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