Phases and Processes in the ISM

The Astrophysical Journal, 1995

Much of the interstellar medium in disk galaxies is in the form of neutral atomic hydrogen, H i. This gas can be in thermal equilibrium at relatively low temperatures, Td300 K (the cold neutral medium [CNM]), or at temperatures somewhat less than 10 4 K (the warm neutral medium [WNM]). These two phases can coexist over a narrow range of pressures, P min P P max . We determine P min and P max in the plane of the Galaxy as a function of Galactocentric radius R using recent determinations of the gas heating rate and the gas-phase abundances of interstellar gas. We provide an analytic approximation for P min as a function of metallicity, far-ultraviolet radiation field, and the ionization rate of atomic hydrogen. Our analytic results show that the existence of P min , or the possibility of a two-phase equilibrium, generally requires that H + exceed C + in abundance at P min . The abundance of H + is set by EUV/soft X-ray photoionization and by recombination with negatively charged polycyclic aromatic hydrocarbons. In order to assess whether thermal or pressure equilibrium is a realistic assumption, we define a parameter Ç t cool =t shock , where t cool is the gas cooling time and t shock is the characteristic shock time or '' time between shocks in a turbulent medium.'' For Ç < 1 gas has time to reach thermal balance between supernova-induced shocks. We find that this condition is satisfied in the Galactic disk, and thus the two-phase description of the interstellar H i is approximately valid even in the presence of interstellar turbulence. Observationally, the mean density n H i h i is often better determined than the local density, and we cast our results in terms of n H i h i as well. Over most of the disk of the Galaxy, the H i must be in two phases: the weight of the H i in the gravitational potential of the Galaxy is large enough to generate thermal pressures exceeding P min , so that turbulent pressure fluctuations can produce cold gas that is thermally stable; and the mean density of the H i is too low for the gas to be all CNM. Our models predict the presence of CNM gas to R ' 16 18 kpc, somewhat farther than previous estimates. An estimate for the typical thermal pressure in the Galactic plane for 3 kpcdRd18 kpc is P th =k ' 1:4  10 4 expðÀR=5:5 kpcÞ K cm À3 . At the solar circle, this gives P th =k ' 3000 K cm À3 . We show that this pressure is consistent with the C i*/C i tot ratio observed by Jenkins & Tripp and the CNM temperature found by Heiles & Troland. We also examine the potential impact of turbulent heating on our results and provide parameterized expressions for the heating rate as a function of Galactic radius. Although the uncertainties are large, our models predict that including turbulent heating does not significantly change our results and that thermal pressures remain above P min to R ' 18 kpc.

The Astronomical …, 2005

New and archival interferometric 12 CO(1→0) datasets from six nearby galaxies are combined with H 2 2.122 µm and Hα maps to explore in detail the interstellar medium in different star-forming galaxies. We investigate the relation between warm (H 2 at T ∼ 2000 K) and cold (CO at T ∼ 50 K) molecular gas from 100 pc to 2 kpc scales. On these scales, the ratio of warm-to-cold molecular hydrogen correlates with the fν (60µm) fν (100µm) ratio, a ratio that tracks the star formation activity level. This result also holds for the global properties of galaxies from a much larger sample drawn from the literature. The trend persists for over three orders of magnitude in the mass ratio, regardless of source nuclear activity.

2009

1999

The thermal and chemical phases of the cool component of interstellar gas are discussed. Variations with galactocentric radius and from galaxy to galaxy are mostly the result of changes in the ambient interstellar pressure and radiation field. Interstellar structure that is hierarchical or fractal in the cloudy parts and has large and connected empty regions between these clouds is probably the result of turbulence. Such structure opens up the disk to the transmission of OB star light into the halo, and it provides for a diffuse ionized component that tapers away gradually from each dense HII region. Fractal cloud structure may also produce the cloud and clump mass functions, and perhaps even the star cluster mass function.

2017

Astronomy and Astrophysics, 2007

Aims. We present new radio continuum observations of ten BIMA SONG galaxies, taken at 1.4 GHz with the Very Large Array. These observations allow us to extend the study of the relationships between the radio continuum (RC) and CO emission to 22 CO luminous galaxies for which single dish CO images have been added to interferometric data. New Spitzer infrared (IR) images of six of these galaxies have been released. The analysis of these high resolution images allowed us to probe the RC-IR-CO correlations down to linear scales of a few hundred pc. Methods. We compare the point-by-point RC, CO and mid-IR intensities across entire galaxy disks, producing radial profiles and spatially resolved images of the RC/CO and RC/mid-IR ratios. Results. For the 22 galaxies analysed, the RC-CO correlation on scales from ∼ 10 kpc down to ∼ 100 pc is nearly linear and has a scatter of a factor of two, i.e. comparable to that of the global correlations. There is no evidence for any severe degradation of the scatter below the kpc scale. This also applies to the six galaxies for which high-resolution mid-IR data are available. In the case of NGC 5194, we find that the non-thermal radio spectral index is correlated with the RC/FIR ratio. Conclusions. The scatter of the point-by-point correlations does not increase significantly with spatial resolution. We thus conclude that we have not yet probed the physical scales at which the correlations break down. However, we observe local deviations from the correlations in regions with a high star formation rate, such as the spiral arms, where we observe a flat radio spectrum and a low RC/FIR ratio. In the intra-arm regions and in the peripheral regions of the disk, the RC/FIR is generally higher and it is characterized by a steepening of the radio spectrum.

