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 h _Z ∼ 0. 7 kpc for the scale height of the thick disk, associated with f _thick ∼ 0. 1 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 ∼ 0. 6 prescribed in star-count modeling. This value of h _Z for the thick disk is supported by the kinematic constraint of ∼ 40 km s^{− 1} 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 all-sky near-infrared surveys have arrived at a convergent result, indicating that the thick disk has a scale length h _R ∼ 3. 5 kpc and has a greater radial extension compared to the thin disk, with h _R ∼ 2. 5 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^{− 1} 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 ∼ 10 Gyr. 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.