Figure 1: An
artistic impression of a corrugation pattern in a galactic stellar disc.
As the distance from the galactic centre increases, and the surface
density of the disc decreases, the amplitude of the vertical oscillation
increases. Credit: Rensselaer Polytechnic Institute/Dana Berry
The stellar discs of nearby spiral galaxies are generally not flat and often show waves and warps. Even our own Galactic disc seems to be corrugated. It is still not clear what causes these structures. A research team at MPA, together with external collaborators, have revisited this question by analyzing new simulations of spiral galaxy formation. Their study shows that close encounters with satellite galaxies and more distant flybys of massive companions are the most common drivers. However, in some cases, bending patterns in discs can also be driven by the accretion of cold gas. The vertical motions produced by these patterns can be as large as 60 km/s. Such perturbations should be easily detectable in line-of-sight velocity fields of nearly face-on galaxies. This provides a new way to study the structure of galactic stellar discs, allowing us to understand how and how often such corrugation patterns arise in the nearby universe.
Large observational surveys of spiral galaxies have revealed that
their stellar discs are often not flat but rather show a perturbed
vertical structure. There is strong evidence that even our own Galactic
disc has been significantly perturbed. The most common morphology for
these vertical perturbations is what is known as an S-shaped or
‘integral sign’ warp which is frequently visible in edge-on galaxies.
However, although less common, other types of distortion have also been
observed. For example, rather than a flat plane, our Galactic disc
presents a corrugated structure, resembling the pattern observed on a
pond after dropping a rock (see Figure 1).
At least two major mechanisms are known to be capable of producing
vertical perturbations of the otherwise flat outer discs of spirals. The
first mechanism is tidal distortion of a pre-existing disc by an
external perturber. The standard paradigm of hierarchical structure
formation not only predicts that galaxies are surrounded by a
non-spherical distribution of dark matter, but also that they grow in
mass and evolve morphologically thanks to mergers with satellite
galaxies. Strong tidal torques are exerted on a pre-existing disc as
relatively massive satellites pass by and can induce the formation of
vertical perturbations such as warps. In addition, a dark matter halo
that is misaligned with respect to the embedded disc can also drive the
formation of such vertical features. Another mechanism is misaligned
accretion of cold gas. Such accretion can result from a close encounter
with a gas-rich satellite or from misaligned infall from the cosmic web,
or from a cooling hot gas halo.
Figure 2: Top
panels: Two examples of simulated stellar discs obtained from the Auriga
Project. The colour bar indicates surface brightness values in the
V-band. Bottom panels: Maps of the mean deviation from a flat plane for
the same two galaxies shown above. The colour bar indicates the mean
deviation. Blue (red) colors indicates that the stellar disc is, on
average, below (above) the mid-plane.
Credit: Gómez et al 2016b
This strong connection between stellar discs and the outer regions of
galaxies indicates that it is possible to study unseen structure in
galaxy halos by characterizing their disc's vertical structure. To date,
however, it has not been established what is the dominant mechanism
driving the observed vertical perturbations. Furthermore, the frequency
with which corrugation patterns (such as those observed in the Milky Way
disc) arise in a cosmological context has not been quantified. To shed
light on this problem a team of scientists at MPA, together with
external collaborators, have analyzed a suite of 17 state-of-the art
fully-cosmological hydrodynamical simulations of the formation of disc
galaxies. These simulations, known as the Auriga Project, include the
main physical processes responsible for the formation and evolution of
galaxies and self-consistently follow the evolution of gas, stars and
dark matter over time. Overall, they are one of the best currently
available simulation sets for studies of the formation of Milky Way
sized galaxies.
This work shows that, at the present day, about 70% of the simulated
galactic discs show strong vertical distortions, with amplitudes that
can exceed 2 kpc – almost seven times the thickness of the disk. Half of
these are typical ‘integral sign’ warps while the rest are corrugation
patterns (see Figure 2). Such structures are thus predicted to be
common.
Figure 3: Top panel: Map of the mean deviation
from a plane in kpc for one of our simulated galaxies. The vertical
pattern seen in this stellar disc was excited by a distant fly-by
encounter with a relatively massive satellite. Bottom panel: The mean
vertical velocity field in km/s for the same galactic disc. The
amplitude of the perturbations can be as large as 60 km/s. Credit: Gómez et al. 2016a
The vertical perturbations have a variety of causes. The most common
is encounters with satellite galaxies which can be effective from
surprisingly large distances. In some cases, however, the disc’s
vertical patterns are clearly driven by the accretion of misaligned cold
gas from halo infall or from mergers. Tidally induced vertical patterns
can be identified in both young and old disc stellar populations,
whereas those originating from cold gas accretion are seen mainly in the
younger populations.
The team also characterized the mean vertical motions that arise due
to these patterns. They find that satellites can induce vertical
velocities as large as 60 km/s, quite substantial when compared to the
rotational velocity of Milky Way of 220 km/s (see Figure 3). Such
perturbations should be easily detectable in nearly face-on galaxies
from line-of-sight stellar or gas velocity fields obtained by integral
field spectroscopy or radio interferometry. This may be the easiest way
to study the vertical structure of galactic discs in nearby galaxies,
providing a direct way to assess the frequency with which oscillating
vertical patterns arise in the nearby universe.
Authors:
Postdoc
Phone:
2014
Phone: 2211
Email: swhite@mpa-garching.mpg.de
Original publications
Gómez et al 2016a
A fully cosmological model of a Monoceros-like ring
MNRAS, 456, 2779
Source - DOGómez et al 2016b
Warps and waves in fully cosmological models of galactic discs
MNRAS, submitted
Source
Xu et al. 2015
Rings and Radial Waves in the Disk of the Milky Way ApJ, 801, 105
Source - DOI
