Hot Gas Fraction Research Articles

The Multiphase Intracluster Medium in Galaxy Groups Probed by the Ly Forest

The case is made that the intracluster medium (ICM) in present-day spiral-rich galaxy groups probably has undergone a much slower evolution than that in elliptical-rich groups and clusters. The environments of protoclusters and protogroups at z > 2 are likely similar to spiral-rich group environments at lower redshift. Therefore, like the ICM in spiral-rich groups today, the ICM in protogroups and protoclusters at z > 2 is predicted to be significantly multiphased. The QSO Lyα forest in the vicinity of galaxies is an effective probe of the ICM at a wide range of redshift. Two recent observations of Lyα absorption around galaxies by Adelberger et al. and by Pascarelle et al. are reconciled, and it is shown that observations support the multiphase ICM scenario. Galaxy redshifts must be very accurate for such studies to succeed. This scenario can also explain the lower metallicity and lower hot gas fraction in groups.

Gas in Groups and Clusters of Galaxies

Groups and clusters contain a large fraction of hot gas which emits X-ray radiation. This gas yields information on the dynamical state and on the total mass of these systems. X-ray spectra show that heavy elements are present in the gas. As these metals must have been produced in the cluster/group galaxies and later transported into the gas, the metallicity is a good tracer for the transport processes. Several possible processes, that transport gas from the small potential wells of the galaxies into the clusters and groups, are discussed.

X‐Ray Scaling Relations of Galaxy Groups in a Hydrodynamic Cosmological Simulation

We examine the scalings of X-ray luminosity, temperature, and dark matter or galaxy velocity dispersion for galaxy groups in a ΛCDM cosmological simulation, which incorporates gravity, gas dynamics, radiative cooling, and star formation, but no substantial nongravitational heating. In agreement with observations, the simulated LX-σ and LX-TX relations are steeper than those predicted by adiabatic simulations or self-similar models, with LX ∝ σ4.4 and LX ∝ T for massive groups and significantly steeper relations below a break at σ ≈ 180 km s-1 (TX ≈ 0.7 keV). The TX-σ relation is fairly close to the self-similar scaling relation, with TX ∝ σ1.75, provided that the velocity dispersion is estimated from the dark matter or from 10 galaxies. The of hot gas in low-mass groups is higher than predicted by self-similar scaling or adiabatic simulations, and it agrees with observational data that suggest an entropy floor. The steeper scalings of the luminosity relations are driven by radiative cooling, which reduces the hot (X-ray-emitting) gas fraction from 50% of the total baryons at σ ≈ 500 km s-1 to 20% at σ ≈ 100 km s-1. A secondary effect is that hot gas in smaller systems is less clumpy, further driving down LX. A smaller volume simulation with 8 times higher mass resolution predicts nearly identical X-ray luminosities at a given group mass, demonstrating the insensitivity of the predicted scaling relations to numerical resolution. The higher resolution simulation predicts higher hot gas fractions at a given group mass, and these predicted fractions are in excellent agreement with available observations. There remain some quantitative discrepancies: the predicted mass scale of the LX-TX and LX-σ breaks is somewhat too low, and the luminosity-weighted temperatures are too high at a given σ, probably because our simulated temperature profiles are flat or rising toward small radii, while observed profiles decline at r 0.2Rvir. We conclude that radiative cooling has an important quantitative impact on group X-ray properties and can account for many of the observed trends that have been interpreted as evidence for nongravitational heating. Improved simulations and observations are needed to understand the remaining discrepancies and to decide the relative importance of cooling and nongravitational heating in determining X-ray scalings.

