The unmixed kinematics and origins of diffuse stellar light in the core of the Hydra I cluster (Abell 1060)

0.67 MB·English

by  G. Ventimiglia

Astronomy&Astrophysicsmanuscriptno.15982 c ESO2011
(cid:13)
January21,2011
The unmixed kinematics and origins of diffuse stellar light in
the core of the Hydra I cluster (Abell 1060)
G.Ventimiglia1,2,M.Arnaboldi2,3,andO.Gerhard1
1 Max-Plank-Institutfu¨rExtraterrestrischePhysik,Giessenbachstraβe1,D-85741GarchingbeiMu¨nchen,Germany.
2 EuropeanSouthernObservatory,Karl-Schwarzschild-Straβe2,85748GarchingbeiMu¨nchen,Germany.
3 INAF,OsservatorioAstronomicodiPinoTorinese,I-10025PinoTorinese,Italy.
ReceivedOctober22,2010;acceptedDecember16,2010
1
1
ABSTRACT
0
2
Context. Diffuse intracluster light (ICL) and cD galaxy halos are believed to originate from galaxy evolution and disruption in

n clusters.
a Aims.Theprocessesinvolvedmaybeconstrainedbystudyingthedynamical stateoftheICLandthegalaxiesintheclustercore.
J HerewepresentakinematicstudyofdiffuselightintheHydraI(Abell1060)clustercore,usingplanetarynebulas(PNs)astracers.

9 Methods.Weusedmulti-slitimagingspectroscopywithFORS2onVLT-UT1todetect56PNsassociatedwithdiffuselightinthe
1 central100×100kpc2oftheHydraIcluster,atadistanceof∼50Mpc.Wemeasuredtheir[OIII]m5007magnitudes,skypositions,
andline-of-sightvelocitydistribution(LOSVD),andcomparedwiththephase-spacedistributionofnearbygalaxies.
] Results.TheluminosityfunctionofthedetectedPNsisconsistentwiththatexpectedatadistanceof∼50Mpc.Theirnumberdensity
O is∼4timeslowerforthelightseenthanexpected,andwediscussrampressurestrippingofthePNsbythehotintraclustermedium
C asoneofthepossibleexplanations.TheLOSVDhistogramofthePNsishighlynon-Gaussianandmultipeaked:itisdominatedbya
broadcentralcomponentwithσ∼500kms−1ataroundtheaveragevelocityofthecluster,andshowstwoadditionalnarrowerpeaks
h. at1800kms−1and5000kms−1.Themaincomponentisbroadlyconsistentwiththeoutwardcontinuationoftheintraclusterhaloof
p NGC3311,whichwasearliershowntohaveavelocitydispersionof∼470kms−1atradiiof∼>50′′.Galaxieswithvelocitiesinthis
- rangeareabsentinthecentral100×100kpc2andmayhavebeendisruptedearliertobuildthiscomponent.ThePNsinthesecond
o peakintheLOSVDat5000kms−1arecoincidentspatiallyandinvelocitieswithagroupofdwarfgalaxiesintheMSISfield.They
r
t maytracethedebrisfromtheongoingtidaldisruptionofthesegalaxies.
s Conclusions.MostofthediffuselightinthecoreofAbell1060isstillnotphase-mixed.Thebuild-upofICLandthedynamically
a
hotcDhaloaroundNGC3311areongoing,throughtheaccretionofmaterialfromgalaxiesfallingintotheclustercoreandtidally
[
interactingwithitspotentialwell.

1
Key words. galaxies:clusters:general – galaxies:clusters:individual (Hydra I) – galaxies:cD – galaxies:individual (NGC 3311) –
v
planetarynebulae:general
6
8
7
3 1. Introduction proachto deep photometryfor studyingthe ICL, also enabling
. kinematic measurements for this very low surface brightness
1 Intracluster light (ICL) consists of stars that fill up the cluster
population.
0
space amonggalaxiesand thatare notphysicallyboundto any
1 An important open question is the relation between the
galaxyclustermembers.Forclustersinthenearbyuniverse,the
1 ICL and the extended outer halos of brightest cluster galaxies
morphologyand quantitativephotometryof the ICL have been
: (BCGs), whether they are independentcomponentsor whether
v studiedwithdeepphotometricdataorbydetectionofsinglestars
theformeris a radialextensionofthe latter. Usinga sampleof
i inlargeareasofsky.
X 683SDSS clusters, Zibettietal. (2005) founda surfacebright-
Deep large-field photometry shows that ICL is common in
r nessexcesswithrespecttoaninnerR1/4profileusedtodescribe
clusters of galaxies and it has morphological structures with
a
the mean profile of the BCGs, but it is not known yet whether
different angular scales. The fraction of light in the ICL with
this cD envelope is simply the central part of the cluster’s dif-
respect to the total light in galaxies is between 10% and
fuselightcomponentorwhetheritisdistinctfromtheICLand
30%, depending on the cluster mass and evolutionary status
partofthehostgalaxy(Gonzalezetal.2005).
(Feldmeieretal. 2004; Adamietal. 2005; Mihosetal. 2005;
Both the ICL and the halos of BCGs are believed to have
Zibettietal.2005;Krick&Bernstein2007;Pierinietal.2008).
formedfromstarsthatweretidallydissolvedfromtheirformer
The detection of individualstars associated with the ICL, such
host galaxiesor from entirely disrupted galaxies. A number of
asplanetarynebulas(PNs)(Arnaboldietal.2004;Aguerrietal.
processeshavebeendiscussed,startingwithearlyworksuchas
2005; Gerhardetal. 2007; Castro-Rodrigue´zetal. 2009), glob-
Richstone(1976);Hausman&Ostriker(1978).Contributionsto
ular clusters (GCs) (Hilker 2002; Leeetal. 2010), red giants
the ICL are thought to come from weakly bound stars gener-
stars(Durrelletal.2002;Williamsetal.2007),andsupernovae
ated by interactionsin galaxy groups,subsequentlyreleased in
(Gal-Yametal.2003;Neilletal.2005)isacomplementaryap-
thecluster’stidalfield(Rudicketal.2006,2009;Kapfereretal.
Send offprint requests to: G. Ventimiglia, e-mail: gven- 2010),interactionsofgalaxieswitheachotherandwiththeclus-
[email protected] ter’stidalfield(Mooreetal.1996;Gnedin2003;Willmanetal.
1
Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
2004), and from tidal dissolution of stars from massive gal- sphere, whose central region is dominated by a pair of non-
axies prior to mergers with the BCG (Muranteetal. 2007; interactinggiantellipticalgalaxies,NGC3311andNGC3309.
Puchweinetal.2010).StarsinBCG halosmayhaveoriginated NGC3309isa regulargiantelliptical(E3)andNGC3311is a
in both such major mergers as well as through minor mergers cDgalaxywithanextendedhalo(Vasterbergetal.1991).
with the BCG. Which of these processesare most importantis
X-raypropertiesofHydraI-Exceptfortwopeaksassociated
stillanopenissue.
with the bright elliptical galaxies NGC 3311 and NGC 3309,
Kinematic studies of the ICL and the cD halos are instru-
the X-ray emission from the hot intracluster medium (ICM)
mentalinansweringthesequestions.ThekinematicsoftheICL
in the Hydra I (A 1060) cluster is smooth and lacks promi-
contains the fossil records of past interactions, due to the long
nent spatial substructures. The center of the nearly circularly
dynamical timescale, and thus helps in reconstructing the pro-
symmetric emission contours roughly coincides with the cen-
cessesthatdominatetheevolutionofgalaxiesinclustersandthe
ter of NGC 3311 (Tamuraetal. 2000; Yamasakietal. 2002;
formation of the ICL (Rudicketal. 2006; Gerhardetal. 2007;
Hayakawaetal.2004,2006).Afaintextendedemissionwithan-
Muranteetal.2007;Arnaboldi&Gerhard2010).Thekinemat- gularscale<1′trailingNGC3311tothenortheastern,overlap-
ics in the cD halos can be used to separate cluster from gal-
pingwithanFeexcess,couldbeduetogasstrippedfromNGC
axy components, as shown in simulations (Dolagetal. 2010);
3311 if the galaxy moved towards the south-west with veloc-
sofar,however,theobservationalresultsarenotunanimous:in
ity > 500kms−1, according to Hayakawaetal. (2004, 2006).
