arXiv subjects

C. Firmani

Publications and source records attributed to C. Firmani.

At least 73 records · Page 4Linked to original sources

arXiv2001

Formation and structure of halos in a warm dark matter cosmology

(Abridged) Using high-resolution cosmological N-body simulations, we study how the density profiles of dark matter halos are affected by the filtering of the density power spectrum below a given scale length and by the introduction of a thermal velocity dispersion. In the warm dark matter (WDM) scenario, both the free-streaming scale, R_f, and the velocity dispersion, v_w, are determined by the mass m_w of the WDM particle. We find that v_w is too small to affect the density profiles of WDM halos. Down to the resolution attained in our simulations, there is not any significant difference in the density profiles and concentrations of halos obtained in simulations with and without the inclusion of v_w. The density profiles of halos with masses down to ~0.01 the filtering mass M_f can be described by the NFW shape; significant soft cores are not formed. Nevertheless, the concentrations of these halos are lower than those of the CDM counterparts and are approximately independent of mass. The lower concentrations of WDM halos with respect to their CDM counterparts can be accounted for their late formation epoch. Overall, our results point to a series of advantages of a WDM model over the CDM one. In addition to solving the substructure problem, a WDM model with R_f~0.16 Mpc (m_w~0.75 kev; flat cosmology with Omega_L=h=0.7) also predicts concentrations, a Tully-Fisher relation, and formation epochs for small halos which seems to be in better agreement with observations, relative to CDM predictions.

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arXiv2001

Star formation history in the solar neighborhood: the link between stars and cosmology

Using a cosmological galactic evolutionary approach to model the Milky Way, we calculate the star formation history (SFH) of the solar neighborhood. The good agreement we obtain with the observational inferences suggests that our physical model describes accurately the long term/large spatial trends of the local and global Milky Way SFH. In this model, star formation is triggered by disk gravitational instabilities and self-regulated by an energy balance in the ISM. The drivers of the SFH are the cosmological gas infall rate and the gas surface density determined by the primordial spin parameter. A LambdaCDM cosmology was used throughout.

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arXiv2000

Modelling Self-Interacting CDM Haloes with a Cosmological Boltzmann Code

We investigate the density profiles and evolution of weakly self-interacting cold dark matter haloes using a numerical code based on the collisional Boltzmann equation. This approach is alternative to N-body techniques in following the dynamical evolution of haloes in the cosmological context and taking into account particle self-interaction. The physical case with a cross section inversely proportional to the relative velocity of the colliding particles is modelled with an unprecedented resolution, spanning five orders of magnitude on the radius for each halo. The modelled haloes cover a mass range from dwarf galaxies to galaxy clusters. We find that for σv_{100} \approx 10^{-24} cm^2/GeV, where σis the cross section per unit mass and v_{100} is the collision velocity in units of 100 km/s, soft cores in good agreement with observations on galactic as well as on galaxy cluster scales are obtained. Remarkably, the observed nearly invariance of the halo central density with mass is reproduced.

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arXiv2000

Properties and evolution of disk galaxies in a hierarchical formation scenario

We highlight some results from disk galaxy evolution models conceived within a cosmological context. When disk mergers and strong disk-halo feedback are omitted, several properties and correlations of disk galaxies seem to be related to initial conditions given by the CDM model, for example, the intensive galaxy properties, the disk Hubble sequence, and the Tully-Fisher relation in the infrared bands.

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arXiv2000

Constraints on dark matter physics from dwarf galaxies through galaxy cluster haloes

One of the predictions of the standard CDM is that dark haloes have centrally divergent density profiles. An extensive body of rotation curve observations of dwarf and low surface brightness galaxies shows the dark haloes of those systems to be characterized by soft constant density central cores. Several physical processes have been proposed to produce soft cores in dark haloes, each one with different scaling properties. With the aim of discriminating among them we have examined the rotation curves of dark matter dominated dwarf and low surface brightness galaxies and the inner mass profiles of two clusters of galaxies lacking a central cD galaxy and with evidence of soft cores in the centre. The core radii and central densities of these haloes scale in a well defined manner with the depth of their potential wells, as measured through the maximum circular velocity. As a result of our analysis we identify self-interacting CDM as a viable solution to the core problem, where a non-singular isothermal core is formed in the halo center surrounded by a Navarro, Frenk, & White profile in the outer parts. We show that this particular physical situation predicts core radii in agreement with observations. Furthermore, using the observed scalings, we derive an expression for the minimum cross section (σ) which has an explicit dependence with the halo dispersion velocity (v). If m_x is the mass of the dark matter particle: σ/m_x ~4 10^-25 (v/100 km s^-1)^-1 cm^2/Gev.

