Physical properties of X-ray gas in galaxy cluster CL0024+17

We analyzed the X-ray data obtained by the Chandra telescope for the galaxy cluster CL0024+17 (z = 0.39). The mean temperature of the cluster is estimated (kT = 4.35 _?0.44 ^+0.51 keV) and the surface brightness profile is derived. We generated the mass and density profiles for dark matter and gas using numerical simulations and the Navarro-Frenk-White dark matter density profile (Navarro et al., 1995) for a spherically symmetric cluster in which gas is in hydrostatic equilibrium with the cluster field. The total mass of the cluster is estimated to be M ₂₀₀ = 3.51 _?0.47 ^+0.38 × 10 _Sun ¹⁴ within a radius of R ₂₀₀ = 1.24 _?0.17 ^+0.12 Mpc of the cluster center. The contribution of dark matter to the total mass of the cluster is estimated as ${{M_{200_{DM} } } \mathord{\left/ {\vphantom {{M_{200_{DM} } } {M_{tot} }}} \right. \kern-0em} {M_{tot} }} = 0.89$ .     

On the detectability of turbulence and bulk flows in X-ray clusters

Coolingflows, cluster mergers, and the motions of galaxies through cluster gas with supersonic and sonic velocities must lead to large scale motions of the intracluster medium (ICM). A high-resolution numerical simulation of X-ray cluster formation by Norman and Bryan (1999) predicts cluster-wide turbulence with ν_turb ≈300–600 km s^?1 and eddy scales l_outer ≈100–500 kpc, the larger numbers being characteristic of turbulence near the virial radius, while the smaller numbers pertain to the core. The simulation also predicts the existence of ordered bulk flows in the core with v≈400 km s^?1 on scales of several hundred kpc. In this paper, we consider the observability of such fluid motions via the distortions they induce in the CMB via the kinematic SZ effect, as well as via Doppler broadening and shifting of metal lines in the X-ray spectrum. We estimate |ΔT/T|_kinematic?6—at or below current limits of detectability. However, we find that an energy resolution of a few eV is sufficient to detect several Doppler shifted components in the 6.7 keV Fe line in the cluster core.     

The hydrogen emission spectrum of a shock wave in a stellar atmosphere

Based on a self-consistent solution of the equations of gas dynamics, kinetics of hydrogen atomic level populations, and radiative transfer, we analyze the structure of a shock wave that propagates in a partially ionized hydrogen gas. We consider the radiative transfer at the frequencies of spectral lines by taking into account the effects of a moving medium in the observer's frame of reference. The flux in Balmer lines is shown to be formed behind the shock discontinuity at the initial hydrogen recombination stage. The Doppler shift of the emission-line profile is approximately one and a half times smaller than the gas flow velocity in the Balmer emission region, because the radiation field of the shock wave is anisotropic. At Mach numbers M₁?10 and unperturbed gas densities σ₁=10^?10 g cm^?3, the Doppler shift is approximately one third of the shock velocity U₁. The FWHM of the emission-line profile δ _? is related to the shock velocity by δ _? ≈ k _? U₁, where k _? = 1, 0.6, and 0.65 for the Hα, Hβ, and Hγ lines, respectively.     

Dwarf-to-giant galaxy ratio in superclusters and in the field at 0.02 &lt; <Emphasis Type="Italic">z</Emphasis> &lt; 0.05

We report the results of the study of red-sequence (RS) galaxies in 47 galaxy clusters (0.023 < z < 0.047) located in different environments: in the superclusters Hercules and Leo, and in the field, based on the SDSS catalog data. In the

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interval, the number of bright RS dwarf galaxies in galaxy clusters increaseswith the X-ray luminosity of the cluster as logN ∝ log _X ^0.64 . The dwarf-to-giant ratio (DGR) does not depend on the surroundings, mass, or richness of the cluster. This ratio is seen to increase for galaxy clusters with log L _X > 43.5 erg/s or σ > 520 km/s. The compositeDGR of galaxy clusters, determined both from the membership in different structures and the X-ray luminosity along the radius R ₂₀₀, is minimum in the central regions of the clusters (about 0.6 ± 0.06), reaches a maximum within 0.3–0.9R ₂₀₀ (about 0.9 ± 0.10), and decreases approximately to 0.7 ± 0.03 upon reaching the radius 1.4 R ₂₀₀.

