CLOUDY is a large‐scale spectral synthesis code designed to simulate fully physical conditions within an astronomical plasma and then predict the emitted spectrum. Here we describe version 90 (C90) of the code, paying particular attention to changes in the atomic database and numerical methods that have affected predictions since the last publicly available version, C84. The computational methods and uncertainties are outlined together with the direction future development will take. The code is freely available and is widely used in the analysis and interpretation of emission‐line spectra. Web access to the Fortran source for CLOUDY, its documentation Hazy, and an independent electronic form of the atomic database is also described.

https://iopscience.iop.org/article/10.1086/316190/pdf

CLOUDY 90: Numerical Simulation of Plasmas and Their Spectra

We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxies (LIR ≥ 1012 L☉). We combine the survey results with a detailed case study, based on arcsecond resolution, near-IR, and millimeter imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultraluminous galaxies to ISO and literature data of 30 starburst and active galactic nuclei (AGN) template galaxies. We find the following: 1. Of the ultraluminous IRAS galaxies in our sample, 70%-80% are predominantly powered by recently formed massive stars, and 20%-30% are powered by a central AGN. These conclusions are based on a new infrared diagnostic diagram involving the ratio of high- to low-excitation mid-IR emission lines on the one hand, and the strength of the 7.7 μm PAH feature on the other hand. 2. At least half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circumnuclear disk/ring. 3. The mid-IR emitting regions are highly obscured [Av(screen) ~ 5-50 or Av(mixed) ~ 50-1000]. In a model where star-forming regions and dense molecular clouds are fully mixed, the ISO-derived, V-band dust extinctions approach the dust column densities inferred from CO millimeter measurements. After correction for these extinctions, we estimate that the star-forming regions in ultraluminous infrared galaxies have ages between 107 and 108 yr, similar to but somewhat larger than those found in lower luminosity starburst galaxies. 4. In the sample we have studied there is no obvious trend for the AGN component to dominate in the most compact, and thus most advanced mergers. Instead, at any given time during the merger evolution, the time-dependent compression of the circumnuclear interstellar gas, the accretion rate onto the central black hole, and the associated radiation efficiency may determine whether star formation or AGN activity dominates the luminosity of the system.

https://iopscience.iop.org/article/10.1086/305576/pdf

What Powers Ultraluminous IRAS Galaxies

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, PminPPmax. We determine Pmin and Pmax 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 Pmin as a function of metallicity, far-ultraviolet radiation field, and the ionization rate of atomic hydrogen. Our analytic results show that the existence of Pmin, or the possibility of a two-phase equilibrium, generally requires that H + exceed C + in abundance at Pmin. 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 � � tcool=tshock, where tcool is the gas cooling time and tshock is the characteristic shock time or '' time between shocks in a turbulent medium.'' For � < 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 nHi hi is often better determined than the local density, and we cast our results in terms of nHi hi as well. Over most of the disk of the Galaxy, the Hi must be in two phases: the weight of the Hi in the gravitational potential of the Galaxy is large enough to generate thermal pressures exceeding Pmin, 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 Pth=k ' 1:4 � 10 4 expð� R=5:5 kpcÞ Kc m � 3 . At the solar circle, this gives Pth=k ' 3000 K cm � 3 . We show that this pressure is consistent with the C i*/C itot 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 Pmin to R ' 18 kpc. Subject headings: ISM: clouds — ISM: general — ISM: structure

https://iopscience.iop.org/article/10.1086/368016/pdf

Neutral Atomic Phases of the Interstellar Medium in the Galaxy

We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key program that will map the inner Galactic plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2° × 2° tiles approximately centered at l = 30° and l = 59°. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores' in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around AV ~ 1 is exceeded for the regions in the l = 59° field; a AV value between 5 and 10 is found for the l = 30° field, likely due to the relatively higher distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm-2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.

/pdf/clouds-filaments-and-protostars-the-herschel-hi-gal-milky-4cbi9k6503.pdf

Clouds, filaments, and protostars: The Herschel Hi-GAL Milky Way

The mass of molecular gas in an interstellar cloud is often measured using line emission from low rotational levels of CO, which are sensitive to the CO mass, and then scaling to the assumed molecular hydrogen H2 mass. However, a significant H2 mass may lie outside the CO region, in the outer regions of the molecular cloud where the gas-phase carbon resides in C or C+. Here, H2 self-shields or is shielded by dust from UV photodissociation, whereas CO is photodissociated. This H2 gas is "dark" in molecular transitions because of the absence of CO and other trace molecules, and because H2 emits so weakly at temperatures 10 K <T < 100 K typical of this molecular component. This component has been indirectly observed through other tracers of mass such as gamma rays produced in cosmic-ray collisions with the gas and far-infrared/submillimeter wavelength dust continuum radiation. In this paper, we theoretically model this dark mass and find that the fraction of the molecular mass in this dark component is remarkably constant (~0.3 for average visual extinction through the cloud ) and insensitive to the incident ultraviolet radiation field strength, the internal density distribution, and the mass of the molecular cloud as long as , or equivalently, the product of the average hydrogen nucleus column and the metallicity through the cloud, is constant. We also find that the dark mass fraction increases with decreasing , since relatively more molecular H2 material lies outside the CO region in this case.

https://iopscience.iop.org/article/10.1088/0004-637X/716/2/1191/pdf

The Dark Molecular Gas

A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.

D. Hollenbach

Papers

Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region