Table Of Content(DOI:willbeinsertedbyhandlater)
A search for Interstellar absorptions towards Sgr dSph
S.Monai1,P.Bonifacio1,andL. Sbordone2,3 ⋆
1 IstitutoNazionalediAstrofisica-OsservatorioAstronomicodiTrieste,ViaTiepolo1134131Trieste-Italy
2 EuropeanSouthernObservatory,Casilla19001,Santiago,Chile
3 Universita`diRoma2”TorVergata”,viadellaRicercaScientifica,Rome,Italy
Abstract.WesearchedforNaID1,D2interstellarabsorptionlinestowards12giantstarsoftheSgrdwarfspheroidaland3
starsoftheassociatedglobularclusterTerzan7,observedathighresolutionwiththeUVESspectrographatthe8.2mKueyen
VLTtelescope.Thesestarsarenotanidealbackgroundsourcefortheanalysisofinterstellar(IS)absorptionlinessincethey
5
havebeenobservedintheframeworkofastellarabundancesstudy.Howevertheyaresufficientlyluminoustoallowadecent
0
0 S/Nratioataresolutionofabout43000,i.e. 7kms−1,whichallowsanexploratorystudy.Wedetectedtwodistinctgroupsof
2 ISabsorptions, a“local”group, withradialvelocitiesranging from-43kms−1 to50kms−1,andahigh velocitygroup with
radialvelocitiesrangingfrom150km/sto165km/s.LikelythehighvelocitycomponentsareduetogasfallingontheSgrdSph
n
sincehavebeenobservedonlyin2starswhichareseparatedbyonly1.′5inthesky.Wearguethattheobservationssuggesta
a
J strippingofgasduetothepassageofSgrthroughtheGalacticdisc.
3
Keywords.InterstellarMedium–galaxies:individual:SgrdSphstars:GlobularClusters:individual:Terzan7
1
v
0 1. Introduction siderably fainter, which implies that their use as background
1
sourcesrequiresamajorinvestmentintelescopetime.Itisthus
0 The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is one of
ofparamountimportancetomakethebestpossibleuseofthe
1 the nearest satellites of the Milky Way. It offers an interest-
0 availablespectra.
ingopportunityforinvestigatingthestructureoftheinterstellar
5
medium(ISM)intheGalacticHaloand,inprinciple,alongall
0
In general dSph galaxies are considered to be gas poor,
/ thelineofsightuptothedwarfgalaxy.TheUVESspectraof
h as opposed to dwarf irregulars. With regard to the gas con-
12giantstarsinSgrand3giantstarsintheassociatedglobular
p tent of the Sgr dSph, pointed HI observations of its central
- clusterTerzan7,whichhavebeenrecentlyusedtoinvestigate
region indicate that it does not contain a significant amount
o the chemical composition of the Sgr dSph (Bonifacioetal.,
r ofneutralgas(Koribalskietal.,1994).ThesearchforHIwas
t 2000, 2004; Sbordoneetal., 2004) give us the opportunity
s continuedbyBurtonandLockman (1999), whofoundnode-
to perform the first study of this kind. The Sgr giants are, of
a tectable emission and providedan upperlimit to the HI mass
: course,notidealbackgroundsourcesforthestudyoftheISM,
v M(HI)<7000M⊙over18deg2,betweenb=−13◦and−18◦.5.
