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Paper text:
optical emission - line gas is detected in at least 50% of the centers of nearby early - type ( e / s0 ) galaxies ( e.g. , phillips et al .
1986 ; goudfrooij et al .
1994 ; macchetto et al .
1996 ; ho et al .
the gas velocity dispersion @xmath0 ( corrected for instrumental broadening ) typically increases towards the nucleus , reaching values similar to the the central @xmath1 as measured through 100s of pc scale sized apertures ( e.g. , zeilinger et al . , 1996 ) .
closer to the nucleus , @xmath0 often exceeds @xmath1 ( e.g. , ferrarese et al . 1996 ; macchetto et al .
, 1997 ; van der marel & van den bosch 1998 ; cappellari et al .
2002 ; verdoes kleijn et al .
potential contributors to @xmath2 are motions due to ( i ) a black hole ( bh ) gravitational potential or ( ii ) shocks / turbulence in the gas . in the former case ,
@xmath2 could be caused by three dimensional motion of collisionless cloudlets of gas .
more likely , the collisional gas is expected to have settled in a disk . in that case , @xmath0 will be caused by differential rotation over the aperture .
however , shocks and/or turbulence can contribute to @xmath0 if there is a permanent input of kinetic energy ( perhaps the active galactic nucleus ) to sustain them .
thus , determining the driver of @xmath2 has implications for measurements of the bh mass ( @xmath3 ) and the flow properties of potential bh accretion material . here
we present preliminary modeling of @xmath0 and @xmath4 in the nuclei of 16 active and 4 quiescent early - type galaxies to constrain the relative importance of gravitation and shocks / turbulence .
the galaxy sample consists of 16 active galaxies with radio - jets taken from the ugc fr  i sample ( see noel - storr et al . , these proceedings , for selection criteria ) and four relatively quiescent early - type galaxies ( ngc 3078 , ngc 4526 , ngc 6861 and ic 1459 ) ( table  1 ) .
the four quiescent early - type galaxies were selected to have central dust and/or gas disks .
four of the 21 ugc fr  i galaxies were not included here because either @xmath1 is not available or the location of the nucleus could not be determined reliably , mainly due to dust obscuration       the galaxies are taken from the ugc fr  i radio galaxy sample ( noel - storr et al .
, these proceedings ) except for ngc 3078 , ngc4526 , ngc 6861 and ic 1459 .
col.(2 ) : hubble classification from ned .
col.(3 ) : absolute blue magnitude from leda .
col.(4 ) : distances from faber et al .  ( 1989 ) , tonry et al .  (
2001 ) , or , if not available , directly from recession velocity and @xmath5kms@xmath6mpc@xmath6 .
col.(5 ) : galaxy is part of the ugc fr  i sample ( 1 ) or not ( 0 ) .
[ table1 ]    hst / stis ( hst / fos for ic 1459 ) emission - line spectra are available for the sample . they include the and 6716,6731 lines .
a central @xmath0 per galaxy is determined by fitting single gaussians to each of the lines in and 6716,6731 at the nucleus .
an analytic emission - line flux profile is determined by fitting a double exponential to the emission - line surface brightness measurements from the hst / stis spectra and hst / wfpc2 emission - line images ( where available ) .
the central stellar dispersion @xmath1 for the sample galaxies was obtained from the leda catalog .
can the gravitational potential of a bh mass induce gas motions which explain @xmath0 ?
current observations are consistent ( e.g. , kormendy & gebhardt 2001 , for a review ) with most ( and perhaps all ) early - type galaxies containing a black hole with a mass related to @xmath1 , i.e. , the @xmath7 relation ( gebhardt et al . 2000 ; ferrarese & merritt 2000 ) .
we compute the expected @xmath0 for such a bh mass under idealized circumstances .
we assume the gas is a collisionless isotropic spherical distribution of gas cloudlets orbiting the bh .
the predicted gas velocity dispersion @xmath0(sphere,1 ) takes into account aperture size , psf and instrumental line width .
[ figure2]a shows that @xmath0 typically exceeds @xmath2(sphere,1 ) .
however , the calculation neglects the stellar mass present , which will increase @xmath0(sphere,1 ) . to illustrate the possible effect of this , fig .
[ figure2]b shows @xmath2(sphere,2 ) assuming an isothermal spherical stellar mass distribution in addition to the bh mass , i.e. , @xmath8 .
@xmath9 typically agrees within a factor 2 to @xmath9(sphere,2 ) . in other words
, the @xmath3 needed to account for @xmath9 agrees within a factor @xmath10 with @xmath3 as predicted by the @xmath11 relation ( because @xmath12 ) , under these simplifying assumptions .
the estimated intrinsic scatter in the relation is also a factor 2 ( tremaine et al .
thus , an additional contribution to @xmath0 from collisions ( e.g. , shocks / turbulence ) is not required in this case .     as a function of predicted velocity dispersions for radio galaxies ( open circles ) and relatively quiescent galaxies ( filled circles ) .
( a ) : the model for @xmath0(sphere,1 ) assumes an isotropic spherical distribution of gas cloudlets orbiting a bh with a mass according to the @xmath13 relation .
( b ) : similar to ( a ) but now including the stellar mass distribution as well .
the dashed and solid lines indicate @xmath0(sphere)= @xmath14 respectively ( to facilitate comparison to the @xmath13 ) .
the figure suggests the gas might be largely collisionless : see text for details . ]     as a function of predicted velocity dispersions for radio galaxies ( open circles ) and relatively quiescent galaxies ( filled circles ) .
