Hot Interstellar Medium Research Articles

X-ray emission line gas in the LINER galaxy M 81

We present the soft X-ray spectrum of the LINER galaxy M 81 derived from a long observation with the XMM- Newton RGS. The spectrum is dominated by continuum emission from the active nucleus, but also contains emission lines from Fe L, and H-like and He-like N, O, and Ne. The emission lines are significantly broader than the RGS point-source spectral resolution; in the cross dispersion direction the emission lines are detected adjacent to, as well as coincident with, the active nucleus. This implies that they originate in a region of a few arcminutes spatial extent (1 arcmin ∼ 1 kpc in M 81). The flux ratios of the O VII triplet suggest that collisional processes are responsible for the line emission. A good fit to the whole RGS spectrum is obtained using a model consisting of an absorbed power law from the active nucleus and a 3 temperature optically thin thermal plasma. Two of the thermal plasma components have temperatures of 0.18 ± 0.04 keV and 0.64 ± 0.04 keV, characteristic of the hot interstellar medium produced by supernovae; the combined luminosity of the plasma at these two temperatures accounts for all the unresolved bulge X-ray emission seen in the Chandra observation by Tennant et al. (2001). The third component has a higher temperature (1.7 +2.1 −0.5 keV), and we argue that this, along with some of the 0.64 keV emission, comes from X-ray binaries in the bulge of M 81.

Accretion onto the Supermassive Black Hole in M87

Chandra X-ray observations of the giant elliptical galaxy M87 resolve the thermal state of the hot interstellar medium into the accretion (Bondi) radius of its central 3 10^9 Msun black hole. We measure the X-ray gas temperature and density profiles and calculate the Bondi accretion rate, Mdot_Bondi \sim 0.1 Msun/yr. The X-ray luminosity of the active nucleus of M87 observed with Chandra is L_{x, 0.5-7 \keV} \sim 7 \times 10^{40}erg/s. This value is much less than the predicted nuclear luminosity, L_{Bondi} \sim 5 \times 10^{44} erg/s, for accretion at the Bondi rate with a canonical accretion radiative efficiency of 10%. If the black hole in M87 accretes at this rate it must do so at a much lower radiative efficiency than the canonical value. The multiwavelength spectrum of the nucleus is consistent with that predicted by an advection-dominated flow. However, as is likely, the X-ray nucleus is dominated by jet emission then the properties of flow must be modified, possibly by outflows. We show that the overall energetics of the system are just consistent with the predicted Bondi nuclear power. This suggests that either most of the accretion energy is released in the relativistic jet or that the central engine of M87 undergoes on-off activity cycles. We show that, at present, the energy dumped into the ISM by the jet may reduce the accretion rate onto the black hole by a factor \propto (v_j/c_s)^{-2}, where v_j is the jet velocity and c_s the ISM sound speed, and that this is sufficient to account for the low nuclear luminosity.

A 15 Kiloparsec X‐Ray Disk in the Elliptical Galaxy NGC 1700

We present Chandra observations of the young elliptical galaxy NGC 1700. The X-ray isophotes are highly flattened between semimajor axes of 30'' and 80'', reaching a maximum ellipticity X ≈ 0.65 at 60'' (15 kpc). The surface brightness profile in the spectrally soft flattened region is shallower than that of the starlight, indicating that the emission comes from hot gas rather than stellar sources. The flattening is so extreme that the gas cannot be in hydrostatic equilibrium in any plausible potential. A likely alternative is that the gas has significant rotational support. A simple model, representing isothermal gas distributed about a particular angular momentum, can reproduce the X-ray morphology while staying consistent with stellar kinematics. The specific angular momentum of the gas matches that of the stars in the most isophotally distorted outer part of the galaxy, and its cooling time matches the time since the last major merger. We infer that the gas was acquired in that merger, which involved a preexisting elliptical galaxy with a hot interstellar medium. The hot gas carried the angular momentum of the encounter and has since gradually settled into a rotationally flattened cooling disk.

ChandraObservation of the Central Galaxies in the A1060 Cluster of Galaxies

A Chandra observation of the central region of the A1060 cluster of galaxies resolved X-ray emission from two giant elliptical galaxies, NGC 3311 and NGC 3309. The emission from these galaxies consists of two components, namely, the hot interstellar medium (ISM) and the low-mass X-ray binaries (LMXBs). We found that the spatial extent of the ISM component was much smaller than that of stars for both galaxies, while the ratios of X-ray to optical blue-band luminosities were rather low but within the general scatter for elliptical galaxies. After subtracting the LMXB component, the ISM is shown to be in pressure balance with the intracluster medium of A1060 at the outer boundary of the ISM. These results imply that the hot gas supplied from stellar mass loss is confined by the external pressure of the intracluster medium, with the thermal conduction likely to be suppressed. The cD galaxy NGC 3311 does not exhibit the extended potential structure that is commonly seen in bright elliptical galaxies, and we discuss the possible evolution history of the very isothermal cluster A1060.

