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PreprinttypesetusingLATEXstyleemulateapjv.11/10/09 THE DISPLACED DUSTY ISM OF NGC3077: TIDAL STRIPPING IN THE M81 TRIPLET. F. Walter1 K. Sandstrom1 G. Aniano2 D. Calzetti3 K. Croxall4 D. A. Dale5 B. T. Draine2 C. Engelbracht6 J. Hinz6 R. C. Kennicutt7 M. Wolfire8 L. Armus9 P. Beiraõ9 A. D. Bolatto8 B. Brandl10 A. Crocker3 M. Galametz7 B. Groves10 C.-N. Hao11 G. Helou12 L. Hunt13 J. Koda14 O. Krause1 A. Leroy15 S. Meidt1 E. J. Murphy9 N. Rahman8 H.-W. Rix1 H. Roussel16 M. Sauvage17 E. Schinnerer1 R. Skibba6 J. D. Smith4 C. D. Wilson18 S. Zibetti19 Draft version January 24, 2011 1 ABSTRACT 1 0 We presentthe detection of extended (∼30kpc2) dust emissionin the tidal HI armnear NGC3077 (member of the M81 triplet) using SPIRE on board Herschel. Dust emission in the tidal arm is 2 typically detected where the HI column densities are >1021cm−2. The SPIRE band ratiosshow that n the dust in the tidal arm is significantly colder (∼13K) than in NGC3077 itself (∼31K), consistent a J with the lower radiation field in the tidal arm. The total dust mass in the tidal arm is ∼1.8×106M⊙ 0 (assumingβ=2),i.e. substantiallylargerthanthedustmassassociatedwithNGC3077(∼2×105M⊙). Where dust is detected, the dust–to–gas ratio is 6±3×10−3, consistent within the uncertainties with 2 what is found in NGC3077 and nearby spiral galaxies with Galactic metallicities. The faint HII regions in the tidal arm can not be responsible for the detected enriched material and are not the ] O main source of the dust heating in the tidal arm. We conclude that the interstellar medium (atomic C HI, molecules and dust) in this tidal feature was pre–enriched and stripped off NGC3077 during its recent interaction (∼3×108yr ago) with M82 and M81. This implies that interaction can efficiently . h remove heavy elements and enriched material (dust, molecular gas) from galaxies. As interactions p were more frequent at large lookback times, it is conceivable that they could substantially contribute - (along with galactic outflows) to the enrichment of the intergalactic medium. o r Subject headings: galaxies: individual (NGC 3077, Garland) — ISM: general — galaxies: ISM — t s galaxies: interactions — infrared: ISM a [ 1 1Max-Planck-Institut fu¨r Astronomie, Königstuhl 17, v D-69117 Heidelberg, Germany [e-mail: [email protected], sand- 3 [email protected]] 2 2DepartmentofAstrophysicalSciences,PrincetonUniversity, 0 Princeton,NJ08544, USA 3Department of Astronomy, University of Massachusetts, 4 Amherst,MA01003,USA 1. 4Department of Physics and Astronomy, University of Toledonew, Toledo,OH43606, USA 0 5Department of Physics & Astronomy, University of 1 Wyoming,Laramie,WY82071, USA 1 6Steward Observatory, University of Arizona, Tucson, AZ : 85721, USA v 7InstituteofAstronomy,UniversityofCambridge,Madingley i Road,CambridgeCB30HA,UK X 8Department of Astronomy, Universityof Maryland, College ar Pa9rkS,pMitzDer20S7c4ie2n,cUeSCAenter, California Institute of Technology, MC314-6,Pasadena, CA91125, USA 10Leiden Observatory, Leiden University, P.O. Box 9513, 2300RALeiden,TheNetherlands 11Tianjin Astrophysics Center, Tianjin Normal University, Tianjin300387, China 12NASA Herschel Science Center, IPAC, CaliforniaInstitute ofTechnology, Pasadena, CA91125, USA 13INAF-OsservatorioAstrofisicodiArcetri,LargoE.Fermi 5,50125Firenze,Italy 14Department of Physics and Astronomy, SUNY Stony Brook,StonyBrook,NY11794-3800, USA 15Hubble Fellow, National Radio Astronomy Observatory, 520EdgemontRoad,Charlottesville,VA22903,USA 16Institut d’Astrophysique de Paris, UMR7095 CNRS, UniversitéPierre&MarieCurie,98bisBoulevardArago,75014 Paris,France 