Dieter Breitschwerdt

Volume filling factors of the ISM phases in star forming galaxies - I. The role of the disk-halo interaction

The role of matter circulation between the disk and halo in establishing the volume filling factors of the different ISM phases in the Galactic disk (|z |≤ 250 pc) is investigated, using a modified version of the three-dimensional supernova- driven ISM model of Avillez (2000). We carried out adaptive mesh refinement simulations of the ISM with five supernova rates (in units of the Galactic value), σ/σGal = 1, 2, 4, 8 and 16 (corresponding to starburst conditions) using three finer level resolutions of 2.5, 1.25 and 0.625 pc, allowing us to understand how resolution would affect the volumes of gas phases in pressure equilibrium. We find that the volume filling factors of the different ISM phases depend sensitively on the existence of a duty cycle between the disk and halo acting as a pressure release valve for the hot (T > 10 5.5 K) phase in the disk. The amount of cold gas (defined as the gas with T < 10 3 K) picked up in the simulations varies from a value of 19% for σ/σGal = 1 to ∼5% for σ/σGal = 4a nd≤1% for higher SN rates. Background heating prevents the cold gas from immediate collapse and thus ensures the stability of the cold gas phase. The mean occupation fraction of the hot phase varies from about 17% for the Galactic SN rate to ∼28%, for σ/σGal = 4, and to 44% for σ/σGal = 16. Overall the filling factor of the hot gas does not increase much as we move towards higher SN rates, following a power law offv, hot �∝ (σ/σGal) 0.363 . Such a modest dependence on the SN rate is a consequence of the evacuation of the hot phase into the halo through the duty cycle. This leads to volume filling factors of the hot phase considerably smaller than those predicted in the three-phase model of McKee & Ostriker (1977) even in the absence of magnetic fields.

The nonthermal energy content and gamma-ray emission of starburst galaxies and clusters of galaxies

The nonthermal particle production in contemporary starburst galaxies and in galaxy clusters is estimated from the Supernova rate, the iron content, and an evaluation of the dynamical processes which characterize these objects. The primary energy derives from SN explosions of massive stars. The nonthermal energy is transformed by various secondary processes, like acceleration of particles by Supernova Remnants as well as diffusion and/or convection in galactic winds. If convection dominates, the energy spectrum of nonthermal particles will remain hard. At greater distances from the galaxy almost the entire enthalpy of thermal gas and Cosmic Rays will be converted into wind kinetic energy, implying a fatal adiabatic energy loss for the nonthermal component. If this wind is strong enough then it will end in a strong termination shock, producing a new generation of nonthermal particles which are subsequently released without significant adiabatic losses into the external medium. In clusters of galaxies this should only be the case for early type galaxies, in agreement with observations. Clusters should also accumulate their nonthermal component over their entire history and energize it by gravitational contraction. The pion decay γ-ray fluxes of nearby contemporary starburst galaxies is quite small. However rich clusters should be extended sources of very high energy γ-rays, detectable by the next generation of systems of air Cherenkov telescopes. Such observations will provide an independent empirical method to investigate these objects and their cosmological history.

The Generation and Dissipation of Interstellar Turbulence: Results from Large-Scale High-Resolution Simulations

We study, by means of adaptive mesh refinement hydro- and magnetohydrodynamic simulations that cover a wide range of scales (from kiloparsec to subparsec), the dimension of the most dissipative structures and the injection scale of turbulent interstellar gas, which we find to be about 75 pc, in agreement with observations. This is, however, smaller than the average size of superbubbles but consistent with significant density and pressure changes in the ISM, which leads to the breakup of bubbles locally and hence to the injection of turbulence. The scalings of the structure functions are consistent with log-Poisson statistics of supersonic turbulence, where energy is dissipated mainly through shocks. Our simulations are different from previous ones by other authors, since (1) we do not assume an isothermal gas but have temperature variations of several orders of magnitude, and (2) we have no artificial forcing of the fluid with some ad hoc Fourier spectrum but drive turbulence by stellar explosions at the Galactic rate, self-regulated by density and temperature thresholds imposed on the ISM gas.