( a ) : the model assumes an infinitely thin gas disk in circular rotation , inclined by @xmath15 to the line of sight and a bh mass @xmath3 according to the @xmath13 relation .
( b ) : same as ( a ) , but now for @xmath3 inferred from published , detailed modeling of the gas mean velocities @xmath16 .
the figure suggests that @xmath17 and @xmath0 can not be simultaneously fit by the gas disk model : see text for further details . ]
alternatively , we can assume that the collisional gas settles in a disk .
we idealize the disk as an infinitely flat disk of gas particles / cloudlets , inclined by 60@xmath18 to the line of sight , and in circular rotation in the same combined bh and stellar potential as used in the spherical case . on the nucleus
, the differential rotation will broaden the emission - line flux profile .
[ figure3]a shows that the predicted gas velocity dispersion @xmath0(disk ) roughly agrees with @xmath2 .
hence no shocks / turbulence are required on average ; a similar result as for the gas spherical model .
gas dynamical models of galactic nuclei , based on gas mean velocity @xmath17 instead of @xmath0 often reach a different conclusion . for five sample galaxies ,
the central bh mass has been measured directly from gas mean velocities @xmath17 in extended gas disks . fig .
[ figure3]b shows that the velocity dispersion @xmath19 predicted by our gas disk model for _ these _ bh masses underpredicts @xmath0 in four cases .
hence , an additional contributor to the velocity dispersion is needed to explain the observed values in our gas disk model .
this agrees with the mean velocity modeling which invokes an additional source of dispersion , gas turbulence , to explain @xmath0 ( maciejewski & binney 2001 ; van der marel & van den bosch 1998 ; cappellari et al .
2002 ; verdoes kleijn et al .
2002 ) . only for ngc 4486
does a detailed gas disk modeling by macchetto et al .
( 1997 ) account for both @xmath17 and @xmath0 , consistent with our modeling .
it is reassuring that our simplistic modeling is consistent with the more detailed gas dynamical models on the difference between ngc 4486 and the other four galaxies .
it is less reassuring that fig .  [ figure3]b suggests that a thin , circular gas disk model in general can not explain simultaneously observed gas velocities and dispersions . for a simultaneous fit one
has to invoke gas turbulence .
alternatively , the extended gas might somehow rotate at subcircular velocities , which would lead the models to infer a lower bh mass and hence lower nuclear differential rotation .
qualitatively , fig .
[ figure2 ] suggests another possible solution to this problem : the gas changes from a flat disk configuration outside the nucleus to a ( more ) spherical distribution closer to the nucleus .
the nuclear spherical gas distribution explains the central @xmath0 .
furthermore , the new configuration implies that ( i ) the flat gas disk effectively has an inner radius and ( ii ) only part of the observed nuclear emission - line flux is coming from a disk .
both changes put the gas associated with the circular disk at a larger mean distance from the bh , compared to the continous disk model , and hence at lower circular velocities for the same bh mass .
the simple models discussed above suggest that the observed excess in velocity dispersion in nuclear emission - line gas disks could be explained by a more spherical gas distribution towards the nucleus .
in such a model , the gas remains collisionless ( as in the customary single gas disk model ) and thus avoids invoking ad hoc turbulence or shocks which require a permanent input of kinetic energy to persist .
nevertheless , it could well be that activity associated with the bh can provide such a continuous input of energy .
the outliers in figs .
[ figure2 ] &  [ figure3 ] and the tendency for the collisionless models to underpredict @xmath0 more for active galaxies rather than for quiescent galaxies perhaps favor this scenario .
more sophisticated modeling and further observations can help to determine between different gas configurations .
on the one hand , more accurate modeling of the stellar mass density and the emission - line velocity profiles are expected to decrease the predicted @xmath2 at fixed bh mass .
the central density profile of bright ellipticals is generally shallower than an isothermal distribution .
emission - lines often show extended wings : the true 2nd moment is then larger than the gaussian 2nd moment .
this effect is taken into account in the gas disk model , but not in the spherical model .
on the other hand , emission - line flux profiles are typically marginally resolved .
they may contain a significant contribution from an unresolved component which can be arbitrarily close to the bh .
this might increase the predicted @xmath0 .
uv spectroscopy can determine directly if shocks are present .
in the presence of shocks the gas can be shock - ionized .
if absent , the emission - line gas is only photo - ionized .
the two ionization mechanisms produce different uv emission - line ratios ( e.g. , dopita et al .
1997 ) .    in summary
, the current results can not distinguish between the various gas configurations .
however , accurate bh mass measurements ( and hence bh demography ) and bh accretion models depend on such a distinction .
improvement of the simplistic gas dynamical modeling presented here and uv observations can help to resolve this .
the majority of nearby early - type galaxies contains detectable amounts of emission - line gas at their centers . 
 the emission - line ratios and gas kinematics potentially form a valuable diagnostic of the nuclear activity and gravitational potential well . 
 the observed central gas velocity dispersion often exceeds the stellar velocity dispersion . 
 this could be due to either the gravitational potential of a black hole or turbulent shocks in the gas . 
 here we try to discriminate between these two scenarios .    
 [ 1996/06/01 ]    a&a