Spectacular tails of ionised gas in the Virgo cluster galaxy NGC 4569

12.6 MB·

by  A. Boselli

Astronomy&Astrophysicsmanuscriptno.N4569˙ha˙rev (cid:13)c ESO2016
January22,2016
Spectacular tails of ionised gas in the Virgo cluster galaxy NGC
4569(cid:63)
A.Boselli1,J.C.Cuillandre2,M.Fossati3,4,S.Boissier1,D.Bomans5,G.Consolandi6,G.Anselmi7,L.Cortese8,P.
Coˆte´9,P.Durrell10,L.Ferrarese9,M.Fumagalli11,G.Gavazzi6,S.Gwyn9,G.Hensler12,13,M.Sun14,E.Toloba15,16
1 AixMarseilleUniversite´,CNRS,LAM(Laboratoired’AstrophysiquedeMarseille),UMR7326,F-13388,Marseille,Francee-
mail:[email protected], [email protected]
2 CEA/IRFU/SAP, Laboratoire AIM Paris-Saclay, CNRS/INSU, Universit Paris Diderot, Observatoire de Paris, PSL Research
University,F-91191Gif-sur-YvetteCedex,Francee-mail:[email protected]
3 Universita¨ts-SternwarteMu¨nchen,Scheinerstrasse1,D-81679Mu¨nchen,Germany
6 4 Max-Planck-Institut fu¨r Extraterrestrische Physik, Giessenbachstrasse, 85748, Garching, Germany e-mail:
1 [email protected]
0 5 Astronomical Institute of the Ruhr-Universita¨t Bochum, Universita¨tsstr. 150, 44801 Bochum, Germany e-mail:
2 [email protected]
6 Universita´ di Milano-Bicocca, piazza della scienza 3, 20100, Milano, Italy e-mail: [email protected],
n
[email protected]
a 7 CoelumAstronomia,viaAppia20,30173Venezia,Italye-mail:[email protected]
J
8 InternationalCentreforRadioAstronomyResearch,TheUniversityofWesternAustralia,35StirlingHighway,CrawleyWA6009,
1 Australiae-mail:[email protected]
2 9 NRC Herzberg Astronomy and Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada e-mail:
[email protected], [email protected], [email protected]

] 10 DepartmentofPhysiscsandAstronomy,YoungstownStateUniversity,Youngstown,OH,USAe-mail:[email protected]
A
11 InstituteforComputationalCosmologyandCentreforExtragalacticAstronomy,DepartmentofPhysics,DurhamUniversity,South
G Road,DurhamDH13LE,UKe-mail:[email protected]
12 Department of Astrophysics, University of Vienna, Trkenschanzstrasse 17, 1180, Vienna, Austria e-mail:
.
h [email protected]
p 13 NationalAstronomyObservatoryofJapan,2-21-1Osawa,Mitaka-shi,Tokyo181-8588,Japan
- 14 PhysicsDepartment,UniversityofAlabamainHuntsville,Huntsville,AL35899,USAe-mail:[email protected]
o 15 UCO/Lick Observatory, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA e-mail:
r
t [email protected]
s 16 TexasTechUniversity,PhysicsDepartment,Box41051,Lubbock,TX79409-1051,USA
a
[

2 ABSTRACT
v
WeobtainedusingMegaCamattheCFHTadeepnarrowbandHα+[NII]widefieldimageofNGC4569(M90),thebrightestlate-type
8
galaxyintheVirgocluster.Theimagerevealsthepresenceoflongtailsofdiffuseionisedgaswithoutanyassociatedstellarcomponent
7
extendingfromthediscofthegalaxyupto(cid:39)80kpc(projecteddistance)withatypicalsurfacebrightnessofafew10−18ergs−1cm−2
9
arcsec−2.ThesefeaturesprovidedirectevidencethatNGC4569isundergoingarampresurestrippingevent.Theimagealsoshowsa
4
prominent8kpcspurofionisedgasassociatedtothenucleusthatspectroscopicdataidentifyasanoutflow.Withsomeassumptions
0
onthe3Ddistributionofthegas,weusetheHαsurfacebrightnessoftheseextendedlowsurfacebrightnessfeaturestoderivethe
.
1 density and the mass of the gas stripped during the interaction of the galaxy with the intracluster medium. The comparison with
0 ad-hocchemo-spectrophotometricmodelsofgalaxyevolutionindicatesthatthemassoftheHαemittinggasinthetailiscomparable
6 tothatofthecoldphasestrippedfromthedisc,suggestingthatthegasisionisedwithinthetailduringthestrippingprocess.The
1 lackofstarformingregionssuggeststhatmechanismsotherthanphotoionisationareresponsiblefortheexcitationofthegas(shocks,
: heatconduction,magnetohydrodynamicwaves).Thisanalysisindicatesthatrampressurestrippingisefficientinmassive(M (cid:39)
v star
1010.5M )galaxieslocatedinintermediatemass((cid:39)1014M )clustersunderformation.Italsoshowsthatthemassofgasexpelledby
i (cid:12) (cid:12)
X thenuclearoutflowis∼1%thanthatremovedduringtherampressurestrippingevent.Alltogethertheseresultsindicatethatram
pressurestripping,ratherthanstarvationthroughnuclearfeedback,canbethedominantmechanismresponsibleforthequenchingof
r
a thestarformationactivityofgalaxiesinhighdensityenvironments.
Keywords.Galaxies:individual:NGC4569;Galaxies:clusters:general;Galaxies:clusters:individual:Virgo;Galaxies:evolution;
Galaxies:interactions;Galaxies:ISM
1. Introduction
(cid:63) BasedonobservationsobtainedwithMegaPrime/MegaCam,ajoint
project of CFHT and CEA/DAPNIA, at the Canadian-French-Hawaii
The environment plays a fundamental role in galaxy evolution.
Telescope(CFHT)whichisoperatedbytheNationalResearchCouncil
(NRC)ofCanada,theInstitutNationaldesSciencesdel’Universofthe Since the early works of Hubble & Humason (1931), Abell
CentreNationaldelaRechercheScientifique(CNRS)ofFranceandthe (1965),andOemler(1974)itbecameevidentthatgalaxiesinrich
UniversityofHawaii. environments are systematically different from those located in
2 Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569
thefield.Quiescentobjects(ellipticalsandlenticulars)aredomi- ieswithlongtailsofgaswithoutanyassociatedstellarcompo-
natinghigh-densityregionssuchasclustersandcompactgroups, nent at large clustercentric distances seems to corroborate this
whilelate-typesystemsaremostlylocatedinthefield(Dressler scenario (e.g. Scott et al. 2012; Yagi et al. 2010; Fossati et al.
1980; Binggeli et al. 1988; Whitmore et al. 1993; Dressler et 2012). However, it remains unclear what is the contribution of
al. 1997). It also became clear that the physical properties of the nuclear feedback to the stripping process, in particular in
star forming systems inhabiting rich environments are system- massive spiral galaxies where both the gravitational potential
atically different from those of their isolated analogues, with a well and the nuclear activity are maximal. To date, the direct
reduced atomic (e.g. Cayatte et al. 1990; Solanes et al. 2001; observations of the feedback process in clusters is still limited
Vollmer et al. 2001; Gavazzi et al. 2006a) and molecular gas to a few central early-type galaxies with cooling flows (Fabian
content(Fumagallietal.2009;Bosellietal.2014c),dustcontent 2012).Thedetailedstudyofstarformingsystemsisthusurgent
(Corteseetal.2010;2012a),andstarformation(e.g.Kennicutt tounderstandtheroleoffeedbackintheenvironmentalquench-
1983;Gavazzietal.1998,2006b,2013;Lewisetal.2002;Goto ingofthestarformationactivity.
etal.2003;Bosellietal.2015). Afullunderstandingofthegasstrippingprocessinhighden-
As reviewed in Boselli & Gavazzi (2006), several physi- sity regions requires the comparison of multifrequency obser-
cal mechanisms have been proposed to explain the origin of vationscoveringthedifferentphasesoftheinterstellarmedium
thesedifferences.Theseprocessesbelongstotwomainfamilies, (ISM;atomicandmolecular,ionised,hotgas,dust)andthedif-
those related to the gravitational interactions between galaxies ferentstellarpopulationswithtunedchemo-spectrophotometric
or with the potential well of the overedense region (tidal in- and hydrodynamic models of gas stripping. This comparison
teractions - Merritt 1983; Byrd & Valtonen 1990, harassment has been done in the Virgo cluster (Boselli et al. 2014b), the
-Mooreetal.1998),andthoseexertedbythehotanddensein- closest concentration of galaxies to the Milky Way (17 Mpc,
tracluster medium on galaxies moving at high velocity within Gavazzietal.1999;Meietal.2007),wheremultifrequencydata
the cluster (ram pressure stripping - Gunn & Gott 1972, vis- coveringthewholeelectromagneticspectrumarenowavailable
cous stripping - Nulsen 1982, thermal evaporation - Cowie & down to the dwarf galaxy population (GUViCS, Boselli et al.
