Energy input from quasars regulates the growth and activity of black holes and their host galaxies

TL;DR

Simulations that simultaneously follow star formation and the growth of black holes during galaxy–galaxy collisions find that, in addition to generating a burst of star formation, a merger leads to strong inflows that feed gas to the supermassive black hole and thereby power the quasar.

Abstract

In the early Universe, while galaxies were still forming, black holes as massive as a billion solar masses powered quasars. Supermassive black holes are found at the centers of most galaxies today, where their masses are related to the velocity dispersions of stars in their host galaxies and hence to the mass of the central bulge of the galaxy. This suggests a link between the growth of the black holes and the host galaxies, which has indeed been assumed for a number of years. But the origin of the observed relation between black hole mass and stellar velocity dispersion, and its connection with the evolution of galaxies have remained unclear. Here we report simulations that simultaneously follow star formation and the growth of black holes during galaxy-galaxy collisions. We find that in addition to generating a burst of star formation, a merger leads to strong inflows that feed gas to the supermassive black hole and thereby power the quasar. The energy released by the quasar expels enough gas to quench both star formation and further black hole growth. This determines the lifetime of the quasar phase (approaching 100 million years) and explains the relationship between the black hole mass and the stellar velocity dispersion.

Figures (6)

• Figure 1: Snapshots of the simulated time evolution in mergers of two galaxies with and without black holes. The model with black holes is shown in the top panels. The full time sequence for this simulation can be viewed in the Supplementary Video. The bottom row shows the corresponding simulation without the inclusion of black holes. In both cases, four snapshots at different times in the simulations are shown. The images visualise the projected gas distribution in the two galaxies, colour-coded by temperature (blue to red). The colliding galaxies have the same initial mass corresponding to a 'virial velocity' of $V_{\rm vir} = (M_{\rm tot} \times 10 G H_{0} )^{1/3} = 160$ km/s, and consist of an extended dark matter halo, a stellar bulge, and a disk made up of stars and 20% gas. Each individual galaxy in the simulations is represented with 30000 particles for the dark matter, 20000 for the stellar disk, 20000 for the gaseous disk, and 10000 for the bulge component. Two such galaxies were set up on a parabolic, prograde collision course, and then evolved forward in time numerically with GADGET-2SH03, a parallel TreeSPH simulation code. The first snapshot ($t=1.1$ Gyr) shows the systems after the first passage of the two galaxies. The second snapshot ($t=1.4$ Gyr) depicts the galaxies distorted by their mutual tidal interaction, just before they merge. The peak in the star formation and black hole accretion (see also Fig. 2) is reached at the time of the third snapshot ($t=1.6$ Gyr), when the galaxies finally coalesce. At this time, a strong wind driven by feedback energy from the accretion expels much of the gas from the inner regions in the simulation with black holes. Finally, the last snapshots show the systems after the galaxies have merged ($t=2.5$ Gyr), leaving behind quasi-static spheroidal galaxies. In the simulation with black holes, the remnant is very gas poor and has little gas left dense enough to support ongoing star formation. This highlights that the presence of supermassive black holes, which accrete from the surrounding gas and heat it with the associated feedback energy, dramatically alters the merger remnant.
• Figure 2: Black hole activity, star formation and black hole growth plotted as a function of time during a galaxy-galaxy merger. The star formation rate (SFR) and black hole accretion rate (BHAR) are shown in the top and middle panels, respectively, and are given in units of solar masses per year. The black hole mass ($M_{\rm BH}$). is given in units of solar masses. The three lines in each panel correspond to models with galaxies of virial velocity $V_{\rm vir} = 80$, $160$, and $320$ km/s, (bottom to top lines in each panel, also labelled in the bottom panel). For comparison, we also show the evolution of the star formation rate for the model without a black hole that is shown in Fig. \ref{['fig1']} (dashed line -- for the $V_{\rm vir} = 160$ km/s galaxy). We note, in particular, that owing to AGN feedback, the peak amplitude of the starburst during the merger is lowered by a significant factor. The black solid circles in the individual panels identify the times of the corresponding snapshots shown in Figure 1.
• Figure 3: The relation between the final black hole mass, $M_{\rm BH}$, and the velocity dispersion of stars, $\sigma$, of our galaxy merger simulations compared with observational measurements. The solid circles show the masses of the black holes and the bulge velocity dispersions measured for the final remnants of six merger simulations of galaxies with disk gas fraction of 20%, but different total mass, parameterised by virial velocities of $V_{\rm vir} = 50$, $80$, $160$, $320$, and $500\;{\rm km/s}$ (shown by the dark to light red, from low to high mass galaxies respectively). Open circles and open squares with the same colour give results for gas fractions of 40% and 80%, respectively. We have also checked that our results are insensitive to the orbits of the galaxy collisions. Mimicking the observational data, we calculate $\sigma$ as the line-of-sight stellar velocity dispersion of stars in the bulge within the effective radius, $R_{e}$, of the galaxy. Black symbols show observational data for the masses of supermassive black holes and the velocity dispersions of their host bulges. Measurements based on stellar kinematics are denoted by filled stars, those on gas kinematics by open squares, and those on maser kinematics by filled triangles. Details for all the displayed measurements are given in ref. 3 and 28. The observed BH sample has been fit by a power law relation, yieldingT02: $M_{\rm BH}=(1.5 \pm 0.2) \times 10^{8} {\rm M}_{\odot} ({\sigma}/{200\, {\rm km/s} })^{4.02 \pm 0.32}$. The inset shows the relation between the circular velocity $V_{\rm vir}$ and $\sigma$ measured for the merger remnants in the simulations. The same colour coding is used as in the main panel to indicate corresponding mass objects.
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