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:
thick disks are made up of accretion debris (Abadi et al. 2003). From the mass-metallicity relation for galaxies, the accreted galaxies that built up the thick disk of the Galaxy would need to be more massive than the SMC to get the right mean [Fe/H] abundance (∼ -0.7). The possible discovery of a counter-rotating thick disk (Yoachim & Dalcanton 2008) in an edge-on galaxy would favor this mechanism.
thick disks come from the heating of the thin disk via disruption of its early massive clusters (Kroupa 2002). The internal energy of large star clusters is enough to thicken the disk. Recent work on the significance of the high redshift clump structures may be relevant to the thick disk problem: the thick disk may originate from the merging of clumps and heating by clumps (e.g. Bournaud et al. 2009). These clumps are believed to form by gravitational instability from turbulent early disks: they appear to generate thick disks with scale heights that are radially approximately uniform, rather than the flared thick disks predicted from minor mergers.
thick disks come from early partly-formed thin disks, heated by accretion events such as the accretion event which is believed to have brought omega Centauri into the Galaxy (Bekki & Freeman 2003). In this picture, thin disk formation began early, at z = 2 to 3. The partly formed thin disk is partly disrupted during the active merger epoch which heats it into thick disk observed now, The rest of the gas then gradually settles to form the present thin disk, a process which continues to the present day.
a recent suggestion is that stars on more energetic orbits migrate out from the inner galaxy to form a thick disk at larger radii where the potential gradient is weaker (Schönrich & Binney 2009)
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.