Central Gas Density Research Articles

Galaxy Bulge Formation: Interplay with Dark Matter Halo and Central Supermassive Black Hole

We present a simple scenario where the formation of galactic bulges was regulated by the dark halo gravity and regulates the growth of the central supermassive black hole. Assuming the angular momentum is low, we suggest that bulges form in a runaway collapse due to the "gravothermal" instability once the central gas density or pressure exceeds a certain threshold. We emphasize that the threshold is nearly universal, set by the background NFW dark matter gravity gDM ~ 1.2 × 10-8 cm s-2 in the central cusps of halos. Unlike known thresholds for gradual formation of galaxy disks, we show that the universal "halo-regulated" star formation threshold for spheroids matches the very high star formation rate and star formation efficiency seen in high-redshift observations of central starburst regions. The starburst feedback also builds up a pressure shortly after the collapse. This large pressure could act both outward to halt further infall of gas from larger scales and inward to counter the Compton-thick wind launched from the central black hole in an Eddington accretion. Assuming the feedback balances inward and outward forces, our scenario naturally gives rise to the black hole-bulge relationships observed in the local universe.

Turbulent gas motions in galaxy cluster simulations: the role of smoothed particle hydrodynamics viscosity

Smoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamic shock waves. In its original formulation, the resulting numerical viscosity is large enough to suppress structure in the velocity field on scales well above the nominal resolution limit, and to damp the generation of turbulence by fluid instabilities. This could artificially suppress random gas motions in the intracluster medium (ICM), which are driven by infalling structures during the hierarchical structure formation process. We show that this is indeed the case by analysing results obtained with an SPH formulation where an individual, time-variable viscosity is used for each particle, following a suggestion by Morris & Monaghan. Using test calculations involving strong shocks, we demonstrate that this scheme captures shocks as well as the original formulation of SPH, but, in regions away from shocks, the numerical viscosity is much smaller. In a set of nine high-resolution simulations of cosmological galaxy cluster formation, we find that this low-viscosity formulation of SPH produces substantially higher levels of turbulent gas motions in the ICM, reaching a kinetic energy content in random gas motions (measured within a 1-Mpc cube) of up to 5‐30 per cent of the thermal energy content, depending on cluster mass. This also has significant effects on radial gas profiles and bulk cluster properties. We find a central flattening of the entropy profile and a reduction of the central gas density in the low-viscosity scheme. As a consequence, the bolometric X-ray luminosity is decreased by about a factor of 2. However, the cluster temperature profile remains essentially unchanged. Interestingly, this tends to reduce the differences seen in SPH and adaptive mesh refinement simulations of cluster formation. Finally, invoking a model for particle acceleration by magnetohydrodynamics waves driven by turbulence, we find that efficient electron acceleration and thus diffuse radio emission can be powered in the clusters simulated with the low-viscosity scheme provided that more than 5‐10 per cent of the turbulent energy density is associated with fast magneto-sonic modes.

The X-ray Fundamental Plane of Clusters of Galaxies

We analyze the relations among the central gas density, core radius, and temperature of X-ray clusters by plotting the observational data in the three-dimensional (log ρgas,0, log rc, and log Tgas) space and find that the data lie on a ‘fundamental plane’. We discuss the implications of the plane.

Inhomogeneity in the Hot Intracluster Medium of Abell 1060 Observed with Chandra

Abstract A Chandra observation of the non-cooling flow cluster A 1060 has confirmed that the hot intracluster medium has fairly uniform distributions of temperature and metal abundance from a radius of about 230 kpc to the central 5 kpc region ($H_0 = 75 \,\mathrm{km} \,\mathrm{s}^{-1} \,\mathrm{Mpc}^{-1}$). The radial temperature profile shows a broad peak at 30–40 kpc from the center at a level $\sim 20\%$ higher than that in the outer region. Assuming spatially uniform temperature and abundance distributions, we derived a 3-dimensional density structure by iteratively correcting the $\beta$ model, and obtained the central gas density to be $8.2^{+1.8}_{-1.0} \times 10^{-3} \,\mathrm{cm}^{-3}$. The distribution of gravitational mass was estimated from the density profile, and a central concentration of mass within a radius of 50 kpc was indicated. The data also suggest several high-abundance regions. The most significant blob adjacent to the central galaxy NGC 3311 has a radius of about 9 kpc, which indicates a metallicity of $\sim 1.5$ solar. We consider that this blob may be produced by the gas stripped off from NGC 3311.

