arXiv subjects

P. B. Tissera

Publications and source records attributed to P. B. Tissera.

At least 37 records · Page 2Linked to original sources

arXiv2014

Morphology of galaxies with quiescent recent assembly history in a Lambda-CDM universe

The standard disc formation scenario postulates that disc forms as the gas cools and flows into the centre of the dark matter halo, conserving the specific angular momentum. Major mergers have been shown to be able to destroy or highly perturb the disc components. More recently, the alignment of the material that is accreted to form the galaxy has been pointed out as a key ingredient to determine galaxy morphology. However, in a hierarchical scenario galaxy formation is a complex process that combines these processes and others in a non-linear way so that the origin of galaxy morphology remains to be fully understood. We aim at exploring the differences in the formation histories of galaxies with a variety of morphology, but quite recent merger histories, to identify which mechanisms are playing a major role. We analyse when minor mergers can be considered relevant to determine galaxy morphology. We also study the specific angular momentum content of the disc and central spheroidal components separately. We used cosmological hydrodynamical simulations that include an effective, physically motivated supernova feedback that is able to regulate the star formation in haloes of different masses. We analysed the morphology and formation history of a sample of 15 galaxies of a cosmological simulation. We performed a spheroid-disc decomposition of the selected galaxies and their progenitor systems. The angular momentum orientation of the merging systems as well as their relative masses were estimated to analyse the role played by orientation and by minor mergers in the determination of the morphology. We found the discs to be formed by conserving the specific angular momentum in accordance with the classical disc formation model. The specific angular momentum of the stellar central spheroid correlates with the dark matter halo angular momentum and determines a power law. Abridged

astro-ph.GA↗

arXiv2013

Stellar haloes of simulated Milky Way-like galaxies: Chemical and kinematic properties

We investigate the chemical and kinematic properties of the diffuse stellar haloes of six simulated Milky Way-like galaxies from the Aquarius Project. Binding energy criteria are adopted to defined two dynamically distinct stellar populations: the diffuse inner and outer haloes, which comprise different stellar sub-populations with particular chemical and kinematic characteristics. Our simulated inner- and outer-halo stellar populations have received contributions from debris stars (formed in sub-galactic systems while they were outside the virial radius of the main progenitor galaxies) and endo-debris stars (those formed in gas-rich sub-galactic systems inside the dark matter haloes). The inner haloes possess an additional contribution from disc-heated stars in the range $\sim 3 - 30 %$, with a mean of $\sim 20% $. Disc-heated stars might exhibit signatures of kinematical support, in particular among the youngest ones. Endo-debris plus disc-heated stars define the so-called \insitu stellar populations. In both the inner- and outer-halo stellar populations, we detect contributions from stars with moderate to low [$α$/Fe] ratios, mainly associated with the endo-debris or disc-heated sub-populations. The observed abundance gradients in the inner-halo regions are influenced by both the level of chemical enrichment and the relative contributions from each stellar sub-population. Steeper abundance gradients in the inner-halo regions are related to contributions from the disc-heated and endo-debris stars, which tend to be found at lower binding energies than debris stars. (Abridged).

astro-ph.GA↗

arXiv2012

The Halo Shape and Evolution of Polar Disc Galaxies

We examine the properties and evolution of a simulated polar disc galaxy. This galaxy is comprised of two orthogonal discs, one of which contains old stars (old stellar disc), and the other, containing both younger stars and the cold gas (polar disc) of the galaxy. By exploring the shape of the inner region of the dark matter halo, we are able to confirm that the halo shape is a oblate ellipsoid flattened in the direction of the polar disc. We also note that there is a twist in the shape profile, where the innermost 3 kpc of the halo flattens in the direction perpendicular to the old disc, and then aligns with the polar disc out until the virial radius. This result is then compared to the halo shape inferred from the circular velocities of the two discs. We also use the temporal information of the simulation to track the system's evolution, and identify the processes which give rise to this unusual galaxy type. We confirm the proposal that the polar disc galaxy is the result of the last major merger, where the angular moment of the interaction is orthogonal to the angle of the infalling gas. This merger is followed by the resumption of coherent gas infall. We emphasise that the disc is rapidly restored after the major merger and that after this event the galaxy begins to tilt. A significant proportion of the infalling gas comes from filaments. This infalling gas from the filament gives the gas its angular momentum, and, in the case of the polar disc galaxy, the direction of the gas filament does not change before or after the last major merger.

