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Clues on the origin of thick disks in galaxies

» Thursday 17 February 2011

Spiral galaxies have not only a thin disk of stars, but also a thick disk, which is less bright. The mechanisms invoked to form the thick disk range widely, from a turbulent initial phase in the galaxy formation, through radial migrations of stars due to bars, to mergers between galaxies. A team of astronomers from Paris Observatory has carried out numerical simulations to test whether minor mergers of galaxies with their small satellites could provide clues on this formation. They find a thick disk at the end of the simulation, formed from perturbed stars coming from the main galaxy disk. The thick disk is flaring with radius, and has a larger radial scale length than the thin disk. They also simulated the thick disk formation due to secular evolution, and do not find in this case a larger radial scale length for the thick disks. The different resulting characteristics indicate observations than can be made that could help to determine the actual formation mechanism(s).

Thick disks are ubiquitous features in disk galaxies and are
observed all along the Hubble sequence — from lenticular early type
spirals to ones that are bulgeless. As the name suggests, the thick
disk is much thicker than the high surface brightness thin disk that
make images of spiral galaxies so spectacular. Interestingly, even
though the thin disk of galaxies show a wide variety of structural
properties, thick disks seem to have many features in common no matter
what the properties of their host disk galaxy. The stars in thick disks
in galaxies are generally old, metal poor, and show a rotational lag,
that is, they rotate around the galaxy center more slowly, than the disk
stars in the thin disk. In some galaxies, a high fraction (up to 50%) of
stars in thick disks counter-rotate compared to stars in the thin disk.
Thick disks are seen in a majority of spiral galaxies, including
our Galaxy. Its stellar mass fraction is estimated to be
around 10% of the thin disk mass, its stars have a lower rotational
support than thin disk stars, they are old, enhanced in α-elements,
and with average metallicities intermediate between those of thin disk
stars and halo stars.

Even though thick disks have been known and studied for over 30 years, their origin is still hotly debated, and any
mechanisms have been invoked to explain their formation. It is well known that external mechanisms, like minor galaxy mergers, i.e., mergers of small satellites with a more massive galaxy, are viable processes to thicken a pre-existing thin stellar disk, although internal processes such as scattering of stars by spiral waves, by molecular clouds, or by massive clumps forming in unstable gas-rich disks in the early universe and radial migration of stars from the inner to the outer disk, are also able to heat a thin disk enough to produce a thicker component.

How can we discriminate between these different processes for explaining
the origin of the thick disk? What are the typical and (perhaps)
unique signatures that these processes leave in the stars — in their
distribution, metal abundances and ratios, and kinematics?

Modeled scale heights as function of radius of various components in a galaxy with a thick disk. Note that the thick disk gets thicker with radius, while the stellar excess has a constant thickness.
The scale heights z0 of the merger-induced thick disk and stellar excess at great heights
as function of radius in units of disk scale length, r/rd, for
dissipationless minor merger simulations. The shaded regions indicate the scatter in
the scale heights, most of which is due to differences in initial orbital configurations. Also shown are
the scale heights of the original stellar disk at the start (dotted line) and after
its evolution in isolation for 3 Gyr (dashed line). The stellar excess has
morphological and kinematical properties which are distinct from those of thick disk stars: its scale height is constant in radius, while that of the thick disk increases with
radius — this disk flaring is a characteristic of thick disks formed by
minor mergers.

By means of tens of N-body/SPH simulations, a group of researchers
at the Paris Observatory have studied the imprints minor mergers leave
on the distribution of stars in the vertical direction with respect to
the galaxy mid-plane. Their results show that during the interaction and
subsequent merger of a satellite, stars in the thin disk of the primary
galaxy can be dynamically heated and scattered to very large distances
from the mid-plane. This leaves an unique signature in the vertical
surface density profile of the post-merger disk galaxy: the thick disk
formed during the merger has an "excess’’ in the regions furthest away
from the disk mid-plane (z> 2 kpc). The distribution of stars in the
thick disk appear to follow a double sech function. The first of these
functions, necessary to fit the stellar distribution closest to the
mid-plane is the classical thick disk. The second component, necessary
to fit the stars at the largest mid-plane distances defines the excess.
The scale height of the excess is larger than that of the main thick
disk component. Stars in the excess have a rotational velocity lower than
that of stars in the thick disk, and thus may be confused with stars in
the inner galactic halo, which can have a similar lag.

Interestingly, some of the excess stars may already have been
observed. Recently, Nissen & Schuster (2010, A&A, 511, L10) discovered the presence
in the solar neighborhood of stars with halo kinematics, but [α/Fe]
abundances similar to those of thick disk stars. These stars may well be
part of the excess, as these stars show a significant rotational lag with
respect to thick disk stars. If the Milky Way thick disk formed through
the heating of a pre-existing thin component by minor mergers at early
in the universe, we would expect to find an inner halo population (the
excess), with abundances similar to those of stars in the thick disk,
but lagging rotationally behind them — just as observed.

In addition, the models showed that minor mergers result in a thick
disk that has a radial scale length that is 10-50% larger than that of
the thin disk. This is a natural outcome of minor merger in that the
loss of angular momentum in the gas during the merger causes both the
deepening of the gravitational potential and enhanced star-formation in
the densest regions of the disk, which is expected to lead to a larger
scale length of the thick disk than of the thin disk newly formed during
the merger. This also appears supported by observations of thick disks,
by for example, Pohlen et al. (2007, MNRAS, 378, 594), which show similar ratios of thin
to thick disk scale lengths as the simulations.

The thick disks formed through internal processes do not show any stellar
excess at large heights or a thin disk scale length which
is significantly greater than that of the thin disk, unlike the merger
simulations. Thus a possible way of distinguishing between a thick disk
formed via secular processes and one formed through minor mergers is
to investigate the radial distribution of scale heights and the ratio
of scale lengths, as well as the presence and properties of the excess
component in the vertical stellar profile at large disk heights.

Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths, Qu, di Matteo, Lehnert, van Driel, A&A in press

Yan Qu
Paola Di Matteo
Matthew Lehnert
Wim van Driel

So-called Toomre diagrams, comparing the total kinetic energy of stars with their rotational energy. The minor merger models agree with observations of the Milky Way; in particular, the stellar excess stars lie exactly in the area of the Toomre diagram where alpha-enhanced stars are found.
Toomre diagram of stars between radii of 2-3 times the disk scale length, rd, after a prograde minor merger (left)
and in an instable gas-rich clumpy
disk evolved in isolation for 3 Gyr (right). The diagram shows
the relationship between the total
radial and vertical kinetic energy of stars and their rotational energy.
In minor merger models the stellar excess stars lie exactly in the area
of the Toomre diagram where α-enhanced stars are found (Nissen &
Schuster 2010, A&A, 511, L10), thus strengthening the hypothesis that they may
originally have been in the thin disk. Note that the clumpy disk model
does not reproduce the Toomre diagram of the Milky Way, since it cannot
heat disk stars as efficiently in the vertical direction as minor
mergers do.
In both panels, the dashed vertical line marks a total velocity of 180 km s-1,
which serves as a potential criteria to separate thick disk stars and
halo stars (Venn et al. 2004, AJ, 128, 1177), and the dotted line is the Local Standard
of Rest in these models. Also shown are the observed thick disk stars
(triangles), halo stars with high α/Fe abundance (squares) and low α/Fe
value (crosses) in the solar neighborhood, see Nissen & Schuster (2010)
for further details. The halo stars in retrograde motion, against the sense of rotation of the bulk of the galaxy, are shown in
green whereas those in prograde motion are in blue.