Paola Mazzei

Abundances for metal-poor stars with accurate parallaxes - II.

Abundances for α−elements and Fe in about 150 field subdwarfs and early subgiants with accurate parallaxes and kinematic data are used to discuss the run of abundance ratios in metal-poor stars in the solar neighborhood. Based on kinematics, we separated stars into two populations: the first one has a positive velocity of rotation around the galactic center, and it is likely to be related to the dissipational collapse of the galaxy; the second one has either negligible or negative rotational velocity, and it is likely related to an accretion component. The two populations show a large overlap in metallicity. However, they show distinct chemical properties. For the first population we found that there are close correlations (with small scatters around) of the rotational velocity with metallicity and with the Fe/α abundance ratio: this might be a signature of a not very fast collapse of the progenitor clouds, with enough time for a significant contribution by SNe Ia, although this result needs to be confirmed by a 3-D/non-LTE study. On the other side, the second population exhibits a larger scatter in both the above mentioned relations, and on average, a larger Fe/α ratio at a given metallicity, suggesting a larger scatter in ages. We argue that the lack of stars with moderate rotational velocities and high Fe/α abundance ratios is due to the short merging time for protogalactic clouds with prograde motion, while the presence of a group of counter-rotating stars with this characteristics indicates a much longer typical lifetimes for protogalactic fragments having such a motion. Finally, we found that perigalactic distances correlate with the Fe/α abundance ratios better than the apogalactic distances.

\alpha-

We present maps of dust emission at 7 ma nd 15m/7m intensity ratios of selected regions in 43 spiral galaxies observed with ISOCAM. This atlas is a complement to studies based on these observations, dealing with star formation in centers of barred galaxies and in spiral disks. It is accompanied by a detailed description of data reduction and an inventory of generic morphological properties in groups dened according to bar strength and HI gas content.

elements in the halo

We study the mid-infrared properties of a sample of 69 nearby spiral galaxies, selected to avoid Seyfert activity contributing a signicant fraction of the central energetics, or strong tidal interaction, and to have normal infrared luminosities. These observations were obtained with ISOCAM, which provides an angular resolution of the order of 10 00 (half-power diameter of the point spread function) and low-resolution spectro-imaging information. Between 5 and 18m, we mainly observe two dust phases, aromatic infrared bands and very small grains, both out of thermal equilibrium. On this sample, we show that the globalF15=F7 colors of galaxies are very uniform, the only increase being found in early-type strongly barred galaxies, consistent with previous IRAS studies. The F15=F7 excesses are unambiguously due to galactic central regions where bar-induced starbursts occur. However, the existence of strongly barred early-type galaxies with normal circumnuclear colors indicates that the relationship between a distortion of the gravitational potential and a central starburst is not straightforward. As the physical processes at work in central regions are in principle identical in barred and unbarred galaxies, and since this is where the mid-infrared activity is mainly located, we investigate the mid-infrared circumnuclear properties of all the galaxies in our sample. We show how surface brightnesses and colors are related to both the available molecular gas content and the mean age of stellar populations contributing to dust heating. Therefore, the star formation history in galactic central regions can be constrained by their position in a color-surface brightness mid-infrared diagram.

An atlas of mid-infrared dust emission in spiral galaxies

Aims. We want to investigate the growth of bar instability in stell ar disks embedded in a suitable dark matter halo evolving in a fully consistent cosmological framework. Methods. We perform seven cosmological simulations to emphasise the role of both the disk-to-halo mass ratio and of the Toomre parameter, Q, on the evolution of the disk.We also compare our fully cosmological cases with corresponding isolated simulations where the same halo, is extracted from the cosmological scenario and evolved in physical coordinates. Results. A long living bar, lasting about 10 Gyr, appears in all our simulations. In particular, disks expected to be stable accord ing to classical criteria, form indeed weak bars. We argue that such a result is due to the dynamical properties of our cosmological halo which is far from stability and isotropy, typical of the classical halos used in literature; it is dynamically active, endowed of substru ctures and infall. Conclusions. At least for mild self-gravitating disks, the study of the ba r instability using isolated isotropic halos, in gravitati onal equilibrium, can lead to misleading results. Furthermore, the cosmological framework is needed for quantitatively investigating such an instability.

The impact of bars on the mid-infrared dust emission of spiral galaxies: global and circumnuclear properties ?

We have worked out evolutionary synthesis models of the broad-band spectral energy distribution of elliptical galaxies over the whole frequency range from UV to far--IR. Internal extinction and far--IR re--emission by interstellar dust have been taken into account in a self--consistent way. Diffuse dust emission has been modelled in terms of two components: warm dust, located in regions of high radiation intensity, and cold dust, heated by the general radiation field. Emission from circumstellar dust clouds at different galactic ages was also taken into account. The models reproduce well the present average broad--band spectrum of nearby ellipticals over about four decades in frequency. Under the assumption of a dust--to--gas ratio proportional to the metallicity, the fraction of bolometric luminosity coming out at far-IR wavelengths strongly evolves during the galaxy lifetime, ranging from very low local values (

Bar instability in cosmological halos

\lsim 0.5\%

MODELS FOR THE EVALUATION OF THE SPECTRAL ENERGY DISTRIBUTION OF ELLIPTICAL GALAXIES FROM ULTRAVIOLET TO FAR-INFRARED WAVELENGTHS

) to

FAR-INFRARED PROPERTIES OF EARLY-TYPE GALAXIES

\sim 30\%

ISOCAM observations of the deep IRAS 60 micron sample in the NEP region: II. Comparison of ISO and IRAS galaxy counts☆

or more in the first billion years. Some models even imply that early phases are optically thick; these models provide an excellent fit of the observed spectral energy distribution of the high redshift galaxy IRAS F10214

SPH simulations of galaxy evolution including chemo-photometric predictions

+