M. B. Mosconi

Chemical Enrichment at High Redshifts: Understanding the Nature of Damped Lyα Systems in Hierarchical Models

We use cosmological hydrodynamical simulations including star formation and metal enrichment to study the evolution of the chemical properties of galaxy-like objects at high redshift in the range 0.25 < z < 2.35 in a hierarchical clustering scenario. We find that as the galactic objects are assembled, their gaseous components exhibit neutral hydrogen column densities with abundances and scatter comparable to those observed in damped Lyα systems (DLAs). The unweighted mean of abundance ratios and least-square linear regressions through the simulated DLAs yield intrinsic metallicity evolution for [Zn/H] and [Fe/H] consistent with results obtained from similar analyses of available observations. Our model statistically reproduces the mild evolution detected in the metallicity of the neutral hydrogen content of the universe, given by mass-weighted means, if observational constraints are considered (as suggested in 1998 by Boissee and co-workers). For the α-elements in the simulated DLAs, we find neither enhancement nor dependence on metallicity. Our results support the hypotheses that DLAs trace a variety of galactic objects with different formation histories and that both Type I and Type II supernovae are contributing to the chemical enrichment of the gas component, at least since z ≈ 2. This study indicates that DLAs could be understood as the building blocks that merged to form current normal galaxies within a hierarchical clustering scenario.

Building blocks in hierarchical clustering scenarios and their connection with damped Lyα systems

We carried out a comprehensive analysis of the chemical properties of the interstellar medium (ISM) and the stellar population (SP) of current normal galaxies and their progenitors in a hierarchical clustering scenario. We compared the results with obser- vations of Damped Lyman-� systems (DLAs) under the hypothesis that, at least, part of the observed DLAs could originate in the building blocks of today normal galax- ies. We used a hydrodynamical cosmological code which includes star formation and chemical enrichment. Galaxy-like objects are identified at z = 0 and then followed back in time. Random line-of-sights (LOSs) are drawn through these structures in order to mimic Damped Lymansystems. We then analysed the chemical properties of the ISM and SP along the LOSs. We found that the progenitors of current galaxies in the field with mean L < 0.5L ∗ and virial circular velocity of 100 −250 km/sec could be the associated DLA galaxies. For these systems we detected a trend for h L/L ∗ i to increase with redshift. We found moderate metallicity evolution for the (Zn/H), (Fe/H) and (Si/H). However, when we applied the observational filter suggested by Boisse et al. (1998) in order to restrict the sample to the observed limits in densities and metallicities, we found mild evolution consistent with observational results that include dust corrections. The (Si/Fe) and (S/Fe) show weak �-enhancement in agree- ment with observations corrected by dust depletion. We found the �/Fe in the ISM and SP to have more homogeneous abundances than the (Fe/H) and (Zn/H) ones. In our models, the global metallicity evolution is driven by the high metallicity and high column density simulated DLAs which have low impact parameters (b < 5 kpc) and SPs with more than 10 8 M⊙. Our results suggest that geometrical effects could be the mechanism responsible for the non-detectability of high metallicity and high column density DLAs. We found sub-DLAs to map preferentially the outskirts of the simu- lated DLA galaxies. Hence, they can contribute to the study of the metallicity of the galactic structure as a function of redshift. An analysis of the metallicity content of the ISMs and SPs of the galaxy-like objects as a function of redshift show the forma- tion of a central stellar mass concentration with nearly solar metallicity at all redshift while stars in the outer parts of these objects have lower metallicities. The gas content gets enriched progressively with redshift and at all radii. The abundance properties of the galaxy-like objects and the simulated DLAs are the results of the contribution of supernovae type Ia and type II and gas infall from the dark matter haloes with a timing settled by their particular history of evolution in a hierarchical clustering sce- nario. Our results suggest that the mild evolution detected in the observations could arise from a conspiracy of all these processes.

