Dense Interstellar Medium Research Articles

Detection of Magnesium-rich Ejecta in the Middle-aged Supernova Remnant N49B

The middle-aged supernova remnant (SNR) N49B in the Large Magellanic Cloud has been observed with the Chandra X-Ray Observatory. The superb angular resolution of Chandra resolves the complex structure of X-ray-emitting filaments across the SNR. All observed features are soft (E < 3 keV), and we find no evidence of either pointlike or extended hard emission within the SNR. Spectral lines from O, Ne, Mg, Si, S, and Fe are present. Equivalent width images for the detected elemental species and spatially resolved spectral analysis reveal the presence of Mg-rich ejecta within the SNR. We find no such enrichment in O or Ne, which may reflect details of the nucleosynthesis process or the heating and cooling of the ejecta as it evolved. The bright circumferential filaments are emission from the shocked dense interstellar medium (ISM). We detect faint diffuse X-ray emission that extends beyond the X-ray-bright filaments toward the west and southeast. These features appear to be the blast wave shock front expanding into lower density portions of the ISM seen in projection. We set an upper limit of ~2 × 1033 ergs s-1 on the 0.5-5 keV band X-ray luminosity of any embedded compact object.

Discovery of Radio/X-Ray/Optical-resolved Supernova Remnants in the Center of the Andromeda Galaxy

We have detected a spatially resolved supernova remnant (SNR) in the center of the Andromeda galaxy, in radio, X-ray, and optical wavelengths. These observations provide the highest spatial resolution imaging of a radio/X-ray/optical SNR in that galaxy to date. The multiwavelength morphology, radio spectral index, X-ray colors, and narrowband optical imaging are consistent with a shell-type SNR. A second SNR is also seen resolved in both radio and X-ray. By comparing the morphological structure of the SNRs in different wavelengths and with that in our own Galaxy, we can study the shock morphologies of SNRs in the Andromeda galaxy. The proximity of the SNRs to the core suggests a high interstellar medium density in the vicinity of the SNRs in the center of the Andromeda galaxy.

Soft X-ray sources at the centers of the elliptical galaxies NGC 4472 and NGC 4649

Analysis of recent observations of the elliptical galaxies NGC 4472 and NGC 4649 with the Chandra X-ray space telescope has revealed faint soft X-ray sources at their centers. The sources are located at the galactic centers, to within 1″, and are most likely associated with the radiation from the supermassive black holes that are assumed to be at the optical centers of these galaxies. Interest in these and several other similar objects stems from the unusually low luminosity of the supermassive black hole embedded in a dense interstellar medium. The sources have soft energy spectra in the Chandra energy range 0.2–10 keV. The source is detected at a 3σ confidence level only in the range 0.2–0.6 keV with a luminosity of ∼6×1037 erg s−1 in NGC 4649 and in the range 0.2–2.5 keV with a luminosity of ∼ 1.7×1038 erg −1 in NGC 4472.

The Effects of Ion Irradiation on the Evolution of the Carrier of the 3.4 Micron Interstellar Absorption Band