Chandra Observations of “The Antennae” Galaxies (NGC 4038/4039). II. Detection and Analysis of Galaxian X‐Ray Sources

We report the detailed analysis of the X-ray properties of the discrete sources detected in a long (72 ks) Chandra ACIS-S observation of the Antennae galaxies. We detect 49 sources, down to a detection limit of ~1038 ergs s-1; 18 sources have LX > 1039 ergs s-1 (ultraluminous X-ray sources; ULXs). Six of the 49 sources have an extended component. Two sources show evidence for variability during this observation, and three sources exhibit long-term variability in timescales of a few years. The spectra of the discrete sources are diverse, suggesting different emission mechanisms, broadly correlated with the source luminosity: the most luminous sources exhibit harder emission, while the spectra of fainter sources appear softer. Spectra and variability suggest that the ULXs may be binary accretion sources; Supernova remnants or hot interstellar medium in the Chandra beam may be responsible for some of the softer sources.

ChandraObservations of “The Antennae” Galaxies (NGC 4038/4039). III. X‐Ray Properties and Multiwavelength Associations of the X‐Ray Source Population

[abridged] We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae galaxies. We derive the average X-ray spectral properties of sources in different luminosity ranges, and we correlate the X-ray positions with radio, IR, and optical HST data. The X-ray sources are predominantly associated with young stellar clusters, indicating that they belong to the young stellar population. Based on both their co-added X-ray spectrum, and on the lack of associated radio emission, we conclude that the Ultra Luminous X-ray sources (ULXs), with Lx>10^{39} erg\s, are not young compact Supernova Remnants (SNR), but accretion binaries. While their spectrum is consistent with those of ULXs studied in nearby galaxies, and interpreted as the counterparts of intermediate mass black-holes (M >10-1000 \msun), comparison with the position of star-clusters suggests that some of the ULXs may be runaway binaries, thus suggesting lower-mass binary systems. The co-added spectrum of the sources in the 3x10^{38}-10^{39} erg\s luminosity range is consistent with those of Galactic black-hole candidates. These sources are also on average displaced from neighboring star clusters. The softer spectrum of the less luminous sources suggests the presence of SNRs or of hot interstellar medium (ISM) in the Chandra source extraction area. Comparison with HI and CO observations shows that most sources are detected in the outskirts of large concentrations of gas. The absorbing columns inferred from these observations would indeed absorb X-rays up to 5 keV, so there may be several hidden X-ray sources. Associated with these obscured regions we find 6 sources with heavily absorbed X-ray spectra and absorption-corrected luminosities in the ULX range.

A Chandra observation of the interacting pair of galaxies NGC 4485/44901

We report the results of a 20-ks Chandra ACIS-S observation of the galaxy pair NGC 4485/4490. This is an interacting system containing a late-type spiral with an enhanced star formation rate (NGC 4490), and an irregular companion that possesses a disturbed morphology. A total of 29 discrete X-ray sources are found coincident with NGC 4490, but only one is found within NGC 4485. The sources range in observed X-ray luminosity from ∼ 2 × 10 37 to 4 × 10 39 erg s −1 . The more luminous sources appear, on average, to be spectrally harder than the fainter sources, an effect that is attributable to increased absorption in their spectra. Extensive diffuse X-ray emission is detected coincident with the disc of NGC 4490, and in the tidal tail of NGC 4485, which appears to be thermal in nature and hence the signature of a hot interstellar medium in both galaxies. However, the diffuse component accounts for only ∼10 per cent of the total X-ray luminosity of the system (2 × 10 40 erg s −1 , 0.5‐8 keV), which arises predominantly in a handful of the brightest discrete sources. This diffuse emission fraction is unusually low for a galaxy pair which has many characteristics that would lead it to be classified as a starburst system, possibly as a consequence of the small gravitational potential well of the system. The discrete source population, on the other hand, is similar to that observed in other starburst systems, possessing a flat luminosity function slope of ∼− 0.6 and a total of six ultraluminous X-ray sources (ULX). Five of the ULX are identified as probable black hole X-ray binary systems, and the sixth (which is coincident with a radio continuum source) is identified as an X-ray luminous supernova remnant. The ULX all lie in star formation regions, providing further evidence of the link between the ULX phenomenon and active star formation. Importantly, this shows that even in star-forming regions, the ULX population is dominated by accreting systems. We discuss the implications of this work for physical models of the nature of ULX, and in particular how it argues against the intermediate-mass black hole hypothesis.

