Far-infrared loops in the 2nd Galactic Quadrant

0.49 MB·English

by  Cs. Kiss

Astronomy&Astrophysicsmanuscriptno.cfirlg˙astroph February2,2008
(DOI:willbeinsertedbyhandlater)
Far-infrared loops in the 2nd Galactic Quadrant
Cs.Kiss1,2,A.Moo´r1,andL.V.To´th2,3
4
0
0
1 KonkolyObservatoryoftheHungarianAcademyofSciences,P.O.Box67,H-1525Budapest,Hungary
2
2 Max-Planck-Institutefu¨rAstronomie,Ko¨nigstuhl17,D-69117,Heidelberg,Germany

n 3 DepartmentofAstronomy,Eo¨tvo¨sLorndUniversity,P.O.Box32,H-1518Budapest,Hungary
a
J
Received/Accepted...
5
1
Abstract. We present the results of an investigation of the large scale structure of the diffuse interstellar medium in the

2nd GalacticQuadrant (90 l 180 ).145loops wereidentifiedonIRASbasedfar-infraredmaps. Ourcatalogue liststheir
1 ◦≤ ≤ ◦
v basicphysicalproperties.Thedistributionclearlysuggeststhatthereisanefficientprocessthatcangenerateloop-likefeatures
3 at highgalacticlatitudes. Distancesareprovided for 30loops. Wealsogiveanobservational estimateon thevolumefilling
0 factorofthehotgasintheLocalArm,4.6%≤f2nd<6.4%.
3
1 Keywords.Catalogs–ISM:bubbles–Galaxy:structure
0
4
0
1. Introduction mation, and not to the infall of HVCs. Apart from the con-
/
h nection of these features to young massive stars and their as-
p A studyofthe diffuseinterstellarmediumis presented,a step
sociations, they may also provide a nest for the next genera-
- inthewayderivingthenear-pastandnear-futureofthegalac-
o tionofstarsviatheprocessofpropagatingstarformation.The
r tic star formation.The large scale structure of the ISM in the idea was first proposed by Oort (1954) and O¨pik (1953), and
st Galaxyisdiversewiththecomplexdistributionofcavities,fil- later followed by observationalevidences (see Blaauw, 1991,
a aments,arc,loopsandshells,whichisoftenreferredtoasthe
for a review). This mechanism was modelled in detail by
:
v “CosmicBubbleBath”(Brand&Zealey,1975).Ridgesofen-
Elmegreen&Lada (1977) and successfully applied to the se-
i hanced radio contiunuum emission extending to high galac-
X quential formation of subgroups in OB associations. Gamma
tic latitudes have been known since the 60s. The first exten-
r ray bursts may also be responsible for the formation of the
a sivelystudiedexamplesareLoopI(Largeetal.,1966),LoopII largest HI holes, as proposed by Efremovetal. (1998) and
(”Cetus Arc” Largeetal., 1962) and LoopIII (Largeetal.,
Loeb&Perna(1998).Nonlineardevelopmentofinstabilitiesin
1962).LateranomalousHIfeatureslikeworms,chimneys,su-
theISMmayformlargecavitiesaswell,withoutstellarenergy
pershells were identified and catalogued, up to a character-
injection (Wadaetal., 2000; Korpietal., 1999; Klessenetal.,
istic size of 2kpc with expansion velocities of 20kms 1
− 2000;Sa´nchez-Salcedo,2001).
∼ ∼
(Heiles, 1979, 1980, 1984; Hu, 1981; Koo&Heiles, 1991).
The cavities of the ISM contain low density hot gas in
ThesewereassociatedwithvoidsintheHIdistribution(seee.g.
the multiphase model by McKee&Ostriker (1977). The fill-
Palousˇ,1996,forareview).Similarcomplexmorphologyhas
ing factor f of this hot gas in the galactic disc is an impor-
been recognized in some nearby spiral and irregular galaxies
tantparameteroftheISM,whichispoorlyconstrainedobser-
e.g.M31(Brinks,1981),M33(Deul&denHartog,1990)and
vationally. Ferrie`re (1998) and Gazol-Patin˜o&Passot (1999)
theMagellanicClouds(Staveley-Smithetal.,1992;Kimetal.,
estimate f 20%atthesolarcircle,whichdropsto f =3–8%
1998). ≈
for8.5 R 10kpcgalactocentricdistances.
The most studied galactic shells were formed by su- ≤ ⊙≤
The21cmemissionofdiffuseHIcloudsiswell-correlated
pernova (SN) explosions and winds of massive stars
with the 100µm FIR emission (Boulanger&Pe´rault, 1988).
(Tenorio-Tagle&Bodenheimer, 1988). However, high veloc-
This extended emission is often referred to as the galac-
ityclouds(HVCs)mayalsoformlargecavitieswheninfalling
tic cirrus (Lowetal., 1984). A typical far-infrared colour of
fromthegalactichalo andcollidingwith the ISM ofthe disc.