both NGC 6166 in Abell 2199 (Kelsonetal. 2002) as well as
The∼total gas mass and iron mass contained in this region are
NGC3311inAbell1060(Ventimigliaetal.2010b)thevelocity
dispersion profile in the outer halo rises to nearly cluster val- ∼109M⊙and2×107M⊙,respectively(Hayakawaetal.2004,
2006).TheemissioncomponentsofNGC3311andNGC3309
ues,whereasintheFornaxcDgalaxyNGC1399(McNeiletal.
themselvesare small, extendingto only 10” 2.5kpc,sug-
2010) and in the central Coma BCGs (Coccatoetal. 2010) the ∼ ≃
gestingthatbothgalaxieslostmostoftheirgasinearlierinterac-
velocity dispersion profiles remain flat, and in M87 in Virgo
tionswiththeICM.Inbothgalaxies,theX-raygasishotterthan
(Dohertyetal.2009)itappearstofallsteeplytotheouteredge.
the equivalent temperature corresponding to the central stellar
Evidently,moreworkisneededbothtoenlargethesampleand
velocity dispersions, and in approximate pressure equilibrium
tolinktheresultstotheevolutionarystateofthehostclusters.
withtheICM(Yamasakietal.2002).
The aim of this work is to further study the NGC 3311
halo, how it blends into the ICL, and what is its dynamical On cluster scales the X-ray observations show that the
status. NGC 3311 is the cD galaxy in the core of the Hydra hot ICM has a fairly uniform temperature distribution, rang-
I (Abell 1060) cluster. Based on X-ray evidence, the Hydra I ing from about 3.4KeV in the center to 2.2KeV in the outer
clusteristheprototypeofarelaxedcluster(Tamuraetal.2000; region, and constant metal abundances out to a radius of 230
Furushoetal.2001;Christlein&Zabludoff2003).Surfacepho- kpc. Deviations from uniformity of the hot gas temperature
tometry is available in the Johnson B, Gunn g and r bands and metallicity distribution in Hydra I are in the high metal-
(Vasterbergetal. 1991), and the velocity dispersion profile has licity region at 1.5arcmin northeastern of NGC 3311, and
∼
beenmeasuredoutto 100”(Ventimigliaetal.2010b), show- a region at a slightly higher temperature at 7 arcmin south-
ingasteeprise to 47∼0kms−1 in theouterhalo.Herewe use east of NGC 3311 (Tamuraetal. 2000; Furushoetal. 2001;
the kinematics of P∼Ns from a region of 100 100 kpc2 cen- Yamasakietal. 2002; Hayakawaetal. 2004, 2006; Satoetal.
teredonNGC3311,toextendthekinematicstu×dytolargerradii 2007). Based on the overall regular X-ray emission and tem-
andcharacterizethedynamicalstateoftheouterhaloandofthe perature profile, the Hydra I cluster is considered as the pro-
clustercore. totype of an evolved and dynamically relaxed cluster, with the
In Section 2 we summarize the properties of the Hydra I time elapsed since the last major subcluster merger being at
cluster from X-ray and optical observations. In Section 3 we least several Gyr. From the X-ray data the central distribution
discussPNsaskinematicalanddistanceprobes,andthe“Multi- of dark matter in the cluster has been estimated, giving a cen-
Slit Imaging Spectroscopy - MSIS” technique for their detec- tral density slope of 1.5 and a mass within 100 kpc of
tioninclustersinthedistancerange40 100Mpc.Wepresent 1013M⊙ (Tamurae≃tal−. 2000; Hayakawaetal. 2004). Given
theobservations,datareduction,identifi−cation,andphotometry ≃these properties, the Hydra I cluster is an interesting target for
in Sections4and 5. In Section6 we describethe spatialdistri- studyingtheconnectionbetweentheICLandtheextendedhalo
bution,line-of-sight(LOS) velocitydistribution(LOSVD),and ofNGC3311.
magnitude-velocity plane of the PN sample. In Section 7 we Theclusteraveragevelocityandvelocitydispersion-From
use the propertiesof the planetarynebulae luminosity function a deep spectroscopic sample of cluster galaxies extending to
(PNLF) and a kinematic model for the PN population to pre- M 14, Christlein&Zabludoff (2003) derive the average
R
dict its LOSVD in MSIS observations. The simulation allows cluste≤rr−edshift(meanvelocity)andvelocitydispersionofHydra
us to interpret the observed LOSVD and also to determine the I. We adopt their values here: v¯ = 3683 46kms−1, and
luminosity-specific PN number or α parameter for the halo of σ = 724 31kms−1.ThesamHypleofmeas±uredgalaxyspec-
NGC 3311. In Section 8 we correlate the velocity subcompo- Hy ±
tra in Hydra I is extended to fainter magnitudes M > 17
nents in the PN LOSVD with kinematic substructures in the V −
through the catalog of early-type dwarf galaxies published by
HydraI galaxydistributionanddiscussimplicationsforgalaxy
Misgeldetal. (2008); their values for the average cluster ve-
evolution and disruption in the cluster core. Finally, Section 9
locity and velocity dispersion are v¯ = 3982 148kms−1
containsasummaryandtheconclusionsofthiswork. Hy
and σ = 784kms−1, with the average clust±er velocity at
Hy
somewhat higher value with respect to the measurement by
2. TheHydraI clusterofgalaxies(Abell1060) Christlein&Zabludoff (2003). Both catalogs cover a radial
range of 300kpc around NGC 3311. Close to NGC 3311,
∼
The Hydra I cluster (Abell 1060) is an X-ray bright, non- a predominance of velocities redshifted with respect to v¯ is
Hy
cooling flow, medium compact cluster in the southern hemi- seen,butintheradialrange 50 300kpc,thevelocitydistri-
∼ −
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Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
butionappearswell-mixedwith aboutconstantvelocitydisper- Theplanetarynebulaluminosityfunction(PNLF)technique
sion. isoneofthesimplestmethodsfordeterminingextragalacticdis-
Distance estimates - The distance to the Hydra I cluster tances.ThisisbasedontheobservedshapeofthePNLF.Atfaint
is not well constrained yet, as different techniques provide magnitudes, the PNLF has the power-law form predicted from
rather different estimates. The cosmological distance to Abell modelsofuniformlyexpandingshellssurroundingslowlyevolv-
1060 based on the cluster redshift is 51.2 5.7Mpc assum- ing central stars (Henize&Westerlund 1963; Jacoby 1980).
ing H = 72 8km−1 Mpc−1 (Christlein&±Zabludoff2003), However,observationsand simulations have demonstratedthat
0
while direct m±easurements using the surface brightness fluctu- thebrightendofthePNLFdramaticallybreaksfromthisrelation
ation (SBF) method for 16 galaxies give a distance of 41Mpc andfallstozeroveryquickly,within 0.7mag(Ciardulloetal.