X-ray emission from haloes of simulated disc galaxies

Bolometric and 0.2–2 keV X-ray luminosities of the hot gas haloes of simulated disc galaxies have been calculated at redshift z= 0. The TreeSPH simulations are fully cosmological and the sample of 44 disc galaxies span a range in characteristic circular speeds of Vc= 130–325 km s−1. The galaxies have been obtained in simulations with a considerable range of physical parameters, varying the baryonic fraction, the gas metallicity, the meta-galactic ultraviolet field, the cosmology, the dark matter type, and also the numerical resolution. The models are found to be in agreement with the (few) relevant X-ray observations available at present. The amount of hot gas in the haloes is also consistent with constraints from pulsar dispersion measures in the Milky Way. Forthcoming XMM and Chandra observations should enable much more stringent tests and provide constraints on the physical parameters. We find that simple cooling flow models overpredict X-ray luminosities by up to two orders of magnitude for high (but still realistic) cooling efficiencies relative to the models presented here. Our results display a clear trend that increasing cooling efficiency leads to decreasing X-ray luminosities at z= 0. The reason is found to be that increased cooling efficiency leads to a decreased fraction of hot gas relative to total baryonic mass inside of the virial radius at present. At gas metal abundances of one-third the solar value this hot gas fraction becomes as low as just a few per cent. We also find that most of the X-ray emission comes from the inner parts (r≲ 20 kpc) of the hot galactic haloes. Finally, we find for realistic choices of the physical parameters that disc galaxy haloes possibly were one order of magnitude brighter in soft X-ray emission at z∼ 1 than at present.

The steady-state conduction-driven temperature profile in clusters of galaxies

In this paper, I present a theoretical model of a relaxed cluster where the temperature profile (hereafter TP) is structured by electronic thermal conduction. Neglecting cooling and heating terms, the stationary energy conservation equation reduces to a second-order differential equation, the resolution of which requires two boundary conditions, taken here as the inner radius and the ratio between the inner and outer temperature. Thus a two-parameter family of analytical models for the TP is obtained. Once these two constants are chosen, the TP has a fixed analytical expression, which reproduces nicely the observed ‘universal’ TP obtained by Markevitch et al. from ASCA data. Using observed X-ray surface brightnesses for two hot clusters with spatially resolved TP, the local polytropic index and the hot gas fraction profile are predicted and compared with ASCA observations (Markevitch et al. 1999). Moreover, the total density profile derived from observed X-ray surface brightness, hydrostatic equilibrium and the conduction-driven TP is very well fitted by three analytical profiles found to describe the structure of galactic or cluster haloes in numerical simulations of collisionless matter. With the forthcoming availability of spatially resolved high-quality spectroscopic data, the predicted shape of the TP (related to the temperature dependence of the heat flux for a collisionally ionized plasma) will be tested directly against observations.

Parameter tests within cosmological simulations of galaxy formation

ABSTRA C T Numerical simulations of galaxy formation require a number of parameters. Some of these are intrinsic to the numerical integration scheme (e.g., the time-step), while others describe the physical model (e.g., the gas metallicity). In this paper we present results of a systematic exploration of the effects of varying a subset of these parameters on simulations of galaxy formation. We use N-body and ‘Smoothed Particle Hydrodynamics’ techniques to follow the evolution of cold dark matter and gas in a small volume. We compare a fiducial model with 24 different simulations, in which one parameter at a time is varied, focusing on properties such as the relative fraction of hot and cold gas, and the abundance and masses of galaxies. We find that for reasonable choices of numerical values, many parameters have relatively little effect on the galaxies, with the notable exception of the parameters that control the resolution of the simulation and the efficiency with which gas cools.

On the Hot Gas Fraction in Supernova Remnants

On the Hot Gas Fraction in Supernova Remnants

Completing the evolution of supernova remnants and their bubbles

The filling fraction of hot gas in the ISM is reexamined with new calculations of the very long term evolution of SNRs and their fossil hot bubbles. Results are presented of a 1D numerical solution of the evolution of an SNR in a homogeneous medium with a nonthermal pressure corresponding to a 5-micro-G magnetic field and density of 0.2/cu cm. Comparison is made with a control simulation having no magnetic field pressure. It is found that the evolutions, once they have become radiative, differ in several significant ways, while both differ appreciably from qualitative pictures presented in the past. Over most of the evolution of either case, the hot bubble in the interior occupies only a small fraction of the shocked volume, the remainder in a thick shell of slightly compressed material. Column densities and radial distributions of O VI, N V, C IV, and Si IV as well as examples of absorption profiles for their strong UV lines are presented.