Secular Evolution and the Formation of Pseudobulges in Disk Galaxies

▪ Abstract The Universe is in transition. At early times, galactic evolution was dominated by hierarchical clustering and merging, processes that are violent and rapid. In the far future, evolution will mostly be secular—the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. Both processes are important now. This review discusses internal secular evolution, concentrating on one important consequence, the buildup of dense central components in disk galaxies that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges.We begin with an “existence proof”—a review of how bars rearrange disk gas into outer rings, inner rings, and stuff dumped onto the center. The results of numerical simulations correspond closely to the morphology of barred galaxies. In the simulations, gas is transported to small radii, where it reaches high densities and plausibly feeds star formation. In the observations, many barred and oval galaxies have dense central concentrations of gas and star formation. Optical colors and spectra often imply young stellar populations. So the formation of pseudobulges is well supported by theory and observations. It is embedded in a broader evolution picture that accounts for much of the richness observed in galaxy structure.If secular processes built dense central components that masquerade as bulges, how can we distinguish them from merger-built bulges? Observations show that pseudobulges retain a memory of their disky origin. That is, they have one or more characteristics of disks: (a) flatter shapes than those of classical bulges, (b) correspondingly large ratios of ordered to random velocities, (c) small velocity dispersions σ with respect to the Faber-Jackson correlation between σ and bulge lumi nosity, (d) spiral structure or nuclear bars in the “bulge” part of the light profile, (e) nearly exponential brightness profiles, and ( f ) starbursts. All these structures occur preferentially in barred and oval galaxies, where secular evolution should be most rapid. So the cleanest examples of pseudobulges are recognizable.Are their formation timescales plausible? We use measurements of central gas densities and star-formation rates to show that pseudobulges of the observed densities form on timescales of a few billion years.Thus a large variety of observational and theoretical results lead to a new picture of galaxy evolution that complements hierarchical clustering and merging. Secular evolution consists of more than the aging of stellar populations. Every galaxy is dynamically evolving.

An old galaxy group:ChandraX-ray observations of the nearby fossil group NGC 6482

We present the first detailed X-ray observations, using Chandra, of NGC 6482 - the nearest known fossil group. The group is dominated by an optically luminous giant elliptical galaxy and all other known group members are at least two magnitudes fainter. The global X-ray properties (luminosity, temperature, extent) of NGC 6482 fall within the range of other groups, but the detailed properties show interesting differences. We derive the gas temperature and total mass profiles for the central 30 h -1 70 kpc (∼0.1 r 200 ) using ACIS spatially resolved spectroscopy. The unusually high L X /L opt ratio is found to result from a high central gas density. The temperature profile shows a continuous decrease outward, dropping to 0.63 of its central value at 0.1r 200 . The derived total mass profile is strongly centrally peaked, suggesting an early formation epoch. These results support a picture in which fossil groups are old, giving time for the most massive galaxies to have merged (via the effects of dynamical friction) to produce a central giant elliptical galaxy. Although the cooling time within 0.1r 200 is less than a Hubble time, no decrease in central temperature is detected. The entropy of the system lies toward the low side of the distribution seen in poor groups and drops all the way into the centre of the system, reaching very low values. No isentropic core, such as those predicted in simple pre-heating models, is present. Given the lack of any central temperature drop in the system, it seems unlikely that radiative cooling can be invoked to explain this low central entropy. The lack of any signature of central cooling is especially striking in a system that appears to be old and relaxed, and to have a central cooling time ≤10 8 yr. We find that the centrally peaked temperature profile is consistent with a steady-state cooling-flow solution with an accretion rate of 2 M ○. yr -1 , given the large P dV work arising from the cuspy mass profile. However, solutions involving distributed or non-steady heating cannot be ruled out.

