Detection of non-ordered central gas motions in a sample of four low surface brightness galaxies

0.57 MB·English

by  A. Pizzella

Astronomy&Astrophysicsmanuscriptno.ms6580 (cid:13)c ESO2008
February2,2008
Detection of non-ordered central gas motions in a sample of four
⋆ ⋆⋆
low surface brightness galaxies
(ResearchNote)
A.Pizzella1,D.Tamburro2,E.M.Corsini1,3,andF.Bertola1
8
0 1 DipartimentodiAstronomia,Universita`diPadova,vicolodell’Osservatorio3,I-35122Padova,Italy
0 2 Max-Planck-Institutfu¨rAstronomie,Ko¨nigstuhl17,D-69117Heidelberg,Germany
2 3 ScuolaGalileianadiStudiSuperiori,viaVIIIFebbraio2,35122Padova,Italy

n AcceptedJanuary14,2008
a
J
ABSTRACT

5
1 Aims.Wepresentintegral-fieldspectroscopyoftheionizedgasinthecentralregionsoffourgalaxieswithalowsurfacebrightnessdisk
takenwiththeVisibleMultiObjectSpectrographattheVeryLargeTelescopeandaimedattestingtheaccuracyinthedetermination

] ofthecentrallogarithmicslopeαofthemassdensityradialprofileρ(r)∝rαinthisclassofobjects.
h
Methods.Forallthesamplegalaxieswesubtractedfromtheobservedvelocityfieldthebest-fitmodelofgasincircularmotionsand
p
derived the residuals. Only ESO-LV5340200 ischaracterized by aregular velocity field.Weextracted the velocity curves of this
-
o galaxyalongseveralpositionangles,inordertoestimatetheuncertaintyinderivingthecentralgradientofthetotalmassdensityfrom
r long-slitspectroscopy.
t Results.Wereportthedetectionofstrongnon-orderedmotionsoftheionizedgasinthreeoutoffoursamplegalaxies.Thedeviations
s
a havevelocityamplitudesandspatialscalesthatmakenotpossibletodisentanglebetweencuspyandcoredensityradialprofiles.
[
Keywords.galaxies:kinematicsanddynamics–galaxies:spiral–galaxies:structure

1
v
2 1. Introduction 2003)datawiththeCDMpredictions.Despitethisobservational
4 effort, a unique answer to the centralDM density profile slope
3 Lowsurfacebrightness(LSB)galaxiesaredefinedasdiskgalax-
hasnotbeenfound.
2 ies with a central face-on brightness fainter than 22.6 B−mag
1. arcsec−2. In comparison with high surface brightness (HSB) However,measuring the mass density profile using the gas
aspotentialtraceringalacticcentersmayresultproblematicnot
0 galaxies, the LSB galaxies have higher mass-to-light ratios
onlybecauseofthe resolutioneffects.Indeed,thegascanhave
8 and are dominated by the dark matter (DM) even in the cen-
an intrinsic velocity dispersion and does not necessarily move
0 tral regions (e.g., deBloketal. 1996; Courteau&Rix 1999;
on perfectly ballistic orbits (Bertolaetal. 1995; Cinzanoetal.
: Borriello&Salucci2001).Therefore,therotationcurvesofLSB
v 1999). Moreover, the emissivity distribution of the gas is of-
Xi gpareladxicietisonarseotfhNou−gbhotdtyosriempurelasteinotnasninidaecaolldtedsta-rbkedmtaottecrh(eCckDMthe) ten clumpy (vandenBosch&Swaters 2001) and the presence
of asymmetric and/or decoupled structures could remain un-
r universe, where the mass density radial profile of the DM ha-
a los is described by a steep power law ρ(r) ∼ rα (i.e. α . detected in long-slit observations(see Coccatoetal. 2005, and
references therein). More mundanely, the imperfect centering
−1,Navarroetal.1997,2004;Mooreetal.1999;Diemandetal.
andorientationoftheslitin opticalspectroscopyandthe beam
2005).
smearinginradioobservationscontributetothetotaluncertain-
On the other hand, the observations seem to be inconsis-
tiesonthemeasuredkinematics.
