document: low - luminosity active galactic nuclei ( llagns ) , operationally defined as a h@xmath0 luminosity of @xmath1 , reside in about 40  per  cent of nearby bright galaxies , and occupy majority of the population of agns @xcite . 
   its low luminosities are thought to be caused by very low accretion rate on to super - massive black holes . 
   such accretion has often been explained by the model of optically - thin disc such as an advection - dominated accretion flow ( adaf ; @xcite ) , rather than optically - thick disc ( ` standard disc ' ; ) . 
   it is predicted that the adaf has a ` submillimetre bump ' in its spectral - energy distribution ( sed ) , while the standard disc is responsible for a big - blue bump in the sed of luminous agns such as quasars . since a high brightness temperature of @xmath2  k in radio bands 
   is expected from the adaf model , the first imaging investigations for accretion discs must be promising with future very - long - baseline - interferometry ( vlbi ) instruments that will provide micro - arcsec resolutions @xcite . 
   therefore , the observational evidence of the adaf is crucial in the llagn radio sources . 
   although the adaf model successfully explains the broadband spectra of llagns , there is a significant gap between observations and the adaf spectrum especially at low - frequency radio bands ( e.g. , @xcite ) . 
   this indicates that additional unknown emission does exist , and putative emission from the accretion disc may be buried under it . 
   the submillimetre bump means a highly inverted spectrum at centimetre - to - submillimetre wavelengths of spectral index @xmath3 , where @xmath4 , @xmath5 is flux density at frequency @xmath6 @xcite 
   . observations for llagns have been carried out exclusively at centimetre bands where high sensitivities are available , because most of llagns are very faint radio sources . 
   about half of low - luminosity seyfert galaxies and low - ionization nuclear emission - line regions ( liners ; @xcite ) hold a compact radio core @xcite , and at least 25 per cent of transition objects , which are hypothesized to be liner/-nucleus composite systems @xcite , also hold a compact core @xcite , at 15 or 8.4  ghz in 0.152.5 arcsec resolutions . most of the cores show nearly flat spectra ( @xmath7 ) in 0.5-arcsec resolution : that is evidence for jet domination @xcite . 
   although slightly inverted spectra have been found in unresolved compact cores of several llagns in milli - arcsec resolutions of vlbis , their radio - to - x - ray luminosity ratios suggest that another significant radio component seems to contribute in addition to original fluxes from the adaf model @xcite . 
   to explain this radio excess , an jet  adaf model has been proposed @xcite . 
   thus , at centimetre bands , the contamination that is believed to be from jets has prevented the adaf from being revealed .    in the present paper , 
   we report millimetre survey and submillimetre data analyses for many llagns . 
   although technically difficult , a high - frequency observation is very promising to detect the inverted spectrum of the adaf because of following two advantages : ( 1 ) spectral measurements at high frequencies are less affected by non - thermal jets , which generally show power - law spectra ( @xmath8@xmath9 ) , and ( 2 ) the adaf predicts larger flux densities at higher frequencies . 
   in fact , it has been obsevationally confirmed that flux densities at millimetre wavelengths are 510 times larger than those at centimetre wavelengths in sgr  a * ( e.g. @xcite ) , which is the nearest low - accretion rate system from us . 
   however , contamination from diffuse dust may be harmful at @xmath10  ghz when we use a poor spatial resolution . 
   therefore , the use of a beam as small as possible and the estimations of every conceivable contamination are essential . 
   the present paper is structured as follows . 
   sample selection is described in section  [ section : sample ] . 
   observations and data reduction are described in section  [ section : obs&reduction ] . in section 
   [ section : results ] , we report these results and the estimations of diffuse contamination in our millimetre measurements . 
   our results of submillimetre photometry are utilized only for estimation for dust contribution . 
   the origins of the spectra are discussed in section  [ section : discussion ] from the relation between the high - frequency radio spectrum and low - frequency radio core power . 
   finally , we summarize in section  [ section : summary ] . 
   our sample , ` vlbi - detected llagn sample ' , covers 20 out of 25 all - known llagn radio sources that have been detected with vlbis ( column [ 1 ] of table  [ table1 ] ) . 
   the other five llagns are ngc  4235 , ngc  4450 @xcite , ngc  524 , ngc  5354 , and ngc  5846 @xcite , which were recently newly vlbi - detected , but whose reports had not yet been published when we plan the survey . 