17CEA/DSM/DAPNIA/Service d’Astrophysique, UMR AIM,CESaclay,91191GifsurYvette Cedex 18DepartmentofPhysics&Astronomy,McMasterUniversity, Hamilton,OntarioL8S4M1,Canada of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen, 19Dark Cosmology Centre, Niels Bohr Institute, University Denmark 2 Walter et al. 1. INTRODUCTION ucts and algorithms available in HIPE (the Herschel Interactive Processing Environment), version 2.0.0. Ithaslongbeenknownthattidalinteractionscanstrip The determination of the background in NGC3077 is offtheextendedenvelopesofatomichydrogen(HI)ofin- teractinggalaxies. Thisleadstoasubstantialincreaseof complicated by the extended HI filaments around the region of interest. The background was defined away the HI cross section (i.e., areal coverage) in such systems. Given these large cross sections of atomic hydro- from the HI tidal tails (here defined as column densities gen, nearby interacting systems are thought to be remi- >2×1020cm−2, including NGC3077 itself), to avoid niscentofDampedLymanAlpha(DLA) systemsseenin subtracting real dust emission that may be coincident absorption at high redshift. with HI features. The SPIRE data were calibrated in One of the most prominent examples of a nearby in- units of Jybeam−1, which were converted to MJysr−1 teracting system is the M81 triplet, a group of galaxies by assuming beam sizes of 501, 944, and 1924 square at a distance of ∼3.6Mpc (1′=1.05kpc). Tidal HI fea- arcseconds (FWHM: 18′′, 25′′ and 37′′) for the 250, tures are distributed over50×100kpc2 in this systemat 350, and 500 µm bands. For the quantitative analysis HIcolumndensities>2×1020cm−2 andinterconnectthe we convolved all data to the SPIRE 500 µm resolution using the kernels described by Gordon et al. (2008) centralthree galaxiesM81,M82andNGC3077(Yun et and updated by those authors for Herschel. We have al. 1994). Early simulations suggest that the tidal sys- also applied calibration correction factors of (1.02, 1.05, tem was created 3×108 years ago and that the material 0.94)at (250, 350,500) µm to the values as givenin the in the tidal streams eastwards of NGC3077 originally observers manual. We selected 44 background apertures belonged to NGC3077 itself (Yun et al. 1997). The total HI mass of the tidal arm feature around NGC3077 (radius: 1′)awayfromNGC3077andthe tidalHI arms. The respective flux histograms of these apertures at is M(HI)=(3−5)×108M⊙ (van der Hulst 1979, Wal- 250, 350 and 500 µm are centered on zero, as expected ter & Heithausen 1999, Walter et al. 2002a), depending given the background subtraction procedure, and have on the exact integration boundaries. The HI emission 1σ widths of (0.33, 0.16 and 0.086) MJysr−1 at (250, in the tidal feature is kinematically distinct from Galac- 350, and 500) µm which we adopt as the uncertainty tic cirrus (both in systemic velocity as well as velocity in the tidal feature aperture measurements (we do not dispersion, Walter et al. 1999, 2002a). include the 15% SPIRE calibration uncertainty which However,rather than being a passive HI tidal feature, will affect all three bands in a similar way). this region has both star–formation and molecular gas, reminiscentofadwarfgalaxy: Asearlyas1974,Barbieri et al. detected a ‘fragmentary complex of almost stellar objects’ in the tidal arm around NGC3077, and subse- 3. RESULTS quentopticalobservationsrevealedthepresenceofablue stellar component in this region (dubbed the ‘Garland’, 3.1. Dust in the Tidal HI Arm Karachentsevetal. 