tentwiththeCDMscenario.TheDMmassdensitydistribution
To address all these issues, it is crucial to obtain high-
derived by H I rotation curves of most of LSB galaxies is de-
resolutiongaskinematicsalongdifferentaxes.Onlyrecently,the
scribedbya radialprofilewith a constantcoreprofile(i.e.α ≃
two-dimensionalvelocityfieldoftheionizedgasinLSBgalax-
0, McGaugh&deBlok 1998; Salucci 2001). This is true also
ies has been measured by means of integral-field units spec-
for the ionized-gas rotation curves (see McGaughetal. 2001;
troscopy.Simonetal.(2005)studiedasampleoflowmassspiral
deBloketal. 2001; McGaughetal. 2003). The spatial resolu-
galaxieswith a velocity field which can be explainedby either
tion of these data allows to detect cuspy DM density profiles
a cored or a cuspy density radial profile. KuziodeNarayetal.
whentheyarepresent.But,otherauthorsdofindanagreementof
(2006)observedasampleofLSBgalaxies.TheyfoundthatDM
theradio(vandenBoschetal.2000)andoptical(Swatersetal.
haloswithacoreofconstantmassdensityprovideabetterfitto
thedatawithrespecttothosewithadensitycusp.
Sendoffprintrequeststo:[email protected]
⋆ Based on observations carried out at the European Southern In this paper we focus on the intrinsic limitation of the
Observatory(ESO71.B-3050) ionized-gas kinematics in tracing the galactic central potential
⋆⋆ Table 2 is only available in electronic form at the CDS and mass distribution. The work is based on the kinematics in
via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via thecenteroffourgalaxieswithaLSBdiskmeasuredbyintegral-
http://cdsweb.u-strasbg.fr/Abstract.html field spectroscopy.Theobservationsanddata reductionarede-
2 Pizzellaetal.:Non-orderedgasmotioninthecenterofLSBgalaxies.(RN)
scribed in Sect. 2. The measurement and analysis of the kine- 3. Analysis
maticsanddistributionoftheionizedgasarediscussedinSect.
Theionized-gaskinematicswasmeasuredbyfittingthebright-
3. The results are given in Sect. 4. Finally, the conclusionsare
estemissionlinesin thegalaxyspectra.TheyweretheHα and
presentedinSect.5.
[NII]λ6583linesforESO-LV1860550andESO-LV4000370,
Hαand[SII]λ6716linesforESO-LV3520470,andHαlinefor
ESO-LV 5340200.A Gaussian profile and a straight line were
2. Observationsanddatareduction
fitted toeach emissionline andits adjacentcontinuum,respec-
tively.Thelineswereassumedtoshareacommoncentroidve-
The sample galaxies were selected among those observed by
locity and a common velocity width. The heliocentric correc-
McGaughetal.(2001)andPizzellaetal.(2008a)tohaveaLSB
tionwasappliedtothefittedline-of-sightvelocities.Theywere
disk and measured ionized-gas kinematics with long-slit spec-
notcorrectedforthe galaxyinclination.The fitted line-of-sight
troscopy.TheirmainpropertiesarereportedinFig.1.
velocity dispersions were corrected for the instrumental veloc-
The integral-field spectroscopic observations were carried
itydispersion.Nofluxcalibrationwasperformed.Theresulting
out with the Very Large Telescope (VLT) at the European
kinematics of the ionized gas as well as the intensity maps of
Southern Observatory (ESO) in Paranal (Chile) in 2003. The
thestellarcontinuumandfittedemissionlinesareplottedinFig.