∼
(Mieske,Hilker,&Infante2005). 1998;Mendez&Soffner1997).Itistheconstancyofthecutoff
TherelativedistanceofNGC3311andNGC3309alongthe magnitude,M∗ = 4.51,andthehighmonochromaticluminos-
−
lineofsightisalsocontroversial.Distancemeasurementsbased ityofPNs,thatmakesthePNLFsuchausefulstandardcandle.
on the globular cluster luminosity function locate NGC 3311
about 10Mpc in front of NGC 3309, which puts NGC 3309
3.2. TheMulti-Slit ImagingSpectroscopytechnique
at61Mpc(Hilker2003), while SBF measurementssuggestthe
opposite, with NGC 3311 now at shorter distance of about At the distance of the Hydra I cluster, the brightest PNs at
41Mpc and NGC 3309 even closer at 36Mpc, 5Mpc in front the PNLF cutoff have an apparent m magnitude equal to
5007
ofNGC3311(Mieskeetal.2005). 29.0, corresponding to a flux in the [OIII]λ5007A˚ line of
In this work we assume a distance for NGC 3311 and the 8 10−18ergs−1cm−2 accordingtothedefinitionofm b∼y
5007
×
Hydra I cluster of 51Mpc, corresponding to a distance modu- Jacoby (1989). To detectthese faintemissions we needa tech-
lus of 33.54. Then 1” corresponds to 0.247kpc. The systemic nique that substantially reduces the noise from the night sky.
velocity for NGC 3311 and its central velocity dispersion are This is possible by using a dedicated spectroscopic technique
v = 3825(3800) 8kms−1 (heliocentric;withoutandin named“Multi-SlitImagingSpectroscopy”(MSIS,Gerhardetal.
N3311
bracketswithrelativistic±correction),andσ =154 16kms−1 2005;Arnaboldietal.2007).
0
(Ventimigliaetal. 2010b).Thesystemicvelocityof±NGC3309 MSISisablindsearchtechniquethatcombinestheuseofa
is v = 4099kms−1 (Misgeldetal. 2008). The velocities mask of parallelslits, a dispersing element,and a narrowband
N3309
oftheotherHydraIgalaxiesareextractedfromthecatalogsof filter centered at the redshifted [OIII]λ5007A˚ emission line.
Misgeldetal.(2008)andChristlein&Zabludoff(2003). With MSIS exposures, PNs and other emission objects in the
filter’swavelengthrangewhichhappentoliebehindtheslitsare
detected,andtheirvelocities,positions,andmagnitudescanbe
3. Probingthe ICL kinematicsusingplanetary measuredatthesametime.The[OIII]emissionlinefromaPN
is 30kms−1wide(Arnaboldietal.2008),soifdispersedwith
nebulas
∼
a spectral resolution R 6000, it falls on a small number of
∼
3.1. Planetarynebulasaskinematicalprobesand pixels,dependingontheslitwidthandseeing.
distanceindicators Inthiswork we use MSISto locate a sample ofPNs in the
coreoftheHydraIclusterandmeasuretheirvelocitiesandmag-
PNsoccurasabriefphaseduringthelateevolutionofsolar-type nitudes. Our aim is to infer the dynamical state of the diffuse
stars. In stellar populations older than 2 Gyrs, about one star lightintheclustercore,asdescribedbelowinSections7and8.
everyfew millionis expectedto bein the PN phaseat anyone
time(Buzzonietal.2006).StarsinthePNphasecanbedetected
viatheirbrightemissionintheoptical[OIII]λ5007A˚ emission 4. Observations
line, because the nebular shellre-emits 10%of the UV pho-
∼ MSISdataforHydraIwereacquiredduringthenightsofMarch
tonsemittedbythestellarcoreinthissingleline(Ciardulloetal.
26-28,2006,withFORS2onUT1,invisitormode.TheFORS2
2005). When the [OIII] emission line is detected, the line-of-
sightvelocityofthePNcanbeeasilymeasured. field-of-view (FoV) is 6.8 6.8arcmin2, corresponding to
ThenumberdensityofPNstracestheluminositydensityof 100 100kpc2 atthe∼distan×ceoftheHydraIcluster.Theef-
∼ ×
theparentstellarpopulation.Accordingtosinglestellarpopula- fective field area in which it was possible to position slits with
tiontheory,theluminosity-specificstellardeathrateisindepen- theGrismusedhereis44.6arcmin2.TheFoVwascenteredon
dentoftheprecisestarformationhistoryoftheassociatedstellar NGC3311atα = 10h36m42.8s,δ = 27d31m42s(J2000)in
−
population(Renzini&Buzzoni1986;Buzzonietal.2006).This the core of the cluster. The FoV is imaged onto two 2 2 re-
propertyiscapturedinasimplerelationsuchthat binnedCCDs,withspatialresolution0′′.252perrebinned×-pixel.
Themaskusedhas24 21slits,each0′′.8wideand17′′.5long.
N =αL (1) The area covered with×the mask is about 7056arcsec2, corre-
PN gal
spondingtoabout4.4%oftheeffectiveFoV.Tocoverasmuch
whereN isthenumberofallPNsinastellarpopulation,L ofthefieldaspossible,themaskwasstepped15timessoasto
PN gal
fillthedistancebetweentwoadjacentslitsinthemask.Thetotal
isthebolometricluminosityofthatparentstellarpopulationand
α is the luminosity-specific PN number. The predictions from surveyedareaistherefore29.4arcmin2, i.e.,66%oftheeffec-
stellar evolution theory are further supported by empirical evi- tive FoV. Three exposures of 800sec were taken at each mask
dence that the PN number density profiles follow light in late- positiontofacilitatetheremovalofcosmicraysduringthedata
and early-type galaxies (Herrmannetal. 2008; Coccatoetal. reductionprocess.
2009),andthattheluminosity-specificPNnumberαstaysmore The dispersing element was GRISM-600B with a spec-
or less constant with (B-V) color. The empiricalresult that the tral resolution of 0.75A˚ pixel−1 (or 1.5A˚ rebinned-pixel−1) at
rmsscatterofαforagivencolorisaboutafactor2-3remainsto 5075A˚.With theadoptedslit width,themeasuredspectralres-
beexplained,however(Buzzonietal.2006). olution is 4.5A˚ or 270kms−1. Two narrow band filters were
3
Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
used, centered at 5045A˚ and 5095A˚, respectively, both with but two fall in the blue filter in the velocity range between
60A˚ FWHM.ThisensuresthefullcoverageoftheHydraIclus- 1000kms−1 and 2800kms−1, blue-shifted with respect to the
ter LOS velocity range. Each illuminated slit in the mask pro- HydraI cluster. Severalof theunresolvedbackgroundgalaxies
ducesa two-dimensionalspectrumof40 rebinnedpixelsin the in this blue-shifted velocity range have a continuum level just
spectraldirectionand70rebinnedpixelsinthespatialdirection. abovethedetectabilitythreshold,suggestingthatthePNcandi-
Theseeingduringtheobservingnightswasintherangefrom datesamplemaycontainafewbackgroundgalaxycontaminants
0′′.6to1′′.5.Fortheaverageseeing(0′′.9)andwiththespectral inthisvelocityrangewhosecontinuumistoofainttodetect.
resolution of the set-up, monochromaticpoint-like sources ap- Thetwo backgroundgalaxiesseen in the red filter are both
pear in the final spectra as sources with a total width of 5 extended and have medium bright emission fluxes; one has a
∼
pixelsinboththespatialandwavelengthdirections. verybrightcontinuum,theothernodetectablecontinuum.From
Biases and through-maskflat field images were also taken. thisweconcludethattheresidualcontaminationofthePNcandi-
Arc-lampcalibrationframeswithmask,Grismandnarrowband datesampleatvelocities>3000kms−1mustbeminimal.With
filter were acquired for the extraction of the 2D spectra, their this in mind, we will in the following simply refer to the PN
wavelengthcalibrationanddistortioncorrection.Longslit data candidatesasPNs.
forthestandardstarLTT7379withnarrowbandfilterandGrism
wereacquiredforfluxcalibration.