A simple model for dissipative galaxy formation

In the presently analyzed numerical experiment results of interacting gas cloud dynamics in a protogalactic potential, inelastic cloud-cloud collision-induced energy dissipation leads to a strong flattening and a large rotational vs kinetic energy ratio for clouds with several inelastic conditions. The adimensional spin parameter reaches high values for this highly dissipative component; after the collapse, the resulting systems exist in a stationary state in which collisions gradually increase the number of star clusters composed of gas clouds with a threshold condition in the number of inelastic collisions. Energy dissipation/relaxation process-involving gasdynamic effects play a fundamental role in the galaxy-formation model presented. 30 refs.

Nitrogen abundances in damped Lyman α systems: the combined effects of SNII and SNIa in a hierarchical clustering scenario

The combined enrichment of supernovae (SN) types II and I in a hierarchical clustering scenario could produce regions with low N content with respect to α elements, consistent with observed values measured in damped Lyman α systems (DLAs). We have studied the formation of DLAs in a hierarchical clustering scenario under the hypothesis that the building blocks of current field galaxies could be part of the structures mapped by DLAs. In our models the effects of the non-linear evolution of the structure (which produces bursty star formation histories, gas infall, etc.) and the contributions of SNIa and SNII are found to be responsible of producing these N regions with respect to the α elements. Although SNIa are not main production sites for Si or O, because of the particular timing between SNIa and SNII, their contributions can help to produce clouds with such abundances. Consistently, we found the simulated low nitrogen DLAs to have subsolar [Fe/H]. We show that low nitrogen DLAs have experienced important star formation activity in the past with higher efficiency than normal DLAs. Our chemical model suggests that SNIa play a relevant role in the determination of the abundance pattern of DLA and, that the observed low nitrogen DLA frequency could be explained taking into account the time-delay of ≈0.5 Gyr introduced by these supernova to release

Spherical galaxy collisions - Head-on encounters

A series of N-body simulations was performed in order to investigate head-on encounters between equal mass spherical galaxies of similar structure. Two different types of galaxy models were used in the experiments, differing mainly in the velocity distribution and central concentration: in the first kind of model, most of the particles moved predominantly along eccentric orbits, while in the second type most of the halo kinetic energy resided on the tangential velocity components of the particles and the core was rather isotropic. The fractional internal energy gain that a galaxy experiences after a head-on encounter was found to depend slightly on the type of orbits which predominate in the galaxy. The exchange of particles in a head-on collision is negligible. Tandem encounters are progressively more effective in producing escapers. The bounce induced by the overlapping at pericentric passage tends to erase the initial conditions of the galaxy models. 19 references.

The tidal origin of angular momentum in galaxies

We present the results of numerical simulations of relaxing protogalaxies under the tidal action of other similar systems and also clusters of galaxies. It is found that the bimodal behaviour of the observed angular momentum of galaxies can be explained under the assumption of different initial dynamical conditions induced by the evolving structure of the Universe expected in the adiabatic picture.

Astronomical Site Testing in Northwest of Argentina

We present the preliminary results of the astronomical site testing, which the group IATE of the OAC is developing in northwest of Argentine in collaboration with ESO and the Department of Astronomy of Cornell University.We show the results, which we have obtained from the analysis of GOES 8 satellite images in the 10.7 μm band, which allowed us to configurate a map of clear sky regions in an area between 23° and 28° S, and 66° 30′ and 69° W. We also comment logistical and tectonic aspects, and discuss next steps to follow in the research.

Evolution of Galaxy Luminosity in the CDM Model

We analyze the evolution of the luminosity function of galaxies using the CDM model in numerical simulations. There is an observational excess in the number counts of galaxies per square degree in the blur band Nb (i.e. APM, Maddox et. al. 1990). Several authors have tried to reproduce it trying to fit the observations. Our model assumes an instantaneous star formation rate (SFR) proportional to a power of the local density. A ‘single star burst’ is produced each time step and we follow the evolution of the luminosity and colour of each ‘stellar group’. The galaxies are identified with a density criterium. We compute U,V,B,K colours and Nb and our results agree quite well with observations.