UnitTelescope3mountedtheVisibleMultiObjectSpectrograph
1.Theresultingline-of-sightheliocentricvelocitiesandvelocity
(VIMOS) in the Integral Field Unit (IFU) configuration. The
dispersionsarelistedinTab.2.
fieldofviewofthefourVIMOSchannels(Q1-Q4)was13′′×13′′
McGaughetal. (2001) measured the gas kinematics along
and it was projected onto a microlenses array. This was cou-
themajoraxisofESO-LV3520470.Pizzellaetal.(2008a)mea-
pled to optical fibers which were rearranged on a linear set of
sured the gas kinematics along several axes of the remaining
microlenses to produce an entrance pseudoslit to the spectro-
threegalaxies.Acomparisonwiththesedatasetswasperformed
graph. The pseudoslit generated a total of 1600 spectra cover-
to assess the accuracyandreliability of our measurements.We
ingthe fieldof viewwith a spatialresolutionof0′.′33perfiber.
extractedfromourtwo-dimensionalvelocityfieldsthe velocity
EachchannelwasequippedwiththeHR orangehighresolution
curvesalongthesameaxesobservedbyMcGaughetal.(2001)
(R ∼ 2500) grism and a thinned and back illuminated EEV44
and Pizzellaetal. (2008a) mimicking their instrumental setup.
CCDwith2048×4096pixelsof15×15µm2.Thewavelength
For all the galaxieswe foundan agreementwith the errorsbe-
rangebetween5250and7450Åwascoveredwithareciprocal
tween the velocity curves extracted from the two-dimensional
dispersion is 0.65 Å pixel−1. The observing log and values of velocity fields and the long-slit measurements. The best match
thefullwidthathalfmaximumoftheseeingasmeasuredbythe wasfoundforthemajor-axisrotationcurveofESO-LV5340200
ESODifferentialImageMeteoMonitorarereportedinTable1. (Fig.2).Thelargestdeviations(∆v≃40kms−1)wereobserved
alongadiagonalaxis(PA =165◦)ofESO-LV1860550(Fig.2)
andtheycanbeattributedtothedifferentpointspreadfunctions
Table1.Observinglog andspatialsamplingsofthetwodatasets.
Foreachgalaxy,wefittedtheobservedvelocityfieldwiththe
Name Date Exp.Time FWHM modelof a thin disk of rotating gas to investigate the presence
of non-circular and non-orderedmotions. They limit the accu-
ESO-LV1860550 07/08Apr2003 2×45min 0′.′4/0′.′6
racyofthemassdensitydistributionderivedfromtheavailable
ESO-LV3520470 31Jul2003 1×45min N/A
ESO-LV4000370 24Aug2003 1×45min 0′.′5 kinematics.Themodelofthegasvelocityfieldisgeneratedas-
ESO-LV5340200 28/29Jun2003 2×45min 0′.′7/0′.′6 sumingthattheionized-gascomponentismovingontocircular
orbits in an infinitesimally thin disk with a negligible velocity
dispersion.We assumethatthecircularvelocityv atagiven
disk
radiusrofthegaseousdiskis
For each VIMOSchannelall the spectra were traced, iden-
tified, bias subtracted, flatfield corrected, corrected for relative 1
fiber transmission, and wavelength calibrated using the rou- v (r)=v 1− harctan r 2 , (1)
disk ∞
tines of the VIPGI pipeline (Scodeggioetal. 2005). The cos- " r h#
mic rays and bad pixels were identified and cleaned using our
wherev andharetheasymptoticvelocityandascaleradius,re-
routines developed under the IDL environment1. We checked ∞
spectively.FollowingCoccatoetal. (2007),the ionized-gasve-
andcorrectedthewavelengthrebinningbymeasuringthediffer-
locitymeasuredalongthelineofsightatagivenskypoint(x,y)
encebetweenthemeasuredandpredictedwavelengthforthe23
is
brightestnight-skyemissionlinesintheobservedspectralrange
(Osterbrocketal.1996).Thermsofthedifferenceswas0.08Å v (x,y)=v (r)sinicosφ+v , (2)
los disk sys
(Q1),0.05Å (Q2),0.05Å (Q3),and0.07Å (Q4),correspond-
ingtoanaccuracyinwavelengthcalibrationof3km s−1 inthe wherevsysisthesystemicvelocityofthegalaxy.Theanomalyφ
observedspectralrange.Theintensityofthenight-skyemission ismeasuredonthediskplaneanditisdefinedby
