A theory of the interstellar medium - Three components regulated by supernova explosions in an inhomogeneous substrate

“ISM Basics The interstellar medium (ISM) is a complex environment encompassing a very wide range of properties (e.g., McKee & Ostriker 1977; Cox 2005).”

“Here, we attempt to incorporate some of the key aspects of the theoretical picture of the ISM outlined by McKee & Ostriker (1977).”

“Therefore, instead of large-scale gravitational instability or cloud destruction by shear, the ability to form a cold neutral medium (McKee & Ostriker 1977; Wolfire et al. 2003) may regulate GMC formation.”

“The resulting values are in rough agreement with those of McKee & Ostriker (1977).”

“Others are thermal phase transitions (Field, Goldsmith, & Habing 1969, McKee & Ostriker 1977, Wolfire et al. 1995) or gravitational collapse (e.g. Wada & Norman 1999).”

“The ISM is a complex place, with multiple gas phases co-existing at very different densities and temperatures (McKee & Ostriker 1977).”

“(e.g. McKee & Ostriker 1977; Efstathiou 2000).”

“…observation of a continuous distribution of neutral hydrogen temperature (Dickey, Salpeter, & Terzian 1977), and the attribution of gas motions to supernovae (e.g., McKee & Ostriker 1977), there was no compelling reason to dismiss the basic cloud-intercloud model in favor of widespread turbulence.”

“Some of the warm gas is partially ionized, the warm ionized medium (WIM), which also has T 104 K (McKee & Ostriker 1977, hereafter MO; Reynolds 1983; Haffner, Reynolds, & Tufte 1999).”

“This models treats the ISM as a two-phase medium (McKee & Ostriker 1977; Johansson & Efstathiou 2006) where clouds of cold gas are embedded in the hot gas phase at pressure equilibrium.”

“To motivate this model, and to connect with classic treatments of the ISM (e.g., McKee & Ostriker 1977), we estimate the pressure in both the “cold” (pc) and “hot” (ph) phases of the ISM, focusing on the contributions from radiation pressure and supernovae.”

“…values) c© 0000 RAS, MNRAS 000, 000–000 star forming threshold, star formation proceeds in a subresolution two-phase medium where supernova energy released by type II SNe during star formation balances cold cloud formation and cloud evaporation by the hot medium as in McKee & Ostriker (1977).”

“Therefore, we implemented an effective version of supernova feedback that uses the blastwave solution presented in McKee & Ostriker (1977).”

“This simple argument indicates that supernovae alone might determine the thermal and dynamical state of the ISM (McKee & Ostriker 1977).”

“…clouds(A V D 1), are PDR gasÈthus, most of the ISM is in PDRs.(A V [ 1) In addition, the warm neutral medium (n ^ 0.25 cm~3) of the Galaxy (see McKee & Ostriker 1977 ; WolÐre et al. 1995), the neutral gas around planetary nebulae, and photodissociated winds from red giant or asymptotic giant…”

“…solution (Osterbrock 1989), the radius of a shock is given by Rs = 12.8 ( texp 104 )2/5 ( E51 n0 )1/5 pc, (8) where texp is the time since the explosion in years, E51 is the energy released by the SN in units of 1051 erg, and n0 ∼ 10−2 cm−3 is the ambient ISM density (McKee & Ostriker 1977).”

“The processes of evaporation and condensation are followed analytically within each particle using the formalism of McKee & Ostriker (1977).”

“In our algorithm the time-scale for the cooling shut off and the amount of mass affected are physically motivated and chosen following typical values from the McKee & Ostriker (1977) and Ostriker & McKee (1988) blast wave model.”

“Early models (e.g., McKee & Ostriker 1977) classified the ISM into three phases: the Cold Neutral Medium (CNM), often referred to as clouds; the Warm Ionized Medium or Warm Neutral Medium (WIM or WNM), which is sometimes considered the boundary layers of the CNM; and the Hot Ionized Medium (HIM),…”

“The values obtained correspond roughly to the fiducial values of McKee & Ostriker (1977).”

“We also assume that supernovae evaporate cold clouds (McKee & Ostriker, 1977), essentially by thermal conduction, thereby “cooling” the ambient hot gas.”

“The remnant is then in the pressure-driven snowplow (PDS) stage (McKee & Ostriker 1977).”

“…is small, but by mass, they dominate the total gas mass; the hot gas with filling factor near unity has a typical mass fraction f hot 0.1 (see e.g. McKee & Ostriker 1977). act on the cold clouds (in the form of ionization and momentum injection from absorbed photons), but they may be too dense…”

“…and controversial phenomena to model; a detailed treatment should include a multiphase interstellar medium, with hot, cold and possibly warm gas components, tracking collisions between cold clouds and their evaporation by supernova heating (McKee & Ostriker 1977; Efstathiou 2000; Monaco 2004).”

“Gas pressurization from a multiphase ISM by feedback from star formation (McKee & Ostriker 1977; Springel & Hernquist 2003) stabilizes gas disks and allows mergers between gas-dominated galaxies to produce large, smoothly distributed, rapidly rotating stellar disks.”

