arXiv:astro-ph/9811130v1 9 Nov 1998 3 4 ilmtrTlsoe(ET tL il,Chile Silla, La at (SEST) Telescope millimeter oeua a nTeCrwelGalaxy Horellou Cartwheel C. The in Gas Molecular h iko h tre”glx n nue asv massive induces ring. and well-defined a in “target” through formation the propagates of disk that ring-wave the the a of passage triggers The 1976). “intruder” Spiegel (Lynds & disk Theys rotating 1976; Toomre larger & a galaxy of companion centre a the when through form plunges convincingly can systems showed ring by such which elucidated that was simulations, Cartwheel numerical (see the ring of early inner nature the The to 1). ring (1941), spokes Figure outer of bright Zwicky system the prominent connects by the which from discovered nickname its Galaxy, earned Cartwheel The Introduction 1. 10 e:idvda:TeCrwel–Glxe:IM–Radio – ISM : – galaxies Cartwheel lines: The individual: ies: words: Key low Cartwheel. a the or of activity centre temperature star-forming the low weak of in a the observed excitation to with low due consistent The is is density, it region. whether that gas, within the gas atomic of 5 h O10 msinaie ihntecnrl22 central the that within and arises sub-thermal emission is CO(1–0) emission CO(2–1) the sug- widths the line the that and gest ratio line The we position. central Telescope, the Submm expected ESO theoretically Swedish both de- the is detected Using been gas up. had where pile CO Until centre, to no galaxies. the and gas two in atomic tected of little encounter very only head-on now, galaxy the ring collisional in a of formed prototype the Cartwheel, the in Abstract. 1998 October 21 accepted July; 28 Received lu.W ne aso oeua a (H gas molecular of nu- mass and ring a inner infer the We with cleus. associated probably is it kpc); 1 2 edopitrqet to requests offprint Send 10. h athe;1.92 11.19.3 11.09.2; Cartwheel; The 11.09.1 are: codes thesaurus later) Your hand by inserted be (will no. manuscript A&A ⋆ eateto hsc srnm,Iw tt nvriy A University, State Iowa Astronomy, & Physics of Department evc ’srpyiu,CASca,F911GfsrYvet sur Gif F-91191 CEA-Saclay, d’Astrophysique, Service nttt eAtoo´ayFıiadlEpco c6,sc28 suc 67, cc Espacio, F´ısica del Astronom´ıa y de Instituto naaSaeOsraoy hlesUiest fTechnolo of University Chalmers Observatory, Space Onsala bevtied ai,DMR,6 vned l’Observatoire de Avenue 61 DEMIRM, Paris, de Observatoire 9 ae nosrain aewt h wds-S Sub- Swedish-ESO the with made observations on based M ⊙ hc ssgicnl ihrta the than higher significantly is which , epeettefis eeto fmlclrgas molecular of detection first the present We 1 .Charmandaris V. , 12 aais niiul M03-3–Galax- – 0035-33 AM individual: Galaxies: O10 n 21 ieeiso towards emission line (2–1) and CO(1–0) .Horellou C. : 2 .Combes F. , 2 f15t 6 to 1.5 of ) ∼ 10 2 8 ′′ ..Appleton P.N. , M (13 ⊙ 12)Beo ie,Argentina. Aires, Buenos (1428) , eCdx France Cedex, te y -39 naa Sweden Onsala, S-43992 gy, -51 ai,France Paris, F-75014 , cl.Ms bevdrn aaishv leclusand colours large blue a have H galaxies on CO, elevated ring starbursts observed exhibit (the Most area galaxies scale. small ring very centre), a in very enhanced significantly is mation swl stedffs n ntysrcueo h spokes 1998). the al. of et structure Appleton knotty 1997; ring, and al. outer diffuse et the (Borne the 1992). in as al. clusters well young as et the massive detail (Marcum unprecedented of wave with distribution population density ring, revealed have stellar the outer images the of HST the of wake to evolution the interior the in disk trace may the which radial in strong