The Most Distant Known Quasar In The Universe: ULAS J112001.48+064124.3

21/05/2012

Some galaxies like our very own Milky Way seem to be in a dormant state, existing in a seemingly quiet epoch of the Universe. However, what lies at the centre of our Galaxy reveals our not so distant past: our Galaxy was most likely to be active due to the accretion of gas and dust onto the central super massive black hole (SMBH) (Lynden-Bell 1969; Antonucci 1993). This accretion can power certain galaxies to emit very high-energy photons, known as active galaxies. There is no real phenomenological distinction between quasars and active galactic nuclei. They are both believed to be powered by the same accretion processes, where the distinction comes that a quasar is considered to be a distant object. The issue of the activity of nuclei of galaxies (AGN) was first raised by the Armenian physicist Victor Ambartsumian in the early 1950s. However, it wasn’t until the discovery of 3C 273 in 1959, the first confirmed of the quasi-stellar class, that the paradigm of active galactic nuclei was taken with less skepticism.

Since the late 1950s many objects resembling quasars and radio galaxies have been observed. Many have been observed locally, with redshifts equalling z ~ 0.5. But it wasn’t until the Hubble Space Telescope and other modern space telescopes that the super distant objects, such as ULAS J112001.48+064124.3, were detected with redshifts of z > 6. The quasar ULAS J112001.48+064124.3 was first identified in the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) (Lawrence et al. 2007) and published in the Eighth Data Release back in September of 2010, where follow-up observations confirmed it as a quasar with a redshift of z ≥ 6.5 (Mortlock et al. 2011).

In November of 2010 a spectrum was obtained for ULAS J112001.48+064124.3 using the Gemini Multi-Object Spectrograph on the Gemini North Telescope. The absence of significant emission blueward of a sharp break at the λ = 0.98 μm wavelength confirmed that ULAS J112001.48+064124.3 was indeed a quasar with a redshift of z ~ 7.08. With that tentative figure in mind and assuming that the standard model of cosmology (Dunkley et al. 2009) is the most correct (i.e. a flat Universe with a Hubble parameter of at the present epoch) that would mean the light we are seeing from ULAS J112001.48+064124.3 has taken 12.9 billion years (Gyr) to reach us, starting it’s journey when the Universe was as little as 0.77 billion years old.

Further spectroscopic observations of ULAS J112001.48+064124.3 were made using the FOcal Reducer/Low Dispersion Spectrograph 2 (FORS2) on the Very Large Telescope (VLT) Antu and the Gemini Near-Infrared Spectrograph (GNIRS) on the Gemini North Telescope. The spectrum obtained of ULAS J112001.48+064124.3 shows remarkable similarity to lower redshift quasars of comparable luminosity, and comparison to a rest-frame template spectrum (Hewett & Wild 2010) over the wavelength range including the strong Si [III]+C [III] and Mg[II] emission features yielding an accurate redshift of z = 7.085±0.003. The most unusual feature of the spectrum is the 2800±250 km s^-1 blueshift of the C [IV] emission line, which is greater than that seen in 99.9 % of z ≳ 2 quasars (Richards et al. 2011). There is associated absorption at various other spectral doublets indicating the presence of material in front of the quasar flowing out at 1100±200 km s^-1. There is also a narrow absorption line at the Lyman-α emission wavelength that is consistent with a cloud of H[I] gas close to the quasar. Aside from its existence, the most striking aspect of ULAS J112001.48+064124.3 is the almost complete lack of observed flux blueward of its Lyman-α emission line, which can be attributed to absorption by H[I] along the line of sight.

Although this is not the highest redshift object in the Universe [The Discovery Of UDFy-38135539: Z ~ 8 In The Hubble Ultra-Deep Field], ULAS J112001.48+064124.3 is the furthest known quasar in the Universe. Quasars are not expected to have formed so early in the galaxy, due to the current understanding of the formation of SMBHs. The light from ULAS J112001.48+064124.3 was emitted during a time period before the end of the theoretically predicted transition of the interstellar medium from an electrically neutral to an ionized state. Quasars may have been an important energy source in this process, known as reionization, which marked the end of an epoch known as the Cosmic Dark Ages, so a quasar from before the transition is of significant theoretical interest in both how it interacted with the Universe around it but also in the manner of the formation of the central SMBH engine, e.g. do they form slowly and gradually over time or do they form instantaneously at approximately the same mass that they are recorded at the present epoch? The disovery of ULAS J112001.48+064124.3 and the subsequent research it will enable will almost certainly allow astrophysicists to shed light on these age old questions of SMBH formation.

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