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.
Colours in this image of the galaxy J090543.56+043347.3 indicate whether there is gas moving towards us or away from us, and at what speed. Using this information, the researchers reconstructed the galaxy's dynamical mass. The star shape indicates the position of the galaxy's active nucleus; the surrounding contour lines indicate brightness levels for light emitted by the nucleus. Image: K. J. Inskip/MPIA
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.
Contact informationKatherine Inskip (lead author)
 There are different characteristic masses for a galaxy, and there is currently no consensus about whether the key property related to the black hole mass is the host galaxy's total mass, the mass of its stars (leaving aside dark matter and interstellar gas), or its bulge mass (the mass contained in a central thickening observed in many galaxies known as their bulge).
 In the hierarchical model of galaxy evolution, galaxies evolve and grow by ingesting smaller galaxies, or through mergers with galaxies of comparable size. The prediction is published as Jahnke & Macciò 2011, Astrophysical Journal, vol. 734, article ID 92
 This is because light travels at a finite speed. Every time we look at the Sun, we see our mother star as it was eight minutes ago, simply because it took the light we perceive eight minutes to travel from the Sun to Earth.
 In an expanding universe, a distant galaxy's distance from us and the redshift of that galaxy's light (the amount that the light is shifted towards lower frequencies) are directly related. Although distances are very hard to measure directly, it is typically straightforward to determine a galaxy's redshift, and so astronomers frequently quote a galaxy's redshift, z, as an (indirect) measure of its distance.
 In active galaxies, the central black hole habitually swallows surrounding matter, emitting enormous amounts of electromagnetic radiation in the process. Well-established methods allow astronomers to determine the black hole's mass by studying specific properties of this radiation.
 Both in the title and here, "weighing" is a reference to the standard everyday method for determining a body's mass (a measure for the amount of matter contained within the body) by determining its weight (that is, measuring the force by which the Earth's gravity pulls the body downwards). The concept of weight is not applicable to large celestial objects such as stars or galaxies; the work described in this release is strictly about determining a specific galaxy's mass.
 There is a simple analogue within our own Solar System: Kepler's 3rd Law of Planetary Motion states that a planet's orbital period T is related to its mean distance from the Sun a, the Sun's mass M and the gravitational constant by T2 = 4 π2a3/MG. Once you know the planet's orbit and its distance from the Sun, you can determine the Sun's mass. For much closer galaxies than the one examined here, studies of dynamic mass are a common astronomical tool. Notably, such studies are a key part of the evidence for the existence of Dark Matter
Dr. Markus Pössel | Max-Planck-Institut
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