1985;Sharina1991;Sakai&Madore Figure1 presents a multi–wavelengthview ofthe tidal 2001,Mouhcine & Ibata 2009). The presence of a young featuresaroundNGC3077. Thetopleftpanelshowsthe stellar population in the tidal feature has recently been SPIRE map at 250µm (convolved to 40′′ resolution, i.e. unambigiously established by Weisz et al. (2008) using approximately the 500µm beam). The brightest source deep Hubble Space Telescope ACS observations. in the field is NGC3077 itself, but significant emission Molecular gas has been detected and mapped in is detected east of the main body of the galaxy. The this region through observations of carbon monoxide nuclear starburst in NGC3077 is distributed over scales (CO, Walter et al. 1998, Heithausen et al. 2000). of ∼10′′ (Martin et al. 1997, Ott et al. 2003, Walter The total star formation rate (SFR) in the tidal fea- et al. 2006), which is unresolved at SPIRE’s resolution. ture is 2.3×10−3M⊙yr−1 distributed overmany square- The extendeddust emissiontowardsthe eastis spatially kiloparsec,comparabletowhatisseeninotherlow–mass coincident with HI column densities ≥1021cm−2 in the dwarf galaxies in the M81 group of galaxies (Walter, tidal feature (Fig. 1, top right; HI data are taken from Martin & Ott 2006). Recent optical spectroscopy of THINGS, ‘The HI Nearby GalaxySurvey’, Walter et al. theseHIIregionsindicatedthattheirmetallicityismuch 2008). TheapparentcorrelationofHIemissionwithdust higher than expected based on the total blue magnitude seen in the SPIRE data (in particular towards high HI of the stellar system in the tidal arm, possibly reaching column densities) implies that the dust emission is in- a value as high as found in NGC3077 itself, i.e. close to deed physically coincident with the tidal arm and does the Galactic one (Croxall et al. 2009). Here we report not arise from Galactic cirrus which is observedtowards the detection ofsubstantialamounts ofcold dust onkpc the direction of the M81 group (e.g. Davies et al. 2010, scales in the tidal feature around NGC3077 based on Sollima et al. 2010). Herschel SPIRE observations. 3.2. Selection of Apertures 2. OBSERVATIONS To quantify the dust properties, we have selected 25 NGC3077 was observed with the SPIRE instrument apertures of radius 1′ (∼1kpc) each from visual inspec- on Herschel (Pilbratt et al. 2010) in ‘scan map’ mode tionoftheSPIRE250µmmap. Thelocations(andnum- as part of the Open Time Key Project KINGFISH bers) of the apertures are shown in the bottom right (PI: R. Kennicutt) on March 28th, 2010. The galaxy panel of Fig. 1 (green circles) and the positions are tab- and its surroundings were homogeneously covered using ulated in Tab. 1. The red circle indicates the aperture 11 arcmin scan–legs (AOR length: 0.65 hours). Data (#1)centeredonNGC3077. For eachaperture,wehave were reduced using the standard calibration prod- derived the flux in the SPIRE bands (Tab. 1). The er- Cold Dust in the Tidal HI Arms of the M81 Triplet 3 Fig.1.—Multi–wavelengthviewofNGC3077andthesurroundingtidalHIsystem(Northistop,Eastisleft). Topleft: SPIRE250µm imageofNGC3077anditssurrounding. Thecontours aretheHIintensities showninthetoprightpanel. Top right: Distributionofthe tidal HI complex around NGC3077 at 40′′ resolution (greyscale). Contours are shown at HI columns of 2, 5, 10 and 20×1020cm−2 at thatresolution(fromWalteretal.2008). Bottom left: DSSimageofNGC3077anditssurroundingswiththeHIcontourssuperimposed. Bottom right: SPIRE500µm image,withHIcontours andaperturessuperimposed. Aperture#1iscentredonNGC3077itself. ror in this measurement is derived from the dispersion reflecting a lowertemperature in the tidal region. Given of 44 background apertures of the same size that are the lower temperature in the tidal arm and the compa- located away from the HI tidal arms (at column den- rable total fluxes