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Most spiral galaxies, including out Galaxy, have a second thicker disk component. For example, the thick disk and halo of the edge-on spiral galaxy NGC 891, which is much like the Milky Way in size and morphology, has a thick disk nicely seen in star counts from HST images (Mouhcine et al. 2010). Its thick disk has scale height ∼ 1.4 kpc and scalelength ∼ 4.8 kpc, much as in our Galaxy. The fraction of baryons in the thick disk is typically about 10 to 15 percent in large systems like the Milky Way, but rises to about 50% in the smaller disk systems (482008Yoachim & Dalcanton 2008).

The Milky Way has a significant thick disk, discovered by Gilmore & Reid (1983). Its vertical velocity dispersion is about 40 km s-1; its scale height is still uncertain but is probably about 1000 pc. The surface brightness of the thick disk is about 10% of the thin disk's, and near the Galactic plane it rotates almost as rapidly as the thin disk. Its stars are older than 10 Gyr and are significantly more metal poor than the stars of the thin disk; most of the thick disk stars have [Fe/H] values between about -0.5 and -1.0 and are enhanced in alpha-elements relative to Fe. This is usually interpreted as evidence that the thick disk formed rapidly, on a timescale ∼ 1 Gyr. From its kinematics and chemical properties, the thick disk appears to be a discrete component, distinct from the thin disk. Current opinion is that the thick disk shows no vertical abundance gradient (e.g. Gilmore et al. 1995, Ivezić et al. 2008).

The old thick disk is a very significant component for studying Galaxy formation, because it presents a kinematically and chemically recognizable relic of the early Galaxy. Secular heating is unlikely to affect its dynamics significantly, because its stars spend most of their time away from the Galactic plane.

How do thick disks form ? Several mechanisms have been proposed, including:

How can we test between these possibilities for thick disk formation? Sales et al. (2009) looked at the expected orbital eccentricity distribution for thick disk stars in different formation scenarios. Their four scenarios are:

Preliminary results from the observed orbital eccentricity distribution for thick disk stars may favor the gas-rich merger picture (Wilson et al. 2011). This is a potentially powerful approach for testing ideas about the origin of the thick disk. Because it depends on the orbital properties of the thick disk sample, firm control of selection effects is needed in the identification of which stars belong to the thick disk. Kinematical criteria for choosing the thick disk sample are clearly not ideal.

To summarize this section on the thick disk: Thick disks are very common in disk galaxies. In our Galaxy, the thick disk is old, and is kinematically and chemically distinct from the thin disk. It is important now to identify what the thick disk represents in the galaxy formation process. The orbital eccentricity distribution of the thick disk stars will provide some guidance. Chemical tagging will show if the thick disk formed as a small number of very large aggregates, or if it has a significant contribution from accreted galaxies. This is one of the goals for the upcoming AAT/HERMES survey: see section 5.

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