Presence Of Hot Gas Research Articles

Searching for the shadows of giants – II. The effect of local ionization on the Ly α absorption signatures of protoclusters at redshift z ∼ 2.4

ABSTRACT Local variations in the intergalactic medium (IGM) neutral hydrogen fraction will affect the Ly α absorption signature of protoclusters identified in tomographic surveys. Using the IllustrisTNG simulations, we investigate how the AGN proximity effect and hot, collisionally ionized gas arising from gravitational infall and black hole feedback changes the Ly α absorption associated with $M_{z=0}\simeq 10^{14}\, {\rm M}_\odot$ protoclusters at z ≃ 2.4. We find that protocluster galaxy overdensities exhibit a weak anticorrelation with Ly α transmission in IGM transmission maps, but local H$\, \rm \scriptstyle I$ ionization enhancements due to hot $T\gt 10^{6}\rm \, K$ gas or nearby AGN can disrupt this relationship within individual protoclusters. On average, however, we find that strong reductions in the IGM neutral fraction are limited to within $\lesssim 5h^{-1}\, \textrm {cMpc}$ of the dark matter haloes. Local ionization enhancements will therefore have a minimal impact on the completeness of protocluster identification in tomographic surveys if smoothing Ly α transmission maps over scales of $\sim 4 h^{-1}\, \textrm {cMpc}$, as is typically done in observations. However, if calibrating the relationship between the matter density and Ly α transmission in tomographic maps using simple analytical models for the Ly α forest opacity, the presence of hot gas around haloes can still result in systematically lower estimates of Mz = 0 for the most massive protoclusters.

Search for H i emission from superdisk candidates associated with radio galaxies

Giant gaseous layers (termed “superdisks”) have been hypothesized in the past to account for the strip-like radio emission gap (or straight-edged central brightness depression) observed between twin radio lobes, in over a dozen relatively nearby powerful Fanaroff-Riley Class II radio galaxies. They could also provide a plausible alternative explanation for a range of observations. Although a number of explanations have been proposed for the origin of the superdisks, little is known about their material content. Some X-ray observations of superdisk candidates indicate the presence of hot gas, but a cool dusty medium also seems to be common. If they are entirely or partly composed of neutral gas, then it may be directly detectable and we report here a first attempt to detect/image any neutral hydrogen gas present in the superdisks that are inferred to be present in four nearby radio galaxies. We have not found a positive H i signal in any of the four sources, resulting in tight upper limits on the H i number density in the postulated superdisks, estimated directly from the central rms noise values of the final radio continuum subtracted image. The estimated ranges of the upper limit on neutral hydrogen number density and column density are 10−4-10−3 atoms per cm3 and 1019−1020 atoms per cm2, respectively. No positive H i signal is detected even after combining all the four available H i images (with inverse variance weighting). This clearly rules out an H i dominated superdisk as a viable model to explain these structures, however, the possibility of a superdisk being composed of warm/hot gas still remains open.

The Dynamics of Truncated Black Hole Accretion Disks. II. Magnetohydrodynamic Case

Abstract We study a truncated accretion disk using a well-resolved, semi-global magnetohydrodynamic simulation that is evolved for many dynamical times (6096 inner disk orbits). The spectral properties of hard-state black hole binary systems and low-luminosity active galactic nuclei are regularly attributed to truncated accretion disks, but a detailed understanding of the flow dynamics is lacking. In these systems the truncation is expected to arise through thermal instability driven by sharp changes in the radiative efficiency. We emulate this behavior using a simple bistable cooling function with efficient and inefficient branches. The accretion flow takes on an arrangement where a “transition zone” exists in between hot gas in the innermost regions and a cold, Shakura & Sunyaev thin disk at larger radii. The thin disk is embedded in an atmosphere of hot gas that is fed by a gentle outflow originating from the transition zone. Despite the presence of hot gas in the inner disk, accretion is efficient. Our analysis focuses on the details of the angular momentum transport, energetics, and magnetic field properties. We find that the magnetic dynamo is suppressed in the hot, truncated inner region of the disk which lowers the effective α-parameter by 65%.