A Thermal Bremsstrahlung Model for the Quiescent X‐Ray Emission from Sagittarius A*

I consider the thermal bremsstrahlung emission from hot accretion flows (Bondi/ADAFs), taking into account the finite size of the observing telescope's beam (Rbeam) relative to the Bondi accretion radius (RA). For Rbeam ≫ RA, soft X-ray emission from the hot interstellar medium surrounding the black hole dominates the observed emission, while for Rbeam ≪ RA, hard X-ray emission from the accretion flow dominates. I apply these models to Chandra observations of the Galactic center, for which Rbeam ≈ RA. I argue that bremsstrahlung emission accounts for most of the "quiescent" (nonflaring) flux observed by Chandra from Sgr A*; this emission is spatially extended on scales ~RA ~ 1'' and has a relatively soft spectrum, as is observed. If accretion onto the central black hole proceeds via a Bondi or ADAF flow, a hard X-ray power law should be present in deeper observations with a flux ~ of the soft X-ray flux; nondetection of this hard X-ray component would argue against ADAF/Bondi models. I briefly discuss the application of these results to other low-luminosity active galactic nuclei.

Thermal Instability in a Cooling and Expanding Medium Including Self‐Gravity and Conduction

A systematic study of the linear thermal stability of a medium subject to cooling, self-gravity, and thermal conduction is carried out for the case when the unperturbed state is subject to global cooling and expansion. A general, recursive WKB solution for the perturbation problem is obtained that can be applied to a large variety of situations in which there is a separation of timescales for different physical processes. Solutions are explicitly given and discussed for the case in which sound propagation and/or self-gravity are the fastest processes, with cooling, expansion, and thermal conduction operating on slower timescales. A brief discussion is also added for the solutions in the cases in which cooling or conduction operate on the fastest timescale. The general WKB solution obtained in this paper permits solving the problem of the effect of thermal conduction and self-gravity on the thermal stability of a globally cooling and expanding medium. As a result of the analysis, the critical wavelength (often called the length) above which cooling makes the perturbations unstable against the action of thermal conduction is to the case of an unperturbed background with net cooling. As an astrophysical application, the generalized Field length is calculated for a hot (104-108 K), optically thin medium (as pertains, for instance, for the hot interstellar medium of supernova remnants or superbubbles) using a realistic cooling function and including a weak magnetic field. The stability domains are compared with the predictions made on the basis of models for which the background is in thermal equilibrium. The instability domain of the sound waves in particular is seen to be much larger in the case with net global cooling.

Spectra of the \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\frac{1}{4}$ \end{document} keV X‐Ray Diffuse Background from the Diffuse X‐Ray Spectrometer Experiment

The Diffuse X-ray Spectrometer (DXS) flew as an attached payload on the STS-54 mission of the space shuttle Endeavour in 1993 January and obtained spectra of the soft X-ray diffuse background in the 148-284 eV (84-44 ?) band using a Bragg-crystal spectrometer. The spectra show strong emission lines, indicating that the emission is primarily thermal. Since the observations were made at low Galactic latitude, this thermal emission must arise from a nearby hot component of the interstellar medium, most likely the Local Hot Bubble, a region within ~100 pc of the Sun characterized by an absence of dense neutral gas. The DXS spectrum of the hot interstellar medium is not consistent with either collisional equilibrium models or with nonequilibrium ionization models of the X-ray emission from astrophysical plasmas. Models of X-ray emission processes appear not yet adequate for detailed interpretation of these data. The DXS data are most nearly consistent with models of thermal emission from a plasma with a temperature of 106.1 K and depletions of refractory elements magnesium, silicon, and iron to levels ~30% of solar.

Observations of OviEmission from the Diffuse Interstellar Medium

We report the first Far Ultraviolet Spectroscopic Explorer (FUSE) measurements of diffuse O VI (lambda,lambda 1032,1038) emission from the general diffuse interstellar medium outside of supernova remnants or superbubbles. We observed a 30arcsec x 30arcsec region of the sky centered at l = 315 and b = -41. From the observed intensities (2930+/-290(random)+/-410(systematic) and 1790+/-260(random)+/-250(systematic) photons/cm/cm/s/sr in the 1032 and 1038 Angstrom emission lines, respectively), derived equations, and assumptions about the source location, we calculate the intrinsic intensity, electron density, thermal pressure, and emitting depth. The intensities are too large for the emission to originate solely in the Local Bubble. Thus, we conclude that the Galactic thick disk and lower halo also contribute. High velocity clouds are ruled out because there are none near the pointing direction. The calculated emitting depth is small, indicating that the O VI-bearing gas fills a small volume. The observations can also be used to estimate the cooling rate of the hot interstellar medium and constrain models. The data also yield the first intensity measurement of the C II 3s2 S1/2 to 2p2 P3/2 emission line at 1037 Angstroms and place upper limits on the intensities of ultraviolet line emission from C I, C III, Si II, S III, S IV, S VI, and Fe III.