Gas clouds from tidally disrupted stars in active galactic nuclei

view Abstract Citations (35) References (111) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gas Clouds from Tidally Disrupted Stars in Active Galactic Nuclei Roos, Nico Abstract A direct implication of the presence of massive black holes in galactic nuclei is the occasional tidal disruption of a star in the vicinity of the hole. The tidal disruption rate will be enhanced as a result of perturbations of the star cluster around the hole (for instance during the final stage of a merger with a smaller galaxy) leading to prolonged periods of activity. Recent investigations of the tidal break- up of solar-type stars near massive black holes have shown that debris gas will be distributed throughout the galactic nucleus. About half of the stellar debris mass is strongly bound to the hole and will quickly be swallowed while the rest is expelled with typical velocities of order 5000M^1/6^_6_ km s^-1^, where M_6_ = M_hole_/10^6^ M_sun_. In this paper the structure and properties of freely expanding (unbound) remnant clouds irradiated by the central continuum source of an AGN are explored. It is argued that the remnant cloud fragments into cold clumps of gas (T~10^4^K) surrounded by hot gas at the Compton temperature (T_c_~10^7-8^K) when the ionization parameter {XI} (defined as the ratio of the ionizing radiation pressure to the gas pressure) reaches the critical value {XI}_c_ ~ 10. Subsequently the fraction of hot gas slowly increases at the expense of the cold component while the ionization parameter of both components remains equal to {XI}_c_. Photoionization of cold gas in the outmoving and radially elongated remnant clouds produces a broad line region at a distance of ~10^16-17^ cm in Seyferts and ~10^17-18^ cm in quasars. The broad line region (BLR) has an outer boundary where the remnant clouds become optically thin to the ionizing radiation from the central source. The densities and column densities in the BLR are in the range ~10^9-11 cm^-3^ and ~10^24-27^ cm^-2^, respectively ({XI}~10). Bound debris orbiting at ~10^15-16^M^2/3^_6_ cm with characteristic densities >= 10^11^ cm^-3^ may contribute to the inner part of the broad line region yielding symmetric lines, whereas a relatively small number (~10-100) of large outmoving remnant clouds produces a more asymmetric and bumpy line profile. There are two potential problems with this new model for the BLR: First, a high tidal disruption rate of order >= 0.1 M_sun_ yr^-1^ seems required to replenish the gas in the BLR in Seyferts as well as in QSOs. Second, in some AGN the central continuum source may be obscured by optically thick remnant clouds along our line of site. A very attractive property of the tidal disruption model is that it offers a natural interpretation not only for the broad emission lines but for a number of other features in AGN spectra as well. Tidal disruption will provide cold, dense gas close to the central source which can reprocess the radiation from the central source and produce a thermal bump in the optical/UV as well as several characteristic X-ray features. Optically thin remnants moving along our line of site outside the BLR at about 10^18-19^ cm may produce broad absorption lines similar to those observed in broad absorption line QSOs. The radially outmoving remnants are decelerated and compressed by the interstellar medium at radii >= 10^19^ cm, where they will emit narrow forbidden lines. Publication: The Astrophysical Journal Pub Date: January 1992 DOI: 10.1086/170919 Bibcode: 1992ApJ...385..108R Keywords: Active Galactic Nuclei; Black Holes (Astronomy); Gravitational Collapse; High Temperature Gases; Interstellar Gas; Star Clusters; Gas Ionization; Gas Pressure; Perturbation Theory; Photoionization; Quasars; Seyfert Galaxies; Spectral Line Width; Tides; Astrophysics; BLACK HOLE PHYSICS; GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: NUCLEI; GALAXIES: SEYFERT; GALAXIES: QUASARS: GENERAL full text sources ADS | data products SIMBAD (3) MAST (1)