The ATLAS3D Project – XIV. The extent and kinematics of the molecular gas in early-type galaxies

We use interferometric 12CO(1–0) observations to compare and contrast the extent, surface brightness profiles and kinematics of the molecular gas in CO-rich ATLAS3D early-type galaxies (ETGs) and spiral galaxies. We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scalelengths. Amongst ETGs, we find that the extent of the gas is independent of its kinematic misalignment (with respect to the stars), but does depend on the environment, with Virgo cluster ETGs having less extended molecular gas reservoirs, further emphasizing that cluster ETGs follow different evolutionary pathways from those in the field. Approximately half of ETGs have molecular gas surface brightness profiles that follow the stellar light profile. These systems often have relaxed gas out to large radii, suggesting they are unlikely to have had recent merger/accretion events. A third of the sample galaxies show molecular gas surface brightness profiles that fall off slower than the light, and sometimes show a truncation. These galaxies often have a low mass, and either have disturbed molecular gas or are in the Virgo cluster, suggesting that recent mergers, ram pressure stripping and/or the presence of hot gas can compress/truncate the gas. The remaining galaxies have rings, or composite profiles, that we argue can be caused by the effects of bars. We investigated the kinematics of the molecular gas using position–velocity diagrams, and compared the observed kinematics with dynamical model predictions, and the observed stellar and ionized gas velocities. We confirm that the molecular gas reaches beyond the turnover of the circular velocity curve in ≈70 per cent of our CO-rich ATLAS3D ETGs, validating previous work on the CO Tully–Fisher relation. In general we find that in most galaxies the molecular gas is dynamically cold, and the observed CO rotation matches well model predictions of the circular velocity. In the galaxies with the largest molecular masses, dust obscuration and/or population gradients can cause model predictions of the circular velocity to disagree with observations of the molecular gas rotation; however, these effects are confined to the most star forming systems. Bars and non-equilibrium conditions can also make the gas deviate from circular orbits. In both these cases, one expects the model circular velocity to be higher than the observed CO velocity, in agreement with our observations. Molecular gas is a better direct tracer of the circular velocity than the ionized gas, justifying its use as a kinematic tracer for Tully–Fisher and similar analyses.

Observations of eclipses of UU Sge

We have performed spectroscopy and photometry of eclipses of the pre-cataclysmic variable UUSge using the 6-m telescope of the Special AstrophysicalObservatory and the 1.5-mRussian-Turkish telescope. Our analysis of variations of the B-V and V-R color indices during the eclipses indicates that the temperature of the secondary is Teff,2 = 6000−6300 K. A similar value, Teff,2 = 6200 ± 200 K, follows from our comparison of the observed spectrum of UU Sge at the total eclipse phase and theoretical spectra of late-type stars. We identify 27 absorption lines of 11 chemical elements in the secondary’s spectrum. Their abnormal intensities indicate possible high-velocity turbulent motions (up to ξturb = 10.0 km/s) in the atmosphere of the star and the presence of hot gas above its surface.

The diverse X-ray properties of four truly isolated elliptical galaxies: NGC 2954, NGC 6172, NGC 7052, and NGC 7785

Aims. We investigate the X-ray properties of four isolated elliptical galaxies, selected from the Updated Zwicky Catalog according to strict isolation criteria. Isolated galaxies are not influenced by the group/cluster environment, and their X-ray emission can be studied independently of the often overwhelming contribution of the hot intergalactic medium. They are therefore suited to studying the X-ray characteristics relative to their intrinsic properties. Methods. We analyzed our own XMM- Newton and archival Chandra data in detail for three objects, and derived, when possible, the spatial and spectral characteristics of each source. An upper limit for the fourth one was obtained from archival ASCA data. We compared their characteristics with those of other 23 isolated objects for which X-ray and optical data are available in the literature. We explored possible theoretical explanations to interpret our results. Results. In spite of our attempt to select very homogeneous objects, both in terms of optical properties and environmental characteristics, we find a wide range in X-ray luminosities and L X / L B ratios for the four objects: two of them show a hot gaseous halo, whereas no gas is detected in the other two, to a factor >10 in luminosity. In fact, we find a large spread in the L X / L B for all galaxies considered, suggesting that the presence of hot gas is not easily related to the optical luminosity or to the mass, even in isolated systems. Younger objects tend to be less luminous in X-rays than older systems. However, it appears that older objects could span a wide range in luminosities.

