Dense Interstellar Medium Research Articles

H2Pure Rotational Lines in the Orion Bar

Photodissociation regions (PDRs), where UV radiation dominates the energetics and chemistry of the neutral gas, contain most of the mass in the dense interstellar medium of our Galaxy. Observations of H2 rotational and rovibrational lines reveal that PDRs contain unexpectedly large amounts of very warm (400-700 K) molecular gas. Theoretical models have difficulty explaining the existence of so much warm gas. Possible problems include errors in the heating and cooling functions or in the formation rate for H2. To date, observations of H2 rotational lines smear out the structure of the PDR. Only by resolving the hottest layers of H2 can one test the predictions and assumptions of current models. Using the Texas Echelon Cross Echelle Spectrograph (TEXES) we mapped emission in the H2 v = 0-0 S(1) and S(2) lines toward the Orion Bar PDR at 2'' resolution. We also observed H2 v = 0-0 S(4) at selected points toward the front of the PDR. Our maps cover a 12'' by 40'' region of the bar where H2 rovibrational lines are bright. The distributions of H2 0-0 S(1), 0-0 S(2), and 1-0 S(1) line emission agree in remarkable detail. The high spatial resolution (0.002 pc) of our observations allows us to probe the distribution of warm gas in the Orion Bar to a distance approaching the scale length for FUV photon absorption. We use these new observational results to set parameters for the PDR models described in a companion paper in preparation by Draine et al. The best-fit model can account for the separation of the H2 emission from the ionization front and the intensities of the ground-state rotational lines, as well as the 1-0 S(1) and 2-1 S(1) lines. This model requires significant adjustments to the commonly used values for the dust UV attenuation cross section and the photoelectric heating rate.

The Large-Scale, Decelerating X-ray Jets from the Microquasar Xte J1550−564: Evidence for External Shocks Caused by the Jet-Ism Interaction?

Large-scale, decelerating, relativistic X-ray jets from microquasar XTE J1550—564 has been recently discovered with Chandra by Corbel et al. (2002). We find that the dynamical evolution of the approaching jet at the late time is consistent with the well-known Sedov evolutionary phase R ∝ t2/5. A trans-relativistic external shock dynamic model by analogy with the evolution of gammaray burst remnants, is shown to be able to fit the proper-motion data of the approaching jet reasonably well. The inferred interstellar medium density around the source is well below the canonical value nISM ∼ 1 cm−3. The rapidly fading X-ray emission can be interpreted as synchrotron radiation from the non-thermal electrons in the adiabatically expanding ejecta. These electrons were accelerated by the reverse shock (moving back into the ejecta) which becomes important when the inertia of the swept external matter leads to an appreciable slowing down of the original ejecta.

Turbulence in nearly incompressible fluids: density spectrum, flows, correlations and implication to the interstellar medium

Abstract. Interstellar scintillation and angular radio wave broadening measurements show that interstellar and solar wind (electron) density fluctuations exhibit a Kolmogorov-like k-5/3 power spectrum extending over many decades in wavenumber space. The ubiquity of the Kolmogorov-like interstellar medium (ISM) density spectrum led to an explanation based on coupling incompressible magnetohydrodynamic (MHD) fluctuations to density fluctuations through a "pseudosound" relation within the context of "nearly incompressible" (NI) hydrodynamics (HD) and MHD models. The NI theory provides a fundamentally different explanation for the observed ISM density spectrum in that the density fluctuations can be a consequence of passive scalar convection due to background incompressible fluctuations. The theory further predicts generation of long-scale structures and various correlations between the density, temperature and the (magneto) acoustic as well as convective pressure fluctuations in the compressible ISM fluids in different thermal regimes that are determined purely by the thermal fluctuation level. In this paper, we present the results of our two dimensional nonlinear fluid simulations, exploring various nonlinear aspects that lead to inertial range ISM turbulence within the context of a NI hydrodymanics model. In qualitative agreement with the NI predictions and the in-situ observations, we find that i) the density fluctuations exhibit a Kolmogorov-like spectrum via a passive convection in the field of the background incompressible fluctuations, ii) the compressible ISM fluctuations form long scale flows and structures, and iii) the density and the temperature fluctuations are anti-correlated.

