Discovery of the most luminous quasar of the last 9 Gyr (2206.04204v2)

Abstract: We report the discovery of a bright (g = 14.5 mag (AB), K = 11.9 mag (Vega)) quasar at redshift z = 0.83 -- the optically brightest (unbeamed) quasar at z > 0.4. SMSS J114447.77-430859.3, at a Galactic latitude of b = +18.1deg, was identified by its optical colours from the SkyMapper Southern Survey (SMSS) during a search for symbiotic binary stars. Optical and near-infrared spectroscopy reveals broad MgII, H-beta, H-alpha, and Pa-beta emission lines, from which we measure a black hole mass of log10(M_BH/M_Sun) = 9.4 +/- 0.5. With its high luminosity, L_bol = (4.7 +/- 1.0) * 10^47 erg/s or M_i(z=2) = -29.74 mag (AB), we estimate an Eddington ratio of ~1.4. As the most luminous quasar known over the last ~9 Gyr of cosmic history, having a luminosity 8 times greater than 3C 273, the source offers a range of potential follow-up opportunities.

• The paper reports SMSS J1144 as the most luminous quasar beyond z=0.4, with a bolometric luminosity of (4.7 ± 1.0)×10^47 erg/s.
• It derives a black hole mass of approximately 2.6×10^9 M☉ using broad emission line analysis, indicating efficient accretion with an Eddington ratio near 1.4.
• The quasar, detected via optical colors in the SMSS and eight times brighter than 3C 273, highlights the value of comprehensive surveys in uncovering rare phenomena.

Discovery of the Most Luminous Quasar in the Last 9 Billion Years

The paper "Discovery of the most luminous quasar of the last 9~Gyr" presents the identification and characterization of SMSS J114447.77-430859.3, a remarkably bright quasar discovered at a redshift of $z = 0.83$. This object is highlighted as the optically brightest quasar beyond redshift $z = 0.4$ and represents a rare discovery in the exploration of cosmic history over a substantial 9 billion-year period.

Key Observations and Results

• Luminosity and Redshift: SMSS J1144 has been identified at a redshift of $z = 0.83$ with a bolometric luminosity measured at $L_{\rm bol} = (4.7 \pm 1.0) \times 10^{47}$ erg s$^{-1}$. Its absolute magnitude was extrapolated to $M_{i}(z=2) = -29.74$ mag (AB), underscoring its unparalleled brightness relative to comparably distant sources.
• Massive Black Hole: Spectroscopic analysis revealed prominent broad emission lines, including #1{Mg}{ii}, H$\beta$, H$\alpha$, and Pa$\beta$, from which the authors derived a black hole mass of $$\log_{10} (M_{\rm BH}/\text{M}_\odot) = 9.4 \pm 0.5$$. This mass estimate corresponds to approximately $2.6 \times 10^9$ M$_\odot$, noting a substantial accretion scenario with an Eddington ratio of approximately 1.4, suggesting a highly efficient accretion process for its luminosity.
• Comparison with Other Quasars: SMSS J1144 is characterized as possessing a luminosity eight times greater than that of 3C 273, another notable but fainter cosmic lighthouse. This comparison highlights SMSS J1144's significance as the most luminous quasar observed within the considered timeline.
• Discovery Methodology: The quasar was initially detected due to its optical colors in the SkyMapper Southern Survey (SMSS) during a search intended for symbiotic binary stars. Its serendipitous identification as a quasar underscores the value of comprehensive survey methodologies in uncovering rare astronomical phenomena.

Implications and Future Research

The discovery of SMSS J1144 bears significant consequences both in practical observational astronomy and theoretical cosmology. Its exceptional luminosity offers a unique opportunity to probe the high-luminosity end of quasar activity, providing insights into black hole accretion dynamics and the evolution of supermassive black holes over cosmic time. Moreover, its brightness presents an attractive target for further spectroscopic and variability studies which could further unravel its structure and perhaps provide a more refined understanding of its surrounding environment, notably the circumgalactic medium of its host galaxy.

From a theoretical stance, this find invites reevaluation of the existence and distribution of such luminous quasars within and beyond the observable universe. It posits questions regarding the frequency of such quasars and the conditions conducive to their formation, potentially driving advancements in simulation models of galaxy formation and evolution. Additionally, its apparent lack of strong X-ray or radio emissions could prompt further inquiries into the diversity of quasar emission characteristics and their linkage to underlying black hole and host galaxy properties.

In conclusion, this discovery not only expands the catalog of known high-luminosity quasars but also serves as a valuable observable for validating cosmological models and enhancing our understanding of the phenomenology of active galactic nuclei in the context of the broader cosmic web. As such, SMSS J1144 stands as a beacon of cosmological inquiry, inviting ongoing research facilitated by existing and forthcoming astrophysical instrumentation and surveys.