     

Nature of the optical and ultraviolet variability of the nucleus of NGC 4151: A new concept

We have analyzed the optical (U BV) and ultraviolet (λ1000–2700 Å) observations of the nuclear variability of the Seyfert galaxy NGC 4151 in the period 1987–2001 (the second cycle of activity). The fast (tens of days) and slow (～10 years) components of the nuclear variability, F and S, respectively, are shown to be completely different, but thermal in nature. We associate the S component with the formation and evolution of an accretion disk and the F component (flares) with instabilities in the accretion disk and their propagation over the disk in the form of a shock wave. The S component is present not only in the optical, but also in the ultraviolet range, with its amplitude being comparable over the entire range λ1000–5500 Å under study. The amplitude of the average flare (the F component) doubles as the wavelength decreases from 5500 to 1000 Å, while the rise time of the brightness to its maximum Δt (the variability time scale) decreases from

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to 6^d ± 2^d. The brightness decline (flare decay) time decreases by a factor of 16. The extinction in the ultraviolet is shown to have been grossly underestimated: beginning from the first IUE data, only the extinction in our Galaxy,

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, has been taken into account. A proper allowance for the total extinction, i.e., for the extinction in the nucleus of NGC 4151 as well

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leads to a large increase in the luminosity of the variable source in the nucleus of NGC 4151: L = (6–8) × 10⁴⁶ erg s^?1. The spectral energy distribution for the variable source (λ950–5500 Å) agrees well with two Planck distributions: T_e = 65 000 (λ_max = 450 Å) and 8000 K. The radiation with T_e = 8000 K is the reprocessing of the bulk of the ultraviolet radiation by the accretion disk with a lag of 0.5–0.6 days in the V band. The lag in the U-B variability of the slow component revealed the existence of an extended broad line region (EBLR) at an effective distance of 1.5 lt-years, as confirmed by spectroscopic data obtained at the Crimean Astrophysical Observatory. This yields the following mass of the central object in NGC 4151: M_c = (1–3) × 10⁹M_⊙. The luminosity of the variable source then accounts for 50–60% of L_Edd rather than 1–2%, as has been thought previously. In general, the pattern of ultraviolet and optical variability in NGC 4151 agrees excellently with the theory of disk accretion instability for a supermassive black hole suggested by N. Shakura and R. Sunyaev 30 years ago: the energy release is at a maximumin the ultraviolet (in the case under consideration, at λ450 Å), the luminosity is ～10⁴⁷ erg s^?1 for M_c ～ 10⁹M_⊙ (several tens of percent of L_Edd), and the variability time scale ranges from several days to many years.

     

Comparison of the X-ray and optical variabilities in the Seyfert galaxy 3C 120

Based on our UBV RI observations and X-ray data from the RXTE satellite, we have investigated the variability of the galaxy 3C 120 over the period 1996–2008. The relative variability amplitude in the U and B bands without any subtraction of the contribution from the underlying galaxy is 23 and 22%, respectively, against 21% in the X-ray band. The autocorrelation function based on the B-band data is considerably wider than that based on the X-ray data. The structure functions on a time scale from 1 to ～100–300 days in the X-ray and optical spectral ranges have the form of a power law (SF ～ τ ^b ). However, their indices differ significantly: b = 0.42 in the X-ray band and b = 1.36 in the B band. Considering the X-ray and optical variabilities as a superposition of independent flares in a wide range of durations, we may conclude that the amplitudes of short flares in the X-ray band are higher than those in the optical one and, conversely, the relative amplitudes of long flares in the X-ray band are slightly lower than those in the optical one, i.e., short events dominate in the X-ray band. The optical flux variations in the R _c and I _c bands lag significantly behind those in the B band, by 3.9 _?0.7 ^+1.0 and 6.2 _?0.6 ^+1.1 days, respectively, if the lag is estimated from the centroid of the cross-correlation function. The X-ray variability on a time scale of about 1800 days (～5 yr) lags behind the B-band variations by 5.3 _?3.3 ^+2.7 days, but the confidence level of this estimate is only 87%. A more detailed analysis of the correlation between the X-ray and optical emissions has revealed a fairly complex picture: different degrees of correlation between the optical and X-ray fluxes are observed at different times.     

