Figure 17.
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)
42
Once the binary nature of the occultation
star was revealed by Gemini/DSSI, the two
observed occultations, combined with non-
detections at the other sites, allowed the
team to place a tight constraint of 443 ± 10
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.
GeminiFocus
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 Explorer
data in the mid-infrared. Follow-up optical
spectroscopy with the 6.5-meter (m) Mul-
tiple Mirror Telescope and 10-m Keck I tele-
scope showed a prominent spectral 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, yield-
ing a firm redshift measurement of z = 6.51.
Figure 17 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 pow-
ering the quasar, and the photometric mea-
surements implied an astounding total lumi-
nosity of 5.8 × 10 14 solar luminosities.
However, imaging obtained with adap-
tive optics on the 2 × 8.4-m Large Binocular
Telescope indicated that J0439 + 1634 was
broader than a point source, suggesting the
presence of either a host galaxy or multiple
January 2020 / 2019 Year in Review