An Einstein Ring – Courtesy of ALMA

Just back from a short Easter holiday, I thought I’d resume blogging activities by showing you this remarkable image.

 

SDP81_ALMA3bands

What you see is a near-perfect example of an Einstein Ring which is a result of a chance alignment between a background galaxy and a foreground concentration of mass, sometimes a cluster of galaxies but in this case another galaxy. A more usual effect is the formation of a number of bright arcs; here there are two bright segments, but there is enough detail to see the rest of the circle. The lensed galaxy has a redshift about 3, so that light from it was emitted when the Universe was about one-quarter its current size, about 12 billion years in the past.

This object, codenamed SDP81, was initially detected as a potential lens system by the Herschel Space Observatory, which turned out to be superb at identifying gravitational lenses. I posted about this here, in fact. Working in the far-infrared makes it impossible to resolve the detailed structure of lensed images with Herschel – even with a 3.5m mirror in space, λ/D isn’t great for wavelengths of 500 microns! However, the vast majority of sources found during the Herschel ATLAS survey with large fluxes at this wavelengths can be identified as lenses simply because their brightness tells us they’ve probably been magnified by a lens. Candidates can then be followed up with other telescopes on the ground. A quick look during the Science Demonstration Phase of Herschel produced the first crop of firmly identified gravitational lens systems published in Science by Negrello et al. This one was followed up last year by the Atacama Large Millimetre Array (ALMA), itself a remarkable breakthrough in observational technology; the image was actually made in an extended configuration during the commissioning tests of ALMA’s long-baseline interferometric capability, which gives it stunning resolving power of about 23 milli-arcseconds. It’s absolutely amazing to see such detail in an image made in the submillimetre region of the spectrum.

The press release accompanying this can be found here and the full scientific paper by Vlahakis et al. is already on the arXiv here.

For the specialists the abstract of the journal paper reads:

We present initial results of very high resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations of the z=3.042 gravitationally lensed galaxy HATLAS J090311.6+003906 (SDP.81). These observations were carried out using a very extended configuration as part of Science Verification for the 2014 ALMA Long Baseline Campaign, with baselines of up to 15 km. We present continuum imaging at 151, 236 and 290 GHz, at unprecedented angular resolutions as fine as 23 milliarcseconds (mas), corresponding to an un-magnified spatial scale of ~180 pc at z=3.042. The ALMA images clearly show two main gravitational arc components of an Einstein ring, with emission tracing a radius of ~1.5″. We also present imaging of CO(10-9), CO(8-7), CO(5-4) and H2O line emission. The CO emission, at an angular resolution of ~170 mas, is found to broadly trace the gravitational arc structures but with differing morphologies between the CO transitions and compared to the dust continuum. Our detection of H2O line emission, using only the shortest baselines, provides the most resolved detection to date of thermal H2O emission in an extragalactic source. The ALMA continuum and spectral line fluxes are consistent with previous Plateau de Bure Interferometer and Submillimeter Array observations despite the impressive increase in angular resolution. Finally, we detect weak unresolved continuum emission from a position that is spatially coincident with the center of the lens, with a spectral index that is consistent with emission from the core of the foreground lensing galaxy.

ALMA will only work in long baseline mode for a small fraction of its time, and it is bound to be in very heavy demand, so it’s not clear how many of the hundreds of candidate lenses flagged up by Herschel will ever be mapped in such detail, but this is definitely one for the album!

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