On the nature of EXO 0423.4−0840

We present a Chandra observation of the candidate BL Lac object EXO 0423.4-0840. The X-ray emission from EXO 0423.4-0840 is clearly extended, and is associated with an optical early-type galaxy, MCG-01-12-005, at the centre of cluster ClG 0422-09. We do not detect a point source which can be associated with a BL Lac, but we found a small radio source in the centre of MCG-01-12-005. The cluster gas temperature mapped by the Chandra observation drops continuously from 80 kpc towards the centre, and is locally single phase. We measure a metallicity profile which declines outwards with a value 0.8 Z_sun in the centre, dropping to 0.35 Z_sun at larger radius, which we interpret as a superposition of cluster gas and a dense interstellar medium (ISM) in the central galaxy. Although the temperature profile suggests that conduction is not efficient, the ISM and intra-cluster medium seem not to have mixed. The entropy profile declines continuously towards the centre, in agreement with recent results on groups and clusters. The radio source appears to have had some effect in terms of gas heating, as seen in the small scale (~10 kpc) entropy core, and the asymmetric hard emission on the same scale.

Can CCM law properly represent all extinction curves?

We present the analysis of a large sample of lines of sight with extinction curves covering wavelength range from near-infrared (NIR) to ultraviolet (UV). We derive total to selective extinction ratios based on the Cardelli, Clayton & Mathis (1989, CCM) law, which is typically used to fit the extinction data both for diffuse and dense interstellar medium. We conclude that the CCM law is able to fit most of the extinction curves in our sample. We divide the remaining lines of sight with peculiar extinction into two groups according to two main behaviors: a) the optical/IR or/and UV wavelength region cannot be reproduced by the CCM formula; b) the optical/NIR and UV extinction data are best fit by the CCM law with different values of Rv. We present examples of such curves. The study of both types of peculiar cases can help us to learn about the physical processes that affect dust in the interstellar medium, e.g., formation of mantles on the surface of grains, evaporation, growing or shattering.

Molecular Absorption Lines in Galaxies

AbstractMolecular absorption lines have become an important tool in studying the astrochemistry of the dense and cold interstellar medium in both our own Galaxy and high redshift systems. The sensitivity is to first order only dependent on the observed continuum flux. Apart from a few nearby galaxies, molecular absorption lines have been used to study the molecular ISM in 4 galaxies at redshifts 0.25-0.89. A large number of molecular species and transitions have been observed, allowing a detailed comparison with molecular gas in our own Galaxy. Planned instruments, such as ALMA, will allow studies of a larger number of molecular absorption line systems.

Late‐Time Observations of the Afterglow and Environment of GRB 030329

We present Very Long Baseline Interferometry (VLBI) observations 217 days after the {gamma}-ray burst of 2003 March 29. These observations provide further measurements of the size and position of GRB 030329 that are used to constrain the expansion rate and proper motion of this nearby GRB. The expansion rate appears to be slowing down with time, favoring expansion into a constant density interstellar medium, rather than a circumstellar wind with an r{sup -2} density profile. We also present late time Arecibo observations of the redshifted Hi and OH absorption spectra towards GRB 030329. No absorption (or emission) is seen allowing us to place limits on the atomic neutral hydrogen of N{sub H} < 8.5 x 10{sup 20} cm{sup -2}, and molecular hydrogen of N{sub H{sub 2}} < 1.4 x 10{sup 22} cm{sup -2}. Finally, we present VLA limits on the radio polarization from the afterglow of <2% at late times.

