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dynamical studies have established the existence of supermassive black holes ( bhs ) with masses @xmath6  @xmath7  in the centers of most , if not all , local galaxies with bulges ( magorrian et al .
1998 ; richstone 2004 ) .
a significant challenge to any model of cosmological bh growth is the nature of seed bhs .
observations of intermediate - mass bhs ( @xmath8  @xmath7 ) in the local universe would provide the most direct empirical constraints on such seeds . their number and mass distribution will impact the expected integrated background in gravitational radiation ( e.g. , hughes 2002 ) .
intermediate - mass bhs have practical value in that they offer tremendous leverage in anchoring local bh - galaxy scaling correlations such as the @xmath5  relation .
recently discovered intermediate - mass bh candidates in galactic nuclei offer tantalizing hints as to the nature of this population .
ngc 4395 is a very late - type ( sdm ) spiral with no bulge component .
nevertheless , it has the emission properties of a type 1 active galactic nucleus ( agn ; filippenko & sargent 1989 ; filippenko , ho , & sargent 1993 ) .
mass estimates based on the h@xmath9  linewidth - luminosity - mass scaling relation and x - ray variability suggest a bh mass of @xmath10  @xmath7 .
interestingly , these agree with the limit of @xmath11  @xmath7  derived from the @xmath5  relation ( filippenko & ho 2003 ; tremaine et  al . 2002 ) .
pox 52 , whose optical spectrum is remarkably similar to that of ngc 4395 , has a dwarf elliptical host ( barth et al .
the galaxy has a central velocity dispersion of 36  km s@xmath12 , which yields a mass estimate of @xmath13  @xmath7 , again consistent with the value of @xmath14 @xmath7  derived from the h@xmath9linewidth - luminosity - mass scaling relation .
ngc 4395 and pox 52 fall on the @xmath5  relation while their hosts are anomalous compared to the local agn population ( ho , filippenko , & sargent , 1997a , 2003 ; kauffmann et al . 2003 ) , which is invariably affiliated with massive , bulge - containing galaxies .
unfortunately , it is currently technically impossible to obtain direct mass measurements for @xmath15 @xmath7 , because of our inability to resolve the bh sphere of influence for all but the nearest galaxies in the local group ( e.g. , m33 ; gebhardt et al . 2001 ) .
thus , the best hope to find intermediate - mass bhs is through an agn survey . however , it may be that objects like ngc 4395 are quite rare . to accumulate
a significant sample will require a large survey .
also , both the radiative signatures of accretion and the host galaxies themselves will be intrinsically faint . as a result ,
detection of the faint agn will be limited by sensitivity and host galaxy contamination .
-0.1 mm    -3.mm    a large - area , sensitive , and uniform optical spectroscopic galaxy survey , such as the sloan digital sky survey ( sdss ; york et  al .
2000 ) , offers the best opportunity for finding a significant number of new intermediate - mass bh candidates .
we present our initial sample from the sdss first data release ( dr1 ; abazajian et  al .
2003 ) .
we begin with the 153,000 galaxy and quasar spectra from the dr1 .
we use the principal component analysis code of hao ( 2004 ) to model and remove contaminating starlight .
we then select all broad - line agn with @xmath16 based on the presence of broad h@xmath17  emission ( ho et  al .
we estimate bh `` virial '' masses using a variant of the empirically determined linewidth - luminosity - mass scaling relation of kaspi et al .
( 2000 ) , which relates @xmath18 , where @xmath19 is the agn luminosity at 5100   and @xmath20 is the fwhm of ( typically ) broad h@xmath9 . to determine @xmath21 we use the stronger h@xmath17  as a surrogate . to properly deconvolve the narrow components of the h@xmath17+[nii ]  complex we model the [ s  ii ] @xmath226716 , 6731 doublet with a superposition of gaussian components and
then scale this model to fit the complex ( ho et al .
the continuum luminosity is derived from a power - law fit ( see fig .