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arXiv2000

Shallow cores in the dark matter halos: self-interaction in action?

Using observational data for a sample of dark matter dominated galaxies and two cluster of galaxies, we have found that the central halo density does not depend on its mass, and the core radius is roughly proportional to the maximum rotation velocity. A good agreement with these scaling laws is obtained for CDM halos whose dense inner parts were expanded by gravothermal instabilities if the particles efficiently self-interact only in these parts. We find that the particle cross-section is inversely proportional to the velocity dispersion.

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arXiv2000

The evolution of disk galaxies

We study the evolution of disk galaxies using galaxy evolutionary models with initial and boundary conditions linked to the hierarchical formation scenario. Disks galaxies are modeled locally within growing cold dark matter halos and including a physical model for star formation. We focus our attention on predictions of the star formation history, size and surface brightness evolution and the evolution of the H- and B-band Tully-Fisher relations. Comparisons with available observational data are presented.

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arXiv2000

Evidence of self-interacting cold dark matter from galactic to galaxy cluster scales

Within the framework of the cold dark matter (CDM) cosmogony, a central cusp in the density profiles of virialized dark haloes is predicted. This prediction disagrees with the soft inner halo mass distribution inferred from observations of dwarf and low surface brightness galaxies, and some clusters of galaxies. By analysing data for some of these objects, we find that the halo central density is nearly independent of the mass from galactic to galaxy cluster scales with an average value of around 0.02 M_sun/pc^3. We show that soft cores can be produced in the CDM haloes by introducing a lower cut-off in the power spectra of fluctuations and assuming high orbital thermal energies during halo formation. However, the scale invariance of the halo central density is not reproduced in these cases. The introduction of self-interaction in the CDM particles offers the most attractive alternative to the core problem. We propose gravothermal expansion as a possible mechanism to produce soft cores in the CDM haloes with self-interacting particles. A global thermodynamical equilibrium can explain the central density scale invariance. We find a minimum cross section capable of establishing isothermal cores in agreement with the observed shallow cores. If σand m_x are the cross section and mass of the dark matter particle, and v is the halo velocity dispersion, then (σ/m_x) \~ 4 10^{-25} (100 km s^{-1}/v) cm^2/GeV.

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arXiv2000

Formation and evolution of disk galaxies within cold dark matter halos

We present results of extensive model calculations of disk galaxy evolution within an hierarchical inside-out formation scenario. We first compare properties of the dark halos identified in a cosmological N-body simulation with predictions of a seminumerical method based on an extended collapse model and find a good agreement. We also study the properties of the halos in dependence on their environment. We then describe detailed modelling of the formation and evolution of disks within the growing isolated cold dark matter halos and predictions for the main properties, correlations and evolutionary features of normal disk galaxies. The possible shortcomings of the scenario are discussed.

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arXiv2000

On the origin of the color Tully-Fisher and color-magnitude relations of disk galaxies

We propose and show that the dependence of the internal face-on dust absorption upon B-band luminosity reported by Wang & Heckman (1996) for a sample of late-type galaxies may explain the empirical color-magnitude and color Tully-Fisher relations of disk galaxies. Thus, in order to explain these relations, it is not necessary to evoke star formation and gas infall efficiencies dependent on the mass of the galaxy system. After applying the Wang & Heckman's luminosity-dependent extinction to hierarchical inside-out galaxy formation and evolution models where the star formation and gas infall efficiencies do not significantly depend on mass, we succesfully predict the observed Tully-Fisher relations in the H and B bands as well as the color-magnitude relation.