The Intracluster Medium inz > 1 Galaxy Clusters

The Chandra X-ray Observatory was used to obtain a 190 ks image of three high redshift galaxy clusters in one observation. The results of our analysis of these data are reported for the two z > 1 clusters in this Lynx field, including the most distant known X-ray selected cluster. Spatially-extended X-ray emission was detected from both these clusters, indicating the presence of hot gas in their intracluster media. A fit to the X-ray spectrum of RX J0849+4452, at z=1.26, yields a temperature of kT = 5.8^{+2.8}_{-1.7} keV. Using this temperature and the assumption of an isothermal sphere, the total mass of RX J0849+4452 is found to be 4.0^{+2.4}_{-1.9} X 10^{14} h_{65}^{-1} M_{\sun} within r = 1 h_{65}^{-1} Mpc. The T_x for RX J0849+4452 approximately agrees with the expectation based on its L_{bol} = 3.3^{+0.9}_{-0.5} X 10^{44}$ erg s$^{-1} according to the low redshift L_x - T_x relation. The very different distributions of X-ray emitting gas and of the red member galaxies in the two z > 1 clusters, in contrast to the similarity of the optical/IR colors of those galaxies, suggests that the early-type galaxies mostly formed before their host clusters.

Tracking Down the Processes that Shape the ISM: The Case of the Supergiant Shell in IC 2574

We present new X-ray data (obtained with the Chandra telescope) as well as FUV data of the supergiant shell (SGS) in IC 2574, a dwarf galaxy in the M 81 group. This region has proven to be a unique target for studying the interplay of massive star formation with the surrounding interstellar medium. The Chandra data confirm the presence of soft X-ray emission, indicative of the presence of hot gas. Using optical and FUV data, we estimate the age and the energy input of the stellar cluster inside the SGS. The age estimate agrees well with the age estimated based on the HI data alone — providing further support for the view that the stellar cluster interior the SGS is powering its expansion. We conclude that indeed massive stellar clusters can create supergiant shells in galaxies (even at large galactocentric distance) as predicted by the ‘standard’ picture (creation by SN explosions and strong stellar winds), a scenario which has recently been questioned by some authors.

The Supergiant Shell LMC 2. I. The Kinematics and Physical Structure

LMC 2 has the brightest, most coherent filamentary structure of all known supergiant shells in the Large Magellanic Cloud. The optical emission-line images show active star formation regions along the western edge and long filaments to the east. ROSAT PSPC and HRI images show bright X-ray emission from within the shell boundary, indicating the presence of hot gas. Counterintuitively, neither high-resolution echelle spectra in the Hα line nor aperture synthesis H I 21 cm emission-line observations show LMC 2 to have the kinematics expected of an expanding shell. Rather, LMC 2 appears to consist of hot gas confined between H I sheets. The interior surfaces of these sheets are ionized by the UV flux of massive stars in the star formation regions along the periphery of LMC 2, while the heating is provided by outflows of hot gas from the star formation regions and by SNRs interior to LMC 2. We have compared LMC 2 to other supergiant shells in the LMC and in more distant galaxies. When the spatial resolution of our data are degraded, we find that LMC 2 resembles supergiant shells observed at a distance of 4 Mpc that have previously been interpreted as expanding shells. Therefore, great caution should be exercised in the analysis and interpretation of the kinematics of distant supergiant shells to prevent overestimates of their velocities and total kinetic energies.

AnASCAObservation of M51 (NGC 5194): Iron K Emission from an Obscured Active Galactic Nucleus

