Vertical Metallicity Research Articles

Numerical simulation of a possible origin of the positive radial metallicity gradient of the thick disk

We analyze the radial and vertical metallicity and [α/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemo-dynamical galaxy evolution code GCD+. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity ([α/Fe]) gradient becomes more positive (negative) with increasing height, changing sign around 1.5 kpc above the plane. At the largest vertical height (2 < |z| < 3 kpc), our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity gradient is caused by the age-metallicity and age-velocity dispersion relation, where younger stars have higher metallicity and lower velocity dispersion. Due to the age-velocity dispersion relation, a greater fraction of younger stars reaches |z| > 2 kpc at the outer region, because of the lower gravitational restoring force of the disk, i.e. flaring. As a result, the fraction of younger stars with higher metallicity due to the age-metallicity relation becomes higher at the outer radii, which makes the median metallicity higher at the outer radii. Combining this result with the recently observed age-metallicity and age-velocity dispersion relation for the Milky Way thick disk stars suggested by Haywood et al., we argue that the observed (small) positive radial metallicity gradient at large heights of the Milky Way disk stars can be explained by flaring of the younger thick and/or thin disk stars.

THE VERTICAL METALLICITY GRADIENT OF THE MILKY WAY DISK: TRANSITIONS IN [α/Fe] POPULATIONS

Using G dwarfs from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey, we have determined a vertical metallicity gradient over a large volume of the Milky Way's disk, and examined how this gradient varies for different [a/Fe] subsamples. This sample contains over 40,000 stars with low-resolution spectroscopy over 144 lines of sight. We employ the SEGUE Stellar Parameter Pipeline (SSPP) to obtain estimates of effective temperature, surface gravity, [Fe/H], and [a/Fe] for each star and extract multiple volume-complete subsamples of approximately 1000 stars each. Based on the survey's consistent target-selection algorithm, we adjust each subsample to determine an unbiased picture of the disk in [Fe/H] and [a/Fe]; consequently, each individual star represents the properties of many. The SEGUE sample allows us to constrain the vertical metallicity gradient for a large number of stars over a significant volume of the disk, between ~0.3 and 1.6 kpc from the Galactic plane, and examine the in situ structure, in contrast to previous analyses which are more limited in scope. This work does not pre-suppose a disk structure, whether composed of a single complex population or a distinct thin and thick disk component. The metallicity gradient is -0.243 +0.039 -0.053 dex/kpc for the sample as a whole, which we compare to various literature results. Each [a/Fe] subsample dominates at a different range of heights above the plane of the Galaxy, which is exhibited in the gradient found in the sample as a whole. Stars over a limited range in [a/Fe] show little change in median [Fe/H] with height. If we associate [a/Fe] with age, our consistent vertical metallicity gradients with [a/Fe] suggest that stars formed in different epochs exhibit comparable vertical structure, implying similar star-formation processes and evolution.

The age structure of stellar populations in the solar vicinity

We analyze high quality abundances data of solar neighborhood stars and show that there are two distinct regimes of [alpha/Fe] versus age which we identify as the epochs of the thick and thin disk formation. A tight correlation between metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk stars, implying that this population formed from a well mixed ISM, over a time scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with age, with the youngest objects having as small scale heights as those of thin disk stars. A natural consequence of these two results is that a vertical metallicity gradient is expected in this population. We suggest that the thick disk set the initial conditions for the formation of the inner thin disk. This provides also an explanation of the apparent coincidence between the step in metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk at about R<10 kpc. We suggest that the outer thin disk developped outside the influence of the thick disk, but also that the high alpha-enrichment of the outer regions may originate from a primordial pollution by the gas expelled from the thick disk. Local metal-poor thin disk stars, whose properties are best explained by an origin in the outer disk, are shown to be as old as the youngest thick disk (9-10 Gyr), implying that the outer thin disk started to form while the thick disk formation was still on-going in the inner Galaxy. We point out that, given the tight age-abundance relations in the thick disk, an inside-out process would give rise to a radial gradient in abundances in this population which is not observed. Finally, we argue that the data discussed here leave little room for radial migration, either to have contaminated the solar vicinity, or, to have redistributed stars in significant proportion across the solar annulus.

Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS

We investigate the large scale metallicity distribution in the Galactic bulge, using a large spatial coverage, in order to constrain the bulge formation scenario. We use the VISTA variables in the Via Lactea (VVV) survey data and 2MASS photometry, covering 320 sqdeg of the Galactic bulge, to derive photometric metallicities by interpolating of the (J-Ks)0 colors of individual Red Giant Branch stars based on a set of globular cluster ridge lines. We then use this information to construct the first global metallicity map of the bulge with a resolution of 30'x45'. The metallicity map of the bulge revealed a clear vertical metallicity gradient of ~0.04 dex/deg (~0.28 dex/kpc), with metal-rich stars ([Fe/H]~0) dominating the inner bulge in regions closer to the galactic plane (|b|<5). At larger scale heights, the mean metallicity of the bulge population becomes significantly more metal-poor. This fits in the scenario of a boxy-bulge originated from the vertical inestability of the Galactic bar, formed early via secular evolution of a two component stellar disk. Older, metal-poor stars dominate at higher scale heights due to the non-mixed orbits from the originally hotter thick disk stars.

METALLICITY GRADIENTS THROUGH DISK INSTABILITY: A SIMPLE MODEL FOR THE MILKY WAY'S BOXY BULGE

Observations show a clear vertical metallicity gradient in the Galactic bulge, which is often taken as a signature of dissipative processes in the formation of a classical bulge. Various evidence shows, however, that the Milky Way is a barred galaxy with a boxy bulge representing the inner three-dimensional part of the bar. Here we show with a secular evolution N-body model that a boxy bulge formed through bar and buckling instabilities can show vertical metallicity gradients similar to the observed gradient, if the initial axisymmetric disk had a comparable radial metallicity gradient. In this framework the range of metallicities in bulge fields constrains the chemical structure of the Galactic disk at early times, before bar formation. Our secular evolution model was previously shown to reproduce inner Galaxy star counts and we show here that it also has cylindrical rotation. We use it to predict a full mean metallicity map across the Galactic bulge from a simple metallicity model for the initial disk. This map shows a general outward gradient on the sky as well as longitudinal perspective asymmetries. We also briefly comment on interpreting metallicity gradient observations in external boxy bulges.

METALLICITY DISTRIBUTION FUNCTIONS, RADIAL VELOCITIES, AND ALPHA ELEMENT ABUNDANCES IN THREE OFF-AXIS BULGE FIELDS

We present radial velocities and chemical abundance ratios of [Fe/H], [O/Fe], [Si/Fe], and [Ca/Fe] for 264 red giant branch (RGB) stars in three Galactic bulge off-axis fields located near (l,b)=(-5.5,-7), (-4,-9), and (+8.5,+9). The results are based on equivalent width and spectrum synthesis analyses of moderate resolution (R~18,000), high signal-to-noise ratio (S/N~75-300) spectra obtained with the Hydra spectrographs on the Blanco 4m and WIYN 3.5m telescopes. The targets were selected from the blue side of the giant branch to avoid cool stars that would be strongly affected by CN and TiO; however, a comparison of the color-metallicity distribution in literature samples suggests our selection of bluer targets should not present a significant bias against metal-rich stars. We find a full range in metallicity that spans [Fe/H]\approx-1.5 to +0.5, and that, in accordance with the previously observed minor-axis vertical metallicity gradient, the median [Fe/H] also declines with increasing Galactic latitude in off-axis fields. The off-axis vertical [Fe/H] gradient in the southern bulge is estimated to be ~0.4 dex/kpc. The (+8.5,+9) field exhibits a higher than expected metallicity, with a median [Fe/H]=-0.23, that might be related to a stronger presence of the X--shaped bulge structure along that line-of-sight. All fields exhibit an identical, strong decrease in velocity dispersion with increasing metallicity that is consistent with observations in similar minor-axis outer bulge fields. Additionally, the [O/Fe], [Si/Fe], and [Ca/Fe] versus [Fe/H] trends are identical among our three fields, and are in good agreement with past bulge studies. [abridged]

