Warm Interstellar Medium Research Articles

Enhanced Hα Emission in Early-Type Galaxies Belonging to Compact Groups

The results of Hα photometry of a sample of early-type galaxies belonging to nearby compact groups are presented. Over 75% of the galaxies in the sample were detected in Hα, a result that we have interpreted as proof of the presence of a warm interstellar medium within them. Comparing the Hα luminosities of the galaxies in our sample with those of a sample of early-type galaxies from the field, we have found that field early-type galaxies are systematically fainter in Hα than the compact group ones for a given B absolute magnitude by Δlog(LHα/LB) ≈ 0.5. We suggest that the excess in the Hα emission within the groups is due to photoionization of gas by Lyman continuum photons emitted by massive stars. Recent massive star formation would be a consequence of the accretion by early-type galaxies of gas from the outer envelopes of gas-rich galaxies experiencing close passages across the group.

The Polarizing Power of the Interstellar Medium in Taurus

We present a study of the polarizing power of the dust in cold dense regions (dark clouds) compared to that of dust in the general interstellar medium (ISM). Our study uses new polarimetric, optical, and spectral classification data for 36 stars to carefully study the relation between polarization percentage (p) and extinction (A_V) in the Taurus dark cloud complex. We find two trends in our p-A_V study: (1) stars background to the warm ISM show an increase in p with A_V; and (2) the percentage of polarization of stars background to cold dark clouds does not increase with extinction. We detect a break in the p-A_V relation at an extinction 1.3 +/- 0.2 mag, which we expect corresponds to a set of conditions where the polarizing power of the dust associated with the Taurus dark clouds drops precipitously. This breakpoint places important restrictions on the use of polarimetry in studying interstellar magnetic fields.

Interstellar Dust in the Solar System

A fraction of the dust in the interstellar medium (ISM) enters the heliosphere and is measured in situ from spacecraft. This review surveys the in situ measurements and discusses a hence derived model of dust in the local interstellar cloud (LIC). The LIC dust model bears similarities to pristine cometary dust and is characteristic of the warm ISM clouds that fill a part of the ISM in the vicinity of the Sun. Recent and future dust in situ measurements provide a basis for closely studying physical processes in the ISM surrounding the Solar System. The LIC dust is the only dust component measurable in the Solar System that was not previously incorporated in larger Solar System objects. Issues for future LIC dust studies are measuring dust fluxes at the outer heliosphere, measuring the mass distribution with meteor observations and observations from spacecraft, and measuring the LIC dust composition in situ in space.

Kinetic theory of cosmic rays and gamma rays in supernova remnants. I. Uniform interstellar medium

Abstract Kinetic models of particle acceleration in supernova remnants (SNRs) are used to determine the cosmic ray (CR) nucleon and, for the first time, also the associated γ-ray spectrum during SN shock propagation in a uniform interstellar medium. SNR evolution is followed numerically taking into account the backreaction of accelerated CRs on the overall dynamics. The high energy CRs also produce π0-decay γ-rays. The model for SNRs includes injection of suprathermal particles at the shock front and heating of the thermal plasma due to the dissipation of Alfven waves in the precursor region. It is shown that the CRs are accelerated with very high efficiency. About 50% of the explosion energy is absorbed by CRs at maximum during the SNR evolution even for relatively low injection rates. The maximum energy achieved by accelerated CR protons is about 1014 eV for a Bohm-limit diffusion coefficient. The main flux of high energy γ-rays is produced during the early Sedov phase and decreases thereafter. The results are compared with earlier models based on the hydrodynamic approximation for CR transport and test particle estimates. For a moderate ambient gas density of 0.3 H-atoms cm−3, corresponding to a warm interstellar medium, a magnetic field strength of 5 μG, and an explosion energy of 1051 erg, the integral TeV γ-ray flux from a SNR at a distance of 1 kpc exceeds 10−11 photons cm−2s−1 at peak luminosity. This is higher than previously estimated peak fluxes by factors of the order of 7, and is primarily due to the large shock compression during the sweep-up phase. Spatially, SNRs without a central compact object are shell sources also in γ-rays throughout all their evolutionary phases. Given that such SNRs exist close-by they should be observable in particular with sensitive ground based instruments.