5.2. Photometry
MagnitudesofthePNcandidatesarecomputedusingthem
5. Data reductionandanalysis 5007
definitionbyJacoby(1989),m = 2.5logF 13.74,
5007 5007
− −
The data reduction is carried out in IRAF as described in whereF istheintegratedfluxinthelinecomputedincircular
5007
Arnaboldietal.(2007)andVentimigliaetal.(2008).Theframes aperturesofradius0′′.65 0′′.85inthe2Dspectra,measuredus-
−
are registered and co-added after bias subtraction. The con- ingtheIRAF task.noao.digiphot.aphot.phot.The1σlimiton
tinuum light from the bright galaxy halos is subtracted us- thecontinuuminthesespectrais7.2 10−20ergcm−2s−1A˚−1.
ing a median filtering technique implemented in the IRAF ×
task .images.imfilter.median,with a rectangularwindowof
19 35 pixels. Then emission line objects are identified, and 5.2.1. Photometric errorsandcompletenessfunction
×
2D-spectra around the emission line positions are extracted,
The photometric errors are estimated using simulations on a
rectified, wavelength and flux calibrated, and backgroundsub-
sample of 2D wavelength,flux calibratedand backgroundsub-
tracted.Finallythewavelengthoftheredshifted[OIII]λ5007A˚
tractedspectra.Foreachsimulation100artificialPNsourcesare
emission line for all the identified sources is measured via a
generatedusingtheIRAF task.noao.artdata.mkobject.The
Gaussian fit. The heliocentric correction for the PN velocities
is 5.44kms−1. adopted PSF is a Gaussian with a dispersion obtained by fit-
− ting a 2D Gaussian to the profile of a detected PN candidate
with adequate signal-to-noise. The σ value is 1.1 pixels, i.e.,
5.1. IdentificationofEmission-LineObjects FWHM 0′′.7, and the FWHM in wavelength is 4A˚. The
∼ ∼
simulatedPNsampleshaveluminosityfunctions(LFs)givenby
All emission line objects found are classified according to the
a deltafunctionatoneoffivedifferentinputmagnitudes(29.3,
followingcriteriaas
29.7, 30.1, 30.5 and 30.9 mag). The output magnitudes on the
2D spectra are measured with .noao.digiphot.aphot.phot us-
– PN candidates: unresolved emission line objects, both in
ingcircularapertures,inthesamewayasforrealPNcandidates.
wavelengthandspatialdirection,withnocontinuum;
Intheseexperiments,nosignificantsystematicshiftinthemag-
– backgroundgalaxycandidates:unresolvedemissionlineob-
nitudes was found, and the standard deviation of the retrieved
jectswithcontinuumorresolvedemissionlineobjectsboth
magnitude distribution is adopted as the measured error at the
withandwithoutcontinuum.
respectiveoutputmagnitude.
The total numberof detected emission line sources in our data Onthebasisofthese simulations,we thusmodelthe errors
setis82,ofwhich56areclassifiedasPNcandidatesand26as fortheMSISm photometry,whichincreaseapproximately
5007
backgroundgalaxycandidates,ofwhich6areclassifiedas[OII] linearlytowardsfaintermagnitudes,by
emittersandtheremaining20ascandidateLyαgalaxies1.
For details on the background galaxy candidates see ǫ 0.25(m5007 28.5) [29.0,30.4]. (2)
≃ −
Ventimigliaetal.(2010a).Notethatthebackgroundgalaxyclas-
sification is independent of luminosity and that these objects We thenevaluatea completenesscorrectionfunction,usingthe
have a broad equivalent width distribution. Therefore, the fact fractionofobjectsretrievedateachmagnitudeasthesebecome
thatthePNcandidates(unresolvedemissionlineobjectswithout fainter. This fraction is nearly100%at 29.0 mag, the apparent
detectable continuum) have a luminosity function as expected magnitudeofthePNLFbrightcutoffat51Mpc,anddecreases
for PNs observed with MSIS at a distance of 50 Mpc (see linearlydownto10-20%at30.4 mag,the detectionlimitmag-
Section7.1), impliesthatthe largemajorityof t∼hese PN candi- nitudeofourobservations.Wemodelthisdependenceby
datesmustindeedbePNs.Inaddition,Fig.1ofVentimigliaetal.
(2010a) shows that all of the background galaxy candidates 1 ifm5007 29.0,
≤
f 0.64( m +30.55) if29.0<m 30.4, (3)
1 Note that the equivalent widths (EWs) of the PN candidates are c ≃ − 5007 5007 ≤
mostlydistributedbetween30A˚ <EW <100A˚ ,similartotheEWs 0 ifm5007 >30.4.
ofthebackground galaxycandidates, andcannot thereforebeusedto 
discriminatebetween both types of emission sources. Thisisbecause Theerrordistributionandthecompletenessfunctionareusedin
thesedistantPNsarefaintandthecontinuumlevelintheMSISimages Section 7 below to performsimulations of the LOSVD for the
isgivenbythe1σlimitfromtheskynoise;seeSection5.2. PNsample.
4
Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
Fig.1. PNs in the Hydra I cluster core. Left panel: the PN velocity-magnitude distribution. The black crosses show the entire
sampleof56PNcandidates.Theblueandredlinesarethemeasuredtransmissioncurvesoftheblueandtheredfilter,respectively,
normalizedsothatthemaximumtransmissionisnearthetheoreticalbrightcutoffofthePNLFatthedistanceofHydraI.Central
panel:thePNLOSVD(blackhistogram).Thebinsinvelocityare270kms−1wide.Theblueandtheredsolidlinesshowagainthe
suitablynormalizedtransmissioncurvesoftheblueandredfilters.Theverticalmagenta,greenandgraylinesinbothpanelsmarkthe
systemicvelocityofHydraI,NGC3311andNGC3309,respectively.Rightpanel:SpatialdistributionofthePNs(blackdiamonds)
intheMSISfield.ThefieldiscenteredonNGC3311andhassize 100 100kpc2;northisupandeasttotheleft.Thetwoopen
∼ ×
trianglesindicatethepositionsofNGC3311(center)andNGC3309(upperright).ThePNindicatedbythegraysymbolistheonly
objectcompatiblewithaPNboundtoNGC3309,basedonitspositionontheskyandLOSvelocity,vgrayPN =4422kms−1.
6. ThePN samplein HydraI Hydra I cluster. In the blue filter velocity range there is a sec-
Our PN catalog for the central (100kpc)2 of the Hydra I ondary peak at ∼ 1800kms−1 that falls 2 − 3σHy from the
systemicvelocityofHydraI.Thisbluepeakmaycontainafew
cluster contains 56 candidates, for which we measure v ,
LOS
background galaxy contaminants, as discussed in Section 5.1
x , y and m . The detected PN velocities cover a
raPnNge frPomN 970kms5−0017to 6400kms−1 with fluxes from 2.2 above. Finally a red peak at 5000kms−1 within 2σHy of
10−18ergcm−2s−1 to 7.6 10−18ergcm−2s−1. The detecte×d theclustermeanvelocityisde∼tectedinthevelocityin∼tervalfrom
sampleofobjectshaveama×gnitudedistributioncompatiblewith 4600to5400kms−1,andtherearesomePNswithevenhigher
thePNLFatthedistanceofHydraI;seealsoSection7.1. LOSvelocities.