∆I /∆I =0.25 was derived from IRAS/ISSA maps when
AremarkableexampleofaHVC–galacticdiscinteractionis 60 100
the gains are corrected according to Wheelocketal. (1994).
theNorthCelestialPole(NCP)loop(Meyerdierksetal.,1991).
AccordingtoCOBE/DIRBEresultsitsdustcolourtemperature
Ehlerova´&Palousˇ (1996) investigated the origin of galactic
isT 18K(Lagacheetal.,1998).
HI shells and found, that those are likely related to star for- d
≈
Studies of the large scale structure of the cirrus emis-
Sendoffprintrequeststo:Cs.Kiss,[email protected] sion and loop- or arc-like features were restricted so far to
2 Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant
low galactic latitudes in the far-infrared (Schwartz, 1987;
Marston, 1996), and reached only medium galactic latitudes
in radio surveys. Detailed investigations of individual loops
were performed only in the vicinity of the galactic plane.
RecentlyDaigleetal.(2003)presentedthefirstautomatictool
for detecting expanding HI shells using artificial neural net-
works.Theirdataisrestrictedtotheskyarea75 l 145 and
◦ ◦
≤ ≤
3 b 5 andtheirdetectionstrategyisbasedonthevelocity
◦ ◦
− ≤ ≤
informationoftheHIdataratherthanmorphology.
Inordertogetacomplete,full-scaleviewonthelarge-scale
structure of the cold diffuse ISM, in our study we searched
thewhole2ndgalacticquadrant(90 l 180 ;–90 b 90 )
◦ ◦ ◦ ◦
≤ ≤ ≤ ≤
forintensityenhancementsintheinterstellarmedium,showing
loop-likestructures.
2. Dataanalysis
2.1.Questforloopsonfar-infraredmaps
We investigated the 60 and 100µm ISSA plates (IRAS Sky
Survey Atlas, Wheelocketal., 1994) in order to explore the Fig.1. Intensity profile before (top) and after (bottom) back-
distributionofdustemissioninthe2nd GalacticQuadrant.We groundremoval(graysolidline)withthemainderivedparam-
created composite images of the 12◦.5×12◦.5 sized individual eters(Iex,σex,ain,aout,seeSect.2.2)
ISSA plates using the ”geom” and ”mosaic” procedures of
theIPAC-Skyviewpackage,bothat60and100µm.Theseim-
fromtheDIRBE100and240µmmaps,andatemperaturecor-
ageswerebuiltuptypicallyfrom 10-15ISSAplates,reaching
∼ rectedmapwasusedtoconvertthe100µmcirrusmaptoamap
a size of 40 40 . Loop-like intensity enhancements were
◦ ◦
∼ × proportionaltodustcolumndensity.
searchedbyeyeonthe100µmmosaicmaps.Loopsbyourdefi-
nitionmustshowanexcessFIRintensityconfinedtoanarc-like
feature,atleast60%ofacompleteellipse-shapedring.Aloop Shape: We approximated the shape of a possible loop by
mayconsistofa set ofbright,moreorless isolated,extended an ellipse, which was then fitted using a 2D least-square fit
spots,ormaybeadiffuseringorpartofaring.Thesizeofthe method.AnellipseshapeisexpectedfromSNorstellar wind
mosaicimagelimitsthemaximalsizeoftheobjectsfound.On shells, since(1)non-sphericalexplosion(wind)mayoccur,in
the other hand, due to the relatively large size of the investi- the mostextremecase creatinga ring,ratherthana shell, and
gatedregions,loop-likeintensityenhancementswitha size of (2)originallysphericalshellsaredistortedtoellipsoidal(i)due
≤1◦werenotsearched.TheoriginalISSAII6S0SA andII1S0S0A sur- to the shear in the direction of galactic rotation (Palousˇetal.,
facebrightnessvaluesweretransformedtotheCOBE/DIRBE 1990)and(ii)duetotheverticalgravitationalfieldinthegalac-
photometricsystem,usingtheconversioncoefficientsprovided ticdisc(seee.g.Ehlerova´&Palousˇ,1996).
byWheelocketal.(1994):
– I =0.87 IISSA+0.13MJysr 1 The fitted ellipse is defined with the central (galactic) coor-
– I60 =0.72× 6I0ISSA 1.47MJys−r 1 dinates, the minorand major semi-axis of the ellipse, and the
100 × 100 − − positionangleofthemajoraxistothecircleofgalacticlatitude
Dust IR emission maps by Schlegeletal. (1998) (SFD) at the centre of the ellipse. This latter was defined to be ’+’
wereinvestigatedtoderiveparametersdescribingourloopfea- fromEasttoNorth(orcounter-clockwise).
