We present a Hubble Space Telescope ( HST ) study of the nuclear region of the E4 radio galaxy NGC 7052 , which has a nuclear disk of dust and gas .
The Second Wide Field and Planetary Camera ( WFPC2 ) was used to obtain B , V and I broad-band images and an H \alpha + [ NII ] narrow-band image .
The images yield the stellar surface brightness profile , the optical depth of the dust , and the flux distribution of the ionized gas .
The Faint Object Spectrograph ( FOS ) was used to obtain H \alpha + [ NII ] spectra at six different positions along the major axis , using a 0.26 ^ { \prime \prime } diameter circular aperture .
The emission lines yield the rotation curve of the ionized gas and the radial profile of its velocity dispersion .
The observed rotation velocity at r = 0.2 ^ { \prime \prime } from the nucleus is V = 155 \pm 17 \ > { km } { s } ^ { -1 } .
The Gaussian dispersion of the emission lines increases from \sigma \approx 70 \ > { km } { s } ^ { -1 } at r = 1 ^ { \prime \prime } , to \sigma \approx 400 \ > { km } { s } ^ { -1 } on the nucleus .

To interpret the gas kinematics we construct axisymmetric models in which the gas and dust reside in a disk in the equatorial plane of the stellar body , and are viewed at an inclination of 70 ^ { \circ } .
It is assumed that the gas moves on circular orbits , with an intrinsic velocity dispersion due to turbulence ( or otherwise non-gravitational motion ) .
The latter is required to fit the observed increase in the line widths towards the nucleus , and must reach a value in excess of 500 \ > { km } { s } ^ { -1 } in the central 0.1 ^ { \prime \prime } .
The circular velocity is calculated from the combined gravitational potential of the stars and a possible nuclear black hole .
Models without a black hole predict a rotation curve that is shallower than observed ( V _ { pred } = 92 \ > { km } { s } ^ { -1 } at r = 0.2 ^ { \prime \prime } ) , and are ruled out at > 99 % confidence .
Models with a black hole of mass M _ { \bullet } = 3.3 ^ { +2.3 } _ { -1.3 } \times 10 ^ { 8 } \ > { M _ { \odot } } provide an acceptable fit .
The best-fitting model with a black hole adequately reproduces the observed emission line shapes on the nucleus , which have a narrower peak and broader wings than a Gaussian .

NGC 7052 can be added to the list of active galaxies for which HST spectra of a nuclear gas disk provide evidence for the presence of a central black hole .
The black hole masses inferred for M87 , M84 , NGC 6251 , NGC 4261 and NGC 7052 span a range of a factor 10 , with NGC 7052 falling on the low end .
By contrast , the luminosities of these galaxies are identical to within \sim 25 % .
Any relation between black hole mass and luminosity , as suggested by independent arguments , must therefore have a scatter of at least a factor 10 .