Determining the cosmic ray ionization rate in dynamically evolving clouds
Astronomy and Astrophysics 448:2 (2006) 425-432
Abstract:
The ionization fraction is an important factor in determining the chemical and physical evolution of star forming regions. In the dense, dark starless cores of such objects, the ionization rate is dominated by cosmic rays; it is therefore possible to use simple analytic estimators, based on the relative abundances of different molecular tracers, to determine the cosmic ray ionization rate. This paper uses a simple model to investigate the accuracy of two well-known estimators in dynamically evolving molecular clouds. It is found that, although the analytical formulae based on the abundances of H 3+, H2, CO, O, H2O and HCO + give a reasonably accurate measure of the cosmic ray ionization rate in static, quiescent clouds, significant discrepancies occur in rapidly evolving (collapsing) clouds. As recent evidence suggests that molecular clouds may consist of complex, dynamically evolving sub-structure, we conclude that simple abundance ratios do not provide reliable estimates of the cosmic ray ionization rate in dynamically active regions. © ESO 2006.Massive elliptical galaxies : From cores to haloes
ArXiv astro-ph/0512175 (2005)
Abstract:
In the context of recent observational results that show massive ellipticals were in place at high redshifts, we reassess the status of monolithic collapse in a LCDM universe. Using a sample of over 2000 galaxies from the Sloan Digital Sky Survey, by comparing the dynamical mass and stellar mass (estimated from colours) we find that ellipticals have `cores' which are baryon-dominated within their half-light radius. These galaxies correspond to 3-sigma peaks in the spherical collapse model if the total mass in the halo is assumed to be 20 times the dynamical mass within the half-light radius. This value yields stellar mass to total mass ratios of 8%, compared to a cosmological baryon fraction of 18% derived from WMAP3 alone. We further develop a method for reconstructing the concentration halo parameter c of the progenitors of these galaxies by utilizing adiabatic contraction. Although the analysis is done within the framework of monolithic collapse, the resulting distribution of c is log-normal with a peak value of c~3-10 and a distribution width similar to the results of N-body simulations. We also derive scaling relations between stellar and dynamical mass and the velocity dispersion, and find that these are sufficient to recover the tilt of the fundamental plane.Determining the cosmic ray ionization rate in dynamically evolving clouds
ArXiv astro-ph/0511064 (2005)
Abstract:
The ionization fraction is an important factor in determining the chemical and physical evolution of star forming regions. In the dense, dark starless cores of such objects, the ionization rate is dominated by cosmic rays; it is therefore possible to use simple analytic estimators, based on the relative abundances of different molecular tracers, to determine the cosmic ray ionization rate. This paper uses a simple model to investigate the accuracy of two well-known estimators in dynamically evolving molecular clouds. It is found that, although the analytical formulae based on the abundances of H3+,H2,CO,O,H2O and HCO+ give a reasonably accurate measure of the cosmic ray ionization rate in static, quiescent clouds, significant discrepancies occur in rapidly evolving (collapsing) clouds. As recent evidence suggests that molecular clouds may consist of complex, dynamically evolving sub-structure, we conclude that simple abundance ratios do not provide reliable estimates of the cosmic ray ionization rate in dynamically active regions.Hot cores: Probes of high-redshift galaxies
Monthly Notices of the Royal Astronomical Society 360:4 (2005) 1527-1531
Abstract:
The very high rates of second generation star formation detected and inferred in high-redshift objects should be accompanied by intense millimetre-wave emission from hot core molecules. We calculate the molecular abundances likely to arise in hot cores associated with massive star formation at high redshift, using several different models of metallicity in the early Universe. If the number of hot cores exceeds that in the Milky Way Galaxy by a factor of at least 1000, then a wide range of molecules in high-redshift hot cores should have detectable emission. It should be possible to distinguish between different models for the production of metals and hence hot core molecules should be useful probes of star formation at high redshift. © 2005 RAS.Hot Cores : Probes of High-Redshift Galaxies
ArXiv astro-ph/0504040 (2005)