The magnitude-velocity plane - The properties of the PN The spatial distribution of the PNs - The locations of the
sample in the velocity-magnitude plane are shown in the left detectedPNson thesky areshownin the rightpanelofFig. 1.
panelofFig.12.Inthisplot,theapparentmagnitudeofthePNLF Theirspatialdistributioncanbecharacterizedasfollows:
brightcutoffatthedistanceoftheHydraIclustercorrespondsto
ahorizontallineat29.0mag.Theblueandredlinesarethefilter – most PNs follow an elongated north-south distribution ap-
transmission curves, as measured from the spectra, normalized proximatelycenteredonNGC3311;
sothatthemaximumtransmissionoccursnearthePNLFbright – there is no secondary high density concentration around
cutoff. The PNs are indeed all fainter than m = 29.0 and NGC 3309. Only one PN, indicated by the gray symbol in
5007
extend to the detection limit magnitude, m . This is slightly the right plot of Fig. 1, has a combination of velocity and
dl
differentforthetwofilters;thefaintestPNsdetectedthroughthe positionthatarecompatiblewithaPNboundtothehaloof
bluefilterhavem =30.45,andthosedetectedwiththered NGC3309;
B,dl
– a possibly separate concentration of PNs is present in the
filterhavem =30.3.
R,dl
northeasterncornerofthefield.
ThePNLOSVD-ThemeasuredLOSVDofthePNsampleis
shownbytheblackhistograminthecentralpanelofFig.1.The Wesummarizeourmainresultssofar:
velocitywindowcoveredbythetwofiltersisalsoshownandthe
systemic velocitiesof HydraI, NGC 3311andNGC 3309(see 1. ThePNcandidatesdetectedintheMSISfieldhaveluminosi-
Section2)areindicatedbythemagenta,greenandgrayvertical tiesconsistentwithapopulationofPNsatthedistanceofthe
lines,respectively.Thesevelocitiesfallinthemiddleoftheve- HydraIcluster.
locitywindowallowedbythefilters,wherebothfiltersoverlap. 2. The distribution of PNs in the MSIS field is centered on
ThemeanvelocityofallPNcandidatesis¯v =3840kms−1 NGC 3311. Only one candidate is consistent with being
PNs
andthestandarddeviationisrms = 1390kms−1.Thedis- boundto NGC 3309,even thoughNGC 3309 is of compa-
PNs
tribution is highly non Gaussian and dominated by several in- rableluminositytoNGC3311and,onaccountoftheX-ray
dividual components. The main peak appears in the range of results(seeSection2),islikelylocatedintheinnerpartsof
velocities from 2400 to 4400kms−1 and its maximum is at theclusterwithinthedenseICL,atsimilardistancefromus
3100kms−1, within 1σHy of the systemic velocity of the asNGC3311.
∼ 3. Thereisnoevidenceofasingle,well-mixeddistributionof
2 Thisplotisbased onmoreaccuratephotometry thanandupdates PNs in the central 100kpc of the Hydra I cluster, contrary
Fig.1ofVentimigliaetal.(2008). towhatonewouldexpectfromthedynamicallyrelaxedap-
5
Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
pearance of the X-ray emission. Instead, the observed PNs whereG(v)isnormalizedsothat G(v )∆v =1.
i i i
separateintothreemajorvelocitycomponents. SimulatingtheMSISobservatiPons-Themagnitude-velocity
diagram for such a model population is modified by a number
of effects in the MSIS observations, which we simulate as de-
7. Kinematicsubstructuresandα parameterfor
scribedbelow.TheMSISsimulationprocedureimplementsthe
the observedPN samplein HydraI: followingsteps:
comparisonwith a simulatedMSISmodel
– thethrough-slitconvolutionofthePNLF;
Atthispoint,we wouldlike toreinforcethe lastpointbycom- – theconvolutionwiththefiltertransmission;
paring the observed velocity distribution with a simple model. – thephotometricerrorconvolution;
The model is obtained by assuming a phase-mixed PN popu- – thecompletenesscorrection;
lation placed at the distance (51 Mpc) and mean recession ve- – thecomputationoftheLOSVD.
locity of NGC 3311, and simulating its line-of-sight velocity
distribution by convolving with the MSIS instrumental set up. Thethrough-slitPNLF-TheMSIStechniqueisablindsur-
The velocity dispersion of the PN population is taken to be veytechnique.Thereforethepositionsoftheslitsontheskyare
464kms−1, the highestvaluemeasuredfrom the long-slitdata not centered on the detected objects, and the further away an
inVentimigliaetal.(2010b).Inthiswaywecantestmorequan- objectis fromthe centerof its slit, the fainterit becomes.This
titativelywhethertheobservedmultipeakedLOSVDforPNsin effectisafunctionofbothseeingandslitwidth,anditmodifies
ourfieldisbiasedbytheMSISobservationalset-uporwhetherit the functionalform of the PNLF, which needs to be accounted
providesevidenceofun-mixedcomponentsintheHydraIclus- forwhenusingtheLFfromMSISPNdetectedsamples.
tercore. Inprinciple,somePNsmaybedetectedintwoadjacentslits
ofthemask,andthiswouldneedto becorrectedfor.However,
atthe depthofthe presentHydraI surveythis effectis notim-
7.1. PredictingtheluminosityfunctionandLOSVDwith portant for the predicted PNLF, and indeed no such object has
MSISforamodelPNpopulation beenfoundinthesample.
Given a “true” PNLF LF(m), the “through slit PNLF”
Wefirstcharacterizethemodelintermsoftheintrinsicluminos-
sLF(m)caneasilybecomputed,anddependsonslitwidthand
ityfunctionandLOSVDofthePNpopulation.Thenwedescribe
seeing; for further details see Gerhard et al. (2011, in prepa-
thestepsrequiredtopredictthecorrespondingm magnitude
5007
ration). The effect of the through-slit correction is to shift the
vs.LOSvelocitydiagramandLOSVDthatwouldbemeasured
sLF(m) faintwards in the observable bright part, compared to
with the MSIS set up. In the next subsection we compare the
the”true”PNLF.
resultsobtainedwiththeobservedHydraIPNsample.
Convolutionwith filtertransmission- Whenthefiltertrans-
Model for the intrinsic PN population - The intrinsic
mission T(v ) is less then 1 (100%), it shifts the through-slit
PNLFcanbeapproximatedbythe analyticalfunctiongivenby i
PNLF to fainter magnitudes.The ∆m dependson the value of
Ciardulloetal.(1989):
the filter transmission curve at the wavelength λ or equivalent
N(m)=Ce0.307m 1 e3(m∗−m) (4) binned velocity vi, and is equal to ∆m(vi) = 2.5logT(vi).
h − i The resulting instrumental PNLF, the distribu−tion of source
wheremistheobservedmagnitude,m∗ = 29.0istheapparent magnitudesbefore detection, becomes velocity dependent,i.e.,
magnitudeof the brightcutoff at the adopteddistance of NGC iLF(m,vi).
3311, and C is a multiplicative factor. The integral of N(m) For the present MSIS Hydra I observations, the combined
from m∗ to m∗ + 8 gives the total number of PN associated filtertransmissioncurvefrombothfiltersisdefinedas
with the bolometric luminosity of the parent stellar population
(N in Eq. 1), and the C parameter can be related to the T(v)i = max[TB(vi),TR(vi)], (7)
PN
luminosity-specificPNnumberα(Buzzonietal.2006).Forour
whereBandRdenotetheblueandredfilters.Itis1wherethe
modelwedistributethemagnitudesofaPNpopulationaccord- transmissionis100%,approximatelyfrom 1500kms−1 to
ingto a verysimilar formulafitted byMe´ndeztothe resultsof 3300kms−1 and from 4200kms−1 to ∼6300kms−1; it∼is
Mendez&Soffner(1997). < 1 in the filter gaparo∼und 3800kms−∼1 andat the low and
Next we assume that this PN population is dynamically ∼
highvelocityendsoftheobservedrange.
phase-mixedandthatitsintrinsicLOSVDisgivenbyaGaussian
Photometric error convolution - Once the instrumental LF
centeredonthesystemicvelocityofNGC3311,¯v,
iLF(m,v ) is computed, it must be convolved with the photo-
i
1 (v ¯v)2 metric errors which, for the case of the Hydra I observations,
G(v)= exp − (5) aregivenbythelinearfunctioninEq.2.Becauseofthephoto-
σcore√2π (cid:20) 2σ2core (cid:21)
metricerrors,PNsthatareintrinsicallyfainterthanthedetection
wherehereweadopt¯v=3830kms−1(Ventimigliaetal.2010b, limit (heremag 30.4)may be detected if they happento fall
∼
correctedtothefilterframe),andforthevelocitydispersionwe on a positive noise peak on the CCD image, and PNs that are
take σcore = 464kms−1, the highest value measured from intrinsicallybrighterthanmag∼30.4maybelostfromthesam-
ple.Generally,becausethethrough-slitPNLFsLF(m)increases
the long-slit data in this paper. This approximates the velocity
towardsfaintermagnitudes,thephotometricerrorsscattermore
dispersion for the intracluster component in the outer halo of
faintobjectstobrightermagnitudesthanvice-versa;sotheeffect
NGC3311,atcentraldistances 20 30kpc(Ventimigliaetal.
