Star Counts and Nature of the Galactic Thick Disk

Abstract

Modern star counts at high Galactic latitudes played a major role in revealing the existence of a thick disk as the third stellar component of the Milky Way Galaxy in addition to the old thin disk and halo. A number of star count observations and models showed that the thick disk is represented well by a double exponential density law in the vertical and radial directions. The thick-disk structural parameters determined to date from star count analysis are reviewed, and their limitations are described in terms of the correlation among the derived parameters. The recent preference for hZ ∼. kpc for the scale height of the thick disk, associated with fthick ∼. for its normalization relative to the thin disk, is likely a consequence of the recent popularity of the flattened inner halo with an axial ratio of q ∼. prescribed in starcount modeling. This value of hZ for the thick disk is supported by the kinematic constraint of ∼40 km s− for the measured vertical velocity dispersion of candidate thick-disk stars more than 1 kpc from the disk plane. Furthermore, star counts in multiple directions and from allsky near-infrared surveys have arrived at a convergent result, indicating that the thick disk has a scale length hR ∼ . kpc and has a greater radial extension compared to the thin disk, with hR ∼ . kpc. Other constraints have arisen from high-resolution spectroscopic observations of the kinematics, chemical abundances, and ages of candidate thick-disk stars, confirming the rotational lag of ∼40 km s− as well as the vertical gradients of the mean rotation and velocity dispersions in three directions, the constant ratio of alpha to the iron abundances [α/Fe] of ∼+0.4 dex up to [Fe/H]∼−0.4 dex, a large scatter of metallicity around the mean [Fe/H] ∼−0.8 dex with little or no spatial gradient, and a fairly old thick-disk age of ∼10Gyr. The star counts and other constraints together indicate dissipational contraction and spin-up of an extended disk-like gas component or early gas-rich mergers in which thick-disk stars formed in situ with more rapid chemical enrichment than in the thin disk. Successful scenarios of thick-disk formation and evolution must address all these constraints and furthermore involve a self-regulating mechanism that produces a universal double exponential stellar structure for both thin and thick disks in spiral galaxies.