The Thick Disks of Spiral Galaxies as Relics from Gas-Rich,

Turbulent, Clumpy Disks at High Redshifts

Frédéric Bournaud, Bruce G. Elmegreen, and Marie Martig

2009, ApJ, 707, L1

What are the thick disks?

• Detected as exponential excess of flux at a few thin disc scale-heights above the mid-plane.

• Made of older and metal-poorer stars than the thin disk.

• They are ubiquitous.• Scale-length similar to

that in thin disks.

Burstein 1979

How do they form?

Possibility 1: Dynamical heating of a cold disk by disk overdensities like giant molecular clouds or spiral arms.

How do they form?

Possibility 2: Dynamical heating of the thin disk due to satellites crossing it.

Thickening + flare!

How do they form?

Possibility 3: in situ formation. Disks form originally thick.

Disk forms thick and thin disk forms from gas accreted through cold flows or which has not been spent in the first

star formation burst.

Cold gas flow

Cold gas flow

How do they form?

Possibility 4: Accretion of a thick disk by the thin disk.

Kinematical signatures!

What is this Letter about?

Possibility 1: Dynamical heating of a cold disk by disk overdensities like

giant molecular clouds or spiral arms.

Possibility 2: Dynamical heating of the thin disk

due to satellites crossing it.

vs

Simulation properties for Possibility 1

• Simulations designed for creating plausible z = 2 progenitors of Milky Way-like spirals.

• N-body simulation with sticky-particle algorithm for representing cold gas.

• Initial disk with a constant disk and gas density truncated at R = 6 kpc and with scale-height hz = 500 pc.

• Code run in isolation for 1 Gyr and then, for 6 Gyr, includes cold disk flows which increase galaxy mass.

Simulation properties for Possibility 2

• Initially contains a fully-formed spiral galaxy.• The galaxy suffers one or a few minor mergers with varying

orbital parameters.

Possibility 1: results

Gravitational instabilities appear soon, scatter stars and form clumps with masses up to a few 108 Mʘ.

The stellar scattering caused by the clumps increase even more the disk thickening.

hz = 500 pchz = 2 kpc

Clumps merge and form a bulge.

hz = 1.5 kpc

Cold flows increase the thin disk mass and reduce the thick disk scale-height.

Cold gas flow

Possibility 1: results

Possibility 1: results

Possibility 1: results

Possibility 2: results

Possibility 2: results

Points in favour of Possibility 1

• Observed thick disks do not flare substantially (in general).• Although minor mergers occur, they are far more frequent at

high redshift, where a large disk gas fraction prevents the thin disk to be thickened by this mechanism (Moster et al. 2010).

• Kinematical studies suggest that thick disks have formed in one single event and not due to a succession of mergers (Moni Bidin 2009).

• Thick disk formation scale-time in Possibility 1 (around 1 Gyr) is compatible with thick disk high α/Fe abundances.

• Observations of very clumpy edge-on galaxies in the HUDF.

And the winner is...

Possibility 1: Dynamical heating of a cold disk by disk overdensities like

giant molecular clouds or spiral arms.

BUT...

• Thick disks created in this Letter are not as massive as what recent analysis suggest the thick disks are (Comerón et al. 2011, sent to ApJ).

• Elemgreen & Elmegreen (2006) found clumpy edge-on disks in the HST Ultra-Deep Field, but those clumps are as large as the thick disk scale-heights, so thick disks could be (partly) created thick and not be the result of a heated thin disk.

• At least a part of the material in thick disks comes from the accretion from external material as a significant fraction of counterrotating stars has been found in one thick disk (Yoachim & Dalcanton 2008).

Thank you for your attention!