AstrophysicsandSpaceScience DOI10.1007/sXXXXX-XXX-XXXX-X Far UV and X-ray observations: a hot view of shell galaxies A. Marino1, R. Rampazzo1, G. Trinchieri2, R. Grützbauch3, M.S. Clemens1 8 0 0 2 n a (cid:13)c Springer-Verlag•••• J 5 Abstract Shell galaxies are considered the debris of (homogeneously) old, the field early-type galaxy pop- 1 recent accretion/merging episodes. Their high fre- ulation seems to contain genuinely, recently rejuve- quency in low density environments suggest that such natedobjects(seee.g.Clemens et al.2006). Early-type ] h episodes could drive the secular evolution for at least galaxies showing fine structure, like shells, occupy a p some fraction of the early-type galaxy population. We special position since they are believed to fill the gap - o present here the preliminary results of ultraviolet and betweenongoingmergersandnormalellipticalgalaxies. r X-ray data for a sample of three shell galaxies, namely Shells are faint, sharp-edged stellar features char- t s NGC 474, NGC 7070A and ESO 2400100. The Far acterizing ≈ 16.5% of the field early-type galaxy a [ UV morphology and photometry are derived using the population and avoiding cluster environments (e.g. observations obtained with the Galaxy Evolution Ex- Malin & Carter1983;Reduzzi et al.1996;Colbert et al. 1 plorerandtheXMM-NewtonOpticalMonitor. Weaim 2001). Two major scenarios for their origin have v at investigating the rejuvenation processes in the stel- 7 been proposed: a) a weak interaction between galax- lar population using the UV information as well as at 2 ies (Thomson & Wright 1990; Thomson 1991), b) 3 gaining information about the possible evolution with merging/accretion events (Dupraz & Combes 1986; 2 timeoftheX-rayemissiondueinteraction/mergingpro- Henquist & Quinn 1987a,b). In the former scenario, . 1 cesses. weakinteractioncanformlong lastingazimuthallydis- 0 tributedshellsthroughtheinterferenceofdensitywaves 8 Keywords Ultraviolet: galaxies — X-ray:galaxies — 0 Galaxies: elliptical, lenticular — Galaxies: individ- producing a thick disc population of dynamically cold : stars. However, this requires a cold thick disc, not v ual(NGC 474, NGC 7070A, ESO 2400100 — Galaxies: i evolution found in ellipticals. In the merging/accretion events X between galaxies of different masses (mass ratios typi- r cally1/10-1/100,b) scenarioabove)shellsaredensity a 1 Introduction waves formed from infalling stars from the companion during a minor merger. Major merger can also pro- In a hierarchical evolutionary scenario, galaxies expe- duce shells (Barnes 1992; Hernquist & Spergel 1992; rience accretion/merging events during their lifetime. Hernquist & Mihos 1995). Whatever the mechanism While early-type galaxies in nearby clusters appear is, interaction/merging events seem to have played a significantrole in the formation/evolutionof the early- A. Marino, R. Rampazzo, G. Trinchieri, R. type class in the field and activated a new star forma- Grützbauch, M.S. Clemens tion event (Longhetti et al. 1999, 2000). 1 INAF - Osservatorio Astronomico di Padova, Vicolo Whether there is a link between shell galaxies, the dell’Osservatorio 5, I-35122 Padova, Italy earlyphasesofmergingprocesses,andtheclassof’nor- 2INAF-OsservatorioAstronomicodiBrera,ViaBrera mal’ early-type galaxies is still an open question. To 28, I-20121 Milano, Italy test the depth of the link, a multiwavelength approach 3 Institut für Astronomie der Universität Wien, is of paramount importance. The UV data probe the Türkenschatzstrasse 17, 1180 Wien, Austria ongoing/recentstarformationprocessesandstudytheir distributionacrossthe galaxywhile the X-rayemission is connected to the past star formation and metal en- 2 richment history of the bulk of the galaxy and may DuringX-rayobservations,shellgalaxiesweresimul- disclose hidden AGN activity. taneously imaged in the ultraviolet and optical bands We present Galaxy Evolution Explorer (GALEX), with the Optical Monitor (Mason et al. 2001)). Obser- XMM-Newton OpticalMonitor(OM)andXMM-EPIC vations have been performed using UVW1 and UVM2 observations of NGC 474, NGC 7070A, and ESO bands which cover the ranges 245-320 nm and 205-245 2400100, three shell galaxies in the Malin & Carter nm, respectively. Galaxies have been observed also in (1983) catalogue. the U (300-390 nm) and B (390-490 nm) bands. The Point Spread Function – FWHM – is ≈2.0′′ in UVW1, and 1.8′′ in UVM2 sampled with 0′.