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arXiv2000

Properties of disk galaxies in a hierarchical formation scenario

We used galaxy evolutionary models in a hierarchical inside-out formation scenario to study the origin of the main properties and correlations of disk galaxies. We found that most of these properties and correlations are the result of three (cosmological) initial factors and their dispersions: the virial mass, the halo mass aggregation history (MAH), and the angular momentum given through the spin parameter λ. The MAH determines mainly the halo structure and the color indexes while λdetermines mainly the surface brightness and the bulge-to-disk ratio. We calculated star formation (SF) using a gravitational instability criterion and a self-regulation mechanism in the turbulent ISM. The efficiency of SF in this model is almost independent from the mass. We show that the luminosity-dependent dust absorption empirically determined by Wang & Heckman explains the observed color-magnitude and color Tully-Fisher (TF) relations without the necessity of introducing a mass-dependent SF efficiency. The disks in centrifugal equilibrium form within growing CDM halos with a gas accretion rate proportional to the MAH. The disks present exponential surface density and brightness profiles, negative radial color index gradients, and nearly flat rotation curves. We also calculated the secular formation of a bulge due to gravitational instabilities in the stellar disk. The intensive properties of our models agree with the observational data and the trends of the Hubble sequence are reproduced. The predicted infrared TF and luminosity-radius relations also agree with observations. The main shortcomings of our inside-out hierarchical models are the excessive radial color gradients and the dark halo dominion in the rotation curve decompositions.

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arXiv2000

Disc galaxy evolution models in a hierarchical formation scenario: structure and dynamics

We predict the structure and dynamics of disc galaxies using galaxy evolution models within a hierarchical formation scenario The halo mass aggregation histories, for a Lambda CDM model, were generated and used to calculate the virialization of dark matter (DM) haloes. A diversity of halo density profiles were obtained, the most typical one being close to the NFW profile. We modeled the formation of discs in centrifugal equilibrium within the evolving DM haloes using gas accretion rates proportional to the halo mass aggregation rates, and assuming detailed angular momentum conservation. We calculated the gravitational interactions between halo and disc, and the hydrodynamics, star formation, and evolution of the galaxy discs. We found that the slope and zero-point of the infrared Tully-Fisher relations (TFR) may be explained as a direct consequence of the cosmological initial conditions. This relation is almost independent of the assumed disc mass fraction. The rms scatter of the TFR originates mainly from the scatter in the DM halo structure and, to a minor extension, from the dispersion of the primordial spin parameter. The scatter obtained does not disagree with the observational estimates. Our models allow us to understand why the residuals of the TFR do not correlate significantly with disc size or surface brightness (SB), and why low and high SB galaxies have the same TFR. The correlations between gas fraction and SB, and between scale length and V_max agree with those observed. The discs present nearly exponential SB distributions. The shape of the rotation curves changes with the SB and is nearly flat for most cases. The rotation curve decompositions show a dominance of DM down to very small radii. The introduction of shallow cores in the DM halo attenuates this difficulty.

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arXiv1999

Structure, dynamics and evolution of disk galaxies in a hierarchical formation scenario

Using galaxy evolutionary models in a hierarchical formation scenario, we predict the structure, dynamics and evolution of disk galaxies in a LCDM universe. Our models include star formation and hydrodynamics of the ISM. We find that the Tully-Fisher relation (TFR) in the I and H bands is an imprint of the mass-velocity relation of the cosmological dark halos. The scatter of the TFR originates mainly from the scatter in the dark halo structure and, to a minor extension, from the dispersion of the primordial spin parameter lambda. Our models allow us to explain why low and high surface brightness galaxies have the same TFR. The disk gas fractions predicted agree with the observations. The disks formed within the growing halos have nearly exponential surface brightness and flat rotation curves. Towards high redshifts, the zero-point of the TFR in the H band increases while in the B-band it slightly decreases.

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arXiv1999

Density profiles of dark matter haloes: diversity and dependence on environment

(Abridged) We study the outer density profiles of dark matter haloes predicted by a generalized secondary infall model and observed in a N-body cosmological simulation of a ΛCDM model. We find substantial systematic variations in shapes and concentrations of the halo profiles as well as a strong correlation of the profiles with the environment. In the N-body simulation, the average outer slope of the density profiles, β(ρ\propto r^{-β}), of isolated haloes is \approx 2.9; 68% of these haloes have values of βbetween 2.5 and 3.8. Haloes in dense environments of clusters are more concentrated and exhibit a broad distribution of βwith values larger than for isolated haloes . Contrary to what one may expect, the haloes contained within groups and galaxy systems are less concentrated and have flatter outer density profiles than the isolated haloes. The concentration decreases with M_h, but its scatter for a given mass is substantial. The mass and circular velocity of the haloes are strongly correlated: M_h \propto V_m^α with α~ 3.3 (isolated) and ~3.5 (haloes in clusters). For M_h=10^12M_sun the rms deviations from these relations are ΔlogM_h=0.12 and 0.18, respectively. Approximately 30% of the haloes are contained within larger haloes or have massive companions (larger than ~0.3 the mass of the current halo) within 3 virial radii. The remaining 70% of the haloes are isolated objects. The distribution of βas well as the concentration-mass and M_h-V_m relations for the isolated haloes agree very well with the predictions of our seminumerical approach which is based on a generalization of the secondary infall model and on the extended Press-Schechter formalism.

