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in standard models of active galactic nuclei , seyfert 2 galaxies are interpreted as systems viewed at sufficiently high inclination angles that the central continuum source is obscured by the surrounding dusty torus ( e.  g. , krolik 1999 ) . however
, residual soft x - ray emission is observed from many of these systems .
this must originate in the unobscured nuclear vicinity , or perhaps in other regions of the galaxy .
while one expects some soft x - ray emission to be produced by reprocessing of the strong continuum flux in the circumnuclear environment , collisionally - heated gas associated with starburst regions might also provide an important contribution ( e.  g. , wilson et  al .
1992 ) .
high resolution x - ray spectroscopy provides the least ambiguous means of distinguishing between these two interpretations .
discrete spectral features formed in collisional plasmas are qualitatively different from those formed in photoionized gas ( liedahl 1999 ) . in collisional plasmas ,
emission lines are primarily produced by electron impact excitation from the ground state , which favors strong resonance transitions .
fe l - shell transitions are especially prominent . in photoionized plasmas ,
the electron temperature and density are too low for excitation by electron impact , and the lines are produced mostly via radiative cascades following recombination , which favors the lowest - lying multiplet levels , thereby producing forbidden transitions .
also , fe l lines are much weaker than hydrogenic and heliumlike k - shell transitions of lower z elements .
another characteristic difference involves the radiative recombination continua ( rrc ) . in collisional plasmas , these are broad weak features , barely discernible above the bremsstrahlung continuum . in photoionized plasmas ,
the rrc are much narrower and far more prominent ( liedahl and paerels 1996 ) .
the grating experiments on chandra and xmm - newton have provided the first x - ray spectra of seyfert 2 s with sufficient resolution and sensitivity to investigate these issues in detail . here
we present chandra hetg spectra of markarian 3 and xmm - newton rgs spectra of ngc 1068 .
the data are consistent with all emission arising in photoionized gas .
we can use these spectra to derive sensitive constraints on the structure of the circumsource environment .
the chandra hetg spectrum of markarian 3 is shown in figure 1 ( sako et  al .
a variety of narrow features are readily discernible and are explicitly labeled in the figure .
these include ly series transitions of hydrogenic o , ne , mg , si , s , and fe , heliumlike complexes of o , ne , mg , si , s , ar , and fe , near - neutral fluorescence k@xmath0 lines of mg , si , s , and fe , and a few weak fe l lines for a range of charge states ( fe xvii through xxiv ) .
the he@xmath0 complexes are especially interesting . for o vii ,
the forbidden line is clearly the strongest , and the relative line ratios agree with the values expected for pure recombination ( porquet & dubau 2000 ) . for both ne ix and si xiii ,
the resonance lines are roughly comparable to the forbidden lines , but the ratios are still not as high as expected for electron impact excitation .
this suggests possible contributions from both recombination and collisional excitation emission from the same plasma or spatially - distinct plasmas .
the presence of the fe l features might also bolster that interpretation .    however , we are firmly convinced that this is _ not _ what is happening in this spectrum .
both the heliumlike resonance lines and the fe l lines are not produced collisionally , but instead are photoexcited by the central continuum radiation .
two major clues lead us to this interpretation : ( 1 ) the fe l lines are too weak relative to the heliumlike resonance lines to be consistent with electron impact excitation , unless we significantly reduce the iron abundance in the putative collisional component .
( 2 ) the specific fe l lines observe are all 3d2p and 3p2s .
these have high oscillator strengths , and are the most prominent features expected for photoexcitation .
there are no accompanying 3s 2p transitions , which have much lower oscillator strengths . in collisional plasmas ,
the two sets of transitions have near comparable line intensities .    at low to intermediate line
optical depths , photoexcitation and recombination provide comparable contributions to the x - ray line spectrum of photoionized gas .
this interpretation suffices for markarian 3 .
we see no conclusive evidence for a contribution from collisional gas .
the xmm - newton rgs spectrum of ngc 1068 is shown in figure 2 ( kinkhabwala et  al .
we see a very line - rich spectrum dominated by hydrogenic and heliumlike lines ( c through si ) , accompanied by weak fe l - shell transitions .
in addition , we see very prominent rrc .
all rrc for the hydrogenic and heliumlike charge states of c , n , o , and ne are detected .
the narrow rrc provide unambiguous evidence that most of the x - ray emission we are observing is produced in photoionized gas .
this conclusion is supported by the he@xmath0 complexes , in which the forbidden line is brighter than the resonance .
the ratios of the forbidden line intensities to the rrc intensities agree with expectations for recombination cascades .
nevertheless , as for markarian 3 , recombination alone does not provide a good quantitative fit to the entire ngc 1068 spectrum .
the heliumlike resonance lines are still too bright relative to the forbidden lines , and the fe l lines are brighter than would be expected for purely recombining gas . here
again , we can explain all of these features by allowing for additional contributions from photoexcitation .
conclusive evidence for this comes from the hydrogenic line series : the higher series lines are observed to be significantly brighter relative to ly@xmath0 than is expected for either recombining gas or collisionally - excited gas .
this is a natural consequence of photoexcitation at intermediate optical depths due to saturation of the highest oscillator strength lines .
we have developed a fully self - consistent model for the ngc 1068 spectrum in terms of a photoionized medium which correctly accounts for both the photoexcitation and radiative recombination contributions ( kinkhabwala et  al .
2001a , behar et  al .
individual column densities and velocity distributions for each ion are left as free parameters .
the model provides an amazingly good fit to these high statistics data .
fits to the earlier chandra hetg spectra of markarian 3 , ngc 4151 ( ogle et  al .
2000 ) , and the circinus galaxy ( sambruna et  al . 2000 ) as well as to a recent observation of ngc 4507 yield similar results ( kinkhabwala et  al .
2001b ) . in particular , we find that the ogle et  al .
( 2000 ) conclusion that the ngc 4151 spectrum indicates the presence of hot collisional gas is incorrect .
all of the features in the spectra of all seyfert 2 galaxies so far observed with chandra and xmm are consistent with fluorescence of cold gas and photoexcitation and recombination cascades in photoionized gas .
the high resolution x - ray spectra of seyfert 2 galaxies display a wealth of emission features produced by fluorescence , photoexcitation , and recombination cascades in radiatively - driven gas .
there is no evidence for measureable contributions from hot collisionally - ionized gas in any of the sources which have been studied to date .
further observations of this kind should be very helpful in elucidating the nature of the circumsource medium in active galactic nuclei .
x - ray spectroscopy of seyfert 2 galaxies provides an excellent probe of the circumnuclear environment in active galactic nuclei . 
 the grating experiments on both chandra and xmm - newton have now provided the first high resolution spectra of several of the brightest seyfert 2 s . 
 we present chandra hetg data on markarian 3 , and xmm - newton rgs data on ngc 1068 . in both cases , 
 the spectra are dominated by emission lines due to radiative recombination following photoionization , photoexcitation , and fluorescence . 
 there is no evidence for any significant contribution from collisionally - heated gas .    
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