Detailed studies of the motions of stars in the Galactic Center have all but clinched the case that our own Milky Way contains a black hole with a mass of 3 million suns. Astronomers have also been measuring black holes in a few dozen nearby galaxies and have discovered that the mass of the black hole is very tightly correlated with the mass of the surrounding galactic spheroid or bulge (see fig. 1). The bulge mass is always close to a thousand times the mass of the black hole. Moreover, galaxies with no bulge component do not appear to contain a black hole. This discovery is tantalizing because it suggests that galactic spheroids and black holes must form together. The question is then how and when did this formation occur? Is it possible to find direct evidence that bulges and black holes are still forming in some galaxies or did most formation activity end a long time in the past?

Scientists at the Max-Planck-Institute for Astrophysics (MPA) and at Johns Hopkins University (JHU) in the United States have been attempting to answer these questions by studying a large sample of galaxies with active nuclei in the Sloan Digital Sky Survey (see fig. 2). In these galaxies, material accreting onto the central black hole (see fig. 3) produces ionizing radiation that leads to characteristic signatures in the emission line spectrum of the galaxy. The MPA/JHU team carried out a detailed census of 22,000 such systems in the local Universe in order to calculate the rate at which black holes grow at the present day. The results indicate that on average, "low mass" black holes of less than a hundred million solar masses are still growing at a significant rate. The team also measured the star formation in nearby low mass bulges and deduced that rate at which new stars form in these systems is a thousand times larger than the rate at which their black holes are growing. This factor of a thousand is in gratifying agreement with the ratio between bulge and black hole mass observed in inactive galaxies! By contrast, the largest black holes in the local Universe, which have masses of up to 10 billion solar and which reside in giant elliptical galaxies, are hardly growing at all, indicating that they must have formed at significantly earlier cosmic epochs.

The results of the MPA/JHU team give further credence to the concept of "cosmic downsizing". Cosmic downsizing describes a scenario in which active star formation and black hole growth occurs shifts to lower and lower mass galaxies as the Universe evolves. This is seen as something of a paradox by theoreticians who try to understand how galaxies may have formed from the evolution of tiny density perturbations generated in the earliest moments of the Universe following the Big Bang. According to now-standard theory, the dominant matter component of the Universe is not in the form of the ordinary baryons that make up ordinary human beings and stars, but consists of unseen dark matter that interacts only via gravitation. Dark matter does not undergo cosmic downsizing. The collapse of dark matter starts on small scales and proceeds to ever more massive structures. Understanding exactly why the behaviour of galaxies and dark matter should be so different remains one of the major challenges for cosmologists today.

Guinevere Kauffmann

Further Information:
A scientific article
SDSS data page