   it is difficult to estimate resultant selection bias including into our sample , because of multiplicity in selection criteria across the past vlbi surveys . 
   however , at least we can say that 16 out of 20 sources of our sample are all known llagns that are at @xmath11  mpc with @xmath12  mjy at 15  ghz , and the other four sources ( ngc 266 , ngc 3147 , ngc 3226 , and ngc 4772 ) are more distant . 
   it may be , therefore , as a moderately good distance - limited and radio - flux - limited sample for llagns with a compact radio core . 
   we made millimetre continuum observations for 16 out of 20 targets using the nobeyama millimetre array ( nma ) , at the nobeyama radio observatory ( nro ) , with d configuration that is the most compact array configuration . 
   we had precluded the other four sources from our observation list because they had already been observed at @xmath13  mm in the past ( ngc  266 , @xcite ; ngc  3031 , @xcite ; ngc  4258 , doi  et  al . in prep . ; ngc  4486 , @xcite ) . 
   our campaign spent more than 20 observational days between 2002 november  28 and 2003 may  25 . 
   most of weak targets were observed over several days for integration . 
   visibility data were obtained with double - sided - band receiving system at centre frequencies of 89.725 and 101.725 ghz , which were doppler - tracked . 
   we used the ultra wide - band correlator ( uwbc ; * ? ? ? 
   * ) , which can process a bandwidth of 1  ghz per each sideband , i.e. , 2  ghz in total . the wide bandwidth gives the nma a very high sensitivity for continuum observations . 
   even if the systemic velocities of our targets and galaxies rotations of several hundred km  s@xmath14 are accounted for , the observing band can avoid possible contaminations from several significant line emissions : @xmath15co(@xmath16 ) , c@xmath17o(@xmath16 ) , hcn(@xmath16 ) , hco@xmath18(@xmath16 ) , and sio(@xmath19 ) . 
   a system noise temperature , @xmath20 , was typically about 150  k in the double - sided bands . for observations of each target , we made scans of a reference calibrator close to the target every 20 or 25  minutes for gain calibration . 
   bandpass calibrators of very bright quasars were scanned once a day . 
   the data were reduced using the uvproc - ii package @xcite , developed at the nro , by standard manners , including flagging bad data , baseline correction , opacity correction , bandpass calibration and gain calibration . to achieve higher sensitivity , visibilities of both 
   the sidebands were combined in the same weight , which resulted in a centre frequency of 95.725 ghz . 
   each daily image was individually made in natural weighting and deconvolved using the aips software , developed at the national radio astronomy observatory ( nrao ) . 
   statistically significant variability was detected from the daily images in several llagns , which will be reported in a separate paper . in the present paper , 
   the visibilities of all days were combined by weighting with @xmath21 , and then imaged . 
   the half - power beam widths ( hpbws ) of synthesized beams were typically @xmath22  arcsec , corresponding to @xmath23660  pc at a mean distance of 18.9  mpc in our sample . 
   the flux scales of the gain calibrators were derived with the uncertainty to 10  per  cent by relative comparisons to the flux - known calibrators , such as uranus , neptune , or mars . 
   these were scanned quasi - simultaneously when they were at almost the same elevations . 
   the rms of noise on the images were estimated from statistics on off - source blank sky with the imean task of the aips . 
   source identifications and measurements of flux densities were done on image domain by elliptical gaussian profile fitting with the jmfit task of the aips . 
   we derived total errors in the flux measurements from root sum square of the errors in the gaussian fitting ( including thermal noise ) and the flux scaling . 
   we attempted to measure flux densities at submillimetre band from the nuclear regions using archival data . 
   we searched the data that had been obtained at 347.38 ghz ( @xmath24 ) using submillimetre common user bolometer array ( scuba ; @xcite ) on the james clerk maxwell telescope ( jcmt ) . 
   the data of 14 out of 20 objects of the vlbi - detected sample were available to the public on 2004 . 
   the jcmt measurements for five objects have already been reported ( ngc 266 , @xcite ; ngc 4258 , doi et  al . in  prep . 