The search for the origin of the Local Bubble redivivus

ABSTRACT We present anew unbiasedsearchand analysisofall B starsin the solarneighbourhood(within a volume of 400 pc diameter) using the Arivel data base to track down theremains of the OB associations, which hosted the supernovae responsible for the LocalBubble in the interstellar gas. We find after careful dereddening and by comparisonwith theoretical isochrones, that besides the Upper Scorpius the Upper CentaurusLupus and Lower Centaurus Crux subgroups are the youngest stellar associations inthe solar neighbourhood with ages of 20 to 30 Myr, in agreement with previous work.In search for the “smoking gun” of the origin of the Local Bubble, we have traced thepaths of the associations back into the past and found that they entered the presentbubble region 10 to 15 Myr ago. We argue that the Local Bubble began to form thenand estimate that 14 to 20 supernovae have gone off since. It is shown that the impliedenergy input is sufficient to excavate a bubble of the presently observed size.Key words: solar neighbourhood - open clusters and associations: individual:ScoOB2 - ISM: individual: local bubble - Local ISM

The History and Future of the Local and Loop I Bubbles

Context. The Local and Loop I superbubbles are the closest and best investigated supernova (SN) generated bubbles and serve as test laboratories for observations and theories of the interstellar medium. Aims. Since the morphology and dynamical evolution of bubbles depend on the ambient density and pressure distributions, a realistic modelling of the galactic environment is crucial for a detailed comparison with observations. Methods. We have performed 3D high resolution (down to 1.25 pc on a kpc-scale grid) hydrodynamic simulations of the Local Bubble (LB) and the neighbouring Loop I (L1) superbubble in a realistically evolving inhomogeneous background ISM, disturbed already by SN explosions at the Galactic rate for 200 Myr before the LB and L1 are generated. The LB is the result of 19 SNe occurring in a moving group, which passed through the present day local Hi cavity. Results. We can reproduce (i) the Ovi column density in absorption within the LB in agreement with Copernicus and recent FUSE observations, giving NOV I < 2 × 10 13 cm 2 and NOV I < 7 × 10 12 cm 2 , respectively, (ii) the observed sizes of the Local and Loop I superbubbles, (iii) the interaction shell between LB and L1, discovered with ROSAT, (iv) constrain the age of the LB to be 14.5± 0.7 0.4 Myr, (v) predict the merging of the two bubbles in about 3 Myr, when the interaction shell starts to fragment, (vi) the generation of blobs like the Local Cloud as a consequence of a dynamical instability. Conclusions. We find that evolving superbubbles strongly deviate from idealised self-similar solutions due to ambient pressure and density gradients, as well as due to turbulent mixing and mass loading. Hence, at later times the hot interior can break through the surrounding shell, which may also help to explain the puzzling energy “deficit” observed in LMC bubbles.

Chandra detection of a hot gaseous corona around the edge-on galaxy ngc 4631

We present a Chandra X-ray observation that shows, unambiguously for the first time, the presence of a giant diffuse X-ray-emitting corona around the edge-on disk galaxy NGC 4631. This corona, with a characteristic thermal temperature of × 106 K, extends as far as 8 kpc away from the galactic plane. The X-ray morphology resembles the radio halo of the galaxy, indicating a close connection between outflows of hot gas, cosmic rays, and the magnetic field from the galactic disk. Enhanced diffuse X-ray emission is apparently enclosed by numerous Hα-emitting loops blistered out from the central disk of the galaxy, as is evident in a comparison with our deep Hubble Space Telescope imaging.

The origin of the young stellar population in the solar neighborhood — A link to the formation of the Local Bubble?

We have analyzed the trajectories of moving stellar groups in the solar neighborhood in an attempt to estimate the number of supernova explosions in our local environment during the past 20 million years. Using Hipparcos stellar distances and the results of kinematical analyses by Asiain et al. (1999a) on the Pleiades moving groups, we are able to show that subgroup B1, consisting of early type B stars up to 10 M, but lacking more massive objects, has passed through the local interstellar medium within less than 100 pc. Comparing the stellar content of B1 with the initial mass function derived from the analysis of galactic OB associations, we estimate the number of supernova explosions and find that about 20 supernovae must have occurred during the past10 20 million years, which is suggested to be the age of the Local Bubble; the age of the star cluster is about20 30 million years. For the first time, this provides strong evidence that the Local Bubble must have been created and shaped by multi-supernova explosions and presumably been reheated more than 1 million years ago, consistent with recent findings of an excess of 60 Fe in a deep ocean ferromanganese crust. Calculating similarity solutions of an expanding superbubble for time-dependent energy input, we show that the number of explosions is sucient to explain the size of the Local Bubble. The present energy input rate is about ˙ ESN 5 10 36 erg/s, in good agreement with the estimated local soft X-ray photon output rate. It seems plausible that the origin of the Local Bubble is also linked to the formation of the Gould Belt, which originated about 30-60 Myrs ago.

