black hole.
Thaisa Storchi-Bergmann (Instituto de Fisica, UFRGS, Brazil) and colleagues
have used the Cassegrain Spectrograph on the Blanco 4-m telescope to discover
and subsequently monitor a double peaked H profile at the center of the
active galaxy NGC 1097. This profile appears to be direct evidence for a
central accretion disk. Its shape and variations with time imply that the
central engine in NGC 1097 is a
black hole.
In the most widely accepted picture for active galactic nuclei (AGNs), a
nuclear, supermassive black hole is fed by an accretion disk. Nevertheless,
observational evidence for the presence of the disk has been limited and
indirect until very recently. There was no clear, dynamical signature of the
disk in the form of double-peaked emission lines, such as those found in
cataclysmic variable stars. It is now known that the X-ray spectra of most
Seyfert 1 galaxies possess broad, disk-like iron K lines and about 10% of
broad-line radio galaxies show disk-like Balmer lines with twin peaks or
twin shoulders. These line profiles are very well reproduced by models of
gas rotating at relativistic speeds in a Keplerian accretion disk. The
relative strengths of the narrow lines of these radio-loud "disk-like"
emitters are similar to those found in LINERs (low-ionization nuclear
emission-line regions). The association of broad, double-peaked Balmer lines
(whatever their origin) with objects displaying LINER-like narrow lines has
been underscored by the recent abrupt appearance of double-peaked Balmer
lines in three objects that did not previously have them: NGC 1097, Pictor
A, and M81.
The initial abrupt appearance of a double-peaked profile in NGC 1097 was
consistent with the tidal disruption of a star by a black hole.
Significantly, the profile showed a flux asymmetry, with the red peak
stronger than the blue one. Using models by Michael Eracleous (Berkeley) and
collaborators, Storchi-Bergmann and colleagues reproduced the shape of the
double-peaked profile with a model of an elliptical accretion ring, and
predicted the likely profile evolution. These predictions have been borne
out. Subsequent monitoring showed that after approximately two years, the
profile had become more symmetric, with the two peaks showing similar
heights. This change was interpreted as a result of relativistic precession
of the ring, which yielded an estimate of the black hole mass of about
. The most recent observations, obtained at CTIO
in January 1996 and
illustrated in Figure 1, show that the profile has continued to vary and is
now almost a mirror image of the discovery profile.
The fact that the emission line profiles have been varying smoothly and
continuously over several years reinforces the general idea of origination
in a single structure in the accretion flow. As shown in Figure 2, the
eccentric precessing accretion disk model can fit the line profiles quite
well. Equally important, at least one alternative scenario for the origin of
broad double-peaked lines now appears unlikely: the binary black hole
hypothesis. The separation of the two peaks in the NGC 1097 profile implies
two black holes of roughly equal mass. If such a binary is to form from the
merger of two parent galaxies, then the two parents should have roughly
equal masses, and the result of the merger would not be a spiral galaxy like
NGC 1097. A third class of models creates double peaked profiles in the
oppositely directed parts of a bipolar outflow; these are purely kinematic
and so notoriously difficult to constrain. Such a model might possibly
accommodate the observed profile variability but does not offer an
explanation in terms of a specific physical mechanism. The precessing
ellipse model of Figure 2, derived from seven observational epochs, implies
a period of 25 years and a central black hole mass of 
. This work
will appear in the Astrophysical Journal.
A preprint can be accessed via the WWW at astro-ph/9705176.
The future evolution of the line profiles in NGC 1097 will further constrain this or other models of its origin. Among the few AGN objects in which double-peaked lines have abruptly appeared, NGC 1097 is the most spectacular performer. Frequent and careful spectroscopic monitoring is called for, so as not to miss the next act of this unfolding drama.