High-resolution CO Survey of Virgo Spirals at Nobeyama — Massive Cores inferred from Central Rotation Curves

We present the results of a high-resolution CO-line survey using the Nobeyama Millimeter-wave Array at high-angular (⋐ 2–3″) and high spectral resolutions for the 15 CO-richest Virgo spirals. We derived exact rotation curves using position-velocity diagrams by applying the iteration method. the obtained RCs rise steeply in the central 100 pc. Surface-mass-density distributions by direct deconvolution of the RCs show massive cores of ⋐ 109M⊙ within 100 pc, suggesting the existence of DM cusps in the centres. Five galaxies were found to nest single-peaked extremely high-density molecular cores, where star formation is currently suppressed by the high differential rotation. We show that the deeper is the gravitational potential, the higher is the central gas density.

Starbursts from the Strong Compression of Galactic Molecular Clouds due to the High Pressure of the Intracluster Medium

We demonstrate that the high pressure of the hot intracluster medium (ICM) can trigger the collapse of molecular clouds in a spiral galaxy, leading to a burst of star formation in the clouds. Our hydrodynamical simulations show that the high gaseous (ram pressure and static thermal) pressure of the ICM strongly compresses a self-gravitating gas cloud within a short timescale (~107 yr), dramatically increasing the central gas density and consequently causing efficient star formation within the cloud. The stars developed in the cloud form a compact, gravitationally bound star cluster. The star formation efficiency within such a cloud is found to depend on the temperature and the density of the ICM and the relative velocity of the galaxy with respect to it. Based on these results, we discuss the origin of starburst/post-starburst populations observed in distant clusters, the enhancement of star formation for galaxies in merging clusters, and the isolated compact H II regions recently discovered in the Virgo Cluster.

Clusters of galaxies: a cosmological probe.

The constraints on cosmological parameters presently obtained and those that can be obtained from X-ray cluster surveys are presented. Extremely strong bounds on the values of Omega, Lambda, sigma(8) and the power spectrum of fluctuations, as well as constraints on the equation of state of dark energy, can be determined. Recent Advanced Satellite for Cosmology and Astrophysics and XMM results on the chemical composition of clusters show that the Fe abundance is not universal, but is correlated with cluster mass and central gas density. The Si, S and Fe abundances do not resemble those seen in Milky Way Halo stars or those in the Lyman-limit galaxies. The XMM RGS abundances for gas in elliptical galaxies are subsolar and the abundance pattern is not alpha-element rich, in contradiction with all models of elliptical-galaxy gas abundances.

Projection effects in X-ray cores of cooling flow galaxy clusters

Recent analyses of XMM and Chandra data of the cores of X-ray bright clusters of galaxies show that modeling with a multi-phase gas in which several temperatures and densities are in equilibrium might not be appropriate. Instead, a single-phase model seems able to reproduce properly the spectra collected in annuli from the central region. The measured single-phase temperature profiles indicate a steep positive gradient in the central 100-200 kpc and the gas density shows a flat profile in the central few tens of kpc. Given this observational evidence, we estimate the contribution to the projected-on-the-sky rings from the cluster emissivity as function of the shell volume fraction sampled. We show that the observed projected X-ray emission mimics the multi-phase status of the plasma even though the input distribution is single-phase. This geometrical projection affects (i) analyses of data where insufficient spatial resolution is accessible, (ii) the central bin when its dimension is comparable to the extension of any flatness in the central gas density profile.

Femtosecond laser energy deposition in strongly absorbing cluster gases diagnosed by blast wave trajectory analysis

An intense ultrafast laser pulse can be very strongly absorbed in a moderate density gas composed of van der Waals bonded clusters. In this paper, the deposition of the energy of intense 30 fs light pulses in a gas of deuterium clusters has been diagnosed using a technique based on analysis of the trajectories of the resulting cylindrically symmetric blast waves. Using the well-known relation between blast wave velocity and energy deposition in gas, the laser energy deposited per unit length as a function of distance in gas jet plume was measured. These measurements were conducted in jets containing either deuterium clusters or simple deuterium molecules.

