Home  About us  Contact
 

Gas Clouds (gas + cloud)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

Ionization, shocks and evolution of the emission-line gas of distant 3CR radio galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2000
P. N. Best
An analysis of the kinematics and ionization state of the emission-line gas of a sample of 14 3CR radio galaxies with redshifts z,1 is carried out. The data used for these studies, deep long-slit spectroscopic exposures from the William Herschel Telescope, are presented in an accompanying paper. It is found that radio sources with small linear sizes (,150 kpc) have lower ionization states, higher emission-line fluxes and broader line widths than larger radio sources. An analysis of the low-redshift sample of Baum et al. demonstrates that radio galaxies at low redshift show similar evolution in their velocity structures and emission-line ratios from small to large radio sources. The emission-line ratios of small radio sources are in agreement with theoretical shock ionization predictions, and their velocity profiles are distorted. Together with the other emission-line properties, this indicates that shocks associated with the radio source dominate the kinematics and ionization of the emission-line gas during the period that the radio source is expanding through the interstellar medium. Gas clouds are accelerated by the shocks, giving rise to the irregular velocity structures observed, whilst shock compression of emission-line gas clouds and the presence of the ionizing photons associated with the shocks combine to lower the ionization state of the emission-line gas. By contrast, in larger sources the shock fronts have passed well beyond the emission-line regions; the emission-line gas of these larger radio sources has much more settled kinematical properties, indicative of rotation, and emission-line ratios consistent with the dominant source of ionizing photons being the active galactic nucleus. This strong evolution with radio size of the emission-line gas properties of powerful radio galaxies mirrors the radio size evolution seen in the nature of the optical,ultraviolet continuum emission of these sources, implying that the continuum alignment effect is likely to be related to the same radio source shocks. [source]

Simulations of the formation and evolution of dwarf galaxies

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
S. Valcke
Abstract We present models of the formation and evolution of isolated dwarf galaxies. The models follow the evolution of an initially homogeneous gas cloud collapsing in a pre-existing dark-matter halo. These simplified initial conditions are supported by the merger trees of isolated dwarf galaxies extracted from the milli-Millennium Simulation. An extensive comparison of the models to observations was made. The models' surface brightness profiles are well fitted by Sérsic profiles and the correlations between the models' Sérsic parameters and luminosity agree with the observations. We have also compared model predictions for the half-light radius Re, central velocity dispersion ,c, broad band colour B , V, metallicity [Z/Z,] versus luminosity relations and for the location relative to the fundamental plane with the available data. In all cases the models give the correct slope, in most cases we also get the zero-point right (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

What in the world is your object, Hanny?

ASTRONOMY & GEOPHYSICS, Issue 4 2010
Article first published online: 23 JUL 2010
Dutch schoolteacher Hanny van Arkel made headlines when she discovered a new type of object, a greenish gas cloud, as part of the Galaxy Zoo project in which citizen scientists can search the Sloan Digital Sky Survey database. [source]

Ionization, shocks and evolution of the emission-line gas of distant 3CR radio galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2000
P. N. Best
An analysis of the kinematics and ionization state of the emission-line gas of a sample of 14 3CR radio galaxies with redshifts z,1 is carried out. The data used for these studies, deep long-slit spectroscopic exposures from the William Herschel Telescope, are presented in an accompanying paper. It is found that radio sources with small linear sizes (,150 kpc) have lower ionization states, higher emission-line fluxes and broader line widths than larger radio sources. An analysis of the low-redshift sample of Baum et al. demonstrates that radio galaxies at low redshift show similar evolution in their velocity structures and emission-line ratios from small to large radio sources. The emission-line ratios of small radio sources are in agreement with theoretical shock ionization predictions, and their velocity profiles are distorted. Together with the other emission-line properties, this indicates that shocks associated with the radio source dominate the kinematics and ionization of the emission-line gas during the period that the radio source is expanding through the interstellar medium. Gas clouds are accelerated by the shocks, giving rise to the irregular velocity structures observed, whilst shock compression of emission-line gas clouds and the presence of the ionizing photons associated with the shocks combine to lower the ionization state of the emission-line gas. By contrast, in larger sources the shock fronts have passed well beyond the emission-line regions; the emission-line gas of these larger radio sources has much more settled kinematical properties, indicative of rotation, and emission-line ratios consistent with the dominant source of ionizing photons being the active galactic nucleus. This strong evolution with radio size of the emission-line gas properties of powerful radio galaxies mirrors the radio size evolution seen in the nature of the optical,ultraviolet continuum emission of these sources, implying that the continuum alignment effect is likely to be related to the same radio source shocks. [source]

