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the standard model of an active galactic nucleus ( agn ) consists of a supermassive black hole being fed via an accretion disk . the model also postulates the presence of a dusty molecular torus surrounding this inner region , and blocking it from view in agns where the line - of - sight intercepts the torus @xcite .    for several years , the only plausible signature of the accretion disk was the `` big blue bump , '' observed in the ultraviolet ( uv ) spectra of active galaxies , which was supposed to be the thermal emission from the disk @xcite . a more telling feature , such as the kinematic signature of rotating gas , in the form of double - peaked line emission as observed in cataclysmic variables , was only discovered in optical spectra in the 1980s @xcite , and systematic searches were later published by @xcite and by @xcite . broad , double - peaked emission lines from the nucleus of ngc1097 were reported by @xcite , and similar lines were found in other nearby galaxies with the _ hubble space telescope ( hst ) _ @xcite . ngc1097 is particularly interesting because the double - peaked line originates in a low - luminosity agn and is a transient phenomenon , as it was not seen in previous observations . these characteristics indicate that we would have a good chance of observing temporal variations in a reasonable ( not too long ) time interval . indeed , monitoring of the double - peaked emission over the years 19922001 @xcite has put further constraints on the accretion - disk model and provided evidence against other possible origins for the double - peaked emission in this particular object , such as double jets and a binary black hole @xcite .    in this _ letter _ , we report the presence of absorption features in small - aperture @xmath7 uv spectra of ngc1097 , and argue that they are characteristic of young o and b stars and of a low - ionization interstellar medium , all within a radius of at most 9pc from the nucleus . such dimensions are typical of those of star clusters and also of those expected for obscuring tori in the standard model of agns . besides the absorption features , the uv continuum also reveals the spectral energy distribution of a young starburst extinguished by a dusty medium . uv and optical spectra of ngc1097 were obtained with the space telescope imaging spectrograph ( stis ) aboard @xmath7 on 2001 february 7 and 11 ut , using a @xmath8 slit and the g140l , g230l , g430l , and g750l gratings . these gratings have resolving powers @xmath9 , which allow the separation of absorption features in the galaxy from those in the milky way . the exposure times were 8302s for each of the two uv gratings , 1008s for grating g430l , and 936s for grating g750l . in order to obtain the best possible angular resolution achievable with the observations , we extracted a nuclear spectrum within a window of @xmath10 , corresponding to a 9pc radius region centered on the nucleus . we then combined the various spectral segments to construct one uv / optical spectrum spanning the spectral range 11009000   . the combined spectrum was corrected for the foreground reddening @xmath11 mag from @xcite . the spectrum , in the rest frame of the galaxy , is shown in figure [ fig1 ] , where we have edited out the geocoronal ly@xmath12 and oi@xmath131302.08 emission lines . the main features readily observed in this spectrum are a `` hump '' in the uv continuum peaking near 2500   , and several double - peaked emission lines , identified in the figure : mgii@xmath132800 , h@xmath14 , hei@xmath135876 , and h@xmath12 . broad components are also observed in ly@xmath12 , civ@xmath131550 , and ciii]@xmath131909 , although the double - peaked nature is not clear because of the lower signal - to - noise ratio . the uv spectra of nuclei with broad double - peaked emission lines ( hereafter dubbed `` doublepeakers '' ) show a number of common absorption features in the spectral range 20003000   , mostly metastable feii absorption lines @xcite . the absorption spectrum of ngc1097 in the above spectral range is similar to that of the prototypical doublepeaker , arp102b @xcite , which @xcite have proposed to originate in thin sheets or filaments embedded in an outflowing wind that overlays the accretion disk ( and thus is close to it , within roughly 5000 gravitational radii ) . @xcite propose that many doublepeakers may have these filaments , and ngc1097 probably also fits within this scenario . here we will focus on the range 11001600  of the nuclear spectrum of ngc1097 . in this spectral range , ngc1097 shows a number of absorption lines , some of which are superimposed on the broad ly@xmath12 and civ emission lines , as illustrated in the top panel of figure [ fig2 ] . the absorption features are