Multi stage three-dimensional sweeping and annealing of disc galaxies in clusters

We present new three dimensional, hydrodynamic simulations of the ram pressure stripping of disc galaxies via interaction with an hot intracluster medium (ICM). The simulations were carried with the smoothed-particle hydrodynamics, adaptive mesh 'Hydra' code (SPH-A$P^3$M), with model galaxies consisting of dark halo, and gas and stellar disc components. The simulations also include radiative cooling, which is important for keeping the warm, diffuse gas of moderate density from being unrealistically heated by the ICM. We examine the role that wind velocity, density and galaxy tilt play in gas stripping. The onset of the ICM wind has a profound effect on the disc gas that is not immediately stripped. This remnant disc is displaced relative to the halo center and compressed. This can trigger gravitational instability and the formation of numerous flocculent spirals. These waves transport angular momentum outward, resulting in further compression of the inner disc and the formation of a prominent gas ring. This 'annealing' process makes the inner disc, which contains much of the total gas mass, resistant to further stripping, but presumably susceptible to global starbursts. When the mass loss and annealing processes go to completion we find that the total amount of mass lost from a fixed target galaxy is well fit by a simple power-law function of a dimensionless parameter that combines the ram pressure and internal properties of the galaxy. (abs. abbreviated)

Molecular Evolution in Collapsing Prestellar Cores

We have investigated the evolution and distribution of molecules in collapsing prestellar cores via numerical chemical models, adopting the Larson-Penston solution and its delayed analogs to study collapse. Molecular abundances and distributions in a collapsing core are determined by the balance among the dynamical, chemical, and adsorption timescales. When the central density nH of a prestellar core with the Larson-Penston flow rises to 3 × 106 cm-3, the CCS and CO column densities are calculated to show central holes of radius 7000 and 4000 AU, respectively, while the column density of N2H+ is centrally peaked. These predictions are consistent with observations of L1544. If the dynamical timescale of the core is larger than that of the Larson-Penston solution owing to magnetic fields, rotation, or turbulence, the column densities of CO and CCS are smaller, and their holes are larger than in the Larson-Penston core with the same central gas density. On the other hand, N2H+ and NH3 are more abundant in the more slowly collapsing core. Therefore, molecular distributions can probe the collapse timescale of prestellar cores. Deuterium fractionation has also been studied via numerical calculations. The deuterium fraction in molecules increases as a core evolves and molecular depletion onto grains proceeds. When the central density of the core is nH = 3 × 106 cm-3, the ratio DCO+/HCO+ at the center is in the range 0.06-0.27, depending on the collapse timescale and adsorption energy; this range is in reasonable agreement with the observed value in L1544.

Infrared Spectroscopy of Molecular Supernova Remnants

We present Infrared Space Observatory spectroscopy of sites in the supernova remnants W28, W44, and 3C391, where blast waves are impacting molecular clouds. Atomic fine-structure lines were detected from C, N, O, Si, P, and Fe. The S(3) and S(9) lines of H2 were detected for all three remnants. The observations require both shocks into gas with moderate (~ 100 /cm3) and high (~10,000 /cm3) pre-shock densities, with the moderate density shocks producing the ionic lines and the high density shock producing the molecular lines. No single shock model can account for all of the observed lines, even at the order of magnitude level. We find that the principal coolants of radiative supernova shocks in moderate-density gas are the far-infrared continuum from dust grains surviving the shock, followed by collisionally-excited [O I] 63.2 and [Si II] 34.8 micron lines. The principal coolant of the high-density shocks is collisionally-excited H2 rotational and ro-vibrational line emission. We systematically examine the ground-state fine structure of all cosmically abundant elements, to explain the presence or lack of all atomic fine lines in our spectra in terms of the atomic structure, interstellar abundances, and a moderate-density, partially-ionized plasma. The [P II] line at 60.6 microns is the first known astronomical detection. There is one bright unidentified line in our spectra, at 74.26 microns. The presence of bright [Si II] and [Fe II] lines requires partial destruction of the dust. The required gas-phase abundance of Fe suggests 15-30% of the Fe-bearing grains were destroyed. The infrared continuum brightness requires ~1 Msun of dust survives the shock, suggesting about 1/3 of the dust mass was destroyed, in agreement with the depletion estimate and with theoretical models for dust destruction.