Metal Enrichment Processes in the Intra-Cluster Medium

Institut fu¨r Astronomie, Universit¨at Wien, Tu¨rkenschanzstr. 17, 1180 Vienna, AustriaReceived / AcceptedAbstract. We present numerical simulations of galaxy clusters which include interaction processes between thegalaxies and the intra-cluster gas. The considered interaction processes are galactic winds and ram-pressurestripping, which both transfer metal-enriched interstellar medium into the intra-cluster gas and hence increase itsmetallicity. We investigate the efficiency and time evolution of the interaction processes by simulated metallicitymaps, which are directly comparable to those obtained from X-ray observations. We find that ram-pressurestripping is more efficient than quiet (i.e. non-starburst driven) galactic winds in the redshift interval between 1and 0. The expelled metals are not mixed immediately with the intra-cluster gas, but inhomogeneities are visiblein the metallicity maps. Even stripes of higher metallicity that a single galaxy has left behind can be seen. Thespatial distribution of the metals transported by ram-pressure stripping and by galactic winds are very differentfor massive clusters: the former process yields a centrally concentrated metal distribution while the latter resultsin an extended metal distribution.Key words. Galaxies:clusters:general – Galaxies:abundances – Galaxies:interactions – Galaxies:ISM – X-ray:galaxies:clusters

Simulations of galactic winds and starbursts in galaxy clusters

School of Mathematics, University of Edinburgh, Edinburgh EH9 3JZ, Scotland, UK-/-Abstract. We present an investigation of the metal enrichmentof the intra-cluster medium (ICM) bygalactic windsand merger-driven starbursts. We use combined N-body/hydrodynamic simulations with a semi-numerical galaxyformation model. The mass loss by galactic winds is obtained by calculating transonic solutions of steady stateoutflows, driven by thermal, cosmic ray and MHD wave pressure. The inhomogeneities in the metal distributioncaused by these processes are an ideal tool to reveal the dynamical state of a galaxy cluster. We present surfacebrightness, X-ray emission weighted temperature and metal maps of our model clusters as they would be observedby X-ray telescopes like XMM-Newton. We show that X-ray weighted metal maps distinguish between pre- orpost-merger galaxy clusters by comparing the metallicity distribution with the galaxy-density distribution: pre-mergers have a metallicity gap between the subclusters, post-mergers a high metallicity between subclusters. Weapply our approach to two observed galaxy clusters, Abell 3528 and Abell 3921, to show whether they are pre-or post-merging systems. The survival time of the inhomogeneities in the metallicity distribution found in oursimulations is up to several Gyr. We show that galactic winds and merger-driven starbursts enrich the ICM veryefficiently after z=1 in the central (∼ 3 Mpc radius) region of a galaxy cluster.Key words. Galaxies: clusters: general - Galaxies: abundances - Galaxies: interactions - Galaxies: ISM - X-ray:galaxies: clusters

Stephan's Quintet with XMM-Newton

The prototype compact group known as Stephans Quintet (SQ) was observed with XMM-Newton in order to complement the excellent resolution of Chandra with high sensitivity to extended emission. SQ is a dynamic environment whose main effect, at both X-ray and optical wavelengths, appears to be ISM stripping. This is manifested by: 1) secular evolution of morphological types towards earlier types and 2) growth of diffuse emission. Virtually all cold, warm, and hot gas in SQ is found outside of the member galaxies. XMM-Newton offers the opportunity to study the hot gas with unprecedented sensitivity. We find two main components: 1) extended high surface brightness emission from shocked gas associated with an ongoing collision and 2) even more extended and unrelaxed diffuse emission that follows the stripped stellar envelope of the group.