tures(seeSect.2.2).ThemaindifferencesoftheSFD100µm
mapcomparedtotheISSAmapsarethefollowing:
Intensityprofile: Forallofourloopsweextractedradiallyav-
eraged surface brightness profiles, extended to a distance of
(1) Fourier-destrippingwasapplied,
twicethemajor(andminor)axisofthefittedellipse,using40
(2) asteroidsandnon-Gaussiannoisewereremoved,
concentric ellipsoidal rings. These surface-brightness profiles
(3) IRASandDIRBE100µmmapswerecombined,preserving
(ISSA100and60µm,SFD100µmandSFDreddeningmaps)
theDIRBEzeropointandcalibration,
were used in the following to determine the basic parameters
(4) starsandgalaxieswereremoved.
oftheFIRemissionintheloop.AnexampleisshowninFig.1
Weanalysedtheradialsurfacebrightnessprofilesoftheloops
ontheSFD100µmmapinordertochecktheeffectofthere-
2.2.Derivedparameters
movalofthesourcesmentionedabove.WealsousedtheSFD
E(B V) maps derived from the dust column density maps. Significance: Thelocalbackgroundwasdeterminedusingthe
−
In the case of these maps the colour temperaturewas derived ’non-loop’points in the radial surface brightness profiles, fit-
Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant 3
Fig.2.Examplesofloopsfoundinthe2ndGalacticQuadrant.left:GIRLG134–28,firstdescribedbyHeiles(1984)asGS137–
27–17.This supershellis located at a distance at 1.5kpc and has a size of 0.41 0.31kpc; right: GIRL G117+00,supershell
∼ ×
aroundtheCasOB5associationinadistanceof2.5kpcwithasizeof 0.31 0.27kpc(Moo´r&Kiss,2003).
∼ ×
tinga3rdorderpolynomial.Thisappropriatebackgroundwas
removed from each surface brightness profile points. The in-
tensity excess I was derived as the maximum value of this
ex
backgroundremovedprofile.In orderto checkif thisvalueis
abovethe ’noise’,we calculated the standard deviationof the
background removed intensity in the ’non-loop’ values, σ ,
ex
and defined the significance of the loop as Ψ = I /σ . We
ex ex
derivedsignificanceparameterson60and100µmISSAmaps
and on the SFD 100µm point source removedsky brightness
and reddening maps. The higher the value of Ψ is the higher
theintensityexcessoftheloopoverthebackground,therefore
weusethisparameterasa’qualityindicator’inthefollowing.
Relative width: Innerand outer edges of the loop wall along
the major axis (a and a , respectively) are defined as the
in out
radial distance at the full width at half power of the back-
groundremovedintensityprofile,I /2(Fig.1)Wedistinguish
ex
three regions for a specific loop: (1) loop interior (a<a ),
in
(2) loop wall (a a a ), (3) outer region (a>a ). The
in out out
≤ ≤
relative width of the wall of the fitted ellipse is defined as
W = 1 a /a .
in out
−
Colour index: We derived a colour index for our loops
∆I /∆I , from the radially averaged 60 and 100µm surface
60 100
brightness profile. This is defined as the slope of the I vs.
60
I100 scatterplotusingthedatapointsofthesurfacebrightness Fig.3. Distribution of GIRLs in the sky in the 2nd Galactic
profilesinthepositionsoftheloopwall(ain a aout)only. Quadrant(Aitoffprojection),representedbythefittedellipses
≤ ≤
call these objects ’GIRL’-s, abbreviating ’Galactic InfraRed
3. Results
Loops’.
3.1.Thecatalogue Theentriesofthecataloguearethefollowing:
We identified 145 loops in the 2nd Galactic Quadrant. The 1.) Name of the loop, derived from the galactic coordinates
parameters of these loops are summarized in Table A.1. We of the loop centre. The format is : GIRL GLLL BB, the
±
4 Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant
’GIRL’ prefix stands for ’Galactic InfraRed Loops’, ’G’
marksthegalacticcoordinatesystem,’LLL’isthegalactic
longitude in degrees, is the sign of the galactic latitude
±
(+/–),and’BB’istheabsolutevalueofthegalacticlatitude
oftheloopcentre.
2.) Centralgalacticcoordinatesoftheloop(landb)
3.) Semimajorandminoraxisofthefittedellipse
4.) Position angle of the fitted ellipse. The position angle is
defined as ’+’ from East to North. Zero position angle is
pointedtotheEastingalacticcoordinates.
