Galaxy Formation and Evolution: I. The Padua TreeSPH code (PD-SPH)

In this paper we report on PD-SPH the new tree-sph code developed in Padua. The main features of the code are described and the results of a new and independent series of 1-D and 3-D tests are shown. The paper is mainly dedicated to the presentation of the code and to the critical discussion of its performances. In particular great attention is devoted to the convergency analysis. The code is highly adaptive in space and time by means of individual smoothing lengths and individual time steps. At present it contains both dark and baryonic matter, this latter in form of gas and stars, cooling, thermal conduction, star formation, and feed-back from Type I and II supernovae, stellar winds, and ultraviolet flux from massive stars, and finally chemical enrichment. New cooling rates that depend on the metal abundance of the interstellar medium are employed, and the differences with respect to the standard ones are outlined. Finally, we show the simulation of the dynamical and chemical evolution of a disk-like galaxy with and without feed-back. The code is suitably designed to study in a global fashion the problem of formation and evolution of elliptical galaxies, and in particular to feed a spectro-photometric code from which the integrated spectra, magnitudes, and colors (together with their spatial gradients) can be derived.Comment: 25 pages, 24 figures, to be published in MNRA

Is the galactic disk older than the halo?

Aim of this study is to infer the age of the Galactic Disk by means of the ages of old open clusters, and comment on some recent claims that the Galactic Disk can be older than the Halo. To this purpose, we analyze the Color-Magnitude Diagrams (CMDs) of six very old clusters, namely NGC 188, NGC 6791, Collinder 261, Melotte 66, Berkeley 39 and Berkeley 17, and determine their ages. For each cluster we use the most recent photometric and spectroscopic data and metallicity estimates. The ages are derived from the isochrone fitting method using the stellar models of the Padua library (Bertelli et al . 1994, Girardi et al. 1999). We find that the ages of these clusters fall in the range 4 to 9-10 Gyr: Melotte 66 is the youngest whereas NGC 6791 and Berkeley 17 have ages of about 9-10 Gyr. Previous estimates for Berkeley 17 indicated an age as old as 12 Gyr, almost falling within the range of classical globular clusters. In our analysis, this cluster is always very old but perhaps somewhat younger than in previous studies. However we call attention on the fact that the above ages are to be taken as provisional estimates, because of the many uncertaintes still affecting stellar models in the mass range 1.0 to 1.5 Mo. Despite this drawback of extant theory of stellar structure, if NGC 6791 and Berkeley 17 set the limit to the age of the Galactic Disk, this component of the Milky Way can be as old as 9-10 Gyr, but surely younger than the Galactic Halo, at least as inferred from recent determinations of the age of globular clusters. Finally, it is worth recalling that open clusters can only provide a lower limit to the age of the Galactic Disk, while other indicators - like White Dwarfs - are perhaps more suited to this task.Comment: 16 pages, 11 figure, accepted for publication in MNRA

Galaxy Formation and Evolution. II. Energy Balance, Star Formation and Feed-back

In this paper we present a critical discussion of the algorithms commonly used in N-body simulations of Galaxy Formation to deal with the energy equation governing heating and cooling, to model star formation and the star formation rate, and to account for energy feed-back from stars. First, we propose our technique for solving the energy equation in presence of heating and cooling, which includes some difference with respect to the standard semi-implicit technique. Second, we examine the current criteria for the onset of the star formation activity. We suggest a new approach, in which star formaiton is let depend on the total mass density - baryonic (gas and stars) and dark matter - of the system and on the metal-dependent cooling efficiency. Third, we check and discuss the separate effects of energy (and mass) feed-back from several sources - namely supernovae, stellar winds from massive stars, and UV flux from the same objects. All the simulations are performed in the framework of the formation and evolution of a disk galaxy. We show that the inclusion of these physical phenomena has a signigicant impact on the evolution of the galaxy model.Comment: 11 pages, 6 figures, to be pubblished in MNRA

Cosmological interpretation of the color-magnitude diagrams of galaxy clusters

We investigate the color-magnitude diagram (CMD) of cluster galaxies in the hierarchical $\Lambda$-CDM cosmological scenario using both single stellar populations and simple galaxy models. First, we analyze the effect of bursts and mergers and companion chemical pollution and rejuvenation of the stellar content on the integrated light emitted by galaxies. The dispersion of the galaxy magnitudes and colors on the $M_V-(B-V)$ plane is mainly due to mixing of ages and metallicities of the stellar populations, with mergers weighting more than bursts of similar mass fractions. The analysis is made using the Monte-Carlo technique applied to ideal model galaxies reduced to single stellar populations with galaxy-size mass to evaluate mass, age and metallicity of each object. We show that separately determining the contributions by bursts and mergers leads to a better understanding of observed properties of CMD of cluster galaxies. Then we repeat the analysis using suitable chemo-photometric models of galaxies whose mass is derived from the cosmological predictions of the galaxy content of typical clusters. Using the halo mass function and the Monte-Carlo technique, we derive the formation redshift of each galaxy and its photometric history. These are used to simulate the CMD of the cluster galaxies. The main conclusion is that most massive galaxies have acquired the red color they show today in very early epochs and remained the same ever since. The simulations nicely reproduce the Red Sequence, the Green Valley and the Blue Cloud, the three main regions of the CMD in which galaxies crowd.Comment: Accepted for publication in Ap

