EVIDENCE FOR AGE DELAY BETWEEN THE FORMATION OF THE OLD HALO AND THE THICK DISK.

Borkova T.V. Marsakov V.A.

2000, in Join European and Nattional Atronomy Meeting "JENAM-2000". 9^th European and 5^th Euro-Aian Astron. Soc. Conf. - Moscow: GEOS, 2000 .

Scenario for the evolution of the Galaxy.

     The metallicity function of globular clusters of thick disk and old halo is shown on Fig.1Ю There are a two approximately the same normal distribution with well-defined gap between them with width [Fe/H]0.2 dex. Distribution of these clusters on age also demonstrates two practically not intersected gaussians (see Fig.1b ). It is follows from the figure that age dispersion among halo clusters is almost the same as accuracy of age determinations: 0.8±0.2 Gyr. But an age scatter among disk clusters is larger: 1.4±0.3 Gyr. In other words, halo clusters are formed practically simultaneously, whereas it took at least several Gyr in the disk. Rent in the ages between subsystems on the histogram (if available) probably can be blended by the errors of age determinations.

Fig. 1.Metallicity function (left panel), and age distribution (right panel) for the old halo and thick disk globular clusters. Gaussian curves of different GC samples are seen to do not intersects almost.

Fig. 2.Metallicity (a), galactocentric distance (b), and distance from the galactic plane (c) are plotted against age for the related to the Galaxy globular clusters of the thick disk (triangles) and the old halo (circles). The lines gives the linear least squares bisector fits for GC of the thick disk, old halo, and whole the Galaxy.

     On Fig. 2 it is shown relationships between age with metallicity, galactocentric distance, and module of distance from the galactic plane for these two subsystems. Straight lines on diagrams are least squares regressions for each subsystem separately and for whole the Galaxy. Correlation coefficients testifies that on extremely measure first two dependencies exists for whole the Galaxy (r=0.8±0.1; 0.6±0.1; 0.3±0.2, accordingly). Correlation between age and metallicity is not observed on the separate thick disk subsystem (r=0.3±0.3), whereas on the old halo, in spite of the small range of ages, it is possible to note weakening dependency (r=0.4±0.2) with the slopping close to general for the Galaxy. Correlation between age and galactocentric distance in the same halo is absent completely (r=0.1±0.3). On the other hand in the disk such dependency is very strong (r=0.7�.2) and coincides with ones for whole the Galaxy (Fig. 2b). Interesting picture is observed on Fig. 2c, where it is more correct to speak about on dependency between height of clusters positions above the galactic plane and age, but about existence of sharp jump on |Z| between subsystems with complete absence of such correlations inside each separate subsystems.

Fig. 3.Plots of VS against cos for the thick disk and the old halo samples. In this graph a constant Vrot appears as a straight line. Higher-weight points are plotted with larger symbols.

     For forming a true notion on subsystem properties we also define else their rotational velocities and metallicity gradients. In the case of constant systematic rotation peed, V_rot, the latter is possible to calculate by the least squares method. We have to know for this only distance to clusters, their positions on the sky, and radial velocities with respect to a stationary observe at the location of the Sun, V_S. Simultaneously, method allows to calculate residual line-of-sight velocity dispersions of the sample, _los. In detail on the method see, for instance, in Armandroff (1989). On the Fig. 3 it is shown kinematical diagrams (cos - V_S) for GCs of thick disk and old halo. Where y is the angle between the line-of-sight on GC and the direction of galactic rotation of the clusters. Values of cos and V_S we have calculated on the Harris (1997) data. Throughout this paper the value of 225 km/s and 8.0 kpc were adopted for the rotational velocity of the LSR and for the San's galactocentric distance. Straight lines on diagrams are least squares regressions, which slop defines a value of rotational velocity of subsystem, V_rot. Residual line-of-site velocity dispersion, _los, is standard deviation of points about line. Main uncertainty on diagrams appears because of inaccuracy of calculation of cos , which coursed by the errors of clusters distance. Inaccuracy of cos is particularly great near the galactic centre. For its account we have assigned a weight to each cluster, which is inversely proportional to inaccuracy of cos. Sizes of symbols on the plot are proportional to the weight. Least-squares fits to the velocity data leads for the thick disk V_rot=165±38 km/s with _los=88±15 km/s, whereas for old halo V_rot=77±33km/s with _los=129±19 km/s.