METALLICITY GRADIENTS OF THICK DISK DWARF STARS

We examine the metallicity distribution of the Galactic thick disk using F, G, and K dwarf stars selected from the Sloan Digital Sky Survey, Data Release 8. Using the large sample of dwarf stars with proper motions and spectroscopically determined stellar parameters, metallicity gradients in the radial direction for various heights above the Galactic plane and in the vertical direction for various radial distances from the Galaxy center have been found. In particular, we find a vertical metallicity gradient of -0.113 +/- 0.010 (-0.125 +/- 0.008) dex/kpc using an isochrone (photometric) distance determination in the range 1 < |Z| < 3 kpc, which is the vertical height range most consistent with the thick disk of our Galaxy. In the radial direction, we find metallicity gradients between +0.02 and +0.03 dex/kpc for bins in the vertical direction between 1 < |Z| < 3 kpc. Both of these results agree with similar values determined from other populations of stars, but this is the first time a radial metallicity gradient for the thick disk has been found at these vertical heights. We are also able to separate thin and thick disk stars based on kinematic and spatial probabilities in the vertical height range where there is significant overlap of these two populations. This should aid further studies of the metallicity gradients of the disk for vertical heights lower than those studied here but above the solar neighborhood. Metallicity gradients in the thin and thick disks are important probes into possible formation scenarios for our Galaxy and a consistent picture is beginning to emerge from results using large spectroscopic surveys, such as the ones presented here.

Radial migration in barred galaxies

AbstractIn this talk, I will present the result of high resolution numerical simulations of disk galaxies with various bulge/disk ratios evolving isolated, showing that: •Most of migration takes place when the bar strength is high and decreases in the phases of low activity (in agreement with the results by Brunetti et el. 2011, Minchev et al. 2011).•Most of the stars inside the corotation radius (CR) do not migrate in the outer regions, but stay confined in the inner disk, while stars outside CR can migrate either inwards or outwards, diffusing over the whole disk.•Migration is accompanied by significative azimuthal variations in the metallicity distribution, of the order of 0.1 dex for an initial gradient of ~-0.07 dex/kpc.•Boxy bulges are an example of stellar structures whose properties (stellar content, vertical metallicity, [α/Fe] and age gradients, ..) are affected by radial migration (see also Fig. 1).

The stellar metallicity distribution in intermediate-latitude fields with BATC and SDSS data

Based on the Beijing-Arizona-Taiwan-Connecticut (BATC) and Sloan Digital Sky Survey (SDSS) photometric data, we adopt SEDs fitting Method to evaluate the metallicity distribution for \sim40,000 main-sequence stars in the Galaxy. According to the derived photometric metallicities of these sample stars, we find that the metallicity distribution shift from metal-rich to metal-poor with the increase of distance from the Galactic center. The mean metallicity is about of 1.5 \pm 0.2dex in the outer halo and 1.3 \pm 0.1 dex in the inner halo. The mean metallicity smoothly decreases from -0.4 to -0.8 in interval 0 < r \leq 5 kpc. The fluctuation in the mean metallicity with Galactic longitude can be found in interval 4 < r \leq 8 kpc. There is a vertical abundance gradients d[Fe/H]/dz\sim -0.21 \pm 0.05 dex kpc-1 for the thin disk (z \leq 2 kpc). At distance 2 < z \leq 5 kpc, where the thick disk stars are dominated, the gradients are about of -0.16 \pm0.06 dex kpc-1, it can be interpreted as a mixture of stellar population with different mean metallicities at all z levels. The vertical metallicity gradient is - 0.05 \pm0.04 dex kpc-1 for the halo (z > 5 kpc). So there is little or no metallicity gradient in the halo.