since the spectrum is complicated by the many stellar photo-
i This result is somewhat surprising since the chemical abun-
X sphericabsorptionlines.TheonlysuitableISMlinesforwhich
dances (Bonifacioetal., 2000, 2004) and the colours of the
r suchastudyispossible,withtheabovedata,aretheNaIDdou-
a Sgrstarsshowthatitcomprisesametal-richyoungpopulation
blet lines. The observed stars are K giants and display strong
implying that the Sgr dSph was forming stars within the last
photosphericabsorptionsintheselines. Howeverthelargera-
Gyr.Bonifacioetal. (2004)suggestedthatthepassagesofSgr
dialvelocityof Sgr(∼ 140kms−1) guaranteesthatat least all
throughtheGalacticdisctriggerstarformationepisodes,how-
thelowradialvelocityGalacticabsorptionsarenotaffectedby
everthisimpliesthatthereisgasavailableinSgrorthatitcan
thecorrespondingstellarabsorptions.Luckily,thespectralre-
be stripped from the disc. The “Blue Plume” stars are mem-
gion adjacent to the Na I D lines is relatively free from ab-
bersofSgr(Zaggiaetal.,2004)andthereislittlequestionthat
sorptions in the stellar spectrum, these two conditions allow
theyaretheMainSequenceoftheyoungpopulation.Stillthe
to use these spectra for an exploratory study of the ISM to-
questionlingers:wherehasthegasassociatedtothisrecentstar
wards Sgr. We further note that the hot stars in Sgr (the so
formationgone?ArecentinvestigationofPutmanetal. (2004)
called“BluePlume”andtheHBstars),whichwouldbemore
hasshownthatgasisassociatedwith thestreamofstellar de-
desirablebackgroundsourcesforthestudyoftheISM,arecon-
bris found to extend, along the Sgr dSph orbit, for over 150◦
Sendoffprintrequeststo:S.Monai across the south Galactic hemisphere.It thus appearsthat the
⋆ BasedonobservationsobtainedwithUVESatVLTKueyen8.2m tidalinteractionofSgrwiththeMilkyWayresultsinthestrip-
telescopeinprogram67.B-0147 pingofgas.
Table1.Basicdataonthestarsusedasbackgroundsources
Stara α(2000)b δ(2000)b V (V−I)c v σ
0 rad v
kms−1 kms−1
432 18 53 50.75 −30 27 27.3 17.55 0.965 160.8 0.3
628 18 53 47.91 −30 26 14.5 18.00 0.928 145.7 0.6
635 18 53 51.05 −30 26 48.3 18.01 0.954 125.2 0.2
656 18 53 45.71 −30 25 57.3 18.04 0.882 135.0 0.3
709 18 53 38.73 −30 29 28.5 18.09 0.917 125.2 0.5
716 18 53 52.97 −30 27 12.8 18.10 0.902 136.0 0.4
717 18 53 48.05 −30 29 38.1 18.10 0.872 142.0 0.4
772d 18 53 48.13 −30 32 0.8 18.15 0.947 143.8 0.8
867 18 53 53.02 −30 27 29.2 18.30 0.933 154.3 0.2
879e 18 53 48.59 −30 30 48.7 18.33 0.965 133.8 0.8
894 18 53 36.84 −30 29 54.1 18.34 0.940 129.5 0.8
927 18 53 51.69 −30 26 50.7 18.39 0.937 144.0 0.3
1282f 19 17 39.36 −34 39 6.4 16.08 1.324 158.2 1.0
1515f 19 17 38.16 −34 29 16.8 16.76 1.156 157.8 1.0
1272f 19 17 37.11 −34 39 11.8 16.62 1.188 158.9 1.0
aStarnumbersforSgrstarsarefromMarconietal.(1998)field1,availablethroughCDS
atcdsarc.u-strasbg.fr/pub/cats/J/A+A/330/453/sagit1.dat
baccurateto0′.′3(Ferraro&Monaco2002,priv.comm.forSgrandZaggia2004priv.comm.forTerzan7)
cTheadoptedreddeningisE(V−I)=0.22
dthisisstar[BHM2000]143ofBonifacioetal. (2000)
ethisisstar[BHM2000]139ofBonifacioetal. (2000)
f StarnumbersforTerzan7starsarefromthecatalogueofBuonannoetal. (1995)availablethroughCDS
atcdsarc.u-strasbg.fr/pub/cats/J/AJ/109/663/table2.dat.
