Figure 10.
GMOS spectrum of the AGN in the low-mass
spiral NGC 4395, showing the narrow [NII] and
Hα lines superposed on the broad Hα emission
used for the reverberation measurement (left).
The narrow [SII] lines at longer wavelength
were used as proxies for the central stellar
dispersion (right).
Figure reproduced from Woo et al., Nature
Astronomy, 2019, in press (arXiv 1905.00145).
Figure 11.
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).
36
minosity AGNs known. The AGN resides
within a nuclear star cluster at the center of
the nearby dwarf spiral NGC 4395, and the
study was led by Jong-Hak Woo of Seoul Na-
tional University. Using spectroscopic data
from the Gemini Multi-Object Spectrograph
(GMOS) at Gemini North, Woo’s team mea-
sured a line-of-sight velocity dispersion of
426 kilometers per second (km/s) from the
width of the broad Hα line (Figure 10). Com-
bined with a reverberation time delay of 83
GeminiFocus
minutes based on a combination of broad-
and narrow-band imaging collected at sev-
eral small telescopes, the implied black hole
mass is about 9,100 M B . Previous estimates
ranged from 5 to 40 times higher, but were
much more poorly constrained. The new re-
sult is securely within the realm of the elu-
sive “intermediate-mass” black holes, which
may be the seeds from which supermassive
black holes grow.
There are well established relations for mas-
sive galaxies between central
black hole mass and the prop-
erties of the stellar bulge; it
is interesting to ask how NGC
4395, a pure disk galaxy without
any bulge, fits into these. The
new study estimated the cen-
tral stellar velocity dispersion
σ ★ from the width of the nar-
row [SII] emission line, finding
σ ★ ≈ 18 km/s, consistent with a
previous upper limit. Using this
value, they place NGC 4395 on
the diagram of M BH versus ve-
locity dispersion for high-mass
galaxies (Figure 11), conclud-
ing it is broadly consistent with
a simple extrapolation to lower
masses. This suggests that the
observed relations between M BH
January 2020 / 2019 Year in Review