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early - type galaxies ( etgs ) , comprising ellipticals and lenticulars ( e / s0s ) , are generally considered `` red and dead '' , with the bulk of their stars homogeneously old , slightly less so at the lower masses and in the field ( e.g.  ( * ? ? ?
* thomas et al .  2005 ) ) .
this is clearly an oversimplification , however , and residual star formation is still present in at least @xmath3 of local etgs .
this is easily quantified with near - ultraviolet imaging , particularly sensitive to even small amounts of star formation ( e.g.  ( * ? ? ?
* yi et al.2005 ) ) , but is also clearly detected through optical absorption lines ( e.g.  ( * ? ? ? * kuntschner et al .  2010 ) ) .    in this paper , we study the properties of the molecular gas in etgs , the fuel for star formation , through the @xmath4co(1 - 0 ) line .
we focus on the complete , volume - limited sample of @xmath1 local etgs from the atlas@xmath0 survey ( @xmath5  mag ; ( * ? ? ?
* cappellari et al.2011 ) ) , as well as results from its predecessor ( sauron ; ( * ? ? ?
* de zeeuw et al .  2002 ) ) .
in ( * ? ? ? * young et al .  ( 2011 ) ) , we present a complete co single - dish survey of the atlas@xmath0 sample carried out with the iram 30 m telescope . with a sensitivity of @xmath6  mk @xmath7 per @xmath8  km  s@xmath9 channel ( @xmath10  @xmath11 of molecular hydrogen at the median distance ) , the main result is a surprisingly high detection rate of @xmath2 . equally important
, detections are present throughout the so - called red sequence , formed of objects with presumably low specific star formation rates .
the molecular gas masses span the range @xmath12@xmath13  @xmath11 , and both the detection rate and the molecular - to - stellar mass ratios are independent of luminosity and most structural and dynamical parameters probed .
there is however a clear dependence on the specific stellar angular momentum ( as quantified by the @xmath14 parameter ; ( * ? ? ?
* emsellem et al.2007 ) ) , in the sense that slow - rotating galaxies are deficient in molecular gas , and a possible weak dependence on environment for fast - rotating galaxies .    we have strived to obtain follow - up interferometric line - imaging of all co detections , first with bima and pdbi , and now with carma ( alatalo et al .
, in preparation ) .
figure  [ fig : mosaic ] shows a subset of the nearly @xmath15 galaxies observed so far , and it illustrates the range of distributions detected .
co is generally centrally - concentrated , with central disks , bars , rings , and occasional disturbed morphologies . a careful comparison with the spiral galaxies of the bima - song survey ( ( * ? ? ?
* regan et al.2001 ) ) reveals that while the co extent is generally smaller in an absolute sense in etgs , the extent distributions are identical once scaled by a characteristic size ( effective radius , scalelength , isophotal diameter , etc ) .
taking full advantage of the three - dimensional nature of the sauron optical integral - field data _ and _ the co synthesis observations , we can compare the kinematics of the stars , ionised gas , and molecular gas ( fig .
[ fig : kin ] ; ( * ? ? ?
* davis et al.2011 ) ) .
the kinematic major axes of the stars and molecular gas are aligned in about @xmath16 of the co - detected galaxies , implying that the molecular gas is consistent with an internal origin ( e.g.stellar mass loss ) in @xmath16 of the cases overall , while it must have an external origin ( e.g.  external accretion , minor merger ) in at least @xmath17 of the cases ( some of the kinematically - aligned gas can also have an external origin ) .
crucially , these statistics are strongly dependent on environment .
the stars and molecular gas are nearly always kinematically - aligned in clusters ( here virgo ) , while about half the galaxies are misaligned in the field .
this implies that external accretion of gas is very important in the field but is possibly totally shut down in clusters .
interestingly , the ionised and molecular gas are always kinematically - aligned , so that they must share a common origin . as our ionised gas detection rate is much higher ,
our conclusions are put on a much firmer basis by the ionised gas .     of the objects
are misaligned .
_ bottom - left : _ kinematic misalignment distribution between the ionised and molecular gas .
the two gas phases are always aligned . _
right : _ kinematic misalignment distribution between the ionised gas and stars , for fast - rotating galaxies in the virgo cluster ( top ) and in the field ( bottom ) .
the galaxies are nearly always aligned in virgo but show a range of misalignments in the field . adapted from (
* davis et al .