showed gradients imaging Hernquist colour near-IR & and Mihos 1993; Optical Weil & 1994). of observational & (Struck-Marcell several prototype Hernquist modeling of 1993; dynamical the Higdon subject as as the well as been considered studies has is It rings, class. its outer and ner ex- the in and star-formation galaxy the a ring. of a of panding chronology that medium the effects interstellar follow the the to they study on interaction, to has the examples collision of review). textbook geometry as a simple for appear the 1996 of Struck-Marcell & Because Appleton (see sion h athe emgspo n ni eety oevi- no recently, until and gas-poor seem Cartwheel the rdcdispeetdyapaac Hgo 1996). and the (Higdon Cartwheel of appearance the present-day compan- one with its than a collided produced rather of companions G3 direction nearer that the two suggests in (G3) projection galaxy in ion kpc H 80 massive than a of (H detection 1976). The gas ring. Hawarden atomic & the (Fosbury of metallicity Most low a indicates ato h ig hr h H quad- the southern where the ring, in the occurs 1998). of formation al. rant star et the massive Amram of by 1995, Most populated (Higdon is regions and star-forming colours blue has expanding, 0c n mrdocniumeiso ek(Higdon peak H emission few continuum a radio of Spectroscopy cm 1996). 6 and cm 20 e A 01,U.S.A. 50011, IA, mes ncnrs oms egn aaisi hc trfor- star which in galaxies merging most to contrast In h ue igo h athe ( Cartwheel the of ring outer The in- well-defined its of because Galaxy, Cartwheel The h ne ig( ring inner The 3 .Casoli F. , α a-nrrdadrdocniumemis- continuum radio and far- , ⋆ ∼ 2 18 n ..Mirabel I.F. and ′′ i sascae ihteouter the with associated is ) ndaee)adncesof nucleus and diameter) in α ii ASTROPHYSICS ASTRONOMY msin swl sthe as well as emission, ein nteotrring outer the in regions i lm xedn more extending plume ∼ 70 AND ′′ ndaee)is diameter) in 4 , 5 2 Horellou et al.: Molecular Gas in The Cartwheel Galaxy dence of star formation could be found. This is in disagree- 3. Results and Discussion ment with the models that predict an infall of gas towards In Figure 1 we present the CO(1–0) and CO(2–1) spectra the centre and vigorous star formation in the inner ring obtained towards the centre of the Cartwheel, as well as (Struck-Marcell & Appleton 1987). However, a number of the HST image (Borne et al. 1996) on which the sizes of recent observations has revealed a richer environment in the CO beams have been superimposed. The parameters the inner regions of the Cartwheel: of the lines, derived from gaussian fits after smoothing i) The HST images resolve a network of dust lanes −1 to a final velocity resolution of 15 km s , are given in in the inner ring and luminous kiloparsec-size cometary Table 1. The central velocities agree with the systemic structures which are suggestive of massive dense clouds −1 velocity of 9089 km s determined from H i observations traveling through the ambient gas (Struck et al. 1996). (Higdon 1996). An indicative molecular gas mass within ii) Hα emission has been detected at a low level the CO(1–0) beam is given in the last column of Table 1. throughout the inner ring and nucleus (Amram et al. 1998, It has been computed using a standard CO-H2 conversion Higdon et al. 1997). factor (see below). iii) ISOCAM images show strong 7 µm and15 µm emis- sion in the inner ring and nucleus. The mid-IR fluxes indi- cate the presence of hot dust and are consistent with weak Table 1. Observational results. star formation activity (Charmandaris et al. 1998).