identified at the bottom of the figure . we include in this figure , for comparison , the stis spectrum of the seyfert galaxy akn564 ( representative of seyfert galaxy spectra ) and of the starburst nuclei of ngc7552 and ngc5253 , all in the rest frame of the galaxies . upon inspection of the uv spectra of many other seyferts and starbursts , we conclude that the absorption features in the uv spectrum of ngc1097 are not common in agns , but are frequently observed in the spectra of starburst galaxies . we also include in figure [ fig2 ] a synthetic spectrum of a starburst obtained using the code starburst99 @xcite , with the following parameters : instantaneous burst of mass 10@xmath15 , age 10@xmath2yr , salpeter initial mass function ( imf ) , lower and upper limits for the imf of @xmath16 and @xmath17 ( respectively ) , and solar metallicity . the spectra of figure [ fig2 ] have been normalized at 1350  to allow a better comparison among them , and have been shifted vertically for clarity .    in order to further emphasize the similarity of the nuclear uv spectrum of ngc1097 with those of starbursts , we show in the lower panels of figure [ fig2 ] the ngc1097 spectra on top of those of ngc7552 and ngc5253 . we decided to keep in the spectra the geocoronal ly@xmath12 and o  i @xmath131302 emission lines , in order to avoid introducing artificial discrepancies . the following stellar absorption features , characteristic of the spectra of early - type stars ( o , b ) , are observed in the spectra of ngc1097 , ngc7552 , and ngc5253 @xcite : ciii@xmath131175.7 , nv@xmath181238.8,1242.8 , siiv@xmath181393.8,1402.8 , and civ@xmath181548.2,1550.8 . the last three are resonance lines and can also originate in the interstellar medium . other interstellar lines present in the spectra are siii@xmath181190.4 , 1193.3 , ni@xmath131199.9 , siii@xmath181259.5,1260.4 , oi@xmath131302.1 , and siii@xmath131304.4 ( the last two partially filled by the geocoronal o  i emission line ) , cii@xmath181334.5,1335.7 , and siii@xmath131526.7 . there are also in the nuclear spectrum of ngc1097 a few absorption lines from our galaxy , but which can be separated from the absorption lines of ngc1097 because of the redshift of the latter . among the stellar absorption features , only ciii@xmath131175 is not a resonance line and must originate in the atmosphere of young stars . this line is sharp and deep in the starburst synthetic spectrum , but is broader and shallower in the spectra of the actual starbursts ngc5253 and ngc7552 . in the ngc1097 spectrum , it has a similar profile to those observed in the starburst galaxies , as can be seen in fig . the presence of young stars is also supported by the p - cygni profile of the line nv@xmath131240 , which can be clearly observed in ngc1097 , ngc5253 and in the synthetic starburst spectra , although only a hint of it appears in the ngc7552 spectrum . notice , in particular , the similarity between the emission portion of this line in ngc1097 and ngc5253 ( fig [ fig2 ] ) .    a p - cygni profile is also observed in the civ@xmath131550 line of the ngc5253 and synthetic starburst spectra , but not in ngc7552 . in ngc1097 , there is only a hint of a p - cygni profile in the civ line , superimposed on the broad emission - line profile . the large depth of the absorption , which reaches practically zero flux , indicates that this line probably originates not exclusively in the stellar atmospheres of young stars . a large depth is also observed in the mgii@xmath132800 absorption . these two deep absorption lines suggest that some of the absorbing gas is along our line of sight to the agn . we thus conclude that at least part of the absorption must come from a region very close to the agn . the absorber could be the starburst superwind whose filaments cross our line of sight to the nucleus [ e.g. @xcite ] , or filaments embedded in an accretion - disk wind [ as is the case favored for the other double peakers @xcite ] , or both . is the spectral energy distribution ( sed ) compatible with the presence of a young starburst in the nucleus of ngc1097 ? in order to answer this question , we have used the code starburst99 @xcite to construct model seds of continuous and single bursts of star formation of different ages . inspection of the nuclear continuum of ngc1097 in figure [ fig1 ] shows that the flux increases from the optical to the uv , as expected for a young starburst , but then , at @xmath192500 , decreases to shorter wavelengths . the only way to have this flux decrease in the uv is through reddening . we have tried different combinations of starburst models and reddening laws . for the starbursts , we have also experimented with different slopes for the initial mass function ( imf ) , and our conclusions are as follows . continuous