Havingthisinmind,ourattentionhasbeenfocusedonthe of a synthetic spectrum, since atmospheric parameters and
searchofISMcomponentsatradialvelocitiesclosetothoseof chemical abundances of the background sources are known.
the Sgr dSphstars. Thisis certainlya necessaryconditionfor For each star we computed a synthetic spectrum using the
gas belonging to Sgr, although radial velocity by itself is not Linux version of SYNTHE (Sbordoneetal., 2004; Kurucz,
sufficienttoprovethatthegasisactuallyatthatdistance. 1993) and the same atmospheric parameters and abundances
given in Bonifacioetal. (2000), Bonifacioetal. (2004) or
Sbordoneetal. (2004), as appropriate. An example of syn-
2. ObservationsandAnalysis thetic spectra is shown in Fig. 2, with this aid we rejected
possible identifications of interstellar lines for which one of
Details on the observations are reported in Bonifacioetal.
thecomponentsappearedtobeseverelyblendedwithastellar
(2000), Bonifacioetal. (2004) and Sbordoneetal. (2004).
absorptionline.Thewavelength,columndensityandbcoeffi-
For the convenienceof the readerwe reportthe basic data on
cientwerethereforederivedonlywhenthelinesweredetected
thestarsusedasbackgroundsources,inTable1.Thespectral
in both componentsand beyonda 3σ leveloverthe synthetic
resolutionisabout43000,i.e.thevelocityresolutionisabout7
spectra, i.e. only when the latter were not able to completely
kms−1attheNaID2linewavelength.
reproducethefeatures.Theanalysiswasperformedbymeans
We used the verysame spectra used for stellar abundance
of the code fitvoigtminuit(E. Caffau, 2004 private com-
analysis in the above quoted papers and we refer the reader
munication).Thiscode,starting frominitial guessesprovided
to those papersfor details on the data reduction.For our pur-
bytheuser,performsavoigtprofile(χ2)fit;priortothecom-
pose the main problem in our data is that the terrestrial Na I
putationofthe(χ2)thecomputedprofileisconvolvedwithan
D emission lines, when present, cannot be satisfactorily sub-
instrumentalprofile,inourcaseweusedagaussianprofilewith
tracted.ThisimpliesthatthelowestspeedlocalIScomponents aFWHMof7kms−1,whichisappropriateforourdata,ascan
areaffected,sometimesseverely.However,thecomponentsin
beverifiedfromtheprofilesofunblendedThIlinesinthecal-
the Sgr dSph velocity range are always clean from the atmo-
ibration arc spectrum. The minimum is found by the MINUIT
sphericemission.Theheliocentriccorrectionswerealwaysap-
routine(James,2001).Thefittingparametersforeachcompo-
pliedbeforesummingthespectra.
nentarefour:centralwavelength,columndensity,bfactorand
Another serious problem is posed by the stellar absorption
gf value.OneoftheusefulfeaturesofMINUITisthatanyofthe
lines, though these may be reliably identified with the aid
Table 2. Column densities, b factors and velocities, for our Table3.atomicparametersusedinthefittingprocedure
sample.*meansthatbandcolumndensitywereassumedand
notfitted. line λ(Å) fvalue
NaID1 5889.9510 0.65459
Star logN b v NaID2 5895.9240 0.32732
kms−1 kms−1
867 13.42 0.50 −17
inFig.2),toproperlyrecoveralltheparametersoftheISlines
14.63 1.57 −2
(central wavelength, b and column density) it would be nec-
716 11.58 6.05 11
11.48 7.33 34 essary to fit simultaneously the stellar and interstellar lines.