X-ray analysis of the cluster 3C 129

The cluster 3C 129 is classified as a rich cluster. An analysis of the properties of the cluster 3C 129 from ROSAT PSPC and HRI, Einstein IPC, and EXOSAT ME observations is presented. The mean temperature from a joint fit of the ROSAT PSPC and EXOSAT ME data is 5.5(±0.2) keV. The luminosity is 0.6×1044 erg s−1 in 0.2–2.4 keV and 2.7×1044 erg s−1 in 0.2–10 keV. We find a cooling flow with a rate of ∼84 M⊙ yr−1. The central gas density is 6×10−3 cm−3, and the ICM mass is 3.6×1013 M⊙. The total cluster mass is ∼5×1014 M⊙. The X-ray morphology shows an east–west elongation, which is evidence for a recent merger event. The radio source 3C 129.1 is located near the X-ray centre. Another cluster member galaxy (the radio galaxy 3C 129) is a prototype of head-tailed radio galaxies, and is located in the west part of the cluster. The tail points along the gradient of intracluster gas pressure. There are no significant point X-ray sources associated with the AGNs of the two radio galaxies.

The Variation of Gas Mass Distribution in Galaxy Clusters: Effects of Preheating and Shocks

We investigate the origin of the variation of the gas mass fraction in the core of galaxy clusters, which was indicated by our work on the X-ray fundamental plane. Applying a spherical-collapse model of cluster formation and considering the effect of shocks on preheated intracluster gas, we construct a simple model to predict the spatial gas distribution of clusters. As suggested by our previous work, we assume that the core structure of clusters determined at the cluster collapse has not changed much after that. The adopted model supposes that the gas distribution characterized by the slope parameter is related to the preheated temperature. Comparison with observations of relatively hot (3 keV) and low-redshift clusters suggests that the preheated temperature is about 0.5-2 keV, which is higher than expected from the conventional galactic wind model and possibly suggests the need for additional heating, such as from quasars or gravitational heating on the largest scales at high redshift. The dispersion of the preheated temperature may be attributable to gravitational heating in subclusters. We calculate the central gas fraction of a cluster from the gas distribution, assuming that the global gas mass fraction is constant within a virial radius at the time of the cluster collapse. We find that the central gas density thus calculated is in good agreement with the observed one, which suggests that the variation of the gas mass fraction in cluster cores appears to be explained by breaking the self-similarity in clusters due to preheated gas. We also find that this model does not change major conclusions regarding the fundamental plane and its cosmological implications obtained in previous papers, which strongly suggests that the core structure preserves information about the cluster formation not only for the dark halo, but also for the intracluster gas.

Environmental Dependence of the Fundamental Plane of Galaxy Clusters

Galaxy clusters approximate a planar distribution in a three-dimensional parameter space that can be characterized, for example, by an optical luminosity, half-light radius, and X-ray luminosity. We find the nearest neighbor clusters for those common to either of two previous fundamental-plane studies and a high-quality cluster redshift catalog. Examining scatter about one plane in parameter space, we find a 2 sigma result that it is dependent on the nearest neighbor distance. Study of another sample of X-ray clusters shows that those with high central gas densities are systematically (2.5 sigma) closer to neighbor clusters. These results suggest a possible explanation for recent evidence that X-ray cooling flow clusters reside in crowded environments.