5.) Major axes corresponding to the inner and outer edge of
the loopwall, derivedfromthe 100µm surfacebrightness
profile
6.) ISSA 60 and 100µm, SFD 100µm and reddeningsignifi-
canceparameters(Ψ ,Ψ ,Ψ andΨ ,respectively
I60 100 S100 EBV
7.) colourindex∆I /∆I oftheloopwall
60 100
In the electronic version of the catalogue (URL:
“http://astro.elte.hu/CFIRLG”) we provide the following
additionalfeatures:
8.) 100µmISSAimageoftheloopwiththepossibleassociated
objectsoverlaid
9.) ISSA60and100µm,SFD100µmandSFDE(B-V)surface
brightnessprofilesinFITSformat
10.) Listofobjectsapparentlyassociatedwiththeloop
Anassociatedobjecthastobeplacedin thewallorinthe
interioroftheloop(definedbyconcentricellipses,asdescribed
above). We considered the following type of possible associ-
atedobjects(referencesareindicated)
– darkclouds(Dutra&Bica,2002)
– supernovaremnants(Green,1994)
– OB-associations(Lang,1992)
– pulsars(Tayloretal.,1993) Fig.4.DistributionofGIRLsinthe2ndGalacticQuadrant.(a)
– HIIregions(Sharpless,1959) Loop-countsvs. galacticlongitude.Graylineswith lightgray
– IRASpointsourcewithmolecularcoreFIRcolours filled area: northern galactic hemisphere (b>0 ). Dash-dotted
◦
– IRASpointsourcewithTTaustar-likeFIRcolours black lines: southern galactic hemisphere (b<0 ). (b) Loop-
◦
countsvs.the’sin’ofthegalacticlatitude(equalareabelts)
MolecularcoresandTTauristarswereselectedfromtheIRAS
PointSourceCatalogue(JointIRASScience WorknigGroup,
1988)accordingtothefollowingcriteria: 3.2.Skydistributionandfundamentalstatistics
– pointsourcesassociatedwithgalaxieswereexcluded
Sky distribution: Fig. 3 presents the distribution of the far-
– photometricqualitiesare2orbetterat12,25and60µm
infrared loops found in the 2nd Galactic Quadrant. The dis-
– – molecularcores:
tributionofGIRLsseemstoberathercomplex.However,there
0.4≤log10(cid:18)FF2152(cid:19)≤1.0&0.4≤log10(cid:18)FF2650(cid:19)≤1.3 areslightlymoreloopsatlow(b≤30◦)galacticlatitudesthan
– TTauristars: athighones(b>30◦).
0.0 log F25 0.5&–0.2 log F60 0.4 ItisobviousfromFig4athatthetwogalactichemispheres
≤ 10(cid:18)F12(cid:19)≤ ≤ 10(cid:18)F25(cid:19)≤ represent different loop distributions. We have tested the hy-
followingthe definitionsbyEmerson(1998).F , F and
12 25 pothesis that the galactic latitudes of the loop centres on the
F arethe12,25and60µmuncorrectedIRASfluxes,re-
60 Northernand Southernhemispheresare drawnfromthe same
spectively.
distribution.AKolmogorov–Smirnovtestshowedthattheho-
Thislistofpossibleassociatedobjectdoesnottakeintoac- mogeneousdistributionhypothesiscouldnotbeacceptedatthe
countthe distancesof the individualobjects, thereforeall ob- 80%confidencelevel.Atestperformedforthegalacticlongi-
jectsprojectedtotheloopwallortotheinteriorareincluded.In tude distribution on the two hemispheres(Fig.4b) yielded the
thecaseofafew(30)loops,wewereabletoderiveadistance sameresult.
fortheloopbasedonthedistancesofassociatedobjects.This There is a hint in Fig. 3, that most of the loops are part
isdiscussedindetailinSect.5. Thelistofassociatedobjects of a structurewith scalessignificantlylargerthanthe individ-
ispresentedinAppendixC. ual loop diameters. The most remarkable feature of this kind
Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant 5
is a huge chain of loops seen in the l 90 –130 , b=–35 –
◦ ◦ ◦
≈
+5 region. In the ranges l 130 –140 , b +45 – +65 nine
◦ ◦ ◦ ◦ ◦
≈ ≈
loopscan be foundsqueezedto a relativelysmallarea, which
correlates well with the position of the Ursa Maior molecu-
larcomplexconnectedtotheexpandingshelloftheNCPloop
(Pound&Goodman,1997).
Fig.6.Distributionofthewidthoftheloopwallsrelativetothe
effectiveradiusoftheloop(W=1–a /a )
in out
Fig.5.ApparentsizedistributionofGIRLs.Sizeispresentedin
effective radius, r , the geometricalmean of semi-minor and
eff
-majoraxesofthefittedellipse
Sizedistribution: Wedefinedtheeffectiveradiusofourloops
asthegeometricalmeanofthesemi-minorand-majoraxisof
thefittedellipse,r = √a b.Forthewholesampleofloops
eff
×
we obtainedm(r ) = 3.2 and E(r ) = 2.7 for the sample
eff ◦ eff ◦
meanandexpectationvalueoftheeffectiveradius,respectively.
ThesizedistributionispresentedinFig.5.Majority(78%)
of the loopsare in the apparenteffective radiusr 4 , and
eff ◦
≤
there are only a few (9) loopsr 7 . The lower end of the
eff ◦
≥
sizedistributionislimitedbyourdetectionmethod,whichex-
cludesfeatureswithasizesmallerthan 1 indiameter. Fig.7. Comparison of the expected distribution of minor to
◦
∼
major axis ratio (assuming randomlyoriented 2D rings, solid
line)withthedistributionoftheobservedaxisratios(grayhis-
Distribution of relative width: Fig. 6 shows a Gaussian- togram).
like distribution, with an average value of W =0.29 and a
h i
FWHM=0.04fortherelativewidthW oftheloopwalls.
