Black Hole Masses from Nearinfrared Observations of AGN
Black holes are intimately tied to the growth
and evolution of galaxies, and active galaxies offer some of the best examples in which
to measure the mass of the central black
hole, the quantitatively significant property. In a new work, Hermine Landt (Durham
University and University of Melbourne) and
collaborators expanded the sample of wellmeasured galaxies to allow determination of
black hole masses from single near-infrared
(NIR) spectra of active galaxies.
The underlying physical relationship is between the velocity of emitting material and
its distance from the central black hole. The
observational proxies for these properties
are the spectral width of the broad emission
lines and the active galactic nucleus (AGN)
continuum luminosity, where the distance is
expected to go as the square root of luminosity (assuming the line is produced at a
location of fixed ionizing flux).
Reverberation mapping at optical wavelengths establishes this relationship, where
the continuum variability is observed after a delay in the broad line emission. This
technique has the disadvantage of being
observationally time-consuming, and fewer
than 50 AGN have been measured. Once the
radius-luminosity relationship is established,
however, further measurements are observationally easier.
This new work provides the observational
correlations in the NIR, using observations
with the Gemini Near-infrared Spectrograph
(GNIRS). This wavelength regime offers advantages over the optical and ultraviolet,
including being less contaminated by host
galaxy stellar emission, having lines that are
less confused by blending, and being less affected by dust obscuration. The sample is restricted to galaxies that have reverberation
mapping results, and the new data especial-
ly help to fill out the high-luminosity range.
Figure 14.
Figure 14 shows the resulting radius-luminosity relationship, where the radius, R, is
based on previous measurements, and the
NIR provides the luminosity, L. The observed
scatter and lack of change with the enlarged
sample here suggest that some of the scatter is intrinsic to the relationship, not measurement uncertainty. With a direct measurement of the velocity spread from the
width of Paschen a or b lines, the black hole
mass can be calculated.
The location of broad
Hb emission (measured
in light-days, from
reverberation mapping)
versus 1-micron
continuum luminosity.
Previous observations
are plotted in black,
new results are shown
in red, and upper limits
result when the host
galaxy dominates the
emission (green). The
different lines show fits
obtained using various
techniques, all of which
are consistent with a
slope of 0.6±0.1.
Alternatively, the combination of NIR luminosity and line width together can be related to the previously measured black hole
mass. The complete paper, to be published
in Monthly Notices of the Royal Astronomical
Society, provides the resulting quantitative
relationships, including consideration of different techniques for determining the velocity spread. A preprint is available at: http://
arxiv.org/abs/1303.1923
Nancy A. Levenson is Deputy Director and Head
of Science at Gemini Observatory and can be
reached at: [email protected]
January2014 2013 Year in Review
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