Cosmic weight watching reveals black hole-galaxy history

September 29, 2011

Using state-of-the-art technology and sophisticated data analysis tools, a team of astronomers from the Max Planck Institute for Astronomy has developed a new and powerful technique to directly determine the mass of an active galaxy at a distance of nearly 9 billion light-years from Earth. This pioneering method promises a new approach for studying the co-evolution of galaxies and their central black holes. First results indicate that for galaxies, the best part of cosmic history was not a time of sweeping changes.

One of the most intriguing developments in astronomy over the last few decades is the realization that not only do most galaxies contain central black holes of gigantic size, but also that the mass of these central black holes are directly related to the mass of their host galaxies. This correlation is predicted by the current standard model of galaxy evolution, the so-called hierarchical model, as astronomers from the Max Planck Institute for Astronomy have recently shown.

When astronomers look out to greater and greater distances, they look further and further into the past. Investigating this black hole-galaxy mass correlation at different distances, and thus at different times in cosmic history, allows astronomers to study galaxy and black hole evolution in action.

For galaxies further away than 5 billion light-years (corresponding to a redshift of z > 0.5), such studies face considerable difficulties. The typical objects of study are so-called active galaxies, and there are well-established methods to estimate the mass of such a galaxy's central black hole. It is the galaxy's mass itself that is the challenge: At such distances, standard methods of estimating a galaxy's mass become exceedingly uncertain or fail altogether.

Now, a team of astronomers from the Max Planck Institute for Astronomy, led by Dr Katherine Inskip, has, for the first time, succeeded in directly "weighing" both a galaxy and its central black hole at such a great distance using a sophisticated and novel method. The galaxy, known to astronomers by the number J090543.56+043347.3 (which encodes the galaxy's position in the sky) has a distance of 8.8 billion light-years from Earth (redshift z = 1.3).

The astronomers succeeded in measuring directly the so-called dynamical mass of this active galaxy. The key idea is the following: A galaxy's stars and gas clouds orbit the galactic centre; for instance, our Sun orbits the centre of the Milky Way galaxy once every 250 million years. The stars' different orbital speeds are a direct function of the galaxy's mass distribution. Determine orbital speeds and you can determine the galaxy's total mass.

This is much easier said than done. In order to secure their measurement, the cosmic weightwatchers had to pull out all the stops of observational astronomy before finally obtaining a reliable value for the dynamical mass of J090543.56+043347.3. Combining this result with the mass value of the galaxy's central black hole, which the researchers measured from the same dataset, the result is the same that would be expected for a present-day galaxy. Apparently, nothing major has changed between now and then: At least out to this distance, 9 billion years into the past, the correlation between galaxies and their black holes appears to be the same as for their modern-day counterparts.

Inskip and her colleagues are already hard at work to expand their novel kind of analysis to a larger set of 15 further galaxies. If this confirms their conclusions from J090543.56+043347.3, that would indicate that, over the past 9 billion years – for more than half of the age of our Universe! – most galaxies have lived comparatively boring lives, subject to only very limited and slow change.


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.

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