These new results strengthened our expectation that Line vhel,opt ∆v R Tmbdv σmb M(H2) −1 −1 −1 9 molecular gas must be present in the central region of the km s km s K km s mK 10 M⊙ 12 Cartwheel. Previous searches for CO(J=1–0) line emis- CO(1–0) 9123±13 218±23 0.82±0.10 1.0 1.5 sion had been unsuccessful, and the Cartwheel was one of CO(2–1) 9136±12 211±27 0.95±0.11 1.2 the two galaxies that remained undetected in the survey of 16 ring galaxies of Horellou et al. (1995). Here, we present the first 12CO(1–0) and 12CO(2–1) detections towards the −1 −1 centre of the Cartwheel. For H0=75kms Mpc , the distance to the galaxy is 121 Mpc. 3.1. Molecular gas mass and CO(2–1)/(1–0) line ratio

To convert CO intensities into H2 column densities, we use the factor of Strong et al. (1988): 2. Observations and Data Reduction 20 −2 −1 −1 We have observed at α=00h37m40.8s, δ=−33◦42′56.9′′ X = N(H2)/Imb(CO)=2.310 mol cm (Kkms ) (1) (J2000), which is the peak of emission of the Cartwheel where Imb(CO) = R Tmbdv is the main-beam line area. nucleus in the HST I–band image. This yields a mass The observations have been carried out in July 1998 in 5 ′′ 2 2 La Silla (Chile) with the 15m Swedish-ESO Submillimeter M(H2) (M⊙)=1.2510 (θ/43 ) Imb(CO) (D/1Mpc) (2) Telescope (SEST) (Booth et al. 1989). We used the IRAM 115 and 230 GHz receivers to observe simultaneously at where θ is the half-power beam width of the telescope. the frequencies of the 12CO(1–0) and the 12CO(2–1) lines. From the CO(1–0) line intensity, we estimate a mass 9 ′′ At 115 GHz and 230 GHz, the telescope half-power beam of molecular gas (H2) of 1.5 10 M⊙ within the 43 beam. widths are 43′′ and 22′′, respectively. The main-beam ef- This value should be interpreted with caution since it is ∗ ficiency of SEST is ηmb = TA/Tmb=0.68 at 115 GHz and known that the CO-to-H2 conversion factor (X) is depen- 0.46 at 230 GHz (SEST handbook, ESO). The typical sys- dent on the metallicity (e.g. Maloney & Black 1988). Due ∗ tem temperature varied between 220 and 320 K (in TA to the very weak line emission in the centre, the metal- unit) at both frequencies. The total on-source integration licity could be measured only in the outer star-forming time was 6 hours. A balanced on-off dual beam switching ring of the Cartwheel, and it is 12+log[O/H]=8.1 (Fos- mode was used, with a frequency of 6 Hz and two symmet- bury & Hawarden 1976), compared with 8.9 in Orion. If ric reference positions offset by 12′ in azimuth. The point- the nucleus has the same metallicity as the outer ring, ing was regularly checked on the SiO maser R Aqr. The then the conversion factor would be higher than the stan- ′′ pointing accuracy was 4 rms. The backends were low- dard one by a factor of about 4 (Wilson 1995), and the H2 resolution acousto-optical spectrometers. The total band- mass as well. However, it is also known that most galactic width available was 500 MHz at 115 GHz and 1 GHz at disks present a metallicity gradient, with metallicities in 230 GHz, with a velocity resolution of 1.8 km s−1. The the nucleus up to ∼ 10 times higher than in the outer parts data were reduced with the software CLASS. Only first- of the disk (Smartt & Rolleston 1997). Although X may order baselines were subtracted from the spectra. vary linearly with the reciprocal of metallicity when the metallicity is low, X is expected to become less sensitive to metallicity at values above that of Orion (Sakamoto Horellou et al.: Molecular Gas in The Cartwheel Galaxy 3

     30''   18 kpc       

b

  N     c  E a

Fig. 1. a) HST I–band image of the Cartwheel galaxy (Borne et al. 1996). The circles on the optical image represent the resolution of the CO(1–0) and CO(2–1) observations (43′′ and 22′′ respectively). b) The CO(1–0) spectrum of the nuclear regions. The y-axis shows the main-beam temperature in K. The velocity resolution is 15 km s−1. c) The CO(2–1) spectrum. Units and velocity resolution are the same as in b).

1996). Thus, if we take into account the metallicity gra- the CO(2–1) emission is sub-thermal, possibly because of dient across the disk, the value of X for the nucleus of a low temperature or a low gas density, and that the CO the Cartwheel might be not much lower than the stan- emission is concentrated within the 13 kpc CO(2–1) beam dard value. We can bracket the conversion factor in the (which is consistent with the fact that both lines have nucleus: X

line ratio is low, suggesting sub-thermal excitation, which is consistent with the low level of star-forming activity observed in the central region of the Cartwheel. These CO measurements make it possible to justify future inter- ferometric observations to study the distribution and the dynamics of the molecular gas with a better spatial reso- lution and to establish whether it is associated with the inner ring and/or the dust-lanes, and whether it is of pre- collisional origin or infalling onto the nucleus as numerical simulations predict.