in the SPIRE bands this immediately sities <2×1020cm−2). This dispersion is mainly driven implies that the dust mass in the tidal feature is larger by unresolved background sources at high redshift. The than in NGC3077 itself (see more detailed discussion in HI column densities (at 40′′ resolution) for each aper- Sec. 3.6). ture are also given in Tab. 1 — all column densities are We have attempted to use Spitzer MIPS 160µmimag- >4M⊙pc−2 (5×1020cm−2). Significant dust emission ing to further constrain the SED towards shorter wave- (here defined as a >3σ detection at 250µm) is detected lengths. ThemainbodyofNGC3077isverybright,and in 8 apertures, i.e. an area of ∼30kpc2. These aper- weplotthecorrespondingfluxdensityinFig.2. Because tures are labeled in blue in Fig. 1 (bottom right) and ofcoverageandextendedcirrusemission,thebackground marked with a boldface font in Tab. 1. Averaging the oftheMIPS160µmishoweverbadlybehavedattheflux other apertures also yields a statistical detection of dust levels of interest, leading to significant error bars in the emission. flux determination. We nevertheless derived a 160µm flux density for the brightest aperture (#10, the error 3.3. SPIRE Dust SED bar is dominated by the background uncertainties) — the complete SED for this aperture is also plotted in Figure2showstheSPIREmeasurementsforNGC3077 Fig. 2 for comparison. (aperture#1,redcurve)andthesumofalldetectedtidal regions (green curve, boldface aperture numbers in Ta- 3.4. Dust Temperature ble 1). A comparison of the two SEDs directly shows that the 250µm/500µmratio is significantly lowerin the We have used a blackbody fit and the Li & Draine tidal feature compared to the main body of NGC3077, (2001) dust emissivity (equivalent to a modified black- 4 Walter et al. TABLE 1 ApertureMeasurements #a RA DEC F(250µm)b F(350µm)b F(500µm)b Σdustc ΣHId ΣSFRe J2000.0 J2000.0 MJysr−1 MJysr−1 MJysr−1 M⊙pc−2 M⊙pc−2 10−4M⊙yr−1kpc−2 1 100317.18 +684353.0 23.61 8.41 3.07 0.054±0.005 12.42 220 2 100305.57 +684549.4 0.73 0.34 0.15 ··· 2.95 ··· 3 100249.53 +684412.2 1.54 0.91 0.48 0.053±0.015 4.88 ··· 4 100251.35 +684215.7 1.71 0.99 0.53 0.058±0.016 7.04 ··· 5 100252.28 +684019.3 0.02 0.11 0.09 ··· 6.04 ··· 6 100310.96 +684112.8 2.63 1.32 0.58 0.075±0.020 10.18 ··· 7 100316.30 +683916.3 -0.10 0.05 0.05 ··· 5.63 ··· 8 100333.21 +684132.2 2.53 1.35 0.65 0.078±0.021 10.69 0.65 9 100338.53 +683940.5 0.29 0.31 0.17 ··· 7.69 ··· 10 100357.30 +684050.1 2.74 1.73 0.95 0.101±0.027 14.78 4.00 11 100350.72 +684251.7 2.65 1.68 0.91 0.097±0.026 16.23 1.23 12 100342.18 +684443.2 0.76 0.57 0.34 ··· 8.16 ··· 13 100327.96 +684615.9 0.84 0.51 0.31 ··· 3.30 ··· 14 100420.44 +684045.2 0.63 0.41 0.21 ··· 6.39 ··· 15 100413.43 +684244.4 1.54 1.02 0.52 0.057±0.016 15.62 0.29 16 100435.72 +684229.4 0.59 0.41 0.21 ··· 6.30 1.09 17 100426.79 +684426.1 0.19 0.16 0.10 ··· 5.23 ··· 18 100404.49 +684438.4 0.63 0.42 0.26 ··· 12.60 ··· 19 100416.16 +684622.8 -0.23 -0.07 0.01 ··· 6.49 ··· 20 100350.70 +684637.3 0.54 0.43 0.31 ··· 8.93 ··· 21 100411.77 +684824.2 0.04 0.11 0.13 ··· 4.71 ··· 22 100349.41 +684848.8 0.89 0.51 0.29 ··· 5.94 ··· 23 100328.82 +684932.7 0.72 0.39 0.19 ··· 4.58 ··· 24 100321.66 +685129.2 0.78 0.42 0.23 ··· 5.35 ··· 25 100314.47 +685325.6 0.78 0.38 0.24 ··· 4.36 ··· a Aperture number in bold indicate regions with significant emission (i.e. >3σ at 250µm). Dust mass densities have been derived for thoseregionsassumingatemperatureofT=12.6K(onlyexception: aperture#1[NGC3077,T=30.6K])andβ=2(seeSec.3.4). b The uncertainties in the SPIRE measurements are (0.33, 0.16 and 0.086) MJysr−1 at (250, 350, and 500) µm, respectively, based on backgroundaperturemeasurement. Theseuncertainties