Anna Curir

Bar instability in cosmological halos

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 thick disk rotation-metallicity correlation as a fossil of an “inverse chemical gradient” in the early Galaxy

Context. The thick disk rotation-metallicity correlation, ∂Vφ/∂[Fe/H] = 40 ÷ 50 km s −1 dex −1 represents an important signature of the formation processes of the galactic disk. Aims. We use nondissipative numerical simulations to follow the evolution of a Milky Way (MW)-like disk to verify if secular dynamical processes can account for this correlation in the old thick disk stellar population. Methods. We followed the evolution of an ancient disk population represented by 10 million particles whose chemical abundances were assigned by assuming a cosmologically plausible radial metallicity gradient with lower metallicity in the inner regions, as expected for the 10-Gyr-old MW. The two cases of a disk with and without a bar were simulated to compare the evolution of their kinematics and radial chemical properties. Results. Migration processes act in both cases and appear to be enhanced in the presence of a central bar. Essentially, inner disk stars move towards the outer regions and populate layers located at higher |z|. In the case of an evolved barred disk, a rotation-metallicity correlation appears, which well resembles the behaviour observed in our Galaxy at a galactocentric distance between 8 kpc and 10 kpc. In particular, we measure a correlation of ∂Vφ/∂ [Fe/H] � 60 km s −1 dex −1 for particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6G yr. Conclusions. Our pure N-body models can account for the Vφ vs. [Fe/H] correlation observed in the thick disk of our Galaxy, suggesting that processes internal to the disk such as heating and radial migration play a role in the formation of this old stellar component. In this scenario, the positive rotation-metallicity correlation of the old thick disk population would represent the relic signature of an ancient inverse chemical (radial) gradient in the inner Galaxy, which resulted from accretion of primordial gas.

SPH simulations of galaxy evolution including chemo-photometric predictions

Abstract We performed smooth particle hydrodynamical simulations, including star formation and chemo-photometric evolution from the U to K band, of isolated collapsing triaxial systems. We analysed the dependence of the evolutionary paths on some initial conditions so far unexplored, such as the geometry of the collapsing system and the effect of a different ratio of baryonic mass to dark matter. We also outline the role of several parameters connected with the star formation rate, such as the initial mass function (IMF), which influences the energy input by supernovae explosions per stellar generation. By generating integrated colours and brightness profiles, which can be compared with the same observed galaxy properties, our results give further insight into the role of different parameters examined, such as the dependence of the star formation rate and related chemical enrichment on the IMF for systems with different total mass.

Evolution of stellar-gaseous disks in cosmological halos

INAF-Osservatorio Astronomico di Padova. Vicolo Osservatorio 5 - 35122 Padova. Italy. e-mail: paola.mazzei@oapd.inaf.itReceived/AcceptedABSTRACTAims.We explore the growth and the evolution of the bar instability in stellar-gaseous disks embedded in a suitable dark matter halo evolvingin a fully consistent cosmological framework. The aim of this paper is to point out the impact of different gas fractions on the bar formation,inside disks of different disk-to-halo mass ratio, and the role of the cosmological framework.Methods.We perform cosmological simulations with the same disk-to-halo mass ratios as in a previous work where the gas was not taken intoaccount. We compare results of the new simulations with the previous ones to investigate the effect of the gas by analysing the morphology ofthe stellar and gaseous components, the stellar bar strength and the behaviour of its pattern speed.Results.In our cosmological simulations, inside dark-matter dominated disks, a stellar bar, lasting 10 Gyr, is still living at z = 0 even if thegaseous fraction exceeds half of the disk mass. However, in the most massive disks we find a threshold value (0.2) of the gas fraction able todestroy the bar.The stellar bar strength is enhanced by the gas and in the more massive disks higher gas fractions increase the bar pattern speed.Key words. galaxies: spirals, structure, evolution, halos, kinematics and dynamics

Star formation and bar instability in cosmological halos

Context. This is the third in a series of papers presenting the first attempt to analyse the growth of the bar instability in a consistent cosmological scenario. In the previous two articles, we explored the role of the cosmology on stellar disks and the impact of the gaseous component on a disk embedded in a cosmological dark matter halo. Aims. The aim of this paper is to point out the impact of the star formation on the bar instability inside disks having different gas fractions. Methods. We performed cosmological simulations of the same disk-to-halo mass systems as in the previous works where the star formation was not triggered. We compared the results of the new simulations with the previous ones to investigate the effect of the star formation by analysing the morphology of the stellar components, the bar strength, the behaviour of the pattern speed. We followed the gas and the central mass concentration during the evolution and their impact on the bar strength. Results. A stellar bar, lasting 10 Gyr, is still living at z = 0 in all our cosmological simulations. The central mass concentration of gas and of the new stars has a mild effect on the ellipticity of the bar but is not able to destroy it, and at z = 0 the stellar bar strength is enhanced by the star formation. The bar pattern speed decreases with the disk evolution.

Self-Consistent Evolution of Ring Galaxies

Ring galaxies are commonly known as objects where a burst of star formation was triggered by a close encounter with an intruder, maybe a satellite galaxy. BVRI CCD observations of five ring galaxies have been performed. Here we present the results of a self-consistent approach to reproduce their observed morphology and spectral energy distribution using updated

NEW SIGNATURES OF THE MILKY WAY FORMATION IN THE LOCAL HALO AND INNER-HALO STREAMERS IN THE ERA OF GAIA

N

The dual nature of the Milky Way stellar halo

--body simulations and evolutionary population synthesis models extending from UV to far--IR wavelengths. Some suggestions about the evolutionary properties of these starburst galaxies are then derived.

Halo Kinematic Streams in the Era of Gaia

We explore the vicinity of the Milky Way through the use of spectro-photometric data from the Sloan Digital Sky Survey and high-quality proper motions derived from multi-epoch positions extracted from the Guide Star Catalogue II database. In order to identify and characterise streams as relics of the Milky Way formation, we start with classifying, select, and study

Evolution and instabilities of disks harboring super massive black holes

2417