   ; ngc 4374 , @xcite ; ngc 4472 , @xcite ; ngc 4486 , @xcite ) . 
   four objects had been observed in jiggle - map mode . 
   we reduced these data using the scuba user reduction facility ( surf ) package . 
   standard reduction procedures were used , including flat fielding , flagging of transient spikes , correction for extinction , pointing correction , sky removal and flux - density scaling . 
   flux calibrators ( uranus , mars , or crl  618 ) gave us flux scaling factors ( we assumed its uncertainty to @xmath2315  per  cent ) and an effective beam size of 15.1  arcsec . 
   the derivations of the rms of noise on images , source identifications , measurements of flux densities , and total error estimations were done in the same manners as the nma reductions described in section  [ subsection:3 mm ] . 
   the other five objects had been observed in photometry mode . 
   we followed standard reduction procedures , including flat fielding , flagging of transient spikes , correction for extinction , sky removal , averaging , and flux - density scaling , using the surf package . 
   the photometry can measure a flux density from a beam - size region at the nucleus , i.e. , an intensity . 
   we derived total errors in flux measurements from root sum square of the errors of the image noise and the flux scale . 
   [ cols="<,>,^,^,>,>,^,^,^ " , ]     [ table4 ]    note . 
   column are : * ( 1 ) * galaxy name ; * ( 2 ) * core power detected in vlbi observations at 5 ghz , except for ngc  4258 at 22  ghz ; * ( 3 ) * presence of jet feature in the vlbi image . ` s ' represents sub - pc - scale jets . 
   ` p ' represents pc - scale jets ; * ( 4 ) * reference for the vlbi observation , as listed below ; * ( 5 ) * presence of jet feature in vla image . ` 
   k ' represents kpc - scale jets ; * ( 6 ) * reference for the vla observation , as listed below ; * ( 7 ) * spectral category , the same as table  [ table3 ] . 
   the sources showing steep spectra have exclusively high vlbi - core powers , @xmath25  w  hz@xmath14 .    note . 
   reference : * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite ; * * : @xcite    ) . 
   the dot - dashed line represents the radio power limit that the adaf on a black hole with @xmath26  m@xmath27 can generate . ] 
   we have suggested in the previous section that jets and the adaf seem to coexist in llagns . 
   however , only an unresolved structure has been found in milli - arcsec images for most of the vlbi - detected llagns @xcite . on the other hand , 
   pc / sub - pc - scale jet structures have been found in several llagns . 
   we find from the core power ranking in table  [ table4 ] that the presence of vlbi - detectable pc - scale jets depends on the core radio powers . 
   the pc - scale jets have been found exclusively in high - core - power sources . in low - core - power sources , 
   no pc - scale jet has been detected in vlbi images even with moderately high dynamic ranges of @xmath28 @xcite . 
   can not the low - core - power sources generate high - brightness jets ? 
   the cases of ngc  3031 and ngc  4258 are very suggestive . 
   sub - pc - scale jets have been revealed in their nuclei ( e.g. , * ? ? ? 
   * ; * ? ? ? 
   * ) , nevertheless , they are low - core - power llagns . 
   note that the two llagns are the nearest two sources in the vlbi - detected sample ( table  [ table3 ] ) . 
   we speculate that all low - core - power sources also have sub - pc - scale or smaller jets , and that such structures would be revealed if they were at less than several  mpc like ngc  3031 and ngc  4258 . 
   the detectability of small - scale jets might merely depend on distance . 
   if that is true , the 5-ghz vlbi - core power should contain jet contributions significantly . 
   additionally , ngc  3031 and ngc  4258 are very suggestive cases also about coexistence with the adaf disc . 
   although they show clear evidence of jets , their high - frequency spectra are highly inverted , @xmath29 and @xmath30 , respectively . 
   these are the highest two spectral indices in the vlbi - detected llagns with negligible diffuse contaminations . 
   if their millimetre emissions originate in the adaf discs , the two llagns would be examples for the coexistence of the jets and adaf . 
   we mention apparent exceptions . 
   ngc  266 and ngc  3147 show steep spectra , but have no detectable jet in spite of their high core powers  ( table  [ table4 ] ) . since the two llagns are the most two distant llagns in our sample , it could be somewhat hard to reveal their putative small - scale jet structures . 
   their high radio powers might be also responsible for the dominance of such small - scale jets . 
   ngc  5866 show a definitely steep spectrum , in spite of a low core power . 
   its nuclear activity type , ` transition ' , might be related to the jet domination ; another transition object , ngc  4552 , also shows a definitely steep spectrum . 
