M. A. de Avillez

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.

Non-equilibrium ionization modeling of the Local Bubble - I. Tracing Civ, Nv, and Ovi ions

Aims. We present the first high-resolution non-equilibrium ionization simulation of the joint evolution of the Local Bubble (LB) and Loop I superbubbles in the turbulent supernova-driven interstellar medium (ISM). The time variation and spatial distribution of the Li-like ions Civ ,N v ,a nd Ovi inside the LB are studied in detail. Methods. This work uses the parallel adaptive mesh refinement code EAF-PAMR coupled to the newly developed atomic and molecular plasma emission module E(A+M)PEC, featuring the time-dependent calculation of the ionization structure of H through Fe, using the latest revision of solar abundances. The finest AMR resolution is 1 pc within a grid that covers a representative patch of the Galactic disk (with an area of 1 kpc 2 in the midplane) and halo (extending up to 10 kpc above and below the midplane). Results. The evolution age of the LB is derived by the match between the simulated and observed absorption features of the Li-like ions Civ ,N v ,a nd Ovi. The modeled LB current evolution time is bracketed between 0.5 and 0.8 Myr since the last supernova reheated the cavity in order to have N(Ovi) < 8 × 10 12 cm −2 ,l og[N(Civ)/N(Ovi)] < −0.9 and log[N(Nv)/N(Ovi)] < −1 inside the simulated LB cavity, as found in Copernicus, IUE, GHRS-IST and FUSE observations.

ISM simulations: an overview of models

Until recently the dynamical evolution of the interstellar medium (ISM) was simulated using collisional ionization equilibrium (CIE) conditions. However, the ISM is a dynamical system, in which the plasma is naturally driven out of equilibrium due to atomic and dynamic processes operating on different timescales. A step forward in the field comprises a multi-fluid approach taking into account the joint thermal and dynamical evolutions of the ISM gas.

Theory and Models of the Disk-Halo Connection - Modelling the Disk-Halo Interaction in Galaxies

We review the evolution of the interstellar medium in disc galaxies, and show, both analytically and by numerical 3D hydrodynamic simulations, that the disc-halo connection is an essential ingredient in understanding the evolution of star forming galaxies. Depending on the star formation rate of the underlying gaseous disc, a galactic fountain is established. If the star formation rate is sufficiently high and/or cosmic rays are well coupled to the thermal plasma, a galactic wind will be formed and lead to a secular mass loss of the galaxy. Such a wind leaves a unique imprint on the soft X-ray spectra in edge-on galaxies, with delayed recombination being one of its distinctive features. We argue that synthetic spectra, obtained from self-consistent dynamical and thermal modelling of a galactic outflow, should be treated on an equal footing as observed spectra. We show that it is thus possible to successfully fit the spectrum of the starburst galaxy NGC3079.

The Turbulent Interstellar Medium: Insights and Questions from Numerical Models

“The purpose of numerical models is not numbers but insight.” (Hamming) In the spirit of this adage, and of Don Cox’s approach to scientific speaking, we discuss the questions that the latest generation of numerical models of the interstellar medium raise, at least for us. The energy source for the interstellar turbulence is still under discussion. We review the argument for supernovae dominating in star forming regions. Magnetorotational instability has been suggested as a way of coupling disk shear to the turbulent flow. Models make evident that the unstable wavelengths are very long compared to thermally unstable wavelengths, with implications for star formation in the outer galaxy and low surface brightness disks. The perennial question of the factors determining the hot gas filling factor in a SN-driven medium remains open, in particular because of the unexpectedly strong turbulent mixing at the boundaries of hot cavities seen in the models. The formation of molecular clouds in the turbulent flow is also poorly understood. Dense regions suitable for cloud formation clearly form even in the absence of self-gravity, although their ultimate evolution remains to be computed.

Deep XMM-Newton observations of the northern disc of M31. I. Source catalogue

We carried out new observations of two fields in the northern ring of M31 with XMM-Newton with two exposures of 100 ks each and obtained a complete list of X-ray sources down to a sensitivity limit of ~7 x 10^34 erg s^-1 (0.5 - 2.0 keV). The major objective of the observing programme was the study of the hot phase of the ISM in M31. The analysis of the diffuse emission and the study of the ISM is presented in a separate paper. We analysed the spectral properties of all detected sources using hardness ratios and spectra if the statistics were high enough. We also checked for variability. We cross-correlated the source list with the source catalogue of a new survey of the northern disc of M31 carried out with Chandra and Hubble (Panchromatic Hubble Andromeda Treasury, PHAT) as well as with other existing catalogues. We detected a total of 389 sources, including 43 foreground stars and candidates and 50 background sources. Based on the comparison to the Chandra/PHAT survey, we classify 24 hard X-ray sources as new candidates for X-ray binaries (XRBs). In total, we identified 34 XRBs and candidates and 18 supernova remnants (SNRs) and candidates. Three of the four brightest SNRs show emission mainly below 2 keV, consistent with shocked ISM. The spectra of two of them also require an additional component with a higher temperature. The SNR [SPH11] 1535 has a harder spectrum and might suggest that there is a pulsar-wind nebula inside the SNR. We find five new sources showing clear time variability. We also studied the spectral properties of the transient source SWIFT J004420.1+413702, which shows significant variation in flux over a period of seven months (June 2015 to January 2016) and associated change in absorption. Based on the likely optical counterpart detected in the Chandra/PHAT survey, the source is classified as a low-mass X-ray binary.

RECENT RESULTS ON INTERSTELLAR TURBULENCE

The statistical properties of interstellar turbulence are studied by means of three-dimensional high-resolution HD and MHD simulations of a SN-driven ISM. It is found that the longitudinal and transverse turbulent length scales have time averaged (over a period of 50 Myr) ratios of 0.5-0.6, almost similar to the one expected for isotropic homogeneous turbulence. The mean characteristic size of the larger eddies is found to be ~75 pc. Furthermore, the scalings of the structure functions measured in the simulated disk show unambiguous departure from the Kolmogorv (1941) model being consistent with the latest intermittency studies of supersonic turbulence (Politano & Pouquet 1995; Boldyrev 2002). Our results are independent of the resolution, indicating that convergence has been reached, and that the unresolved smaller dissipative scales do not feed back on the larger ones.

3D HD and MHD adaptive mesh refinement simulations of the global and local ISM

We have performed high resolution 3D simulations with adaptive mesh refinement, following the ISM evolution in a star forming galaxy both on small ( 10 kpc) scales, enabling us to track structures in cooling shock compressed regions as well as the entire Galactic fountain flow. It is shown in an MHD run that the latter one isnot inhibited by a large scale disk parallel magnetic field. The fountain plays a vital role in limiting the volume filling factor of the hot gas. Contrary to classical models most of the gas between 100 K and 8000 K is found to be thermally unstable. On scales of superbubbles we find that the internal temperature structure is rather inhomogeneous for an old object like our LocalBubble, leading to low OVI column densities, consistent with observations.

Chimneys and Worms in the Galaxy

A three-dimensional model for the evolution of the disk gas is presented here. This model reproduces many of the features that have been observed in the Galaxy, by means of 21 cm line emission surveys. The cold structures obtained in the model are associated with supernova shells and chimney walls.