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the dust properties in nls1 galaxies may play a key role in explaining their peculiar properties .
indeed , the relative weakness of low density forbidden line emission and the absence of broad hydrogen wings in nls1 galaxies may be understood if most of the ionizing flux is absorbed in the inner forbidden line region , thus reducing the amount of ionizing radiation reaching the low density clouds , and absorbing the internal broad line region ( blr ) emission .
the presence of dust is indicated by several observations .
spectropolarimetry observations suggest that dust scatters the optical photons in several nls1 galaxies  @xcite , and soft x - ray observations indicate the presence of a dusty warm absorber along the line of sight to some nls1 galaxies  @xcite . however , in the far - infrared ( fir ) domain , nls1s show a high degree of similarity with normal seyfert 1 galaxies  @xcite,@xcite , suggesting that their dust properties are not unique among seyfert galaxies .
our purpose is to investigate the dust properties in nls1 galaxies through the analysis of their infrared ( ir ) emission , a direct tracer of dust  @xcite,@xcite .
the spectral energy distribution ( sed ) from near - ir ( nir ) to millimetre wavelengths  @xcite of four nls1 galaxies ( tons180 , rxj0323@xmath149 , iras13224@xmath13809 , and pg1404 + 226 ) are analyzed here and shown in the objects rest - frames in fig .
[ figsed ] .
the grey body radiation model  @xcite was used to fit the observed seds .
the parameters of the model are the radius @xmath2 of the projected source , the dust temperature t , and the dust optical depth @xmath3 .
the observed ir seds are smooth , describable by several grey body components characterized by different temperatures and opacities . since the data do not permit us to constrain @xmath3 , we fixed it at two extreme values and then we derived the other parameters of the model .
first , we fixed @xmath3 at one of the highest hydrogen column density values measured in seyfert galaxies ( 1.8@xmath410@xmath5 atoms @xmath6 ) , and , secondly , we chose @xmath3 corresponding to the optical extinction estimated from the balmer decrement of each object  @xcite ,  @xcite ,  @xcite ,  @xcite ( see best fitting curves in fig .  [ figsed ] ) . in the case of large opacities ,
the estimated sizes of the dust emission regions vary from 0.2 pc for the hottest component ( 1000 k ) , to @xmath7500 pc for the coldest one ( 20 k ) , and in the case of low opacities , the minimum estimated size is 0.3 pc , and the maximum 20 kpc . the comparison between the temperatures and sizes measured in the two extreme regimes , and the observed values in seyfert galaxies suggests that the cold dust is characterized by low opacity , while the warm / hot dust may be more opaque . as a consequence , the ir reradiation at long wavelengths ( t @xmath8 60 k ) will be isotropic , and orientation - independent .
conversely , at shorter wavelengths ( t @xmath9 60 k ) the dust may self - absorb its emitted radiation , and the observed emission may depend on orientation .
the steepness of the power law obtained by fitting all the ir data at wavelengths shorter than 60 @xmath0 m , where the effects of opacity become important , is an indicator of dust extinction . we compared the balmer decrement values ( h@xmath10/h@xmath11 ) , indicators of extinction at optical wavelengths , of the nls1 galaxies studied here with the slope of the power law obtained by fitting the observed data between 3 and 60 @xmath0 m ( see best - fit power laws in fig .
[ figsed ] ) .
the measured power law slopes range between 0.99 and 1.77 , which corresponds to a range in the visual extinction of 61 mag , provided that all objects have the same intrinsic ir power law .
the variation in the h@xmath10/h@xmath11 values corresponds instead to a range in the visual extinction of only 2.8 mag .
such a difference indicates that all the emitting dust can not be responsible for the reddening observed in the balmer lines .
however , the two parameters , h@xmath10/h@xmath11 , and @xmath12 , are strongly correlated ( the spearman s correlation rank is 1.0 , with a null associated probability to obtain such a value from uncorrelated values ) .
the corresponding values and their linear interpolation are shown in fig .
[ red_ir]a , with filled diamonds and a dashed line , respectively .
we selected 12 additional nls1 galaxies from a sample of 148 objects for which the balmer decrement and the ir spectrum were measured .
the observed correlation is still present for the additional 12 nls1 galaxies ( the spearman s correlation rank is 0.61 , and the probability to obtain such a value from uncorrelated values is 1.13% ) , as shown in fig .
[ red_ir]a ( the solid line represents the best fit line obtained considering all the 16 nls1 galaxies which are marked by empty diamonds ) . a correlation between @xmath12 and the line intensity ratio
[ oiii]/h@xmath11 was also discovered ( see fig .  [
red_ir]b ) .
the corresponding spearman s coefficient rank is 0.78 , and the probability to obtain such a value from uncorrelated values is 0.04% .
+ the steepening of the ir power law and the enhancement of the balmer decrement and of the line ratio [ oiii]/h@xmath13 can be interpreted in terms of inclination - dependent obscuration .
the central regions of these objects are probably surrounded by a distribution of dust that becomes gradually less opaque along some directions .
the blr must be more extended than the inner hot dust , since its emission is less absorbed than the  broad " lines . towards the direction where dust is less opaque
, the blr and the hottest dust component will be directly visible , while towards the direction where dust is more opaque , the broad emission lines and the hottest dust radiation will be partially absorbed . since the h@xmath11 emission is produced in both the blr and in the narrow line region , when the blr is obscured the ratio [ oiii]/h@xmath11 will be higher than in unobscured objects .
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broad infrared spectra ( 7200 @xmath0 m ) of four nls1 galaxies , obtained with the imaging photo - polarimeter ( isophot ) on board the infrared space observatory ( iso ) , are presented . 
 the infrared luminosities and temperatures , opacities and sizes of the emitting dust components are derived .    
 a comparison between the observed infrared spectra and the optical emission line fluxes of a sample of 16 nls1 galaxies suggests that these objects suffer different degrees of dust absorption according to the inclination of the line of sight with respect to the dust distribution .    and    galaxies : seyfert ; galaxies : photometry ; infrared : galaxies