Figure 1.
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).]
tween variations in the intensity of the con-
tinuum light from the AGN, which excites
the gas within the BLR, and the line emission
itself: R = c τ , where c is the speed of light. Be-
cause lines of different ionization show dif-
ferent delays, the same line should be used
for determining both σ and τ . Typical AGNs
powered by supermassive black holes of mil-
lions of solar masses (M B ) have delay times
measured from Balmer lines ranging from a
few days to many months.
A new study published in Nature Astronomy
has measured the mass of the black hole as-
sociated with one of the lowest luminosity
AGNs known. The AGN resides within a nu-
clear star cluster at the center of the nearby
dwarf spiral NGC 4395, and the study was
led by Jong-Hak Woo of Seoul National Uni-
versity. Using spectroscopic data from the
Gemini Multi-Object Spectrograph (GMOS)
at Gemini North, Woo’s team measured a
line-of-sight velocity dispersion of 426 kilo-
meters per second (km/s) from the width of
July 2019
the broad Hα line (Figure 1). Combined with
a reverberation time delay of 83 minutes
based on a combination of broad- and nar-
row-band imaging collected at several 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 result
is securely within the realm of the elusive
“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 properties 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 central stellar
velocity dispersion σ ★ from the width of the
narrow [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 velocity disper-
GeminiFocus
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