Thick gas discs in faint dwarf galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010
Sambit Roychowdhury
ABSTRACT We determine the intrinsic axial ratio distribution of the gas discs of extremely faint MB < ,14.5 dwarf irregular galaxies. We start with the measured (beam corrected) distribution of apparent axial ratios in the H i 21-cm images of dwarf irregular galaxies observed as part of the Faint Irregular Galaxy GMRT Survey (FIGGS). Assuming that the discs can be approximated as oblate spheroids, the intrinsic axial ratio distribution can be obtained from the observed apparent axial ratio distribution. We use a variety of methods to do this, and our final results are based on using Lucy's deconvolution algorithm. This method is constrained to produce physically plausible distributions, and also has the added advantage of allowing for observational errors to be accounted for. While one might a priori expect that gas discs would be thin (because collisions between gas clouds would cause them to quickly settle down to a thin disc), we find that the H i discs of faint dwarf irregulars are quite thick, with mean axial ratio ,q,, 0.6. While this is substantially larger than the typical value of ,0.2 for the stellar discs of large spiral galaxies, it is consistent with the much larger ratio of velocity dispersion to rotational velocity (,/vc) in dwarf galaxy H i discs as compared to that in spiral galaxies. Our findings have implications for studies of the mass distribution and the Tully,Fisher relation for faint dwarf irregular galaxies, where it is often assumed that the gas is in a thin disc. [source]

Self-consistent simulations of star cluster formation from gas clouds under the influence of galaxy-scale tidal fields

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
Jarrod R. Hurley
ABSTRACT We present the first results of a project aimed at following the formation and long-term dynamical evolution of star clusters within the potential of a host galaxy. Here, we focus on a model evolved within a simplified potential representing the Large Magellanic Cloud. This demonstrates for the first time the self-consistent formation of a bound star cluster from a giant molecular cloud. The model cluster reproduces the density profiles and structural characteristics of observed star clusters. [source]

Helium pre-enrichment in the first stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2006
Leonid Chuzhoy
ABSTRACT We show that element diffusion can produce large fluctuations in the initial helium abundance of stars. Diffusion time-scale, which in stellar cores is much larger than the Hubble time, can fall below 108 yr in the neutral gas clouds of stellar mass, dominated by collisionless dark matter or with dynamically important radiation or magnetic pressure. Helium diffusion may therefore explain the recent observations of globular clusters, which are inconsistent with initially homogeneous helium distribution. [source]

Modelling the broad-band spectra of X-ray emitting GPS galaxies

ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009
L. Ostorero
Abstract The study of the broad-band emission of GHz-Peaked-Spectrum (GPS) radio galaxies is a powerful tool to investigate the physical processes taking place in the central, kpc-sized region of their active hosts, where the jets propagate and the lobes expand, interacting with the surrounding interstellar medium (ISM). We recently developed a new dynamical-radiative model to describe the evolution of the GPS phenomenon (Stawarz et al. 2008): as the relativistic jets propagate through the ISM, gradually engulfing narrow-line emitting gas clouds along their way, the electron population of the expanding lobes evolves, emitting synchrotron light, as well as inverse-Compton radiation via up-scattering of the photon fields from the host galaxy and its active nucleus. The model, which successfully reproduces the key features of the GPS radio sources as a class, provides a description of the evolution of their spectral energy distribution (SED) with the lobes' expansion, predicting significant and complex X-ray to , -ray emission. We apply here the model to the broad-band SED's of a sample of known, X-ray emitting GPS galaxies, and show that (i) the free-free absorption mechanism enables us to reproduce the radio continuum at frequencies below the turnover; (ii) the lobes' non-thermal, inverse-Compton emission can account for the observed X-ray spectra, providing a viable alternative to the thermal, accretion-dominated scenario. We also show that, in our sample, the relationship between the X-ray and radio hydrogen column densities, NH and NHI, is suggestive of a positive correlation, which, if confirmed, would support the scenario of high-energy emitting lobes (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]