Fig. 4.Metallicity is plotted against galactocentric distance (a) and distance from the Galactic plane (b) for the globular clusters of thick disk (triangles) and old halo (circles). The slopes of linear regressions gives the values of radial and vertical metallicity gradients, corresponding. The points denoted with squares have been excluded from regressions according to the 3 rule.

     On diagrams of Fig. 4 the clusters of thick disk and old halo are denoted by different symbols. Straight lines are least squares regressions. Radial metallicity gradient in the disk is absent, but metallicity gradient with the distance from the plane of the Galaxy ist very great; in halo the values of both gradients coincides within errors and are.

Fig. 5.The distribution of distance from the Galactic plane for the thick disk (a) and old halo (b) globular clusters. The solid curves shows an approximations of histograms by an exponential low. The numbers gives the scale heights and their errors.

     The height distribution of stars is usually represented by an exponential law:

n(Z)=Ce^-Z/Z₀

     Inspection of calculated properties of globular clusters brings us to the following scenario for early history of the Milky Way Galaxy. The first globular clusters was formed when protogalactic cloud collapsed already up to modern sizes 12 kpc. (Here, however, it is remains indefinite the question of belonging slightly reddened most metal-poor old globular clusters, to the old halo - which lies on galactocentric distance 15÷25 kpc.) The old halo subsystem was formed for a short period of time, so we are not able to reveal a changing with an age neither sizes, nor metallicity in this subsystem. Existance of radial and vertical metallicity gradients inside of this mostly old galactic subsystem evidences that most probably the first acts of the enrichment of the proto-galaxy by heavy elements had occurred before the formation of the clusters of this subsystem else. Moreover an activity of these processes was stronger nearer the galactic centre. (Notice that clusters save orbits of parent protoclouds. In particular, the existence of great number of sufficiently old and simultaneously very distant younger halo clusters points absence of relaxation processes in the Galaxy.) Spatial and chemical characteristics of globular clusters are suddenly changed when turning to the thick disk subsystem. Probably it was sufficiently long delay between formation of these subsystems. Delay had allowed the gaseous protogalactic greatly enriched by heavy elements and had time to mix up them and cloud contracted up to vastly smaller sizes. As a result it was formed rather flat, metal-rich thick disk subsystem. Thick disk rotates faster, than old halo, and its line-of-site velocity dispersion is less. Neither mean metallicity, nor thickness of thick disk did not change with a time. Star formation in protoclusters of this subsystem has began on the most distant galactocentric distances. Then process become to spread to the galactic centre. Significant contraction of the protodisk cloud after formation of the halo subsystem led to increasing of rotation velocity and hence to quick flattening of it. Interstellar matter herewith, probably, was not deeply mixed, as far as under the full absence of the radial metallicity gradient in the thick disk we observe strong negative vertical metallicity gradient in it. In other words, performed analysis shows that quick contraction of gaseous matter to the galactic plane with simultaneous enrichment by heavy elements and partial homogenization of it had occurred basically for a period between the formation end of the old halo and the beginning of the thick disk formation. Presented scenario requires further revision as far as modern quality observation data does not allow to advance deeper into understanding of the processes, occurred inside each globular clusters subsystems of the Galaxy for a period of its formation.

References

• Armandroff, T.E., 1993, in "The Globular Cluster-Galaxy Connection", ASP Conf. Ser., eds. Smith, H., Brodee, J., v.48, p.48 //First quotation in article
• Harris, W.E, Bell, R.A., VandenBerg, D.A., Bolte, M., Stetson, P.B., Hesser, J.E., van den Bergh, S., Fahlman, H.E., Richer, H.B., 1997, AJ, v.114, p.1030//First quotation in article