The Quasar That Acts Like a Reverse Gravitational Lens


Late last week, a paper appeared in the journal Astronomy and Astrophysics, announcing the discovery of the first known case of a quasar acting as a gravitational lens, magnifying a distant galaxy and splitting it into two distinct images. The new result is based on spectral data collected by the Sloan Digital

Sky Survey, augmented by observations at the W.M. Keck Observatory on Hawaii’s Mauna Kea.

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This is surprising because the effect usually occurs the other way around: the first gravitational lens observed in 1979 resulted because a large galaxy (or cluster of galaxies) interrupted the line of sight to a distant galaxy. Its mass warped the surrounding space time, so it produced an image of a distant quasar that was magnified and split by the foreground galaxy.

Quasars are very bright objects, a thousand times brighter than a galaxy of stars, and astronomers believe they are powered by supermassive black holes in the cores of their host galaxies.

Astrophysicist Fritz Zwicky was the first to propose that a gravitational lens could be used to measure the masses of far-away galaxies, and since the first such lens was discovered, these cosmic lenses have become very important tools for astrophysicists.

I managed to snag a few comments from Caltech co-author George Djorgovski over the weekend about the significance of these results. My first question: why haven’t we seen such a thing before? Apparently this is because quasars are so incredibly bright, it’s a bit like staring into the headlights of an oncoming car when they’re on high beam and — in the words of lead author Frederic Courbin of Switzerland’s EPFL (Ecole Polytechnique Federale de Lausanne) — “trying to discern the color of their rims.”

So it’s tough to even see the host galaxies in all that glare, even using the Hubble Space Telescope, so “seeing some faint lensed arcs lost in the glare of the quasar is effectively hopeless for a regular, seeing-limited ground-based imaging ,” Djorgovski says.

How did the scientists manage it? There were two stages. First, they looked for interesting spectra — particular emission lines at a higher redshift than normal, super-imposed onto a quasar’s spectrum. That reduced the candidates from over 20,000 quasar spectra down to around a dozen.

Then it was just a matter of combining the Keck telescope with some cutting-edge adaptive optics to “untwinkle the stars” so that the lensing effect was finally visible. And voila! They had a fascinating breakthrough for the astronomy archives.

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One of the reasons this result is so significant is that it enables astronomers to “weigh” the host galaxies of the quasars. Scientists have been able to weigh regular galaxies and clusters — indeed, they’ve even weighed the entire universe — but this is the first time it’s been used for galaxies playing host to quasars.

Once again, the issue is their brightness: you need a technique that doesn’t depend on brightness, so gravitational lensing is an ideal tool. It also complements other methods for studying how quasars and galaxies co-evolve.

And for those (like me) who assume it must be incredibly fun and exciting to work at the Keck Observatory in Hawaii, Djorgovski was quick to dispel those illusions.

They’re not on the actual peak with the telescope — the altitude is too high, so they monitor the data from a station at Waimea, where “oxygen is more abundant.” It’s nowhere near the beach, so lunchtime surfing breaks and luaus are out, at least for the experimentalists, who “slug it out in front of the computers, calibrating finicky instruments.”

As for my romantic notions of staring at the night sky far removed from city lights? Djorgovski quips, “Ah, yes, the romance of staring at the computer screen showing the view of the guide star camera…” Killjoy.

Image: Image of the first-ever foreground quasar (blue) lensing a background galaxy (red), taken with the Keck II telescope. Credit: Courbin, Meylan, Djorgovski, et al., EPFL/Caltech/WMKO

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