Abundance Patterns of Heavy Elements in Damped L[CLC]yα[/CLC] Galaxies

We present a quantitative analysis of the abundances of heavy elements in damped Lyα galaxies in the sample of Lu et al. In particular, we compare the observed gas-phase abundances with those expected when the intrinsic (i.e., nucleosynthetic) pattern is the same as that in either the Sun or in Galactic halo stars and when the depletion pattern is the same as that in the warm Galactic interstellar medium, but with various values of the dust-to-metals ratio. We find that the observations are equally consistent with the solar and halo-star intrinsic patterns and that they favor some depletion, the typical dust-to-metals ratio being 40%-90% of that in the Milky Way today. However, neither intrinsic pattern matches the observations perfectly. For the solar pattern, the discrepancy is mainly with [Mn/Fe], while for the halo-star pattern, the discrepancy is with [Zn/Fe], [Ni/Fe], and possibly [Al/Fe]. Our analysis does not support the claim by Lu et al. that the damped Lyα galaxies have halo-star abundance patterns and no dust depletion.

Global Models of the Interstellar Medium in Disk Galaxies

We have computed six two-dimensional simulations with two cospatial, interacting fluids that represent the stars and gas in a disk galaxy. The two fluids interact through star formation, mass loss, stellar winds, and supernovae, and the gas includes an optically thin cooling function. Our previous simulations have been able to produce the multiphase nature and the overall topology of the cold and warm interstellar medium (ISM) and here we extend the previous work by allowing for million-degree gas and an impulsive form of heating that represents supernovae. The six simulations differ in the form and rates of energy injection; for simulations with heating only from stellar winds and from both supernovae and stellar winds, we have run three simulations with low, moderate and high energy injection rates (with a factor of 4 difference between adjacent rates), where the moderate energy injection rate corresponds to that of the Galaxy. The three values of Edot in our simulations correspond to 2 (low), 8 (moderate), and 32 (high) × 10<SUP>41</SUP> ergs s<SUP>-1</SUP> for a disk of 16 kpc and are equivalent to supernova rates of 0.0075, 0.03, and 0.12 yr<SUP>-1</SUP>, respectively (assuming E<SUB>SN</SUB> = 6 × 10<SUP>50</SUP> ergs, and a ratio of energy released from supernovae to that from stellar winds of 3:1). We use a grid that is 2 kpc across and ±15 kpc in the vertical direction and impose a constant gravitational potential along this direction. <P />Our simulations create a three-phase medium with filaments of dense, cold and warm gas surrounding bubbles of hot gas, which are usually hundreds of parsecs across and can exceed 1 kpc in size. This filamentary topology is very similar to that inferred for our Galaxy and others, based largely on H I observations. The evolution of the cold, dense filaments is dominated by a loss of identity from filament-filament collisions, although in higher energy injection cases a Rayleigh-Taylor instability can cause a filament to fragment. <P />We calculate central densities, scale heights for the density, and filling factors of the three phases of gas, and demonstrate that the vertical density distribution of each phase is usually best fit with more than one component. The calculated central densities, scale heights, and filling factors for each gas phase reproduce the observational values made for the Galaxy in the moderate energy injection rate simulations. Also, the computed median pressure and pressure scale height best reproduce the Galactic values in the moderate energy injection rate simulations. <P />Velocity information for the cold (or neutral) gas is analyzed at one or two times in each simulation. The occasional multicomponent nature of H I emission profiles and the holes of H I in position-velocity plots occur in the simulations, although we fail to recreate quantitatively the amount and velocities of high-velocity gas that are observed in the Galaxy. Specifically, only the highest energy injection rate cases have significant amounts of neutral gas (more than 0.25% of the mass of cold gas) at |υ| ≥ 50 km s<SUP>-1</SUP>, and the maximum velocities are smaller than those observed in H I (≳100 km s<SUP>-1</SUP>) We attribute these shortcomings to numerical viscosity in the simulations. <P />Further analysis of the cold gas velocities reveals an anticorrelation between the column density of cold gas and its velocity dispersion for a galaxy viewed face-on. Also, one calculation of the net mass flux of each gaseous phase as a function of height demonstrates that most of the hot gas is rising, while cooler gas is falling, which is consistent with the galactic fountain model. <P />Varying the rate of energy injection has a large effect on the nature of the ISM, especially the extent of the cold gas, so we can constrain to a relatively narrow range the energy injection rate within the Galaxy. Our results suggest that neither the low-Edot simulations nor the high-Edot case can reproduce the distribution (i.e., central densities, scale heights, and filling factors) of the multiphase ISM in the Galaxy, although the high-E simulations are more effective at reproducing the high-velocity H I observed in the Galaxy.