Significances: InFig.9wecomparethesignificancesderived
from ISSA 100µm, ISSA 60µm, SFD 100µm and SFD red-
Axisratiodistribution: AsmentionedinSect.1,itiswidelyac- deningmaps,Ψ ,Ψ ,Ψ ,Ψ (seealsoTableA.1).
I100 I60 SFD100 EBV
cepted,thatgalacticloops–identifiedinanywavelengthrange The Pearsoncorrelationcoefficientsarelisted in Table 1. The
–are2D projectionsof3Dcavitiesin theISM.We compared Ψ ,Ψ ,Ψ andΨ significancesarewellcorrelated,
I100 I60 S100 SEBV
theexpecteddistributionoftheratioofminortomajoraxesof and in all 145 cases resulted in a definite confirmation of the
thesky-projectionof2Dringtothedistributionoftheobserved suspectedellipsoidalstructure.
minortomajoraxesratiosinFig.7.2Dringsareassumedtobe Fig. 8 presents the distribution of ISSA 100µm and SFD
randomlyorientedin φandθ polarangles.Thisfigureclearly E(B-V)significances(Ψ andΨ ,respectively).Basedon
I100 EBV
showsasignificantdifferencebetweenthetwodistributions.It thedefinitionofΨ(seeSect2.2)onecansetalimitofΨ=3as
isnotlikelythatmostofourobjectsarejusttheprojectionsof avalueseparatingweakandprominentloops(prominentloops
2Drings. areabovethe“3σ”levelofbackgroundfluctuations).However,
6 Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant
correlated correlation
significances coefficients
Ψ –Ψ 0.77
I100 I60
Ψ –Ψ 0.82
I100 S100
Ψ –Ψ 0.68
I100 SEBV
Ψ –Ψ 0.79
S100 SEBV
Table 1. Linear Pearson correlation coefficients of signifi-
cancesderivedfromISSA100µm(Ψ ),ISSA60µm(Ψ ),
I100 I60
SFD100µm(Ψ )andSFDE(B–V)(Ψ )maps.
S100 SEBV
Fig.9.Comparisonofsignificancesextractedfrommapsofdif-
ferent data sets. (a) ISSA 100µm vs. ISSA 60µm; (b) ISSA
100µmvs.SFD100µm;(c)ISSA100µmvs.SFDreddening
maps;(d)SFD100µmvs.reddening
that of the extended background,especially in the vicinity of
the galactic plane (b 10 , see Fig. 10), where loops are
◦
| | ≤
”warmer” according to their FIR colours. A rather high scat-
terofFIRcolourswithhigherrorbarscanbeobservedforthe
highest galactic latitudes, which could also be related to the
less reliable accuracy of the surface brightness calibration in
these fields.We concludethatthe loopsdescribethestructure
ofcirrusinthefewto 20 scales.
Fig.8. Distribution of the Ψ (gray solid line) and Ψ ◦
I100 EBV ∼
(blackdash-dottedline)significancevalues
Excess extinction in the loop walls: Using the radially av-
eraged SFD E(B-V) (reddening) profiles extracted from the
as seen in Fig. 8, the majority of our loops show high sigma appropriate maps we estimated the excess visual extinction
values, regardlessof the significancy type (ΨI100, ΨI60, ΨS100 of the loop walls. Visual extinction values were calculated as
orΨEBV)andonlyveryfewloopshaveΨ≤3. AV =R×E(B−V),whereRistheratiooftotaloverselective
Brightfore-orbackgroundsourcesnotrelatedto theloop extinctionandE(B-V)isthereddeningvalueinaspecificsky
may cause low significances.An exampleis GIRL G125+09, position.Rwaschoosentobeuniformandsettothestandard
whichhaslowΨ andΨ (2.4and2.6,respectively)butit valueof3.1ofthediffusemedium(Kru¨gel,2003).Theaverage
I100 S100
isoutstandingonthereddeningmaps(Ψ =6.0)andisvery valueofA wasderivedfortheinner-,wall-andouterregions
EBV V
conspicuous at 60µm (Ψ =17.2). In addition, this loop is (seeSect.2.2),separatelyforallloops(Ain,Awall andAout,re-
I60 V V V
clearly detected in the HI 21cm line (Kissetal., 2000). The spectively).We also derivedanexcessvisualextinctionvalue
mainreasonbehindthelowsignificancesat100µmisthepres- fortheloopwall,whichwasdefinedas
enceofanextremelybrightregioninthesouth-easternpartof
2 Awall
thelooprelatedtoNGC7822,S171andtotheCepOB4asso- ∆A = · V (1)
ciation, which dominates the backgroundof GIRL G125+09. V Ain+Aout
V V
∆A values show a clear dependence (despite the relatively
V
large scatter around the main trend) on the galactic latitude
Far-infraredcolouroftheloopwalls: AsseeninFig.10most (Fig.11a),widthvalues<0m.01athighgalacticlatitudes,while
of the loops have a far-infrared colour similar to that of the reaching∆AV 10minthevicinityofthegalacticplane.