The theory of synchrotron emission from supernova remnants

The time-dependent nonlinear kinetic theory for cosmic ray (CR) acceleration in supernova remnants (SNRs) is ap- plied studying the properties of the synchrotron emission from SNRs, in particular, the surface brightness-diameter (Σ − D )r e- lation. Detailed numerical calculations are performed for the expected range of the relevant physical parameters, namely the ambient density and the supernova explosion energy. The magnetic field in SNRs is assumed to be significantly amplified by the efficiently accelerating nuclear CR component. Due to the growing number of accelerated CRs the expected SNR luminosity increases during the free expansion phase, reaches a peak value at the beginning of the Sedov phase and then decreases again, since in this stage the overall CR number remains nearly constant, whereas the effective magnetic field diminishes with time. The theoretically predicted brightness-diameter relation in the radio range in the Sedov phase is close to ΣR ∝ D −17/4 . It fits the observational data in a very satisfactory way. The observed spread of ΣR at a given SNR size D is the result of the spread of supernova explosion energies and interstellar medium densities.

Is the Cepheus E Outflow Driven by a Class 0 Protostar?

New early release observations of the Cepheus E outflow and its embedded source, obtained with the Spitzer Space Telescope, are presented. We show the driving source is detected in all 4 IRAC bands, which suggests that traditional Class 0 classification, although essentially correct, needs to accommodate the new high sensitivity infrared arrays and their ability to detected deeply embedded sources. The IRAC, MIPS 24 and 70 microns new photometric points are consistent with a spectral energy distribution dominated by a cold, dense envelope surrounding the protostar. The Cep E outflow, unlike its more famous cousin the HH 46/47 outflow, displays a very similar morphology in the near and mid-infrared wavelengths, and is detected at 24 microns. The interface between the dense molecular gas (where Cep E lies) and less dense interstellar medium, is well traced by the emission at 8 and 24 microns, and is one of the most exotic features of the new IRAC and MIPS images. IRS observations of the North lobe of the flow confirm that most of the emission is due to the excitation of pure H2 rotational transitions arising from a relatively cold (Tex~700 K) and dense (N{H}~9.6E20 cm-2 molecular gas.

The Radio Continuum of the Metal‐deficient Blue Compact Dwarf Galaxy SBS 0335−052

We present new Very Large Array observations at five frequencies, from 1.4 to 22 GHz, of the extremely low metallicity blue compact dwarf SBS 0335-052. The radio spectrum shows considerable absorption at 1.49 GHz and a composite thermal plus nonthermal slope. After fitting the data with a variety of models, we find the best-fitting geometry to be one with free-free absorption homogeneously intermixed with the emission of both thermal and nonthermal components. The best-fitting model gives an emission measure EM ~ 8 × 107 pc cm-6 and a diameter of the radio-emitting region D ≈ 17 pc. The inferred density is ne ~ 2000 cm-3. The thermal emission comes from an ensemble of ~9000 O7 stars, with a massive star formation rate (≥5 M☉) of 0.13-0.15 yr-1 and a supernova (SN) rate of 0.006 yr-1. We find evidence for ionized gas emission from stellar winds, since the observed Brα line flux significantly exceeds that inferred from the thermal radio emission. The nonthermal fraction at 5 GHz is ~0.7, corresponding to a nonthermal luminosity of ~2 × 1020 W Hz-1. We derive an equipartition magnetic field of ~0.6-1 mG and a pressure of ~3 × 10-8 to 1 × 10-7 dynes cm-2. Because of the young age and compact size of the starburst, it is difficult to interpret the nonthermal radio emission as resulting from diffusion of SN-accelerated electrons over 107-108 yr timescales. Rather, we attribute the nonthermal radio emission to an ensemble of compact SN remnants expanding in a dense interstellar medium. If the radio properties of SBS 0335-052 are representative of star formation in extremely low metallicity environments, derivations of the star formation rate from the radio continuum in high-redshift primordial galaxies need to be reconsidered. Moreover, photometric redshifts inferred from "standard" spectral energy distributions could be incorrect.