We present an ASCA observation of the nearby spiral galaxy M51 (NGC 5194). We detected hard X-ray emission with a photon index of ~1.4 and a luminosity of LX ~ 1.1 × 1040 ergs s-1 in the 2-10 keV band (assuming a distance of 9.6 Mpc). A strong fluorescent iron K line (equivalent width ~900 eV) was detected at 6.4 keV in the X-ray spectra. Such an intense iron line is characteristic of Seyfert 2 galaxies and strongly suggests the presence of a heavily obscured active galactic nucleus (AGN). However, the X-ray image is extended even at energies above 2 keV. From the strong iron line and the extended hard X-ray image we speculate that the AGN is obscured by matter with a hydrogen column density more than several times 1023 cm-2 and that the observed 2-10 keV X-ray flux is not dominated by emission from the AGN but rather by other components, such as low-mass X-ray binaries, which typically dominate the X-ray emission of normal spiral galaxies. Emission lines from O K, Ne K, Fe L, Mg K and Si K were detected in the soft energy spectra, which indicate the presence of hot gas. The soft component is well represented by a Raymond-Smith thermal plasma model (with kT ~ 0.4 keV), which suggests a lower iron abundance (<0.1 solar) than other elements (~0.1-0.4 solar), or by a two temperature (kT ~ 0.3 keV and kT ~ 0.8 keV) model with ~0.1 solar abundance, which is reminiscent of the X-ray-emitting gas in starburst galaxies.

Detection of Compact Nuclear X‐Ray Emission in NGC 4736

We report the results from a deep ROSAT Position Sensitive Proportional Counter (PSPC) observation of LINER galaxy NGC 4736. Two bright sources are detected, separated by only about 1', with the brighter one coinciding with the center of the galaxy. Neither source shows apparent X-ray variability on timescales of minutes to hours in the ROSAT band. Simple power-law models, typical of active galactic nucleus (AGN) X-ray spectra, produce poor fits to the observed X-ray spectrum of the nuclear source. The addition of a Raymond-Smith component improves the fits significantly. This is consistent with the presence of hot gas in the nuclear region with kT 0.3 keV, in addition to a compact nuclear source. However, a careful examination of the residuals reveals apparent features at low energies (<0.25 keV). We find that the addition of a narrow emission line at about 0.22 keV is a significant improvement to the parameterization of the spectrum. We examine the results in the light of the accuracy of the PSPC spectral calibration. The derived photon index is about 2.3, which is similar to those for Seyfert 1 galaxies measured in the ROSAT energy range. On the other hand, the 0.1-2 keV luminosity for the compact source is only about 3.4 × 1039 ergs s-1, much fainter than typical Seyfert galaxies. We discuss the implications of these results on the connection between LINERs and AGNs. The off-center source is transient in nature. It has a hard X-ray spectrum, with a photon index of about 1.5, so is likely to be an X-ray binary. There is still some ambiguity regarding its association with the galaxy. If it is indeed located in the galaxy, the 0.1-2 keV luminosity would be greater than 5.1 × 1038 ergs s-1, making it a stellar mass black hole candidate.

ULTRAVIOLET ABSORPTION STUDIES OF THE INTERSTELLAR GAS

ABSTRACT Copernicus and IUE observations of UV absorption lines of interstellar gas (IG) are reviewed. The historical context, major findings, and IG observation proposals presented in a paper by Spitzer and Zabriskie (1959) are recalled and compared with the campaigns actually undertaken and their results. Consideration is given to the performance of the satellite instruments, the observed IG depletion of most atomic species relative to hydrogen, the dominance of H2 over atomic H in dense molecular clouds, the presence of hot gas in the Galactic disk, D/H abundance ratios, neutral H distributions, and the distribution of IG in the solar vicinity. Problems encountered in interpreting UV absorption data are discussed, and the important role of the IG in the evolution of galaxies is indicated.

Measuring EUV Line Emission from the Hot Interstellar Medium

AbstractIn the local interstellar medium, the presence of hot gas (log T ~ 5 to 6) has been inferred from measurements of oxygen VI absorption in the ultraviolet, and from diffuse emission in the extreme ultraviolet (EUV) and soft X-ray regions. Here we describe a spectrometer that is very sensitive to gas in this temperature range. The spectrometer uses an array of plane-ruled gratings at grazing incidence in the extreme off plane mount. A set of Kirkpatrick-Baez mirrors focuses the conically diffracted light on to one of two microchannel plate detectors. The field of view of the instrument is 0.2° by 12°. The predicted sensitivity ranges from 50 to 200 ph/(cm2 sec str) with a resolving power (λ/Δλ) of 15 to 50 over the 50 to 700A wavelength band. The instrument is currently under construction for a sounding rocket flight.