Songaila1977,starvation-Larsonetal.1980).Sincetheselarge 2011;NGVS,Ferrareseetal.2012;HeViCS,Daviesetal.2010;
dynamically-bounded structures observed in the local universe ALFALFA,Giovanellietal.2005).Theseworksconsistentlyin-
form through the accretion of smaller groups of galaxies, these dicaterampressureasthedominantprocessresponsibleforthe
processesmightstarttoactwellbeforegalaxiesenterrichclus- gasstrippingandthequenchingofthestarformationactivityof
ters(pre-processing;Dressler2004). star forming systems recently accreted by the cluster (Cayatte
The identification and the physical understanding of the et al. 1990; Solanes et al. 2001; Gavazzi et al. 2013; Boselli et
dominantprocessaffectinggalaxiesinrichenvironmentsatdif- al.2008ab,2014b).TheVirgoclusterhasanothermajorquality:
ferent epochs is fundamental for tuning cosmological models thankstoitsproximitytheangularresolutionofmultifrequency
of galaxy evolution. At present, observations and simulations data is comparable to that obtained in high-resolution simula-
give discordant results whenever large statistical samples ex- tions.Thecomparisonofthekinematicandspectrophotometric
tracted from blind surveys are compared to targeted observa- propertiesofseveralbrightVirgogalaxieshasbeenindeedcru-
tions of nearby clusters and groups. Most hydrodynamic cos- cial for the identification of the perturbing process (Vollmer et
mologicalsimulationssuggestthattheenvironmentalquenching al. 1999, 2000, 2004, 2005, 2006, 2008a, 2008b, 2009, 2012;
ofthestarformationismainlyregulatedbystarvation.Oncein Vollmer 2003; Kenney et al. 2004; Boselli et al. 2005, 2006;
high-densityregions,galaxieslosetheirhotgashaloduringthe Crowl & Kenney 2008; Abramson et al. 2011; Kenney et al.
dynamical interaction with the hostile environment. AGN and 2014;Abramson&Kenney2014;Cortesetal.2015).
supernovae feedback becomes particularly efficient in ejecting A representative case is NGC 4569, the most massive late-
the gas out from the galactic disc, quenching on several Gyr type galaxy of the cluster (M (cid:39) 1010.5 M(cid:12)) located at ∼ 1.7
∗
theactivityofstarformation(McGeeetal.2009;Weinmannet degree north-east from M87 (corresponding to 0.32 virial radii
al. 2010). This scenario is supported by the analysis of SDSS R from the cluster core, see Fig. 1). The study of the kine-
vir
data,whichsuggestaquenchingtimescaleof∼5Gyrforgalax- matic properties of the galaxy derived from HI data combined
ies in dense environments (Wetzel et al. 2012, 2013). It is also with simulations suggest that the galaxy has undergone a ram
supportedbytheobservationsofseveralclustersatintermediate pressure stripping event with a peak of efficiency ∼ 300 Myr
redshift(e.g.Hainesetal.2013,2015).Initscurrentform,how- ago(Vollmeretal.2004).Asimilarresult(∼100Myr)hasbeen
ever,thisscenarioover-predictsthefractionofreddwarfgalax- obtainedbycomparingtheobservedradialtruncationofthedif-
iescomparedtowhatisobservedinnearbyclusters(Kang&van ferent gaseous and stellar components with multizone chemo-
denBosch2008;Fontetal.2008;Kimmetal.2009;DeLucia spectrophotometricmodelsofgalaxyevolutionthataretailored
2011; Weinmann et al. 2011; Taranu et al. 2014). At the same to take into account the effects of ram pressure and starvation
timechemo-spectrophotometricmultizonemodelsofstarvation (Bosellietal.2006;Crowl&Kenney2008).Thisgalaxy,how-
failtoreproducetheobservedradialprofilesofthecoldgasand ever, as most of the massive galaxies in the nearby universe, is
of the young stars of star forming galaxies in nearby clusters also characterised by a nuclear activity with an associated out-
(Bosellietal.2006). flowofgas(seesect.6.2).Itisthusanidealcandidatetostudy
Recenthydrodynamicsimulationsofindividualgalaxiesin- the relative contribution of ram pressure stripping and nuclear
dicate ram pressure as a compelling alternative process to ex- feedback to the stripping process of cluster galaxies. For this
plain the observed peculiarities of cluster members (Roediger purpose we have obtained a deep narrow band Hα+[NII] im-
& Bruggen2007, 2008;Tonnesen &Bryan 2009,2010, 2012). ageofNGC4569anditssurroundingregionswithMegaCamat
Thesesimulationsshowthat,wheneverthedifferentgasphases theCFHT,thatwecombineherewithanumpublishedlong-slit
are properly simulated at high resolution, ram pressure is the spectrum obtained at Calar Alto in 2001. Deep Hα imaging is
dominant mechanism responsible for the gas stripping and for usedtosearchforlongtailsofionisedgas,themostdirectwit-
thefollowingquenchingofstarformationupto∼1virialradius nessofanongoingrampressurestrippingevent(e.g.Gavazziet
ofthecluster.Therecentobservationofseverallate-typegalax- al.2001).Atthesametime,theimpactoffeedbackcanbequan-
Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569 3
observationofthesestandardstarsgivesconsistentresultswithin
2%.Theimagesweretakeningoodseeingconditions(0.80arc-
secinrand0.86inHα+[NII]).
2.2. Longslitspectroscopy
NGC4569wasobservedin2001withtheCalarAlto3.5mtele-
scope using the Twin spectrograph. The galaxy was observed
with the T05 and T06 gratings in the blue (4430 Å) and red
(6562Å)armofthespectrographwithadispersionof36Å/mm
and a spatial sampling of 0.56 arcsec/pixel. Observations were
obtained with a 30 min exposure using a 4 arcmin long slit of
width1.2arcseccenteredonthenucleusofthegalaxyandori-
entedtoaPAof105oasindicatedinFig.6.Duetothelowsignal
tonoiseandsensitivityinthebluearmwewerenotabletoobtain
reliablemeasurementsforadetailedandsatisfactoryanalysisof
thedata.
3. Datareduction
3.1. Narrowbandimaging
Fig.3. The Hα+[NII] image of NGC 4569 and IC 3583 ob-
The data have been reduced using the Elixir-LSB package
tained with a 30 min ON-band exposure at the 1.2m telescope
(Cuillandreetal.inpreparation),apipelineexpresslydeveloped
oftheObservatoiredeHauteProvence,fromBoselli&Gavazzi
tominimisethecontributionofthescatteredlightcomponentin
(2002). The image covers the area shown as the white box in MegaCamimages.Theefficiencyofthisobservingstrategyand
Figure2.
datareductionprocedureshavebeenprovenbythedetectionof
verylowsurfacebrightnessfeaturesinthetidaltailsassociated
tifiedbystudyingthepropertiesoftheionisedgasassociatedto to early-type galaxies in the MATLAS (Duc et al. 2011; 2015)
thenuclearoutflow. and in the NGVS surveys (Ferrarese et al. 2012; Mihos et al.
2015).