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arXiv1998

Disk galaxy formation and evolution: models up to intermediate redshifts

Making use of a seminumerical method we develop a scenario of disk galaxy formation and evolution in the framework of inflationary cold dark matter (CDM) cosmologies. Within the virializing dark matter halos, disks in centrifugal equilibrium are built-up and their galactic evolution is followed through an approach which considers the gravitational interactions among the galaxy components, the turbulence and energy balance of the ISM, the star formation (SF) process due to disk gravitational instabilities, the stellar evolution and the secular formation of a bulge. We find that the main properties and correlations of disk galaxies are determined by the mass, the hierarchical mass aggregation history and the primordial angular momentum. The models follow the same trends across the Hubble sequence than the observed galaxies. The predicted TF relation is in good agreement with the observations except for the standart CDM. While the slope of this relation remains almost constant up to intermediate redshifts, its zero-point decreases in the H-band and slightly increases in the B-band. A maximum in the SF rate for most of the models is attained at $z\sim 1.5-2.5$.

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arXiv1998

Dark Halos and Galaxy Evolution

We study the evolution of disk galaxies within the frame of the cold dark matter (CDM) cosmologies. The hydrodynamics of a centrifugally supported gaseous disk and the growth of a stellar disk are calculated in detail taking into account the energy balance of the ISM and the gravitational instabilities that concern gas and stars. The halo density profile is derived from the primordial cosmological conditions and its gravitational contraction produced by the disk is included. Several features of the spiral galaxies at different redshifts are predicted, and the main factors which influence on these features are found. A strong evidence is provided that the Tully-Fisher relation is an imprint of the primordial cosmological conditions.

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arXiv1998

On the Formation of Bulges and Elliptical Galaxies in the Cosmological Context

We study the formation of hot spheroidal systems within the frame of a scenario where galaxy formation and evolution is related to the gentle mass aggregation history and primordial angular momentum of protogalaxies, both defined by the cosmological initial conditions. We explore two cases: (1) the hot spheroidal system forms from the dynamical instabilities of the stellar disks, and (2) the spheroidal systems are formed during the dissipative collapse of the gas before falling to the disk in centrifugal equilibrium. In the former case a good agreement with observations for late type galaxies is found. In the second case, contrary to recent claims, we find that the tidal stability criterion is not easily reached. The gas that dissipatively collapses within the dark matter halos should be very clumpy, and the clumps very dense, in order to avoid the tidal destruction of the star formation unities.

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arXiv1998

On Formation and Evolution of Disk Galaxies: Cosmological Initial Conditions and the Gravitational Collapse

We use a semianalytical approach and a CDM cosmological model to study the gravitational collapse and virialization, the structure, as well as the global and statistical properties of isolated dark matter galactic halos which emerge from primordial Gaussian fluctuations. From the statistical properties of the density fluctuation field the possible mass aggregation histories (MAHs) are generated, and these histories are used as the initial conditions of the gravitational collapse. To calculate the structure of the virialized systems we have generalized the secondary infall model. Although a range of halo structures is obtained, the average density profiles agree with the profile derived as a fitting formula to results of N-body cosmological simulations. The comparison of the density profiles with the observational data is disscused, and some possible solutions to the disagreement found in the inner regions are discussed. The results of our approach, after considering the gravitational dragging of the baryon matter that forms a central disk in centrifugal equilibrium, show that the Tully-Fisher relation and its scatter can be explained through the initial cosmological conditions. The sigma8=1 SCDM model produces galaxies with high velocities when compared to observations, but when the SCDM power spectrum is normalized to sigma8=0.57 an excellent agreement with the observable TF relation is found, suggesting that this relation is the natural extension to galactic scales of the observed galaxy distribution power spectrum. The theoretical TF scatter is close to the measured one. The slope of the TF relation is practically invariant with respect to the spin parameter lambda.

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