10,000 galaxies plus the dark matter and inter-galactic gas could be such a small number, when you consider how big some galaxies (like the milky way and M31) are.

  2. These images always make me ponder, is there anyone looking back? Is our little corner of the universe under observation by other minds, other souls?

  3. There might be ETI looking back. However, it will be in two billion years plus some additional time due to the further expansion of the universe. In that time the light from our galaxy will have reached them.

    I am not sure, but I think the total mass is because most galaxies are dwarf galaxies.


  4. @Surak:
    the mass of 10^14 solar masses is only the mass of the galaxy cluster Abell 315, which contains about 100 galaxies. actually, of those 10^14 solar masses, only about 10% is made up by the yellowish galaxies you see in the central part of the image – another 10% is hot gas and the rest is dark matter.

    The other, over 10,000 galaxies visible in the image do not belong to Abell 315, but are either in the background or in the foreground. Those in the background, being gravitationally lensed by the cluster, tell you how massive the latter is.

    Hope this helps!


  5. @ LBC:

    I’m not sure I followed that.

    The more distant galaxies are 5 Gy old, which evidently would be enough time to establish both a planet biosphere and intelligent species thriving in it.

    The constraint is that the first stars don’t have metals. One would have to be lucky with early supernovas (~ 100 My, I believe) seeding some volumes, so a galaxy habitable zone starts out very spotty at a guess. But considering the available volume, it would still be expected to happen, wouldn’t it?

  6. In the lower left corner of the image a blue arc appears beside a foreground star. Does anyone know if this object is an artifact of the imaging process or, if not, what it might be?

  7. @ Torbjorn Larsson OM: My point is that if there are ETI which look back on our galaxy at this time they will do so in the future, for it will take over 2 billion years for light emitted from our galaxy to get out there. If there are ETI on the Hubble frame which defines the “now,” they see our galaxy as it was some 2 billion years ago.

    The galaxies we witness are from photons emitted on the past light cone centered around our region of spacetime. So this is why the further out we look the younger galaxies are. Further back enough and we see the region of last electron-photon scatter we call the CMB.

    What Claudia makes sense, for this gives about 10^{12} stars per galaxy. The article above is easily mis-read to think the 10.000 galaxies are what are “weighed,” but rather it is how their light is gravitationally deflected which is used to do the weighing.


  8. Beautiful. My first thought was “Hubble Deep Field” image?

    As Spaceninja already pointed out, very humbling.

  9. I think this is even more entertaining than the Hubble Deep Field! Definitely looking for a nerdy poster of this to plaster in my house somewhere…

  10. Unlike the Hubble deep field image, I am having a difficulty seeing even a hint of distortion. What am I missing? Are the lensed objects outside the perimeter of the cluster and every everything inside the perimeter are all Abel 315 galaxies, gas and DM?
    @LC I see ~a dozen large/bright galaxies in what is described as the location of Abel 315. Are you saying the remainder of the 100 are dwarfs?

  11. Sorry, I re-read the ESO article. We can’t visually see the weak x-ray lensing in this image.

  12. Are those red, blue and green trails added in post processing?

    If not, I’m very curious how they occur 🙂

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