The parallel lives of super-massive black holes and their host galaxies

Researchers at the Max Planck Institut für Astrophysik have used the most recent constraints from X-ray and optical/Infrared observations to trace the parallel growth of black holes and galaxies back to the time when the Universe was five times younger than its current age. They have found that the supermassive black holes commonly observed in galactic nuclei must have assembled earlier, and evolved slower, than the stars in their host galaxies.

Fig. 1: A schematic representation of the correlation between black hole masses and host spheroids, observed in the local Universe. Credit: NASA and A. Feild (STScI)

Fig. 2: The top panel shows the evolution in the stellar mass density, solid line with dark-red shaded area) as a function of redshift, z (where z=0 corresponds to today and z=3 to approximately 10 billion years ago). The density is given as a ratio to today's value. The predicted stellar mass density has been calculated from the evolution of the black hole mass density. Also shown are the relative decomposition of the total stellar mass density into spheroids (dashed line, orange shaded area) and disks and irregulars (dotted line, light-orange shaded area). This shows that the stellar mass in galaxies was almost equally divided between bulges and disks/irregulars in the past, but is dominated by the bulges today. The lower panel shows the evolution in the star formation rate (solid line and dark blue shaded area) and the corresponding black hole accretion rate density (dot-dashed line) rescaled by a factor of about one thousand. Since z=2, black holes grew more slowly than the stars, as indicated by the dot-dashed line rapidly declining towards z=0. The data points are a collection of all the available measurements of the total stellar mass density (upper panel) and star formation rate density (lower panel) in the Universe as a function of time.

Black holes, with masses from a few millions to a billion times that of the sun reside in the centers of nearby galaxies. Even more remarkably, recent observational evidence indicates that the mass of the central black hole is tightly correlated with the total mass (and luminosity) of the stars in the central, spheroidal component of galaxies, called bulge. The larger the black hole, the larger the bulge (see Fig.1). These discoveries imply that the formation of the galactic spheroids (and of galaxies themselves) must be intimately connected to the growth of the central black hole, as recent numerical simulations performed at the Max Planck Institut für Astrophysik have indeed been able to show (see linkPfeil.gifColliding galaxies light up dormant black holes).

In the local Universe, black holes not only comprise a fixed fraction (about 1 part in a thousand) of the mass of their host galaxies, as indicated by the black hole--bulge correlations, but they are also known to be growing at a rate proportional (still a factor of about one part in a thousand) to the rate at which stars are being formed (see linkPfeil.gifLow Mass Black Holes Still Grow Today). But was this true also in the past?

From observations of Active Galactic Nuclei (AGN) and Quasars (QSOs), the most powerful class of AGN, we know that black holes must have assembled at the time when galaxies were still forming. According to the standard picture, AGN are powered by the release of gravitational energy from matter that falls onto a supermassive black hole. Therefore, measuring the amount of radiation emitted by the active galactic nuclei gives a measure of how fast the black holes grow. Andrea Merloni, Gregory Rudnick and Tiziana Di Matteo at the Max Planck Institut für Astrophysik have collected the most up to date information on the evolution over cosmic time of the AGN population, and derived from that the history of black holes mass assembly in galaxies. Then, they have compared it with all available measurements of the total stellar mass residing in galaxies and of the rate at which it was assembled since early times (about 10 billion years ago) till today.

Figure 2 illustrates the results they have obtained in details. The upper panel shows the data of the evolution in the observed stellar mass density in the Universe (total mass in stars per unit volume) as a function of time, together with the best approximation calculated from the known evolution of the black hole mass density. The prediction shown by the solid curve implies a non-constant ratio between black hole mass and host spheroid mass. The lower panel shows the evolution in the star formation rate and the corresponding black hole growth rate (rescaled by a factor of about one thousand).

This study demonstrates, for the first time, that the correlation between black hole mass and galactic bulge properties observed in the local Universe has evolved and was different in the past. Black holes and bulges do indeed grow together, but not at the same rate. The researchers have found that supermassive black holes assembled earlier than bulges: 10 billion years ago they were about 1.7 times larger than their host spheroids compared to today. Since then, black holes have grown less compared to the rate at which their hosts have.

These results, combined with future sensitive observations of even more distant galaxies and Quasars, will help scientists to better understand how the first stars and black holes assembled in the early Universe.

Andrea Merloni

Further Information:

A. Merloni, G. Rudnick & T. Di Matteo: Tracing the cosmological assembly of stars and supermassive black holes in galaxies, MNRAS, 356, L1, 2004