2. Observations
The best ON- and OFF-band images are then combined to
produceaHα+[NII](continuum-subtracted)frame(Fig.2).The
2.1. Narrowbandimaging
ON- and OFF-band frames are scaled using ad-hoc normalisa-
TheobservationswerecarriedoutinMay2015usingMegaCam tion factors n depending on the typical colour of the galaxy
at the CFHT. NGC 4569, which has a recessional velocity of (Spector et al. 2012). The colour-dependent normalisation has
-221 km s−1, was observed in the narrow band filter MP9603 been determined using 15 stars of different colour in the ob-
centered on the Hα line (λ = 6590 Å; ∆λ = 104 Å, T = 93%). servedfield.Thephotometriccalibrationoftheimageobtained
The transmissivity T of the filter (-1140 < vel < 3600 km s−1) using the spectrophotometric standard stars was checked us-
perfectly covers the range in recessional velocity of the whole ing a few galaxies in the field observed during previous tar-
Virgoclusterregion(Binggelietal.1993;Bosellietal.2014a). getednarrow-bandimagingobservationsorusingnuclearspec-
Becauseofitswidth,thefilterencompassesthetwo[NII]lines troscopy from the SDSS (see Table 1). This comparison gives
at λ = 6548 and 6584 Å. The stellar continuum was measured fairlyconsistentresults(0.25dex).
throughthebroad-bandrfilter.Sincethepurposeofthepresent The pixel size in the raw images is of 0.187 arcsec/pixel.
observationswastodetectlowsurfacebrightnessfeaturesasso- To increase the signal-to-noise we rebinned the images by
ciatedtothegalaxy,theobservationsweredoneusingthepoint- a factor of three (0.561 arcsec/pixel), and later smoothed
ingmacroQSOLDP-CCD7optimisedforElixir-LSBobserving them using a median 5x5 pixel filter. At low level counts the
and processing mode (see sect. 3). This macro makes 7 differ- continuum-subtracted image also shows several low surface
ent pointings around the galaxy using a large dithering as gen- brightness features. Some of them are clearly associated to the
erally done in near-infrared imaging of extended sources. The reflection of bright stars in the field, not fully removed by the
final fully co-added image covers the central 40’×30’ at max- Elixir-LSB data reduction procedure. Fortunately these are far
imum depth, while a larger area is mapped at lower sensitiv- from the target galaxy and only marginally affect the present
ity. This mode has been extensively used over the recent years analysis. The image also shows filamentary structures in the
and applies well to narrow-band imaging data with long expo- west of NGC 4569 forming a double tail clearly associated to
sures (medium sky background levels). Integration times were thegalaxy(seenextsection).Thepixel1σrmsoftheresulting
of 660 sec per pointing in the ON-band image, and 66 sec in rebinned image is of 3.8 × 10−18 erg cm−2 sec−1 arcsec−2.
the OFF-band r-band filter, thus the resulting integration time Becausethesignalisextendedonscalesofafewarcsecondsin
onthestackedimageisof4620secinHα+[NII]and462secin the extended filaments detected in Hα+[NII], once smoothed,
r.Thephotometriccalibrationofthedata,takeninphotometric the image is sufficiently deep to detect features with a surface
conditions, has been done in the r band following the standard brightness of (cid:39) 10−18 erg cm−2 sec−1 arcsec−2 (Fig. 2). The
MegaCamprocedures.Inthenarrowbandfilterthephotometric comparison of the continuum-subtracted image of the galaxy
calibrationwassecuredwiththeobservationofthespectropho- (Fig. 2) with the one obtained in a 30 min exposure with the
tometricstandardsFeige34andHz44(Masseyetal.1988).The 1.2 meter OHP telescope (Fig. 3, Boselli & Gavazzi 2002)
4 Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569
underlines the exquisite quality of the present image. The total
Hα+[NII] flux of the stellar disc measured within an elliptical
aperture arbitrary defined to minimise the contribution of IC
3583(Fig.4)islogf(Hα+[NII])=-11.85±0.03ergs−1 cm−2,
whichisconsistentwithpreviousestimates(seeTable1).
3.2. Longslitspectroscopy
Reduction was performed using the longslit package in IRAF.
We used dome flats to create the spectroscopic flat field.
Wavelength calibration was performed using the Thorium-
Argon lamp. The tilt of the dispersion axis with respect to the
CCDrowswascorrectedusingatemplatecreatedfromobserva-
tionsofstandardstarsandthegalaxycoreatdifferentpositions
alongtheslit.
From the sky subtracted, continuum subtracted, and wave-
Fig.5. The 2D continuum subtracted spectrum of NGC 4569
length calibrated 2D spectrum (shown in Figure 5) we extract
zoomed on the Hα and [NII] lines (left) and on the [SII] dou-
1D spectra in the regions given in Table 2 using median statis-
blet (right) obtained at Calar Alto with a slit of width 1.2 arc-
tics. Variance spectra are extracted in the same positions along
sec positioned along the minor axis on the spur of ionised gas
the slit by assuming Poisson statistics on the un-skysubtracted
as indicated in Fig. 6. The red labels shown on the left panel
2Dspectrum.Thespectralresolution(R=λ/∆λ)isobtainedby
correspond to those shown in Fig. 6 and indicate the range of
fitting18brightisolated skylinesinthespectrumina position
pixels used to extract 1D spectra to derive the physical param-
close to the galaxy nucleus. We then fitted a 3rd order polino-
eters given in Table 2. The blue vertical arrow shows the mean
mialtothosepointstoobtainamodelofRvs.wavelengthwhich recessionalvelocityofthegalaxy(-221kms−1)derivedfromHI
is then used in the fitting procedure to correct the width of the
data.
lines for the instrumental resolution. The spectral resolution of
thedataisR=6290(FWHM=46kms−1or1.02Å)atHα.
We fit the emission line of the spectrum with the kubeviz
kpc projected distance to the west. These filaments are domi-
software1(Fossatietal.2015).Thiscodeuses“linesets”,defined
nated by two main features as often observed in cluster galax-
asgroupsoflinesthatarefittedsimultaneously.Eachlinesetis
ies with extended tails of ionised gas (Sun et al. 2007, 2010;
describedbyacombinationof1DGaussianfunctionswherethe
Yagietal.2010).TheHα+[NII]fluxesofNGC4569andofits
relativevelocityseparationofthelinesiskeptfixed.Inthiswork
associated low surface brightness extended features (Table 3),
wefittwolinesets,thefirstmadeofHαand[NII]λλ6548,6584
as well as that of the other galaxies in the frame (Table 1), are
and the second made of [SII] λλ6716,6731. Furthermore, the
determined by measuring the counts in the ON- and OFF-band
flux ratio of the two [NII] lines is kept constant in the fit to
frames(afterremovingthecontributionofunwantedforeground
the ratios in Storey & Zeippen (2000). The continuum level is
starsinthefield).Thefluxofallthesetargetshasbeenextracted
evaluatedduringthefitprocedurewithaninitialguessestimated
using the QPHOT IRAF task whenever possible. For sources
in two symmetric windows around each lineset. During the fit,
with asymmetric distributions or with evident nearby compan-
kubeviz takes into account the noise from the variance spectra,
ionswhichmightsignificantlycontaminatethefluxweusedthe
thusoptimallysuppressingskylineresiduals(whichbracketthe
FUNTOOLS analysis package on DS9. For these sources we
HαlineattheredshiftofNGC4569).However,theadoptedvari-
manuallydefinedellipticalaperturesadjustedtofitthefulllight
anceunderestimatestherealerror,mostnotablybecauseitdoes
profile of each target galaxy and to select uncontaminated re-
notaccountforcorrelatednoiseintroducedinthereductionpro-
gionsforthedeterminationoftheskybackground.Usingpoly-
cedure and extraction of 1D spectra. We therefore renormalise
gons,weidentifiedintheimagesmoothedwithamedianfilterof
thefinalerrorsonthelinefluxesandkinematicalparametersas-
5×5pixelsanumberoflowsurfacebrightnessfeatureslabeledin
sumingaχ2 = 1perdegreeoffreedom.Theresultingemission
Fig. 4. Their flux has been extracted using FUNTOOLS, while
linemeasurementsaregiveninTable2.Weverifiedthatthekine-
the local sky background was estimated in adjacent polygons.
maticalparametersareconsistentwithintheuncertaintiesifwe
We also extracted the total flux of all the extended filaments
comparethetwolinesets(Hα+[NII]and[SII]).Nonethelessthe
using a large polygon covering the western part of the galaxy,
valueslistedinTable2areobtainedfromthefirstlinesetbecause
withitsassociatedskybackgroundmeasuredinseveralcircular
theuncertaintiesaresmallerduetothebrighterlines.Diagnostic
apertures located in uncontaminated regions around it. The un-
lineratiosarealsogivenwheneverthesignal-to-noiseofallthe
certaintyonthesemeasurementshasbeendeterminedusingthe
linesinvolvedintheratioisgreaterthan3.
prescriptiongiveninBosellietal.(2003)whichisoptimisedfor
extended sources where the uncertainty is dominated by large
scale fluctuations of the sky background (see also Ciesla et al.