Properties of the highly ionized disk and halo gas toward two distant high-latitude stars

Goddard High Resolution Spectrograph (GHRS) intermediate -resolution observations of S III, Si III, Al III, Si IV, C IV, and N V absorption along the sight lines to HD 18100 (l = 217.9 deg, b = -62.7, d = 3.1 kpc, z = -2.8 kpc) and HD 100340 (l = 258.9 deg, b = +61.2 deg, d = 5.3 kpc, z = 4.6 kpc) are presented. These small science aperture spectra have resolutions ranging from 11 to 20 km/s full width at half maximum (FWHM) and S/N from 30 to 65 per diode substep. Strong absorption by moderately and highly ionized gas is seen in each direction. The absorption in the direction of the south Galactic polar region (HD 18100) is kinematically simple, while the absorption in the direction of north Galactic polar region (HD 100304) is kinematically complex. In each case the absorption by the highly ionized gas lies within the velocity range of absorption by neutral and weakly ionized gas. Along each sight line, the velocity dispersion determined from the unsaturated absorption lines increases with the energy required to create each ion. The logarithmic column densities for Al III, Si IV, C IV, and N V are log N(atoms/sq cm = 12.71, 13.10, 13.58, and 12.75 toward HD 18100 and log N = 12.88, 13.31, 13.83, and 13.04 toward HD 100340. Average ionic ratios among these species are very similar along the two sight lines. Differences in profile shape between the absorption for AL II, Si IV, C IV, and N V provide additional support for the claim of Savage, Sembach, & Cardelli (1994) that there exists two types of highly ionized gas in the interstellar medium. One type of highly ionized gas is responsible for the structured Si IV absorption and part of the C IV absorption. In this gas N(C IV)/N(Si IV) approximately 3.0 and N(C IV)/N(N V) greater than 6. The absorption by this gas seems to be associated with some type of self-regulating interface or mixing layer between the warm and hot interstellar medium. The other type of highly ionized gas is responsible for most of the N V absorption, part of the C IV absorption, and has very little associated Si IV absorption. In this gas N(C IV)/N(N V) is approximately 1 to 3. This gas is hot (T greater than 2 x 10(exp 5) K) and may be tracing the cooling gas of supernova (SN) bubbles or a Galactic fountain. The relative mixture of these two types of highly ionized gas varies from one sight line to the next. The two sight lines in this study sample halo gas in the solar neighborhood and have a smaller percentage of the more highly ionized gas than inner Galaxy sight lines.

Highly ionized gas absorption in the disk and halo toward HD 167756 at 3.5 kilometers per second resolution

High-resolution spectra of interstellar Si IV, C IV, and N V absorption lines along the 4 kpc path to the inner Galaxy star HD 167756 at z = -0.85 kpc are presented. The spectra were obtained with the echelle mode of Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) and have signal-to-noise ratios ranging from 23 to 38. The high resolution of the measurements full width at half maximum (FWHM = 3.5 km/s) results in fully resolved line profiles for the highly ionized gas absorption. The measurements provide information on the column density per unit velocity, N(v), as a function of velocity for Si IV, C IV, and N V. The C IV and N V profiles extend from -70 to +70 km/s, while the Si IV profiles extend from -40 to +70 km/s. The integrated logarithmic column densities are long N(Si IV) = 13.09 +/- 0.02, log N(C IV) = 13.83 +/- 0.02, and log N(N V) = 13.56 +/- 0.03. The N V profile is broad, asymmetric, and featureless, while the Si IV profile contains narrow absorption components near V(sub LSR) = -19, 0, +20, and +52 km/s with Doppler spread parameters, b about = 10-12 km/s. The C IV profile contains both broad and narrow structure. The high ion feature near +52 km/s is also detected in the low-ionization lines of Ca II, O I, Si II, and Fe II. The other narrow Si IV and C IV components occur within several km/s of components seen in low-ionization species. The sight line contains at least two types of highly ionized gas. One type gives rise to a broad N V profile, and the other results in the more structured Si IV profile. The C IV profile contains contributions from both types of highly ionized gas. The broad but asymmetric N V profile is well represented by a large Galactic scale height gas which is participating in Galactic rotation and has a combination of thermal and turbulent broadening with b(sub tot) about = 42 km/s. The C IV to N V abundance ratio of 1.0 +/- 0.3 for the gas implies T about 1.6 x 10(exp 5) K or about 8 x 10(exp 5) K if the gas is in collisional ionization equilibrium and has a solar carbon to nitrogen abundance ratio. This absorption may be associated with cooling hot gas situated in Galactic shells and supershells along the sight line. The gas producing the narrow Si IV and C IV absorption components has line widths that are compatible with origins in conductive interfaces between the warm and hot interstellar medium. Kinematic flows associated with the photoionized edges of clouds might also produce Si IV and C IV lines with Doppler spread parameters similar to those observed, but the C IV to Si IV ratio in this gas is 3.5, which leads us to favor the conductive interface interpretation.