lineswasusedtocorrectforthedifferentrelativetransmissionof
cosφ= (x−x )cosθ+(y−y )sinθ /R, (3)
theVIMOSchannels.Thewidthofthenight-skyemissionlines 0 0
wasusedto estimatethe instrumentallinewidth.We measured where(x(cid:2),y ),i,andθarethecoordin(cid:3)atesofthecenter,inclina-
0 0
FWHM = 1.86±0.19Å(Q1),1.81±0.19Å(Q2),1.88±0.14 tion,andpositionangleofthelineofnodesofthegaseousdisk,
Å(Q3),and1.84±0.14Å(Q4),correspondingtoainstrumental respectively.Theparametersofourmodelaretheasymptoticve-
velocitydispersionσinst =36kms−1. locity, velocity scale radius, and position angle of the gaseous
disk, and the systemic velocity of the galaxy. The disk incli-
1 TheInteractiveDataLanguage isaproduct ofResearchSystems, nation was not fitted because the field of view was too small
Inc.(RSI) to properly constrain it. We adopted the inclination given by
Pizzellaetal.:Non-orderedgasmotioninthecenterofLSBgalaxies.(RN) 3
Fig.1.Mapsofthedistributionandkinematicsoftheionizedgasforthesamplegalaxies.EastisupandNorthisright.Theranges
areindicatedattherightofeachpanel.Upperpanel:reconstructedimageofthegalaxyobtainedbyintegratingthestellarcontinuum
between5728and5758Å.Middleleftpanel:Hαintensitymap.Middlerigthpanel:[NII]λ6583(or[SII]λ6716)intensitymap.
Lowerleftpanel:heliocentricline-of-sightvelocitywithoutapplyinganycorrectionforgalaxyinclination.Lowerrightpanel:line-
of-sightvelocitydispersioncorrectedforinstrumentalvelocitydispersion.Themorphologicalclassification,inclination,major-axis
positionangle,diameterofthe25 B−magarcsec−2 isophote,andtotalbluemagnitudearetakenfromLauberts&Valentijn(1989,
ESO-LV).Thesystemicvelocitiesarefromthispaper.ThedistancesarederivedfromthesystemicvelocitiescorrectedtotheCMB
referenceframefollowingFixsenetal.(1996)andassumingH =75kms−1 Mpc−1.
0
4 Pizzellaetal.:Non-orderedgasmotioninthecenterofLSBgalaxies.(RN)
dropsto0.7andthedistributionoftheionizedgasbecomesmore
patchy with the emission regionsaligned in a sort a spiral arm
structure.Wearguedthatthesefeaturesareduetogasexhaustion
bymassivestellarformationandtoshocksinducedbyradiation
andwinds,asfoundinthecenterofNGC7331byBattaneretal.
(2003).
Thevelocityfieldexhibitstheoverallshapeofarotatingdisk
(Fig. 1). However,there are signaturesof non-orderedmotions
in the inner few arcsec. Theyare clearly visible in the residual
map(Fig.3)obtainedbysubtractingthecircularvelocitymodel
from the observed velocity field. In particular, there is a struc-
Fig.2.Therotationcurvesmeasuredinthispaper(crosses)along ture receding with v ≃ 120 km s−1, which is characterized by
the major-axis of ESO-LV 5340200 (left panel) and a diago- the highestvelocitydispersion measuredin the frame (σ ≃ 80
nalaxisofESO-LV1860550(rightpanel)comparedwiththose km s−1). It is located in the southeastern quadrant of the field
obtained from long-slit spectra by Pizzellaetal. (2008a, dia- ofviewatabout3′′fromthegalacticcenter.Inthesameregion,
monds). wemeasuredanarrow(σ≃50kms−1)Hαabsorptionline.Itis
likelythatitisduetoa(relativelycold)hydrogencloud,whichis
locatedbetweentheobserverandthegalaxyandfallingtowards
the ESO-LV catalog (Fig. 1). The center of the gaseous disk
thegalacticcenter.Thisregionwasmaskedincalculatingthecir-
was assumed to be coincident with the position of the inten-
cular velocitymodel to minimize systematic errors. Except for
sitypeakofthestellarcontinuumofthereconstructedimagefor
such a remarkable feature, the velocity dispersion field is reg-
ESO-LV 1860550and ESO-LV 5340200(Fig. 1). No intensity
ular and symmetric. The velocity dispersion peaks at about 60
peakwasobservedforESO-LV3520470andESO-LV4000370.