4. Physicalparameters
2012).ThepropertiesoftheseregionsarelistedinTable3.
The Hα luminosity of the different galaxies and of the ex-
4.1. Narrowbandimaging
tendedfeaturescanbedeterminedoncetheobservedfluxesare
The deep Hα+[NII] image of NGC 4569 shows several fila- correctedfor[NII]contamination.Thisisrelativelyeasyforthe
mentaryfeaturesextendingfromthediscofthegalaxyto∼80 emission over the discs of the two galaxies NGC 4569 and IC
3583,anirregulargalaxyataprojecteddistanceof∼6arcmin,
1 http://www.mpe.mpg.de/˜mfossati/kubeviz/ for which integrated spectroscopy is available from Gavazzi et
Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569 5
al. (2004) and Boselli et al. (2013)2. The physical conditions Ifweassumethatthegasisfullyionised,thusthatn =n ,
e p
of the stripped interstellar medium might be significantly dif- and αeff = 1.17 × 10−13 cm3 s−1 (Osterbrock & Ferland 2006)
ferentthanoverthediscofthegalaxy,thusthisratiomightsig- thedeHnαsityofthegascanbederivedfromeq.(1):
nificantly change. In particular, the radiation of the hot gas of
theintraclustermediumisexpectedtoincreasethe[NII]/Hαra- (cid:115)
L(Hα)
tio.RecentspectroscopicobservationsdonewiththeMultiUnit n = (2)
Spectroscopic Explorer (MUSE) of ESO 137-001, a galaxy in e αeHfαfVfhνHα
theNormaclusterundergoingastrippingprocesssimilartothe
one occurring in NGC 4569, suggest that the [NII]/Hα ratio in Under these assumptions, the mean density of the ionised gas
the tail is not typical of a photoionisation region ([NII]/Hα (cid:39) is ne (cid:39) 5 × 10−3 cm−3, and the total mass of the ionised gas
0.3), but is slightly higher likely because the gas is partly ex- is M(tail(Hα) (cid:39) 3.2 × 109 M(cid:12). Given the large uncertainty on
cited by shocks or heat conduction ([NII]/Hα (cid:39) 0.5, Fossati et the geometry of the gas, this is a very rough estimate. We can
al. 2015). Similar results have been derived from the spectro- also calculate the typical density of the gas within the different
scopic observations of the tails of other cluster galaxies (Yagi filaments labeled in Fig. 4. Assuming a cylindrical geometry,
etal.2007;Yoshidaetal.2012;Merluzzietal.2013).Wethus thetypicaldensitywithinthedifferentfilamentsrangesbetween
assume[NII]/Hα=0.5inthetails. 2 and 4 × 10−2 cm−3, while the total mass in the 13 identified
regionsis(cid:39)3.1×108M 4.
Anestimateofthedensityoftheionisedgascanbederived (cid:12)
usingtherelation:
4.2. Longslitspectroscopy
L(Hα)=n n αeffVfhν (1) 4.2.1. Kinematics
e p Hα Hα
The spectral resolution of the Calar Alto observations is suffi-
(Osterbrock & Ferland 2006) where n and n are the number cienttostudythekinematicsoftheionisedgasalongtheslit(see
e p
densityofelectronsandprotons,αeff istheHαeffectiverecom- Table2).The2D-spectrumofthegalaxy(Fig.5)showsanoffset
binationcoefficient,VisthevolumHeαoftheemittingregion, f the invelocityintheSE(regionA,extendingupto(cid:39)20arcsec)and
NW(regionB,upto(cid:39)30arcsec).Thisoffset,whichisobserved
fillingfactor,hthePlanck’sconstant,andν thefrequencyof
Hα
alsointheHαFabry-PerotdataofCheminetal.(2006),isdue
theHαtransition.ThetwovariablesVand f canbeonlycrudely
totherotationofthegalaxy.ThisregioncorrespondstotheHα
estimatedfromobservationsorfromsimulations.
main body of the galaxy in Fig. 6. The nuclear spectrum also
Thestrippedmaterialisassumedtobedistributedinacylin-
showsalargevelocitydispersion(132kms−1)duetotheturbu-
derofdiameter((cid:39)50kpc)andofheightcomparabletotheex-
lenceinthegasprobablyinducedbyanuclearactivity(Hoetal.
tension of the observed tail of ionised gas. In the case of NGC
1997).Ahighsurfacebrightnessregionisdetectedat∼50arc-
4569, the tail extends up to (cid:39) 80 kpc on the plane of the sky.
secfromthenucleusintheNWdirection(regionD).Thisregion,
Sincethegalaxyisblue-shifted(vel=-221kms−1),weexpect
whichislocatedintheprojectionoftheextendedNWspiralarm,
that it is crossing the cluster from the backside. It is conceiv-
ispresentalsointheFabry-Perotdata.Thevelocitydispersionof
able that the observed tail of ionised gas is just a projection on
thisregionislow((cid:39)10kms−1)andistypicalofanHIIregion.
theplaneofthesky,thusthat80kpcisalowerlimittothereal
Further out, emission isdetected along the filaments of ionised
heightofthecylinder.Thecomparisonofmultifrequencyobser-
gas at high velocity (up to (cid:39) 130 km s−1 in region E) with re-
vationsofthegalaxywithtunedmodelsofgasstrippingsuggest
specttothenucleus.Thekinematicofthegas,asshowninFig.
that the galaxy underwent the peak of the stripping process (cid:39)
5, is totally disconnected from the rotation of the disc. Further
100Myrago(Bosellietal.2006).Ataradialvelocityof(cid:39)1100
out(regionsF,G),therecessionalvelocitydecreasessmoothly,
kms−1withrespecttotheclustercentre3,thegalaxywouldhave
thenrapidlyinregionHreachingnegativevalues.Thevelocity
traveled ∼ 120 kpc along the line of sight. Summing these val-
dispersion in these outer and diffuse regions is relatively high
ues quadratically, we expect that the physical extension of the
(70-90kms−1),indicativeofthepresenceofturbulentmotions.
strippedgasis∼145kpc,correspondingto∼5.5timestheopti-
calradiusofthegalaxy.
The filling factor f is another unconstrained parameter. In 4.3. Lineratios
allgalaxieswheretailshavebeenobserved,theionisedgashas
We can also estimate how the [NII], Hα, and [SII] line ratios
astructureddistribution,withhighdensityclumpsofcondensed
change along the slit in the different positions listed in Table
materialoftenassociatedtostarformingregions,otherwiseex-
2. The [NII]/Hα line ratio in the nucleus is 1.29±0.04, while
tended in filamentary structures (Yagi et al. 2007, 2010, 2013).
[SII]/Hα=0.67±0.02.ThesevaluesaretypicalofLINERgalax-
Similarstructuresarealsopresentinhydrodynamicsimulations
ies (e.g. Ho et al. 1997). The values of region D ([NII]/Hα =
(e.g. Tonnesen & Bryan 2010). In particular, the filamentary
0.37±0.03; [SII]/Hα = 0.26±0.02) are typical of HII regions,
structureswithdoubletailsarereproducedinsimulationswhen-
confirmingthatthishighsurfacebrightnessspotisastarform-
ever magnetic fields are taken into account (Ruszkowski et al.
ing regions associated to the western spiral arm. The values of
2014,Tonnesen&Stone2014).Itisthusmostlikelythat f <1,
[NII]/Hαand[SII]/Hαmeasuredinallotherregions,onthecon-
but its exact value is highly uncertain. Consistently with previ-
trary,areintherange0.7(cid:46)[NII]/Hα(cid:46)1.4and0.4(cid:46)[SII]/Hα
ousworks,whichgenerallytake0.05< f <0.1,weassume f =
(cid:46) 0.9. Although uncertain, these values are too high to be pro-
0.1.
ducedbystellarphotoionisationonly,butratherrequirethecon-
tribution of an hard radiation field as the one produced by a
2 Weuseforthispurposetheupdatedvalueof[NII]/Hα=0.97for
NGC4569giveninBosellietal.(2015). 4 Thisvalueshouldbetakenasalowerlimittothetotalmassofthe
3 The mean velocity of cluster A, the Virgo substructure to which ionisedgasgiventhatthe13regionsindicatedinFig.4donotinclude
NGC4569belongs,isvel=955kms−1(Bosellietal.2014a). allthelowsurfacebrightnessfilamentsassociatedtothegalaxy.