donotincludetheSPIREcalibrationuncertainties of∼15%. cTheuncertaintyindustmasssurfacedensitieshavebeenderivedusingdifferenttemperatures(±1K)asthefittingerrorsgiveunrealistically smallnumbers. d HIsurfacedensityuncertainties aretypically<10%. e SFRsurfacedensitiesarederivedfromthevaluesgiveninWalteretal.2006(tidalarm)andKennicuttetal.2008(NGC3077). body with β=2) to derive temperatures and dust their Hα luminosities using the values given in Wal- masses. The temperature for NGC3077 (aperture #1) ter et al. (2006) and give the star formation rate sur- is 30.6±2K and is in agreement with high temperatures face densities (averaged over the size of our apertures) expectedforstarburstsandthe centraltemperaturesde- in Tab. 1. We plot the 250µm/500µm ratios for these rived for a sub–sample of the KINGFISH galaxies pre- aperatures as a function of the Hα luminosity surface sentedbyEngelbrachtetal.(2010)ofT =25.7±1K. density in Figure 3. Star formation rates can be es- center The temperature in the tidal feature is much lower. timated using SFR[M⊙yr−1]=7.9×10−42L(Hα)[ergs−1] Based on the SPIRE data only, we derive an average (Kennicutt 1998, see top x-axis in Fig. 3; choosing a temperature of T=12.0±1K. In the brightest aperture Kroupa IMF would decrease the SFRs by ∼30%) and of the tidal feature (#10) the addition of the 160µm thebrightestregionoutsideofNGC3077(aperture#10) fluxestimateincreasesthetemperatureslightly(12.6K), accountsfor∼50%ofthe totalSFRinthe tidalcomplex but is consistent with the ‘SPIRE–only’ value. Chang- (1.3×10−3M⊙yr−1, or a SFR surface density for this ingβ toavalueof1.5increasesthetemperatureby∼3K aperture of 4×10−4M⊙yr−1kpc−2). For reference, sin- (see dashed SED fits in Fig. 2). In the following we will glemassivestarscreateHIIregionswithHαluminosities adoptβ=2andT=12.6±2forthetidalfeatureapertures ofL ≈5×1036ergs−1andL ≈5×1037ergs−1 Hα,O7 Hα,O5 but note that the temperature (and the corresponding (e.g., Devereux et al. 1997), i.e. the total Hα flux in the masses) depend on the exact choice of dust model. tidalfeaturecaninprinciplebecreatedbyafewmassive stars. Figure3showsthatthedusttemperaturedoesnot 3.5. Radiation Field change as a function of the star formation rate surface density over two orders of magnitude of the latter. This Findinglowdusttemperaturesmaynotbeunexpected suggeststhatthe currentlyobservedstar formationdoes as the intensity of the radiation field in the tidal arm is not provide the main heating source for the dust on kpc presumably low: the total star formation in the entire scales and we speculate that other mechanisms may be tidal feature is only 2.3×10−3M⊙yr−1 (based on Hα responsible for the dust heating. observations, Walter et al. 2006). This SFR is con- Assuming G ∼4×10−3 for the intergalactic radiation 0 sistent with the rate determined in Weisz et al. (2008) field(Sternbergetal.2002,notethatG =1fortheGalac- 0 through stellar population studies, and GALEX mea- tic interstellar radiation field), that the temperature of surements that cover regions #10 and #11, implying dust goes as T ∼ G1/(4+β) and β=2, and scaling from that the SFR in the tidal feature was roughly constant 0 the results by Li & Draine (2001) for the Galactic inter- over the recent past (few hundred million years). For stellarradiationfield(16Kforasilicategrain,20Kfora those apertures that include HII regions we sum up Cold Dust in the Tidal HI Arms of the M81 Triplet 5 Fig.2.— SPIRE dust SEDs for NGC3077 (aperture #1, red Fig.3.