“Star-forming gas particles have a subgrid recipe containing cold clouds embedded in a warm ionized medium to simulate the processes of evaporation and condensation seen in our own galaxy (McKee & Ostriker 1977).”

“The opposite extreme, qeos = 1, represents the ‘maximal feedback’ sub-resolution model of Springel et al. (2005b), based on the multiphase ISM model of McKee & Ostriker (1977); in this case, 100 per cent of the energy from supernovae is assumed to stir up the ISM.”

“Star formation is modelled using a subgrid recipe introduced in Springel & Hernquist (2003a) where a gas particle above a critical density is modelled as a fraction of cold clouds embedded in a warm ionised medium as in McKee & Ostriker (1977).”

“Stars are formed from dense gas via a subresolution multi-phase model that tracks condensation and evaporation in the interstellar medium following McKee & Ostriker (1977).”

“As argued by Ceverino & Klypin, gas cooling rates drop by orders of magnitude at lower gas temperatures, making it possible for thermal feedback to maintain greater pressure gradients between dense star forming regions and the ambient ISM.”

“In relatively low density gas, heating by photoionization, stellar winds and SNe is critical (McKee & Ostriker 1977; Matzner 2002).”

“…massive stars, depositi n of momentum by expanding bubbles of photoionized HII region gas, deposition of momentum by the shocked winds from massive stars, and jets from protostars (e.g., McKee & Ostriker 1977; Silk 1997; Wada & Norman 2001; Matzner 2002; Li & Nakamura 2006; Cunningham et al. 2008).”

“The atomic gas in a galaxy generally comprises regions of both cold and warm gas (cold neutral medium and warm neutral medium, or CNM and WNM, respectively) in approximate pressure balance (e.g. McKee & Ostriker 1977; Wolfire et al. 2003).”

“340 P̃ −0.7004 yr given in McKee & Ostriker (1977).”

“In the classical two-phase picture, both phases exist in pressure equilibrium with stable phases for temperatures of T 300 K (CNM) and T 5000 K (WNM) embedded in a hot ionized phase (McKee & Ostriker 1977).”

“Outflows from the AGN will preferentially propagate through the less resistive hot gas that fills the space between the clouds and leave the cold clouds behind instead of blowing them away (e.g., McKee & Ostriker 1977, in the case of supernova blast waves.).”

“3251 n 0.34 0 P̃ −0.70 04 yr (3) given in McKee & Ostriker (1977).”

“Star formation is modeled using a subgrid recipe introduced in Springel & Hernquist (2003) where a gas particle above a density threshold of nH = 0.13 cm −3 is modeled as a fraction of cold clouds embedded in a warm ionized medium following McKee & Ostriker (1977).”

“There are indications that supernova feedback is the most important agent for truncating the star formation process on these scales (Larson 1974; McKee & Ostriker 1977; Korpi et al. 1999; Joung & Mac Low 2006; Dobbs, Burkert & Pringle 2011a).”

“The explosion of these stars is treated using the analytic model for blastwaves presented in McKee & Ostriker (1977) as described in detail in Stinson et al. (2006).”

“McKee & Ostriker (1977) presented an estimate of q for the ISM and found that it was very large, demanding restructuring be present.”

“After shell formation, a pressure-driven snowplow (PDS) stage occurs while the interior hot gas has non-negligible pressure (e.g., Cox 1972; McKee & Ostriker 1977).”

“Norman (1991) and Koo et al. (1992) associate the pervasive H emission with the walls of superbubbles, ‘‘ chimneys,’’ and ‘‘ worms,’’ while McKee & Ostriker (1977) suggest that prominent H emission is confined to the surface of neutral clouds bathed in a hot medium.”

“…the various components (such as multiphase neutral E-mail: barbara.catinella@uwa.edu.au and ionized gas, and dust) of the interstellar medium (ISM; McKee & Ostriker 1977; Cox 2005) and star formation as a function of galaxy properties, environment and cosmic time is a formidable task, which…”

“Galactic H I is an important component of the interstellar medium (McKee & Ostriker 1977), and much e†ort has been devoted to its study (see Burton 1992).”

“The ISM is modelled as multi-phase, with cold clouds embedded in a hotter phase (McKee & Ostriker 1977) which is practically implemented in the code via hybrid SPH particles (Springel & Hernquist 2003).”

“An effect in this direction is also expected in detailed models of the effects of feedback of supernovae on the interstellar medium (McKee & Ostriker 1977; Efstathiou 2000).”

“Photons that escape from the porous interstellar medium (ISM,Clarke & Oey 2002), driven by supernova (SN) explosions (McKee & Ostriker 1977), to the intergalactic medium (IGM) create HII bubbles, which expand as more stars form.”

