Pointing VLT/X-shooter at the most luminous high-redshift quasars

By Samuel Lai
[email protected]

Despite their reputation as invisible gluttons which can devour light itself, black holes are also responsible for the brightest non-transient phenomena in the universe. Quasars are extremely luminous active galactic nuclei powered by supermassive black holes, with masses ranging from millions to billions of suns. Their radiative output and high energy outflows play a significant role in regulating the evolution of the host galaxy. Furthermore, their ultra-luminous nature allows them to be observed at high redshift, allowing us to study conditions in the early universe and of the intervening material along the quasar sightlines.

However, observations of high redshift quasars reveal their black holes to be more massive than expected based on stellar seeds and accretion-dominated evolution. Working out the black hole mass evolution pathway requires detailed studies of the underlying population of supermassive black holes at high redshift. By examining quasar demographics across a wide redshift range, we can gain insights on their rapid growth and whether more exotic formation or evolutionary scenarios may be responsible.

X-shooter spectrum of a quasar
Fig. 1: Example X-shooter spectrum of quasar SDSS J001115.23+144601.8 at redshift z=4.96. Strong emission features are identified and labeled. The estimated black hole mass of this quasar is 5.5 billion solar masses based on velocity broadening of the Mg II line.

In our study recently published in MNRAS, we make use of the Very Large Telescope’s X-shooter instrument, which is an echelle spectrograph with broad simultaneous optical to near-infrared wavelength coverage. We also utilise the Southern Astrophysical Research (SOAR) Telescope’s TripleSpec4.1 instrument and the Australian National University’s 2.3m Telescope’s Wide-Field Spectrograph. In all, we studied 83 extreme quasars between redshifts 4.5 and 5.3, constructing the most luminous quasar sample at high redshift with high completeness. The wide wavelength coverage enabled us to observe broad emission features from Lyman-alpha to Mg II, where we utilise the luminosity of the accretion disc and the Doppler-broadened Mg II to estimate black hole masses across the entire sample (Fig. 1 above).

A key result from this study is the compilation of a highly complete ultra-luminous quasar sample, called XQz5, with robustly-determined black hole masses. The publicly-available reduced and post-processed data has widespread community value as a resource for studying the properties and evolution of quasars in the early universe. Due to its high completeness, the sample has legacy significance and will likely remain the brightest known sample at this redshift range within the surveyed area (Fig. 2 below). Another key result is an observed boost in black hole masses with decreasing redshift within XQz5 and in comparison to other high-redshift quasar samples, indicating a rising branch stage of cosmic evolution, as the black holes feed continuously from an earlier seed. In an upcoming study, we present a demographic analysis of ultra-luminous quasars with high-mass black holes to quantitatively explore the evolution of the underlying quasar population.

Black hole mass vs quasar luminosity distribution
Fig. 2: Black hole mass and quasar luminosity distribution, comparing our sample (XQz5) with the Trakhtenbrot et al. 2011 (T11 = ApJ, 730:7) sample, which is located at a comparable redshift range. XQz5 is both a higher mass and higher luminosity sample than T11. The Eddington ratio distributions are similar, suggesting both samples reflect rapidly accreting active quasars.


Michael Murphy is the Australian representative on the ESO Science Technical Committee. Contact: [email protected]

Sarah Sweet is the Australian representative on the ESO Users Committee. Contact: [email protected]

Stuart Ryder is a Program Manager with AAL. Contact: [email protected]

Guest posts are also welcome – please submit these to [email protected]