The structure of clusters with bimodal distributions of galaxy radial velocities. II: A1775

We analyze the structure of the cluster of galaxies Abell 1775 (α = 13 ^h 42 ^m , δ = +26°22′, cz ≈ 21000 km/s), which exhibits a bimodal distribution of radial velocities of the containing galaxies. The difference of the subcluster radial velocities is ΔV ≈ 2900 km/s. We use the results of our photometric observations made with the 1-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences and the spectroscopic and photometric data from the SDSS DR6 catalog to determine independent distances to the subclusters via three different methods: the Kormendy relation, photometric plane, and fundamental plane. We find that the A1775 cluster consists of two independent clusters, A1775A (cz = 19664 km/s) and A1775B (cz = 22576 km/s), each located at its own Hubble distance and having small peculiar velocities. Given the velocity dispersions of 324 km/s and 581 km/s and the dynamic masses within the R ₂₀₀ radius equal to 0.6 × 10¹⁴ and 3.3 × 1014 M _⊙, the A1775A and A1775B clusters have the K-band luminosity-to-mass ratios of 29 and 61, respectively. A radio galaxy with an extended tail belongs to the A1775B cluster.     

Spectroscopic and photometric characteristics of stars in the field of the cluster vdB 130

The observational data for 24 stars toward the young cluster vdB 130 are analyzed. The spectroscopic and photometric BV RIJHK observations have been carried out with the following telescopes: 6-m at the Special Astrophysical Observatory of the Russian Academy of Sciences, 60-cm at the Southern Station of the Moscow State University, and 2.5-m at the Caucasus Observatory of the Sternberg Astronomical Institute of theMoscow State University. Nine stars previously selected as cluster members have been found to belong to different subtypes of type B. A minimum color excess toward the cluster, E(B ? V) = 0.9 mag, has been revealed for the vdB 130 stars lying outside the molecular cloud. Maximum color excesses, E(B ? V) = 1.3?1.4 mag, have been found in the spectra of cluster stars 1r and 5r observed in dust blobs. Inside the cluster R _v is shown to differ from the standard one. The overwhelming majority of the remaining investigated stars belong to late types and have minor color excesses (≤0.3) typical of close distances.     

Optical observations of the interstellar absorption lines towards the M8 Nebula

High resolution optical observations (FWHM ～ 10–13 km s^?1) of the I-S gas towards the early-type stars HD 164 794, HD 164816, and HD 165052 in the M8 Nebula are presented. A high velocity componentV _LSR=?26 km s^?1 has been detected in all 3 stars' spectra. A line profile fitting analysis has been carried out on the observed Caii and Nai absorption lines to determine cloud component column densities and to subsequently determine the physical and chemical conditions of the associated I-S gas.     

Thermal evolution of excited pre-galactic gas

Thermal evolution and formation of H₂ molecules in hot pre-galactic gas processed by fast shock waves are investigated both in pancake and hydrodynamical scenarios of galaxy formation. It is pointed out that an essential feature of thermodynamics of shocked gas is inequality of temperatures of electrons and heavy particles (atoms and ions): for wide range of conditions electrons are more cold than atoms. Particularly, it makes difficult collisional ionization of matter and provides predominantly neutral flow of gas behind shock front with shock velocityD<500 km s^–1. As a consequence, the major part of thermal energy of shocked gas is radiated in L line. It allows to made an assumption that bright high-redshifted L cloud near radiosource 3C 326.1 is a region of colliding high-velocity gaseous streams.It is shown that at such conditions cooling of post-shock gas is more efficient than in single-temperature approximation, and the possibilities for development of thermal instability become more favorable. In addition, the EUV and X-ray emissivity of two-temperature gas is depressed because cold electrons are not able to produce fast photons.The luminosity of pre-galactic aggregates of excited gas is estimated, and possible observational properties of it are discussed.     