“Subject headings: galaxies: halos — galaxies: individual (NGC 253, NGC 891, NGC 1482, NGC 3034, NGC 3073, NGC 3079, NGC 3628, NGC 4244, NGC 4631, NGC 4945, NGC 6503) — galaxies: starburst — ISM: bubbles — ISM: jets and outflows — X-rays: galaxies”

“Further, this ubiquity may run contrary to the conventional notion that the surface density of star formation activity must reach a critical threshold in order to launch a large-scale wind (McKee & Ostriker 1977; Heckman et al. 1990; Heckman 2002; Kornei et al. 2012).”

“Following McKee & Ostriker (1977) and Tielens (2005), we can estimate the VFF of the hot phase that we expect for a given SN rate and mean gas density.”

“An Origin of “Turbulence” in Interstellar Clouds Diffuse ISM in the Galaxy is frequently compressed by supernova explosions (McKee & Ostriker 1977).”

“Is the WIM the ionized portion of a low density “intercloud medium” (Miller and Cox, 1993) or is it mostly confined to the surfaces of neutral clouds, the transition region between cooler gas and a much hotter “coronal” temperature medium (McKee and Ostriker, 1977)? Do the ionized and neutral phases cycle from one to…”

“Despite many years of observations of the ISM, our understanding of the medium is still simplistic, relying heavily on a three-phase model (e.g., McKee & Ostriker 1977).”

“The situation is analogous to the hot phase of the interstellar medium (ISM) which is thermally unstable but is maintained by supernova heating (McKee & Ostriker 1977).”

“From the theoretical point of view (Field et al. 1969; McKee & Ostriker 1977; Wolfire et al. 1995) as well as from Appendix is only available in electronic form at http://www.edpsciences.org the observational one (Low et al. 1984; Boulanger & Pérault 1988; Kulkarni & Heiles 1987; Troland & Heiles…”

“Our discussion below will mostly follow Efstathiou (2000; see also McKee and Ostriker 1977).”

“Star formation is modeled using a subgrid recipe introduced in Springel & Hernquist (2003) where a gas particle above a critical density is modeled as a fraction of cold clouds embedded in a warm ionised medium following McKee & Ostriker (1977).”

“The ISM in this regime is thermally unstable (Field 1965; McKee and Ostriker 1977).”

“The shape of the ‘cooling curve’ of diffuse astrophysical plasmas – i.e., the cooling rate as a function of density and temperature – is responsible for the thermal instability that generates a multiphase interstellar medium (McKee & Ostriker 1977; Spitzer 1978).”

“Dense gas above the threshold is modeled as a hybrid of cold, dense clouds embedded in a hot, diffuse medium as envisioned by McKee & Ostriker (1977).”

“Further, the temperature of the shocked ISM is in the right range for thermal instability (McKee & Ostriker 1977), and Richtmyer-Meshkov instabilities (Kane et al. 1999) induced by the forward shock mean that new cold clumps may form in the outflow behind it.”

“TC model suggests that the WIM has about the same properties over the entire Galactic disk.”

“3251 n−0.160 P̃ −0.2004 pc (1) (Chevalier & Gardner 1974; McKee & Ostriker 1977), where E51 = E/1051 erg, n0 is the density in CGS, and P̃04 = 10−4 P/[erg cm−3] k−1B , intended to reflect the radius of the energy-conserving (Sedov) phase of the blastwave (often much larger than our softening…”

“The three-phase (McKee & Ostriker 1977) model so much revolutionized our understanding of the ISM that it naturally led to widespread interest in galaxy formation through intergalactic shocking from starbursts (e.g., Ostriker & Cowie 1981) and even ‘‘quasar explosions’’ (Ikeuchi 1981).”

“III (e.g. Ferriere, 44 1998; Ferrière, 1999, 2001; McKee and Ostriker, 1977) is needed.”

“This value, comparable to the typical size of galactic superbubbles (McKee & Ostriker 1977), is however larger than individual supernovae remmants, whose size just before the snow plow phase varies from 10 to 50 pc, depending on the exact early shock wave dynamics, whether adiabatic or…”

“These values of fh are lower than the estimate by McKee & Ostriker (1977) ( fh ≈ 70%).”

“Following that, the evolution enters a snowplow phase driven by the pressure of a still hot interior gas; McKee & Ostriker 1977; Ostriker & McKee 1988.)”

“In the model of Springel & Hernquist (2003), the effective pressure is motivated by the ISM model in McKee & Ostriker (1977).”

“At low resolution, this energy would quickly radiate away (Katz 1992) in the dense gas, without allowing for an adiabatic expansion of the supernova blast-wave (McKee & Ostriker 1977).”

“While the clouds do largely reside in the plane of the disk (which is in essence a two-dimensional system), cloud collisions and feedback can eject gas from the surface, a process that controls the pressure of the ISM from which the clouds are forming (McKee & Ostriker 1977; Cox 2005).”

“This is because tmax defined in McKee & Ostriker (1977) is roughly the time-scale for the SNR to reach pressure equilibrium with its surroundings.”

“Feedback from supernovae (SNe) is treated using the blastwave model described by Stinson et al. (2006) , which is based on the analytic treatment of blastwaves described by McKee & Ostriker (1977).”