Annual Report 2011 Max Planck Institute for Astronomy

y
30
20
10
0
–10
–20
–30
30
20
10
0
–10
–20
Vobs [km/s] sobs [km/s]
–193 193 0 210
V rms, obs [km/s]
–30
–30 –20 –10 0 10 20 30 –30
x
–20 –10 0 10 20 30
Fig. III.3.2: Dark matter content in nearby galaxies. Top: Oblate
axisymmetric dynamical model of NGC 488. Maps of the observed
line-of-sight mean velocity Vobs and velocity dispersion
σobs are combined into the second velocity moment VRMS
V 2 σ2 , which is then fitted by a solution of the Jeans equa-
tions based on the de-projected stellar light distribution multiplied
with a constant mass-to-light ratio (M/L) DYN . Right: The
resulting total mass-to-light ratios (M/L) DYN are compared
with stellar mass-to-light ratio (M/L) POP estimated based on
fitting stellar population models to multi-band photometry. The
(M/L) DYN for the circles/triangles are based on sauroN/califa
stellar kinematics. The coloring and symbol size represent galaxy
type and luminosity as indicated in the legend. The dashed
line indicates the increase in (M/L) POP when going from a
Chabrier to Salpeter initial mass function, reflecting the effect
of increasing the relative fraction of low-mass stars.
circular velocity of the gas as function of radius from
the center of the galaxy, tracing the total mass distribution.
The contribution of the gas to the total mass distribution
as indicated by the lower curve is minimal. As
can be readily seen from the first panel, the distribution
of the stars drops with radius, so that – in the framework
of Newtonian dynamics – we need a dark matter halo
to explain why the observed circular velocity curve remains
flat.
Inferring the total mass distribution directly from the
observed mean velocity only works for tracers on circu-
III.3 Dynamics of Galaxies: inferring their mass distribution and formation history 57
Vrms, mod [km/s]
0 229 0 229
(M/L) DYN
12
10
8
6
4
2
0
0
E
S0
Sab
Sb
Sbc
Sc
Scd
Sd
Salpeter
Chabrier
r-band
1 2
(M/L) POP
lar orbits, like cold gas. Cold gas can be detected through
CO and HI emission at radio wavelengths, but (1) is typically
not present at all radii in galaxies and sometimes
not at all, (2) settles in a plane (equatorial and/or polar)
and, hence, is insensitive to the mass distribution perpendicular
to it, and (3) is dissipative and, therefore, easily
disturbed by perturbations in the plane from e.g. a bar
or spiral arm. Stars, on the other hand, are present in all
galaxy types, distributed in all three dimensions, and as
Credit: Glenn van de Ven
(00.5 – 10.0) 10 10 L
(10.0 – 20.0) 10 10 L
(20.0 – 40.0) 10 10 L
Error bar
3 4
Credit: Glenn van de Ven