The Spatial Distribution of the Galactic First Stars II: SPH Approach

We use cosmological, chemo-dynamical, smoothed particle hydrodynamical simulations of Milky-Way-analogue galaxies to find the expected present-day distributions of both metal-free stars that formed from primordial gas and the oldest star populations. We find that metal-free stars continue to form until z~4 in halos that are chemically isolated and located far away from the biggest progenitor of the final system. As a result, if the Population III initial mass function allows stars with low enough mass to survive until z=0 (< 0.8 Msol), they would be distributed throughout the Galactic halo. On the other hand, the oldest stars form in halos that collapsed close to the highest density peak of the final system, and at z=0 they are located preferentially in the central region of the Galaxy, i.e., in the bulge. According to our models, these trends are not sensitive to the merger histories of the disk galaxies or the implementation of supernova feedback. Furthermore, these full hydrodynamics results are consistent with our N-body results in Paper I, and lend further weight to the conclusion that surveys of low-metallicity stars in the Galactic halo can be used to directly constrain the properties of primordial stars. In particular, they suggest that the current lack of detections of metal-free stars implies that their lifetimes were shorter than a Hubble time, placing constraints on the metal-free initial mass function.Comment: Accepted by ApJ. Emulate ApJ styl

Passive Scalar Structures in Supersonic Turbulence

We conduct a systematic numerical study of passive scalar structures in supersonic turbulent flows. We find that the degree of intermittency in the scalar structures increases only slightly as the flow changes from transonic to highly supersonic, while the velocity structures become significantly more intermittent. This difference is due to the absence of shock-like discontinuities in the scalar field. The structure functions of the scalar field are well described by the intermittency model of She and L\'{e}v\^{e}que [Phys. Rev. Lett. 72, 336 (1994)], and the most intense scalar structures are found to be sheet-like at all Mach numbers.Comment: 4 pages, 3 figures, to appear in PR

Dark matter response to galaxy formation

We have resimulated the six galaxy-sized haloes of the Aquarius Project including metal-dependent cooling, star formation and supernova feedback. This allows us to study not only how dark matter haloes respond to galaxy formation, but also how this response is affected by details of halo assembly history. In agreement with previous work, we find baryon condensation to lead to increased dark matter concentration. Dark matter density profiles differ substantially in shape from halo to halo when baryons are included, but in all cases the velocity dispersion decreases monotonically with radius. Some haloes show an approximately constant dark matter velocity anisotropy with $\beta \approx 0.1-02$, while others retain the anisotropy structure of their baryon-free versions. Most of our haloes become approximately oblate in their inner regions, although a few retain the shape of their dissipationless counterparts. Pseudo-phase-space densities are described by a power law in radius of altered slope when baryons are included. The shape and concentration of the dark matter density profiles are not well reproduced by published adiabatic contraction models. The significant spread we find in the density and kinematic structure of our haloes appears related to differences in their formation histories. Such differences already affect the final structure in baryon-free simulations, but they are reinforced by the inclusion of baryons, and new features are produced. The details of galaxy formation need to be better understood before the inner dark matter structure of galaxies can be used to constrain cosmological models or the nature of dark matter.Comment: 14 pages, 9 figures. Accepted MNRAS. Revised version includes discussion on resolution effects and minor changes

Detecting the Gravitational Redshift of Cluster Gas

We examine the gravitational redshift of radiation emitted from within the potential of a cluster. Spectral lines from the intracluster medium (ICM) are redshifted in proportion to the emission-weighted mean potential along the line of sight, amounting to approximately 50 km/s at a radius of 100 kpc/h, for a cluster dispersion of 1200 km/s. We show that the relative redshifts of different ionization states of metals in the ICM provide a unique probe of the three-dimensional matter distribution. An examination of the reported peculiar velocities of cD galaxies in well studied Abell clusters reveals they are typically redshifted by an average of $\sim +200$ km/s. This can be achieved by gravity with the addition of a steep central potential associated with the cD galaxy. Note that in general gravitational redshifts cause a small overestimate of the recessional velocities of clusters by an average of $\sim$ 20 km/s.Comment: 6 pages, 3 figures, accepted to the Astrophysical Journal Letter

The role of tidal interactions in driving galaxy evolution

We carry out a statistical analysis of galaxy pairs selected from chemical hydrodynamical simulations with the aim at assessing the capability of hierarchical scenarios to reproduce recent observational results for galaxies in pairs. Particularly, we analyse the effects of mergers and interactions on the star formation (SF) activity, the global mean chemical properties and the colour distribution of interacting galaxies. We also assess the effects of spurious pairs.Comment: to appear in "Groups of galaxies in the nearby Universe" ESO Workshop, (Dec 2005) Santiago, Chil

Fingerprints of the hierarchical building up of the structure on the gas kinematics of galaxies

Recent observational and theoretical works have suggested that the Tully-Fisher Relation might be generalised to include dispersion-dominated systems by combining the rotation and dispersion velocity in the definition of the kinematical indicator. Mergers and interactions have been pointed out as responsible of driving turbulent and disordered gas kinematics, which could generate Tully-Fisher Relation outliers. We intend to investigate the gas kinematics of galaxies by using a simulated sample which includes both, gas disc-dominated and spheroid-dominated systems. Cosmological hydrodynamical simulations which include a multiphase model and physically-motivated Supernova feedback were performed in order to follow the evolution of galaxies as they are assembled. Both the baryonic and stellar Tully-Fisher relations for gas disc-dominated systems are tight while, as more dispersion-dominated systems are included, the scatter increases. We found a clear correlation between $\sigma / V_{\rm rot}$ and morphology, with dispersion-dominated systems exhibiting the larger values ($> 0.7$). Mergers and interactions can affect the rotation curves directly or indirectly inducing a scatter in the Tully-Fisher Relation larger than the simulated evolution since $z \sim 3$. Kinematical indicators which combine rotation velocity and dispersion velocity can reduce the scatter in the baryonic and the stellar mass-velocity relations. Our findings also show that the lowest scatter in both relations is obtained if the velocity indicators are measured at the maximum of the rotation curve. Moreover, the rotation velocity estimated at the maximum of the gas rotation curve is found to be the best proxy for the potential well regardless of morphology.Comment: 16 pages, 10 figures, accepted for publication in A&

AGN Feedback Causes Downsizing

We study the impact of outflows driven by active galactic nuclei (AGN) on galaxy formation. Outflows move into the surrounding intergalactic medium (IGM) and heat it sufficiently to prevent it from condensing onto galaxies. In the dense, high-redshift IGM, such feedback requires highly energetic outflows, driven by a large AGN. However, in the more tenuous low-redshift IGM, equivalently strong feedback can be achieved by less energetic winds (and thus smaller galaxies). Using a simple analytic model, we show that this leads to the anti-hierarchical quenching of star-formation in large galaxies, consistent with current observations. At redshifts prior to the formation of large AGN, galaxy formation is hierarchical and follows the growth of dark-matter halos. The transition between the two regimes lies at the z ~ 2 peak of AGN activity.Comment: 6 pages, 2 figures, ApJL in pres