bursts provide too much flux in the optical , as do single bursts older than a few times 10@xmath2  yr . when combined with any of the known reddening laws , they fail to produce the observed shape of the uv sed . the most suitable starburst model is the one of a single burst of age 10@xmath2yr . the seds of younger bursts or bursts with different imfs are only slightly different from that of a single burst of age 10@xmath2  yr , and do not provide significant improvement to the fit of the observed spectrum . we have also concluded that models including the contribution of the gas produce a strong balmer jump in emission which is not observed in the data . the best fit to the ngc1097 continuum we could obtain from the available synthetic spectra includes only the stellar contribution to the continuum .    in order to produce the observed flux decrease in the uv , we have tried the calzetti reddening law @xcite , the milky way ( mw ) reddening law @xcite , the large magellanic cloud ( lmc ) reddening law @xcite , and the small magellanic cloud ( smc ) reddening law @xcite . the calzetti reddening law did not have enough uv extinction to reproduce the data , both the mw and lmc laws introduced the characteristic 2200    absorption feature which is not present in the data , while the smc reddening law produced better agreement with the data for @xmath20 mag and @xmath21 . although it is surprising that the best reddening law is that of the smc ( as ngc1097 is a much more luminous and a more metal - rich galaxy than the smc ) , we point out that extinction by an `` smc - like '' reddening law has also been found to best explain the seds of red quasars in the sdss quasar survey @xcite . not only the sed but also the flux of the reddened 10@xmath2yr starburst is in approximate agreement with the observations , indicating that the mass of the starburst is roughly the one adopted in the model , 10@xmath22 . this is illustrated in figure [ fig3 ] , where we show the ngc1097 spectrum compared to the reddened starburst sed . an inset in the figure shows the sed of the unreddened starburst in comparison with the galaxy spectrum in order to illustrate the effect of reddening . figure [ fig3 ] also shows the contribution of the continuum emitted by the agn , borrowed from a paper in preparation @xcite , but with preliminary results already published @xcite . in @xcite the nuclear sed  from x - ray to radio wavelengths  is found to be well reproduced by a model consisting of the emission from a radiatively - inefficient accretion flow ( riaf ) plus that of a thin accretion disk . although this model produces a good overall description of the sed , it does not reproduce the excess in the uv , as illustrated in figure [ fig3 ] . we point out that in @xcite , the thin - disk emission is self - consistently derived from the incidence of the riaf radiation on the disk . the thin - disk continuum emission peaks in the optical / infrared , and can not reproduce the excess uv emission . although the introduction of a reddened starburst continuum can approximately reproduce the uv and its decrease to shorter wavelengths , there is still a small remaining `` bump '' around 2500  which is not reproduced by the riaf + thin disk + reddened starburst ( fig . [ fig3 ] ) . in this wavelength region there is a blend of fe  ii emission lines which is frequently present in the uv spectra of seyfert  1 galaxies and quasars @xcite .    in order to investigate the possibility that the above bump is due to the blend of fe lines , we have used an archival spectrum of i  zw  1 ( kindly provided to us by karen leighly ) as a template for the fe emission . although this template covers only the 20003000  spectral region , it includes the most intense feii emission lines . [ the contribution of feii lines in the optical is much smaller  with peak intensities @xmath1910% of that in the uv @xcite . detailed modelling of the optical feii emission is beyond the scope of this work . ]    under the assumption that the feii emission is coming from the accretion disk , we have convolved the template with the double - peaked profile of the emission - lines . combining this component with the others , we could successfully reproduce the remaining bump around 2500 . this is also illustrated in fig . [ fig3 ] , where we further include the feii template and a model ( thick line ) for the ngc1097 continuum comprising all the components : the reddened starburst , the riaf + thin disk model and the feii template , properly scaled to fit the data . the reddened starburst model was scaled by 0.85 , corresponding to a uv + optical flux of 1.7@xmath110@xmath23ergs@xmath24s@xmath25 and luminosity of 1.5@xmath110@xmath5l@xmath4 for an adopted distance to ngc1097 of 17mpc .    