Carbon grains in the interstellar medium evolve through exposure to UV photons, heat, gas, and cosmic rays. Understanding their formation, evolution, and destruction is an essential component of evaluating the composition of the dust available for newly forming planetary systems. The 3.4 lm absorption band, attributed to the aliphatic C”H stretch vibration, is a useful probe of the degree to which energetic processing affects hydrogenated carbon grains. Here we report on the effects of ion bombardment of two different kinds of nano-size hydrogenated carbon grains with different hydrogen content. Grain samples, both with and without a mantle of H2O ice, were irradiated with 30 keV He + to simulate cosmic-ray processing in both diffuse and dense interstellar medium conditions. The ion fluences ranged between 1:5 � 10 13 and 7:9 � 10 15 ions cm � 2 . Infrared and Raman spectroscopy were used to study the effects of ion irradiation on grains. In both the dense and diffuse interstellar medium simulations, ion bombardment led to a reduction of the 3.4 lm band intensity. To discuss the effects of cosmic-ray irradiation of interstellar hydrogenated carbon materials we adopt the approximation of 1 MeV monoenergetic protons. An estimate of the C”H bond destruction cross section by 1 MeV protons was made based on experiments using 30 keV He + ions and model calculations. In combination with results from our previous studies, which focused on UV irradiation and thermal H atom bombardment, the present results indicate that the C”H bond destruction by fastcolliding charged particles is negligible with respect to that of UV photons in the diffuse ISM. However, in dense cloud regions, cosmic-ray bombardment is the most significant C”H bond destruction mechanism when the optical depth corresponds to values of the visual extinction larger than � 5 mag. The results presented here strengthen the new interpretation of the evolution of the interstellar aliphatic component (i.e., the C”H bonds in the CH2 and CH3 groups) as evidenced by the presence of the 3.4 lm absorption band in the diffuse medium and the absence of such a signature in the dense cloud environment. The evolutionary transformation of carbon grains, induced by H atoms, UV photons, and cosmic rays, indicates that C”H bonds are readily formed, in situ, in the diffuse interstellar medium and are destroyed in the dense cloud environment.

The Host Galaxies of Optically Bright Quasi-stellar Objects: Molecular Gas in [CLC][ITAL]z[/ITAL][/CLC] ≤ 0.1 Palomar-Green Quasi-stellar Objects

We present results of a CO (1-0) line survey in a complete sample of 12 low-redshift (z ≤ 0.1), optically bright Palomar-Green quasi-stellar objects (PG QSOs with MB ≤ -23 mag). Six new CO detections are reported here at levels exceeding ICO 2 Jy km s-1. Combined with three previously reported detections, we find that nine of the 12 QSOs have abundant, dense interstellar medium (ISM) characteristic of late-type galaxies. In all nine of the detected QSOs, the derived molecular gas masses are M ≥ 1.0 × 109 M☉, with the most massive being 1010 M☉ (PG 0050+124; I Zw 1). In the three sources not yet detected in CO, the upper limits on the gas masses are ~109 M☉, and thus we cannot rule out abundant ISMs even in these objects. Since our sample was chosen entirely on the basis of low redshift and optical luminosity (and not selected for strong infrared emission), we conclude that the majority of luminous, low-redshift QSOs have gas-rich host galaxies and therefore cannot be normal elliptical galaxies.

Is the Afterglow of Gamma-Ray Burst GRB 021004 Unusual?

The afterglow (AG) of gamma-ray burst GRB 021004 has been claimed to be unusual. In the cannonball (CB) model of GRBs, that is not the case. The very early AG's shape is, like for GRB 990123, a direct tracer of the expected circumburst density profile. The unprecedented precision of the data allows for the resolution of two CBs in the AG. These two CBs correspond to the two pulses in the GRB and to the two wide shoulders in the AG light curve. The smaller wiggles in the AG are, like for GRB 000301c and for GRB 970508, to be expected: they trace moderate deviations from a constant density interstellar medium. The observed evolution of the optical spectrum is that predicted in the CB model. The X-ray and radio emissions of GRB 021004 are also normal.

