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the advent of integral - field spectrographs has opened a whole new window of possibilities for extra - galactic studies . coupled with large telescopes and advances in adaptive optics systems ( ao ) , integral - field units ( ifus )
offer a unique opportunity to go further in the exploration of astrophysical phenomena earlier in the history of the universe and at smaller scales .
it is therefore not surprising that ifus are quickly becoming standard instruments on all the 8m - class telescopes . besides the efforts to investigate the formation and evolution of galaxies in the early universe , much
can be learned by studying in detail the _ fossil _ record of nearby galaxies using ifus on intermediate - size telescopes .
the ` sauron `  project ( de zeeuw et al . 2002 ) , is a first step in this direction .
the project is designed to perform a systematic study of a representative sample of early - type , nearby galaxies using the ` sauron`(bacon et al .
2001 ) and ` oasis `  ( bacon et al . 1995 ; mcdermid et al .
2004 ) ifus at the 4.2 m william herschel telescope .
early results of the survey have revealed a variety of structures much richer than usually recognised in early - type galaxies ( de zeeuw et al .
2002 ; mcdermid et al .
2004 ; emsellem et al . 2004
; see also r. mcdermid s contribution in this workshop ) .    here
we focus on the distribution and kinematics of the ionised gas of a subset of galaxies to illustrate the diversity of structures and ionisation processes present in the ` sauron `  representative sample of 48 elliptical and lenticular galaxies .
the sample contains an equal fraction of elliptical and lenticular galaxies , with equal number of field and cluster galaxies in each morphological group .
we will make use of the two - dimensional information delivered by ` sauron `  to compare the distribution and kinematics of the stars and gas and to study the nature and origin of the ionised gas in these systems .
a more extended analysis of the properties of the ionised gas for the entire sample is presented in sarzi et al .
furthermore , the study of the stellar and gas properties of the 24 sa bulges in the ` sauron `  sample is addressed in falcn - barroso et al .
early - type galaxies were traditionally thought to be uniform stellar systems with little or no gas and dust . however , a considerable number of imaging and spectroscopy studies have changed this view ( sadler & gerhard 1985 ; van dokkum & franx 1995 ; goudfrooij 1999 ; tran et al 2001 ) . in the spectral range delivered by ` sauron `  there are three potential emission lines that can be measured ( i.e. , @xmath04959 , 5007 , @xmath05198,5200 ) .
given that the stellar contribution to the overall spectrum in our sample of galaxies remains significant , it is important to perform an accurate continuum subtraction in order to determine reliably the fluxes and kinematics of the emission lines .
we investigated several methods to perform this task and settled on a procedure that _ simultaneously _ fits both the stellar and gaseous contributions to derive the fluxes , velocities and velocity dispersions of the emission lines.-1    traditional methods to separate the stars and gas consisted of the subtraction of the best matching stellar template , in the regions free of emission , to the full spectrum of the galaxy .
as described in sarzi et al .
( 2005 ) , this methodology can significantly overestimate the flux and velocity dispersion of the measured emission lines .
extensive simulations show that the new procedure is much more accurate and appears to be superior to previous ones , allowing us to detect emission down to an equivalent width of 0.1  .
the distribution of the ionised gas in our sample shows diverse morphologies . in general
the distribution follows that of the stars , although there are several cases where the situation is much more complex ( i.e. filaments , rings , spiral arms , lanes ) .
the incidence of ionised - gas emission in our sample is 75% .
seven galaxies show only weak traces of emission , and there are five cases with no detection . by morphological type
, lenticular galaxies display a slightly higher content of ionised gas than elliptical galaxies ( 83% versus 66% ) .
similar percentages were found when the sample was divided according to environment ( 83% field , 66% cluster ) .
one important remark is that the fraction of galaxies with clearly detected emission in the virgo cluster drops to only 55% ( 10/18 ) , with just 3/9 ellipticals showing the presence of gas .
the incidence of ionised - gas shows no correlation with either luminosity or the presence of a bar in the galaxy .
the detection rates found for our sample are in good agreement with those of macchetto et al .