The Astrophysical Journal, 2000

We investigate numerically the role of thermal instability (TI) as a generator of density structures in the interstellar medium (ISM), both by itself and in the context of a globally turbulent medium. We consider three sets of numerical simulations: a) flows in the presence of the instability only; b) flows in the presence of the instability and various types of turbulent energy injection (forcing), and c) models of the ISM including the magnetic field, the Coriolis force, self-gravity and stellar energy injection. Simulations in the first group show that the condenstion process which forms a dense phase ("clouds") is highly dynamical, and that the boundaries of the clouds are accretion shocks, rather than static density discontinuities. The density histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak at low densities plus a slope change at high densities. Final static situations may be established, but the equilibrium is very fragile: small density fluctuations in the warm phase require large variations in that of the cold phase, probably inducing shocks into the clouds. Combined with the likely disruption of the clouds by Kelvin-Helmholtz instability (Murray et al. 1993), this result suggests that such configurations are highly unlikely. Simulations in the second group show that large-scale turbulent forcing is incapable of erasing the signature of the TI in the density PDFs, but small-scale, stellar-like forcing causes the PDFs to transit from bimodal to a single-slope power law, erasing the signature of the instability. However, these simulations do not reach stationary regimes, the TI driving an ever-increasing star formation rate. Simulations in the third group show no significant difference between the PDFs of stable and unstable cases, and reach stationary regimes, suggesting that the combination of the stellar forcing and the extra effective pressure provided by the magnetic field and the Coriolis force overwhelm the TI as a density-structure generator in the ISM, the TI becoming a second-order effect. We emphasize that a multi-modal temperature PDF is not necessarily an indication of a multi-phase medium, which must contain clearly distinct thermal equilibrium phases, and that this "multi-phase" terminology is often inappropriately used.

Astronomy &amp; Astrophysics, 2014

This paper is a numerical study of the condensation of the warm neutral medium (WNM) into cold neutral medium (CNM) structures under the effect of turbulence and thermal instability. It addresses the specific question of the CNM formation in the physical condition of the local interstellar medium (ISM). We use the properties of the H  deduced from observations and theoretical work to constrain the physical conditions in the WNM. Using low resolution simulations we explored the impact of the WNM initial density and properties of the turbulence (stirring in Fourier with a varying mix of solenoidal and compressive modes) on the cold gas formation to identify the parameter space that is compatible with the well established observational constraints of the H  in the local ISM. Two sets of initial conditions that match the observed quantity of CNM in mass were selected to produce high resolution simulations (1024 3) that allowed the properties of the produced dense structures to be studied in detail. We show that for typical values of the density, pressure and velocity dispersion of the WNM in the solar neighborhood, the turbulent motions of the H  cannot provoke the phase transition from WNM to CNM, regardless of their amplitude and their distribution in solenoidal and compressive modes. On the other hand, a quasi-isothermal increase in WNM density of a factor of 2 to 4 is enough to induce the phase transition, leading to the transition of about 40% of the gas to the cold phase within 1 Myr. This suggests that turbulence only induces the formation of the CNM when the WNM is pressured and put in a thermally unstable state. At the same time turbulence is regulating the formation of the CNM by preventing some of the WNM from moving toward the cold phase; indeed, tests performed on decaying simulations have shown that the fraction of CNM increases slowly in the decaying phase. In general, these results show that turbulence is playing a key role in the structure of the cold medium. The high resolution simulations show that the velocity field shows evidence of subsonic turbulence with a 2D power spectrum following a power law (P(k) ∝ k −2.4) close to Kolmogorov (P(k) ∝ k −2.67), while the density is highly contrasted with a significantly shallower 2D power spectrum (P(k) ∝ k −1.3), reminiscent of what is observed in the cold ISM. The cold structures denser than 5 cm −3 are characterized by power laws, M ∝ L 2.25 and σ(|u|) ∝ L 0.41 , that are similar to the ones observed in molecular clouds. The CNM structures are sub-or transonic, and their dynamic is tighly coupled to the WNM velocity field with a clump-to-clump velocity dispersion close to the velocity dispersion of the WNM. From this we conclude that suprathermal linewidth for CNM, inferred from 21 cm observations, might be the result of relative velocity between cold structures along the line of sight.

The Astrophysical Journal, 2001

The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/Blue ratios (indicating star-formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L [CII] /L FIR with increasing F ν (60 µm)/F ν (100 µm) and L FIR /L B , spanning a range of a factor of more than 50. Two galaxies, at the warm and active extreme of the range have L [CII] /L FIR < 2 × 10 −4 (3σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L [OI] /L [CII] shows a tight correlation with F ν (60 µm)/F ν (100 µm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II](122 µm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. (1999) with observed ratios of (a) L [OI] /L [CII] and (L [CII] + L [OI])/L FIR and (b) L [OI] /L FIR and F ν (60 µm)/F ν (100 µm) yields far-UV flux G 0 and gas density n. The G 0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5 respectively in more than half the sources. The derived G 0 and n correlate with each other, and G 0 increases with n as G 0 ∝ n α , where α ≈ 1.4. We interpret this correlation as arising from Strömgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270 − 900 K) and pressure (6 × 10 4 − 1.5 × 10 7 K cm −3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G 0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G 0 and n have larger and/or denser star-forming clouds.