View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server A&A manuscript no. (will b e inserted by hand later) ASTRONOMY AND Your thesaurus co des are: 01(09.02.1, 11.09.1 M 82, 11.09.4, 11.11.1, 11.19.3, 13.25.4) ASTROPHYSICS Evidence for an Expanding Molecular Sup erbubble in M 82 Axel Wei, Fabian Walter, Nikolaus Neininger, and Uli Klein Radioastronomi sches Institut der Universitat Bonn, Auf dem Hugel 71, 53121 Bonn, Germany Received 23 February 1999 / Accepted 06 April 1999 Abstract. We present evidence for an expanding sup er- 1. Intro duction bubble in M 82 (diameter: 130 p c, expansion velo city: 1 6 12 M 82 is the b est studied nearby starburst galaxy (D = 45 km s , mass: 8 10 M ). It is seen in the CO 12 13 3:25 Mp c). The central few hundred parsecs of this galaxy (J =1!0), CO(J =2!1), CO(J =1!0) and 18 are heavily obscured by dust and gas which hides the cen- C O(J =1!0) lines. The sup erbubble is centred around tral starburst region against direct observations at optical the most p owerful sup ernova remnant, 41.9+58. The CO wavelengths. Evidence for strong star{forming activityin observations show that the molecular sup erbubble already the central region comes from radio (e.g. Kronb erg et al. broke out of the disk. This scenario is supp orted byROSAT 1981) and infrared observations (e.g. Telesco et al. 1991) HRI observations which suggest that hot coronal gas orig- and also from the prominent bip olar out ow visible in H inating from inside the shell is the main contributor to (e.g. Bland & Tully 1988, McKeith et al. 1995, Shopb ell the di use X{ray out ow in M 82. We brie y discuss ob- & Bland{Hawthorn 1998) and in X-rays (e.g. Bregman servations of the same region at other wavelengths (radio et al. 1995). The massive star formation (SF) is b elieved continuum, optical, H I, X{rays, ionized gas). From our to b e fuelled by the large amount of molecular gas which sp ectral line observations, we derive a kinematic age of 6 is present in the centre of M 82. ab out 10 years for the sup erbubble. Using simple theo- On the other hand, SF e ects the distribution and retical mo dels, the total energy needed for the creation of 54 kinematics of the surrounding interstellar medium (ISM). this sup erbubble is of order 2 10 ergs. The required 1 Recent millimetre continuum observations (Carlstrom & energy input rate (0.001 SN yr ) is reasonable given the 1 Kronb erg 1990) suggested that the HII regions in M 82 high sup ernova (SN) rate of 0:1SNyr in the central haveswept up most of the surrounding neutral gas and part of M 82. As much as 10% of the energy needed to dust into dense shells. This is in agreement with the stan- create the sup erbubble is still present in form of the ki- dard picture: shells are created byyoung star{forming re- netic energy of the expanding molecular shell. Of order gions through strong stellar winds of the most massive 10% is conserved in the hot X{ray emitting gas emerging astro-ph/9904081 7 Apr 1999 stars in a cluster and through subsequenttyp e{I I sup er- from the sup erbubble into the halo of M 82. This newly novae (e.g. Tenorio{Tagle & Bo denheimer 1991). These detected expanding molecular sup erbubble is b elieved to pro cesses are thoughttoblowhuge cavities lled with be powered by the same ob jects that also lie at the origin coronal gas into their ambient ISM (e.g. Cox & Smith of the prominent X{ray out ow in M 82. It can therefore 1974,Weaver et al. 1977). This hot interior is then b e- b e used as an alternativetooltoinvestigate the physical lieved to drive the expansion of the outer shell of swept{up prop erties of these sources. material. Once sup erbubbles reach sizes that are comparable to Key words: ISM: bubbles { galaxies: individual: M 82 the thickness of a galaxy's disk, the bubble will eventu- { galaxies: ISM { galaxies: kinematics and dynamics { ally break out into the halo. This then leads to an out ow galaxies: starburst { X-rays: ISM of the hot gas with velo cities much higher than the ex- pansion