6 Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569
nearbynucleusorbyslowshocks(Tuelmannetal.2000;Allen
et al. 2008; Rich et al. 2011). In the spur of ionised gas the
[SII]λ6716/[SII]λ6731 line ratio is (cid:39) 1.4 suggesting that the
electron density of gas is low (n (cid:46) 10 cm−3). The exact value
e
ispoorlyconstrainedgiventhelargeuncertaintyonthe[SII]ra-
tio and the saturation in the intensity ratio vs. electron density
relation(Osterbrock&Ferland2006).
5. Comparisonwithmultifrequencydata
High-quality multifrequency data covering the whole elec-
tromagnetic spectrum, from X-ray to radio, of NGC 4569
are available in the literature. These data are crucial for a
comparison with the Hα data obtained in this work to identify
theperturbingmechanismaffectingthegalaxy.Excellentquality
ugiz imaging data of NGC 4569 have been obtained as part of
theNGVSsurveyusingMegaCamattheCFHT(Ferrareseetal.
2012).Thepseudo-colouropticalimageofthegalaxyisshown
in Fig. 7 and 8. The optical image does not show any diffuse Fig.11. The pseudo-colour image of NGC 4569 and IC 3583
stellar emission down to a surface brightness limit of (cid:39) 29 µ obtainedcombainingXMM0.4-1.3keVX-rays(blue),GALEX
g
mag arcsec−2 (AB system) associated to the stripped ionised NUV (green), and CFHT MegaCam Hα+[NII] (red) images.
gas located in the west of the galaxy. The lack of low surface Northisup,eastleft.
brightness stellar features, which are generally formed during
gravitationalinteractionswithnearbycompanions,suggeststhat
Rosat, XMM-Newton, Chandra X-ray and VLA 1.4, 4.86
the gaseous component is removed from the galaxy through
GHzradiocontinuumimagesofNGC4569showextendedfea-
the interaction with the hot and dense intracluster medium.
tures perpendicular to the stellar disc of the galaxy (Tschoeke
The comparison of the Hα frame with the optical NGVS (Fig.
etal.2001;Chyzyetal.2006;Grieretal.2011;Wezgowiecet
7) and the FUV and NUV (Figs. 9 and 11) images of the
al.2011,2012).Theextensionofthehotgastracedbythe0.2-1
galaxyobtainedaspartofthetheGUViCSsurveyofthecluster
keV XMM-Newton X-ray image, however, is smaller than that
(Boselli et al. 2011) also indicates that the ionised gas is only
diffuse and does not have any clumpy structure suggesting oftheionisedgas(Fig.10and11).The20cmradiocontinuum
emission, instead, is limited to the inner 22 kpc and does not
the presence of extraplanar HII regions. The smallest resolved
features in the Hα+[NII] image have a filametary structure of morphologicallymatchtheionisedgas.
thicknesslargerthan200pc.WeremarkthattheNUVimageof
thegalaxyanditssurroundingregionshasbeenobtainedwitha
6. Discussion
very long exposure (16993 sec) and it is thus very sensitive to
extraplanarstarformingregionssuchasthoseobservedaround 6.1. Thegalaxy
M49 (Arrigoni-Battaia et al. 2012) or VCC 1217 (Hester et al.
2010;Fumagallietal.2011;Kenneyetal.2014).TheHα+[NII] NGC 4569 shows a truncated star forming disc with respect
to the distribution of the old stellar populations (Boselli et al.
surface brightness of the extraplanar HII regions associated
to these two galaxies is ∼ 2 × 10−17 erg cm−2 sec−1 arcsec−2, 2006),withaprominentspiralarmstartingfromthenorthofthe
galaxyandextendinginthewestdirection.Theexcellentquality
thus well above the detection limit of our image. The limiting
sensitivityintheNUVimageisof(cid:39)29ABmagarcsec−2,thus of the CFHT image shows resolved HII regions along the disc
and the western spiral arm. A concentration of HII regions is
deep enough to detect low surface brightness features such as
presentonthenorth-eastandsouth-weststarformingdisc,sug-
those observed in the tidal tails of NGC 4438 (Boselli et al.
gestingagranddesignspiralpatterndrawnbytwomajorspiral
2005).
arms. Independent tracers of star formation, including the Hα
fluxderivedinthiswork,canbeconvertedundersomeassump-
The distribution of the atomic gas has been mapped using
tions into star formation rates (e.g. Kennicutt 1998, Boselli et
the VLA in C short configuration by Chung et al. (2009). The
al.2009).Thestarformationratemeasuredoverthewholedisc
HIimage,whichhasanangularresolutionof15-16arcsecanda
column density sensitivity of 3-5 × 1019 cm−2 (3 σ per chan- of NGC 4569 is SFR (cid:39) 2 M(cid:12) yr−1 assuming a Salpeter IMF
(Bosellietal.2015).
nel), shows that the HI gas is located within the stellar disc
Thewavelengthdependenttruncationofthestellardiscand
of the galaxy and has a truncated radial distribution typical of
ofthegas(atomicandmolecular)anddustcomponentsobserved
HI-deficient cluster galaxies (Cayatte et al. 1994; Chung et al.
inNGC4569hasbeenexplainedasduetoarecent((cid:39)100Myr)
2009).Thisisalsothecaseforthedistributionofthemolecular
rampressurestrippingeventabletoradiallyremovethegaseous
gas(Helferetal.2003)andofthehotandcolddustcomponents
as derived from WISE, Spitzer, and Herschel5 images (Boselli component and gradually quench the activity of star formation
ofthegalaxyintheouterregions(Bosellietal.2006).Asimilar
etal.2006;Corteseetal.2014;Cieslaetal.2012;Fig.10).The
timescale ((cid:39) 300 Myr) has been derived from the study of the
atomic gas and the dust component are not detected along the
kinematicpropertiesoftheatomicgas(Vollmeretal.2004)and
tailsofionisedgas.
fromtheanalysisof2-Dopticalspectra(Crowl&Kenney2008).
5 NGC 4569 has been observed during the Herschel Reference ThephysicalandkinematicpropertiesofthemainbodyofNGC
Survey, Boselli et al. (2010). Most of these multifrequency data are 4569 thus consistently indicate that the galaxy underwent a re-
availableonadedicateddatabase:http://hedam.lam.fr/HRS/ cent ram pressure stripping event. We want to see whether this
Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569 7
dh
evolutionarypicturecanalsoexplainthepresenceofthelowsur- =G(ρ +ρ )(πr2)2Hρ (3)
face brightness features detected in the Hα+[NII] narrow band N4569 out out h
imagedescribedintheprevioussection.
where ρ and ρ are the mean mass density of the galaxy
N4569 out
and of the gas in the outflow. The density of the galaxy can be
6.2. Thenuclearoutflow inferredfromitsrotationalvelocityvel (r)(cid:39)250kms−1 at
N4569
a radius r = 8 kpc (Rubin et al. 1989) using the virial theorem
The deep MegaCam Hα+[NII] image shows a diffuse and ex- (ρ (cid:39)4.9×10−24gcm−3),whilethatoftheoutflowistheone
tended halo of ionised gas around the galaxy. It also shows a preNv45io69usly derived from the Hα luminosity (ρ (cid:39) 1.4 × 10−25
prominent plume in the western side perpendicular to the disc. gcm−3).ThepotentialenergyoftheoutflowisoutthusΦ (cid:39)9×
This plume is located south of the minor axis in the region be- out
1055ergs.Thekineticenergyoftheoutflowis:
tween the star forming disc and the prominent western spiral
arm, and along the minor axis outside it, identified with a blue vel2
polygon in Fig. 4. Its total extension is (cid:39) 100 arcsec (8 kpc), E = outM (4)
kin,out out
anditstotalfluxislogf(Hα+[NII])=-13.07ergs−1cm−2(see 2
Table 3). The presence of a prominent dust lane in the western whereM isthemassoftheoutflow(M =3.4×107M ,see
out out (cid:12)
side of the galaxy (Fig. 8), hidden by the bulge in the eastern
Table4)andvel itsvelocitycorrectedfortheorientationofthe
out
side, suggests that the western side is the near side. The spur galaxyontheplaneofthesky(vel (cid:39)260kms−1,seeTable2).