kms−1inverycenterandthenitdecreasesoutwards.Theasym-
Therefore,forthesegalaxies x andy wereleftasfreeparam-
0 0 metryand irregularityof the observedvelocityfield, whichare
eters. The best-fitting parameters were obtained by iteratively
possibly due to an ongoing acquisition event, prevented us to
fitting a model velocity field to the observed one using a non-
derivetheradialprofileofthemassdensity.
linearleast-squaresminimizationmethod.Itisbasedonthero-
bust Levenberg-Marquardtmethodimplementedby More´etal.
(1980).TheactualcomputationwasdoneusingtheMPFITalgo-
4.2.ESO-LV3520470
rithmundertheIDLenvironment.Theseeingeffectsweretaken
into account by convolving the model with a Gaussian kernel The field of view covers3.2×3.2 kpc2 at the galaxy distance.
with a FWHM as givenin Tab.1. Finally,the modelwas com-
We excludedthe [N II]λ6583line in measuringthe kinematics
paredtotheobservedvelocityfieldexcludingbadordeadfibers.
and distribution of the ionized-gas in ESO-LV 3520470, since
WewerenotabletoconstructanyreliablediskmodelforESO-
it was in blend with a night-sky emission line. The Hα and
LV3520470,becauseitsvelocityfieldturnedouttobeveryir-
[S II]λ6716 intensity maps show that the emitting regions are
regular.
aligned along the galaxy major axis (Fig. 1). The ionized-gas
Assuming a spherical mass distribution, the mass density
distributionischaracterizedbythreeprominentblobs:a fainter
profileisgivenby
oneinthegalacticcenterandtwobrighteronesatthetwosides.
1 v dv v 2 There is no velocity gradient along the galaxy major axis.
ρ(r)= 2 c c + c , (4)
4πG " r dr (cid:18) r (cid:19) # This is consistent with the long-slit observations in the same
wavelength range by McGaughetal. (2001). They found that
where vc is the circular velocity (e.g., deBloketal. 2001; the gas rotation is very low (v ≤ 10 km s−1) out to 20′′. We
Swatersetal.2003).Toavoidpossiblebiases,wederivedvc by measureda ∆v ≃ 50 km s−1 overa radialrangeof 16′′ (corre-
deprojectingtheline-of-sightvelocitiesvlos measuredalongdif- sponding to about 4 kpc) along a direction close to the galaxy
ferentaxes withoutadoptingany parametricfunction(e.g.,Eq. minoraxis(PA=160◦).Themisalignedvelocitygradientisnot
1) to describe the velocity field. Finally, we fitted the resulting associatedtoanyfeatureoftheintensitymaps,sinceitscenterof
massdensityprofileswithapowerlawρ(r) ∼ rα.Thelogarith- symmetryisoffsetbyabout2′′westwardwithrespecttothecen-
micslopeαwasderivedexcludingthedatapointsinsidethesee- tralintensitypeak.Thesefeaturescouldbeinterpretedasdueto
ingdisk(Tab.1).ESO-LV5340200wastheonlysamplegalaxy thepresenceofakinematically-decoupledcomponent,similarto
with a velocity field suitable for such a kind of analysis. The theinnerpolardisksfoundinanumberofearly-typediskgalax-
gasvelocityfieldsoftheotherthreegalaxiesweredominatedby ies (Corsinietal. 2003; Coccatoetal. 2005;Sil’Chenko2006).
non-circularandnon-orderedmotions. After masking the central region, negligible rotation velocities
areobserved.Therefore,wewerenotabletofindthekinematic
centerandtheradialprofileofthemassdensitywasnotderived.
