and central dispersion does not originate
from the process of hierarchical growth, but
that the galaxy mergers that produce central
bulges preserve a relation that may already
be present for the seed intermediate-mass
black holes.
Testing this scenario will require more stud-
ies of the incidence and masses of black
holes in the centers of low-mass galaxies. In
addition, such studies can determine wheth-
er the familiar supermassive black holes like-
ly originated from “light” seeds of order 100
to 1,000 M B (possible remnants of massive
Population III stars) or “heavy” seeds of order
10 4 M B or more (formed via the direct col-
lapse of giant gas clouds). As demonstrated
by the impressive results on NGC 4395, rever-
beration mapping remains the most promis-
ing method for building up the required data
samples to address these questions.
Divergent Demographics of
Planets and Brown Dwarfs in
the GPI Exoplanet Survey
Soon after the Gemini Planet Imager (GPI) was
commissioned at Gemini South, the interna-
tional team behind the instrument embarked
on a major systematic survey for substellar
companions and protoplanetary disks around
the youngest, closest stars in the southern
sky. Earlier this year, the GPI Exoplanet Survey
(GPIES) observed its 531st target star, bringing
the main survey to a close after more
than four years, although follow-up
observations of promising candidates
have continued. Now, the team has
published preliminary results from a
statistical analysis of the first 300 stars
surveyed. The study, published in the
July issue of The Astronomical Journal,
was led by Eric Nielsen of Stanford Uni-
versity and represents the largest direct
imaging survey for giant planets pub-
lished to date.
January 2020 / 2019 Year in Review
GPIES is sensitive to young, self-luminous
planets with masses above about 2 Jupiter
masses and orbital semi-major axes from 3
to 100 AU. The detections thus far include
six giant planets and three brown dwarfs.
Although only about 40% of the stars in-
cluded in the analysis have masses greater
than 1.5 M B , all of the detected planets orbit
stars above this mass. This is even more strik-
ing because it would be easier to see such
planets orbiting fainter, lower mass stars.
While there have been previous indications
of a correlation with stellar mass, the GPIES
results confirm to better than 99.9% confi-
dence that high-mass stars are more likely
to host planets within the explored range of
planetary masses and orbital separations.
Accounting for the detection sensitiv-
ity curves and combining their results with
those from radial velocity studies (sensitive
to companions at smaller radii), the team
concluded that the most likely location for
giant planets to occur is between 1 and 10
AU from their host stars. The occurrence rate
drops steeply at larger separations. The num-
ber of giant planets also declines significant-
ly with increasing planetary mass.
Although brown dwarfs are often consid-
ered transitional objects between planets
and stars, they appear to have quite different
demographics than giant planets, as shown
in Figure 12. The study concludes that only
about one in ten stars hosts a brown dwarf
GeminiFocus
Figure 12.
GPIES sensitivity contours
for companion mass (in
units of Jupiter masses)
and orbital semi-major
axis (Astronomical Units)
for planetary (left) and
brown dwarf (right) com-
panions. The six giant
planets and three brown
dwarfs detected in the
survey are overlaid on
the contours. Although
the majority of these
companions were not
discovered by GPIES,
their host stars were part
of the unbiased sample
and were not selected
because of the pres-
ence of the companions;
thus, the detections are
included in the statistical
analysis. The curves indi-
cate the numbers of stars
in the sample for which
the sensitivity allowed
detection of compan-
ions with the plotted
combinations of param-
eters; very few stars had
sensitivity sufficient to
detect planets of masses
< 3 M Jup , but two were
detected.
Figure reproduced from
Nielsen et al., The Astro-
nomical Journal, 158:
13, 2019.
37