( 1996 ) , who found emission in 85% of the lenticular and 68% of the ellipticals in their sample .    in figure  [ fig2 ]
, we present maps of the stellar and ionised - gas distribution and kinematics for four of the 48 galaxies in our sample .
ngc2974 reveals a fast rotating gas disk co - rotating with respect to the stars . despite the regular appearance of the ionised - gas distribution , the equivalent width ( ew ) map of the  line
( see fig .
[ fig2 ] ) highlights the presence of an inner ring and two spiral arms that extend all the way out to the limits of our field - of - view ( see krajnovi ' c et al .
2005a for a detailed analysis of this galaxy using ` sauron `  data ) .
a similarly regular case is that of ngc4526 , although in this case the ionised gas is confined to a well defined , fast rotating disk in the central kpc of the galaxy .
ngc2768 is a well - known galaxy where the distribution of the ionised - gas appears to be perpendicular to that of its stellar counterpart ( i.e. polar - ring , fried & illingworth 1994 ; bertola et al .
the ` sauron `  ew map shows that the gas distribution has a filamentary morphology along the galaxy minor axis .
the gas distribution in ngc4278 displays a peculiar integral - sign pattern that is closely followed by the gas velocity field .
the stellar and gas kinematics appear to be misaligned by increasingly wider angles , as they twist in opposite directions towards the outer parts of the field - of - view .
diagnostic diagrams of /  vs /  ( veilleux & osterbrock 1987 ; kauffmann et al .
2003 ) have been used extensively in the past to investigate the trigger of the ionisation of the gas ( i.e. agn , star - formation ) .
many scenarios have been invoked to explain the presence of ionised gas in early - type galaxies : central agn , hot ( @xmath1k ) gas ( sparks et al .
1989 ; de jong et al . 1990 ) , young stars ( shields 1992 ) , post - agb stars ( binette et al . 1994 ) , or shocks ( dopita & sutherland 1995 ) . within the wavelength range delivered by ` sauron `  only the  and  lines can be used for this purpose . despite this limitation
, the /  ratio serves as a good indicator to locate regions where emission is due to young stars and also to trace variations of the ionisation mechanisms within a single galaxy .
regions displaying low /  ratios ( i.e. @xmath21 ) are usually interpreted as indicative of star - formation ( ho et al . 1997 ) .
however , higher ratios ( i.e. @xmath31 ) could still indicate star - formation if the metallicity of the gas is sufficiently high ( veilleux & osterbrock 1987 ) . additionally other mechanisms can lead to high ratios . since it is unlikely that the metallicity of the ionised gas varies abruptly within galaxies ( unless there has been some recent accretion of material ) , large variations across maps of the /  ratio are more likely to be produced by changes in the ionisation mechanism rather than changes in the metallicity .
figure  [ fig2 ] shows the /  ratios for the galaxies discussed in section  [ sec : distrib ] .
the /  ratio maps clearly reveal the presence of radial gradients and substructures in the ionisation properties of the gas in the four galaxies .
the ratio is moderately high in ngc2768 , ngc2974 , and ngc4278 , while it is very low in ngc4526 . in this last galaxy
, the location of the low /  values corresponds to the location of a prominent dust disk ( as shown in the unsharp - masked image ) , and suggests ongoing star formation . in this respect , there seems to be a clear link between the ionised structures found in the /  maps with those seen in the  equivalent width and kinematic maps ( i.e. ring in ngc2974 , or integral - sign pattern in ngc4278 ) .
the remaining sample of 44 galaxies also displays a great diversity of /  ratios within galaxies , but also across the sample .
this suggests that either there are many mechanisms at play in the ionisation of the gas , that the metallicity of the ionised gas is very heterogeneous in those galaxies , or both .
in order to study the dust distribution and compare it with that of the ionised gas we have generated unsharp - masked images using archival _ hubble space telescope
we found that , consistent with previous results , dust generally follows the ionised - gas distribution ( goudfrooij et al .
1994 ; tomita et el .
2000 ; tran et al . 2001 ) .
the opposite situation is not always true , as we have found galaxies with clear presence of emission with no traces of dust in the unsharp - masked images . as found by previous authors ( ho et al .
