Astronomers have discovered two supermassive black holes actively consuming matter in the distant galaxy MG B2016+112. The distance between them is only 175 parsecs, or about 570 light-years. We see the galaxy as it was when the Universe was only about 2 billion years old.
The galaxy is at a redshift of 3.273. Redshift is a measure showing how much light has been stretched by the expansion of the Universe; the larger the number, the farther away and more ancient the object. The work was published on the arXiv preprint server on September 25 and has not yet undergone full peer review in a scientific journal.
According to models, when galaxies merge, gas rushes toward their central supermassive black holes. Therefore, merging galaxies should sometimes produce dual active nuclei - regions where a black hole greedily consumes matter and shines brightly. Spotting such pairs is difficult: close bright spots are easily confused.
Gravitational lensing comes to the rescue. This is an effect in which a massive foreground galaxy bends space-time so much that light from a distant object behind it is deflected and magnified. As a result, a distant source may appear as several images, arcs or rings. This is exactly how MG B2016+112, an active galaxy with powerful radio emission, appears to us as three images - A, B and C.
Researchers led by Jianghao Huyan and Julia M. Sisk-Reynes analyzed archival spectra obtained with the MUSE instrument on the VLT telescope in 2018. A spectrum is the decomposition of light by wavelength, from which the chemical composition and motion of matter can be determined. The scientists measured the strength of bright emission lines of hydrogen, nitrogen, carbon and helium in each of the three images, after first removing absorption of light by gas in our Galaxy and by gas near the quasar itself.
Most of the lines turned out to be too distorted by absorption to compare their ratios. But one ratio did work: the values for images A and B matched, while for C it was noticeably lower. This is a strong argument that C is illuminated by a different black hole, not the same one as A and B.
Estimates of the black hole masses from the width of the carbon C IV line gave similar values. For the source associated with A and B, the mass is at least 20-23 million solar masses; for C, at least 15 million. Similar masses fit the picture of a merger of two comparable galaxies, each of which brought its own black hole.
The rates of matter consumption, however, differ greatly between the pair. For A and B they are estimated at about 47% and 51% of the maximum, while for C it is only about 4%. This supports the idea of a binary system but conflicts with some simulations that predict the less massive black hole should feed faster. The authors warn that the estimate for C is unreliable because it depends on the lensing magnification factor, and they took only an average value, which may be inaccurate. Independent confirmation of the masses requires near-infrared observations.
For now, the data indicate that images A and B are produced by one black hole and C by another. At a redshift of 3.273, this is the tightest confirmed pair of active nuclei known at such an early epoch. The system offers a rare chance to see how black holes grew during a possible major galaxy merger in the young Universe.