out
ofgasobservedinthewesternsidehasahigherrecessionalve- ThekineticenergyoftheoutflowisE (cid:39)2.4×1055ergsand
kin,out
locity with respect to the nucleus of the galaxy and is thus an thetotalenergyis E (cid:39)1.1×1056 ergs.Thisenergywould
tot,out
outflow. The nuclear outflow is probably powered by a nuclear require 1.1 × 105 supernovae of energy 1051 ergs, or a larger
starburst (Barth et al. 1998; Maoz et al. 1998; Barth & Shields
numberifweassumeamorerealistic1-10%energytransferef-
2000,Tschoekeetal.2001,Chyzyetal.2006).Thespectralsyn-
ficiency.Toprovidethisnumberofsupernovae,thenuclearstar
thesisanalysisofthenuclearHSTspectracarriedoutbyBarth& cluster should have a mass (cid:38) 2.4 × 107 M and a star forma-
Shields(2000),whoclassifieditasatypicalLINER/HIItransi- tionrateof(cid:38)24M yr−1.Therequiredmass(cid:12)andstarformation
tionnucleus,datedthenuclear((cid:46)30pc)starburstto∼3-6Myr (cid:12)
activity of the nuclear starburst are a few order of magnitudes
(Barth & Shields 2000, Gabel & Bruhweiler 2002). The pres-
higher than those observed in the galaxy (Keel 1996; Barth &
ence of A-type supergiants in the inner ∼ 300 pc suggests that
Shield2000,Bosellietal.2015).Wecaution,however,thatram
asecondstarburstoccurredmorethan15Myrago(Keel1996).
pressurecanproducelow-densitysuperbubbleholesintheinner
AcontributionfromanAGNcannotbefullyexcludedgiventhe
discandthussupplyextraenergytotheoutflowthroughKelvin-
presenceofacompactsourceinthesoftChandraX-rayimageof
Helmholtzinstabilities(Roediger&Hensler2005)andviscous
NGC4569(Grieretal.2011).AdominantAGNactivity,how-
stripping(Roediger&Bruggen2008).Despitethispossiblein-
ever, is ruled out by the lack of a point-like nuclear source in creaseoftheefficiencyintheenergytransfertotheoutflowdue
the radio continuum (Hummel et al. 1987; Neff & Hutchings
torampressure,thissimplecalculationsuggestsaprobablesu-
1992) and in the ROSAT X-ray hard band image of the galaxy
plyofenergybyanAGN.
(Tschoekeetal.2001). Thestrikingvelocitydifferencebetweentheadjacentregions
If we assume that the gas is mainly photoionised we can G and H (∼ 60 km s−1) can be explained by the fact that while
use eq. 2 to estimate the total density and mass of the ionised
region G is associated to the nuclear outflow, in region H the
gas in the outflow. Assuming that the gas in the outflow is in
ionisedgasmighthavebeenstrippedfromthediscofthegalaxy
a cylinder of 60 arcsec (5 kpc) diameter and 96 arcsec (8 kpc)
andisonlylocatedinprojectionclosetotheouterextensionof
projectedlength,[NII]/Hα(cid:39)1,andafillingfactor f =0.1,the
theoutflow.Indeed,adeepinspectionofFig.6showsthatregion
density of the ionised gas is n (cid:39) 9.7 × 10−2 cm−3 and its total
e Hisafewarcsecbelowtheprojectionofthenuclearoutflow.
mass M (Hα) (cid:39) 3.4 × 107 M (see Table 4). This density is
out (cid:12)
consistentwiththemeanelectrondensityn (cid:46)10cm−3 derived
e
by the poorly constraining [SII]λ6716/[SII]λ6731 line ratio of 6.3. Thediffusegas
the outflow ((cid:39) 1.4). At this density, the recombination time is
Theroughestimateofthe totalmassoftheionisedgasderived
t (cid:39) 1 Myr, shorter than the age of the nuclear starburst. For
rec from Hα in the diffuse tail that we have derived in sect. 4 is
an outflow velocity of (cid:39) 130 km s−1, the ejected gas should
M (Hα)(cid:39)3.2×109M (Table4).Thismasscanbecompared
have traveled only a distance of (cid:39) 0.65 kpc in 5 Myr, and (cid:38) 2 tail (cid:12)
tothetotaldynamicalmassofthegalaxy(1.2×1011 M ,Haan
kpc if powered by an older (> 15 Myr) starburst episode that (cid:12)
etal.2008)andtothemassofgasintheothergaseousphases.
is necessary to explain the presence of A-type supergiants in
This galaxy has a total mass of molecular hydrogen of M(H )
the inner 300 pc (Keel 1996). These distances are too small 2
= 4.9 × 109 or 2.2 × 109 M depending whether the molecu-
comparedtothetypicalextensionofthespurofionisedgas((cid:39) (cid:12)
lar gas mass is derived from CO observations using a constant
8kpc).Itisthusconceivablethatthegasintheoutflowhasbeen
or variable CO-to-H conversion factor (Boselli et al. 2014b).
ionised by other mechanisms than photoionisation from the 2
ThetotalHImassofNGC4569is7.6×108 M (Haynesetal.
centralstarburst.These canbeidentifiedasthenuclear activity (cid:12)
2011)andisaboutafactoroftensmallerthantheoneofisolated
ortheshockinducedbytheturbulenceintheoutflow.
galaxies of similar type and luminosity, as indeed indicated by
itslargeHI-deficiencyparameter(HI −def6 =1.05,Boselliet
We can also make a rough estimate of the total (potential
al.2014b).ThissuggeststhatNGC4569haslost(cid:39)7.7×109M
andkinetic)energyoftheoutflow.Consideringtheoutflowasa (cid:12)
ofHI.Amoreaccurateestimateofthetotalmassofgaslostby
cylinder of physical length H and radius r, its potential energy
thegalaxyduringitsinteractionwiththeclusterenvironmentcan
canbederivedusingtherelation:
6 TheHI-deficiencyparameterisdefinedasthedifferenceinlogarith-
dΦout =GMN45h69+outdMout = mgivicenscaanlegubleatrwseizeenathnedemxoprepchteodloagnidcatlhteyopbes(eHrvaeydneHsI&mGasisovoafnaegllail1a9x8y4o)f.
8 Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569
bederivedfrommultizonechemo-spectrophotometricmodelof other mechanisms. These might be ionisation by the hot gas of
galaxyevolution(Bosellietal.2006).Thetruncatedgaseousand theintraclustermedium,thermalconductionandturbulentmix-
stellarprofilesofNGC4569canbereproducedifthegalaxylost ing, although a direct evidence of their presence is still lacking
(cid:39) 1.9 × 1010 M of gas during a ram pressure stripping event (Tonnesenetal.2011;Fossatietal.2015).Thelackofanystar
(cid:12)
thatstarted∼100Myrago.Thismassisslightlylargerthanthe forming region in the tail, on the contrary present in ESO 137-
veryroughestimateoftheionisedgasmassderivedfromHαand 001(Jachymetal.2014),suggeststhatthecontributionofthese
suggeststhatalargefractionoftheatomicgas,oncestripped,is othermechanismsmustbeevendominantinNGC4569.
