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while the carriers of the dibs are not known , their correlations with better known constituents of the ism suggest they are likely of molecular origin and reside in diffuse clouds and cloud surfaces ( see the review by daniel welty , these proceedings ) .
we can thus already start using dibs to trace special ism conditions , such as those related to the atomic  molecular transition or cloud irradiation effects , that otherwise may be rather elusive ; hoping meanwhile to expose clues that help identify their carriers .
as no dib has been seen in emission , we rely on background continuum sources to trace their absorption
. fields of stars , such as bright nearby stars , stars in open and globular clusters and stars in nearby resolved galaxies provide a means to construct sparse maps of the dib absorption across a variety of ism volumes . compared to correlation diagrams , these maps add a spatial dimension , even rendering a 3-dimensional picture if stars at different known distances are observed .
i here describe several of our recent and current dib mapping experiments , summarised in table[tab1 ] .
it all started with a spectroscopic survey of the post - main sequence stars in the `` nearby '' ( @xmath1 kpc ) most massive galactic globular cluster , @xmath0centauri .
this programme employed the powerful 2df multi - object spectrograph on the 4-m anglo - australian telescope , which allows to place up to 400 fibres on positions on the sky within a huge , @xmath2-diameter field .
the hot horizontal branch stars in that sample , further aided by their low metal content ( @xmath3 z@xmath4 ) and high velocity ( @xmath5 km s@xmath6 ) with respect to the intervening ism , turned out to be useful in mapping the caiik absorption . i here show ( below ) that in fact those blue spectra also display some dibs . to do this more adequately , we designed a dedicated experiment with the enhanced 2df facility ( aa@xmath7 ) , allowing more spectral coverage at higher spectral resolution .
.overview of spectroscopic dib mapping experiments . [ cols="<,>,<,>,>,<,<",options="header " , ]     likewise
, we have just published dib maps of the tarantula nebula , the famous `` mini - starburst '' region in the large magellanic cloud ( at a distance of 50 kpc , and with @xmath8 z@xmath4 and a recession velocity of @xmath9 km s@xmath6 ) .
this too was a `` by - product '' of a stellar astrophysics programme 
* evans  ( 2011 ) ) .
it utilised the flames spectrograph , offering less multiplexity and coverage but a more sensitive 8-m very large telescope in chile .
we have since conducted a dedicated survey of both the lmc and the small magellanic cloud ( 60 kpc , @xmath10 z@xmath4 and @xmath11 km s@xmath6 ) with aa@xmath7 ; keele university ph.d .  student mandy bailey has now finished the measurements from these spectra , and a publication of the results is expected to appear in 2014 .    inspired by the atomic - line 3-d mapping of the local bubble and its environs by ( * ? ? ? * lallement  ( 2003 ) ) and ( * ? ? ?
* welsh ( 2010 ) ) we have conducted an all - sky survey of dib absorption towards 552 different early - type stars , mostly within a few hundred pc from the sun . in contrast to the above efficient mapping experiments , this meant obtaining one spectrum at a time . because interstellar absorption in such nearby stars is often inconspicuous , we aimed at a signal - to - noise ratio of 1000:1 or better ; this incurs high demands on the calibration measurements as well .
the southern component of the survey was carried out at the 3.5-m new technology telescope in chile , by mandy bailey , whilst the northern component was added by sharif university of technology ph.d .
student amin farhang using iranian time on the 2.5-m isaac newton telescope in spain .
publications arising from this programme are expected to appear in 2014 .
the sightline towards @xmath0centauri is relatively simple , at @xmath12 galactic latitude , with low reddening of @xmath13 mag ( ( * ? ? ?
* mcdonald  2009 ) ) . yet several components are seen in atomic absorption ( ( * ? ? ?
* wood & bates 1994 ) , and references therein ) : hot gas within the local bubble , neutral gas beyond it and including the carina  sagittarius spiral arm , and warm extra - planar gas at greater distances @xmath141 kpc above the galactic plane .
judging from the marginally - resolved kinematics of the caiik line in our spectra , and various correlations , we concluded that the 5780   dib absorption towards @xmath0centauri originates mostly in the extra - planar gas .
the high caii / nai and 5780 /5797   dib ratios in those sightlines indicate a @xmath15-type environment , implying a strong radiation field or perhaps structures embedded within a hotter gas .
however , some more neutral @xmath16-type clouds were seen too , displaying stronger 5797   dib absorption ; these are likely associated with the intervening spiral arm .
intriguingly , the dibs were found to trace small - scale structure down to parsec scales ( or below ) , both in the warm extra - planar gas and neutral gas of the disc .
the top panel is based on the survey of ( * ? ? ?
* van loon et al .  ( 2007 ) )  but we identify the dibs for the first time here ; the bottom panel is based on ( * ? ? ?
* van loon et al .  ( 2009 ) )  who only presented the 5780 and 5797   dibs.,title="fig:",width=377 ] centauri .
the top panel is based on the survey of ( * ? ? ?
* van loon et al .
( 2007 ) )  but we identify the dibs for the first time here ; the bottom panel is based on ( * ? ? ?