Oi] 63 Micron Emission from High‐ and Low‐Luminosity Active Galactic Nucleus Galaxies

The Infrared Space Observatory (ISO) was used to search for a tracer of the warm and dense neutral interstellar medium, the [O I] 63.18 ?m line, in four ultraluminous IRAS sources lying at redshifts between 0.6 and 1.4. While these sources are quasars, their infrared continuum emission suggests a substantial interstellar medium. No [O I] flux was detected after probing down to a 3 ? sensitivity level sufficient for detecting line emission in starbursts with similar continuum emission. However, if the detection threshold is slightly relaxed, one target is detected with 2.7 ? significance. For this radio-quiet quasar, there is likely a substantial dense and warm interstellar medium; the upper limits for the three radio-loud sources do not preclude the same conclusion. Using a new, uniformly processed database of the ISO extragalactic far-infrared spectroscopy observations, we show that nearby Seyfert galaxies typically have higher [O I]-to-far-infrared ratios than do normal star-forming galaxies, so the lack of strong [O I] 63 ?m emission from these high-redshift ultraluminous sources cannot be attributed to their active cores.

Characteristics of Diffuse X‐Ray Line Emission within 20 Parsecs of the Galactic Center

Over the last three years, the Galactic center region has been monitored with the Chandra X-Ray Observatory. Besides the X-ray emission from the target object, Sgr A*, diffuse X-ray emission was detected throughout most of the 17' × 17' field of view. With 11 Chandra observations through 2002 June, the total effective exposure reaches ~590 ks, providing significant photon statistics on much of the detailed structure within the faint, often filamentary, diffuse X-ray emission. The true-color X-ray image and the equivalent-width images for the detected elemental species of the Galactic center region demonstrate that the diffuse X-ray features have a broad range of spatiospectral properties. Enhancements of the low ionization state Fe line emission (E ~ 6.4 keV) to the northeast of Sgr A* can be interpreted as fluorescence within the dense interstellar medium resulting from irradiation by hard, external X-ray sources. They may also be explained by emission induced by the bombardments by high-energy particles on the ambient medium, such as may accompany unresolved supernova ejecta intruding into dense interstellar medium. The detection of molecular cloud counterparts to the 6.4 keV Fe line-emitting features indicates that these Fe line features are associated with dense Galactic center clouds and/or active star-forming regions, which strongly supports the proposed origins of the X-ray reflection and/or supernova ejecta for the Fe line emission. We detect highly ionized S and Si lines that are generally coincident with the low ionization state Fe line emission and the dense molecular clouds in the northeast of Sgr A*. These hot plasmas are then most likely produced by massive star-forming activities and/or supernova remnants in the Galactic center. In contrast, we find that highly ionized He-like Fe line emission (E ~ 6.7 keV) is primarily distributed along the plane instead of being concentrated in the northeast of Sgr A*. The implied high temperature and the relatively uniform, but strong, alignment along the plane are consistent with the magnetic-confinement model, suggesting that this hot gas component has been reheated by the strong interstellar magnetic fields in the Galactic center to produce the observed He-like Fe line emission.

A Characteristic Dense Environment or Wind Signature in Prompt Gamma-Ray Burst Afterglows

We discuss the effects of synchrotron self-absorption in the prompt emission from the reverse shock of gamma-ray burst afterglows in a dense environment, such as the wind of a stellar progenitor or a dense interstellar medium in early galaxies. We point out that when synchrotron losses dominate over inverse Compton losses, the higher self-absorption frequency in a dense environment implies a bump in the reverse-shock emission spectrum, which can result in a more complex optical/IR light curve than previously thought. This bump is prominent especially if the burst ejecta is highly magnetized. In the opposite case of low magnetization, inverse Compton losses lead to a prompt X-ray flare. These effects give a possible new diagnostic for the magnetic energy density in the fireball and for the presence of a dense environment.

