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We developed a Monte Carlo code to generate long-duration gamma ray burst (LGRB) events within cosmological hydrodynamical simulations consistent with the concordance model. As structure is assembled, LGRBs are generated in the substructure that formed galaxies today. We adopted the collapsar model so that LGRBs are produced by single, massive stars at the final stage of their evolution. We found that the observed properties of the LGRB host galaxies (HGs) are reproduced if LGRBs are also required to be generated by low metallicity stars. The low metallicity condition imposed on the progenitor stars of LGRBs selects a sample of HGs with mean gas abundances of 12 + log O/H \~ 8.6. For z<1 the simulated HGs of low metallicity LGRB progenitors tend to be faint, slow rotators with high star formation efficiency, compared with the general galaxy population, in agreement with observations. At higher redshift, our results suggest that larger systems with high star formation activity could also contribute to the generation of LGRBs from low metallicity progenitors since the fraction of low metallicity gas available for star formation increases for all systems with look-back time. Under the hypothesis of our LGRB model, our results support the claim that LGRBs could be unbiased tracers of star formation at high redshifts., Comment: Final revised version with minor changes. 9 pages, 9 figures, mn2e.cls. To appear in MNRAS

We present a Supernova (SN) feedback model that succeeds at describing the chemical and energetic effects of SN explosions in galaxy formation simulations. This new SN model has been coupled to GADGET-2 and works within a new multiphase scheme which allows the description of a co-spatial mixture of cold and hot interstellar medium phases. No ad hoc scale-dependent parameters are associated to these SN and multiphase models making them particularly suited to studies of galaxy formation in a cosmological framework. Our SN model succeeds not only in setting a self-regulated star formation activity in galaxies but in triggering collimated chemical-enriched galactic winds. The effects of winds vary with the virial mass of the systems so that the smaller the galaxy, the larger the fraction of swept away gas and the stronger the decrease in its star formation activity. The fact that the fraction of ejected metals exceeds 60 per cent regardless of mass, suggests that SN feedback can be the responsible mechanism of the enrichment of the intergalactic medium to the observed levels., Comment: 2 pages, 1 figure. Presented at LARIM05; to appear in RevMexAst Conf Series

We study the effects of Supernova (SN) feedback on the formation of disc galaxies. For that purpose we run simulations using the extended version of the code GADGET-2 which includes a treatment of chemical and energy feedback by SN explosions. We found that our model succeeds in setting a self-regulated star formation process since an important fraction of the cold gas from the center of the haloes is efficiently heated up and transported outwards. The impact of SN feedback on galactic systems is also found to depend on virial mass: smaller systems are more strongly affected with star formation histories in which several starbursts can develop. Our implementation of SN feedback is also successful in producing violent outflows of chemical enriched material., Comment: Proceedings of the 2005 Anual Meeting of the Argentinian Astronomical Society, baaa-eng.sty

Gamma ray bursts (GRBs) belong to the most energetic events in the Universe. Recently, the extragalactic nature of these sources has been confirmed with the discovery of several host galaxies (HGs) and the measurement of their redshifts. To explain the origin of GRBs various models have been proposed, among which the coalescence of compact objects and the "collapsar" scenarios are the most representative, being the collapsar model one of the most accepted to explain the long duration GRBs. A natural consequence of this model is that the GRBs would trace the star formation rate (SFR) of their HGs. In this contributed paper we present preliminary results of the development of a Montecarlo-based code for collapsar event formation which is coupled to chemical-cosmological simulations aiming at studying the properties of HGs in a hierarchical scenario., Comment: Minor changes after referee report. To appear in the 48th Bulletin of the Argentine Astronomical Society (BAAA 48, 2005). 4 pages, baaa-eng.sty

We study the mass-metallicity relation of galactic systems with stellar masses larger than 10^9 Mo in Lambda-CDM scenarios by using chemical hydrodynamical simulations. We find that this relation arises naturally as a consequence of the formation of the structure in a hierarchical scenario. The hierarchical building up of the structure determines a characteristic stellar mass at M_c ~10^10.2 Moh^-1 which exhibits approximately solar metallicities from z ~ 3 to z=0. This characteristic mass separates galactic systems in two groups with massive ones forming most of their stars and metals at high redshift. We find evolution in the zero point and slope of the mass-metallicity relation driven mainly by the low mass systems which exhibit the larger variations in the chemical properties. Although stellar mass and circular velocity are directly related, the correlation between circular velocity and metallicity shows a larger evolution with redshift making this relation more appropriate to confront models and observations. The dispersion found in both relations is a function of the stellar mass and reflects the different dynamical history of evolution of the systems., Comment: 4 pages, 4 figures. Accepted MNRAS Letters

We discuss a model for treating chemical enrichment by SNII and SNIa explosions in simulations of cosmological structure formation. Our model includes metal-dependent radiative cooling and star formation in dense collapsed gas clumps. Metals are returned into the diffuse interstellar medium by star particles using a local SPH smoothing kernel. A variety of chemical abundance patterns in enriched gas arise in our treatment owing to the different yields and lifetimes of SNII and SNIa progenitor stars. In the case of SNII chemical production, we adopt metal-dependent yields. Because of the sensitive dependence of cooling rates on metallicity, enrichment of galactic haloes with metals can in principle significantly alter subsequent gas infall and the build up of the stellar components. Indeed, in simulations of isolated galaxies we find that a consistent treatment of metal-dependent cooling produces 25% more stars outside the central region than simulations with a primordial cooling function. In the highly-enriched central regions, the evolution of baryons is however not affected by metal cooling, because here the gas is always dense enough to cool. A similar situation is found in cosmological simulations because we include no strong feedback processes which could spread metals over large distances and mix them into unenriched diffuse gas. We demonstrate this explicitly with test simulations which adopt super-solar cooling functions leading to large changes both in the stellar mass and in the metal distributions. We also find that the impact of metallicity on the star formation histories of galaxies may depend on their particular evolutionary history. Our results hence emphasise the importance of feedback processes for interpreting the cosmic metal enrichment., Comment: 15 pages, 15 figures, MNRAS, modified to match published version

We use cosmological SPH simulations to investigate the effects of mergers and interactions on the formation of the bulge and disc components of galactic systems. We find that secular evolution during mergers seems to be a key process in the formation of stable disc-bulge systems with observational counterparts and contributes to establish the fundamental relations observed in galaxies. Our findings suggest that the secular evolution phase couples the formation mechanisms of the bulge and disc components. According to our results, depending on the particular stability properties and merger parameters, violents events could drive a morphological loop in which the outcome could be a disc or a spheroid., Comment: 2 pages. To appear in the proceedings of the IAUC198 "Near-Field Cosmology with Dwarf Elliptical Galaxies", 14 - 18 March 2005, Les Diablerets, Switzerland

We use cosmological SPH simulations to study the effects of mergers in the star formation history of galactic objects in hierarchical clustering scenarios. We find that during some merger events, gaseous discs can experience two starbursts: the first one during the orbital decay phase, due to gas inflows driven as the satellite approaches, and the second one, when the two baryonic clumps collide. A trend for these first induced starbursts to be more efficient at transforming the gas into stars is also found. We detect that systems which do not experience early gas inflows have well-formed stellar bulges and more concentrated potential wells, which seem to be responsible for preventing further gas inward transport triggered by tidal forces. Our results constitute the first proof that bulges can form as the product of collapse, collisions and secular evolution in a cosmological framework, and they are consistent with a rejuvenation of the stellar population in bulges at intermediate z with, at least, 50% of the stars (in SCDM) being formed at high z. (Abridged), Comment: 17 pages, 6 postscript figures. Accepted MNRAS