   however , our limited sample of transition objects can not allow us to discuss it in detail 
   .      there appears a clear difference in the spectral properties between type  1s ( s1.5 , s1.9 , and l1.9 ) and type  2s ( s2 , l2 , and t2 ) , as seen in table  [ table3 ] . 
   most of the type  1s show inverted spectra , while most of type  2s show steep spectra . 
   we suggest this difference conflicts with the unified scheme @xcite . in this scheme , many observed properties are determined basically only by a viewing angle with respect to the obscuring dusty torus surrounding the nucleus . 
   type-1 seyferts , with broad permitted optical emission lines , are those in which the torus is viewed closer to face - on orientation . 
   type-2 seyferts lack broad permitted lines , except in polarized flux , and thought to be seen with the line of sight passing through the torus in an orientation closer to edge - on . 
   this classification has been expediently used in liners and transitions ( e.g. , @xcite ) . 
   since the radio emission is transparent through the dusty torus , luminosities and spectral indices of type  1s and 2s should look just the same to the observer unless affected by relativistic beaming . 
   however , radio powers of type  2s tend to be significantly larger than those of type 1s in our sample . 
   it seems that the difference is due to larger contribution of jets in type  2s , nevertheless , which would be less affected by relativistic beaming than type  1s . 
   there are also the other reports about differences between type  1s and type  2s in llagns . 
   @xcite found that low - luminosity type-1 seyferts rather show somewhat stronger centimetre radio emission than low - luminosity type-2 seyferts . 
   @xcite have reported that type-2 llagns are underluminous in an x - ray band compared to type-1 ones with the same h@xmath0 luminosity . 
   there are ` intrinsically true type-2 seyferts ( without broad lines even in polarized flux ) ' especially in the low - luminosity range ( @xcite , and reference therein ) . 
   thus , the discrepancy in the unified scheme is commonly seen in llagns . 
   we suggest a possible idea for the discrepancy with the unified scheme in terms of radio properties of llagns . 
   if outflows carry accreting mass away and decrease a density at the innermost disc , then the spectral - peak frequency could decrease to several tens of ghz @xcite . 
   although such a central engine could generate strong jets , neither high - frequency inverted spectra nor a lot of up - scattered photons that will ionize broad - line clouds can be produced . 
   that is , llagns with strong jets would tend to have weak broad - line emission , which could not have been detected in a limited sensitivity of the optical survey , then they could be classified into type 2s , even if the obscuring torus is in a face - on orientation . 
   we have found evidence for inverted high - frequency radio spectra in a number of llagns . 
   such a high - frequency property is potentially the long - sought signatures of the adaf . 
   it is an apparent observational confirmation for the submillimetre bump theoretically - predicted in the adaf model . in past observations , 
   the radio spectra of llagns had been investigated exclusively at centimetre bands where high sensitivities are available . 
   @xcite reported the radio spectra of 16 llagns that had been observed with the vla at 515  ghz ; @xcite reported the radio spectra of three llagns that had been observed with the vlba at 1.78.4  ghz ; @xcite reported the radio spectra measured with the vlba for three llagns at 2.315  ghz and three llagns at 1.743  ghz . 
   the frequency range of submillimetre - bump hunting has greatly expanded by our high - frequency radio survey . 
   our findings will encourage further llagn surveys at higher - frequency ranges with , e.g. , the coming atacama large millimetre and submillimetre array ( alma ) , to prove the adaf . 
   the existence of the adaf will give radio astronomers a chance to make the direct imaging for accretion discs . 
   their area of study had been limited to the nonthermal phenomena in relativistic jets of agns , although optical and x - ray astronomers have observed the direct fluxes from the accretion discs as unresolved sources . 
   radio astronomers have vlbi techniques , which provide the greatest spatial resolutions all over the electromagnetic wavelengths . 
   a high - frequency vlbi instrument will report the first image of morphology of accretion discs in micro - arcsec resolutions . 
   the vsop-2 , a next generation space vlbi mission currently being planned , will provide a 38  micro - arcsec resolution at 43  ghz and a brightness - temperature sensitivity of 10@xmath31  k @xcite . because an electron temperature of 10@xmath32  k at the region within several tens schwarzschild radius is predicted by the adaf model 
   , the vsop-2 can reveal disc structures of more than a dozen nearby llagns . 
