By Christopher Onken
[email protected]
Space is big. It’s so big that the brightest known object in the Universe can be so far away that it looks to us like a dim, red pinpoint as shown in the image above. Not much different from the multitude of small red stars that make up the bulk of the Milky Way. But with the X-shooter instrument on ESO’s Very Large Telescope, we’ve discovered that the black hole powering this brightest of objects, the quasar SMSS J2157-3602, has a mass of 34 billion solar masses – the biggest black hole in the early universe!
The discovery of J2157 two years ago (Wolf et al. 2018) took three wide-field photometric datasets (the SkyMapper Southern Survey, the 2 Micron All Sky Survey, and the AllWISE catalogue of the Wide-field Infrared Survey Explorer) to identify candidates of the right colour. Plus, to avoid all those low-mass stars that like to masquerade as distant black holes, we used the parallax and proper motion measurements from the Gaia satellite, which had just become available. With optical spectroscopy of the candidates from the ANU 2.3m telescope at Siding Spring Observatory, we found that J2157 was indeed a distant quasar.
After crunching the numbers, it became clear that J2157 wasn’t just another quasar, but in fact was more luminous than any other known object. A quasar’s luminosity is governed by the mass of its black hole, so the bright nature of J2157 suggested that it must hold an extremely massive black hole at its heart. But to measure that black hole mass, we needed other tools, and Australia’s access to ESO put the world’s best tools at our fingertips.
The X-shooter spectrograph on the VLT provides an extremely wide wavelength coverage at moderate spectral resolution, and both aspects were critical to measuring the black hole mass in J2157. The two quantities we needed to determine were the black hole’s accretion disk luminosity at a rest-frame wavelength of 300nm, and the width of the Mg II emission line at a rest-frame wavelength of 280nm. But the rest-frame ultraviolet is a rich part of the spectrum in a quasar, and to isolate the accretion disk luminosity and Mg II emission from the other UV components (blended emission from ionised iron, and other strong emission lines), we needed to fit a detailed spectral model across a wide wavelength range. And because J2157 has a redshift of 4.7, that UV spectrum emitted by the quasar spans the optical and near-infrared as observed from the Earth, ideally suited to what X-shooter provides.
With the spectrum above obtained by X-shooter in October 2019, and supplemented by data taken at the Keck Observatory, we measured the continuum luminosity to be (4.7 ± 0.5) × 10^47 erg/s and the Mg II line width to be 5720 ± 570 km/s. The reason that these measurements trace the black hole mass is because the ionised magnesium emission arises close to the black hole, where the Doppler broadening of the line is reflecting how fast the gas is moving due to the gravity of the black hole; while the luminosity of the accretion disk determines how close to the black hole the conditions are right for the magnesium to be singly ionised (too close and the gas is over-ionised and the Mg II emission is suppressed; too far and the gas is neutral and also doesn’t emit the Mg II line). The velocity and orbital distance of the ionised magnesium thus combine to constrain the black hole mass.
With the line width and luminosity measurements, we found that the black hole in J2157 has a mass of (3.4 ± 0.6) × 10^10 solar masses (Onken et al. 2020). At just 1.25 Gyr after the Big Bang, this black hole has acquired as much mass as about half the stars in the entire Milky Way put together! And while it’s growing fast – swallowing the equivalent of a Sun every couple of days – it could probably be growing at twice that rate before it got so bright that the radiation pressure from its accretion disk would overwhelm the black hole’s gravity, blow the gas away, and shut down further accumulation of matter. That self-limiting growth for black holes poses a challenge for understanding how J2157 and the other massive black holes in the early universe got to be as big as we find them. Did they grow unexpectedly fast from a stellar mass black hole, or did they have a leg up by starting from a much more massive “seed”?
We hope to learn more about the galaxy in which J2157 lives by studying the cold gas far from the black hole with ALMA, one of the additional facilities to which Australia would gain access if it becomes a full ESO member (as recommended in the recent Mid-Term Review of Australian Astronomy’s Decadal Plan). In the local universe, we find that black holes and galaxies scale together, with the black hole making up about 0.5% of the galaxy stellar mass. Learning about the host galaxy of J2157 would tell us whether the black hole and galaxy are growing in lock-step from the start, or if this colossal black hole is outpacing its host?
At the same time, we’re expanding the sample of these quasars to understand how black holes grow in more typical galaxies. Selected from SkyMapper and other survey telescopes, confirmed as quasars with the ANU 2.3m, and (we hope) with black hole masses from the VLT, we aim to have robust statistical samples to report on in a future AAL ESO blog – stay tuned!