On the Galactic disc age–metallicity relation

A comparison is made between the age–metallicity relations obtained from four different types of studies: F and G stars in the solar neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be −0.09 dex kpc⁻¹ and most likely constant in time. We do not correct for the vertical gradient, because its existence and value are not firmly established.
Stars and clusters trace a similar age–metallicity relation, showing an excess of rather metal-rich objects in the age range 5–9 Gyr. Galactic structure studies tend to give a more metal-poor relation than chemical evolution models. Neither relation explains the presence of old, relatively metal-rich stars and clusters. This might be caused by uncertainties in the ages of the local stars, or pre-enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.     

Origin and structure of the Galactic disc(s)

We examine the chemical and dynamical structure in the solar neighbourhood of a model Galaxy that is the endpoint of a simulation of the chemical evolution of the Milky Way in the presence of radial mixing of stars and gas. Although the simulation's star formation rate declines monotonically from its unique peak and no merger or tidal event ever takes place, the model replicates all known properties of a thick disc, as well as matching special features of the local stellar population such as a metal-poor extension of the thin disc that has high rotational velocity. We divide the disc by chemistry and relate this dissection to observationally more convenient kinematic selection criteria. We conclude that the observed chemistry of the Galactic disc does not provide convincing evidence for a violent origin of the thick disc, as has been widely claimed.     

The local axisymmetric instability criterion in a thin, rotating, multicomponent disc

Purely gravitational perturbations are considered in a thin rotating disc composed of gas and several stellar components. The dispersion relation for the axisymmetric density waves propagating through the disc is found and the criterion for the local axisymmetric stability of the whole system is formulated. In the appropriate limit of two-component gas we confirm the findings of Jog & Solomon and extend consideration to the case when one component is collisionless. Gravitational stability of the Galactic disc in the solar neighbourhood based on the multicomponent instability condition is explored using recent measurements of the stellar composition and kinematics in the local Galactic disc obtained by the Hipparcos satellite.     

Three-dimensional evolution of worms and chimneys in the Galactic disc

Three-dimensional modelling of the flow of gas and plasma in a section of the Galaxy has been carried out to study the evolution and formation of Galactic chimneys and worms. It is found that clustered supernovae located on either side of the Galactic plane are sources for the formation of well-collimated chimneys, having widths of ∼     . The thick gas disc may have a role in the collimation of chimneys. Channel maps of disc gas, obtained from the simulations, show the presence of sheet-like structures running perpendicular to the Galactic plane and resembling worms. Worms are believed to result from the break-up of the shells and supershells. However, the simulations show that although some worms correlate well with the debris of broken shells/supershells, others do not. They are cold gas that has been accelerated in the disc and rise on to the thick gas disc.     

Galactic mergers and gravitationally unbound populations

Motivated by the observations on the intracluster light and intergalactic stellar populations, N -body simulations are used to model the galactic merging events as a goal to investigate the production and distribution of gravitational unbound populations (GUPs). Both the parabolic and hyperbolic mergers are considered, and each category includes six models with different relative orientations between two galaxies. Our results show that there are more (about a factor of 2) GUPs after a hyperbolic merging event than after a parabolic one. In general, depending on the relative orientation and also on the relative velocity of the two galaxies in a merging pair, a head-on collision of a galaxy pair would only make a tiny fraction (less than 1 per cent) of the initial stellar mass luminous GUP, but a considerable fraction (8–14 per cent) of the dark matter becomes dark GUP.     

Radial mixing and the transition between the thick and thin Galactic discs

The analysis of the kinematics of solar neighbourhood stars shows that the low- and high-metallicity tails of the thin disc are populated by objects which orbital properties suggest an origin in the outer and inner Galactic disc, respectively. Signatures of radial migration are identified in various recent samples, and are shown to be responsible for the high-metallicity dispersion in the age–metallicity distribution. Most importantly, it is shown that the population of low-metallicity wanderers of the thin disc (−0.7 < [Fe/H] < −0.3 dex) is also responsible for the apparent hiatus in metallicity with the thick disc (which terminal metallicity is about −0.2 dex). It implies that the thin disc at the solar circle has started to form stars at about this same metallicity. This is also consistent with the fact that 'transition' objects, which have α-element abundance intermediate between that of the thick and thin discs, are found in the range [−0.4, −0.2] dex. Once the metal-poor thin disc stars are recognized for what they are – wanderers from the outer thin disc – the parenthood between the two discs can be identified on stars genuinely formed at the solar circle through an evolutionary sequence in [α/Fe] and [Fe/H]. Another consequence is that stars that can be considered as truly resulting of the chemical evolution at the solar circle have a metallicity restricted to about [−0.2, +0.2] dex, confirming an old idea that most chemical evolution in the Milky Way have preceded the thin disc formation.     

Galactic satellite systems: radial distribution and environment dependence of galaxy morphology

We have studied the radial distribution of the early (E/S0) and late (S/Irr) types of satellites around bright host galaxies. We made a volume-limited sample of 4986 satellites brighter than   M _r=−18.0  associated with 2254 hosts brighter than   M _r=−19.0  from the Sloan Digital Sky Survey Data Release 5 sample. The morphology of satellites is determined by an automated morphology classifier, but the host galaxies are visually classified. We found segregation of satellite morphology as a function of the projected distance from the host galaxy. The amplitude and shape of the early-type satellite fraction profile are found to depend on the host luminosity. This is the morphology–radius/density relation at the galactic scale. There is a strong tendency for morphology conformity between the host galaxy and its satellites. The early-type fraction of satellites hosted by early-type galaxies is systematically larger than that of late-type hosts, and is a strong function of the distance from the host galaxies. Fainter satellites are more vulnerable to the morphology transformation effects of hosts. Dependence of satellite morphology on the large-scale background density was detected. The fraction of early-type satellites increases in high-density regions for both early- and late-type hosts. It is argued that the conformity in morphology of galactic satellite system is mainly originated by the hydrodynamical and radiative effects of hosts on satellites.