2002 ) , regular dust distributions generally are associated with smooth velocity fields , however the lack of regular dust lanes does not imply the presence of irregular kinematics .    the four cases presented in figure  [ fig2 ] display a wide range of dust morphologies that correlate in different degrees with the structures seen in the distribution of the ionised gas .
the prominent dust disk in ngc4526 traces closely the distribution of the gas . the ring - like structure and spiral arms in ngc2974
are also followed by dusty structures at the same locations .
the dust morphology is less defined in ngc2768 and ngc4278 , but still suggests an orientation for the dust that is similar to that of the gas distribution .
the measurement of misalignments between the kinematics of the gaseous and stellar components in early - type galaxies has often been used to determine the relative importance of accretion events and the internal production of gas through stellar mass - loss ( e.g. bertola et al . 1992 ) .
the orientation of the dust relative to that of the stars has also served this purpose ( e.g. van dokkum & franx 1995 ) . in order to quantify the presence of decoupled gaseous components in our sample and to investigate their dependence on environment ,
we have measured the mean misalignment between the stellar and ionised gas using _ kinemetry _ , a generalisation of surface photometry to the higher - order moments of the line - of - sight velocity distribution of galaxies ( krajnovi ' c et al .
2005b ) .    in figure  [ fig2 ] ( solid line )
we show the distribution of the average misalignments for the galaxies in our sample of 48 galaxies with sufficiently extended emission .
the distribution of values appears to be skewed towards small misalignments .
however , the fact that there is still a significant fraction of galaxies showing mild to strong misalignments suggests that the ionised gas has not a purely internal origin ( in which only small misalignments are expected ) .
the overall shape of the distribution remains unchanged when the sample is divided into elliptical and lenticular galaxies .
no dependency is found on neither environment nor galaxy luminosity .
we do , however , find a strong dependence on apparent flattening of the galaxy ( see fig .
[ fig2 ] , bottom panel ) .
it appears that the distribution of misalignments of the most flattened objects accounts for the observed excess of co - rotating over counter - rotating gas and stellar systems in the whole sample .
the roundest objects ( @xmath40.2 ) display a more uniform distribution .
this result suggests that rotational support might be important to explain the observed dependence .
we have presented some examples of the morphological and kinematical state of the ionised gas in a representative sample of early - type galaxies using data from the ` sauron `  spectrograph at the 4.2 m william herschel telescope .
the results of our analysis reveal a wide range of morphological , but also kinematical structures .
it also shows that there must be several mechanisms responsible for the ionisation of the gas , not only across galaxies , but also within galaxies .
the presence of dust is common in our sample and is usually accompanied by gas emission .
finally , we studied the distribution of misalignments between the stars and gas in our sample , and conclude that the origin of the ionised gas can not be purely internal or external .
jfb acknowledges support from the euro3d research training network , funded by the ec under contract hprn - ct-2002 - 00305 .
this work is based on observations obtained at the wht on the island of la palma , operated by the isaac newton group at the observatorio del roque de los muchachos of the instituto de astrofsica de canarias .
we present results of our ongoing study of the morphology and kinematics of the ionised gas in 48 representative nearby elliptical and lenticular galaxies using the ` sauron `  integral - field spectrograph on the 4.2 m william herschel telescope . making use of a recently developed technique , 
 emission is detected in 75% of the galaxies . 
 the ionised - gas distributions display varied morphologies , ranging from regular gas disks to filamentary structures . 
 additionally , the emission - line kinematic maps show , in general , regular motions with smooth variations in kinematic position angle . in most of the galaxies , 
 the ionised - gas kinematics is decoupled from the stellar counterpart , but only some of them present signatures of recent accretion of gaseous material . 
 the presence of dust is very common in our sample and is usually accompanied by gas emission . 
 our analysis of the /emission - line ratios , both across the whole sample as well as within the individual galaxies , suggests that there is no unique mechanism triggering the ionisation of the gas .    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    
 ,    galaxies : elliptical and lenticular , cd , galaxies : kinematics and dynamics , galaxies : ism , galaxies : individual ( ngc2768 , ngc2974 , ngc4278 , ngc4526 )  98.52.eh , 98.52.lp , 98.58.ay , 98.62.dm