Superbubble breakout and galactic winds from disc galaxies

Abstract

We study the conditions for disk galaxies to produce superbubbles that can break out of the disk and produce a galactic wind. We argue that the threshold surface density of supernovae rate for seeding a wind depends on the ability of superbubble energetics to compensate for radiative cooling. We first adapt Kompaneets formalism for expanding bubbles in a stratified medium to the case of continuous energy injection and include the effects of radiative cooling in the shell. With the help of hydrodynamic simulations, we then study the evolution of superbubbles evolving in stratified disks with typical disk parameters. We identify two crucial energy injection rates that differ in their effects, the corresponding breakout ranging from being gentle to a vigorous one. (a) Superbubbles that break out of the disk with a Mach number of order 2-3 correspond to an energy injection rate of order 10 −4 erg cm −2 s −1 , which is relevant for disk galaxies with synchrotron emitting gas in the extra-planar regions. (b) A larger energy injection threshold, of order 10 −3 erg cm −2 s −1 , or equivalently, a star formation surface density of ∼ 0.1 M ⊙ yr −1 kpc −2 , corresponds to superbubbles with a Mach number ∼ 5-10. While the milder superbubbles can be produced by large OB associations, the latter kind requires super-starclusters. These derived conditions compare well with observations of disk galaxies with winds and the existence of multiphase halo gas. Furthermore, we find that contrary to the general belief that superbubbles fragment through Rayleigh-Taylor (RT) instability when they reach a vertical height of order the scale height, the superbubbles are first affected by thermal instability for typical disk parameters and that RT instability takes over when the shells reach a distance of approximately twice the scale height.

Supernovae driven galactic outflows

“…Using Salpeter IMF, and considering stellar masses in the range of 1-100 M , the corresponding surface density of SFR becomes ∼ 0.1 M yr −1 kpc −2 , consistent with observations. Roy et al (2013) has worked out the dynamics of superbubbles triggered by multiple SNe in a disk galaxy, first analytically and then with hydrodynamic simulations, and they found that this simple estimate is supported by detailed calculations. Furthermore, they studied the clumping and fragmentation of the superbubbles during its evolution, and found that thermal instability plays an important role in clumping of the shell material, and in subsequent seeding of Rayleigh-Taylor instability.…”

“…Recently, it has been argued that these threshold conditions can be understood from the requirement of multiple SNe driven superbubbles to compensate against radiative loss and be able to break out of the disk with sufficient Mach number in order for the hot interior gas to reach a considerable height above the disk (Roy et al 2013). The basic argument can be illustrated with the help of a simple estimate.…”

“…Furthermore, they studied the clumping and fragmentation of the superbubbles during its evolution, and found that thermal instability plays an important role in clumping of the shell material, and in subsequent seeding of Rayleigh-Taylor instability. Without the aid of thermal instability, Roy et al (2013) showed that the clumping due to Rayleigh-Taylor instability usually takes place at a time scale that is longer than the main sequence life time of O stars, the main source of ionizing radiation. This problem is therefore important not only for the hot interior gas to be released into the halo region, but also for the escape of ionizing radiation from massive stars.…”

Narrow escape: how ionizing photons escape from disc galaxies

“…2006), a second order accurate Eulerian hydrodynamics code. We carry out 2-D axisymmetric simulations of the following governing equations (see also Roy et al 2013),…”

“…At t ∼ 2-3t d , after the superbubble breaks out from the disk, the shell begins to fragment due to radiative cooling and Rayleigh-Taylor instability (RTI; Roy et al 2013). These clumps give rise to the zigzag nature of the angular dependence of the escape fraction.…”