Vertical Metallicity Gradient Research Articles

The new Basel high-latitude field star survey of the Galaxy

We discuss star count and three-color data for the two inner-Galaxy elds SA 107 and NGC 6171 observed in the new Basel RGU high-latitude eld star survey. Our analysis is based on the structural models of the Galactic population components that were derived from the previous studies of seven elds mainly in the outer Galaxy (Buser et al. 1998, 1999, hereafter Papers I and II, respectively). Apart from the canonical structural parameters, we here explore some of the special conditions prevailing in the transition region from high to low latitudes approaching the Galactic bulge, such as interstellar reddening, preponderant sampling of giants, and pos- sible large-scale radial metallicity gradients. We show that the data are consistent on a high condence level with a model incorporating an old-thin disk with local mean metallicityh(M=H)i = 0:13 0:21 dex at galactocentric distance R 7:3 kpc and radial abundance gradient @(M=H)=@x = 0:078 0:016 dex/kpc, and a thick disk with mean metallicityh(M=H)i = 0:730:15 dex and vertical metallicity gradient@(M=H)=@z = 0:070:03 dex/kpc. Any radial gradient of the thick disk is shown to be marginal, 0 @(M=H)=@x 0:02 dex/kpc. For the inner halo, we nd a gradient-less metal-weak component with mean metallicity h(M=H)i 1:8 0:3 dex but also evidence of a { possibly localized { metal-rich component havingh(M=H)i = 1:0 0:2 dex. These results provide an important further step toward a comprehensive and detailed mapping of the larger-scale density and metallicity structures of the Galaxy, to be derived from the full-survey data in 14 elds.

Globular cluster subsystems in the galaxy

Data from the literature are used to construct a homogeneous catalog of fundamental astrophysical parameters for 145 globular clusters of the Milky Way Galaxy. The catalog is used to analyze the relationships between chemical composition, horizontal-branch morphology, spatial location, orbital elements, age, and other physical parameters of the clusters. The overall globular-cluster population is divided by a gap in the metallicity function at [Fe/H]=−1.0 into two discrete groups with well-defined maxima at [Fe/H]=−1.60±0.03 and −0.60±0.04. The mean spatial-kinematic parameters and their dispersions change abruptly when the metallicity crosses this boundary. Metal-poor clusters occupy a more or less spherical region and are concentrated toward the Galactic center. Metal-rich clusters (the thick disk subsystem), which are far fewer in number, are concentrated toward both the Galactic center and the Galactic plane. This subsystem rotates with an average velocity of V rot=165±28 km/s and has a very steep negative vertical metallicity gradient and a negligible radial gradient. It is, on average, the youngest group, and consists exclusively of clusters with extremely red horizontal branches. The population of spherical-subsystem clusters is also inhomogeneous and, in turn, breaks up into at least two groups according to horizontal-branch morphology. Clusters with extremely blue horizontal branches occupy a spherical volume of radius ∼9 kpc, have high rotational velocities (V rot=77±33 km/s), have substantial and equal negative radial and vertical metallicity gradients, and are, on average, the oldest group (the old-halo subsystem). The vast majority of clusters with intermediate-type horizontal branches occupy a more or less spherical volume ≈18 kpc in radius, which is slightly flattened perpendicular to the Z direction and makes an angle of ≈30° to the X-axis. On average, this population is somewhat younger than the old-halo clusters (the young-halo subsystem), and exhibits approximately the same metallicity gradients as the old halo. As a result, since their Galactocentric distance and distance from the Galactic plane are the same, the young-halo clusters have metallicities that are, on average, Δ[Fe/H] ≈0.3 higher than those for old-halo clusters. The young-halo subsystem, which apparently consists of objects captured by the Galaxy at various times, contains many clusters with retrograde orbits, so that its rotational velocity is low and has large errors, V rot=−23±54 km/s. Typical parameters are derived for all the subsystems, and the mean characteristics of their member globular clusters are determined. The thick disk has a different nature than both the old and young halos. A scenario for Galactic evolution is proposed based on the assumption that only the thick-disk and old-halo subsystems are genetically associated with the Galaxy. The age distributions of these two subsystems do not overlap. It is argued that heavy-element enrichment and the collapse of the proto-Galactic medium occurred mainly in the period between the formation of the old-halo and thick-disk subsystems.

Tracing the vertical composition of disc galaxies through colour gradients

Optical observations of a statistically complete sample of edge-on disc-dominated galaxies are used to study the intrinsic vertical colour gradients in the galactic discs, in order to constrain the effects of population gradients, residual dust extinction and gradients in the metal abundance of the galaxies. For the majority of our sample galaxies, the colours and colour gradients in the range 1.0hz|z|3.0hz most likely reflect the intrinsic galactic properties (where hz is the vertical scaleheight). It appears that the intrinsic vertical colour gradients are either non-existent, or small and relatively constant as a function of position along the major axes of the galaxies. On average, the earlier-type galaxies exhibit smaller vertical (B−I) gradients than the later types; our results are consistent with the absence of any vertical colour gradient in the discs of the early-type sample galaxies. In most galaxies small-scale variations in the magnitude and even the direction of the vertical gradient are observed: at larger galactocentric distances they generally display redder colours with increasing z height, whereas the opposite is often observed in and near the galactic centres. For a significant fraction of our sample galaxies another mechanism in addition to the effects of stellar population gradients is required to explain the magnitude of the observed gradients. The non-zero colour gradients in a significant fraction of our sample galaxies are likely to be (at least) partially due to residual dust extinction at these z heights, as is also evidenced from the sometimes significant differences between the vertical colour gradients measured on either side of the galactic planes. We suggest that initial vertical metallicity gradients, if any, have probably not been accentuated by accretion or merging events over the lifetimes of our sample galaxies. On the other hand, they may have weakened any existing vertical metallicity gradients, although they also may have left the existing correlations unchanged.

Metallicity Structures of the Milky Way

The metallicity-sensitive (U – G) colors from the new homogeneous catalog of photographic RGU data in seven high-latitude fields have been used to determine the larger-scale metallicity distributions of the Galactic population components. For the thick disk, preliminary analysis based on our best structural models provides a mean metallicity 〈[M/H]〉 = −0.6 ± 0.3dex and a marginal vertical metallicity gradient ≈ −0.1dex/kpc. The observed color distributions are further consistent with the (old) thin disk having mean abundance 〈[M/H]〉 = −0.3 ± 0.2dex and abundance gradient ∂[M/H]/∂z = −0.6dex/kpc.

New subdwarfs. III - On obtaining the vertical galactic metallicity gradient from the kinematics of nearby stars

New subdwarfs. III - On obtaining the vertical galactic metallicity gradient from the kinematics of nearby stars