Physical structure of the local interstellar medium

The physical structure and morphology of the interstellar medium that surrounds our solar system directly effects the heliosphere and the interplanetary environment. High resolution ultraviolet absorption spectra of nearby stars and the intervening interstellar medium, observed by the Hubble Space Telescope, provide important information about the chemical abundance, ionization, temperature, kinematics, density, morphology, and turbulent structures of the local interstellar medium. Fortunately, nearly all observations of nearby stars contain useful local interstellar medium absorption lines. The number of useful observations is large enough that we can start analyzing the local interstellar medium as a three-dimensional object, as opposed to focusing on individual sightlines. We present the results of high resolution observations of nearby gas obtained by the Hubble Space Telescope. Our focus will be on the kinematic, temperature, and turbulent velocity structures in the Local Interstellar Cloud and other nearby clouds. Understanding the physical characteristics of these structures is necessary if we are to discuss the morphology of the local interstellar medium, its evolution, origin, and impact on the heliosphere and our solar system.

Protosolar Carbon Isotopic Composition: Implications for the Origin of Meteoritic Organics

New ion probe isotopic measurements of carbon trapped within the 50 nm thick surface layer of lunar regolith grains strongly suggest that solar wind C is depleted in 13C by at least 10% relative to terrestrial C. In order to account for the general 13C enrichment of planetary C relative to solar C, we propose that the main carriers of C in these objects, i.e., organics, were formed in an environment that allowed a strong isotopic enrichment of 13C in the solid phase. Such an environment is most likely a dense and warm circumstellar or interstellar gas medium, which could well correspond to the nebula surrounding the proto-Sun, where isotopic fractionation could be triggered by photochemical reactions.

Astronomer Wins a Balzan Prize

At a ceremony on 7 November in Bern, Switzerland, the International Balzan Foundation is bestowing four Balzan Prizes, including one to Reinhard Genzel in the category of infrared astronomy. Every year, the foundation awards four Balzan Prizes, two in the sciences and two in the humanities, in various categories. Each prize is worth 1 million Swiss francs (about $715 000).Genzel is being honored for his “fundamental contributions to infrared astronomy,” according to the citation, which adds that he “has developed instrumentation which enabled him and colleagues to make outstanding discoveries, including evidence for a massive black hole in the center of our galaxy.” (See Physics Today, February 2003, page 19.) At the Max Planck Institute for Extraterrestrial Physics, based in Garching, Germany, Genzel directs the infrared and submillimeter astronomy group. Their principal research efforts involve investigating galactic nuclei and massive black holes, examining star formation and the dense interstellar medium, and aiming at a better understanding of how galaxies have formed and evolved. He also is a professor in the physics department at the University of California, Berkeley.Genzel says his group has built instrumentation for studying with unprecedented spatial resolution the motions and orbits of stars in the core of the Milky Way. From those measurements, Genzel’s group could determine the gravitational potential of the core on scales of 10–20 light hours and show that the core contains a central mass concentration of 3 million solar masses. The data provide compelling evidence that this mass must be in the form of a massive black hole. The group performed another experiment at the European Infrared Space Observatory. Genzel says their observations revealed “that the most luminous infrared bright galaxies in the local universe are powered by a fireworks of galactic star formation that followed the collision of two gas-rich galaxies. Those measurements give important clues about the starformation processes that probably dominated during the galaxy formation epoch 1–3 billion years after the Big Bang.”© 2003 American Institute of Physics.

High-Precision Photometry of the Gamma-Ray Burst GRB 020813: The Smoothest Afterglow Yet

We report results of our precise reduction of the high signal-to-noise ratio VRI observations of the optical afterglow of the gamma-ray burst GRB 020813 obtained by Gladders & Hall with the Magellan 6.5 m telescope 3.9-4.9 hr after the burst. These observations are very well fitted locally by a power-law curve, providing the tightest constraints yet on how smooth the afterglows can be in some cases: the rms deviations range from 0.005 mag (0.5%) for the R band to 0.007 mag for the I band, only marginally larger than the rms scatter for nearby nonvariable stars. This scatter is a factor of several smaller than the smallest reported rms of 0.02 mag for GRB 990510 (Stanek et al.). These observations are in strong contrast to those of afterglows of GRB 011211 and GRB 021004, for which large, greater than 10% variability has been observed on timescales from ~20 minutes to several hours. If interstellar medium (ISM) density fluctuations near the GRB are indeed causing the bumps and wiggles observed in some bursts, very uniform regions of ISM near some bursts must be present as well. This result also constrains the intrinsic smoothness of the afterglow itself.