≈
extendedgalacticbackgroundemissionorgalacticcirruswith Therelativestrengthoftheexcessextinction∆AV relative
an average value of ∆I /∆I =0.25 0.12. In the case of a to the backgroundextinction(Abg = 1[Ain+Aout])is usually
60 100 ± V 2 V V
few loops the far-infrared colour significantly deviates from low(inaverage 10%,asshowninFig.11b)exceptinthecase
∼
Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant 7
Fig.10. Distribution of the colour indices of the loop walls (∆I /∆I ) vs. galactic latitude. Error bars in galactic latitude
60 100
representtheextension(r )oftheloop.ErrorbarsofthecolourindicesaretheformalerrorsoftheI vs.I scatterplots.The
eff 100 60
thickgraycurverepresentsthemaintrend.
of a few loops near the galactic plane. This corresponds to a their associations) were taken into account. The sky posi-
10%increaseofcolumndensityintheloopwalls. tionofthestarsmustbewellconfinedinthecentralpartof
∼
theloop(althoughinsomecasesfastpropermotionsmight
haveplacedthestarfarfromitsoriginalposition).
3.3.Distancestoindividualloops
ii) There must be at least two objects (luminous star, pulsar,
Derivationofthedistance: Manyofthemajorphysicalprop- dark/molecularcloud)withsimilarknowndistancesassoci-
erties of a loop(mass, size, particle numberdensity, etc.) can atedwiththeloop,otherwisethedistancehypothesiswould
onlybetackledifthedistanceisknown.Inordertoestimatethe berejected.
distance of the loops, we used associated objects with known iii) The distancesshouldfit each otherwithin their uncertain-
distances. There are two major classes of these objects: (1) ties.
They may be responsible for the formation of the loop itself,
oraretheremnantsoftheloop-creatingevents(weconsidered
Galacticdistribution: Basedonthedistancesderivedforsome
pulsars,luminousstarsand/ortheirassociations;see Sect. 3.1
ofourloopswedrewthe3-dimensionaldistributionofGIRLs
forreferences).(2)Theyarepartofthe loop,butidentifiedin
withknowndistancesinFig.12.
anotherwavelength range (darkcloudsand molecularclouds,
Dutra&Bica,2002). Sinceourdistanceindicatorsaremostlyobjectswhichare
Weappliedthefollowingmaincriteriawhenselectingdis- relatedtothespiralarmsoftheGalaxy,thedistributionreflects
tanceindicators: the position of the spiral arms, introducing a selection effect.
Loops closer than 800pc are related to OB associations and
i) Luminous stars had to have the power to form the loop, luminous stars of the Local Arm and to the nearby interarm
thereforeonlyO stars andB starswith earliersubtype(or region,whileanotherwell-definedgroupcanbeidentifiedata
8 Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant
Fig.11.Excessvisualextinctionintheloopwallsderivedfrom
the SFD E(B-V) surface brightness profiles. (a) galactic lat-
itude vs. excess visual extinction ∆A (b) galactic latitude
V
vs. the ratio of the excess visual extinction ∆A to the av-
V
erage of the inner and outer visual extinction values (Abg =
V
1[Ain+Aout])
2 V V
Fig.12. Loops with known distances in the 2nd Galactic
distance of 1.8–2.7kpc,in agreementwith the distance of the Quadrant,projectedtothegalacticplane,incylindriclatealco-
Perseus Arm in that direction (Humphreys, 1978). There is a ordinatesystem.Thetoppanelshowstheloopswithprojected
hint that the majority of the GIRLs are within a distance of distancesupto4kpc.Thebottompanelisamagnifiedviewof
800pc.Themostprominentloopsremainedobservableupto the region closer than 1kpc. Different gray tones in both the
∼adistanceof 3kpc. upper and lower panels code the vertical distance of the loop
∼ centresfromthegalacticplane(seecolourbar).
4. Discussion
Size distribution: In the Galaxy most of the ISM is con- eration of larger SN- or stellar wind bubbles even possessing
fined to a thin disc, forming the Galactic Disc itself (see e.g. thesameenergyinputastheonesclosertothegalacticplane.
Dickey&Lockman, 1990, for a review). Therefore it is ex- Thus one would expect to observe loops with a large appar-
pected that any object formed in the ISM should somehow entsizeathighgalacticlatitudes.Still,wefoundnocorrelation
reflect the distribution of the originating medium. Accepting betweenthegalacticlatitudeoftheloopcentresandtheeffec-
thatourloopsarefeaturesofthecolddiffuseISM,thesehave tiveradii,neitheronthenorthern,noronthesoutherngalactic
beenformedinthegalacticdiscwithascaleheightof 250pc. hemisphere.
