Interstellar Pressures Research Articles

The Orion OB Association as a Generator for the Hot Circumgalactic Medium

Feedback from star formation may play a key role in energizing the hot, diffuse, X-ray emitting circumgalactic medium (CGM). We observed the diffuse hot gas on the interior of the Orion–Eridanus Superbubble (OES) produced by feedback from the Orion OB association. Using HaloSat, a CubeSat X-ray observatory, we cover the majority of the OES using 11 HaloSat fields, each with a 10° diameter. We find the gas is well described by two thermal plasma components. There are regions of enhanced emission measure (EM) that coincide with the Eridanus X-ray Enhancement and the Orion OB association. Individual field temperatures are statistically consistent with the weighted average of all of the OES fields: a warm temperature kT w = 0.17 ± 0.02 keV and a hot temperature kT h = 0.79 ± 0.12 keV. The gas is overpressured in comparison with typical interstellar medium pressures, and the rate of energy injected by Orion OB1 can sufficiently power growth of the superbubble. The gas’s radiative cooling timescale (∼30 Myr) is long in comparison with the rate of hot gas production. The temperatures and EMs of the gas agree with properties of the bulk CGM elsewhere in the Milky Way. If we take the OES as a typical superbubble, these factors together suggest that the hot CGM is energized by star formation activity.

AN EXPANDEDHST/WFC3 SURVEY OF M83: PROJECT OVERVIEW AND TARGETED SUPERNOVA REMNANT SEARCH

We present an optical/NIR imaging survey of the face-on spiral galaxy M83, using data from the Hubble Space Telescope Wide Field Camera 3 (WFC3). Seven fields are used to cover a large fraction of the inner disk, with observations in nine broadband and narrowband filters. In conjunction with a deep Chandra survey and other new radio and optical ground-based work, these data enable a broad range of science projects to be pursued. We provide an overview of the WFC3 data and processing and then delve into one topic, the population of young supernova remnants. We used a search method targeted toward soft X-ray sources to identify 26 new supernova remnants. Many compact emission nebulae detected in [Fe II] 1.644 micron align with known remnants and this diagnostic has also been used to identify many new remnants, some of which are hard to find with optical images. We include 37 previously identified supernova remnants that the data reveal to be <0.5'' in angular size and thus are difficult to characterize from ground-based data. The emission line ratios seen in most of these objects are consistent with shocks in dense interstellar material rather than showing evidence of ejecta. We suggest that the overall high elemental abundances in combination with high interstellar medium pressures in M83 are responsible for this result. Future papers will expand on different aspects of the these data including a more comprehensive analysis of the overall supernova remnant population.

Power-law models of totally anisotropic scattering

The interstellar scattering responsible for pulsar parabolic arcs, and for intra-day variability of compact radio quasars, is highly anisotropic in some cases. We numerically simulate these observed phenomena using totally anisotropic, power-law models for the electron density fluctuations which cause the scattering. By comparing our results to the scattered image of PSR B0834+06 and, independently, to dual-frequency light curves of the quasar PKS1257-326, we constrain the nature of the scattering media on these lines of sight. We find that models with spectral indices slightly below \beta=3, including the one-dimensional Kolmogorov model, are broadly consistent with both data sets. We confirm that a single physical model suffices for both sources, with the scattering medium simply being more distant in the case of B0834+06. This reinforces the idea that intra-day variability and parabolic arcs have a common cause in a type of interstellar structure which, though obscure, is commonplace. However, the implied gas pressure fluctuations are large compared to typical interstellar pressures, and the magnetic stresses are much larger still. Thus while these scattering media may be commonplace, their underlying dynamics appear quite extraordinary.

Time-Variability in the Interstellar Boundary Conditions of the Heliosphere: Effect of the Solar Journey on the Galactic Cosmic Ray Flux at Earth

During the solar journey through galactic space, variations in the physical properties of the surrounding interstellar medium (ISM) modify the heliosphere and modulate the flux of galactic cosmic rays (GCR) at the surface of the Earth, with consequences for the terrestrial record of cosmogenic radionuclides. One phenomenon that needs studying is the effect on cosmogenic isotope production of changing anomalous cosmic ray fluxes at Earth due to variable interstellar ionizations. The possible range of interstellar ram pressures and ionization levels in the low density solar environment generate dramatically different possible heliosphere configurations, with a wide range of particle fluxes of interstellar neutrals, their secondary products, and GCRs arriving at Earth. Simple models of the distribution and densities of ISM in the downwind direction give cloud transition timescales that can be directly compared with cosmogenic radionuclide geologic records. Both the interstellar data and cosmogenic radionuclide data are consistent with cloud transitions during the Holocene, with large and assumption-dependent uncertainties. The geomagnetic timeline derived from cosmic ray fluxes at Earth may require adjustment to account for the disappearance of anomalous cosmic rays when the Sun is immersed in ionized gas.