The Development of Gas/Star Offsets in Tidal Tails

We present models of interacting galaxies in order to study the development of spatial offsets between the gaseous and stellar components in tidal tails. Observationally, such large-scale offsets are observed in several systems—most notably NGC 3690—suggesting an interaction between the tidal gas and some (unseen) hot interstellar medium (ISM). Instead, our models show that these offsets are a natural consequence of the radially extended H I spatial distribution in disk galaxies, coupled with internal dissipation in the gaseous component driven by the interaction. This mechanism is most effective in systems involved in very prograde interactions and explains the observed gas/star offsets in interacting galaxies without invoking interactions with a hot ISM, starburst ionization, or dust obscuration within the tails.

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.

The first broad-band X-ray images and spectra of the 30 Doradus region in the LMC

We present the XMM-Newton first light image, taken in January 2000 with the EPIC pn camera during the instrument's commissioning phase, when XMM-Newton was pointing towards the Large Magellanic Cloud (LMC). The field is rich in different kinds of X-ray sources: point sources, supernova remnants (SNRs) and diffuse X-ray emission from LMC interstellar gas. The observations are of unprecedented sensitivity, reaching a few 10$\sp{32}$ erg/s for point sources in the LMC. We describe how these data sets were analysed and discuss some of the spectroscopic results. For the SNR N157B the power law spectrum is clearly steeper than previously determined from ROSAT and ASCA data. The existence of a significant thermal component is evident and suggests that N157B is not a Crab-like but a composite SNR. Most puzzling is the spectrum of the LMC hot interstellar medium, which indicates a significant overabundance of Ne and Mg of a few times solar.

Development of the Far-ultraviolet Imaging Spectrograph on KAISTSAT-4

AbstractThe Far-ultraviolet IMaging Spectrograph (FIMS) is a small spectrograph optimized for the observations of diffuse hot interstellar medium in far-ultraviolet wavebands (900–1150Å and 1335–1750Å). The instrument is expected to be sensitive to emission line fluxes an order of magnitude fainter than any previous missions. FIMS is currently under development and is scheduled for launch in 2002.

On the Integrated Spectrum of the X-Ray Binaries and the Origin of Soft X-Ray Emission from the Bulge of M31

Using ROSAT PSPC data, we have performed several tests aimed at understanding the origin of the soft X-ray spectral component detected from the bulge of M31. We find that a significant soft component in the spectrum of the bulge is spatially correlated with the unresolved X-ray emission near the core of M31, which is probably a hot interstellar medium or perhaps a population of multiple faint sources. For the first time, we extracted the spectrum of this unresolved emission, by removing point sources dominating the integral spectrum of the bulge, and found it to be responsible for most of the soft excess. A soft spectral component is not at all needed to fit the point-source spectrum that remains after subtracting the unresolved emission. The integral spectra of bright point sources, both inside and outside of the M31 bulge, can be fitted with a single power law in the ROSAT band. Our analysis rules out the previous suggestion that all bulge emission in M31 may be generated by low-mass X-ray binaries (Irwin & Bregman).

X‐Ray Sources in the Hubble Deep Field Detected byChandra

We present first results from an X-ray study of the Hubble Deep Field North (HDF-N) and its environs obtained using 166 ks of data collected by the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-ray Observatory. This is the deepest X-ray observation ever reported, and in the HDF-N itself we detect six X-ray sources down to a 0.5--8 keV flux limit of 4E-16 erg cm^-2 s^-1. Comparing these sources with objects seen in multiwavelength HDF-N studies shows positional coincidences with the extremely red object NICMOS J123651.74 +621221.4, an active galactic nucleus (AGN), three elliptical galaxies, and one nearby spiral galaxy. The X-ray emission from the ellipticals is consistent with that expected from a hot interstellar medium, and the spiral galaxy emission may arise from a `super-Eddington' X-ray binary or ultraluminous supernova remnant. Four of the X-ray sources have been detected at radio wavelengths. We also place X-ray upper limits on AGN candidates found in the HDF-N, and we present the tightest constraints yet on X-ray emission from the SCUBA submillimeter source population. None of the 10 high-significance submillimeter sources reported in the HDF-N and its vicinity is detected with Chandra ACIS. These sources appear to be dominated by star formation or have AGN with Compton-thick tori and little circumnuclear X-ray scattering.