Critical accretion flow of gas onto compact stars

The critical accretion flow of gas onto compact stars with mass of 0.6M is investigated by numerical integrations of the time-dependent hydrodynamic equations in the sphericallysymmetric and optically thick case. For the compact stars surrounded by such a dense cloud of gas, the radiation pressure force decelerates the infall gas significantly and free fall regime of the gas is not at all attained. This results in incident low velocities at the standing shock front close to the stellar surface, low temperatures of the gas around the compact stars, and no X-ray in white dwarfs but soft X-rays in neutron stars, respectively. Some applications of the results to the X-ray sources are discussed.     

On the velocity of jets powering hot spots similar to those in Cygnus A

Hot spots similar to those in the radio galaxy Cygnus A can be explained by the strong shock produced by a supersonic but classical jet \(\left( {u_{jet}< c/\sqrt 3 } \right)\) . The high integrated radio luminosity (L?2×10⁴⁴ erg s^?1) and the strength of mean magnetic field (B?2×10^?4 G) suggest the hot spots are the downstream flow of a very strong shock which generates the ultrarelativistic electrons of energy ?≥20 MeV. The fully-developed subsonic turbulence amplifies the magnetic field of the jet up to 1.6×10^?4 G by the dynamo effect. If we assume that the post-shock pressure is dominated by relativistic particles, the ratio between the magnetic energy density to the energy density in relativistic particles is found to be ?2×10^?2, showing that the generally accepted hypothesis of equipartition is not valid for hot spots. The current analysis allows the determination of physical parameters inside hot spots. It is found that:

1. The velocity of the upstream flow in the frame of reference of the shock isu ₁?0.2c. Radio observations indicate that the velocity of separation of hot spots isu _sep?0.05c, so that the velocity of the jet isu _jet=u ₁+u _sep?0.25c.
2. The density of the thermal electrons inside the hot spot isn ₂?5×10^?3 e ^? cm^?3 and the mass ejected per year to power the hot spot is ?4M ₀yr^?1.
3. The relativistic electron density is less than 20% of the thermal electron density inside the hot spot and the spectrum is a power law which continues to energies as low as 30 MeV.
4. The energy density of relativistic protons is lower than the energy density of relativistic electrons unlike the situation for cosmic rays in the Galaxy.

     

Surveying the Local Supercluster Plane

We investigate the distribution and velocity field of galaxies situated in a band of 100 by 20 degrees centered on M87 and oriented along the Local supercluster plane. Our sample amounts 2158 galaxies with radial velocities less than 2000 km s^?1. Of them, 1119 galaxies (52%) have distance and peculiar velocity estimates. About 3/4 of early-type galaxies are concentrated within the Virgo cluster core, most of the late-type galaxies in the band locate outside the virial radius. Distribution of gas-rich dwarfs with M_HI >M_* looks to be insensitive to the Virgo cluster presence. Among 50 galaxy groups in the equatorial supercluster band 6 groups have peculiar velocities about 500–1000 km s^?1 comparable with virial motions in rich clusters. The most cryptic case is a flock of nearly 30 galaxies around NGC4278 (Coma I cloud), moving to us with the mean peculiar velocity of ?840 km s^?1. This cloud (or filament?) resides at a distance of 16.1 Mpc from us and approximately 5 Mpc away from the Virgo center. Galaxies around Virgo cluster exhibit Virgocentric infall with an amplitude of about 500 km s^?1. Assuming the spherically symmetric radial infall, we estimate the radius of the zero-velocity surface to be R₀ = (7.0±0.3) Mpc that yields the total mass of Virgo cluster to be (7.4 ± 0.9)× 10¹⁴M_⊙ in tight agreement with its virial mass estimates. We conclude that the Virgo outskirts does not contain significant amounts of dark mater beyond its virial core.     