927 12.43 6.81 −15 Although this is possible, in principle, using the stellar syn-
12.42 3.70 −7 thetic spectra, we felt that considering the rather low S/N ra-
11.94 1.45 9 tio inthe spectraandtheadditionaluncertaintyintroducedby
635 11.80* 0.60 150 the modelling of the stellar absorptions the results would be
11.50* 0.50 165 affected by a large error,too large in fact to be a useful mea-
432 12.47* 0.93 −16
sure.Instead,intheexploratoryspiritofthepresentwork,we
13.84* 1.30 −5
decided in these cases to assume the values of b and column
13.00* 1.10 7
density and fit only the central wavelength of the IS absorp-
879 12.40* 5.30 −1
tion.Althoughthisprocedureissomewhatarbitraryitisfairly
12.60* 1.40 18
robust. As long as column density and b are fixed arbitrarily,
12.00* 0.20 40
12.00* 0.20 50 but in such a way that the resulting synthetic spectrum is not
628 13.90* 0.20 −4 obviously incompatible with the observed spectrum, the fit-
12.40* 0.68 10 ted centroid of the componentdoes not change by more than
11.50* 1.70 28 1 kms−1. Thisis clearly sufficientforthe exploratorypurpose
11.60* 0.30 36 of the present study. The results of our analysis are reported
11.40* 0.30 44 in Table 2. When b and column density were kept fixed, the
717 12.45* 0.28 −15
symbol*appearsnearthelogNvalues.
12.60* 0.33 −5
Afewcommentsareinorderhere.Theavailableresolutionis
12.45* 0.58 14
somewhatlimitedforadetailedanalysisofthekinematicstruc-
772 13.20* 2.08 −2
tureoftheISM.Itisquitelikelythatinhigherresolutionobser-
12.46* 3.43 17
vationsthe“components”wefittedsplitintoseveralnarrower
11.71* 2.11 26
656 12.10* 1.70 −17 components.The numberof componentsto fit to a givenfea-
13.40* 1.90 −5 ture issomewhatarbitraryand,forthe abovereason,maynot
11.50* 0.67 160 haveparticularphysicalmeaning.Weusedthecriteriontouse
709 12.64 0.94 −16 theminimumnumberofcomponentsinthefit. Itisthusclear
13.15 2.41 −2 thatwedonotgiveparticularphysicalsignificancetothecol-
12.18 3.63 15 umndensitiesandbfactorsobtained,keepinginmindthatthey
14.53 0.00 36 reflectthe combinedeffectofseveralgasclouds.Inparticular
894 11.80* 1.40 −16
bfactorsarenotverysignificant;mostofthevalueswefindare
12.40* 2.75 −5
wellbelowourinstrumentalresolution,andthus,notverywell
11.80* 1.70 8
constrained.WestressherethattheFWHMofallthedetected
12.60 0.40 20
1282 11.87 2.82 −42 featuresisabovetheinstrumentalresolution.Averylowfitted
13.72 2.29 1 value of b simply reflects the fact thatthe FWHM of the fea-
1515 11.60 2.04 −40 ture is only very slightly larger than the instrumental profile,
12.70* 3.90 0 howeverattheS/Nratiosofthepresentdataallthevaluesofb
1272 12.38 2.33 −43 upto7kms−1 providealmostequallyacceptablefits.Whatis
13.79 2.34 0 significantisthatwedonotobservebfactorsinlargeexcessof
ourinstrumentalresolution.Thisallowstoconcludethatthere
isnostrongturbulencewithinthecloudsandthatthevelocity
parameterscanbeheldfixed,evenallfour,inwhichcasethere dispersionofgroupsofcloudswhichclusterinvelocityspace
isnofitting,theroutinesimplycomputesalinefortheinputpa- (our“components”)islessthanourinstrumentalresolution,i.e.
rameters.Inourcasethegf values,whicharewellknownfor 7kms−1.
the Na I D lines, were of course always keptfixed (see Table
3).
