The Onset of the Cold HI Phase in Disks of Protogalaxies

We discuss a possible delay experienced by protogalaxies with low column density of gas in forming stars over large scales. After the hydrogen has recombined, as the external ionizing UV flux decreases and the metal abundance $Z$ increases, the HI, initially in the warm phase (T\simgt 5000 K), makes a transition to the cool phase (T\simlt 100 K). The minimum abundance $Z_{min}$ for which this phase transition takes place in a small fraction of the Hubble time decreases rapidly with increasing gas column density. Therefore in the ``anemic'' disk galaxies, where $N_{HI}$ is up to ten times smaller than for normal large spirals, the onset of the cool HI phase is delayed. The onset of gravitational instability is also delayed, since these objects are more likely to be gravitationally stable in the warm phase than progenitors of today's large spiral galaxies. The first substantial burst of star formation may occur only as late as at redshifts $z \sim 0.5$ and give a temporary high peak luminosity, which may be related to the ``faint blue objects". Galaxy disks of lower column density tend to have lower escape velocities and a starburst/galactic fountain instability which decreases the gas content of the inner disk drastically.Comment: TeX file, 24 pages, 4 figures available upon request from [email protected], to appear in The Astrophysical J. (Sept. 1

Selection effects and binary galaxy velocity differences

Measurements of the velocity differences (delta v's) in pairs of galaxies from large statistical samples have often been used to estimate the average masses of binary galaxies. A basic prediction of these models is that the delta v distribution ought to decline monotonically. However, some peculiar aspects of the kinematics have been uncovered, with an anomalous preference for delta v approx. equal to 72 km s(sup-1) appearing to be present in the data. The authors examine a large sample of binary galaxies with accurate redshift measurements and confirm that the distribution of delta v's appears to be non-monotonic with peaks at 0 and approx. 72 km s (exp -1). The authors suggest that the non-zero peak results from the isolation criteria employed in defining samples of binaries and that it indicates there are two populations of binary orbits contributing to the observed delta v distribution

Competition between Pressure and Gravity Confinement in Lyman-Alpha Forest Observations

A break in the distribution function of Ly$\alpha$ clouds (at a typical redshift of $2.5$) has been reported by Petitjean et al. (1993). This feature is what would be expected from a transition between pressure confinement and gravity confinement (as predicted in Charlton, Salpeter, and Hogan (1993)). The column density at which the feature occurs has been used to determine the external confining pressure, $\sim 10 {\rm cm}^{-3} {\rm K}$, which could be due to a hot, intergalactic medium. For models that provide a good fit to the data, the contribution of the gas in clouds to $\Omega$ is small. The specific shape of the distribution function at the transition (predicted by models to have a non-monotonic slope) can serve as a diagnostic of the distribution of dark matter around Ly$\alpha$ forest clouds, and the present data already eliminate certain models.Comment: 10 pages plain TeX, 2 figures available upon request, submitted to ApJ Letters, PSU-jc-

Hi Spin Temperatures and Heating Requirements in Outer Regions of Disk Galaxies

ABSRACT:We show how to use 21-cm emission and absorption studies to estimate the heat inputs to the neutral gas in low pressure environments, such as in outer disks of spiral galaxies, in galactic halos or in intergalactic space. For a range of model parameters we calculate the gas kinetic and spin temperatures ($T_K$ and $T_S$) and the relation between $T_S$ and the heat input to the gas. We outline the conditions for a ``two phase medium'' to exist. We find that although $T_S$ can be much smaller than $T_K$, $T_S$ is always $\gg 3$ K for column densities greater that $5 \times 10^{18}$ cm$^{-2}$. This excludes the possibility that relevant HI masses at the periphery of galaxies are invisible at 21-cm in emission. The outermost interstellar gas in a disk galaxy is more directly affected by external processes and in this paper we estimate the intensity of the extragalactic background at energies close to 0.1 keV by comparing our theoretical results with HI emission/absorption studies. We take into account the possibility that some energy produced in the inner regions affects the energy balance in outer regions. We find that in the absence of any other local heat source QSO dominated background models are still compatible with the spin temperature limits derived for the two best documented HI emission/absorption studies in outer regions.Comment: 24 pages, 8 figures ARCETRI-PR-93-2

Nuclear Astrophysics Before 1957

I discuss especially my summer with Willy Fowler at Kellogg Radiation in 1951, where I did my "triple-alpha" work. I also go back even earlier to Arthur Eddington and Hans Bethe. I also mention the 1953 summer school in Ann Arbor.Comment: 16 page

Sharp HI edges at high z: the gas distribution from Damped Lyman-alpha to Lyman-limit absorption systems