∼ − of the convolution is to shift the PNLF to brighter magnitudes
2010b). We will consider the magnitude-velocity diagram and
again.
theLOSVDashistogramsinvelocity;thenineachvelocitybin
Completeness correction - The completeness correction at
∆v ,thenumberofPNsis
i
a given observed magnitude is a multiplicative function which
LF(v ) N(m)G(v )∆v (6) accountsforthedecreasingfractionofPNsatfaintermagnitudes
i i i
≃
6
Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
detected against the noise on the MSIS image. For the case at 7.2. Realityofobservedkinematicsubstructures
handitisgiveninEq.3.Afterthelasttwosteps,wearriveatthe
final“MSISPNLF”,MSLF(m)forshort. ThesimulatedMSISLOSVDgivenbyN (v )forthesimple
MSIS i
Computation of the simulated LOSVD - For each velocity Gaussianvelocitydistributionmodelandluminosityfunctionof
bintheMSLF(m,vi)isintegratedbetweentheapparentmagni- Eq.4isshownasthegreenhistograminFig.3,withtheobserved
tude of the PNLF bright cut off (m∗ = 29.0 for Hydra I) and PNLOSVDoverplottedinblack.ThesimulatedMSISLOSVD
the detection limit magnitude in the relevant filter, mf,dl (see is scaled to approximatelymatch the observedHydra I sample
Section6),toobtainthe“observed”cumulativenumberofPNs inthecentralvelocitybins.
ineachvelocitybin:
mf,dl
N (v )= MSLF(m,v )dm. (8)
MSIS i i
Zm∗
The most cumbersome step in this procedure is the cor-
rection for the filter transmission, because it makes the final
MSLF(m,v ) velocity-dependent.It must correctly be applied
i
beforetheconvolutionwiththephotometricerrors,becausethe
latter depend on the flux measured at certain positions on the
CCD. So the errors on the through-slit magnitudes depend on
thefiltertransmissionvaluesofthePNs.
However, we have found that the observed MSLF for the
Hydra I PN sample, when obtained from wavelength regions
where the filter transmission is 100%, is very similar to the
∼
oneobtainedbysummingovertheentirefilterbandpass.Theef-
fectofthevelocitydependenceontheoverallMSLFmustthere-
forebesmall,andforthecomparisonofsimulatedandmeasured
LOSVDsbelowwehavethereforeappliedthefiltertransmission
onlyaftertheerrorconvolutionandcompletenesscorrection.
Before we discuss the LOSVD obtainedfromthe complete
Fig.3. LOSVD for the Hydra I PN sample from Fig. 1 (black
model, we show in Fig. 2 the predicted cumulative luminosity
histogram), compared with a simulated MSIS LOSVD (green
functionresultingfromerrorconvolution,completenesscorrec-
histogram) for a Gaussian velocity distribution with σ =
tion,andfilter transmissioncorrectionof the through-slitlumi- core
464kms−1; see text for further details. The blue-redsolid line
nosity function, weighting by the number of observed PNs in
showsthecombinedfiltertransmissioncurveasgiveninEq.7.
each velocity bin. Also shown in Fig. 2 is the cumulative his-
The vertical magenta, green and gray lines mark the systemic
togramofthem magnitudesforthe56observedPNsinthe
5007 velocityofHydraI,NGC3311andNGC3309,respectively.
MSIS field. With a cutoffmagnitudeof 29.0the modelfits the
observedhistogramfairlywell;however,thisisnotaformalbest
fittothedistance.TheimportantpointshownbyFig.2isthatthe
observedMSISluminosityfunctionofthePNemissionsources
The comparison between the simulated LOSVD and the
in the Hydracluster coreis evidentlyconsistentwith a popula-
HydraIPNLOSVDinFig.3identifiesthecentralpeakatabout
tionofPNsat∼50Mpcdistance. 3100kms−1 in the observed PN LOSVD with that of the PN
populationassociatedwiththestellarhaloaroundNGC3311in
the cluster core, with σ 500kms−1. The mean ¯v and
core core
∼
σ ofthiscomponentareapproximatelyconsistentwiththose
core
oftheintraclusterlighthaloofNGC3311derivedfromthelong-
slit kinematic analysis in Ventimigliaetal. (2010b). However,
theasymmetryandoffsetofthepeakoftheobservedhistogram
(by several 100kms−1) relative to the MSIS convolvedmodel
centered at the systemic velocity of NGC 3311 appear signifi-
cant(σ /√N 100kms−1),arguingforsomerealasym-
core core
≃
metry of the central velocity component. We shall refer to the
centralpeakin theHydraI PN LOSVDin Fig.3asthecentral
ICLcomponent.
Two additional velocity peaks are seen in the LOSVD in
Fig.3, onenear1800kms−1 andoneat 5000kms−1,which
∼
do not have any correspondence with the velocity distribution
derived for the simulated MSIS model. These velocity compo-
nents cannot be explained as artifacts of the MSIS set up, in
particular,the filter gapin the B+R filter combination.We will
Fig.2.Cumulativeluminosityfunctionpredictedforthepresent refer to these two velocity components as secondary blue and
MSIS observations and the nominal cutoff magnitude of the redpeaks,respectively.Theyrevealthepossiblepresenceoftwo
HydraIcluster,29.0(fullredline,seetext),comparedwiththe kinematicalsubstructuresinthecoreofAbell1060,whoseori-
cumulativehistogramoftheobservedm5007 magnitudes. ginsmustbeinvestigatedfurther;seeSection8.
7
Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
7.3. Lowα-parameterinthecoreofHydraI consistentwiththecentralICLhaloofNGC3311isafactor 2
∼
lowerthanthenumberofallPNs.Clearlytherefore,someofthe
We now compare the number of observed PNs with the ex-
light at these radii is in a componentdifferentfrom the phase-
pectations from the luminosity distribution and kinematics in
mixedcentralICLhalo,buttheamountisuncertainbecausewe
and around NGC 3311. One issue is the absence of a clear
donotknowwhethertheluminosity-specificα-parameterofthis
subcomponent of PNs with velocity dispersion 150
componentissimilarlylowasfortheNGC3311ICLhalo.For
250kms−1, as would be expected from the central∼ 25” o−f example, agreementbetween observed and predicted PN num-
∼
NGC3311(Ventimigliaetal.2010b).ItisknownthatPNsam- berscouldbeachievedbyscalingonlytheNGC3311halocom-
plesinellipticalgalaxiesaregenerallynotcompleteinthecen- ponentbyafactor 6.Ontheotherhand,scalingonlyanouter
tral regions because of the increasing surface brightness pro- componentwill no∼t work, because the discrepancy in Fig. 4 is
file; PNs are hard to detect against the image noise in the already seen at small radii. Thus we can conclude that the α-
bright centers. E.g., in observations with the Planetary Nebula parameteroftheNGC3311ICLhaloislowbyafactor4 6.