A scaling relation between the SZ decrement and the Thomson depth in clusters of galaxies

In a recent study of dark mater N-body simulations, a scaling relation between the SZ decrement and the Thomson depth of a cluster of galaxies of the form ΔT _r ∝τ _T ² has been found (Diaferio et al. 2000). In this paper, it will be shown that such a scaling relation arises if the intracluster gas is distributed similar to the dark matter density described by the NFW-profile and the finite spatial resolution of the numerical simulation is taken into account. It is furthermore investigated whether the ΔT _r ∝τ _T ² relation holds for analytical models of an isothermal gas sphere in the gravitational potential of a dark matter halo distributed according to the NFW-profile, the available experimental data of SZE observations, and recent results from cosmological gas-dynamical simulations of clusters of galaxies. Combining such a relation with temperature estimates from X-ray observations would provide information about a dependence of T _e on τ_T. The Thomson depth might therefore emerge as another important scaling parameter in studies of clusters of galaxies.     

Preheating of the early universe by radiation from high-mass X-ray binaries

Using a reliablymeasured intrinsic (i.e., corrected for absorption effects) present-day luminosity function of high-mass X-ray binaries (HMXBs) in the 0.25–2 keV energy band per unit star formation rate, we estimate the preheating of the early Universe by soft X-rays from such systems. We find that X-ray irradiation, mainly executed by ultraluminous and supersoft ultraluminous X-ray sources with luminosity L _X > 10³⁹ erg s^?1, could significantly heat (T >T _CMB, where T _CMB is the temperature of the cosmic microwave background) the intergalactic medium by z ~ 10 if the specific X-ray emissivity of the young stellar population in the early Universe was an order of magnitude higher than at the present epoch (which is possible due to the low metallicity of the first galaxies) and the soft X-ray emission from HMXBs did not suffer strong absorption within their galaxies. This makes it possible to observe the 21 cm line of neutral hydrogen in emission from redshifts z < 10.     

Spectroscopy of optical counterparts of ultraluminous X-ray sources

Here we present the results of panoramic and long-slit observations of eight ULX nebular counterparts performed with the 6m SAO telescope. In two ULX nebulae (ULXNe) we detected for the first time signatures of high excitation ([O III]λ5007 / Hβ > 5). Two of the ULXs were identified with young (T ～ 5–10 Myr) massive star clusters. Four of the eight ULXNe show bright high-excitation lines. This requires existence of luminous (～ 10³⁸ ÷ 10⁴⁰ erg s^?1) UV/EUV sources coinciding with the X-ray sources. The other 4 ULXNe require shock excitation of the gas with shock velocities of 20–100 km s ^?1. However, all the studied ULXNe spectra show signatures of shock excitation, but even those ULXNe where the shocks are prevailing show presence of a hard ionizing source with a luminosity of at least ～ 10³⁸ erg s^?1. Most likely shock waves, X-ray and EUV ionization act simultaneously in all the ULXNe, but they may be roughly separated in two groups: shock-dominated and photoionization-dominated ULXNe. The ULXs have to produce strong winds and/or jets (～ 10³⁹ erg s^?1) for powering their nebulae. Both the wind/jet activity and the existence of a bright UV source are consistent with the suggestion that ULXs are high-mass X-ray binaries with supercritical accretion disks of the SS433 type.     

Radial velocity curve of the spectroscopic binary HD 25639 (ADS 2984A)

We present the results of the reduction of our observations for the spectroscopic binary ADS 2984A (B0 II–B0 III), which along with its visual component ADS 2984B (SZ Cam) are the brightest members of the open star cluster NGC 1502. The spectroscopic data were obtained with a fiber-fed echelle spectrograph (R = 15 000) at the 1.2-m telescope of the Astronomical Observatory of the Ural Federal University. The period of ADS 2984A (P _orb = 57.24 ± 0.05 days) has been found for the first time. This spectroscopic binary is shown to belong to the SB1 type. We have determined the parameters of the radial velocity curve for the visible spectroscopic component, V ₀ = ?5.5 ± 1.2 km s^?1 and K = 41.5 ± 1.7 km s^?1. The lower mass limit for the invisible spectroscopic component has been estimated to be ~\(5M_ \odot \). Evidence for the presence of a stellar wind outflowing from the surface of this blue giant is presented.     