4. Results
4.1.ESO-LV1860550
4.3.ESO-LV4000370
Thefieldofviewcovers3.8×3.8kpc2atthegalaxydistance.The
intensity and kinematic maps derived from the two exposures The field of view covers2.4×2.4 kpc2 at the galaxy distance.
availableforthisgalaxyareinagreement.Theintensitymapsof Thecenterofthefielddoesnotcorrespondtothekinematiccen-
theHα and[NII]λ6583linesare shownin Fig. 1. Theyunveil ter, which is located in the northeastern quadrant. It is marked
the presence in the center of the field of view of a region with with a cross in Fig. 1. The Hα and [S II]λ6716intensity maps
a size of about 7′′ ×14′′ and oriented along the galaxy major showaclumpydistributionoftheionizedgas.Anintensitypeak
axis,where I([NII]λ6583)/I(Hα) ≥ 2.Atlargerradiitheratio wasobservedinthesouthwesternquadrantatabout6′′fromthe
Pizzellaetal.:Non-orderedgasmotioninthecenterofLSBgalaxies.(RN) 5
Fig.3. Map of the velocity residuals of ESO-LV 1860550(left panel), ESO-LV 4000370(central panel), and ESO-LV 5340200
(rightpanel)aftersubtractingthebest-fitcircularvelocityfield.Acrossindicatesthekinematiccenter.
galacticcenter,whichischaracterizedbyalowemissioninten- amaximumvalueofabout120km s−1.Thisisduetothecom-
sity. bined effectof the limited spatial resolutionand sharp velocity
The velocity field, although it is not entirely visible, is not gradient.
characterizedbytheexpectedpatternforaregularrotatingdisk. Since the ionized gas of ESO-LV 5340200is characterized
Infact,itdisplaysanS-shapeddistortioninthenucleusandro- by ordered motions, we used its velocity field as a test case to
tationaroundthegalaxyminoraxis.Thecentralkinematically- study the typical uncertainties in estimating the mass density
decoupled component is seen in the residual map (Fig. 3) ob- profilebymeansoflong-slitspectroscopy.We dividedthedisk
tained by subtracting the circular velocity model from the ob- infive22.5◦−widewedges.Thefirstwedgeisorientedalongthe
served velocity field, after masking the central regions. It has line of nodes, two wedges are oriented along directions which
a size of about 4′′ (corresponding to 0.7 kpc) and a ∆v ≃ 60 are±22.5◦offsetfromthelineofnodes,andthelasttwowedges
km s−1. Its position angle is PA = 150◦, whereas the position areorientedalongdirectionswhichare±45◦offsetfromtheline
angleofthegalaxymajoraxisisPA = 68◦ Thevelocitydisper- of nodes. According to galaxy inclination and orientation, on
sionreachesinthecenteritsmaximumvalueofabout45kms−1, the sky plane the wedges are oriented along directions which
anditismildlycorrelatedwiththe I([NII]λ6583)/I(Hα)ratio. are ±12.4◦ and ±27.9◦ from the galaxy kinematic major axis
Its value is about 1 in the very center and it falls to less than (PA = 177◦). We extracted the velocity curve in each wedge
0.5 at largerradii. The small numberof data points, due to the ofthetwo-dimensionalvelocityfield.Tenvelocitycurveswere
poorcenteringofthe galaxyonthefield ofview,didnotallow derived,sincetheapproachingandrecedingsidesofthegalaxy
toderivethecentralmassdensitydistributionforthisobject. were independently considered. For each curve, we computed
the term dv /dr on adjacent points. The density profiles were
c
hencederivedfromEq.4andplottedinFig.4.Thelogarithmic
4.4.ESO-LV5340200 slopeα = 1.2±0.3wasdeterminedatadistancefromthecen-
ter equal to the seeing disk size, where we were confidentthat
Thisisthefarthestgalaxyofthesample.Itwasincludedinour
list due to the wrong systemic velocity (cz = 3551 km s−1) the data were not affected by beam smearing effects. The size
reported in the ESO-LV catalog. We measured v = 17320 of the deprojected seeing disk increases for directions drifting
sys
kms−1andderivedadistanceof227Mpc.Therefore,thefieldof awayfromthemajoraxisasshowninFig.4.Thescatterinthe
measurements is due to random errors in the α determination.