ON THE STRUCTURE OF THE BURST AND AFTERGLOW OF GAMMA-RAY BURSTS I: THE RADIAL APPROXIMATION

We have recently proposed three paradigms for the theoretical interpretation of gamma-ray bursts (GRBs). (1) The relative space–time transformation (RSTT) paradigm emphasizes how the knowledge of the entire world-line of the source from the moment of gravitational collapse is a necessary condition in order to interpret GRB data.1 (2) The interpretation of the burst structure (IBS) paradigm differentiates in all GRBs between an injector phase and a beam-target phase.2 (3) The GRB-supernova time sequence (GSTS) paradigm introduces the concept of induced supernova explosion in the supernovae-GRB association.3 The RSTT and IBS paradigms are enunciated and illustrated using our theory based on the vacuum polarization process occurring around an electromagnetic black hole (EMBH) theory. The results are summarized using figures, diagrams and a complete table with the space–time grid, the fundamental parameters and the corresponding values of the Lorentz gamma factor for GRB 991216 used as a prototype. In the following sections the detailed treatment of the EMBH theory needed to understand the results of the three above paradigms is presented. We start from the considerations on the dyadosphere formation. We then review the basic hydrodynamic and rate equations, the equations leading to the relative space–time transformations as well as the adopted numerical integration techniques. We then illustrate the five fundamental eras of the EMBH theory: the self acceleration of the e+e- pair-electromagnetic plasma (PEM pulse), its interaction with the baryonic remnant of the progenitor star, the further self acceleration of the e+e- pair-electromagnetic radiation and baryon plasma (PEMB pulse). We then study the approach of the PEMB pulse to transparency, the emission of the proper GRB (P-GRB) and its relation to the "short GRBs". Particular attention is given to the free parameters of the theory and to the values of the thermodynamical quantities at transparency. Finally the three different regimes of the afterglow are described within the fully radiative and radial approximations: the ultrarelativistic, the relativistic and the nonrelativistic regimes. The best fit of the theory leads to an unequivocal identification of the "long GRBs" as extended emission occurring at the afterglow peak (E-APE). The relative intensities, the time separation and the hardness ratio of the P-GRB and the E-APE are used as distinctive observational test of the EMBH theory and the excellent agreement between our theoretical predictions and the observations are documented. The afterglow power-law indexes in the EMBH theory are compared and contrasted with the ones in the literature, and no beaming process is found for GRB 991216. Finally, some preliminary results relating the observed time variability of the E-APE to the inhomogeneities in the interstellar medium are presented, as well as some general considerations on the EMBH formation. The issue of the GSTS paradigm will be the object of a forthcoming publication while the relevance of the iron-lines observed in GRB 991216 is shortly reviewed. The general conclusions are then presented based on the three fundamental parameters of the EMBH theory: the dyadosphere energy, the baryonic mass of the remnant, the interstellar medium density. An in depth discussion and comparison of the EMBH theory with alternative theories is presented as well as indications of further developments beyond the radial approximation, which will be the subject of paper II in this series. Future needs for specific GRB observations are outlined.

Radio asymmetry in 3C 99

The N-galaxy 3C 99, a Compact Steep-spectrum Source showing a triple asymmetric structure, has been observed with several arrays of radio telescopes at sub-arcsecond resolution. New images from MERLIN, European VLBI Network and the Very Long Baseline Array show that the source components detected in the central region of 3C 99 have steep spectral indices, making it difficult to determine which of them is the core. The asymmetric radio structure of 3C 99 is explained using the Scheuer-Baldwin continuous streaming model with a rather large difference in the interstellar medium density on the two sides of the central region. The age of 3C 99 has been estimated; the source is found younger than 106 years.

AChandraStudy of Sagittarius A East: A Supernova Remnant Regulating the Activity of Our Galactic Center?

We report on the X-ray emission from the shell-like, nonthermal radio source Sgr A East (SNR 000.0+00.0), located in the inner few parsecs of the Galaxy based on observations made with the ACIS detector on board the Chandra X-Ray Observatory. This is the first time Sgr A East has been clearly resolved from other complex structures in the region. The X-ray-emitting region is concentrated within the central 2 pc of the larger radio shell. The spectrum shows strong Kα lines from highly ionized ions of S, Ar, Ca, and Fe. A simple isothermal plasma model gives electron temperature ~2 keV, absorption column ~1 × 1023 H cm-2, luminosity ~8 × 1034 ergs s-1 in the 2-10 keV band, and gas mass ~2η1/2 M☉ with a filling factor η. The plasma appears to be rich in heavy elements, overabundant by roughly a factor of 4 with respect to solar abundances, and shows a spatial gradient of elemental abundance; the spatial distribution of iron is more compact than that of the lighter elements. The gas mass and elemental abundance of the X-ray emission support the long-standing hypothesis that Sgr A East is a supernova remnant (SNR), perhaps produced by the Type II supernova explosion of a massive star with a main-sequence mass of 13-20 M☉. The combination of the radio and X-ray morphologies classifies Sgr A East as a new metal-rich mixed morphology (MM) SNR. The size of the Sgr A East radio shell is the smallest of the known MM SNRs, which strongly suggests that the ejecta have expanded into a very dense interstellar medium. The ejecta-dominated chemical compositions of the plasma indicate that the ambient materials should be highly homogeneous. We thus evaluate a simplified dynamical evolution model where an SNR was formed about 10,000 yr ago and expanded into an ambient medium with a homogeneous density of 103 cm-3. The model roughly reproduces most of the observed properties in the X-ray and radio wavelengths. A comparison with the radio observations requires the dense ambient medium to be ionized, but a luminous X-ray irradiator with an expected X-ray luminosity of ~1040 ergs s-1 is not currently present. The presence of the ionized gas may be explained if the massive black hole (MBH) associated with the compact, nonthermal radio source Sgr A* was bright in X-rays about 300 yr ago but is presently dim. It is possible that the dust/molecular ridge compressed by the forward shock of Sgr A East hit Sgr A* in the past, and the passage of the ridge may have supplied material to accrete onto the black hole in the past and may have removed material from the black hole vicinity, leading to its present quiescent state. This may be a specific example of the intimate relationship between an SNR and MBH accretion activity in galactic nuclei.