Spectrograph, the threshold surface brightness is typically in SuchananomalousspecificPN numberdensityrequi−resan
the range µV = 20 22mag/arcsec2 (Coccatoetal. 2009). explanation.One possibilityisthatthe stellar populationin the
−
In the current Hydra I data, the PN sample is severely incom- haloofNGC3311isunusuallyPNpoor;thiswillneedstudying
plete at µV = 21.0mag/arcsec2 (only two PNs are seen at thestellarpopulationinthegalaxyoutskirts.Asecondpossibil-
µV 21.0mag/arcsec2,andsixatµV > 21.5mag/arcsec2). ity is that the ram pressure against the hot X-ray emitting gas
Refe∼rring to Fig. 13 of Me´ndezetal. (2∼001), we estimate that inthehaloofNGC3311ishighenoughtoseverelyshortenthe
thecurrentsampleisnotcompleteforµV <22.0mag/arcsec2, lifetime of the PNs (Dopitaetal. 2000; Villaver&Stanghellini
which is reached at a distance of 30∼” from the center of 2005).Intheirsimulations,Villaver&Stanghellini(2005)con-
≃
NGC3311(Arnaboldietal.2011,inpreparation).Atthisradius, sideragaseousmediumofdensityn = 10−4cm−3 andarela-
the projected velocity dispersion has risen to σN3311(30”) tive velocity of 1000kms−1. They find that the inner PN shell
300 400kms−1 (Ventimigliaetal.2010b).ThusthePNsde≃- isnotsignificantlyaffectedbytherampressurestrippingduring
−
tectedinthispaperalmostexclusivelysamplethehot(intraclus- the PN lifetime, and becausethe inner shell dominatesthe line
ter)haloofNGC3311.Thecoldinnergalaxycomponentisnot emissionintheirmodel,thePNvisibilitylifetimeisthereforenot
sampled. shortenedrelativetoanundisturbedPN.However,withadensity
ThesecondissueistheobservedtotalnumberofPNs,given of the ICM inside 5′ around NGC 3311 of 6 10−3cm−3,
thedetectionlimit,theinstrumentalsetupandthelightinNGC and a typical velocity of √3 450kms−1∼ 8×00kms−1 the
3311andNGC3309.Integratingthe simulatedMSISluminos- ram pressure on the NGC 331×1 is 40 time≃s stronger than in
ity functiondownto thedetectionlimitof 30.4mag, we obtain theirsimulatedcase,sotherampre∼ssureeffectscouldbemuch
an effective α parameter for our observations of αMSIS,Hy = stronger. Unfortunately,simulations of the evolution of PNs in
0.012αtot,whereαtot quantifiesthetotalnumberofPNs8mag suchdensemediaarenotyetavailable,toourknowledge.
down the PNLF3. This value is similar to α , the integrated
0.5
value0.5magdownthePNLF.ItisconsistentwithFig.1,even
thoughinthisfigurePNsareseenupto1.5magfainterthanthe
nominalcutoffmagnitude,becauseof(i)theshifttowardsfainter
magnitudesduetotheslitlosses,and(ii)thecompletenesscor-
rection(Eq.3).
We can estimate the bolometric α for NGC 3311 from
tot
its (FUV-V) color, the relation between (FUV-V) and logα
1.0
shown in Fig. 12 of Coccatoetal. (2009), and correcting to
logα by using Fig. 8 of Buzzonietal. (2006). The (FUV-
tot
V) color is determined from the Galex FUV magnitude and
the V band magnitude from RC3, both corrected for extinc-
tion, as described in Coccatoetal. (2009, Section 6.1). The
resulting value, (FUV-V)=6.7, corresponds to logα = 1.1
1.0
and logα = 7.34. This is very similar to the value of
tot
−
logα = 7.30foundfortheFornaxclustercDgalaxyNGC
tot
−
1399 (Buzzonietal. 2006). Using the V band light profile of
Fig.4. Observed and predicted cumulative PN numbers, as a
NGC3311measuredinArnaboldietal.(2011,inpreparation),
function of radial distance from the center of NGC 3311. The
andabolometriccorrectionof0.85mag,wecanthenpredictthe
greenlineshowsthecumulativenumberofPNsassociatedwith
expected cumulative number of PNs within radius R from the
thecentralICLhaloofNGC3311,basedontheirvelocities.The
centerofNGC3311.ThisisshownastheredcurveinFigure4,
blacklineshowsthecumulativenumberofallPNs,withoutve-
aftersubtractingtheluminositywithin20”whichisnotsampled
locity selection. The red curve shows the predicted cumulative
by ourMSIS observations.Also shown are the cumulativehis-
numberofPNscomputedusingtheluminosity-specificparame-
togramsoftheobservednumberofPNsintheMSISdata,both
terαestimatedasexplainedinthetext
forallPNsinthefield,andforPNswithvelocitiesinthecentral
,theMSISobservationalset-up,andtheintegratedbolometric
velocitycomponentonly.
luminosityinincreasingcircularaperturescenteredonNGC3311.
Fig.4showsthatthetotalnumberofPNsdetectedinthefield
fallsshortofthenumberpredictedfromtheluminosityprofileby
afactor 4.Outside 100”,thenumberofPNswithvelocities Ifthisexplanationiscorrect,PNsshouldbemostefficiently
∼ ∼
ram pressure stripped in the innermost, densest regions of the
3 Thisvalueincludesthelightbetweenadjacentslitsforthenormal- ICM.Henceinthiscasewewouldexpectmostoftheobserved
ization. PNs to be located in the outermost halo of NGC 3311, even
8
Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
thoseprojectedontotheinnerpartsofourMSISfield.Atthese brightgalaxiesin the northernpartof the field, but a deficit of
outerradii,dynamicaltime-scalesarelonger,andphase-mixing galaxiestothesouthofNGC3311.
should be less complete. This would fit well with the unmixed The spatial distribution of the PNs associated with the sec-
kinematicsand spatialdistributionof the observedsample (see ondaryredpeakinthePNLOSVDisshownintherightpanelof
alsonextSection). Fig.5.Ithasanorth/southelongation,apparentlyextendingfur-
ThethirdissueisthatwedonotseeaconcentrationofPNs thertowardsthesouthofNGC3311thanthecentralICLcom-
aroundNGC 3309.As shownin Section 6, onlyone PN in the ponent,withahighdensityregionnorth/eastofNGC3311.
sample, shownby the graysymbolin the rightpanelof Fig. 1,
Finally,thespatialdistributionofthePNsassociatedwiththe
hasbothpositionandLOSvelocitycompatiblewithbeingbound secondarybluecomponentat1800kms−1 (leftpanelofFig.5)
to NGC 3309. Whereas using the relative total luminosities of
also appears elongated along the north/south direction, but the
NGC 3309 and NGC 3311 to scale the numberof PNs associ-
smaller number of objects in this subsample makes inferring
ated with the main LOS velocity componentfor NGC 3311 in
theirspatialstructuremoredifficult.
Fig.3(i.e.,27PNs),wewouldexpectabout11PNsassociated
Insummary,thereislittleevidenceofasphericallysymmet-
withthelightofNGC3309ifbothgalaxieswereatthesamedis-
ricwell-mixeddistributionofPNsintheouterhaloofNGC3311
tance. There are two possible explanationsfor this fact. One is
in the cluster core. Several velocity components are seen, and
thatNGC3309isatsignificantlylargerdistancethanNGC3311,
eventhe centralICL componentcenteredon NGC3311shows
suchthatevenPNsatthebrightcutoffwouldbedifficulttosee.
signsofspatialsubstructures.