viewcovers14×14kpc2. Thetwoexposuresavailableforthis
We did not find any trend with the position angle as we would
galaxy differ in the telescope pointing. The intensity and kine-
expectifthenon-circularmotionsinducedbyatriaxialcompo-
matic maps derived from them are in agreement. This ensures
nent were present. It is worth noticing that the galaxydistance
thatthedatareductionwassuccessfullyperformed.Weexcluded
andthepresenceofabrightbulge(Pizzellaetal.2008a)donot
the [N II]λ6583line in measuringthe kinematicsand distribu-
allow to derive the mass density distribution of the DM com-
tionoftheionized-gasinESO-LV5340200,sinceitwasinblend
ponent without applying a full dynamical model (Corsinietal.
with two night-sky emission lines. The distribution of the ion-
1999; Pignatellietal. 2001, e.g.,). The velocity field of ESO-
ized gas seen in the Hα intensity map is similar to that of the
LV5340200isdifferentfromthoseofLSBgalaxiesanalyzedin
starsshowedinthereconstructedimageofthestellarcontinuum
measuringthecentralcontentanddistributionofDMtoaddress
(Fig.1).
thecore/cuspproblem(e.g.,deBloketal.2001;McGaughetal.
The velocity field of the ionized gas exhibits the overall
2003;KuziodeNarayetal. 2006). Inparticular,the centralve-
shapeofarotatingdisk(Fig.1).ESO-LV5340200hasthemost
locitygradientsofESO-LV5340200wasderivedwithlargerun-
regularfieldofthesamplegalaxies.Thermsis8km s−1 inthe
certainties, andthereforethe value of 0.3can be consideredan
central 2× 3 kpc2 of the residual image obtained by subtract-
upperlimit to the typicaluncertaintyin the estimation of loga-
ingthecircularvelocitymodelfromtheobservedvelocityfield
rithmicslopeα.
(Fig.3).Onlykinematicsub-structureswithasizesmallerthan
0.5kpcremainundetected.Foracomparison,wemeasuredthe
rms in the residualmap of ESO-LV 1860550after rebinningit
5. Discussionandconclusions
tothespatialsamplingofESO-LV5340200.Wefoundarmsof
25 km s−1 (26 km s−1 before the rebinning).The velocity dis- The results of the study of the sample galaxiescan be summa-
persionisabout40kms−1.Onlyinthecentralfibersitpeaksthe rized into two remarkable aspects, concerning the presence of
6 Pizzellaetal.:Non-orderedgasmotioninthecenterofLSBgalaxies.(RN)
We concludedthe errorwederivedcanbeconsideredanupper
limit to the typicaluncertaintyin the estimation of logarithmic
slope of the mass density. Althoughit may be substantially re-
ducedwiththeanalysisofthefulltwo-dimensionalvelocityfield
(Simonetal.2005;KuziodeNarayetal.2006),thepresenceof
non-circularandnon-orderedgasmotionremainsanunresolved
issue. A way to circumvent it is using the stellar kinematics.
Themeasurementofthestellarline-of-sightvelocitydistribution
(Pizzellaetal.2008a)allowstobuildreliablemassmodelspro-
vidingavalidalternativetotheusualgas-basedmassdistribution
determinations(Pizzellaetal.2008b;Magorrianetal.2008).
Acknowledgements. The data published in this paper were reduced using the
VIMOS Interactive Pipeline and Graphical Interface (VIPGI) designed by
Fig.4. The mass density profiles along the major (PA = 177◦) the VIRMOS Consortium. We thank Bianca Garilli for her support in using
VIPGIandhospitality attheINAF-IASFMilano. Thisworkwasmadepossi-
and four diagonalaxes of ESO-LV 5340200.In each panelthe ble through grants PRIN 2005/32 byIstituto Nazionale diAstrofisica (INAF)
diamonds and crosses correspond to data measured along the andCPDA068415/06byPaduaUniversity.
approachingand receding side of the galaxy, respectively. The
arrowmarksthesizeoftheseeingdisk.