finally , we point out that it is not our goal to produce a detailed fit to the spectrum , as there are many uncertainties . for example , the reddening law may be somewhat different from that of the smc , and the starburst properties may also differ from those of the starburst99 model . we have shown evidence for an obscured starburst located within 9  pc of the active nucleus of the galaxy ngc1097 . the importance of this finding is two - fold : ( 1 ) the proximity of a young starburst to the nucleus and thus possible association with it ; and ( 2 ) the fact that the starburst is obscured , but the agn is not . the maximum distance between the starburst and the nucleus of ngc1097 ( 9  pc ) is comparable to the sizes of star clusters with masses @xmath19 10@xmath15 @xcite ; hence , the cluster is quite close to the nuclear black hole @xcite . the disruption of a cluster star passing closer to the black hole than its tidal radius could be the origin of the transient accretion disk , as we have proposed in @xcite . flares in the uv @xcite and x - rays @xcite suggest that we are beginning to witness such events . alternatively , mass loss from evolving stars in the cluster could be the origin of the accretion disk . in any case , we may be seeing a rare example of the `` starburst - agn connection ''  the starburst is responsible for feeding the agn !    another interesting characteristic of this starburst is that it is immersed in an obscuring medium . its maximum size is also on the order of that predicted for the torus of the `` unified model '' of agns @xcite . although we can not prove that the geometry is that of a circumnuclear torus , there is no evidence that the nuclear source is obscured , as we can observe the riaf + thin - disk continua and the double - peaked emission - lines with no signs of obscuration ; indeed , the mgii line in the uv is the strongest double - peaked line of the spectrum . thus , the starburst _ may be _ within a dusty toroidal structure which leaves the nuclear source unobscured . if this is the case , we are observing , for the first time , a starburst inside a torus , as has been proposed in the literature as a solution for the stability problem of the torus : the toroidal structure is supported by the pressure produced by the evolving starburst inside @xcite .    finally , we point out that the starburst reddening , @xmath20 mag , is not very high . an additional reddening of 23 mag would render the starburst undetectable . since higher reddening values have been found in the nuclear regions of many agns , our results show that starbursts may be present , but most will not be detected , at least in the uv . we acknowledge valuable discussions with e. bica , c. bonatto , g. kriss , c. leitherer , and p. hall . we thank k. leighly for providing the feii spectrum and the referee for valuable suggestions . this work was supported by nasa grant go-08684 from the space telescope science institute , which is operated by aura , inc . , under nasa contract nas 5 - 26555 , and the brazilian institutions cnpq , capes , and fapergs . filippenko is grateful for a miller research professorship at u.c . berkeley , and eracleous acknowledges support from nasa ltsa grant nag5 - 10817 . antonucci , r. 1993 , , 31 , 473 arp , h. c. 1976 , , 207 , l147 baldwin , j. a. , et al . 2004 , , 615 , 610 barth , a. j. , et al . 2001 , , 546 , 205 bouchet , p. , et al . 1985 , a&a 149 , 330 bower , g. a. , wilson , a. s. , heckman , t. m. , & richstone , d. o. 1996 , , 111 , 1901 calzetti , d. , kinney , a. l. , & storchi - bergmann , t. 1994 , , 429 , 582 calzetti , d. , et al . 1997 , , 114 , 1834 cardelli , j. a. , clayton , g. c. , & mathis , j. s. 1989 , , 345 , 245 chen , k. , halpern , j. , & filippenko , a. v. 1989 , , 339 , 742 cid fernandes , r. , & terlevich , r. 1995 , , 272 , 423 eracleous , m. 2002 , in mass outflows in agns , eds . m. crenshaw et al . 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we report evidence for a recent burst of star formation located within 9pc of the active nucleus of ngc1097 . 
 the observational signatures of the starburst include uv absorption lines and continuum emission from young stars observed in a small - aperture _ hst _ spectrum . 
 the burst is @xmath0 a few @xmath1 10@xmath2yr old , has a mass of @xmath3m@xmath4 , an observed luminosity of 1.5 @xmath1 10@xmath5 l@xmath4 and is obscured by @xmath6 mag . 
 the importance of this finding is two - fold : ( 1 ) the proximity of the starburst to the active nucleus and thus possible association with it ; ( 2 ) its obscuration by and apparent association with a dusty absorbing medium , while the broad emission lines appear unobscured , suggesting that the starburst could be embedded in a circumnuclear torus as predicted in the unified model of active galactic nuclei .