— 250µm/500µm ratio in the tidal feature as a function color),theentiretidalfilament(summingupaperturesthatcontain of Hα luminosity in a given aperture (numbers indicate aperture significantemission,green)andthebrightestaperture(#10,blue). numbers). The temperatures given on the right y–axis are based Thefulllines correspondtofits assumingβ=2,the dashedcurves onthis250µm/500µm ratioandassumeablackbodywithβ=2. indicateβ=1.5(fittedtemperaturesaregivenintheFigure). The SPIRE calibration uncertainties (∼15%) are not included in the errorbars. tures where dust emission was significantly detected, we graphite grain, Fig 3 in Li & Draine 2001), we estimate derive an average value of 11.3M⊙pc−2 (Tab. 1), i.e. more than two orders of magnitude larger than the dust that the dust in the tidal feature should have a temperature of 6-8K in the intergalactic field. This suggests surface densitites. The average dust–to–HI gas ratio is 6.5±3×10−3, and the ratio does not vary within the er- that other heating sources (the radiation field of NGC 3077, an older stellar population in the feature as seen rorsbetweenaperturesandisnotafunctionofHIsurface density(thecorrespondingvalueforNGC3077(aperture in the HST imaging by Weisz et al. 2008and/or shocks) #1) is 0.0043). Within the uncertainties this value is must be partly responsible for heating the dust to our derived temperature of 12±2 K. The 250µm/500µm ra- close to the average dust–to–gas ratio of Mdust/MHI ≈ 0.005derived by Draine et al. (2007)for 12 spiral galax- tio does not change as a function of projected distance ies of the SINGS sample for which Spitzer and SCUBA to NGC3077 within our uncertainties but we note that measurements were available. As noted by Draine et the measurements only span a small range in distances. al. (2007), a M /M ratio between 0.003 and 0.01 dust HI 3.6. Dust Masses and Dust/Gas Ratios is consistent with galactic metallicities between 0.3 and 1 times solar, if a similar fraction of heavy elements is FromtheSEDfitsdiscussedabovewederivedustmass in the form of dust as in the Milky Way. Our derived surface densities for each of our apertures (Tab. 1) with dust–to–gasratiothusindicatesthatthetidalarmissub- anaveragevalueof7.4×10−2M⊙pc−2 (β=2,T=12.6K). stantiallychemicallyenriched,inagreementwiththeHII We derive a total dust mass in the tidal feature (exclud- region metallicities derived by Croxall et al. (2009). For ing NGC3077 and upper limits) of ∼1.8±0.9×106M⊙ comparison, the Mdust/MHI ratio for metal–poor dwarf (the large error bar includes uncertainties in dust prop- irregular galaxies in the (more extended) M81 group of erties,seebelow). Oursimple dustmassestimateiscon- galaxiesisuptooneorderofmagnitudelowerthanfound sistent with results from more detailed modeling using here (Walter et al. 2007). updated Draine & Li (2007) models (Aniano et al., in So far, we have only considered the HI gas. Molecu- prep.). The dust mass of NGC3077 is only 1.9×105M⊙ lar gas has been mapped in the CO(1–0) transition in (β=2,T=30.6K),inreasonableagreementwiththemass the tidal feature over a restricted area (Walter et al. estimates by Price & Gullixson (1989) of the absorbing 1998, Heithausen & Walter 2000, Walter et al. 2006). central dust clouds based on NIR observations (they re- The two main molecular complexes (region number #1 port a mass of ∼105M⊙). Both mass estimates depend and #2 in Heithausen & Walter 2000) are spatially co- on the choice of β and the temperature. For example, if incident with aperture numbers 10 and 11 in this study. wechoseβ=1.5forthetidalfeature,thedustmasswould The implied molecular gas masses are dependent on the godownbyafactorof∼2(asthe temperaturewouldin- choice of the H2–to–CO conversion factor XCO. Hei- crease by ∼3K, Sec. 3.4). Likewise, lowering the dust thausen & Walter (2000) used a conversion factor of temperaturein NGC3077by 5K wouldincreaseits dust X =8×1020cm−2(Kkms−1)−1, a factor of ∼4 larger CO mass by a factor of ∼2. Given these large systematic than the Galactic conversion factor. At the time, their uncertainties involved we assign a conservative error of choicewasdrivenbytheassumptionthatthetidalregion 50% to the dust mass of the tidal feature. wasmetal–poor. Giventheabundanceofdustanddust– For the average HI surface mass densities of the aper- to–gas ratios similar to nearby spiral galaxies, as well 6 Walter et al. as the HII region metallicity measurements (indicating in the tidal arm is expected to lead to a metallicity of metallicities as high as Galactic, Croxall et al. 2009), it only Z∼0.002 Solar (Walter et al. 2006) over the last now seems to be more appropriate to use the Galactic 3×108yr, i.e., much lower than what is measured by conversion factor in this tidal feature. This implies that Croxall et al. (2009) and implied by our dust–to–gas the masses given in Heithausen & Walter (2000) should ratio. We conclude that the tidal arm material was bedividedbyafactorof4. Wethusadoptmoleculargas pre–enriched, and likely belonged to NGC3077 (which massesof∼1×106M⊙ and∼4×106M⊙ forourapertures hasametallicity similarto the tidalHII regions,Croxall # 10 and 11, respectively (comparable to the H2 mass et al. 2009) before the interaction. present in the centre of NGC3077, Meier et al. 2001, Our findings imply that interactions between galax- Walter et al. 2002a). Averaged over our r=1kpc–sized ies can efficiently remove heavy elements, dust and aperturesthiscorrespondstoamolecularsurfacedensity molecules from a galaxy. In the case discussed here, sig- between 1.1–0.3M⊙pc−2 (i.e., the ISM in the tidal fea- nificantly more dust mass is found in the tidal arm than ture is dominated by the atomic gas phase even in the in the parent galaxy NGC3077 (the same holds true for CO–brightest regions. the atomic and moleculargas). Interactionsthus appear to havethe potential to alterthe chemicalevolutionofa 4. CONCLUDINGREMARKS galaxydramaticallyand to expel an interstellarmedium thatis enrichedby heavyelements. As interactionshave The detection of significant amount of dust beenmorefrequentatlargerlookbacktimesitisconceiv- (∼1.8±0.9×106M⊙) inthe tidalfeature nearNGC3077, able that this mechanism can (in addition to outflows, distributed over 30 square–kiloparsecs, which is signifi- e.g., Steidel et al. 2010)effectively enrich the intergalac- cantly larger than the dust mass present in the parent tic medium (see also Roussel et al. 2010, Walter et al. galaxy (irrespective of the choice of β and reasonable 2002b). The tidal system discussed here would be clas- temperatures), raises questions on the origin of the sifiedasadampedLyman–alphaabsorber(DLA) witha enriched material. The ongoing star formation could cross section of roughly 30 kpc2 (e.g. Wolfe, Gawiser & potentially enrich the medium in the tidal feature. If Prochaska 2005, Zwaan et al. 2008). we assumed a constant star formation rate since the creation of the feature (3×108yr ago, Yun et al. 1994) we estimate a total mass of newly formed stars of FW acknowledges the hospitality of the Aspen Center ∼ 7×105M⊙, i.e. less than the total amount of dust for Physics. Herschel is an ESA space observatory with thatispresent. Thisimpliesthatthecurrentrateofstar science instruments provided by European-led Principal formationactivitycannothavecreatedthepresentdust. Investigator consortia and with important participation Also, the chemical enrichment due to the HII regions from NASA. 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