A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios

We re-analyse the kinematics of the system of blue horizontal branch field (BHBF) stars in the Galactic halo (in particular the outer halo), fitting the kinematics with the model of radial and tangential velocity dispersions in the halo as a function of galactocentric distance r proposed by Sommer-Larsen, Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF stars. The basic result is that the character of the stellar halo velocity ellipsoid changes markedly from radial anisotropy at the sun to tangential anisotropy in the outer parts of the Galactic halo (r greater than approx 20 kpc). Specifically, the radial component of the stellar halo's velocity ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/- 10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The rapid decrease in the radial velocity dispersion is matched by an increase in the tangential velocity dispersion, with increasing r. Our results may indicate that the Galaxy formed hierarchically (partly or fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation scenario, which for quite a while has been favoured by most theorists and recently also has been given some observational credibility by HST observations of a potential group of small galaxies, at high redshift, possibly in the process of merging to a larger galaxy (Pascarelle et al 1996).Comment: Latex, 16 pages. 2 postscript figures. Submitted to the Astrophysical Journal. also available at http://astro.utu.fi/~cflynn/outerhalo.htm

Where are the ``Missing'' Galactic Baryons?

Based on 19 high-resolution N-body/gas-dynamical galaxy formation simulations in the LCDM cosmology it is shown, that for a galaxy like the Milky Way, in addition to the baryonic mass of the galaxy itself, about 70% extra baryonic mass should reside around the galaxy (inside of the virial radius), chiefly in the form of hot gas. Averaging over the entire field galaxy population, this ``external'' component amounts to 64-85% of the baryonic mass of the population itself. These results are supported by the recent detection of very extended, soft X-ray emission from the halo of the quiescent, massive disk galaxy NGC 5746. Some of the hot gas may, by thermal instability, have condensed into mainly pressure supported, warm clouds, similar to the Galactic High Velocity Clouds (HVCs). Based on an ultra-high resolution cosmological test simulation of a Milky Way like galaxy (with a gas particle mass and gravity softening length of only 7600 h^-1 Msun and 83 h^-1 pc,respectively), it is argued, that the hot gas phase dominates over the warm gas phase, in the halo. Finally, an origin of HVCs as ``leftovers'' from filamentary, ``cold'' accretion events, mainly occurring early in the history of galaxies, is proposed.Comment: 4 Pages, 4 Figures, Submitted to ApJL, Printing in colour recommende

CDM, Feedback and the Hubble Sequence

We have performed TreeSPH simulations of galaxy formation in a standard LCDM cosmology, including effects of star formation, energetic stellar feedback processes and a meta-galactic UV field, and obtain a mix of disk, lenticular and elliptical galaxies. The disk galaxies are deficient in angular momentum by only about a factor of two compared to observed disk galaxies. The stellar disks have approximately exponential surface density profiles, and those of the bulges range from exponential to r^{1/4}, as observed. The bulge-to-disk ratios of the disk galaxies are consistent with observations and likewise are their integrated B-V colours, which have been calculated using stellar population synthesis techniques. Furthermore, we can match the observed I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies, (M/L_I), is about 0.8. The ellipticals and lenticulars have approximately r^{1/4} stellar surface density profiles, are dominated by non-disklike kinematics and flattened due to non-isotropic stellar velocity distributions, again consistent with observations.Comment: 6 pages, incl. 4 figs. To appear in the proceedings of the EuroConference "The Evolution of Galaxies: II - Basic Building Blocks", Ile de La Reunion (France), 16-21 October 2001 (Slightly updated version). A much more comprehensive paper about this work with links to pictures of some of the galaxies can be found at http://babbage.sissa.it/abs/astro-ph/020436

The galaxy counterpart of the high-metallicity and 16 kpc impact parameter DLA towards Q0918+1636 - a challenge to galaxy formation models?

The quasar Q0918+1636 (z=3.07) has an intervening high-metallicity Damped Lyman-alpha Absorber (DLA) along the line of sight, at a redshift of z=2.58. The DLA is located at a large impact parameter of 16.2 kpc, and has an almost solar metallicity. It is shown, that a novel type of cosmological galaxy formation models, invoking a new SNII feedback prescription, the Haardt & Madau (2012) UVB field and explicit treatment of UVB self-shielding, can reproduce the observed characteristics of the DLA. UV radiation from young stellar populations in the galaxy, in particular in the photon energy range 10.36-13.61 eV (relating to Sulfur II abundance), are also considered in the analysis. It is found that a) for L~L* galaxies (at z=2.58), about 10% of the sight-lines through the galaxies at impact parameter 16.2 kpc will display a Sulfur II column density N(SII)$>$ 10$^{15.82}$ cm$^{-2}$ (the observed value for the DLA), and b) considering only cases where a near-solar metallicity will be detected at 16.2 kpc impact parameter, the probability distribution of galaxy SFR peaks near the value observed for the DLA galaxy counterpart of ~27 Msun/yr. It is argued, that the bulk of the alpha-elements, like Sulfur, traced by the high metal column density, b=16.2 kpc absorption lines, were produced by evolving young stars in the inner galaxy, and later transported outward by galactic winds.Comment: 22 pages, 24 figures, MNRAS in pres