Electromagnetic catastrophe in ultrarelativistic shocks and the prompt emission of gamma-ray bursts

It is shown that an ultrarelativistic shock with the Lorentz factor of order of tens or higher propagating in a moderately dense interstellar medium (density above $\sim 1000 $ cm$^{-3}$) undergoes a fast dramatic transformation into a highly radiative state. The process leading to this phenomenon resembles the first order Fermi acceleration with the difference that the energy is transported across the shock front by photons rather than protons. The reflection of the energy flux crossing the shock front in both directions is due to photon-photon pair production and Compton scattering. Such mechanism initiates a runaway nonlinear pair cascade fed directly by the kinetic energy of the shock. Eventually the cascade feeds back the fluid dynamics, converting the sharp shock front into a smooth velocity gradient and the runaway evolution changes to a quasi-steady state regime. This effect has been studied numerically using the nonlinear Large Particle Monte-Carlo code for the electromagnetic component and a simplified hydrodynamic description of the fluid. The most interesting application of the effect is the phenomenon of gamma-ray bursts where it explains a high radiative efficiencyand gives a perspective to explain spectra of GRBs and their time variability. The results predict a phenomenon of ``GeV bursts'' which arise if the density of the external medium is not sufficiently high to provide a large compactness.

Deep optical observations of the supernova remnant G 78.2+2.1

The wide-field covered by the supernova remnant G 78.2+2.1 was observed in the optical emission lines of H], [] and []. The flux calibrated images reveal several regions in the field which dominate the optical emission but we were able to identify possible areas of shock-heated emission through the H] and [] images. These are mainly found to the north-east of γ Cygni as well as in the south and the morphology of the detected emission is patchy and diffuse. A few patchy structures are also detected in the medium ionization line image of []. Long-slit spectra taken at one of the candidate positions verify that we have detected radiation from shock-heated gas ([]/Hα 0.6). The estimated shock velocity lies below 100 km s-1, while the measured electron density of ~700 cm-3 implies preshock cloud densities of ~20 cm-3. High resolution maps in the infrared show that the optical emission, which may be associated with G 78.2+2.1, lies in areas relatively free of infrared emission. The interstellar extinction measured through the optical spectra is compatible with current estimates of the distance to the remnant. The optical data are in agreement with the explosion energy and interstellar medium density estimated from the X-ray data suggesting that the remnant is still in the adiabatic phase of its evolution. A second set of spectra taken in the north-west suggests that we are probably dealing with a foreground region.

External Shock Model for the Large‐Scale, Relativistic X‐Ray Jets from the Microquasar XTE J1550−564

Large-scale, decelerating, relativistic X-ray jets due to material ejected from the black-hole candidate X-ray transient and microquasar XTE J1550-564 has been recently discovered with Chandra by Corbel et al. (2002). We find that the dynamical evolution of the eastern jet at the late time is consistent with the well-known Sedov evolutionary phase. A trans-relativistic external shock dynamic model by analogy with the evolution of gamma-ray burst remnants, is shown to be able to fit the observation data reasonably well. The inferred interstellar medium density around the source is well below the canonical value $n_ISM \sim 1 cm^{-3}$. We find that the emission from the continuously shocked interstellar medium (forward shock region) decays too slowly to be a viable mechanism for the eastern X-ray jet. However, the rapidly fading X-ray emission can be interpreted as synchrotron radiation from the non-thermal electrons in the adiabatically expanding ejecta. These electrons were accelerated by the reverse shock (moving back into the ejecta) which becomes important when the inertia of the swept external matter leads to an appreciable slowing down of the original ejecta. To ensure the dominance of the emission from the shocked ejecta over that from the forward shock region during the period of the observations, the magnetic field and electron energy fractions in the forward shock region must be far below equipartition. Future continuous, follow-up multi-wavelength observations of new ejection events from microquasars up to the significant deceleration phase should provide more valuable insight into the nature of the interaction between the jets and external medium.