* van loon et al .  ( 2009 ) )  who only presented the 5780 and 5797   dibs.,title="fig:",width=377 ]    a re - analysis of our spectra reveals some more dibs ( figure[fig1 ] ) : at 4428 , 4500 and 4762   in the hot - star spectra of ( * ? ? ?
* van loon ( 2007 ) ) , and near 5850 , 6010 and 6200   in those of ( * ? ? ? * van loon  ( 2009 ) ) . with more targets available
as well , there is plenty of scope for more detailed studies in this interesting direction .
our magellanic dib mapping exercise has thus far concentrated on the tarantula nebula .
this is a complex region characterised by multi - phase ism including molecular clouds ( e.g. , as traced in absorption by ( * ? ? ?
* tatton 2013 ) ) and bubbles filled with hot gas , subject to a strong radiation field from the many massive early - type stars in the central cluster r136 and other young associations ( cf .
* evans  2011 ) ) .
we were able to separate the magellanic absorption from that arising in the galactic foreground .
the latter resembled the extra - planar environment seen in the @xmath0centauri maps .
small - scale structure within the lmc was probed both spatially ( down to @xmath17 pc ) and kinematically ( @xmath17 km s@xmath6 ; up to six components in naid ) . while the 5780 /5797   dib ratio in the tarantula nebula is generally high
, regional variations occur in the dibs indicating particularly harsh environments towards the north and west ( in the direction of super - bubbles ) and in the immediate vicinity of ob - type associations , with more shielded conditions towards the south ( where the molecular ridge starts ) .
the 4428   dib proved relatively strong in and around the tarantula nebula , whilst the 5780   dib could be more strongly confined to cloud skins .    even if accounting for the reduced metal content of the lmc , the 5780 , 5797 and 6614   dibs are weak .
* welty  ( 2006 ) ) and ( * ? ? ?
* cox  ( 2006)@xcite ) attributed this to the strong radiation field .
we proposed to also consider the nitrogen depletion in the magellanic clouds ( cf .
* van loon 2010a ) , ( * ? ? ?
* b ) ; see also veronica bierbaum s contribution to these proceedings ) .
i would also suggest to examine any relation with the depletion of poly - cyclic aromatic hydrocarbons ( cf .
* sandstrom  2012 ) ) .
the relatively little studied set of dibs at 4727 , 4762 and 4780   may prove quite interesting , as we found some tentative evidence for differences among these features between the tarantula nebula and the galactic foreground .    while the results of our galaxy - wide magellanic cloud maps will be presented elsewhere
, we can already reveal that in the smc very few sightlines show dibs .
however , when it does it sometimes is fairly strong .
we must consider that , possibly , the conditions under which dibs are seen might occur in slightly different parts of clouds that have a reduced metal content ( see also the behaviour of interstellar ice 
* oliveira  2013 ) ) .
our magellanic clouds surveys also offer a detailed view of the high galactic latitude dib absorption , adding to our all - sky survey of nearby stars to probe structure on scales of @xmath18 pc in and around the local bubble .
we have managed to obtain reasonable all - sky coverage , maintaining also a range in distance so we can probe the 3-d structure of the local bubble and its surrounding neutral medium ( figure[fig2 ] ; bailey  and farhang , in preparation ; see also amin farhang s contribution to these proceedings ) .    while there is some concentration of 5780   dib absorption towards the galactic plane for stars outside the local bubble ( beyond @xmath19200 pc )
, there also appear to be some dib structures very nearby ( @xmath20 pc ) , well inside the local bubble and including some high - latitude clouds .
no less interesting are the weak or null detections , which are not restricted to nearby or high - latitude stars .
it is not uncommon for the 5780   dib to be stronger in a nearby star seen in a direction close to that of a more distant star with weaker dib absorption . indeed , there is a generally poor correlation between dib equivalent width and distance .
the milky way as seen in the 5780   dib appears rather patchy and porous .
2006 , _ a&a _ , 447 , 991 2007 , _ a&a _
, 470 , 941 2011 , _ a&a _ , 530 , a108 2003 , _
a&a _ , 411 , 447 2009 , _ mnras _ , 394 , 831 2013 , _ mnras _ , 428 , 3001 2012 , _ apj _ , 744 , 20 2013 , _ a&a _ , 554 , a33 2007 , _ mnras _ , 382 , 1353 2009 , _ mnras _ , 399 , 195 2010a , _ aj _ , 139 , 68 2010b , _ aj _ , 139 , 1553 2013 , _ a&a _ , 550 , a108 2010 , _
a&a _ , 510 , a54 2006 , _
apjs _ , 165 , 138 1994 , _ mnras _ , 267 , 660
in a quest to further our understanding of the diffuse interstellar medium ( ism ) as well as the unidentified carriers of the diffuse interstellar bands ( dibs ) , we are mapping dibs across the sky using hundreds of hot stars as background torches  
 globular clusters ( in particular @xmath0centauri ) , nearby stars in and around the local bubble , and stars within the magellanic clouds . 
 i describe the results so far obtained and our current experiments .