the gap between the more well-studied stellar-mass and supermassive varieties.
Full results appear in Monthly Notices of the
Royal Astronomical Society, (viewable here).
Figure 3.
Radial surface
brightness profiles
of NGC 4151. The
Gemini/NIFS data
(red) measure stellar
features only (which
is most relevant to the
determination of black
hole mass), so they do
not show the increase
of central surface
brightness due to the
AGN, which is evident
in the Hubble Space
Telescope (green) and
Ohio State University
Bright Spiral Galaxy
Survey (blue) images.
Testing Black Hole
Mass Measurements in
an Active Galaxy
Supermassive black holes characterize galaxy nuclei, and their masses scale with stellar properties in their hosts. This shows that
black holes are fundamental to galaxy formation and evolution. With quiescent galaxies, astronomers usually employ dynamical
techniques to measure the black holes. Active galactic nuclei (AGN) — which are “active” in the sense of accreting material — offer distinct techniques for the measurement
of their central black holes. Specifically,
reverberation mapping can reveal the size
and motion of nuclear gas, and therefore the
black hole mass.
Each of these approaches is independently
successful, but very few galaxies allow the
results to be compared directly. NGC 4151 is
an exception, being close enough for reliable
dynamical measurements and also having an
AGN subject of reverberation mapping campaigns. It also raised questions, as an apparent outlier from usual relationships between
stellar properties and black hole mass.
Christopher Onken (Australian National
University) and colleagues provide new
dynamical measurements that take advantage of improved spatial resolution from the
Near-infrared Integral Field Spectrometer
and adaptive optics in observations from
Gemini North. They find a black hole mass of
3.8 x 107 MSun, which is lower than previous
measurements (obtained using lower resolution observations) and is consistent with
reverberation mapping results.
Isolating measurements on small spatial
scales emphasizes the region that is within
the black hole’s sphere of influence and
avoids complications from a bar that is dynamically evident on larger scales. The resulting velocity dispersion, σ, is somewhat
larger than previous measurements, with
the net result of putting NGC 4151 closer to
the general relationship between MBH and
σ, though still on the side of lower velocity
dispersion. In addition, the researchers demonstrate the complication of galaxy bars in
such measurements of black hole masses,
predicting a discrepancy in the results obtained depending on the presence of a bar.
Complete results appear in The Astrophysical
Journal (view here).
Nancy A. Levenson is Deputy Director and Head
of Science at Gemini Observatory and can be
reached at: [email protected]
10
GeminiFocus
October 2014