Questions and answers
Which telescope and instrument were used for these projects?
The data were taken with SINFONI at the Very Large Telescope (VLT), belonging to the European Southern Observatory (ESO), stationed on Cerro Paranal in northern Chile. SINFONI is a so-called imaging or integral field spectrograph coupled to an adaptive optics system using the PARSEC laser guide star. Integral field spectrography is the technique of taking a spectrum for each pixel of the region of the sky that is being imaged. Adaptive optics uses deformable mirrors to remove most of the disturbances caused by turbulence in the atmosphere – which is, among other things, responsible for the characteristic "twinkle" of stars in the night-sky, increasing image quality and resolution. The prerequisite for this is a reference star of suitable magnitude; a laser guide star system produces an artificial reference star by exciting sodium atoms in a well-defined layer of the atmosphere. For this single object, around 2 hours of observation time of time were invested. For the whole project, a total of 80 hours have been granted, 30 of which have already been observed.
What made these measurements especially challenging?
At such a great distance, the angular size of the galaxy J090543.56+043347.3 amounts to about one arc-second. An ordinary DVD, viewed from a distance of 25 kilometres, has the same apparent size. In order to determine the motion of the galaxy's gas clouds, one needs to resolve different regions of the galaxy. The unique combination of integral field spectrography and adaptive optics with a laser guide star was the prerequisite for producing the very clear and high-resolution imaging-spectra of the target that were necessary for reconstructing the galaxy's dynamical mass.
Another difficulty is the fact that J090543.56+043347.3 is an active galaxy, whose central region emits intense light. It is necessary for the object under study to be an active galaxy, since these are the only distant galaxies for which it is possible to determine the mass of the central black hole. For determining the dynamical mass of the whole galaxy, the galaxy's activity is a problem, though. For their measurement, the astronomers first had to separate the extremely intense light originating in the central region, which contains the black hole, from the light emitted by the moving gas clouds in the rest of the galaxy. They then analysed the light from more than 400 different image points in order to model the velocity structure of the galaxy's gas and thus derive the dynamical mass of the entire galaxy.
Have galaxies not changed at all over the past 9 billion years?
They have, but apparently not in ways that would change the relationship between their total mass and the mass of the central black hole. In this case, the researchers found that changes in the active galaxy J090543.56+043347.3 likely were restricted to reshuffling the orbits of its stars and gas clouds. The central black hole does not appear to have grown by substantial amounts, and the number of stars newly formed should also have been small.