Title: Star Formation Rate and Dynamical Mass of 108 Solar Mass Black Hole Host Galaxies at redshift 6
Authors: Chris J. Willott, Jacqueline Bergeron, and Alain Omont
First Author’s Institution: Herzberg Institute of Astrophysics
Black holes reside in the centers of most galaxies. Our Milky Way, for instance, hosts a 4×106 M☉ black hole known as Sagittarius A. Black holes interact with their host galaxies through various feedback mechanisms and affect the properties of their host galaxies.
Black holes and Star Formation
Galaxies form stars at different rates; some are slower than the others. The rate at which galaxies create stars is known as the star formation rate or SFR. Black holes are thought to play a role in their host galaxies SFR, primarily through high-energy jets they emit which then send shock waves through their surrounding medium; whether this mechanism trigger or shut down star formation is still an open-ended question.
Black holes and Velocity Dispersion in Galaxy Bulges
Stars do not move in an orderly fashion in stellar populations. Although they are still gravitationally bound as a whole, the local motions of individual stars are random in different directions. This results in a range of stellar velocities in stellar populations. Stars in the bulges of galaxies also demonstrate this velocity spread, usually denoted as σ. A famous relation known as the MBH-σ relation is a linear relation between mass of a black hole and the velocity dispersion of the stars in the galactic bulge. It implies that the growth of black holes and the growth of galaxy bulges are related and not independent of each other. For those who are interested in learning more about velocity dispersion in stellar populations, this is an Astrobites article which explores the relation between velocity dispersion and galaxy evolution.
Black holes and Quasars
Because most structural growth happened at early times, investigating the tight correlation between black hole mass and galaxy properties at early times is important in understanding the origins of these correlations. To do this, astronomers look for the most distant sources in the Universe (that are bright enough to be observed today), and they happened to be high-redshift quasars. Quasars are high-energy sources fueled by accretion of matter onto supermassive black holes (MBH ~ 108-9 M☉), while high-redshift quasars just means they are formed very early on in the Universe. For example, the highest redshift quasar known to date is at z (redshift) = 7.1, corresponding to a time less than 8 Myr after the Big Bang. Studying the properties of high-redshift quasars (z >~6) and their host galaxies will enable us to understand the interaction between black holes and their host galaxies in the early Universe.
In this paper, the authors studied how the star formation rate (SFR), velocity dispersion σ, and dynamical mass Mdyn of high-redshift quasar host galaxies depend upon their black hole accretion rates and masses. A galaxy’s dynamical mass takes into account its dark matter content and so tends to be larger than its stellar mass. Since most dark matter are thought to live in bulges, the dynamical mass of a galaxy is treated as the bulge mass. The authors took observations of two z~6 quasar host galaxies from the Canada-France High-redshift Quasar Survey (CFHQS) using the Atacama Large Millimeter Array (ALMA). ALMA is used for their observations because it has the required sensitivity to probe the SFR and dynamical masses of galaxies at high redshifts. They combined these observations with their previous study of two more high-redshift quasar hosts to give a total sample of four.
The authors discovered that high-redshift quasar hosts have rather low SFR, despite having very high black hole accretion rates. This is different than what is observed in the low-redshift Universe, where star formation rate increases similarly as black hole accretion rate, as studied in this paper. Figure 1 shows the star formation rate against redshifts, where LFIR is used as a star formation tracer. Note the clear rise in LFIR up to a peak at z~2 followed by a decline to z=6, where the rise at z~2 is attributed to the increase in SFR in massive galaxies within this redshift range.
The authors also studied the MBH-σ and MBH-Mdyn relations of their high-redshift quasar hosts. Historically, σ is measured from galaxy bulges but since bulges are less common at high-redshifts, they used [C II] line to determine σ instead. The [C II] line is also used to determine dynamical masses of the host galaxies. Figure 2 illustrates these relations. Their quasars are distributed around the local MBH-σ relation, albeit with a much larger scatter well beyond the size of their error bars. The same can be said of their MBH-Mdyn relation.
The fact that high-redshift quasars lie on the MBH-σ relation suggests that the host galaxies have undergone substantial evolution to acquire their current high dynamical masses. Nevertheless, one wonders why this mass accumulation did not lead to a high SFR, as suggested by their low LFIR. One reason could be strong jets from the central quasar inhibiting star formation. The second reason could be that LFIR is not a good tracer of star formation at such high-redshifts. Another star formation diagnostics is L[C II], and the authors noted that using L[C II] to trace SFR would bump it up by a factor of 3 for one of their quasars. As such, they recommended higher resolution [C II] observations of their quasars to more accurately constrain the various correlations between black holes and their host galaxies at high-redshifts. Until then, their current results showed that galaxies around supermassive black holes in the early Universe are just too cool to be grouped with everybody else.