Acknowledgements. We are grateful to L.-A.˚ Nyman and to the SEST staff for the support during the observations, to S. Leon (Observatoire de Paris), C. Struck (Iowa State Uni- versity) and J.H. Black (Onsala Space Observatory) for useful discussions and comments. C.H. aknowledges financial support from the Swedish Natural Science Research Council (NFR). Fig. 2. The rotation curve of the Cartwheel assuming an V.C. acknowledges the financial support from the TMR fel- inclination of 50◦. The open squares are the H i values lowship grant ERBFMBICT960967. We would like to thank an anonymous referee for critical comments on the manuscript. (Higdon 1996), the filled circles are the Hα points (Am- ram et al. 1998) and the filled star is the velocity derived from the CO(2–1) line width. The arrows on the x-axis in- References dicate the radii of the beams of the CO(1–0) and CO(2–1) Allen, R.J. & Lequeux, J., 1993, ApJ 410, L15 observations. Amram, P., Mendes de Oliveira, C., Boulesteix, J. & Balkowski, C., 1998, A&A 330, 881 8 Appleton, P.N. & Struck-Marcell, C., 1996, Fund. of Cos. Phys. we have calculated, but it is only ∼ 10 M⊙ within the 22′′ CO(2–1) beam. If, as argued in the previous para- 16, 111 Appleton, P.N., et al., 1998, in Galaxy Interactions at Low and graph, the CO emission is concentrated, then the inter- ′′ High , IAU 186, in press. stellar medium within the central 22 is predominantly Booth, R., Delgado, G., Hagstr¨om, M., et al. 1989, A&A 216, molecular. 315 i The H rotation curve rises smoothly from the inner Borne, K.D., Lucas, R., Appleton, P.N. et al., 1996, in Science to the outer ring (see Figure 2). Atomic gas is infalling with the II, edited by P. Benvenuti, near the inner ring, where H i concentrations are seen. As- F.D. Macchetto, and E.J. Schreier, (STScI, Baltimore) suming that the CO gas is distributed in an inclined plane Borne, K. et al., 1997, Rev. Mex. Astron. Astrofis. 6, 141 (i ∼ 50◦), we can use the line widths to estimate the rota- Braine, J., Combes, F., 1992, A&A 264, 433 tional velocity in the central region of the galaxy. For the Charmandaris, V. et al., 1998, A&A, in press −1 Fosbury, R. & Hawarden, T., 1976, MNRAS 178, 473 CO(2–1) line we find vR21=∆v/2sini= 138 km s which is significantly higher than the velocities measured for the Higdon, J.L., 1995, ApJ 455, 524 Higdon, J.L., 1996, ApJ 467, 241 Hi gas at radii R< 20′′ (see Fig. 2). The CO point is more Higdon, J.L., Cecil, G., Bland-Hawthorn, J., Lord, S.D., 1997, consistent with the Hα rotation curve. The difference be- i AAS 191, 8907 tween the H velocity on one hand and the Hα and CO Hernquist, L. & Weil, M., 1993, MNRAS 261, 804 velocities on the other hand is likely to be due to different Horellou, C., Casoli, F., Combes, F. et al., 1995, A&A 298, 743 distributions of the atomic and molecular gas, the latter Lynds, R. & Toomre, A., 1976, ApJ 209, 382 being more concentrated. Maloney, P., & Black, J.H., 1988, ApJ 325, 389 Marcum, P., Appleton, P. N., & Higdon, J., 1992, ApJ 399, 57 Mihos, J.C., & Hernquist, L., 1994 ApJ 437, 611 4. Conclusion Sakamoto, S., 1996, ApJ 462, 215 We have presented the first detection of 12CO(1–0) Smartt, S.J., Rolleston, W.R.J., 1997, ApJ 481, L47 and 12CO(2–1) line emission towards the nucleus of the Strong, A.W., Bloemen, J.B.G.M., Dame, T.M., et al., 1988, A&A 207, 1 Cartwheel galaxy and estimated the mass of molecular ′′ 9 9 Struck, C., Appleton, P.N., Borne, K.D., & Lucas, R.A. 1996, gas within the central 43 (1.5 10 < M(H2) < 610 AJ 112, 1868 M⊙). The limited angular resolution of these single-dish Struck-Marcell, C. & Appleton, P.N., 1987, ApJ 323, 480 observations does not allow us to determine precisely the Struck-Marcell, C. & Higdon, J.L., 1993, ApJ 411, 108 location of the molecular gas, but it is probably concen- Theys, J. & Spiegel, E., 1976, ApJ 212, 616 ′′ trated within the central 22 where little atomic gas is Thronson, H.A.Jr. & Telesco, C.M., 1986, ApJ 311 98 8 found (∼ 10 M⊙, Higdon 1996). The CO(2–1) to (1–0) Wilson, C.D., 1995, ApJ 448, L97 Horellou et al.: Molecular Gas in The Cartwheel Galaxy 5

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