A very special quasar quartet

Quasars are galaxies with unusually large luminosities, much more than can be associated with the typical constituents of galaxies, i.e. their stars, gas and dust. Quasars are known to host supermassive black holes (SMBH) at their centres, which is thought to power the quasar. Their large luminosities are due to physical processes related to the accretion disk of very hot gas which surrounds the SMBH and the formation of extended jets of particles that are most likely collimated by a strong magnetic field. Quasars usually have large UV and X-ray luminosities due to very hot gas in the accretion disk. They can also have large infrared and radio luminosities due to synchrotron radiation associated with relativistic jets and their nuclei. The associated central region of a quasar is typically 100 times brighter than the total brightness of a normal galaxy.

The number of quasars in the Universe is a function of the age of the Universe. There seems to be a special time, often called the ‘quasar epoch’, at a redshift of z~2-3 that has a very high density of quasars. This special time could be related to when the SMBHs were being fed most efficiently by nearby sources of gas. A team of astronomers using the Keck Observatory conducted a survey of 29 quasars at redshift z~2. The survey identified quasars with Lyman-alpha (Lyα) emission, resulting from ionised hydrogen with a rest wavelength of 1216 Angstroms. The Lyα emission is generated by the quasar as it energises the surrounding gas.

However when follow-up spectroscopy was conducted on other objects near one of the quasars SDSSJ0841+3921 (z = 2.0412), three other quasars were detected at the same or similar redshift. The objects were observed with the Keck instrument Low Resolution Imaging Spectrometer (LRIS) for 3 hours in late 2012, using a custom-built narrow-band filter that was tuned to the wavelength of the redshifted Lyα hydrogen gas. This system is the only ‘quadruple quasar’ known, and the probability of finding such a system is very, very small, about ~ 10-7.

Image of the rare quasar quartet from the Keck Observatory. The four quasars are indicated by arrows. The quasars are embedded in a giant nebula of cool dense gas visible in the image as a blue haze. (Credit: Hennawi & Arrigoni-Battaia, MPIA)

Whilst the probability of finding such a unique system of quasars is very small, the system can tell us much about the role of quasars in dense environments. Quasars at high redshift should reside in the most massive galaxies. These massive galaxies should be indicators of the highest density regions of the distant universe, which at redshifts of z~2-3 in theory should indicate regions in the early stages of cluster formation. The light from the quadruple quasar took 10.5 billion years to reach us, and we therefore get a unique view of the universe about 3.3 billion years after the big bang. However quasar surveys do not always find quasars in the highest density areas, so the relationship between quasars and protoclusters, the progenitors of rich clusters we see in the nearby universe, is unclear.

What is clear however is that about 10% of distant quasars and protoclusters do seem to have a common environment – being in or associated with very large, (hundreds of kiloparsecs across) Lyα emission nebula. The nebula associated with SDSSJ0841+3921 has a size of 310 kpc and the 4 quasars appear to be roughly oriented along a line similar to the major axis of the nebula. The protocluster appears special even amongst other protoclusters – as it appears to be about 20 times as rich in Lyα within a radius of 200 kpc. In the case of the SDSSJ0841+3921 quadruple system, there is a physical connection between the nebula, the 4 quasars and a probable (but extremely rare) protocluster. The protocluster has a size of several hundred thousand light years, and within this region there is a galaxy overdensity of ~100 above that expected at an “average” place in the distant universe. However the gas in the SDSSJ0841+3921 system is colder (and denser) than expected. It may be a case of catching a cluster in the earliest stages of formation at a very, special time. For more information, see

[Glen Mackie]

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