astro-ph.GA↗

arXiv2011

Chemical abundances and spatial distribution of Long Gamma-Ray Bursts

We analyse the spatial distribution within host galaxies and chemical properties of the progenitors of Long Gamma Ray Bursts as a function of redshift. By using hydrodynamical cosmological simulations which include star formation, Supernova feedback and chemical enrichment and based on the hypothesis of the collapsar model with low metallicity, we investigate the progenitors in the range 0 < z < 3. Our results suggest that the sites of these phenomena tend to be located in the central regions of the hosts at high redshifts but move outwards for lower ones. We find that scenarios with low metallicity cut-offs best fit current observations. For these scenarios Long Gamma Ray Bursts tend to be [Fe/H] poor and show a strong alpha-enhancement evolution towards lower values as redshift decreases. The variation of typical burst sites with redshift would imply that they might be tracing different part of galaxies at different redshifts.

astro-ph.CO↗

arXiv2008

Rotation curve bifurcations as indicators of close recent galaxy encounters

Rotation curves of interacting galaxies often show that velocities are either rising or falling in the direction of the companion galaxy. We seek to reproduce and analyse these features in the rotation curves of simulated equal-mass galaxies suffering a one-to-one encounter, as possible indicators of close encounters. Using simulations of major mergers in 3D, we study the time evolution of these asymmetries in a pair of galaxies, during the first passage. Our main results are: (a) the rotation curve asymmetries appear right at pericentre of the first passage, (b) the significant disturbed rotation velocities occur within a small time interval, of ~ 0.5 Gyr h^-1, and therefore the presence of bifurcation in the velocity curve could be used as an indicator of the pericentre occurrence. These results are in qualitative agreement with previous findings for minor mergers and fly-byes.

astro-ph↗

arXiv2007

The host galaxies of long-duration GRBs in a cosmological hierarchical scenario

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.

astro-ph↗

arXiv2006

Feedback and metal enrichment in cosmological SPH simulations I. A model for chemical enrichment

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.

astro-ph↗

arXiv2006

Study of the relationship between the gamma ray bursts and their host galaxies

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.

astro-ph↗

arXiv2006

Galaxy Formation and SN Feedback

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.

astro-ph↗

arXiv2006

Impact of Supernova Explosions on Galaxy Formation

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.

astro-ph↗

arXiv2005

Are violent events responsible of a Galaxy Morphological loop?

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.

astro-ph↗

arXiv2003

Galaxy Pairs in the 2dF Survey I. Effects of Interactions in the Field

We study galaxy pairs in the field selected from the 100 K public release of the 2dF galaxy redshift survey. Our analysis provides a well defined sample of 1258 galaxy pairs, a large database suitable for statistical studies of galaxy interactions in the local universe, $z \le 0.1$. Galaxy pairs where selected by radial velocity ($ΔV$) and projected separation ($r_{\rm p}$) criteria determined by analyzing the star formation activity within neighbours (abridged). The ratio between the fractions of star forming galaxies in pairs and in isolation is a useful tools to unveil the effects of having a close companion. We found that about fifty percent of galaxy pairs do not show signs of important star formation activity (independently of their luminosities) supporting the hypothesis that the internal properties of the galaxies play a crucial role in the triggering of star formation by interactions.