The X-Ray Fundamental Plane and [ITAL]L[/ITAL][TINF]X[/TINF]-[ITAL]T[/ITAL] Relation of Clusters of Galaxies

We analyze the relations among the central gas density, the core radius, and the temperature of X-ray clusters by plotting the observational data in the three-dimensional (log ρ0, log R, log T)-space and find that the data lie on a fundamental Its existence implies that the clusters form a two-parameter family. The data on the plane still have a correlation and form a band on the plane. The observed relation LX ∝ T3 turns out to be the cross section of the band perpendicular to the major axis, while the major axis is found to describe the virial density. We discuss the implications of this two-parameter family nature of X-ray clusters.

Einstein ObservatoryImages of Clusters of Galaxies

We present the X-ray characteristics of a sample of 368 clusters of galaxies with redshifts less than 0.2 observed with the Einstein Imaging Proportional Counter. For each cluster, we measure the 0.5-4.5 keV counting rate and compute the 0.5-4.5 keV source luminosity, as well as the bolometric luminosity within fixed metric radii. We detect 85% of Abell clusters with z<0.1, demonstrating that the large majority of these optically selected clusters are not the results of chance superpositions. For 163 clusters, we measure their X-ray surface brightness profiles and determine their core radii. For ~230 clusters, we then use either our measured core radii and β values, or mean values derived for this sample, to measure central gas densities and gas masses. We use estimated or measured cluster gas temperatures, along with the derived gas-density profiles, to estimate total cluster masses, under the assumptions that the gas is isothermal and in hydrostatic equilibrium. We also present contour plots of the X-ray emission, which we use to classify the cluster morphology. We find the percentage of clusters with substructure in their X-ray images is about 40%, with no significant change in this percentage as a function of X-ray luminosity. This implies that a large fraction of all present epoch clusters are still undergoing subcluster mergers. Based on our analysis of surface brightness profiles, we find that most clusters have core radii in the range from 0.1 to 0.3 Mpc (for H0=50 km s-1 Mpc-1), with more massive single clusters having larger core radii. The β values determined from the slope of the surface brightness profiles fall in the narrow range from 0.4 to 0.8, with β values increasing with cluster gas temperatures. No change in the value of β is found in the surface brightness profiles for individual clusters as a function of distance from the cluster center. We compare the β values derived from the surface brightness profiles with the corresponding β values calculated from the gas temperatures and cluster velocity dispersions. We argue that much of the discrepancy between the values of β derived from these two methods results from overestimates of the cluster velocity dispersion due to cluster substructure. Finally, we compare the gas mass to the cluster virial mass and find, for an isothermal gas, that, within a fixed metric radius of 1 Mpc, the gas mass fraction increases as a function of X-ray luminosity from 10% to 20% of the total cluster mass.

A Dynamical Analysis of the Poor Galaxy Clusters Abell 2626 and Abell 2440

We use 189 new radial velocities, an $R$ band CCD mosiac, archival X-ray images, and gas temperatures to study substructure, masses, luminosity functions, and mass-to-light ratios in A2626 and A2440. The symmetric X-ray emission from A2626 suggests a relaxed, single-component system; however, 159 redshifts reveals a complex, three component cluster at cz~17,500km/s. One components is a typical X-ray bright cluster, a second has a cluster-like galaxy population with lower central galaxy and gas density, and the third is a background structure with field galaxy composition. A comparison of the magnitude distributions within the two subclusters suggests that A2626 is a merger in progress. Virial masses and a composite luminosity function for a region with projected radius r=1.5h^{-1}Mpc yield a mass to light ratio M/L_R~610h. Analysis of the X-ray emission from the primary component yields a gas mass fraction of 2.2h^{-3/2}% and a baryon fraction of 3.4%. A radial infall model implies that the virial mass may be an underestimate. In A2440, deep CCD $R$ band photometry indicates a striking correspondence between the galaxy and gas distributions. The galaxy distribution has three main components, each associated with a giant elliptical galaxy. The two larger peaks in the galaxy distribution coincide with the primary peaks in the X-ray emission, and the third is associated with a significant X-ray surface brightness enhancement. We use this galaxy-gas correspondence and 48 redshifts to argue that the subclusters are bound within a single system, and that the two primary components are beginning to merge. The composite luminosity function and estimates of the subcluster virial masses indicate a mass-to-light ratio from 660h to 880h.