∼
ThereforeloopsathighgalacticlatitudesareclosetoourSolar The sky distribution presented in Figs. 3 and 4 shows
System. On the other hand, in an exponential disc the num- clearly that the distribution of the GIRLs found at the 2nd
ber density of the gas and dust is decreasing rapidly with the GalacticQuadrantisfarfromthatexpectede.g.fromtheexpo-
increasingheightfromthegalacticplane.Thisallowsthegen- nentialdischypothesis,inwhichtheregionsneartothegalactic
Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant 9
may be explained by multiple overlapping of the loop walls
projected on the sky, i.e. many loops might be undiscovered
duetoconfusion.Interveningmolecularcloudsmayalsocon-
tributetothisconfusion.
As discussed in Sect. 3.3, there are two separate distance
groupsin the known-distancesample. Acceptingthat the ma-
jorityofourlooparelocatedinthenearbygroup(discussedin
detail in the nextparagraph)at a distance d 800pc, one can
≤
obtaintheexpectedratioofloopcountsaboveandbelow b =
| |
30 forarandomdistributionwithascaleheightofz =250pc
◦ H
(Nakanishi&Sofue, 2003). This results in (N /N ) =
low high calc
2.15 which should be compared with the observed ratio of
(N /N ) =1.28(withoutthecontributionofthePerseus-
low high obs
Armgroup).Ifthisdiscrepancyiscausedbyconfusionasdis-
cussed above, we may miss 40% of the total count in the
∼
galacticmidplane.
Itshouldbenotedthatmoleculargasextendstogalacticlat-
itudeshigher(bothatthenorthern(Hartmannetal.,1998,fig.
2)andthesouthern(Magnanietal.,2000,fig.1)galactichemi-
sphere)inthe2nd GalacticQuadrantthanintheotherpartsof
thesky.Thisanomalousfeaturemayalsoexplaintherichness
ofhighgalacticlatitudeloopsinoursample.
Thelargenumberofloopsathighgalacticlatitudesmight
beaselectioneffect(loopsareeasiertobeidentifiedduetothe
lackofconfusion).Still,oneneedsaprocessthatcangenerate
loops far from the galactic plane. One of these processes can
be the infall of clouds from the halo, and their collision with
thegalacticdisc.Weknowsomeexamples,e.g.theNCP-loop
(Meyerdierksetal.,1991),butthelargenumberofsuchloops
wouldrequireahighrateofcloudinfall,whichisnotsupported
byobservations(Ehlerova´&Palousˇ,1996).
Another possibility is that turbulent processes in the ISM
create ’loop-like’structures(Klessenetal., 2000;Korpietal.,
1999; Wadaetal., 2000). Nonlinear developmentof turbulent
instabilities may form voids in the ISM without the need of
stellarenergyinjection.Theseinstabilitiesmayberesponsible
forthelargenumberofloopfeaturesathighgalacticlatitudes
since these partsof thesky lack suitablestellar sources(mas-
siveOand/orBstars).Fromtheobservationalpointofviewthe
’loops’createdbythesetwodifferentprocessesarepractically
indistinguishable, and therefore we are not able to determine
therelativeimportanceoftheseprocesses.
Itisalsoclearfromthefiguresthatlocaleffectsplayavery
important role forming the distribution. We can mention e.g.
the high latitude group at l=130 –140 , b=45 –65 , which
◦ ◦ ◦ ◦
Fig.13.(a)Distributionofdistancesamongtheloopspresented isrelatedtothehugeNCP-loopandalsobelongstoagroupof
inFig.12(solidlineswithgrayfilledarea).Dashed-dottedlines highlatitudemolecularclouds.Similargroupscontributemuch
represent the distribution of distances projectedto the galac- tothepeaksinFig.4aandb,andmightberesponsibleforthe
ticplane.(b)Distributionofeffectivediameters(2Reff)inthe deviations from the expected distributions. Since these struc-
samesample.(c)Thesameas bbutdisplayedandbinnedona turesarerelativelynearby(e.g.thedistanceoftheNCP-loopis
logarithmicscale. Arelationoflog2Reff–logN wasfitted for 330pc),evenifwecandetectloopsuptoadistanceof 3kpc,
∼ ∼
1.4 2Reff 2.6.Thepointsusedforthefitaremarkedbyas- themaincontributioncomesfromloopsatsmalldistances(see
≤ ≤
terisks(seeSect.4fordetails). Sect.3.3).
