First Quasar Gravitational Lens Discovered (w/video)


Gravitation lensing – a phenomenon that falls out of Einstein’s theory of general relativity – has been observed numerous times, making for some fantastic images of rings, arcs and crosses composed of massive galaxies light years away. As the light from a background object is bent by gravity around a foreground object, multiple, magnified images of the background object are produced from our vantage point.

For the first time, a quasar (quasi-stellar object) has been shown to gravitationally lens a galaxy behind it. About a hundred instances of gravitational lenses that consist of a foreground galaxy and a background quasar have been found, but this is the very first time where the opposite is the case; that is, a quasar bending the light from a background galaxy around it to create a multiple image of that galaxy.

Quasars are thought to be the result of a supermassive black hole at the center of a galaxy attempting to swallow up all of the matter that surrounds it. As the matter bunches up when it gets closer to the black hole, it heats up due to friction and begins to emit light across the electromagnetic spectrum. The light from a quasar can outshine an entire galaxy of stars, making it difficult to separate the light from a background galaxy from the overwhelming glare of the quasar itself.

To make this initial detection (there are surely many to follow), astronomers from the EPFL’s Laboratory of Astrophysics in cooperation with Caltech used data from the Sloan Digital Sky Survey (SDSS). They analyzed 22,298 quasars from the SDSS Data Release 7 catalog, and looked for images that had a strongly redshifted emission spectra. According to the paper announcing the results, “In these spectra, we look for emission lines redshifted beyond the redshift of the [quasar].”

In other words, a quasar that is lensing a galaxy in the background will exhibit a higher redshift than one that is not lensing a background galaxy, since the light from the galaxy and the quasar are combined in the SDSS data. So, quasars that had an expected redshift were thrown out, and a statistical analysis of quasars with emission lines that might mimic a gravitational lens eliminated many more of the objects. This left about 14 objects of the 22,298 analyzed as potential candidates. Of these 14, the team selected one to perform follow-up observations on, named SDSS J0013+1523.

SDSS J0013+1523 lies about 1.6 billion light years away, and is lensing a galaxy that is about 7.5 billion light years away from Earth. Using the Keck II telescope, they were able to confirm that SDSS J0013+1523 was indeed lensing the light from a galaxy located behind it. Hubble images of the discovery are in the works.

Here’s a video produced by the EPFL describing the results.

What is significant about this discovery – besides the novel aspect of a quasar acting as a lens – is that it will allow researchers to better refine their understanding of quasars. When light is bent around an object, it bends because of gravity, and gravity is a result of mass. So, something that is very massive will act as a stronger lens than something that is tiny, and the mass of the object doing all of the lensing work – in this case, the foreground quasar – can be determined.

Their results were published in a letter to Astronomy & Astrophysics on July 16th. The original paper is available for your perusal here.

Source: Eurekalert here and here, Arxiv paper here

19 Replies to “First Quasar Gravitational Lens Discovered (w/video)”

  1. Hmmm, so what is the difference between a galaxy hosting an active quasar being a lensing body, and a galaxy with just a big dormant black hole? Looking at this discovery image, I think this is just a curiosity, and not something that will teach us about quasars.

  2. Agree antoniseb, this only establishes what was already suspected,,, wich is not a bad thing to establish from time to time

  3. Basically there is no fundamental difference between quasar lensing and galactic lensing. This data does permit some estimates of the mass of a quasar. That is one thing gravitational lensing permits you to do, you can weigh the gravitating body. This just took some careful data analysis in order to separate the optical signature of the distant galaxy from the huge amount of radiation produced by the quasar.


  4. In addition, we can look at other quasars knowing they could be used as a possible gravitational lens (provided everything lines up). Difference between quasars and a dormant black hole is obvious… we can see a quasar.
    Because many of the items have been redshifted so far, you can’t see them in the visible spectrum.

  5. “… a quasar that is lensing a galaxy in the background will exhibit a higher redshift than one that is not lensing a background galaxy, since the light from the galaxy and the quasar are combined in the SDSS data. ”

    I don’t figure this. I expect there would be two superimposed spectra, with some lines at a low redshift and others at a higher redshift. How can redshifts add?

    Also, I never see embedded videos on UT. Could someone please link it?

  6. Manu:

    Also, I never see embedded videos on UT. Could someone please link it?

    Sure, click here.

  7. Manu,

    The SSDS uses filters to make thse surveys. So if a quasar is lensing a distant galaxy and it measured by SSDS it will have a large red shift. It is an instrument effect, not anything to do with adding redshifts.


  8. I don’t get how one can determine the expected redshift using this data, ie how does one distinguish between a quasar that is lensing a distant galaxy and a quasar that is just further away?

    Especially if, as LC states, the instrument produces a single value for the redshift.

  9. The SSDS makes surveys of the sky with bandpass filters. It is set to survey the sky at ranges of redshifting. Suppose are looking for galaxies with a certain z to z + delta z, where z is large. Then if you find a galaxy that is Einstein lensed by a quasar that you also image, the redshift of the quasar is within your bandpass.


  10. LBC…
    Your statement would be true, if you were looking for items lensed by a galaxy.

    However, with quasars it is different because the quasar often outshines the lensed background images. Spectra is also difficult to use because quasars iron makeup mimic background emission lines.

    This is why using quasars is news, nobody has been able to reliably use QSOs for lensing.

    On this study, they used known quasars with known masses (it wasn’t a massive ‘sky search’). Excatly how they identified the background images still hasn’t been published. Although, it does look like they were successful in breaking through the noise somehow.

  11. I read the statement as meaning an artifact of instrumentation or a bandpass. Agreed that quasars tend to outshine galaxies they might gravitationally lens. I think in this case the discovery is fortuitous because of the high z of the quasar.


  12. 7.5 billion light year away galaksi and still fairly sharp image almost comparable to the much nearer quasar. This must a be a huge galaksy.

  13. LBC: thanks a lot!
    Checking the article again, I find it somewhat confusing:
    “In these spectra, we look for emission lines redshifted beyond the redshift of the [quasar].” could be understood to apply to the SSDS data, whereas it must be about the subsequent Keck observations. I guess.

    Nedim Ardoga: grav lensing amplifies the faraway object making it look much brighter than it is, typically 10x or so.

  14. LBC..

    Fortuitous is harsh since we don’t have all the information yet, and they did find what they were looking for. They didn’t just stumble accross something.
    Unless you’ve done research which has awed the astronomical and physics community, making such a claim is ignorant. Like a teenager thinking you can do better than someone more experienced.

    Hopefully, I’ve just misunderstood what you meant by the discovery being fortuitous, or you mis-spoke.
    Anyhow…hopefully we’ll get the whole story once the rest of the research is published.

    …and as I’ve said before, “Gravity is some kewl $4!^.”

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