However,asimplecalculationshowsthatthenNGC3309would
beputat 70Mpcwelloutsidethecluster,atvariancewithX-
∼
ray observationsfinding that its gas atmosphere is confined by
8.2. SpatialandvelocitydistributionofHydraIgalaxies:
theICMpressure(seeSection2).Thesecondpossibilityisthat,
comparisonwiththePNssample
similarlyasforNGC3311,also thePNsin NGC3309maybe
severely ram pressure stripped by the galaxy’s motion through The spatial distribution of the galaxies from
thedenseICMintheclustercore.ThiswouldrequirethatNGC Christlein&Zabludoff (2003); Misgeldetal. (2008) in the
3309 moves rapidly through the cluster core, and is physically central 20 20arcmin2 centered on NGC 3311 is shown in
ratherclose toNGC 3311.Againsimulationswouldbeneeded Fig. 6. We w×ould like to analyze their phase-spacedistribution
tocheckthisquantitatively. by dividing into the same velocity componentsas identified in
the PN LOSVD. Therefore,in the image on the left the bright
galaxiesareencircledwith thecolorsof thePN componentsin
8. Thesubstructuresin theHydraI clustercore Fig.5,andin therightpanelallgalaxiesin thefield areshown
schematically as squares and crosses with the same color code
We now turn to a more generaldiscussion of the spatial distri-
for these velocity bins. NGC 3311 and NGC 3309 are marked
butionandkinematicsofPNsandgalaxiesinthecentralregion
inthecenteroftheMSISfield(orangesquare).
of the cluster. ICL is believed to originate from galaxies, so it
In Fig. 7, the left panel shows the velocity distribution of
isinterestingtoaskwhetherthephase-spacesubstructuresseen
in the distribution of the PNs that trace the ICL has some cor- all the galaxies in the 20 20arcmin2 region centered on
×
respondence to similar structures in the distribution of cluster NGC 3311. In the right panel, the velocity histograms for the
galaxies.Thuswewanttoinvestigatethespatialdistributionsof bright galaxies(mR < 15.37, violet color) and dwarf galaxies
the PNs associated with the velocity subcomponentsin the PN (mR >15.37,greencolor)areshownseparately.
LOSVD discussed earlier, and compare them with the spatial The LOSVD for the Hydra I galaxies covers the same ve-
distributionof HydraI galaxiesin similar velocityintervals.In locities as for the PN sample. If we select only galaxies in
thisway,wemayobtainabetterunderstandingofthedynamical the range of velocities of the PNs in the central ICL compo-
evolutionofthegalaxiesintheclustercore,andoftherelevance nent, from 2800kms−1 to 4450kms−1, their LOSVD is con-
ofclustersubstructuresfortheoriginofthediffuseclusterlight. sistent with a Gaussian distribution centered at a velocity of
3723 100kms−1 withadispersionof542 80kms−1.This
± ±
confirms results from long-slit kinematics in the outer halo of
8.1. Spatialdistributions ofthePNvelocitycomponents
NGC3311(Ventimigliaetal.2010b),wherethevelocitydisper-
We first consider the spatial distribution of the PNs associated sionwasfoundtoincreaseto 465kms−1at 70”radius,64%
∼ ∼
withthedifferentvelocitycomponentsinthePNLOSVD.This ofthevelocitydispersionofallclustergalaxies.
is shownin the threepanelsof Fig. 5, dividedaccordingto the This subsample of galaxies also has an interesting spa-
classification in Sect. 7.2. Each panelcoversa region of 6.8 tial distribution: the central 6.8 6.8arcmin2 region of the
6.8arcmin2 100 100kpc2centeredonNGC3311. × cluster (the MSIS field), while do×minated by NGC 3311 and
≃ ×
PNs associated with the central ICL component (middle NGC 3309, contains no other Hydra I galaxies with these ve-
panelofFig.5)canbedividedintotwospatialstructures.There locities.Whereasoutsidethisregion,theyappearuniformlydis-
is a prominent PN group concentrated, as expected, around tributedoverthe field (see the greensquaresandcrossesin the
NGC3311,andanelongatedeast-westdistributioninthenorth- rightpanelofFig.6).NGC3311isatthecenterofthedistribu-
ernpartoftheFoV.Bycontrast,weseealowPNdensityregion tionofthesegalaxiesbothinspaceandinvelocity.Thedistribu-
inthesouthernpartoftheMSISfield. tionofthese galaxies,aswellasthesimilarityoftheirvelocity
Suchanorth/southasymmetryisseenalsointhespatialdis- dispersionwiththatmeasuredinthehaloofNGC3311,supports
tribution of the galaxies. Fig. 6 displays a larger area, 20 the interpretation of Ventimigliaetal. (2010b) that the halo of
20arcmin2,whichincludestheMSISfieldstudiedinthiswor×k, NGC3311isdominatedbyintraclusterstarsthathavebeentorn
asindicatedbytheorangesquare.Wecanseefromthetwopan- fromgalaxiesdisruptedintheclustercore:galaxiesthatpassed
els(photo,andschematic)thatNGC3311andNGC3309dom- throughthecentral100kpcoftheclustercoreatmodestveloci-
inatethecenteroftheMSISfield,thatthereisahighdensityof tieshaveallbeendisrupted.
9
Ventimigliaetal.:KinematicsandoriginsofdiffuselightinHydraIclustercore
Fig.5. Left panel: Spatial distribution of the PNs associated with the blue secondary peak in the PN LOSVD (< 2800kms−1).
Central panel: Spatial distribution of the PNs associated with the central ICL component ( 2800kms−1 to 4450kms−1). Right
panel: Spatial distribution of the PNs associated with the secondary red peak at > 4450kms−1 in the PN LOSVD. The black
trianglesindicateNGC3311(center)andNGC3309(north-westofcenter),respectively.Northisupandeastistotheleft.
Fig.6. Left panel:20 20arcmin2 DSS image of the Hydra I cluster. The two brightgalaxies at the field center are NGC 3311
(center) and NGC 330×9 (north-west of center). The blue circles indicate galaxies with v < 2800kms−1, the green circles
sys
galaxieswith2800kms−1 <v <4450kms−1(onlythosewithin10arcminaroundNGC3311andwithm >15.37),andthe
sys R
redcirclesgalaxieswithv > 4450kms−1.Rightpanel:SpatialdistributionofHydraIgalaxiesinthesameareaof20arcmin2
sys
centeredonNGC3311.SquaresindicategalaxiesfromthecatalogofChristlein&Zabludoff(2003)andcrossesindicategalaxies
fromMisgeldetal. (2008). Thecolorofthe symbolsreferstothe velocitycomponentsin thePN LOSVD asdescribedin Fig.5.
ThetwodiamondslocateNGC3311andNGC3309.TheorangesquareshowstheFoVusedintheFORS2MSISobservations.
Bycontrast,thegalaxieswithLOSvelocities>4450kms−1 regionoccupiedbymanyPNsassociatedwiththesecondaryred
as in the secondaryred peak of the PN LOSVD are mostly lo- peak.
cated withinthe central 100 100 kpc2 region of the cluster Finally, in this region there are only a few galaxies with a
×
(redsquaresandcrossesintherightpanelofFig.6).Inthissub- LOSvelocitylowerthan2800kms−1,compatiblewiththesec-
sample, there are 14 galaxiesin total, 5 are classified as bright
ondary blue peak in the PNs. They are 8 in total (blue squares
galaxiesand9aredwarfs,and3brightgalaxiesand6dwarfsfall
andcrossesintherightpanelofFig.6).Onlyoneofthesefalls
withintheMSISFORS2field.These6dwarfsareconcentrated on the boundary of the central 100 100kpc2 region around
in the northeastern part of the halo of NGC 3311, in the same ×
NGC3311.OneofthesegalaxiesisthegiantspiralNGC3312,
10