of the shell within the disk of the galaxy. In the following we present evidence for a molecular sup erbub- ble in M 82 which already broke out of the disk and seems to contribute signi cantly to the well{known prominent out ow of M 82. Send o print requests to : A.Wei, [email protected]{b onn.de 2 A. Wei et al.: Evidence for an Expanding Molecular Sup erbubbl e in M 82 2. Observations constantvelo city gradient (cf. Shen & Lo 1995) the pv{ diagrams show an expanding ring{like feature centred on 1 2.1. Molecular Lines SNR 41.9+58, with a central velo cityofV 150 km s . lsr The approaching velo city comp onent of the ring is clearly 12 1 For our analysis we used the CO(J =1!0) data cub e seen in all cub es and is centred at 100 km s . The reced- 13 obtained by Shen & Lo (1995) with the BIMA array (spa- ing comp onent is only marginally visible in the CO 00 13 18 tial resolution: 2:5 ) and the CO(J =1!0) data cub e (J =1!0) and C O(J =1 !0) lines; its central ve- 1 from Neininger et al. (1998) observed with the Platau de lo city is ab out 190 km s . An enlargement of the pv{ 00 Bure interferometer (PdBI) (spatial resolution: 4:2 ). In diagram along the ma jor axis is shown in Fig. 3 (left). 12 addition, we used unpublished PdBI data of the CO(J = From this diagram we estimate the radius of the ring to b e 18 1 2 ! 1) and C O(J =1!0) transitions. These observa- (65 5) p c and the expansion velo city (45 5) km s .A tions where carried out in April 1997 in the CD con g- pv{diagram along the minor axis centred on SNR 41.9+58 00 00 uration, resulting in a spatial resolution of 1:4 1:2 is presented in Fig. 3 (right). The orientation of the cut is 12 00 00 18 ( CO(J =2!1)) and 3:7 3:5 (C O(J =1!0)), and shown in Fig. 1. 1 1 avelo city resolution of 3.3 km s and 6.8 km s , re- sp ectively. In order to increase the sensitivity to extended 12 CO emission we combined the CO (J =2!1) data cub e with single dish measurements obtained with the IRAM 30m telescop e. These observations were carried out in May 1998. The combination is essential for this particular study b ecause the receding part of the sup erbubble is only mar- ginally visible in the mere interferometer maps. A full ac- count of the data reduction will b e given elsewhere. 2.2. Evidence for an Expanding Superbubble 12 Fig. 1 shows the integrated CO line emission published by Shen & Lo (1995). The cross corresp onds to the p o- sition of the sup ernova remnant 41.9+58 (SNR 41.9+58) which is the strongest cm continuum p oint{source in M 82 (Kronb erg et al. 1981). It is considered to b e the aftermath ofa'hyp ernova', which exhibits a radio luminosity 50{100 times greater than typical for typ e{I I SNe (Wilkinson & de Bruyn 1990). The line along the ma jor axis indicates the orientation of the p osition{velo city(pv) diagrams shown in Fig. 2. Fig. 2. The pv{diagrams along the ma jor axis of M 82 12 (orientation shown in Fig. 1) for the CO(J =1!0), 12 13 18 CO (J =2!1), CO(J =1!0) and C O(J =1!0) cub es. The slice is centred on SNR 41.9+58. Fig. 3 (right) reveals two emission features centred at the 12 Fig. 1. Integrated CO line emission from Shen & Lo p osition of SNR 41.9+58, which corresp ond to the ap- (1995). The lines along the ma jor and minor axis of 1 proaching (v 100 km s ) and the receding comp onent M 82 indicate the orientations of the pv{diagrams shown 1 (v 190 km s ) of the expanding sup erbubble. Hardly in Figs. 2 and 3. The cross marks the p osition of SNR 1 any CO emission with a velo citybetween v = 100 km s 41.9+58. 1 and v = 190 km s is found south and north of SNR 41.9+58. The pv{diagrams therefore show that the ex- panding molecular shell has already broken out of the disk The pv{cut is orientated in suchaway that the signa- and now only shows the signature of an expanding molec- ture of the expanding sup erbubble is visible most dis- ular ring. It should b e noted that the remaining molecular tinctly. The angular axis of the pv diagrams corresp ond gas in M 82 shows clear solid{b o dy rotation.