Hubble Space TelescopeImaging of Bipolar Nuclear Shells in the Disturbed Virgo Cluster Galaxy NGC 4438

We present broadband and narrowband Hubble Space Telescope (HST) images of the central region of the heavily disturbed Virgo cluster galaxy NGC 4438 (Arp 120), whose nucleus has been described as a type 1 LINER or dwarf Seyfert galaxy. Narrowband Hα and [N II] HST images reveal striking bipolar shell features, 1 kpc in projected length from end to end, that are likely the result of an outflow from the nuclear region experiencing a strong interaction with the interstellar medium (ISM). While these outflow shells share similarities with those in some other starburst or active galactic nucleus (AGN) galaxies, those in NGC 4438 are notable because NGC 4438 harbors neither a luminous circumnuclear starburst nor a luminous AGN. The shells appear to be closed at their outer ends, suggesting that the outflow in NGC 4438 is dynamically younger than those in some other galaxies. The radio continuum emission is strongly enhanced near the outer ends of the shells, suggesting working surfaces arising from collimated nuclear outflows that have impacted and shocked the surrounding ISM. The two shells are quite different, as the northwestern shell is luminous and compact, while the southeastern shell is 2.5 times longer and much fainter in both optical emission lines and the nonthermal radio continuum. The differences between the two shells may be attributed to a difference in ISM density on the two sides of the nuclear disk. Such an ISM asymmetry exists on larger scales in this heavily disturbed galaxy. At the base of the outflow is a nuclear source, which is the highest surface brightness source in the galaxy at optical wavelengths. This source is resolved with a FWHM = 03 = 25 pc and has modest luminosities, uncorrected for extinction, of 5 × 1038 ergs s-1 in Hα and MB = -13. We discuss whether the outflow is powered by a low-luminosity AGN or a compact nuclear starburst. The kinetic energy associated with the ionized gas in the shells is ~1053 ergs, which could be produced by either massive star formation or an AGN. While the northwestern shell, which contributes most of the flux in most ground-based apertures centered on the nucleus, exhibits LINER-type line ratios, the nucleus has an Hα/[N II] ratio consistent with an H II region. Although there appears to be very little massive star formation occurring in the central kpc, the nucleus may contain a partially obscured, young nuclear star cluster. On the other hand, the collimation of the southeastern shell and the strongly enhanced radio continuum emission at the outer ends of the shells are more easily explained by jets associated with a nuclear black hole than by a compact nuclear starburst, although there is no direct evidence of a jet. The 2000 km s-1 broad-line component could be due to an AGN broad-line region but might also be due to high-velocity gas in the outflow. Since NGC 4438 has a large bulge, a large nuclear black hole might be expected.