( 2011)).,title="fig:",scaledwidth=40.0% ]   of the objects are misaligned .
_ bottom - left : _ kinematic misalignment distribution between the ionised and molecular gas .
the two gas phases are always aligned .
_ right : _ kinematic misalignment distribution between the ionised gas and stars , for fast - rotating galaxies in the virgo cluster ( top ) and in the field ( bottom ) .
the galaxies are nearly always aligned in virgo but show a range of misalignments in the field . adapted from (
* davis et al .
( 2011)).,title="fig:",scaledwidth=45.0% ]
comparing our molecular gas maps with the sauron data in a spatially - resolved way , the molecular gas appears to be forming young central disks in a number of objects ( ( * ? ? ?
* crocker et al.2011 ) ) .
this is revealed by fast - rotating decoupled central stellar components , depressed stellar velocity dispersions , ionised gas emission with [ oiii]/h@xmath18 line ratios typical of star formation , enhanced h@xmath18 absorption , etc .
however , this is not always the case .
molecular gas is sometimes associated with [ o iii]/h@xmath18 line ratios more typical of those expected from evolved stellar populations , and occasionally no sign of a young stellar population is detected at all .
as etgs are also ( on average ) significantly different dynamically from disk galaxies ( e.g.  @xmath19 parameter ) , it is clear that they represent a different and unique environment in which to probe star formation , both its usual tracers and possible causal relations .
after carefully accounting for internal extinction , we find that the kennicutt - schmidt relation and a constant star formation efficiency ( relating the surface density of molecular gas to that of star formation ) are both consistent with the data . however , the far infrared ( fir)-radio continuum correlation is not satisfied , with too many fir - bright galaxies ( fig .
[ fig : fir - radio ] ) , and different infrared star formation tracers do not agree , perhaps implying significant dust heating from non - star formation sources .
ghz ) correlation of the sauron etgs ( black dots ) compared to that of the galaxies in the uppsala general catalogue ( red dots ; mostly disk galaxies ) .
black dots with error bars are co - detected and preferentially lie on the fir - bright side of the relation . adapted from (
* crocker et al .
( 2011)).,scaledwidth=60.0% ]    understanding possible differences in the star formation processes of etgs requires a good knowledge of the physical conditions in the interstellar medium .
we have thus started a programme to constrain the opacity , column density , volume density , temperature , and other parameters of the cold gas in our sample galaxies , using multiple molecular line tracers ( e.g.  @xmath20co , hcn , hco@xmath21 ) .
preliminary results indicate occasionally enhanced @xmath20co and suppressed hco@xmath21 ( ( * ? ? ?
* krips et al .
2010 ) ; crocker et al .
, in preparation ) .    observing molecular tracers other than @xmath4co is challenging with the current generation of instruments , but with its vastly improved sensitivity and angular resolution , alma will both broaden and increase the pace of discoveries .
it will also allow to probe trends as a function of both lookback time and environment .
herschel will similarly improve our understanding of the gas ( atomic , ionised and molecular ) and dust associated with the molecular gas , in particular the dust - to - gas ratio .
the molecular gas content of local early - type galaxies is constrained and discussed in relation to their evolution . 
 first , as part of the atlas@xmath0 survey , we present the first complete , large ( @xmath1  objects ) , volume - limited single - dish survey of co in normal local early - type galaxies . 
 we find a surprisingly high detection rate of @xmath2 , independent of luminosity and at best weakly dependent on environment . 
 second , the extent of the molecular gas is constrained with co synthesis imaging , and a variety of morphologies is revealed . 
 the kinematics of the molecular gas and stars are often misaligned , implying an external gas origin in over a third of the systems , although this behaviour is drastically diffferent between field and cluster environments . 
 third , many objects appear to be in the process of forming regular kpc - size decoupled disks , and a star formation sequence can be sketched by piecing together multi - wavelength information on the molecular gas , current star formation , and young stars . 
 last , early - type galaxies do not seem to systematically obey all our usual prejudices regarding star formation , following the standard schmidt - kennicutt law but not the far infrared - radio correlation . 
 this may suggest a greater diversity in star formation processes than observed in disk galaxies . using multiple molecular tracers 
 , we are thus starting to probe the physical conditions of the cold gas in early - types .