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Register a new userObservational tests of the evolution of spheroidal galaxies
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Laura Silva
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Granato et al (2004) have elaborated a physically grounded model exploiting the mutual feedback between star-forming spheroidal galaxies and the active nuclei growing in their cores to overcome, in the framework of the hierarchical clustering scenario for galaxy formation, one of the main challenges facing such scenario, the fact that massive spheroidal galaxies appear to have formed much earlier and faster than predicted by previous semi-analytical models. After having assessed the values of the two parameters that control the effect of the complex radiative transfer processes on the time-dependent SEDs we have compared the model predictions with a variety of IR to mm data. Our results support a rather strict continuity between objects where stars formed and evolved massive early-type galaxies, indicating that large spheroidal galaxies formed most of their stars when they were already assembled as single objects. The model is successful in reproducing the observed z distribution of Kle20 galaxies at z>1, in contrast with both the classical monolithic and the semi-analytic models, the ratio of star-forming to passively evolving spheroids and the counts and z distributions of EROs. The model also favourably compares with the ISOCAM 6.7 mu counts, with the corresponding z distribution, and with IRAC counts, which probe primarily the passive evolution phase, and with the submm SCUBA and MAMBO data, probing the active star-formation phase. The observed fraction of 24mu selected sources with no detectable emission in either the 8mu or R band nicely corresponds to the predicted surface density of star-forming spheroids with 8mu fluxes below the detection limit. Predictions for the z distributions of 24mu sources detected by MIPS surveys are pointed out. [Abridged]
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Granato et al(2004) have elaborated a physically grounded model exploiting the mutual feedback between star-forming spheroidal galaxies and the active nuclei growing in their cores to overcome, within the hierarchical clustering scenario for galaxy formation, one of the main challenges facing such scenario, the fact that massive spheroidal galaxies appear to have formed earlier and faster than predicted by previous models. Adopting the choice by Granato et al for the parameters governing the history of the SF,of chemical abundances and of the gas and dust content of galaxies, we are left with only two parameters affecting the time and mass dependent SED of spheroidal galaxies. After complementing the model with a simple description of evolutionary properties of starburst, normal late-type galaxies and AGNs we have successfully compared the model with a broad variety of observational data, deep K-band, ISO, IRAS, SCUBA, radio counts, the corresponding redshift distributions, the IR background spectrum, and also with data for EROs. We also present detailed predictions for the GOODS and SWIRE surveys with the Spitzer Space Telescope. We find that the GOODS deep survey at 24$mu$m and the SWIRE surveys at 70 and 160$mu$m are likely to be severely confusion limited. The GOODS surveys in the IRAC channels are expected to resolve most of the background, to explore the full passive evolution phase of spheroidal galaxies and most of their active star-forming phase, detecting galaxies up to zsimeq 4. A substantial number of high z star-forming spheroidal galaxies should also be detected by the 24mum SWIRE and GOODS surveys, while the 70 and 160mum will be particularly useful to study the evolution of such galaxies in the range 1 lsim z lsim 2.[abridged]
We present 3D hydrodynamic simulations aimed at studying the dynamical and chemical evolution of the interstellar medium in dwarf spheroidal galaxies. This evolution is driven by the explosions of Type II and Type Ia supernovae, whose different contribution is explicitly taken into account in our models. We compare our results with avaiable properties of the Draco galaxy. Despite the huge amount of energy released by SNe explosions, in our model the galaxy is able to retain most of the gas allowing a long period ($> 3$ Gyr) of star formation, consistent with the star formation history derived by observations. The stellar [Fe/H] distribution found in our model matches very well the observed one. The chemical properties of the stars derive from the different temporal evolution between Type Ia and Type II supernova rate, and from the different mixing of the metals produced by the two types of supernovae. We reproduce successfully the observed [O/Fe]-[Fe/H] diagram.
We analyse the kinematic and chemical evolution of 203 distant spheroidal (elliptical and S0) galaxies at 0.2<z<0.8 which are located in different environments (rich clusters, low-mass clusters and in the field). VLT/FORS and CAHA/MOSCA spectra with intermediate-resolution have been acquired to measure the internal kinematics and stellar populations of the galaxies. From HST/ACS and WFPC2 imaging, surface brightness profiles and structural parameters were derived for half of the galaxy sample. The scaling relations of the Faber-Jackson relation and Kormendy relation as well as the Fundamental Plane indicate a moderate evolution for the whole galaxy population in each density regime. In all environments, S0 galaxies show a faster evolution than elliptical galaxies. For the cluster galaxies a slight radial dependence of the evolution out to one virial radius is found. Dividing the samples with respect to their mass, a mass dependent evolution with a stronger evolution of lower-mass galaxies (M<2x10^{11} M_{sun}) is detected. Evidence for recent star formation is provided by blue colours and weak OII emission or strong Hdelta absorption features in the spectra. The results are consistent with a down-sizing formation scenario which is independent from the environment of the galaxies.
We present a large sample of fully self-consistent hydrodynamical Nbody/Tree-SPH simulations of isolated dwarf spheroidal galaxies (dSphs). It has enabled us to identify the key physical parameters and mechanisms at the origin of the observed variety in the Local Group dSph properties. The initial total mass (gas + dark matter) of these galaxies is the main driver of their evolution. Star formation (SF) occurs in series of short bursts. In massive systems, the very short intervals between the SF peaks mimic a continuous star formation rate, while less massive systems exhibit well separated SF bursts, as identified observationally. The delay between the SF events is controlled by the gas cooling time dependence on galaxy mass. The observed global scaling relations, luminosity-mass and luminosity-metallicity, are reproduced with low scatter. We take advantage of the unprecedentedly large sample size and data homogeneity of the ESO Large Programme DART, and add to it a few independent studies, to constrain the star formation history of five Milky Way dSphs, Sextans, LeoII, Carina, Sculptor and Fornax. For the first time, [Mg/Fe] vs [Fe/H] diagrams derived from high-resolution spectroscopy of hundreds of individual stars are confronted with model predictions. We find that the diversity in dSph properties may well result from intrinsic evolution. We note, however, that the presence of gas in the final state of our simulations, of the order of what is observed in dwarf irregulars, calls for removal by external processes.
We investigate the present-day photometric properties of the dwarf spheroidal galaxies in the Local Group. From the analysis of their integrated colours, we consider a possible link between dwarf spheroidals and giant ellipticals. From the analysis of the V vs (B-V) plot, we search for a possible evolutionary link between dwarf spheroidal galaxies (dSphs) and dwarf irregular galaxies (dIrrs). By means of chemical evolution models combined with a spectro-photometric model, we study the evolution of six Local Group dwarf spheroidal galaxies (Carina, Draco, Sagittarius, Sculptor, Sextans and Ursa Minor). The chemical evolution models, which adopt up-to-date nucleosynthesis from low and intermediate mass stars as well as nucleosynthesis and energetic feedback from supernovae type Ia and II, reproduce several observational constraints of these galaxies, such as abundance ratios versus metallicity and the metallicity distributions. The proposed scenario for the evolution of these galaxies is characterised by low star formation rates and high galactic wind efficiencies. Such a scenario allows us to predict integrated colours and magnitudes which agree with observations. Our results strongly suggest that the first few Gyrs of evolution, when the star formation is most active, are crucial to define the luminosities, colours, and other photometric properties as observed today. After the star formation epoch, the galactic wind sweeps away a large fraction of the gas of each galaxy, which then evolves passively. Our results indicate that it is likely that at a certain stage of their evolution, dSphs and dIrrs presented similar photometric properties. However, after that phase, they evolved along different paths, leading them to their currently disparate properties.
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