Grain-Grain Coagulation in the ISM

We have developed a dust emission model in order to reproduce the evolution of the far-infrared (FIR) to sub-millimetre (sub-mm) dust emission properties which is observed in the transition between the diffuse and the dense interstellar medium (ISM). We propose that the coagulation of grains into fluffy aggregates could be the process responsible for the observed dust evolution from the diffuse to the dense ISM in the Galaxy. In dense clouds, dust coagulation can explain the increase in grain size, the decrease in the grain temperature in excess of that predicted by extinction, the sub-mm emissivity excess and the decrease in the small grain abundance.

Measurement of the Secondary Radionuclides10Be,26Al,36Cl,54Mn, and14C and Implications for the Galactic Cosmic‐Ray Age

We report on abundance measurements of 10Be, 26Al, 36Cl, and 54Mn in the Galactic cosmic rays (GCRs) using the Cosmic-Ray Isotope Spectrometer (CRIS) instrument aboard the Advanced Composition Explorer spacecraft at energies from ~70 to ~400 MeV nucleon-1. We also report an upper limit on the abundance of GCR 14C. The high statistical significance of these measurements allows the energy dependence of their relative abundances to be studied. A steady-state, leaky-box propagation model, incorporating observations of the local interstellar medium (ISM) composition and density and recent partial fragmentation cross section measurements, is used to interpret these abundances. Using this model, the individual galactic confinement times derived using data for each species are consistent with a unique confinement time value of τesc = 15.0 ± 1.6 Myr. The CRIS abundance measurements are consistent with propagation through an average ISM hydrogen number density nH = 0.34 ± 0.04 H atoms cm-3. The surviving fractions, f, for each radioactive species have been calculated. From predictions of the diffusion models of Ptuskin & Soutoul, the values of f indicate an interstellar cosmic-ray diffusion coefficient of D = (3.5 ± 2.0) × 1028 cm2 s-1.

Thermal desorption of water ice in the interstellar medium

Water (H2O) ice is an important solid constituent of many astrophysical environments. To comprehend the role of such ices in the chemistry and evolution of dense molecular clouds and comets, it is necessary to understand the freeze-out, potential surface reactivity, and desorption mechanisms of such molecular systems. Consequently, there is a real need from within the astronomical modelling community for accurate empirical molecular data pertaining to these processes. Here we give the first results of a laboratory programme to provide such data. Measurements of the thermal desorption of H2O ice, under interstellar conditions, are presented. For ice deposited under conditions that realistically mimic those in a dense molecular cloud, the thermal desorption of thin films (~50 molecular layers) is found to occur with zero order kinetics characterised by a surface binding energy, E_{des}, of 5773 +/- 60 K, and a pre-exponential factor, A, of 10^(30 +/- 2) molecules cm^-2 s^-1. These results imply that, in the dense interstellar medium, thermal desorption of H2O ice will occur at significantly higher temperatures than has previously been assumed.

Photoionization Models of NGC 346

We present spherically symmetric and plane parallel photoionization models of NGC 346, an HII region in the Small Magellanic Cloud. The models are based on CLOUDY and on the observations of Peimbert, Peimbert, & Ruiz (2000). We find that approximately 45% of the H ionization photons escape from the HII region providing an important ionizing source for the low density interstellar medium of the SMC. The predicted I(4363)/I(5007) value is smaller than that observed, probably implying that there is an additional source of energy not taken into account by the models. From the ionization structure of the best model and the observed line intensities we determine the abundances of N, Ne, S, Ar, and Fe relative to O.

On the Difference between Type E and Type A OH/IR Stars

The observed spectral energy distributions of a sample of 60 OH/IR stars are fitted using a radiative transfer model of a dusty envelope. Among the whole sample, 21 stars have reliable phase-lag distances, while the others have less accurate distances. L-*-P, (M) over dot-P, and (M) over dot- L* relations have been plotted for these stars. It is found that type E (with an emission feature at 10 mum) and type A (with an absorption feature at 10 mum) OH/IR stars have different L*P and (M) over dot -L* relations, while both follow a single M-P relation. The type E stars are shown to be located in an area without large-scale dense interstellar medium, while the type A stars are probably located in dense interstellar medium. It is argued here that this may indicate that the two types of OH/IR stars do not share the same chemical composition or zero-age main-sequence mass and so evolve in different ways. This conclusion has reinforced the argument by Chen et al., who reached a similar conclusion from the Galactic distribution of about 1000 OH/IR stars, based on IRAS low-resolution spectrometer spectra.