Metallicity gradients in disks

We examine radial and vertical metallicity gradients using a suite of disk galaxy simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient and reconcile differences between extant models and observations within the `inside-out' disk growth paradigm. A sample of 25 disks is used, consisting of 19 from our RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code RAMSES. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code GASOLINE, alongside disks from Rahimi et al. (GCD+) and Kobayashi & Nakasato (GRAPE-SPH). Two chemical evolution models of inside-out disk growth were employed to contrast the temporal evolution of their radial gradients with those of the simulations. We find that systematic differences exist between the predicted evolution of radial abundance gradients in the RaDES and chemical evolution models, compared with the MUGS sample; specifically, the MUGS simulations are systematically steeper at high-redshift, and present much more rapid evolution in their gradients. We find that the majority of the models predict radial gradients today which are consistent with those observed in late-type disks, but they evolve to this self-similarity in different fashions, despite each adhering to classical `inside-out' growth. We find that radial dependence of the efficiency with which stars form as a function of time drives the differences seen in the gradients; systematic differences in the sub-grid physics between the various codes are responsible for setting these gradients. Recent, albeit limited, data at redshift z=1.5 are consistent with the steeper gradients seen in our SPH sample, suggesting a modest revision of the classical chemical evolution models may be required.

Local stellar kinematics from RAVE data - III. Radial and vertical metallicity gradients based on red clump stars

We investigate radial and vertical metallicity gradients for a sample of red clump stars from the RAdial Velocity Experiment (RAVE) Data Release 3. We select a total of 6781 stars, using a selection of colour, surface gravity and uncertainty in the derived space motion, and calculate for each star a probabilistic (kinematic) population assignment to a thin or thick disc using space motion and additionally another (dynamical) assignment using stellar vertical orbital eccentricity. We derive almost equal metallicity gradients as a function of Galactocentric distance for the high probability thin disc stars and for stars with vertical orbital eccentricities consistent with being dynamically young, e_v<=0.07, i.e. d[M/H]/dR_m = -0.041(0.003) and d[M/H]/dR_m = -0.041(0.007) dex/kpc. Metallicity gradients as a function of distance from the Galactic plane for the same populations are steeper, i.e. d[M/H]/dz_{max} = -0.109(0.008) and d[M/H]/dz_{max} = -0.260(0.031) dex/kpc, respectively. R_m and z_{max} are the arithmetic mean of the perigalactic and apogalactic distances, and the maximum distance to the Galactic plane, respectively. Samples including more thick disc red clump giant stars show systematically shallower abundance gradients. These findings can be used to distinguish between different formation scenarios of the thick and thin discs.

Stellar populations in the Milky Way bulge region: towards solving the Galactic bulge and bar shapes using 2MASS data

We re-analyse photometric near-infrared data in order to investigate why it is so hard to get a consensus for the shape and density law of the bulge, as seen from the literature. To solve the problem we use the Besancon Galaxy Model to provide a scheme for parameter fitting of the structural characteristics of the bulge region. The fitting process allows the determination of the global shape of the bulge main structure. We explore various parameters and shape for the bulge/bar structure based on Ferrer's ellipsoids and fit the shape of the inner disc in the same process. The results show that the main structure is a quite standard triaxial boxy bar/bulge with an orientation of about 13 degree with respect to the Sun-centre direction. But the fit is greatly improved when we add a second structure, which is a longer and thicker ellipsoid. We emphasize that our first ellipsoid represent the main boxy bar of the Galaxy, and that the thick bulge could be either a classical bulge slightly flattened by the effect of the bar potential, or a inner thick disc counterpart. We show that the double clump seen at intermediate latitudes can be reproduced by adding a slight flare to the bar. In order to better characterize the populations, we further simulate several fields which have been surveyed in spectroscopy and for which metallicity distribution function (MDF) are available. The model is in good agreement with these MDF along the minor axis if we assume that the main bar has a mean solar metallicity and the second thicker population has a lower metallicity. It then creates naturally a vertical metallicity gradient by the mixing of the two poulations. (abridged)