ionisedwithinthetail.ThedeepVLA21cmobservationsofthe ThelackofstarformingregionsinthetailofNGC4569can
VIVAsurveydetectHIgasonlywithinthestellardiscofNGC result from two main effects as indicated by the simulations of
4569(Chungetal.2009).AcomparisonoftheHIandHα+[NII] Tonnesen&Bryan(2012).Theefficiencywithwhichthegasis
frames of the galaxy does not show the presence of any HI ex- transformedintostarsinthetailsoframpressurestrippedgalax-
traplanarfeatureassociatedtothetailsofionisedgasdowntoa iesdependsonthewaythelow-densitygascoolsandcondenses
column density limit of 3-5 × 1019 cm−2 for a 10 km s−1 spec- intheturbulentwakes.Thisprocessismoreefficientwhenever
tralresolution(Chungetal.2009).IfallthestrippedHIgas((cid:39) thedensityoftheintraclustermediumishighasinmassiveclus-
1.9×1010 M )wasstillinitsneutralphaseandwasdistributed terssuchasComaandA1367wheremostofthetailsofstripped
(cid:12)
withinthesametaildefinedbytheHα+[NII]emission,itscol- materialharborstarformingregions(Yoshidaetal.2008,Yagiet
umndensityshouldbeΣ(HI)(cid:39)0.4M pc−2(∼5×1019atcm−2) al.2010,Fossatietal.2012).Thetypicaldensityoftheintraclus-
(cid:12)
closetothedetectionlimitoftheVLAimages.This,however,is termediuminVirgois∼afactorof10lowerthaninComaand
probablyalowerlimitsinceweexpectthatthegas,asindicated A1367(Brieletal.1992;Bo¨hringeretal.1994).Consideringa
bytheobservationsoftheionisedphaseorbythesimulationof β-model(Cavaliere&Fusco-Femiano1976)totracethedepro-
the HI phase (Tonnesen & Bryan 2010) should have a clumpy jecteddistributionoftheX-rayemittinggaswithinVirgo:
distributionwithpeaksincolumndensitywellabovethislimit.
The MUSE observations of ESO 137-001 show that the kine- (cid:20) r (cid:21)−3β
ρ=ρ 1+( )2 2 (6)
maticalpropertiesofthestrippedgasdonotsignificantlychange 0 r
c
withrespecttothatoftheparentgalaxy(Fumagallietal.2014).
We thus expect that the stripped gas is distributed on the same andassumingacentraldensityoftheintergalacticmediumofρ
0
velocity range than NGC 4569 and it is not spread on a much =2×10−3 cm−3 (Bo¨hringeretal.1994),acoreradiusr =2.7
c
widerrange,reducingtheexpectedsignal-to-noiseperchannel. arcminandβ=0.47(Schindleretal.1999),weestimatethatthe
Wecanthusconcludethat,ifthestrippedgaswasstillinitsneu- density of the intracluster medium near NGC 4569 is ρ (cid:39) 10−5
tralphase,weshoulddetectit. cm−3 (we assume a distance r from the cluster core of 1.7 de-
An independent estimate of the upper limit to the column grees, corresponding to 0.32 R ). This value can be compared
vir
density of the neutral gas in the tail can be inferred using the to the density of the gas used in the simulation of Tonnesen &
deep Herschel observations shown in Fig. 10. Considering the Bryan(2012),ρ=5×10−5cm−3foragalaxyofstellarmass1011
typical sky noise level at 250 µm around NGC 4569 given in M (vs.M =3×1010M forNGC4569).Thesimulationsof
(cid:12) star (cid:12)
Cieslaetal.(2012)((cid:39)0.25mJypixel−2correspondingto(cid:39)0.07 Tonnesen & Bryan (2012) also indicate that for the star forma-
mJyarcsec−2),wecanderivethedetectionlimitindustcolumn tiontotakeplaceinthetailrequiresasufficientamountoftime
density using a modified black body emission with β = 2 and (∼200Myr)tomakethegascoolandcollapse.Thistimescaleis
a grain emissivity parameter k = 2.0 cm2 g−1 (e.g. Boselli comparabletothederivedageoftheinteraction(100-300Myr;
250
2011).Thislimitcanbetransformedintoalimitingascolumn Vollmeretal.2004,Bosellietal.2006;Crowl&Kenney2008).
densityusingatypicalgas-to-dustratio.Thegas-to-dustratioof Itisthuspossiblethatthestrippedgasstilldidnothavethetime
nearbymassivegalaxiesrangesfrom160(Sodroskietal.1994) tocollapseandformnewstars.Weshouldrecall,however,that
to 70 (Sandstrom et al. 2013). With these values, the detection thesecondensedregionsofstarformationappearinthesimula-
limitinthegascolumndensityderivedfrom250µmdatashould tionswheneverthegasisallowedtocoolandthecontributionof
be ∼ 2-5 × 1019 cm−2, comparable to the limits of the VIVA the different heating processes is underestimated. The presence
survey. ofseveralcondensedregionsinanotherVirgoclustergalaxy,IC
Thereareotherindicationssuggestingthatthegasisionised 3418 (Hester et al. 2010; Fumagalli et al. 2011; Kenney et al.
withinthetail.Wecancalculatethetypicaltimescalenecessary 2014), where the conditions of the intracluster medium are ex-
fortheionisedgasinthetailtorecombineusingtherelation: pectedtobesimilartothoseencounteredbyNGC4569,clearly
indicate that the process of formation of these extraplanar HII
1 regionsisstillfarfrombeingunderstood.
τ = (5)
rec n α
e A
6.4. TheevolutionofNGC4569inthecluster
whereα isthetotalrecombinationcoefficient(α =4.2×10−13
A A
cm3s−1;Osterbrock&Ferland2006).Foratypicaldensityofn ThenewsetofextremelydeepHα+[NII]imagingdatacollected
e
(cid:39)2-4×10−2 cm−3,therecombinationtimeis(cid:39)2Myr,ashort inthiswork,combinedwiththoseavailableatotherfrequencies,
timeifcomparedtothetimenecessarytoproduceatails(cid:39)145 and the comparison with model predictions allow us to recon-
kpclong(seesect.4).Itisthusconceivablethatthegasiskept struct the evolution of the galaxy within the Virgo cluster en-
ionised within the tail. The comparison of the Hα+[NII] frame vironment.Allobservationalevidencecollectedsofarsuggests
andtheopticalandUVimagesinFig.7,8,and9showsthelack thatNGC4569underwentarecentrampressurestrippingevent.
of any compact star forming region within the tail ruling out Given the presence of a nearby companion, IC 3583, at a pro-
in situ stellar photoionisation. The detailed comparison of the jecteddistanceof∼6arcmin,however,wecannotexcludegrav-
ionisationmodelswiththespectroscopypropertiesofthegasin itational perturbations. A gravitational interaction between the
the tail of ESO 137-001 suggests that the gas is not only pho- twoobjects,however,seemsruledoutbythefactthattheirmost
toionisedbytheUVradiationemittedbyyoungstarsbutalsoby accurate distance estimate done using the tip of the red giant
Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569 9
branch in HST observations locates IC 3583 in the foreground observedtailsofHα+[NII]gasratherindicateawest-to-eastor-
of the cluster (D = 9.52 Mpc) and NGC 4569 at D (cid:38) 17 Mpc bit,questioningthustheinterpretationoftheX-raydistribution
(Karachentsev et al. 2014). To quantify the importance of any ofthehotgasinaMachconeasproposedbyWezgowiecetal.
possible tidal interaction with that galaxy we can estimate the (2011).