′476×0′.′476 pixels. 2 The sample In order to complete the UV information about our galaxieswe searchedthe GALEX archiveandretrieved Besidethepresenceofshells,clearlyvisibleinallthree, the data for NGC 474. The GALEX mission and in- although more spectacular in NGC 474, these galaxies struments are fully described in Martin et al. (2005) shareotherproperties: theyareinlowdensityenviron- and Morrissey et al. (2005). The spatial resolution of ments and are interacting systems. However, the de- the images is ≈4′.′5 and 6′.′0 FWHM in FUV (135-175 tails of the interaction are quite different: companions nm)andNUV(175-275nm)respectively,sampledwith arewellseparatedinNGC7070A(Ramella et al.1996) 1′.′5×1′.′5 pixels. We found GALEX data only for the andstronglyinteractinginNGC474asshownbyHIob- NGC 474, that was observed the October 5th, 2003 servations (see e.g. Rampazzo et al. 2006). Two nuclei with an exposure of 1477 and 1647 seconds in NUV are embedded and interacting within the ESO 2400100 and FUV bands respectively. envelope(Longhetti et al.1998b). Furthermore,dustis detected in NGC 474 and NGC 7070A, although only forthelatter,thereiskinematicevidenceofanexternal 4 Preliminary results acquisition. Weaimatobtainingcrucialinformationaboutthetime at which the accretion/merging phenomenon has oc- 3 Observations and reduction cured that can be derived from the far UV and optical colours. Suchinformationisimportantforaproperdis- Table 1 gives the Journal of the XMM-Newton obser- cussion of the X-ray data. According to the evidence vations. X-ray data were treated with the standard providedbythesampleofBrassington et al.(2007),the routines provided by SAS version 7.0 as suggested by X-ray luminosity could evolve during the period that XMM-Newton Science Analysis system:Users’ Guide characterizes phases of the galaxy-galaxy encounter, 1 available on line . The X-ray observationsof NGC 474 the merging and the set-up of the merging towards a havebeenalreadydiscussedinRampazzo et al.(2006). relaxed galaxy. Figure 1 shows the isointensity contours from the 1 adaptivelysmoothedXMM-NewtonofNGC7070Aand http://xmm.vilspa.esa.es/external/xmm ESO 2400100 superposed on the DSS plates. It is ev- _user_support/ documentation/sas_usg/USG ident that the emission from these two galaxies has a significantly different morphology. In NGC 7070A the 47:00.0 47:00.0 emission is rather compact both in the hard and soft 48:00.0 MOS 0.5-2.0 keV 48:00.0 MOS 3.0-7.0 keV bands. In ESO 2400100 the soft band emission is sig- 49:00.0 49:00.0 nificantly stronger than the hard one and extends fur- -42:50:00.0 -42:50:00.0 ther than in NGC 7070A.Spectralresults are in agree- 51:00.0 51:00.0 ment with the spatial picture: the nuclear source of 52:00.0 52:00.0 NGC 7070A is the dominant component while in ESO 53:00.0 53:00.0 2400100theemissionisindicativeofalowtemperature 54:00.0 54:00.0 plasma. Rampazzo et al. (2006) showedthat NGC 474 55:00.0 55:00.0 isatthebottomoftheX-rayemissiondistributionofE 10.0 21:32:00.0 50.0 40.0 31:30.0 10.0 21:32:00.0 50.0 40.0 31:30.0 galaxies: its X-ray emission is consistent with the low end of the expected emission from discrete sources. Fig. 1.— Isointensity contours from adaptively smoothedXMM-NewtoncombinedMOSimagesintwo GALEX data of NGC 474 (Figure 2) show that the NUV emission extends to the galaxy bulge, while the bands on DSS2 plate for NGC 7070A (left panels) and ESO2400100(right panels). FUV emission shows up only in the central regions FarUVandX-rayobservations: ahotviewofshellgalaxies 3 Table 1 Journal of the XMM-Newton EPIC and Optical Monitor observations Galaxy EPIC EPIC OM OM OM OM observing observation PN MOS UVM2 UVW1 U B date ident. [sec] [sec] [sec] [sec] [sec] [sec] NGC 474 4300 11400 5000 5000 5000 2004-01-24 0200780101 NGC 7070A 26250 30840 4400 4000 2004-10-28 0200780301 ESO 2400100 19970 26330 4400 4400 4400 2004-05-11 0200780201 XMM-Newton observations of NGC 474 are fully discussed in Rampazzo et al. (2006) Fig. 2.