Neutrino decay and the heat budget of the warm interstellar medium

view Abstract Citations (22) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Neutrino Decay and the Heat Budget of the Warm Interstellar Medium Sciama, D. W. Abstract Although neutrino decay photons may be the primary source of the widespread ionization of the warm interstellar medium, and of its cooling due to the collisional excitation of C II, they are too soft to be its main heating agent. The possibility that photoelectric emission by polycyclic aromatic hydrocarbons (PAHs), induced by stellar UV photons, is the main heating agent of the interstellar medium is considered. Recent observations of the UV spectrum of a halo star are used to show that the cooling rate per H nucleon in each of their 4 slowly moving warm absorption components has the same value to within 10 pct. If 21 pct of the C in each component were locked up in PAHs, the PAH heating mechanism would be adequate to compensate for this cooling. The constancy of the electron density, along with the finding by Spitzer and Fitzpatrick that along their line of sight most of the free electrons and photons are probably completely mixed up with the H I in warm regions opaque to Lyman-continuum photons, strongly supports the neutrino decay theory. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/186851 Bibcode: 1993ApJ...409L..25S Keywords: Dark Matter; Decay; Heat Budget; Interstellar Matter; Neutrinos; Electron Density (Concentration); Hydrocarbons; Ionization; Photoelectric Emission; Astrophysics; ELEMENTARY PARTICLES; ISM: ABUNDANCES; STARS: INDIVIDUAL HENRY DRAPER NUMBER: HD 93521; ULTRAVIOLET: INTERSTELLAR full text sources ADS | data products SIMBAD (2)

AST/RO and submillimetre-wave observations of the Magellanic Clouds from the South Pole

AST/RO (Antarctic Submillimeter Telescope and Remote Observatory), a joint project between AT&amp;T Bell Labs, Boston University and the University of Illinois, will place a 1.7-m submillimetre telescope at the South Pole (a high, dry site) in 1992. The highly automated off-axis parabolic telescope will be equipped initially with two channels of 500 GHz (600 μm) heterodyne receivers and AOS spectrometers. A primary goal of the project is a large-scale survey of CI and CO(4-3) line emission from the southern Galaxy and the Magellanic Clouds, in an effort to further the understanding of the warm interstellar medium and star formation in the two galaxies.

Formation of Interstellar Bubble: A Time Dependent Numerical Simulation

We present results of the numerical simulations for the first 105 years of the development of spherically symmetric interstellar bubbles. We have assumed three phase interstellar medium (ISM) model and estimated the size of the interstellar bubbles (ISB). Our results are based on calculations including 106 virtual stellar wind particles. We discuss the result in the context of Asymptotic Giant Branch (AGB) star, Proto-Planetary Nebula (PPN) phase and Relativistic Wind (RW) star. The size of the ISB is found 0.14 pc to 9.03 pc in the case of AGB (Asymptotic Giant Branch) star. This size increases for the PPN and RW phase to 0.64 - 40.28 pc and 2.83 pc - 178.12 pc, respectively. It is found that the ISB can survive in the case of cold and warm interstellar medium. In the hot ISM, the ISB can not be formed due to the AGB wind. The bubble can survive in the case of PPN phase stellar wind for all kinds of the ISM. In addition, we found that the bubble can be formed in the case of RW when the ISM is hot. In the cold ISM, the ISB can not be formed due to the RW. In the warm ISM, bubble can be formed due to the relativistic (pulsar) wind if the mass loss rate is extremely high. In the warm ISM, the overall size of the bubble increases but do not exceed the recommended limit. Our result indicates that the hot ISM can not be considered as a good environment for the existence of the ISB. The size of the bubble exceeds the recommended limit (i.e., 1 to 5 kpc) in both the PPN and RW phase. However, suitable AGB wind can trigger the environment for a bubble in the hot ISM. The results of this work should be compared with the observations in the future. Key words: Interstellar medium; Stellar wind; Planetary nebula; Pulsar. DOI: 10.3126/sw.v6i6.2626 Scientific World, Vol. 6, No. 6, July 2008 8-15