METALLICITY GRADIENTS IN THE MILKY WAY DISK AS OBSERVED BY THE SEGUE SURVEY

The observed radial and vertical metallicity distribution of old stars in the Milky Way disk provides a powerful constraint on the chemical enrichment and dynamical history of the disk. We present the radial metallicity gradient, \Delta[Fe/H]/\Delta R, as a function of height above the plane, |Z|, using 7010 main sequence turnoff stars observed by the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. The sample consists of mostly old thin and thick disk stars, with a minimal contribution from the stellar halo, in the region 6 < R < 16 kpc, 0.15 < |Z| < 1.5 kpc. The data reveal that the radial metallicity gradient becomes flat at heights |Z| > 1 kpc. The median metallicity at large |Z| is consistent with the metallicities seen in outer disk open clusters, which exhibit a flat radial gradient at [Fe/H] ~ -0.5. We note that the outer disk clusters are also located at large |Z|; because the flat gradient extends to small R for our sample, there is some ambiguity in whether the observed trends for clusters are due to a change in R or |Z|. We therefore stress the importance of considering both the radial and vertical directions when measuring spatial abundance trends in the disk. The flattening of the gradient at high |Z| also has implications on thick disk formation scenarios, which predict different metallicity patterns in the thick disk. A flat gradient, such as we observe, is predicted by a turbulent disk at high redshift, but may also be consistent with radial migration, as long as mixing is strong. We test our analysis methods using a mock catalog based on the model of Sch\"onrich & Binney, and we estimate our distance errors to be ~25%. We also show that we can properly correct for selection biases by assigning weights to our targets.

A spectroscopic survey of thick disc stars outside the solar neighbourhood

We performed a spectroscopic survey of nearly 700 stars probing the galactic thick disc far from the solar neighbourhood towards the galactic coordinates (l~277, b~47). The derived effective temperatures, surface gravities and overall metallicities were then combined with stellar evolution isochrones, radial velocities and proper motions to derive the distances, kinematics and orbital parameters of the sample stars. The targets belonging to each galactic component (thin disc, thick disc, halo) were selected either on their kinematics or according to their position above the galactic plane, and the vertical gradients were also estimated. We present here atmospheric parameters, distances and kinematics for this sample, and a comparison of our kinematic and metallicity distributions with the Besancon model of the Milky Way. The thick disc far from the solar neighbourhood is found to differ only slightly from the thick disc properties as derived in the solar vicinity. For regions where the thick disc dominates, we measured vertical velocity and metallicity trends of d(V_phi)/dZ = 19 +/- 8 km/s/kpc and d[M/H]/dZ = -0.14 +/- 0.05 dex/kpc, respectively. These trends can be explained as a smooth transition between the different galactic components, although intrinsic gradients could not be excluded. In addition, a correlation d(V_phi)/d[M/H] = -45 +/- 12 km/s/dex between the orbital velocity and the metallicity of the thick disc is detected. This gradient is inconsistent with the SDSS photometric survey analysis, which did not detect any such trend, and challenges radial migration models of thick disc formation. Estimations of the scale heights and scale lengths for different metallicity bins of the thick disc result in consistent values, with hR~3.4 \pm 0.7 kpc, and hZ~694 \pm 45 pc, showing no evidence of relics of destroyed massive satellites.

THE METALLICITY GRADIENT OF THE THICK DISK BASED ON RED HORIZONTAL-BRANCH STARS FROM SDSS DR8

Based on SDSS DR8, we have selected a sample of 1728 red horizontal branch stars with |Z|<3 kpc by using a color-metallicity relation and stellar parameters. The sample stars clearly trace a typical thick disk population with peaks at |Z|=1.26 kpc and [Fe/H]=-0.54. The vertical metallicity gradient of the thick disk is estimated in two ways. One is a fit to the Gaussian peaks of the metallicity histograms of the thick disk by subtracting minor contributions from the thin disk and the inner halo based on the Besancon Galaxy model. The resulting gradient is -0.12+-0.01 dex/kpc for 0.5 <|Z| <3 kpc. The other method is to linearly fit the data based on stars with 1 <|Z| <3 kpc being the main component of the thick disk. Five subgroups are then selected in different directions in the X-|Z| plane to investigate the difference in the vertical metallicity gradient between the Galactocenter and anti-Galactocenter directions. We found that a vertical gradient o-0.22+-0.07 dex/kpc is detected for five directions except for one involving the pollution of stars from the bulge. The results indicate that the vertical gradient is dominant, but a radial gradient has a minor contribution for the thick disk population represented by RHB stars with 1 <|Z| <3 kpc. The present work strongly suggests the existence of a metallicity gradient in the thick disk, which is thought to be negligible in most previous works in the literature.

