GeminiFocus April 2019 - Page 12

Figure 3. Gemini was one of several large telescopes that contributed to the study of the lensed quasar J0439+1634, selected as a candidate high-redshift object because it is an r-band “dropout” with little i flux (top). The 6.5-m MMT and 10-m Keck-I telescope obtained optical spectra (outlined in cyan), while the 8.1-m Gemini North telescope obtained an infrared spectrum (outlined in red). The width of the Mg II line near 2100 nm constrains the mass of the black hole powering the quasar. The 2 x 8.4-m Large Binocular Telescope captured an adaptive optics corrected image that suggests the quasar is lensed, later confirmed by HST. Credit: Feige Wang (UCSB), Xiaohui Fan (University of Arizona) km on the diameter of Vanth. Remarkably, this is 60% larger than previous estimates, and roughly half as large as the estimated size of Orcus. The results also placed a limit of a few microbars on any possible atmo- sphere around Vanth. The study has been published in the journal Icarus, and a pre- print is available online. The Mass of the Most Distant Lensed Quasar Observations from the Gemini Near-Infra- red Spectrograph (GNIRS) have confirmed the redshift and constrained the mass of the brightest quasar yet discovered at red- shift z > 5. However, the discovery paper led by Xiaohui Fan of the University of Ari- zona concludes that the object, known as J0439+1634, is not the intrinsically most luminous quasar at this redshift. Rather, its apparent brightness has been boosted by a factor of about 50 by the gravitational magnification of an intervening galaxy. This makes J0439 + 1634 the most distant known strongly lensed quasar, and perhaps the first of many waiting to be revealed through high-resolution imaging. Several decades ago it was proposed that a substantial fraction of the most distant qua- sars found in flux-limited surveys would be brightened above the survey limit by gravi- tational lensing. If this is the case, the result- ing “magnification bias” would cause a sys- tematic overestimation of the masses of the supermassive black hole population power- ing high-redshift quasars. However, no mul- tiply-imaged lensed systems had ever been found above redshift z = 4.8 (a lookback time of about 12.5 billion years), despite intensive high-resolution follow-up of hundreds of quasars known beyond this redshift. It may be that the extended appearance of multi- ply-lensed quasars, and/or color contamina- tion by the lensing galaxy, causes a strong selection bias against these systems. Fan’s team selected J0439 + 1634 as a high- redshift quasar candidate based on a com- bination of imaging data from the Pan- STARRS1 survey in the optical, the UKIRT Hemisphere Survey in the near-infrared, and archival Wide-field Infrared Survey Ex- plorer data in the mid-infrared. Follow-up optical spectroscopy with the 6.5-meter (m) Multiple Mirror Telescope and 10-m Keck I telescope showed a prominent spec- tral break consistent with a redshift near 6.5 (lookback time of 12.9 billion years). A near-infrared spectrum obtained with GNIRS at Gemini North detected strong Mg II emission, yielding a firm redshift mea- surement of z = 6.51. Figure 3 shows the combined spectrum. From the width of the Mg II line in the GNIRS spectrum, the team derived a mass of almost 5 billion solar masses for the black hole powering the quasar, and the photometric mea- surements implied an astounding total luminosity of 5.8 × 10 14 solar luminosities. However, imaging obtained with adap- tive optics on the 2 × 8.4-m Large Binocu- lar Telescope indicated that J0439 + 1634 was broader than a point source, sug- gesting the presence of either a host gal- axy or multiple images. Higher resolution 10 GeminiFocus April 2019