The chemical evolution of gas-rich dwarf galaxies

A numerical double burst model of the chemical evolution of gas-rich dwarf galaxies has been developed. The model is fitted to a sample of N/O, O/H, Y and gas fraction observations, where N/O and O/H are the relative abundances by number of nitrogen to oxygen and oxygen to hydrogen, respectively. Y is the abundance by mass of helium. Closed models as well as models including enriched outflow, ordinary outflow and ordinary outflow combined with inflow are considered.The bursts are assumed to be instantaneous but ordered in pairs to explain the scatter in N/O-O/H. The method of gas fraction fitting is revised, and it is found that it is very important to specify whether dwarf irregulars (dIrrs) or blue compact galaxies (BCGs) are considered. Effective enriched winds fail when fitting N/O, whereas closed models, models with ordinary winds or a combination of ordinary winds and inflow are all viable.Comment: 22 pages, 25 figures, MNRAS LaTeX forma

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 V_c = 130-325 km/s. The galaxies have been obtained in simulations with a considerable range of physical parameters, varying the baryonic fraction, the gas metallicity, the meta-galactic UV 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 over-predict 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 a third solar this hot gas fraction becomes as low as just a few percent. We also find that most of the X-ray emission comes from the inner parts (inner about 20 kpc) of the hot galactic haloes. Finally, we find for realistic choices of the physical parameters that disc galaxy haloes possibly were more than one order of magnitude brighter in soft X-ray emission at z=1, than at present.Comment: 8 pages, 7 figures, MNRAS LaTeX forma

Lyman alpha Resonant Scattering in Young Galaxies - Predictions from Cosmological Simulations

We present results obtained with a 3D, Ly alpha radiative transfer code, applied to a fully cosmological galaxy formation simulation. The developed Monte Carlo code is capable of treating an arbitrary distribution of source Ly alpha emission, neutral hydrogen density, temperature, and peculiar velocity of the interstellar medium. We investigate the influence of resonant scattering on the appearance and properties of young galaxies by applying the code to a simulated "Lyman Break Galaxy" at redshift z = 3.6, and of star formation rate 22 M_sun/yr and total Ly alpha luminosity 2.0 X 10^43 erg/s. It is found that resonant scattering of Ly alpha radiation can explain that young galaxies frequently are observed to be more extended on the sky in Ly alpha than in the optical. Moreover, it is shown that, for the system investigated, due to the anisotropic escape of the photons, the appearent maximum surface brightness can differ by a factor of ~15, and the total derived luminosity by a factor of ~4, depending on the orientation of the system relative to the observer.Comment: Letter updated to match version published in Ap

The Tully-Fisher relation and its evolution with redshift in cosmological simulations of disc galaxy formation

We present predictions on the evolution of the Tully-Fisher (TF) relation with redshift, based on cosmological N-body/hydrodynamical simulations of disc galaxy formation and evolution. The simulations invoke star formation and stellar feedback, chemical evolution with non-instantaneous recycling, metallicity dependent radiative cooling and effects of a meta-galactic UV field, including simplified radiative transfer. At z=0, the simulated and empirical TF relations are offset by about 0.4 magnitudes (1 sigma) in the B and I bands. The origin of these offsets is somewhat unclear, but it may not necessarily be a problem of the simulations only. As to evolution, we find a brightening of the TF relation between z=0 and z=1 of about 0.85 mag in rest-frame B band, with a non-evolving slope. The brightening we predict is intermediate between the (still quite discrepant) observational estimates. This evolution is primarily a luminosity effect, while the stellar mass TF relation shows negligible evolution. The individual galaxies do gain stellar mass between z=1 and z=0, by a 50-100%; but they also correspondingly increase their characteristic circular speed. As a consequence, individually they mainly evolve ALONG the stellar mass TF relation, while the relation as such does not show any significant evolution