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Mass to light ratio of the Milky
Way disc
Chris Flynn, Johan Holmberg, Laura Portinari
Tuorla Observatory
Burkhard Fuchs, Hartmut Jahreiß
Astronomisches Rechen-Inst, Heidelberg
Local mass and local light
Surface density and volume density of mass and light
First measure the mass
Measure vertical velocities
for a few hundred
tracer stars near the Sun.
Measure their vertical density
distribution.
Combining these leads to an estimate of the Galactic vertical
gravitational potential and the surface and volume mass density
Results
Creze et al (1989)
Kuijken and Gilmore (1989)
 ~ 50 M/ pc²
 = 46 +/- 9 M/ pc²
Bahcall, Flynn and Gould (1992)  = 80 +/- 20 M/ pc²
Flynn and Fuchs (1994)
 = 50 +/- 10 M/ pc²
Hipparcos and HST
In the 1990s:
Hipparcos provided superb parallaxes...
... and ... HST nailed down the number of faint
stars in the Galaxy
Holmberg and Flynn (2004)
 = 56 +/- 6 M/ pc²
--- using A stars, F stars, K giants.
Mass model of disc
The total local disc mass is measured via
dynamics...
... and can be compared to the total amount
of visible mass.
Result: The two agree. No significant
evidence for missing disc matter.
Disc surface density
Contributors
to the surface
density
of matter
Roughly : 1/3 gas; 1/3 red dwarfs; 1/3 other stars
Luminosity of the disc
Next step: How much light is generated by
the stellar components?
The sources of nearby V band light
Nearby V luminosity is mainly due to main sequence (MS),
with a healthy contribution by red giants (RG)
Going up
We now integrate the
luminosity sources
vertically (via the disc
mass model and/or
velocity dispersions
locally) to obtain surface
luminosities
Giants have a higher scale
height than turn-off stars,
and contribute much
more to the surface
luminosity
Rise of the giants
B band
V band
I band
Shift from main sequence to giant dominance of luminosity budget
Luminosity results
band
volume luminosity
surface luminosity
B
0.074 L/pc3
26 L/pc2
V
0.056 L/pc3
24 L/pc2
I
0.063 L/pc3
30 L/pc2
Now, for the stellar component, we have
a mass surface density of 36 M/pc2 ... and ...
a local mass density of 0.042 M/pc3 ...
Mass-to-light : the results
band
volume M/L
M/L
surface M/L
M/L
B
0.6 +/- 0.1
1.4 +/- 0.2
V
0.8 +/- 0.1
1.5 +/- 0.2
I
0.7 +/- 0.1
1.2 +/- 0.2
Surface M/L of stars is “one and a bit”!
Including the gas, M/L is ~ 2
Boldly go...
We can extrapolate from
the Solar Neighbourhood and
compute the disc mass and
luminosity...
Turns out to be amazingly
insensitive to the disc's
scalelength...
(Sommer-Larsen and Dolgov, 2001)
Disc I band luminosity ~ 3×1010 L
How many stars?

There are ~70 stars
per square parsec near
the Sun
Stars per square parsec
Main Sequence
Giants

How many in the
Galaxy?

Rough total is 65 billion
stars (including 30%
for binaries) so...

100 billion stars
altogether in the disk,
bulge and halo
White dwarfs
Red dwarfs
Sun-like
Milky Way seen from afar

Surface brightness
fluctuations are a
distance measurement
technique

What does the stellar mix
in the solar column look
like from a distance?

Solar neighbourhood is
quite blue

Still, the SBF fluctuation
magnitude for local
column fits well
Local disk of Milky
Way lies here
Mieske et al 2007
Dunn 2007 (priv. comm.)
Milky Way compared to External discs
Where does the
Milky Way lie on the
Tully-Fisher relation?
Pretty much within the (1-sigma)
scatter.
It's a bit under-luminous for its
rotation rate
Copernicus wouldn't mind too
much, presumably.
Visible baryons are well
understood

mass and light slot nicely
together in the local disc

the disc of the galaxy fits well
to galaxy evolution models

models should reliable for a
range of purposes
Portinari et al 2006
Missing baryons

baryons account for 4% of the
cosmic energy budget (25% is
in both types of matter)

much of the baryons should
be in associated with galaxies

but the visible baryons in the
galaxy only add up to 0.5%
(factor of almost 10 too small!)

where are these local
baryons?
Driver et al , 2007
Baryonic mass fraction
Galaxy clusters

Baryons in
structures versus
dynamical speed

red line is
LambdaCDM
expectation

Very little scatter in
the observed
relation
McGaugh 2007
Milky Way
Baryons in red
halos

stacked SDSS 1047 edge-on
galaxies

seen around blue compact
galaxies, other types too

very red colour is difficult to fit to
ordinary stellar pops

if composed of red dwarfs, they
could represent a lot of newly
found baryons

density profile and M/L poorly
understood
Bergvall et al 2006
Zibetti et al 2004
No red star excess near us

does the Milky Way have a
“red halo”?

red dwarfs account for over
half the nearby mass, but
emit little light

star counts with
Hipparcos/Tycho/HST were
used, and rule out a red star
excess near the Sun

Casagrande/Gardner
working on new, huge M
dwarf sample
Summary
Disc M/L is well constrained by Hipparcos/HST.
M/LB,V,I = '1 and a bit'.
Next comes infra-red (J, H, K) from 2MASS. Work in progress.
Galactic chemical evolutionary models of Solar neighbourhood
fit the data very well. Models gain street credit!
Are there still at least as many baryons to be found in the Milky
Way?