durationofapossibletidalencounterbetweenthetwogalaxies
usingtherelation(Binney&Tremaine1987):
The first interesting result of this work is that ram pressure
strippingcanbethedominantmechanismforremovingtheISM
t (cid:39)max[r ,r ,b]/∆V (7)
enc NGC4569 IC3583 inmassivegalaxies(M (cid:39)3×1010 M )fallingintointerme-
star (cid:12)
diate class clusters (M = 1.4-4.2 × 1014 M , McLaughlin
where r (22.8 kpc) and r (6.9 kpc) are the radii of virial (cid:12)
NGC4569 IC3583 1999, Urban et al. 2011, Nulsen & Bohringer 1995, Schindler
the two galaxies, b their separation, and ∆V their relative ve-
et al. 1999; ∆ = 800 km s−1, Boselli et al. 2014a; ρ = 2 ×
locity.Ifweassumethatbothgalaxiesareatthesamedistance, vel 0
10−3 cm−3 T =2.3 keV, Bohringer et al. 1994), thus extending
thus that b (cid:39) 32 kpc we can calculate t using eq. 7. IC 3583
enc previous finding to a much broader range of environments and
hasaradialvelocityof1120kms−1,significantlydifferentfrom
objects (see also Catinella et al. 2013). If the galaxy has fallen
thatofNGC4569(vel=-221kms−1),thust (cid:39)23Myr.This
enc into the cluster from behind, as indicated by its negative reces-
timescale is very short compared to the time required to NGC
sionalvelocity,inanorbitfromwesttoeast,assuggestedbythe
4569tomakeacompleterevolution((cid:39)370Myr).Althoughan
tails, it has encountered the maximal density of the intracluster
interactionontheirextendedhalosisstillpossible,itisquiteun-
gasnorthtoM87,ataradialdistanceof(cid:38)230kpc((cid:39)0.1R ),
likelythatonsuchshorttimescalethetidalinteractionisableto vir
where the density of the intergalactic medium is ρ (cid:39) 4 ×
remove(cid:39)1.9×1010M ofatomicgas. 230kpc
(cid:12) 10−5 cm−3 as derived using equation 4. It is worth mentioning
We can also estimate the typical truncation radius for two
that NGC 4569 is not an isolated case of massive galaxy with
interactinggalaxiesusingtherelation(Readetal.2006):
tailsofgaswitnessinganongoingrampressurestrippingevent
(cid:20) m (cid:21)1/3 in the Virgo cluster: NGC 4388 is another obvious candidate
r (cid:39)b (8) (Yoshida et al. 2002; Oosterloo & van Gorkom 2005; Kenney
t M(3+e)
et al. 2008), as well as the seven massive spirals with HI tails
observedbyChungetal.(2007).
where m and M are the masses of the two interacting galaxies,
b their separation and e the ellipticity of their orbit. Again as-
suming b = 32 kpc as a lower limit, an ellipticity of e = 1 and This analysis also shows that the diffuse component of the
a stellar mass M = 3 × 1010 M for NGC 4569 and M = ionised gas in the extended tail is a factor of ∼ 90 larger than
star (cid:12) star
6.3×108 M forIC3583weobtainatruncationradiusr (cid:38)74 thatexpelledbythenuclearoutflow.Sincetheionisedgasisthe
(cid:12) t
kpcforNGC4569and5.5kpcforIC3583.Thetruncationra- dominant phase in the tail (the mass of hot gas in the galaxy
diusofNGC4569issignificantlylargerthantheopticalradius halo derived from X-ray data is (cid:46) 2 × 108 M ; Wezgowiec et
(cid:12)
(22.8kpc),itisthusveryunlikelythatgravitationalinteractions al. 2011), this indicates that in massive galaxies the contribu-
havebeenthedominantperturbingmechanismaffectingthere- tion of the nuclear feedback to the ejection of the gas mass is
cent evolution of that galaxy. This result is consistent with the minimal.Furthermoreitcanhardlyreproducethetruncateddisc
fact that we do not observe any significant stellar tidal feature in the gas and dust components observed in NGC 4569 and in
associatedtotheobservedtailsofionisedgas(Fig.7).Wecan- mostofthegasdeficientclustergalaxieswithaquenchedactiv-
not,however,excludethatafly-byencounterofthetwogalaxies ityofstarformation(Corteseetal.2012a;Bosellietal.2014c).
has occurred. This kind of encounter generally induces nuclear Assuggestedbyhydrodynamiccosmologicalsimulations,how-
gasinfall(Mooreetal.1998).Ifthishappened,itcouldexplain ever, it can contribute to make ram pressure stripping efficient
the prodigious nuclear starburst activity occurred 3-6 Myr ago byinjectingkineticenergyintotheISMweakeningthegravita-
(Barth&Shields2000;Grebel&Bruhweiler2002),oranolder tional forces which keep the gas bounded to the potential well
(>15Myr)starburst(Keel1996),withpossiblefeedingofamild ofthegalaxy(Bahe&McCarthy2015).Thequestioniswhether
nuclear activity. The spectacular optical images obtained at the NGC4569isrepresentativeoftypicalmassivegalaxiesinterms
CFHTaspartoftheNGVSsurvey(Fig.8)indicatesmall(afew of nuclear activity. It is classified as a LINER/HII region tran-
kpc) low surface brightness features perpendicular to the disc sition type nucleus by Gabel & Bruhweiler (2002). It is also
in the south east side of the galaxy or in the north suggesting classifiedasastrongAGNusingtheBPTdiagram(Baldwinet
a possible bridge with IC 3583. A minor gravitational interac- al. 1981), as 17 % of the Herschel Reference Survey late-type
tion might also have contributed to flatten the potential well of galaxies with a stellar mass M > 1010 M (Gavazzi et al.,
star (cid:12)
the two galaxies, weakening their gravitational binding forces in preparation), and can thus be considered as a typical active
which keep the diffuse gas of the ISM anchored to the stellar massive galaxy. The feedback process that follows the removal
disc,thusmakingrampressurestrippingmoreefficient(Gavazzi ofthehotX-rayhaloofgalaxiesfallinginhigh-densityenviron-
etal.2001). mentsinastarvationscenario(Larsonetal.1980)doesnotseem
Afurtherevidenceinfavourofarampressurestrippingevent to be as efficient as cosmological simulations or semi-analytic
isthepresenceofapolarisedradioridgesouthwesttothegalaxy models indicate (Weinmann et al. 2006; McCarthy et al. 2008,
centreprobablyproducedbyalocalcompressionofthegasable 2011; Font et al. 2008; Kang & van den Bosch 2008; McGee
toorganisethemagneticfield(Wezgowiecetal.2012).Thispo- et al. 2009; Kimm et al. 2009; Guo et al. 2011; De Lucia et al.
larisedradiocontinuumfeatureislocatedsouthofthemainspur 2012;Bahe&McCarthy2015).Amorerealisticdescriptionof
of ionised gas coming out from the nucleus of the galaxy. The the(nuclear)feedbackprocessshouldbeconsidered.Ourresults
positionofthetailsofionisedgassuggestsaslightlydifferentor- providefurtherevidencethatrampressureisacompellingmech-
bitthantheoneproposedbyVollmer(2009),wherethegalaxyis anismtoexplainthestrippingofthecoldgascomponentofthe
expectedtohavecrossedtheclustercore∼300Myragoandis ISMandthusthequenchingofthestarformationactivityoflate-
nowcomingtowardsusonasouth-westtonorth-eastorbit.The typegalaxiesinhighdensityenvironments.
10 Bosellietal.:SpectaculartailsofionisedgasintheVirgoclustergalaxyNGC4569
7. Conclusion requiresdeephighvelocityandangularresolutionintegralfield
spectroscopicobservationsthatonlyinstrumentssuchasMUSE
We present new deep, narrow-band Hα+[NII] imaging data canprovide.
of NGC 4569 obtained with MegaCam at the CFHT. The
Hα+[NII]imageshowthepresenceoflonglowsurfacebright- Acknowledgements. ThisresearchhasbeenfinancedbytheFrenchANRgrant
ness (Σ(Hα + [NII]) (cid:39) 10−18 erg s−1 cm−2 arcsec−2) tails of VIRAGEandtheFrenchnationalprogramPNCG.WewishtothanktheGALEX
TimeAllocationCommitteeforthegenerousallocationoftimedevotedtothis
ionised gas extending perpendicularly from the disc of the
project and the anonimous referee for constructive comments. M.Fossati ac-
galaxy in the west direction up to (cid:39) 145 kpc. The presence of knowledges the support of the Deutsche Forschungsgemeinschaft via Project
these tails are a clear indication that the galaxy is undergoing ID3871/1-1.LCacknowledgesfinancialsupportfromtheAustralianResearch
a ram pressure stripping event. This observational evidence Council (DP150101734). M.Fumagalli acknowledges support by the Science
suggests that the ram pressure stripping mechanism is efficient and Technology Facilities Council [grant number ST/L00075X/1]. E. Toloba
is supported by the NSF grant AST-1412504. This research has made use of
not only in intermediate-to-low mass galaxies in the core of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet
massive clusters, as previously thought, but also in massive PropulsionLaboratory,CaliforniaInstituteofTechnology,undercontractwith
galaxies located in an unrelaxed cluster of intermediate mass the National Aeronautics and Space Administration and of the GOLDMine
(∼ 1014 M ) still in formation, with characteristics similar to database(http://goldmine.mib.infn.it/)(Gavazzietal.2003).
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