astro-ph↗

arXiv2003

Building Blocks in Hierarchical Clustering Scenarios and their Connection with Damped Ly$α$ 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 observations of Damped Lyman-$α$ systems (DLAs) under the hypothesis that, at least, part of the observed DLAs could originate in the building blocks of today normal galaxies. 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 (LOS) are drawn through these structures in order to mimic Damped Lyman $α$ systems. We then analysed the chemical properties of the ISM and SP along the LOS. We found that the progenitors of current galaxies in the field with mean $L <0.5 L^* $ and virial circular velocity of $100-250 {\rm km/sec}$ could be the associated DLA galaxies. For these systems we detected a trend for $ $ to increase with redshift.(Abridged)

astro-ph↗

arXiv2002

Nitrogen Abundances in DLA Systems: The Combined Effects of SNII and SNIa in a Hierarchical Clustering Scenario

The combined enrichment of Supernovae II and I in a hierarchical clustering scenario could produce regions with low N content respect to $α$-elements consistent with observed values measured in Damped Ly-$α$ (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 Consistently, we found the simulated low nitrogen DLAs to have sub-solar [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 $\approx $ 0.5 Gyr introduced by these supernova to release metals.

astro-ph↗

arXiv2002

Double Starbursts Triggered by Mergers in Hierarchical Clustering Scenarios

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)

astro-ph↗

arXiv2001

Chemical evolution using SPH cosmological simulations. I: implementation, tests and first results

We develop a model to implement metal enrichment in a cosmological context based on the hydrodynamical AP3MSPH code described by Tissera, Lambas and Abadi (1997).The star formation model is based on the Schmidt law and has been modified in order to describe the transformation of gas into stars in more detail. The enrichment of the interstellar medium due to supernovae I and II explosions is taken into account by assuming a Salpeter Initial Mass Function and different nucleosynthesis models.The different chemical elements are mixed within the gaseous medium according to the Smooth Particle Hydrodynamics technique. We present tests of the code that assess the effects of resolution and model parameters on the results. scenario and we present results of the analysis of the star formation and chemical properties of the interstellar medium and stellar population of the simulated galactic objects. We show that these systems reproduce abundance ratios for primary and secondary elements of the interstellar medium, and the correlation between the (O/H) abundance and the gas fraction of galaxies. The numerical simulations performed provide a detailed description of the chemical properties of galactic objects formed in hierarchical clustering scenarios and proved to be useful tools to deepen our understanding of galaxy formation and evolution.

astro-ph↗

arXiv2001

Chemical Enrichment at High Redshifts: Understanding the Nature of Damped Ly$α$ 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. As the galactic objects are assembled we find that their gaseous components exhibit neutral Hydrogen column densities with abundances and scatter comparable to those observed in damped Lyman-$α$ systems (DLAs).The unweighted mean of abundance ratios and least square linear regressions through the simulated DLAs yield intrinsic metallicity evolution for the [Zn/H] and [Fe/H], consistent with results obtained from similar analysis 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 by Boissée et al. 1998). 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 SNI and SNII are contributing to the chemical enrichment of the gas component at least since $z \approx 2$. This study indicates that DLAs could be understood as the building blocks that merged to form today normal galaxies within a hierarchical clustering scenario.

astro-ph↗

arXiv2001

Disc-like Objects in Hierarchical Hydrodynamical Simulations: Comparison with Observations

We present results from a careful and detailed analysis of the structural and dynamical properties of a sample of 29 disc-like objects identified at z=0 in three AP3M-SPH fully consistent cosmological simulations. These simulations are realizations of a CDM hierarchical model, where an inefficient Schmidt law-like algorithm to model the stellar formation process has been implemented. We focus on properties that can be constrained with available data from observations of spiral galaxies, namely, the bulge and disc structural parameters and the rotation curves. Comparisons with data from Broeils (1992), de Jong (1996) and Courteau (1996, 1997) give satisfactory agreement, in contrast with previous findings using other codes. This suggests that the stellar formation implementation we have used has succeded in forming compact bulges that stabilize disc-like structures in the violent phases of their assembly, while in the quiescent phases the gas has cooled and collapsed according with the Fall & Efstathiou standard model of disc formation.

astro-ph↗