We derive the distribution of neutral and ionized gas in high redshift clouds which are optically thick to hydrogen ionizing radiation, using published data on Lyman-limit and Damped Lyman-alpha absorption systems in the redshift range 1.75 < z < 3.25. We assume that the distribution of the hydrogen total (HI+HII) column density in the absorbers follows a power law K N_H^{-alpha}, whereas the observed HI column density distribution deviates from a pure power law as a result of ionization from a background radiation field. Comparison of the models and observations give Maximum Likelihood solutions for the exponent alpha and for X, the value of log(N_H/N_HI) when the Lyman-limit optical depth is unity: alpha=2.7^{+1.0}_{-0.7} and X=2.75\pm0.35. X is much lower than what would be obtained for a gaseous distribution in equilibrium under its own gravity but the ratio of dark matter to gas density is not well constrained being log(eta_0)=1.1\pm 0.8. An extrapolation of our derived power law distribution towards systems of lower column density, the Lyman-alpha forest, favours models with log(eta_0) < 1.1 and alpha=2.7-3.3. With alpha appreciably larger than 2, Lyman-limit systems contain more gas than Damped Lyman-alpha systems and Lyman-alpha forest clouds even more. Estimates of the cosmological gas and dark matter density due to absorbers of different column density around z=2.5 are also given.Comment: 21 pages, 6 figures. Accepted for publication in Ap

Hydrogen Phases on the Surface of a Strongly Magnetized Neutron Star

The outermost layers of some neutron stars are likely to be dominated by hydrogen, as a result of fast gravitational settling of heavier elements. These layers directly mediate thermal radiation from the stars, and determine the characteristics of X-ray/EUV spectra. For a neutron star with surface temperature T\lo 10^6 K and magnetic field B\go 10^{12} G, various forms of hydrogen can be present in the envelope, including atom, poly-molecules, and condensed metal. We study the physical properties of different hydrogen phases on the surface of a strongly magnetized neutron star for a wide range of field strength $B$ and surface temperature $T$. Depending on the values of $B$ and $T$, the outer envelope can be either in a nondegenerate gaseous phase or in a degenerate metallic phase. For T\go 10^5 K and moderately strong magnetic field, B\lo 10^{13} G, the envelope is nondegenerate and the surface material gradually transforms into a degenerate Coulomb plasma as density increases. For higher field strength, $B>> 10^{13}$ G, there exists a first-order phase transition from the nondegenerate gaseous phase to the condensed metallic phase. The column density of saturated vapor above the metallic hydrogen decreases rapidly as the magnetic field increases or/and temperature decreases. Thus the thermal radiation can directly emerge from the degenerate metallic hydrogen surface. The characteristics of surface X-ray/EUV emission for different phases are discussed. A separate study concerning the possibility of magnetic field induced nuclear fusion of hydrogen on the neutron star surface is also presented.Comment: TeX, 35 pages including 6 postscript figures. To be published in Ap

Hydrogen Molecules In Superstrong Magnetic Field: II. Excitation Levels

We study the energy levels of H$_2$ molecules in a superstrong magnetic field (B\go 10^{12} G), typically found on the surfaces of neutron stars. The interatomic interaction potentials are calculated by a Hartree-Fock method with multi-configurations assuming electrons are in the ground Landau state. Both the aligned configurations and arbitrary orientations of the molecular axis with respect to the magnetic field axis are considered. Different types of molecular excitations are then studied: electronic excitations, aligned (along the magnetic axis) vibrational excitations, transverse vibrational excitations (a constrained rotation of the molecular axis around the magnetic field line). Similar results for the molecular ion H$_2^+$ are also obtained and compared with previous variational calculations. Both numerical results and analytical fitting formulae are given for a wide range of field strengths. In contrast to the zero-field case, it is found that the transverse vibrational excitation energies can be larger than the aligned vibration excitation, and they both can be comparable or larger than the electronic excitations. For B\go B_{crit}=4.23\times 10^{13} G, the Landau energy of proton is appreciable and there is some controversy regarding the dissociation energy of H$_2$. We show that H$_2$ is bound even for $B>>B_{crit}$ and that neither proton has a Landau excitation in the ground molecular state.Comment: Revtex (45 pages), 3 postscript figures; Phys. Rev. A in pres

Large Scale Searches for Brown Dwarfs and Free-Floating Planets

Searches of large scale surveys have resulted in the discovery of over 1000 brown dwarfs in the Solar neighbourhood. In this chapter we review the progress in finding brown dwarfs in large datasets, highlighting the key science goals, and summarising the surveys that have contributed most significantly to the current sample.Comment: Accepted to appear in the Handbook of Exoplanets (Springer); Editors: Hans J. Deeg & Juan Antonio Belmont