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relationbetweenthedistributionandkinematicsoftheionized-
deBlok,W.J.G.,McGaugh,S.S.,Bosma,A.,&Rubin,V.C.2001,ApJ,552,
gas of these galaxies and their starlight distribution. Indeed,
L23
there is no hint for the presence of the decoupled components deBlok,W.J.G.,McGaugh,S.S.,&vanderHulst,J.M.1996,MNRAS,283,
inbroad-bandimages(seePizzellaetal.2008a). 18
Thesekinematicdisturbancesseriouslyaffectthedetermina- Diemand,J.,Zemp,M.,Moore,B.,Stadel,J.,&Carollo,C.M.2005,MNRAS,
364,665
tionoftheradialprofileofthemassdensity.Infact,thetypical
Fixsen,D.J.,Cheng,E.S.,Cottingham,D.A.,etal.1996,ApJ,470,63
sizes ofthese features(0.5–1kpc)are comparabletothe radial Kuzio deNaray, R.,McGaugh, S.S.,deBlok, W.J.G.,&Bosma,A.2006,
range where the differences between the core and cuspy den- ApJS,165,461
sity radial profiles are expected to be observed. Their velocity Lauberts,A.&Valentijn,E.A.1989,TheSurfacePhotometryCatalogueofthe
amplitudes (30–50 km s−1) are also of the same order of both ESO-UppsalaGalaxies(EuropeanSouthernObservatory,Garching)
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theobservedrotationvelocitiesandvelocitydifferencesinferred McGaugh,S.S.,Barker,M.K.,&deBlok,W.J.G.2003,ApJ,584,566
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studies on the central mass distribution of LSB galaxies were McGaugh,S.S.,Rubin,V.C.,&deBlok,W.J.G.2001,AJ,122,2381
Moore,B.,Quinn,T.,Governato,F.,Stadel,J.,&Lake,G.1999,MNRAS,310,
based on long-slit spectroscopy, where the presence of kine-
1147
matic irregularities could have been undetected. Non-circular
More´,J.J.,Garbow,B.S.,&Hillstrom,K.E.1980,UserguideforMINPACK-1
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gasmotionsintheircenters.
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ESO-LV5340200wastheonlysamplegalaxycharacterized Pizzella,A.,Corsini,E.,Sarzi,M.,etal.2008a,MNRAS,submitted
byaregularvelocityfield.Itwastoodistanttorecoverthemass Pizzella, A.,Corsini, E.M.,Sarzi, M.,Magorrian, J.,&Bertola, F.2008b,in
density radial profile in the central kpc with the desired accu- FormationandEvolutionofGalaxies,ed.J.G.Funes,S.J.&E.M.Corsini
(ASP,SanFrancisco),inpress
racy. However, it represented a good test-bed to estimate the
Salucci,P.2001,MNRAS,320,L1
uncertainties of the measurement of the central velocity gradi-
Scodeggio,M.,Franzetti,P.,Garilli,B.,etal.2005,PASP,117,1284
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mass density radial profile in objects displaying regular kine- Fridman,M.Y.Marov,&I.G.Kovalenko(Springer,Dordrecht),275
matics. We found α = 1.2 ± 0.3. The error was obtained by Simon,J.D.,Bolatto,A.D.,Leroy,A.,Blitz,L.,&Gates,E.L.2005,ApJ,621,
757
fitting a power law to the density distribution derived from the
Swaters,R.A.,Verheijen,M.A.W.,Bershady,M.A.,&Andersen,D.R.2003,
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vandenBosch,F.C.&Swaters,R.A.2001,MNRAS,325,1017
spectroscopy. On the other hand, the central velocity gradient
ofESO-LV5340200islargerthanthatoftypicalLSBgalaxies.
Pizzellaetal.:Non-orderedgasmotioninthecenterofLSBgalaxies.(RN) 7
ListofObjects
‘ESO-LV1860550’onpage2
‘ESO-LV3520470’onpage2
‘ESO-LV4000370’onpage2
‘ESO-LV5340200’onpage2