Divergent Demographics
of Planets and Brown
Dwarfs in the GPI
Exoplanet Survey
Figure 2.
The new
NGC 4395
black hole
measurement
is plotted in
the context of
the relation
between
central black
hole mass and
stellar velocity
dispersion for more
massive systems. The
stellar velocity dispersion
for NGC 4395 is shown
as a previously published
upper limit (open red
square) and the proxy
value adopted from the
width of the [SII] emission
line [solid red square].
Plotted values for the
higher mass galaxies
are stellar dynamical
measurements in
inactive galaxies [open
black circles] and
reverberation mapping
in active galaxies
[filled blue circles].
The solid and dashed
lines are, respectively,
fits to the combined
high-mass sample
and to the dynamical
measurements only.
[Figure reproduced
from Woo et al., Nature
Astronomy, 2019, in
press (arXiv 1905.00145).]
10
sion for high-mass galaxies (Figure 2), con-
cluding it is broadly consistent with a simple
extrapolation to lower masses. This suggests
that the observed relations between M BH
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
whether the familiar supermassive black
holes likely 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 collapse of giant gas clouds). As
demonstrated by the impressive results on
NGC 4395, reverberation mapping remains
the most promising method for building up
the required data samples to address these
questions.
GeminiFocus
Soon after the Gemini Planet Imag-
er (GPI) was commissioned at Gem-
ini South, the international team
behind the instrument embarked
on a major systematic survey for
substellar companions and proto-
planetary disks around the young-
est, closest stars in the southern sky.
Earlier this year, the GPI Exoplanet
Survey (GPIES) observed its 531st
target star, bringing the main sur-
vey to a close after more than four
years, although follow-up observa-
tions of promising candidates have
continued. Now, the team has pub-
lished preliminary results from a statistical
analysis of the first 300 stars surveyed. The
study, published in the July issue of The As-
tronomical Journal, was led by Eric Nielsen of
Stanford University and represents the larg-
est direct imaging survey for giant planets
published to date.
GPIES is sensitive to young, self-luminous
planets with masses above about 2 Jupiter
masses and orbital semi-major axes from 3
to 100 astronomical units (au). The detec-
tions thus far include six giant planets and
three brown dwarfs. Although only about
40% of the stars included 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 striking because it would
be easier to see such planets orbiting fainter,
lower mass stars. While there have been pre-
vious indications of a correlation with stel-
lar mass, the GPIES results confirm to better
than 99.9% confidence that high-mass stars
are more likely to host planets within the
explored range of planetary masses and or-
bital separations.
July 2019