Metallicities and Physical Conditions in Star‐forming Galaxies atz∼ 1.0–1.51

We present a study of the mass-metallicity (M-Z) relation and H II region physical conditions in a sample of 20 star-forming galaxies at 1.0 < z < 1.5 drawn from the DEEP2 Galaxy Redshift Survey. We find a correlation between stellar mass and gas-phase oxygen abundance in the sample and compare it with the one observed among UV-selected z ∼ 2 star-forming galaxies and local objects from the Sloan Digital Sky Survey (SDSS). This comparison, based on the same empirical abundance indicator, demonstrates that the zero point of the M-Z relationship evolves with redshift, in the sense that galaxies at fixed stellar mass become more metal-rich at lower redshift. Measurements of [O III]/Hβ and [N II]/Hα emission-line ratios show that, on average, objects in the DEEP2 1.0 < z < 1.5 sample are significantly offset from the excitation sequence observed in nearby H II regions and SDSS emission-line galaxies. In order to fully understand the causes of this offset, which is also observed in z ∼ 2 star-forming galaxies, we examine in detail the small fraction of SDSS galaxies that have similar diagnostic ratios to those of the DEEP2 sample. Some of these galaxies indicate evidence for AGN and/or shock activity, which may give rise to their unusual line ratios and contribute to Balmer emission lines at the level of ~20%. Others indicate no evidence for AGN or shock excitation yet are characterized by higher electron densities and temperatures, and therefore interstellar gas pressures, than typical SDSS star-forming galaxies of similar stellar mass. These anomalous objects also have higher concentrations and starformation rate surface densities, which are directly connected to higher interstellar pressure. Higher star formation rate surface densities, interstellar pressures, and H II region ionization parameters may also be common at high redshift. These effects must be taken into account when using strong-line indicators to understand the evolution of heavy elements in galaxies. When such effects are included, the inferred evolution of the M-Z relation out to z ∼ 2 is more significant than previous estimates.

The Distribution of Pressures in a Supernova‐driven Interstellar Medium. I. Magnetized Medium

Observations have suggested substantial departures from pressure equilibrium in the interstellar medium (ISM) in the plane of the Galaxy, even on scales under 50 pc. Nevertheless, multiphase models of the ISM assume at least locally isobaric gas. The pressure then determines the density reached by gas cooling to stable thermal equilibrium. We use numerical models of the magnetized ISM to examine the consequences of supernova driving for interstellar pressures. In this paper we examine a (200 pc)3 periodic domain threaded by magnetic fields. Individual parcels of gas at different pressures reach widely varying points on the thermal equilibrium curve: no unique set of phases is found but rather a dynamically determined continuum of densities and temperatures. A substantial fraction of the gas remains entirely out of thermal equilibrium. Our results appear consistent with observations of interstellar pressures. They also suggest that the high pressures observed in molecular clouds may be due to ram pressures in addition to gravitational forces. Much of the gas in our model lies far from equipartition between thermal and magnetic pressures, with ratios ranging from 0.1 to 104 and ratios of uniform to fluctuating magnetic field of 0.5-1. Our models show broad pressure probability distribution functions with log-normal functional forms produced by both shocks and rarefaction waves rather than power-law distributions produced by isolated supernova remnants. The width of the distribution can be described quantitatively by a formula derived from the work of Padoan, Nordlund, & Jones.

Molecular Cloud Formation behind Shock Waves

Motivated by our previous paper, in which we argued for the formation of molecular clouds from large-scale flows in the diffuse Galactic interstellar medium, we examine the formation of molecular gas behind shocks in atomic gas using a one-dimensional chemical/dynamical model. In our analysis we place particular emphasis on constraints placed on the dynamical evolution by the chemistry. The most important result of this study is to stress the importance of shielding the moleculargas from the destructive effects of UVradiation. For shock ram pressures comparable to or exceeding typical local interstellar medium pressures, self-shielding controls the formation time of molecular hydrogen, but CO formation requires shielding of the interstellar radiation field by dust grains. We find that for typical parameters the molecular hydrogen fractional abundance can become significant well before CO forms. The timescale for (CO) molecular cloud formation is not set by the H2 formation rate on grains, but rather by the timescale for accumulating a sufficient column density or extinction, AV k0:7. The local ratio of atomic to molecular gas (4:1), coupled with short estimates for the lifetimes of molecular clouds (3‐5 Myr), suggests that the timescales for accumulating molecular clouds from atomic material typically must be no longer than about 12‐20 Myr. Based on the shielding requirement, this implies that the typical product of preshock density and velocity must benvk20 cm � 3 km s � 1 . In turn, depending on the shock velocity, this implies shock ram pressures that are a few times the typical estimated local turbulent gas pressure and comparable to the total pressures(gasplusmagneticpluscosmicrays).CoupledwiththerapidformationofCOonceshieldingissufficient, flow-driven formation of molecular clouds in the local interstellar medium can occur sufficiently rapidly to account for observations. We also provide detailed predictions of atomic and molecular emission and absorption that track the formation of a molecular cloud from a purely atomic medium, with a view toward helping to verify cloud formation by shock waves. However, our predictions suggest that the detection of the pre-CO stages will be challenging. Finally, we provide an analytic solution for time-dependent H2 formation that may be of use in numerical hydrodynamic calculations. Subject headingg ISM: clouds — ISM: evolution — ISM: kinematics and dynamics — ISM: molecules — shock waves — stars: formation

Deuterium enrichment of polycyclic aromatic hydrocarbons by photochemically induced exchange with deuterium-rich cosmic ices.

The polycyclic aromatic hydrocarbon (PAH) coronene (C24H12) frozen in D2O ice in a ratio of less than 1 part in 500 rapidly exchanges its hydrogen atoms with the deuterium in the ice at interstellar temperatures and pressures when exposed to ultraviolet radiation. Exchange occurs via three different chemical processes: D atom addition, D atom exchange at oxidized edge sites, and D atom exchange at aromatic edge sites. Observed exchange rates for coronene (C24H12)-D2O and d12-coronene (C24D12)-H2O isotopic substitution experiments show that PAHs in interstellar ices could easily attain the D/H levels observed in meteorites. These results may have important consequences for the abundance of deuterium observed in aromatic materials in the interstellar medium and in meteorites. These exchange mechanisms produce deuteration in characteristic molecular locations on the PAHs that may distinguish them from previously postulated processes for D enrichment of PAHs.