Variations of the X-ray INTEGRAL spectrum of the active galactic nucleus of NGC 4945

Results of 11-year-long X-ray INTEGRAL observations of the nucleus of Seyfert galaxy NGC 4945 in the 3–500 keV range were processed. A two-component spectrum model, which includes strong radiation absorption in the Compton-thick torus around the AGN “central engine” and secondary radiation reflected from the torus walls, was used in the analysis. The following primary spectrum parameters were determined based on the data accumulated throughout the entire exposure period: photon index Γ = 1.60 ± 0.07, exponential cutoff energy E _c =157 _-22 ⁺²⁹ keV, and column density of the medium that absorbs primary radiation N _H,1 =5.0 _-0.9 ^+1.0 × 10²⁴ cm^–2. The column density of the medium absorbing reflected radiation is two orders of magnitude lower. Both the X-ray flux in the ranges of 20–40, 40–60, and 60–100 keV and the shape of the X-ray spectrum of NGC 4945 vary. The spectrum shape variations may be induced by inhomogeneities of the absorbing medium surrounding the AGN. At the same time, there is some evidence for moderate spectrum variations in the highenergy region, which may be associated with changes in the “central engine.”     

Estimation of physical conditions in the cold phase of the interstellar medium in the sub-DLA system at <Emphasis Type="Italic">z</Emphasis> = 2.06 in the spectrum of the quasar J 2123–0050

An independent analysis of the molecular hydrogen absorption system at redshift z _abs = 2.059 in the spectrum of the quasar J 2123?0050 is presented. The H₂ system consists of two components (A and B) with column densities \(\log N_{{H_2}}^A = 17.94 \pm 0.01\) and \(N_{{H_2}}^B = 15.16 \pm 0.02\). The spectrum exhibits the lines of HDmolecules (logN _HD ^A = 13.87±0.06) and the neutral speciesCI and Cl I associated with the H₂ absorption system. For the molecular hydrogen lines near the quasar’s Lyβ and OVI emission lines, we detect a nonzero residual flux, ~3% of the total flux, caused by the effect of partial coverage of the quasar’s broad-line region by an H₂ cloud. Due to the smallness of the residual flux, the effect does not affect the H₂ column density being determined but increases the statistics of observations of the partial coverage effect to four cases. The uniqueness of the system being investigated is manifested in a high abundance of the neutral species H₂ and CI at the lowest HI column density, logN _HI = 19.18 ± 0.15, among the highredshift systems. The H₂ and CI column densities in the system being investigated turn out to be higher than those in similar systems in our Galaxy and theMagellanic Clouds by two or three orders ofmagnitude. The \(N_{HD} /2N_{H_2 }\) ratio for component A has turned out to be also unusually high, (4.26 ± 0.60) × 10^?5, which exceeds the deuterium abundance (D/H) for high-redshift systems by a factor of 1.5. Using the HI, H₂, HD, and CI column densities as well as the populations of excited H₂ and CI levels, we have investigated the physical conditions in components A and B. Component A represents the optically thick case; the gas has a low number density (~30 cm^?3) and a temperature T ~ 140 K. In component B, the mediumis optically thin with n _H ≤ 100 cm^?3 and T ≥ 100 K. The ultraviolet (UV) background intensity in the clouds exceeds the mean intensity in our Galaxy by almost an order ofmagnitude. A high gas ionization fraction, \(n_{H^ + } /n_H \sim 10^{ - 2}\), which can be the result of partial shielding of the systemfrom hard UV radiation, is needed to describe the high HD and CI column densities. Using our simulations with the PDRMeudon code, we can reconstruct the observed column densities of the species within the model with a constant density (n _H ~ 40 cm^?3). A high H₂ formation rate (higher than the mean Galactic value by a factor of 10?40) and high gas ionization fraction and UV background intensity are needed in this case.