In Fig.13cwedisplayedthesizedistributiononalogarith-
micscaleandfittedthelargediameterpart(datapointsmarked
planeshouldhavethelargestnumberofloopsandthisnumber by asterisks) with a power-law (linear in the log–log plot).
shoulddecreasedrasticallybytheincreasinggalacticlatitude. Similar investigation has been carried out by Oey&Clarke
The relatively low number of loops in the galactic midplane (1997) for the Small Magellanic Cloud, Holmberg II, M31,
10 Cs.Kissetal.:Far-infraredloopsinthe2ndGalacticQuadrant
M33 and by Kimetal. (2003) for the HI data of the Large formationhistoryintheMilkyWayorrelatedtothefiniteex-
MagellanicCloud.Wefoundapowerlawindexofs = 0.47 tensionoftheLocalArm.
− ±
0.11(followingKimetal.,2003)orβ = 1.24 0.30usingthe
0
±
transformation by Oey&Clarke (1997), which is lower than
Volume filling factor: Thepresenceofthe Perseusarmgroup
that of the other galaxies mentioned above. We note that the
is probablyrestricted to the galactic plane and representonly
distribution has a large scatter and it is better described by a
a small subsample. Therefore it is plausible to assume that
double-peakonethanbyapower-law.
the majority of the loops with unknown distances belong to
the nearby (D 800pc) group. Applying the distance – size
≤
distribution obtained for the nearby group to all loops with
unknown distances (except the loops of the Perseus group),
it is possible to give a rough estimate for the volume filling
factor (or porosity Q, Cox&Smith, 1974) of our loop interi-
orsinthegalacticdisc.Assumingthattheloops/bubbleswere
formed in the cold neutral interstellar medium (justified by
theirFIRcolours)usingz =250pcforthescaleheightofHI
H
(Nakanishi&Sofue,2003)inthesolarneighbourhood,weob-
tain a volume filling factor of f 4.6%for the 2nd Galactic
2nd
≈
Quadrant for D 800pc. Taking into account an additional
≤
40% of loops in the midplane (see above) an upper limit of
∼
f 6.4% can be obtained. These values confirm the filling
2nd
≈
factors by Ferrie`re (1998) and Gazol-Patin˜o&Passot (1999)
(f =3-8%for8.5 R 10kpcgalactocentricdistances).
≤ ⊙≤
5. Summaryandoutlook
Wesummarizethemainoutcomesofourpaperbelow:
– One fourth of the far-infrared sky was surveyed and 145
loopswereidentified.
– TheloopsampleisrepresentativeintheLocalArm
– TheFIRloopsoutsidethemidplanepossess 10%column
Fig.14. Relationship between distance and effective diameter ∼
densityexcessandacirrus-like∆I /∆I 0.25ratio
(2R ) for loops with known distances (see Table B.1). The 60 100≈
eff – Themidplaneloopsholdseveraltimes denserwalls anda
shaded areas represent the apparent size limits 2r 1 and
d 40 . Dueto the limitationsdiscussed inSect. 2e.f1f ≤we◦were far-infraredcolourof∆I60/∆I100>0.4
≥ ◦ – Distance estimates are given for 30 loops which trace the
notabletodetectfar-infraredfeaturesofthesesizes.
coldISMintheLocalArmandinthePerseusArm
– We estimated the hot gas volume filling factor to be
Correlation of distance and size: A goodcorrelationcan be 4.6% f <6.4% in the Local Arm in the 2nd Galactic
≤ 2nd
foundbetweenthephysicalsizeofthebubbleandthederived Quadrant
distance (for the sample in Sect. 3.3) when we derive linear
Pearson correlation coefficients for the whole available dis- Asacontinuationofthisstudythe1st ,3rd and4th galactic
tance range, up to 3.5kpc (0.66 and 0.73 for the linear-linear quadrants are surveyed as well. The complete sample and a
and log-log plot, respectively). Fig. 14 clearly illustrates the comparisonwithHIdistributionwillbegiveninaforthcoming
limitationsofourstudy:duetothelimitationsinapparentsize paper.
(1 d<40 ) we identified loops in the allowed region only.
◦ ◦
≤ Acknowledgements. This research was partly supported by the
Thisselectioneffectismoresevereforthesmall-sizedpartof
Hungarian Research Fund (OTKA), grants #F–022566 and #T–
the distribution (1 2r ), while even the largest loop is a
◦≤ eff 043773. WearegratefultoA.BurkertandB.G.Elmegreenfortheir
factor of 2 smaller than the largest detectable feature. The
∼ valuablecomments.
distribution shows two groups, one at D 800pc and one at
≤
D>800pc(Fig.13a).Withinthenearbygrouppracticallythere
isnocorrelationbetweenthedistanceandphysicalsize(witha
linearcorrelationcoefficientof0.17),thereforethisgroupisnot
affectedbytheselectioneffect.Accordingly,wemayderivea
statisticalresultforthelocalISM,whilethePerseusArmgroup
is strongly biased. The size distribution of the nearby group
lacks the large bubbles(2R >100pc) which are observable
eff
in the Perseus-group. The lack of large bubbles in the Local
Armgroupmightbetheconsequenceofanon-continuousstar-