— The NGC 470/474 system, also known as Arp 227. Full resolution GALEX FUV (left panel) and NUV (mid panel) backgroundsubtracted images. Pixel by pixel GALEX (FUV-NUV) 2D color map of the NGC 470/474 system (right panel). The (FUV-NUV) color of both the early-type type galaxy, NGC 474, and of the spiral, NGC 470, are typical of their class. Most likely, the NUV and the FUV fluxes have different origins in ‘normal’ early-type galaxies. The NUV flux is partially due to the MS turn-off stars of the evolved population and partially to more evolved and, exotic stars. In contrast, a strong FUV emission is likely due to the presence of one (or more)hot, plausibly highmetallicity, stellar componentgiving originto the wellknownphenomenon of the UV-upturn (e.g. hot-HB and/or post-AGB and AGB-manqué stars). In shell galaxies, the presence of young stars could also contribute in different proportion, to both NUV and FUV fluxes. Fig. 3.—U(left panel),UVW1 (mid panel)andUVM2 (rightpanel) imagesofESO2400100. The two nuclei(we named ESO 2400100athe southern, ES0 2400100bthe northern) embedded in the galaxy body are clearly visible in the UVW1 and UVM2 images. 4 of the galaxy. In the NGC 474 image obtained in Longhetti, M., Rampazzo, R., Bressan, A., Chiosi, C. the UVW1 and UVM2 filters outer shells are visible 1998a, A&AS130, 251 as in the optical image (Rampazzo et al. 2006). Both Longhetti, M., Rampazzo, R., Bressan, A., Chiosi, C. 1998b, A&AS130, 267 ESO 2400100 (Figure 3) and NGC 7070A have exten- Longhetti,M., Bressan,A.,Chiosi, C.,Rampazzo,R.1999, sion similar to that of the optical image in the OM- A&A345, 419 UVW1 and in the GALEX NUV filter. This implies Longhetti M., Bressan A., Chiosi C., Rampazzo R. 2000, that the UV emission comes from the same kind of A&A,353, 917 stellar population. The FUV emission, more concen- Malin D., Carter D.F. 1983, ApJ, 274, 534 trated toward the nucleus is most likely dominated by Martin, D.C., et al. 2005, ApJ,619, L1 the emission of different types of hot stars (see also Mason, K.O., Breeveld, A., Much, R., et al. 2001, A&A, 365, 36 Rampazzo et al. 2007). Morrissey P., et al. 2005, ApJ,619, L7 In Figure 3 (mid and left panels) the UVW1 and Ramella, M., Focardi, P., & Geller, M. J. 1996, A&A,312, UVM2 images show the two nuclei embedded in ESO 745 2400100. The northern nucleus is significantly bluer Rampazzo, R., Alexander, P., Carignan, C., Clemens, than the southern one. Longhetti et al. (2000) noticed M.S., Cullen, H. et al. 2006, MNRAS,368, 851 that the two nuclei have a significantly different Hβ Rampazzo, R., Marino, A., Tantalo, R., Bettoni, D., line-strength indices (2.79 in the northern nucleus vs. Buson, L.M., Chiosi, C., Galletta, G., Grützbauch, R., Rich,M., 2007, MNRAS,in press 1.54 in the southern one) suggesting a different stellar Reduzzi, L., Longhetti, M., Rampazzo R. 1996, MNRAS, population composition. 282, 149 The accretion of faint galaxies seems one of the Sikkema, G., Carter, D., Peletier, R.F., Balcells, M., del driversofthesecularevolutionofgalaxiesinloose,poor Burgo, C., Valentijn E.A. 2007, A&A,467, 1011 groups. Since shells are widely believed to be gener- Thomson, R.C., Wright, A.E. 1990, MNRAS 224, 895; ated by an accretion event, shell galaxies potentially Thomson, R.C. 1991, MNRAS253, 256 trace the typical secular evolution of early-type galax- iesinsuchenvironments. Furthermore,thereisagrow- ing evidence of multiple accretion events in the same galaxy. The complex shell system of NGC 474 is be- lieved to be generated by two distinct accretion events (Sikkema et al. 2007). In this framework the double nucleus in ESO 2400100 is an interesting and puzzling case at the same time. Is the second nucleus the cause of the observed system of shells or another evidence of a new, on-going, accretion event? Acknowledgements This research has been par- tially founded by ASI-INAF contract I/023/05/0. Galex is a NASA Small Explorer, operated for NASA by CaliforniaInstitute of technologyunder NASA con- tract NAS-98034. References Barnes J. 1992, ApJ,393, 484 Brassington, N. J., Ponman, T. J., & Read, A. M. 2007, MNRAS,377, 1439 Clemens, M. S., Bressan, A., Nikolic, B., Alexander, P., Annibali,F.,Rampazzo, R.2006, MNRAS,370, 702 Colbert, J.W., Mulchaey J.S., Zabludoff, A. 2001, AJ, 121, 808 Dupraz, C., Combes, F. 1986, A&AS,166, 53 Hernquist L., Quinn P. 1987a, ApJ,312, 1 Henquist L., Quinn P. 1987b, ApJ,312, 17 Hernquist L., Spergel, D.N.1992, ApJ,399, L117 ThismanuscriptwaspreparedwiththeAASLATEXmacros Hernquist L., Mihos, C. 1995, AJ, 110, 140 v5.2. This figure "fig1a.jpg" is available in "jpg"(cid:10) format from: http://arXiv.org/ps/0801.2327v1