Accretion onto Nearby Supermassive Black Holes: [ITAL]Chandra[/ITAL] Constraints on the Dominant Cluster Galaxy NGC 6166

Chandra observations of low-luminosity supermassive black holes in nearby elliptical galaxies provide tight limits on both their nuclear luminosities and their Bondi accretion rates. We examine Chandra constraints on NGC 6166, the dominant galaxy in the cluster Abell 2199, which hosts an ~109 M☉ black hole. We measure a nuclear X-ray luminosity of LX, 1 keV ~ 1040 ergs s-1 and show that the density and temperature profiles of the hot interstellar medium imply a Bondi accretion rate of Bondi 3 × 10-2 M☉ yr-1. This accretion rate predicts a nuclear luminosity of ~1044 ergs s-1 for a canonical radiative efficiency of 10%. Unless the Bondi estimate is inappropriate and/or the accretion rate onto the black hole is significantly reduced, the observed nuclear flux constrains the radiative efficiency of the accretion flow to be η ~ 10-5. We show that low radiative efficiency accretion flows can explain the observed nuclear X-ray luminosity but that the power output from the jets in NGC 6166 is also important to the energetics of the system.

UV photodestruction of CH bonds and the evolution of the 3.4 $\mathsf{\mu}$m feature carrier

We present the results of a laboratory program aimed at studying the effects induced by energetic UV photons on hydrogenated carbon particles. Experiments have been performed under simulated diffuse and dense interstellar medium conditions. To monitor the effects of UV irradiation on grains IR spectroscopy has been used. In both circumstances UV photons lead to a reduction of the aliphatic 3.4 μm band. An estimation of the destruction cross section by UV photons for the hydrogenated carbon particles has been derived from the reduction of the 3.4 μm intensity band as a function of the UV fluence. The results of the present work, together with previous laboratory data, can shed light on the enigmatic difference observed for the 3.4 μm band between dense and diffuse interstellar medium clouds. This difference is compatible with the transformation of hydrogenated carbon particles produced by UV photons and hydrogen atoms and with the changes of the grain properties in the two environments.

Are anomalous X-ray pulsars and soft gamma-ray repeaters magnetars?

The magnetic fields of soft gamma-ray repeaters and anomalous X-ray pulsars have been estimated, taking into account the appreciable increase in the deceleration of the neutron star if it is embedded in a dense interstellar medium. These estimates yield the usual values of B⋍1012 G.

A Plasma Instability Theory of Gamma‐Ray Burst Emission

A new theory for gamma-ray burst radiation is presented. In this theory, magnetic fields and relativistic electrons are created through plasma processes arising as a relativistic shell passes through the interstellar medium. The gamma-rays are produced through synchrotron self-Compton emission. It is found that shocks do not arise in this theory, and that efficient gamma-ray emission only occurs for a high Lorentz factor and a high-density interstellar medium. The former explains the absence of gamma-ray bursts with thermal spectra. The latter provides the Compton attenuation theory with an explanation of why the interstellar medium density is always high. The theory predicts the existence of a class of extragalactic optical transient that emit no gamma-rays.

The Distribution of Burst Energy and Shock Parameters for Gamma-Ray Bursts

We calculate the luminosity function for gamma-ray burst afterglows in some fixed observed frequency band and at some fixed elapsed time in observer frame (tobs) in two models—one in which the explosion takes place in a uniform density medium and another in which the density falls off as inverse square (expected for stellar winds). For photon energies greater than about 500 eV and tobs 103 s, the afterglow flux is independent of interstellar medium (ISM) density and luminosity functions for wind and uniform ISM are identical. We deduce from the width of the observed X-ray afterglow distribution, 5 hr after the burst, that the FWHM of the distribution for isotropic energy in explosion and the fractional energy in electrons (e) are each less than about 1 order of magnitude and the FWHM for the electron energy index is 0.6 or less.