Oxygen Absorption in M87: Evidence for a Warm+Hot Interstellar Medium

We present a reanalysis of the ROSAT PSPC data within the central 100 kpc of M87 to search for intrinsic oxygen absorption similar to that recently measured in several galaxies and groups. Since M87 is the brightest nearby galaxy or cluster possessing an average temperature (~2 keV) within the PSPC bandpass, it is the ideal target for this study. Using a spatial-spectral deprojection analysis we find the strongest evidence to date for intrinsic oxygen absorption in the hot gas of a galaxy, group, or cluster. Single-phase plasma models modified by intervening Galactic absorption cannot fit the 0.2-2.2 keV PSPC data as they underpredict the 0.2-0.4 keV region and overpredict the 0.5-0.8 keV region where the emission and absorption residuals are obvious upon visual inspection of the spectral fits. These absorption and emission features are significant out to the largest radii investigated. Since the excess emission between 0.2-0.4 keV rules out intrinsic absorption from cold gas or dust, the most reasonable model for the excess emission and absorption features is warm, collisionally ionized gas with a temperature of ~106 K. Simple multiphase models (cooling flow, two phases) modified by both intervening Galactic absorption and by a single oxygen edge provide good fits and yield temperatures and Fe abundances of the hot gas that agree with previous determinations by ASCA and SAX. The multiphase models of M87 inferred from the PSPC can account for the excess EUV emission observed with EUVE and the excess X-ray absorption inferred from Einstein and ASCA data above 0.5 keV. This evidence for a multiphase warm+hot interstellar medium in M87 essentially confirms the original detection by Canizares et al. within the central ~2' using the Einstein FPCS. Although the total mass of the warm gas implied by the oxygen absorption is consistent with the matter deposited by a cooling flow, the suppression of the mass deposition rate and the distortion of the X-ray isophotes in the region where the radio emission is most pronounced suggest some feedback effect from the active galactic nucleus on the cooling gas.

TeV gamma rays expected from supernova remnants in different uniform interstellar media

Calculations of the expected TeV γ-ray emission, produced by accelerated cosmic rays (CRs) in nuclear collisions, from supernova remnants evolving in a uniform interstellar medium (ISM) are presented. The aim is to study the sensitivity of γ-ray production to a physical parameter set. Apart from its general proportionality to N H, it is shown that the γ-ray production essentially depends upon the ratio of the CR diffusion coefficient κ to a critical value κ crit =10(B 0/5 μG)(N H /0.3 cm −3) −1/3κ B , where B 0 and N H are the magnetic field and hydrogen number density of the ISM, and κ B denotes the Bohm diffusion coefficient. If κ is of the same order or lower than κ crit, then the peak TeV γ-ray flux in the Sedov evolutionary phase, normalized to a distance of 1 kpc, is about 10 −10(N H /0.3 cm −3) photons cm −2 s −1 . For a CR diffusion coefficient that is significantly larger than κ crit, the CR cutoff energy is less than 10 TeV, and the expected γ-ray flux at 1 TeV is considerably below the presently detectable level of 10 −12 photons cm −2 s −1 . The same is of course true for a supernova remnant in the rarified, so-called hot ISM.

Dynamics of Cool Fronts in Optically Thin Hot Plasmas

Steady cool fronts supported by nonsaturated thermal conduction in optically thin plasmas are analyzed. Explicit relations for the pressure (p2), number density (n2), temperature (T2), and flow velocity (v2) behind the front are found as functions of the temperature T1 and the intake Mach number M1 ahead of the cool front. Weak cool fronts may exist only for intake velocities less than the isothermal sound speed. Behind these kinds of fronts, the flow retards, cools down, and is compressed. Strong cool fronts exist for intake Mach numbers greater than a threshold value depending on T1. In these fronts, two compression branches appear, a high-compression branch well below the isotherm for which n1/n2 < (γ - 1)/(γ + 1) and a weak-compression branch, close to (but below) the isotherm. In particular, in the high-compression branch for low values of M1 quasi-isobaric cool fronts occur when the ratio n1/n2 is close to the maximum value, where this branch just emerges, beyond which the pressure of the flow behind the front increases when n1/n2 decreases. The implications of these results on the formation of cool condensations in the hot interstellar medium gas and in the solar atmosphere are outlined.