When observed, a galaxy has an age and a past history. Modelling the observed luminosity from its stars, gas and dust with the Pégase evolution model (Figure 1), allows one to estimate the mass of the supernovae that have already exploded and created dense stellar remnants, black holes or neutron stars. This modelling takes into account the history of star formation and that of metal ejecta, which determine the dust absorption and emission in the mid and far infrared, revealed by the Herschel and Spitzer satellite data.

The main result is that the accumulated mass of stellar black holes (and neutron stars) reaches a few billions of solar masses. This value is huge, and moreover of the same order of magnitude as that of the supermassive black hole hosted in the active galaxy center, and measured independently. Hence the idea that stellar black holes, created by past supernovae explosions, migrate towards the galaxy core (Figure 2), thus explaining the growth of supermassive black holes.

Galaxy age and mass with the Pégase model

These new results were obtained by a collaboration driven by a female researcher, professor emeritus, at the IAP. The scientists have fitted the spectrum of a well-known distant radio galaxy 4C41.17 (Figure 1 and Ref. 1), observed at a redshift of 3.8, which is a look-back time of about 12 billion years, when the universe was only one and a half billion years old. The fit is performed with the Pégase evolutionary galaxy model that is developed at the IAP, and which follows the birth, evolution and death of stars, hence the enrichment of metals which then are incorporated into dust grains. These dust grains then can absorb and reemit light at longer wavelengths in the infrared (www2.iap.fr/pegase, and Ref. 3-4).

Similar results are derived from another radio galaxy TN J2007-1316 (Ref. 2), at the same redshift of 3.8, which adds credence to the interpretation for 4C41.17. For these two galaxies, the cumulated mass of stellar remnants (hereafter stellar black holes) that is derived is a few billion times the mass of the Sun. Such a large calculated mass is also close to the mean mass of supermassive black holes in active galaxies, as measured by several independent methods for quasars and radio galaxies observed in the “Sloan Digital Sky Survey” at similar redshifts (Ref. 5).

Figure 1: Observed emission from stars, gas and dust in the radio galaxy 4C41.17 over a large range of wavelengths (Ref. 1): the optical domain are data from the Hubble Space Telescope, the mid-infrared range from the Spitzer satellite, and the far-infrared and submillimeter from the Herschel satellite (red data and error bars). The simulated stellar populations with the Pégase evolution model are an evolved one of elliptical/lenticular type that has an age of about 700 million years (orange line), and one from an intense and rapid star formation episode, which is considerably younger, only 30 million years old (blue line). The sum of these two populations is the thick black line. The emission lines from ionized gas (black dotted vertical lines) are not visible in the observed data. Moreover, this radio galaxy was chosen for its faint active galaxy nucleus activity. This is important, because active galaxy nuclei are thought to be surrounded by a donut like structure of gas and dust, which is called the obscuring “dust torus”. The faintness of the active galaxy nucleus means that the emission from the dust torus surrounding the active nucleus (green dashed line) is relatively weak and has little impact on the modelling results.

Figure 2: Artist rendition of a radio galaxy containing a supermassive black hole in its center [© ESO/M. Kornmesser]. Here, it is proposed that the migration of stellar black holes towards the core of the radio galaxy, by loss of angular momentum from dynamical friction until they reach the central black hole, may explain the growth of supermassive black holes.

Stellar blackhole and neutron star masses

At the galaxy age derived from the best-fit fitting luminosities (Fig. 1) and corresponding metal enrichment (this is also when the galaxy is observed), many supernovae have already exploded: all those whose lifetime duration is shorter than the age of the galaxy itself. Their number and mass depend on the initial stellar mass function (the distribution of masses that a population of stars have at their birth), and the time-dependent star formation law. These functions are determined by the best-fit of the observations with the evolutionary code Pégase.

For each exploded supernova, its residual mass of degenerate dense matter (stellar black hole or neutron star) is derived by subtracting the gas and metal ejecta mass from the initial stellar mass. These are known as “yields”, and are predicted by the models of stellar evolution and internal structure. The dense degenerate matter of stellar black holes is similar to that making the supermassive black holes found in the nuclei of active galaxies (radio galaxies or quasars), just with significantly lower masses.

Migration of stellar black holes towards the center

Stellar black holes being eternal, the process of dynamical friction may explain the selective migration towards the galaxy center (Ref. 6), by a progressive loss of angular momentum during a time interval that is compatible with the ages of these very distant galaxies (less than one and a half billion years). Subsequent processes which may increase the mass of the central supermassive black hole (galaxy mergers leading to the merging of their supermassive blackholes; accretion of matter surrounding the supermassive black hole) are of course not excluded, but they are not required either.

Implications for galaxy evolution

This modeling suggests that the earliest galaxies formed soon after the Big Bang, at a redshift of 5, or about 1.2 billion years after the formation of the Universe. The star formation law for galaxies of elliptical/lenticular spectral types would then peak at this epoch and lead to the formation of the bulk of the stars within an intense star formation episode, shorter than a billion years.

Moreover, such a growth process of supermassive black holes in the most active galaxies would explain the famous relationship between the supermassive black hole masses and the properties of stars in the galaxy bulge on scales of several kiloparsecs (velocity dispersion, luminosity and mass). A more specific analysis is currently being performed.

Finally, the mass ratio of stars and stellar black holes of about 1000 for these distant radio galaxies is plausible, as it is comparable to the values estimated in local galaxies (Ref. 7).

- Ref. 1: Rocca-Volmerange, B., Drouart, G., De Breuck, C., 2015, Astrophysical Journal Letters, 803, L8
- Ref. 2: Rocca Volmerange, ,B., Drouart, G., De Breuck, C., Vernet, J., Seymour, N., Wylezalek, D., Lehnert, M., Nesvadba, N., Fioc, M., 2013, Monthly Notices of the Royal Astronomical Society, 429, 2780
- Ref. 3: Fioc, M.; Rocca-Volmerange, B., 1997, Astronomy and Astrophysics, 326, 950
- Ref. 4: Fioc, M., Rocca-Volmerange, B., Dwek, E., Pegase.3, to be submitted.
- Ref. 5: Vestergaard, M., Fan, X., Tremonti, C. A., Osmer, P. S., Richards, G. T. 2008, Astrophysical Journal Letters, 674, L1
- Ref. 6: Magorrian, J., et al, 1998, AJ, 115, 2285
- Ref. 7: Binney, J. Tremaine, S., 2008, Galactic Dynamics, 2d Ed. Princeton Series in Astrophysics

Brigitte Rocca-Volmerange
rocca at iap dot fr
Tel.: (+33) (0)1 44 32 80 91

May 2015

Institut d'Astrophysique de Paris - 98 bis boulevard Arago - 75014 Paris