A Study of Superbubbles in the ISM : Break-Out, Escape of LYC Photons and Molecule Formation
Multiple coherent supernova explosions (SNe) in an OB association can produce a strong shock that moves through the interstellar medium (ISM). These shocks fronts carve out hot and tenuous regions in the ISM known as superbubbles. The density contour plot at three diﬀerent times (0.5 Myr (left panel), 4 Myr (middle panel), 9.5 Myr (right panel)) showing diﬀerent stages of superbubble evolution for n0 = 0.5 cm−3, z0 = 300 pc, and for NOB = 104. This density contour plot is produced using ZEUS-MP 2D hydrodynamic simulation with a resolution of 512 × 512 with a logarithmic grid extending from 2 pc to 2.5 kpc. For a detailed description of this figure, see Roy et. al., 2015. The evolution of a superbubble is marked by diﬀerent phases, as it moves through the ISM. Consider an OB association at the center of a disk galaxy. Initially the distance of the shock front is much smaller than the disk scale height. The superbubble shell sweeps up the ISM material, and once the amount of swept up material becomes comparable to the ejected material during SNe, the superbubble enters a self-similar phase (analogous to the Sedov-Taylor phase of individual SNe). As the superbubble shell sweeps up material, its velocity decreases, and thus the corresponding post-shock temperature drops. At a temperature of ∼ 2 × 105 K (where the cooling function peaks), the superbubble shell becomes radiative and starts losing energy via radiative cooling. This radiative phase is shown in the left panel of Figure 1. The superbubble shell starts fragmenting into clumps and channels due to Rayleigh-Taylor instabilities (RTI) (which is seeded by the thermal instability; for details see Roy et. al., 2013) when the superbubble shell crosses a few times the scale height. This is represented in the middle panel of the same figure. At a much later epoch, RTI has a strong eﬀect on the shell fragmentation and the top of the bubble is completely blown oﬀ (the right panel). In the first chapter of the thesis (reported in Sharma et. al., 2014), we show using ZEUS-MP hydrodynamic simulations that an isolated supernova loses almost all its mechanical energy within a Myr whereas superbubbles can retain up to ∼ 40% of the input energy over the lifetime of the starcluster (∼ few tens of Myr), consistent with the analytic estimate of the second chapter. We also compare diﬀerent recipes (constant luminosity driven model (LD model), kinetic energy driven model (KE model) to implement SNe feedback in numerical simulations. We determine the constraints on the injection radius (within which the SNe input energy is injected) so that the supernova explosion energy realistically couples to the interstellar medium (ISM). We show that all models produce similar results if the SNe energy is injected within a very small volume ( typically 1–2 pc for typical disk parameters). The second chapter concentrates on the conditions for galactic disks to produce superbubbles which can give rise to galactic winds after breaking out of the disk. The Kompaneets formalism provides an analytic expression for the adiabatic evolution of a superbubble. In our calculation, we include radiative cooling, and implement the supernova explosion energy in terms of constant luminosity through out the life-time of the OB stars in an exponentially stratified medium (Roy et. al., 2013). We use hydrodynamic simulations (ZEUS-MP) to determine the evolution of the superbubble shell. The main result of our calculation is a clear demarcation between the energy scales of sources causing two diﬀerent astrophysical phenomenon: (i) An energy injection rate of ∼ 10−4 erg cm−2 s−1 (corresponding Mach number ∼ 2–3, produced by large OB associations) is relevant for disk galaxies with synchrotron emitting gas in the extra-planar regions. (ii) A larger energy injection scale ∼ 10−3 erg cm−2 s−1, or equivalently a surface density of star formation rate ∼ 0.1 M⊙ yr−1 kpc−2 corresponding to superbubbles with high Mach number (∼ 5–10) produces galactic-scale superwinds (requires superstar clusters to evolve coherently in space and time). The stronger energy injection case also satisfies the requirements to create and maintain a multiphase halo (matches with observations). Roy et. al., 2013 also points out that Rayleigh-Taylor instability (RTI) plays an important role in the fragmentation of superbubble shell when the shell reaches a distance approximately 2–3 times the scale-height; and before the initiation of RTI, thermal instability helps to corrugate the shell and seed the RTI. Another important finding of this chapter is the analytic estimation of the energetics of superbubble shell. The shell retains almost ∼ 30% of the thermal energy after the radiative losses at the end of the lifetime of OB associations. The third chapter considers the escape of hydrogen ionizing (Lyc) photons arising from the central OB-association that depends on the superbubble shell dynamics. The escape fraction of Lyc photons is expected to decrease at an initial stage (when the superbubble is buried in the disk) as the dense shell absorbs most of the ionizing photons, whereas the subsequently formed channels (created by RTI and thermal instabilities) in the shell creates optically thin pathways at a later time (∼ 2–3 dynamical times) which help the ionizing photons to escape. We determine an escape fraction (fesc) of Lyc photons of ∼ 10 ± 5% from typical disk galaxies (within 0 ≤ z (redshift) ≤ 2) with a weak variation with disk masses (reported in Roy et. al., 2015). This is consistent with observations of local galaxies as well as constraints from the epoch of reionization. Our work connects the fesc with the fundamental disk parameters (mid-plane density (n0), scale-height (z0)) via a relation that fescαn20z03 (with a ≈ 2.2) is a constant. In the fourth chapter, we have considered a simple model of molecule formation in the superbubble shells produced in starburst nuclei. We determine the threshold conditions on the disk parameters (gas density and scale height) for the formation of molecules in superbubble shells breaking out of disk galaxies. This threshold condition implies a gas surface density of ≥ 2000 M⊙ pc−2, which translates to a SFR of ≥ 5 M⊙ yr−1 within the nuclear region of radius ∼ 100 pc, consistent with the observed SFR of galaxies hosting molecular outflows. Consideration of molecule formation in these expanding superbubble shells predicts molecular outflows with velocities ∼ 30–40 km s−1 at distances ∼ 100–200 pc with a molecular mass ∼ 106–107 M⊙, which tally with the recent ALMA observations of NGC 253. We also consider diﬀerent combinations of disk parameters and predict velocities of molecule bearing shells in the range of ∼ 30–100 km s−1 with length scales of ≥ 100 pc, in rough agreement with the observations of molecules in NGC 3628 and M82 (Roy et. al., 2016, submitted to MNRAS).
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