   if the accretion disc were an optically - thick standard disc , it will have never been detected with the vlbi because of low brightness temperature .    from our results , both the jets and disc seem to coexist in the core of llagns , and their flux ratios are different from object to object . 
   it suggests that relative radiative efficiencies between the jet and disc would determined by some kind of physical parameter(s ) in the central engine . from the theoretical point of view @xcite 
   , it has been suggested that radiatively inefficient flows of low accretion rate tend to stimulate strong convective instabilities and powerful outflows . 
   the central engines of llagns probably can produce jets . 
   we will discuss the physical parameters that control jet productions in our llagn sample in a separate paper ( doi et  al . 
   in  prep ) . 
   our millimetre survey and analyses of submillimetre archival data for the 20 vlbi - detected llagns reveal high - frequency radio spectra of llagns with compact cores as follows :    ( i ) : :    at least half of the sources show inverted spectra between 15 and    96  ghz . 
   we used the published data at 15  ghz with the vla in a    0.15-arcsec resolution and our measurements at 96  ghz with the nma in    a 7-arcsec resolution . 
   ( ii ) : :    submillimetre fluxes measured with the scuba on the jcmt may be from    the dust associated with host galaxies . 
   these data have been useful to    estimate dust contamination in the nma measurements . 
   ( iii ) : :    we confirm little diffuse contamination in the nma measurements from    dust , free 
   free , and extended nonthermal emissions for llagns showing    inverted spectra . 
   therefore , the inverted spectra are not caused by    resolution effect , but intrinsic properties of the cores . 
   ( iv ) : :    in our sample of 20 llagns , radio cores with inverted high frequency    radio spectra reside in at least ten llagns . 
   ( v ) : :    we find an inverse correlation between the high - frequency radio    spectrum and the low - frequency radio core power . 
   such a property can    be explained by an idea that flux ratios between the jet showing steep    spectrum and the adaf disc showing inverted spectrum are different    from llagn to llagn . 
   the disc components could be seen only if the    jets are faint . 
   consequently , we have found evidence for inverted high - frequency radio spectra in a number of llagns . 
   such a high - frequency property is potentially the long - sought signatures of the adaf . 
   we acknowledge s.  okumura for helpful advices in observations with the nma at the nobeyama radio observatory . the nobeyama radio observatory ( nro ) is a branch of the national astronomical observatory of japan ( naoj ) , which belongs to the national institutes of natural sciences ( nins ) . 
   we have made extensive use of the nvss and first data from the national radio astronomy observatory that is a facility of the national science foundation operated under cooperative agreement by associated universities , inc . 
   we have also made extensive use of jcmt archival data from canadian astronomy data centre , which is operated by the dominion astrophysical observatory for the national research council of canada s herzberg institute of astrophysics .
summary: we investigate the high - frequency radio spectra of twenty low - luminosity active galactic nuclei ( llagns ) with compact radio cores . 
   our millimetre survey with the nobeyama millimetre array ( nma ) and analyses of submillimetre archival data that had been obtained with the submillimetre common user bolometer array ( scuba ) on the james clerk maxwell telescope ( jcmt ) reveal the following properties . 
   at least half of the llagns show inverted spectra between 15 and 96  ghz ; we use published data at 15  ghz with the very large array ( vla ) in a 0.15-arcsec resolution and our measurements at 96 ghz with the nma in a 7-arcsec resolution . 
   the inverted spectra are not artificially made due to their unmatched beam sizes , because of little diffuse contamination from dust , regions , or extended jets in these llagns . 
   such high - frequency inverted spectra are apparently consistent with a ` submillimetre bump ' , which is predicted by an advection - dominated accretion flow ( adaf ) model . 
   we find a strong correlation between the high - frequency spectral index and low - frequency core power measured with very - long - baseline - interferometry ( vlbi ) instruments . 
   the inverted spectra were found exclusively in low - core - power sources , while steep spectra were in high - core - power ones with prominent pc - scale jets . 
   this suggests that the adaf and nonthermal jets may coexists . 
   the flux ratios between disc and jet seem to be different from llagn to llagn ; disc components can be seen in nuclear radio spectra only if the jets are faint . 
   [ firstpage ]    galaxies : active  galaxies : seyfert  radio continuum : galaxies  submillimetre .