3. Resultsanddiscussions
Atthisrelativelylowresolutionthereisnoneedtotakeinto
accountthehyperfinestructureoftheNaIDlines,whichsim- FromTable2wecaninferthepresenceoftwodistinctgroups
ply results in a slightly larger b value. In the cases in which ofIScomponents:onewhichwecall“local”,rangingfrom-43
the detected IS lines are contaminated by the stellar absorp- kms−1 to 50 kms−1, and a high velocity group ranging from
tions(clearexamplesarethehighvelocitycomponentsshown 150kms−1 to165kms−1.
Fig.1.MapoftheobservedstarsinSgrdSph,thestarsinsquaresarethoseinwhichhighvelocityabsorptionshavebeendetected,
thecross(×)indicatesthecenterofthefield,labeledbyitscoordinates.Scalesareinarcminutes.
3.1.Thelocalcomponents bvalues(wherereliable)arenotashomogeneousasthoseob-
servedinthedirectionofTerzan7.
In most cases the sky contaminationspreventus from precise
Othercomponentsbehavein thesame ubiquitousmanner,
column densities or b measures, neverthelessthe velocity de-
temptatively we can identify the following ones: 9-15 kms−1,
terminationsarereliable.
18-28kms−1,34-36kms−1and40-45kms−1thelattertwocom-
Weunderlinethepresenceofthe≃-43km/sand≃0kms−1 to-
ponent are present towards 3 stars. Other absorptions are ob-
wardsallthe Terzan7stars, whichare probablyduetoa local
served towards less than 3 lines of sight, their distances are
cloudcoveringthesmallangularspreadoftheselinesofsight.
probably greater then the previous ones, but we cannot infer
Thecolumndensitiesandbvaluesareconsistentwiththissce-
any more aboutthem. The evidentabsorption features of star
nario.
#635havenotbeenfittedduetosevereskycontaminationsof
The component around –15 kms−1 is also quite ubiquitous,
bothD1andD2lines.
we find it towards 7 out of 12 lines of sight towards the Sgr
dSph. The RedfieldandLinsky (2000) model for the G and
Local Interstellar Cloud in these directions, gives velocities 3.2.Thehighvelocitycomponents
of –24.27 kms−1 and –22.2 kms−1 respectively, at our reso-
lution these two components would appear as a unique fea- As already said, this is the first investigation towards the Sgr
ture at ∼ −23 kms−1. It is not clear whether the component dSph, therefore the finding of high velocity IS absorptions
around−15kms−1foundbyuscouldbeindeedcompatiblewith could revealthe presence of gas in it, or in the Galactic Halo
the RedfieldandLinsky (2000) model. As mentioned above, ontheothersideofGalacticcentre.
we had a heavy contamination by atmospheric NaI emission Only three components of velocities 150, 160, and 165
lines, and considering that the heliocentric corrections are kms−1 havebeenfoundtowardsonlytwolinesofsight,asre-
around–8,–10kms−1,apossiblesystematicerrorinourveloc- portedin Fig. 1. Inparticular,the 150and165kms−1 are re-
itymeasure,duetoanincorrectsubtractionoftheatmospheric vealedtowardsstar#635asshowninFig.2.Thenonubiqui-
emission, leaves open the question of the identification with tous natureof these absorptionsshouldindicate the greatdis-
the above components predicted by the RedfieldandLinsky tance of the absorbinggas, given the relative angular clumpi-
(2000)model. nessofoursample,andinparticularofthesetwostars.Nearby
Another component is revealed between –7 and –2 kms−1. clouds should cover more than 2 or 3 lines of sight, but, at
This velocity spread is not very significant, since our veloc- the other side of our Galaxy, their angular sizes could not. In
ityresolutionis≃7kms−1,perhapsitcouldberelatedtothe0 particular, the nearest star to # 635 is # 927, which shows no
kms−1 componenttowardTerzan7.Thecolumndensitiesand detectableISabsorptionsatthesevelocities,itsangularsepara-
Fig.2.Spectraofstarsshowinghighvelocitycomponents.Thedashed-dottedlineisthesyntheticISfitwhereasthecontinuous
lineisthesyntheticstellarspectrum.Thelocalcomponentsof#635areseverelycontaminatedbytheskyandnofitwasreliable.