Evolutionary models of zero metallicity stars

We present new evolutionary models for zero-metallicity stars, covering a large range of initial masses (from 0.8 to 100 M_sun). Models are computed with overshooting from stellar cores and convective envelopes, and assuming mass-loss from the most massive stars. We discuss the main evolutionary features of these stars, and provide estimates of the amount of newly-synthesized elements dredged-up to the stellar surface, and possibly lost by stellar winds from the most massive stars. Full details of these models will be given in Marigo et al. (2000, in preparation).Comment: 2 pages, to appear in the proceedings of the MPA/ESO Workshop "The first stars

A new framework for understanding the evolution of early-type galaxies

We have recently suggested that the combination of the scalar virial theorem ($M_s \sim R_e \sigma^2$) and the $L=L'_0 \sigma^\beta(t)$ law, with L'_0 and $\beta$ changing from galaxy to galaxy (and with time), can provide a new set of equations valid for investigating the evolution of early-type galaxies (ETGs) (Donofrio & Chiosi, 2022). These equations are able to account for the tilt of the Fundamental Plane (FP) and to explain the observed distributions of ETGs in all its projections. In this paper we analyze the advantages offered by those equations, derive the $\beta$ and $L'_0$ parameters for real and simulated galaxies, and demonstrate that, according to the value of $\beta$, galaxies can move only along some permitted directions in the FP projections. Then, we show that simple galaxy models that grow in mass by infall of gas and form stars with a star formation rate depending on the stellar velocity dispersion nicely reproduce the observed distributions of ETGs in the FP projections and yield $\beta$s that agree with the measured ones. We derive the mutual relationships among the stellar mass, effective radius, velocity dispersion, and luminosity of ETGs as a function of $\beta$ and calculate the coefficients of the FP. Then, using the simple infall models, we show that the star formation history of ETGs is compatible with the $\sigma$-dependent star formation rate, and that both positive and negative values of $\beta$ are possible in a standard theory of galaxy evolution. The parameter $\beta(t)$ offers a new view of the evolution of ETGs. In brief, i) it gives a coherent interpretation of the FP and of the motions of galaxies in its projections; ii) it is the fingerprint of their evolution; iii) it measures the degree of virialization of ETGs; iv) and finally it allows us to infer their evolution in the near past.Comment: 25 pages, 30 figures, 5 table

The scaling relations of galaxies back in time: the road toward virialization

Context. The structural scaling relations (SSRs) of galaxies, i.e. the observed correlations between effective radius, effective surface intensity and velocity dispersion, are important tools for understanding how evolution proceeds. Aims. In this paper we aim to demonstrate that the evolution of the SSRs back in time is governed by the combination of the virial theorem (VT) and the relation $L=L'_0 \sigma^{\beta(t)}$, where the parameters $\beta$ and $L'_0$ vary with time and from galaxy to galaxy. Methods. Using the WINGS database for the galaxies at redshift $z=0$ and the Illustris-1 and Illustris-TNG databases of artificial galaxies, for the galaxies up to redshift $z=4$, we analyse the SSRs back in time and, by means of simple algebraic expressions for $L'_0$ and $\beta$ (functions of time and other physical quantities), we derive the expected paths followed by galaxies in the various SSRs toward the distributions observed at $z=0$. Results. The distribution of galaxies in the SSRs is ultimately related to the evolution in luminosity and velocity dispersion that are empirically mirrored by the $L=L'_0 \sigma^{\beta(t)}$ law. Furthermore, the $\beta$ parameter works as a thermometer of the virialization of a galaxy. This parameter can assume either positive or negative values, and its absolute value attains high values when the galaxy is close to the virial condition, while it tends to zero when the galaxy is far from it. Conclusions. As the SSRs change with time, the method we are proposing allows us to decipher the temporal evolution of galaxies.Comment: 21 pages, 20 figure

Cosmic Star Formation: a simple model of the SFRD(z)

We investigate the evolution of the cosmic star formation rate density (SFRD) from redshift z=20 to z=0 and compare it with the observational one by Madau and Dickinson derived from recent compilations of UV and IR data. The theoretical SFRD(z) and its evolution are obtained using a simple model which folds together the star formation histories of prototype galaxies designed to represent real objects of different morphological type along the Hubble sequence and the hierarchical growing of structures under the action of gravity from small perturbations to large scale objects in \Lambda-CDM cosmogony, i.e. the number density of dark matter halos N(M,z). Although the overall model is very simple and easy to set up, it provides results that well mimic those obtained from large scale N-body simulations of great complexity. The simplicity of our approach allows us to test different assumptions for the star formation law in galaxies, the effects of energy feedback from stars to interstellar gas and the efficiency of galactic winds, and also the effect of N(M,z). The result of our analysis is that in the framework of the hierarchical assembly of galaxies the so-called time-delayed star formation under plain assumptions mainly for the energy feedback and galactic winds can reproduce the observational SFRD(z).Comment: ApJ (accepted for publication