Comment How the technique actually works (Score 1) 116
I'm one of the authors on the paper on which the press release is concerned and thought I'd give a bit more of a description as to how the technique actually works.
Remember the old saying that you can tell the difference between a star and planet because the stars twinkle but planets do not? This is because the angular size of the turbulent fluctuations in Earth's atmosphere responsible for twinkling are comparable to the angular diameter of a planet. So the planet looks as if it's "resolved" and its twinkling is damped out. Stars on the other hand, are point-like and thus twinkle like crazy.
What does this have to do with gas clouds in space? Well, our Galaxy has an "atmosphere" too, and it's called the interstellar medium (ISM), even though its density is about 0.03 particles per cubic centimetre. You can think of the ISM as gas clouds, but it is more appropriate to think of it here as a continuous turbulent medium. The ISM causes twinkling too, but at radio rather than optical wavelengths.
To exhibit twinkling or Interstellar Scintillation (ISS) due to the ISM, a radio source has to have an angular diameter less than a few tens of microarcseconds (1 microarcsecond=2.8x10^-10 deg). The resolving power of interstellar scintillation is so high that no quasars look point-like (unlike stars undergoing atmospheric scintillation). In fact, most quasars do not twinkle at all.
The pattern of intensity fluctuations caused by the ISM is actually a convolution of the scintillation pattern that would occur if the quasar were point-like with the actual brightness distribution of the background quasar. We can find the brightness distribution of the quasar once we know the *statistical* properties of the scattering medium. We do not actually have to know what the exact shape of the interstellar 'lenses' are at any one time. As long as we record enough intensity fluctuations from the scintillating quasar, we can measure the statistical properties of the intensity fluctuations to derive information on the source structure.
But there is a complication. The ISM moves relative to the Earth with some velocity, v, and it turns out that the scintillation is most sensitive to quasar structure along the direction of v. We essentially form a one-dimensional image of source structure. Enter the Earth. As Earth orbits the Sun, its velocity changes (i.e. it changes direction), and thus so does the direction of the ISM _relative to_ Earth. As v changes direction during the course of a year, we can probe the quasar's structure along different directions. This allows us to build up a 2-d image of the source over the course of a year on microarcsecond scales.
We have dubbed this technique Earth Orbit Synthesis.
Remember the old saying that you can tell the difference between a star and planet because the stars twinkle but planets do not? This is because the angular size of the turbulent fluctuations in Earth's atmosphere responsible for twinkling are comparable to the angular diameter of a planet. So the planet looks as if it's "resolved" and its twinkling is damped out. Stars on the other hand, are point-like and thus twinkle like crazy.
What does this have to do with gas clouds in space? Well, our Galaxy has an "atmosphere" too, and it's called the interstellar medium (ISM), even though its density is about 0.03 particles per cubic centimetre. You can think of the ISM as gas clouds, but it is more appropriate to think of it here as a continuous turbulent medium. The ISM causes twinkling too, but at radio rather than optical wavelengths.
To exhibit twinkling or Interstellar Scintillation (ISS) due to the ISM, a radio source has to have an angular diameter less than a few tens of microarcseconds (1 microarcsecond=2.8x10^-10 deg). The resolving power of interstellar scintillation is so high that no quasars look point-like (unlike stars undergoing atmospheric scintillation). In fact, most quasars do not twinkle at all.
The pattern of intensity fluctuations caused by the ISM is actually a convolution of the scintillation pattern that would occur if the quasar were point-like with the actual brightness distribution of the background quasar. We can find the brightness distribution of the quasar once we know the *statistical* properties of the scattering medium. We do not actually have to know what the exact shape of the interstellar 'lenses' are at any one time. As long as we record enough intensity fluctuations from the scintillating quasar, we can measure the statistical properties of the intensity fluctuations to derive information on the source structure.
But there is a complication. The ISM moves relative to the Earth with some velocity, v, and it turns out that the scintillation is most sensitive to quasar structure along the direction of v. We essentially form a one-dimensional image of source structure. Enter the Earth. As Earth orbits the Sun, its velocity changes (i.e. it changes direction), and thus so does the direction of the ISM _relative to_ Earth. As v changes direction during the course of a year, we can probe the quasar's structure along different directions. This allows us to build up a 2-d image of the source over the course of a year on microarcsecond scales.
We have dubbed this technique Earth Orbit Synthesis.