Formation of the Galactic bulge from a two-component stellar disc: explaining cylindrical rotation and a vertical metallicity gradient

Abstract Recent observational studies have revealed that the Galactic bulge has cylindrical rotation and a steeper vertical metallicity gradient. We adopt two representative models for the bulge formation and thereby investigate whether the two models can explain both the observed cylindrical rotation and vertical metallicity gradient in a self-consistent manner. One is the ‘pure disc scenario’ (PDS) in which the bulge is formed from a pure thin stellar disc through spontaneous bar instability. The other is the ‘two-component disc scenario’ (TCDS) in which the bulge is formed from a disc composed of thin and thick discs through bar instability. Our numerical simulations show that although the PDS can reproduce the cylindrical rotation, it shows a rather flatter vertical metallicity gradient that is inconsistent with observations. The derived flatter metallicity gradient is due to the vertical mixing of stars with different initial metallicities by the stellar bar. This result implies that the bulge cannot be simply formed from a pure thin stellar disc. On the other hand, the bulge formed from the two-component disc in the TCDS can explain both the observed cylindrical rotation and vertical metallicity gradient of the Galactic bulge reasonably well. In the TCDS, more metal-poor stars at higher ∣z∣ (vertical distance), which originate from the already dynamically hotter thick disc, cannot be strongly influenced by vertical mixing of the bar so that they can stay in situ for longer time-scales and thus keep the lower metallicity at higher ∣z∣. Consequently, the vertical metallicity gradient of the bulge composed of initially thin and thick disc stars cannot be so flattened, even if the gradient of the thin disc can be flattened significantly by the bar in the TCDS. We therefore suggest that a significant fraction of the present Galactic bulge is composed of stars initially in the inner part of the thick disc and thus that these bulge stars and the thick disc have a common origin. We also suggest that the Galaxy might well have experienced some merger events that could dynamically heat up its inner regions until ∼10 Gyr ago.

The stellar kinematics and populations of boxy bulges: cylindrical rotation and vertical gradients★

Boxy and peanut-shaped bulges are seen in about half of edge-on disc galaxies. Comparisons of the photometry and major-axis gas and stellar kinematics of these bulges to simulations of bar formation and evolution indicate that they are bars viewed in projection. If the properties of boxy bulges can be entirely explained by assuming that they are bars, then this may imply that their hosts are pure disc galaxies with no classical bulge. A handful of these bulges, including that of the Milky Way, have been observed to rotate cylindrically, i.e. with a mean stellar velocity independent of height above the disc. In order to assess whether such behaviour is ubiquitous in boxy bulges, and whether a pure disc interpretation is consistent with their stellar populations, we have analysed the stellar kinematics and populations of the boxy or peanut-shaped bulges in a sample of five edge-on galaxies. We placed slits along the major axis of each galaxy and at three offset but parallel positions to build up spatial coverage. The boxy bulge of NGC 3390 rotates perfectly cylindrically within the spatial extent and uncertainties of the data. This is consistent with the metallicity and α-element enhancement of the bulge, which are the same as in the disc. This galaxy is thus a pure disc galaxy. The boxy bulge of ESO 311−G012 also rotates very close to cylindrically. The boxy bulge of NGC 1381 is neither clearly cylindrically nor non-cylindrically rotating, but it has a negative vertical metallicity gradient and is α-enhanced with respect to its disc, suggesting a composite bulge comprised of a classical bulge and bar (and possibly a discy pseudo-bulge). The rotation of the peanut-shaped bulge of NGC 5746 is difficult to classify, but the peanut-shaped bulge of IC 4767 does not rotate cylindrically. Thus, even this relatively small sample is sufficient to demonstrate that boxy bulges display a range of rotational and population properties, indicating that they do not form a homogeneous class of object.

