Annual Report 2011 Max Planck Institute for Astronomy

56 III. Selected Research Areas
III.3 Dynamics of Galaxies: inferring their mass distribution and formation history
What is the distribution of luminous and dark matter
in galaxies? How much of the formation history of galaxies
has been preserved in their internal dynamical
structure? Is the mass of a central black hole correlated
with the mass of the host galaxy? Is the concordance
cosmological model in agreement with these findings
on the scales of galaxies? These key questions regarding
galaxy formation in a cosmological context can be
answered by studying the dynamics of galaxies. Here
we present our research in this area.
Kinematic tracers
The motion or kinematics of astronomical objects is sensitive
to the underlying gravitational potential, which
is related, through Poisson’s equation, to the density
of all matter, including any possible dark components.
Consider, for example, gas moving in a circular orbit at
radius R in the equatorial plane of a galaxy. The mass
of the galaxy enclosed within this radius M ( R) determines
the circular velocity V c (R) of the gas, because
G M ( R)/R V c 2 , with G Newton’s constant of gravity.
This means that if we can measure the gas velocity
over a range of radii, we can infer the distribution of the
galaxy’s total mass, i.e., including any possible contribution
from unseen components.
This is illustrated in Fig. III.3.1 for the elliptical galaxy
NGC 2974. The top panel shows the projected stellar
distribution, while the middle panel shows the velocity
field of gas in the outer and inner parts. Red/blue
indicates gas moving away/toward us along our lineof-sight,
while green indicates gas that is moving in the
plane of the sky. Taking out these projection effects, the
red circles with error bars in the bottom panel show the
Fig. III.3.1: Dark matter in the elliptical galaxy NGC 2974. Top:
Digital Sky Survey optical image showing the projected stellar
distribution. Middle: Gas velocity map of the outer parts
via atomic hydrogen HI gas obtained with the VLA radiointerferometer,
and of the inner parts via ionized [OIII] gas
obtained with the sauroN integral-field spectrograph. Red/blue
indicates gas moving away/toward along our line-of-sight,
while green indicates gas that is moving in the plane of the sky.
Bottom: The resulting circular velocity curve of red circles with
error bars clearly shows that the gas and stars alone cannot explain
the corresponding total mass distribution. Adding a dark
matter halo which is consistent with cosmological simulations
can explain the observations as indicated by the upper black
solid line. (Extracted from Weijmans, Krajnović, van de Ven et
al. 2008, MNRAS, 383, 1343).
Declination (J2000)
y
V c [km/s]
–340
–42
–44
150
100
50
0
–50
–100
–150
400
300
200
100
9h42m45s 40s 35s 30
Right Ascension (J2000)
s 25s 20s –100
–50
Sauron V [OIII]
0
0
0 20 40 60
radius
x
50
n [km/s]
300
200
100
0
–100
–200
–300
100 150
halo
stars
gas
Credit: Weijmans, Krajnović, van de Ven
80 100 120