tioncorrespondstoalinearseparationof≃2.9pcatthedistance that the gas is at the same distance as Sgr. In the first place
of Sgr dSph. The non detection could be explained by three it shows that there is a small amount of gas in Sgr, possibly
causes: clumpyandconsistingofmanysmallclouds.Thisgasmaybe
theresidualofthegaswhichwasusedupinthelaststarforma-
1. since the radial velocity of star # 927 is 18.8 km/s higher
tion episode of Sgr, which gave birth to the youngmetal-rich
than that of the # 635, it is obvious from Fig.2 that the
populationfoundbyBonifacioetal. (2004),therestbeingnow
stellar features could mask completely the IS ones in this
intheformofstarsorlostalongtheorbit.Inthesecondplace
velocityrange.
sincethisgasischemicallyenrichedand“falling”towardsSgr
2. sincefromtheanalysisofBonifacioetal. (2004)the#927
it could well be gas which has been stripped from the Milky
is moremetalrichthan# 635bya factoroftwo, itshows
Way. If ateach passage Sgr is actually capable of“refueling”
stronger stellar absorptions just around these components
with metal enriched gas stripped from the Galactic disc, this
andthisprobablycould,byitself,preventusfromdetecting
couldbeameanstomaintainthestarformationepisodesovera
themeveniftheywerepresent;
verylongperiodoftime,astestifiedbytheverylargemetallic-
3. thecloudsareveryclumpy,ofsizelessthan2.9pc.
ityspreadofSgr(−3≤[Fe/H]≤0.1Zaggiaetal. 2004),even
Arguments1) and 3) apply also to star # 628, which falls ex-
without a large reservoir of local gas. Moreover it could ex-
actly between the # 635 and # 656, and has a larger angular
plaintheanomalouslyhighmetallicityofSgr,comparedtoits
distance than the angular distance of star # 927 from star #
luminosity;perhapsSgrisnotsochemicallyevolvedbecauseit
635,fromeitherstar#635orstar#656.ThelackofHIdetec-
retainsefficientlytheejectaofitsownSNe,butsimplybecause
tionsintheSgrdSphdirection,(BurtonandLockman,1999),
it “steals” enriched materialfrom the Milky Way. These con-
could be well explained by the small angular size of case 3).
siderationsarestillhighlyspeculative,howevertheyprovidean
Moreover,itisveryinterestingtonotethatthesevelocitiesare
importantthrustfor the continuationof such an investigation.
always greater than the stellar ones, implying that the gas is
Data for more lines of sight using hotter backgroundsources
“falling”towardsthem.
whichspana rangeofdistancesfromtheGalactic disc to Sgr
arebadlyneededtotestthesehypothesis.
4. Conclusions
Acknowledgements. We are grateful to E. Caffau for providing the
The presence of high velocity absorptions towards 2 out of a
fitvoigtminuitcodeandtoS.Zaggiaforprovidingaccuratecoor-
sample of 12 stars of the Sgr dSph is the main result of our
dinatesoftheTerzan7stars.WearealsoindebtwiththerefereeDr.J.
analysis. Given the BurtonandLockman (1999) upper limit Linskybecausethearticlehasbeenimprovedthankstohisconstruc-
on HI mass over the area, one can derive an upper limit for tivecomments.ThisresearchwasdonewithsupportfromtheItalian
the HI column density of logN(HI) ≤ 18.87. We stress the MIURCOFIN2002grant“StellarpopulationsintheLocalGroupasa
fact that the 21′ radio beam integratesthe signal overan area tooltounderstandgalaxyformationandevolution”(P.I.M.Tosi).
whichisconsiderablylargerthanourfield.TheSgrstarswere
initially foundas radialvelocitymembersonan EMMI MOS
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