Probing the Galactic thick disc vertical properties and interfaces

Aims. This work investigates the properties (metallicity and kinematics) and interfaces of the Galactic thick disc as a function of height above the Galactic plane. The main aim is to study the thick disc in a place where it is the main component of the sample. Methods. We take advantage of former astrometric work in two fields of several square degrees in which accurate proper motions were measured down to V-magnitudes of 18.5 in two directions, one near the north galactic pole and the other at a galactic latitude of 46 ◦ and galactic longitude near 0 ◦ . Spectroscopic observations have been acquired in these two fields for a total of about 400 stars down to magnitude 18.0, at spectral resolutions of 3.5 to 6.25 A. The spectra have been analysed with the code ETOILE, comparing the target stellar spectra with a grid of 1400 reference stellar spectra. This comparison allowed us to derive the parameters effective temperature, gravity, [Fe/H] and absolute magnitude for each target star. Results. The Metallicity Distribution Function (MDF) of the thin-thick-disc-halo system is derived for several height intervals between 0 and 5 kpc above the Galactic plane. The MDFs show a decrease of the ratio of the thin to thick disc stars between the first and second kilo-parsec. This is consistent with the classical modelling of the vertical density profile of the disc with 2 populations with different scale heights. A vertical metallicity gradient, ∂[Fe/H]/∂z = −0.068 ± 0.009 dex kpc −1 , is observed in the thick disc. It is discussed in terms of scenarios of formation of the thick disc.

The Metallicity Distribution of Distant F and G Type Stars in the CFHT Legacy Survey Deep Field

AbstractWe study the metal abundances of F and G type stars in the Galactic disk and halo using the Canada–France–Hawaii Telescope Legacy Survey D4 field. For a sample of stars within 7 kpc of the Galactic plane, we derive mean abundance values of [Fe/H] = −0.77 ± 0.36 dex for the thick disk, and [Fe/H] = –1.42 ± 0.98 dex for the stellar halo, respectively. These metallicites are consistent with the metallicity estimates from the Sloan Digital Sky Survey Data Release 6. We do not find a vertical abundance gradient for the thick disk between 1 kpc and 4 kpc. However, stars within 1 kpc of the Galactic plane are saturated in our dataset, and therefore we cannot rule out the observed vertical metallicity gradient for the thick disk stars in the SDSS. Moreover, we find a negative trend in the halo metallicity with increasing distance from the Galactic plane. This trend could be due to a contribution from an increasing number of very metal poor stars that belong to the outer halo. However, systematic effects in photometric metallicities are largest for the most metal poor stars, and therefore these effects cannot be ruled out.

Vertical distribution of Galactic disk stars

We present the parameters of 891 stars, mostly clump giants, including atmospheric parameters, distances, absolute magnitudes, spatial velocities, galactic orbits and ages. One part of this sample consists of local giants, within 100 pc, with atmospheric parameters either estimated from our spectroscopic observations at high resolution and high signal-to-noise ratio, or retrieved from the literature. The other part of the sample includes 523 distant stars, spanning distances up to 1 kpc in the direction of the North Galactic Pole, for which we have estimated atmospheric parameters from high resolution but low signal-to-noise Echelle spectra. This new sample is kinematically unbiased, with well-defined boundaries in magnitude and colours. We revisit the basic properties of the Galactic thin disk as traced by clump giants. We find the metallicity distribution to be different from that of dwarfs, with fewer metal-rich stars. We find evidence for a vertical metallicity gradient of –0.31 dex kpc-1 and for a transition at ~4–5 Gyr in both the metallicity and velocities. The age–metallicity relation (AMR), which exhibits a very low dispersion, increases smoothly from 10 to 4 Gyr, with a steeper increase for younger stars. The age–velocity relation (AVR) is characterized by the saturation of the V and W dispersions at 5 Gyr, and continuous heating in U.