[ obj5 ] ) . from the sample of objects with @xmath23@xmath24  @xmath7  we
remove all objects with a stellar continuum @xmath25 of the total continuum , due to unreliable h@xmath17  fwhm measurements .
the final sample of 19 objects has a mass distribution shown in fig .
[ bhmass ] ( for details see greene & ho 2004 ) .
-0.1 mm    -3 mm
our objects are analogous to the class of agns known as narrow - line seyfert 1s ( nls1 ) , which are thought to possess relatively low - mass bhs emitting at a high fraction of their eddington rates ( e.g. , boroson 2002 ) . by the linewidth criterion ,
all of the objects would technically qualify as `` narrow - line '' sources : the broad h@xmath17  component has fwhm ranging from 464 to 1730 km s@xmath12 . in keeping with expectation the derived eddington ratios mostly cluster around @xmath26 ( fig .
[ bhmass ] ) .
we caution that these values were derived assuming a single bolometric correction of @xmath27 = 9.8@xmath28  ( mclure & dunlop 2004 ) although the spectral energy distributions of agns are known to vary significantly with accretion rate ( pounds et al .
1995 ; ho 1999 ) .
however , our sample shows a greater diversity of fe  ii and [ o  iii ] strengths than in previous samples of nls1s .    1.7 in    -1.mm    -3.mm    the morphology of the host galaxies of intermediate - mass bhs is of fundamental importance for understanding the origin of this class of objects and their relationship to the overall demography of central bhs in galaxies . while we can not obtain detailed morphology from the sdss data , the host absolute magnitudes ( fig .
[ maghist ] ) are relatively low - luminosity and therefore more likely to be late - type galaxies ( c.f .
ngc 4395 and pox 52 above ) .
more detailed morphological information requires deep , high - resolution imaging to disentangle a ( presumably tiny ) bulge component .
we employ a simple technique to exploit the breadth of the sdss galaxy spectroscopy to extend the demography of central bhs at least 1 order of magnitude below the @xmath29 @xmath7  threshold , a regime hardly explored previously .
we hope to use the width of narrow emission lines as a proxy for stellar velocity dispersion ( e.g. , nelson & whittle 1996 ) to select additional candidates .
preliminary stellar velocity dispersions using the esi spectrograph at keck yielded 12 agns with @xmath30  @xmath31 70 km s@xmath12 . as fig .
[ msigma ] shows , the objects with velocity dispersions appear to fall on the extrapolation of the local @xmath5  relation .
the complete data set will test whether the bh mass estimates we derive obey the @xmath5  relation and the reliability of the virial mass estimator for agns .
it would also be valuable to compare the velocity dispersions derived from stars with those derived from gas .
high - resolution , deep imaging is needed to quantify the morphologies and detailed structural parameters of the host galaxies . do these objects have bulges , and if so , what kind and where do they fall on galaxy scaling relations like the fundamental plane ? finally , much work remains to be done , especially at non - optical wavelengths , to further characterize the properties of the agns themselves .
j.  e.  g. acknowledges a aas international travel grant .
we are grateful to lei hao for making available her pca software , and to the entire sdss collaboration for providing the extraordinary database and processing tools that made this work possible .
we present the first homogeneous sample of intermediate - mass black hole candidates in active galactic nuclei . starting with broad - line active nuclei from the sloan digital sky survey 
 , we use the linewidth - luminosity - mass scaling relation to select a sample of 19 galaxies in the mass range @xmath0 . 
 in contrast to the local active galaxy population , the host galaxies are @xmath1 mag fainter than @xmath2 and thus are probably late - type systems . 
 the active nuclei are also faint , with @xmath3 to @xmath4 mag , while the bolometric luminosities are close to the eddington limit . the spectral properties of the sample are compared to the related class of objects known as narrow - line seyfert 1 galaxies . 
 we discuss the importance of our sample as observational analogues of primordial black holes , contributors to the integrated signal for future gravitational wave experiments , and as a valuable tool in the calibration of the @xmath5  relation .