SRG/eROSITA discovers and studies supermassive black holes in the early Universe
In the center of our Galaxy lurks a black hole with a mass of 4 million solar masses. Such black holes are present in almost all galaxies. Typically, they are even larger, in some cases reaching several billion solar masses. Such supermassive black holes (SMBHs) were born many billion years ago, when the first stars and galaxies just started to appear in the Universe, and then have grown by accreting surrounding matter. During this process huge amounts of energy were released, which allows us to observe young, growing massive black holes via their electromagnetic radiation emitted many billion years ago. Such objects are called quasars (quasi-stellar objects).
How SMBHs form and grow is one of the main scientific problems addressed by the Russian orbital observatory SRG, which on July 13 marks its first year in orbit since the launch from the Baikonur Cosmodrome. In early June 2020, eROSITA, one of the two telescopes aboard the observatory, had finished its first (out of the planned eight) survey of the whole sky in X-rays. More than a million X-ray sources have been found during this survey. In addition, to study more distant and fainter objects, eROSITA has conducted a deeper scan of the Lockman Hole, a small region of the sky where absorption of X-rays by the interstellar gas and dust in our Galaxy is minimal and does not hinder observations of extragalactic objects.
A team of scientists from the Space Research Institute (IKI) of the Russian Academy of Sciences led by Marat Gilfanov and Sergey Sazonov are working on the catalog of eROSITA sources and use it to search for the most distant and fastest growing SMBHs in the early Universe. A neural network “SRGz”, developed at IKI by Alexander Mescheryakov, has selected several tens of candidate distant quasars from more than half a million X-ray sources found by eROSITA on the hemisphere allotted to Russian scientists. The most interesting of them were then observed at optical telescopes as part of the ground support program of the SRG sky survey.
Already the first observations have led to the discovery of previously unknown quasars. Among them are
- a quasar at z=4.116, discovered at the AZT-33IK 1.6-meter telescope of the Sayan Observatory of the Institute of Solar-Terrestrial Physics in Buryatia;
- a quasar at z=4.237, discovered at the Russian-Turkish 1.5-meter Telescope (RTT-150) in Turkey;
- a quasar at z=4.576, discovered at the BTA 6-meter telescope of the Special Astrophysical Observatory in Karachay-Cherkessia.
These observations were carried out under the leadership of Rodion Burenin and George Khorunzhev from IKI and Ilfan Bikmaev from the Kazan Federal University (KFU).
In addition, observations performed at the 2.5-meter telescope of the Caucasus Mountain Observatory of the Sternberg State Astronomical Institute of the Moscow State University have confirmed that several other candidates are also quasars.
The letter “z” used above denotes an object’s redshift, which defines its distance. Quasar redshifts are measured through the positions of bright emission lines in their spectra. The most prominent is the Lyman-alpha line known from high-school physics. It results when the electron in a hydrogen atom makes a transition from the second to the first level. In usual conditions, this line appears in the ultraviolet part of the spectrum, but it falls into the visible band in the spectra of distant quasars owing to the large redshift caused by the expansion of the Universe.
Particularly interesting are quasars at redshifts z>6, belonging to an epoch when the Universe was younger than one billion years. It remains unclear how some black holes managed to grow to several billion solar masses in a such a short time by cosmological standards. Another key question in modern astrophysics is the relationship between star formation in the first galaxies and growth of black holes in their nuclei. Finally, it is not fully clear what role quasars played in the reionization of the Universe, which occurred between 200 million and 1 billion years after the Big Bang. More than 200 quasars at z>6 have already been unveiled by optical and infrared observations, and just some 20 of them have been detected in X-rays.
In a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society, Pavel Medvedev and his colleagues from IKI and KFU report the discovery by SRG/eROSITA of X-rays from quasar CFHQS J1429+5447 at z=6.2 (corresponding to an age of the Universe of 900 million years). This quasar was previously known from observations in the visible and radiobands, but its X-ray radiation has been found for the first time. According to eROSITA, the quasar’s X-ray luminosity is about 3×1046 erg per second, which is several times higher than the previous record for z>6 quasars. Since the quasar emits radiation across the electromagnetic spectrum from radio to UV and X-rays, its total luminosity is actually yet higher by an order of magnitude – about 3×1047 erg/s. For comparison, the total luminosity of all two hundred billion stars in our Galaxy is a thousand times lower! This implies that the black hole in this quasar weighs more than 2 billion solar masses and it must have been “swallowing” approximately one Earth mass each second for several tens of millions of years.
Quasar CFHQS J1429+5447 fell into the field of view of eROSITA and was detected by it during scans of the sky on December 10-11, 2019, at the very beginning of the SRG all-sky survey. Follow-up observations at RTT-150 revealed that the optical brightness of the quasar remained nearly the same as it was 10 years ago when it was discovered by the Canada-France-Hawaii Telescope.
Quasar CFHQS J1429+5447 is known to be “radio loud”. Its powerful radio emission presumably originates in a pair of jets launched at almost the speed of light from the vicinity of the black hole. Medvedev and his colleagues suggested that the record-breaking X-ray luminosity of this quasar is associated with the Compton scattering of the relic radiation from the Big Bang by the relativistic matter of the jets. This process should be especially important in the early Universe, where the energy density of the relic radiation was some three orders of magnitude higher than that of the cosmic microwave background around the present-day objects. IKI scientists continue to look for such objects in the eROSITA data.