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Mass to light ratio of the Mass to light ratio of the Milky Way discMilky Way disc
Chris Flynn, Johan Holmberg, Laura PortinariChris Flynn, Johan Holmberg, Laura Portinari
Tuorla ObservatoryTuorla Observatory
Burkhard Fuchs, Hartmut JahreiBurkhard Fuchs, Hartmut Jahreißß
Astronomisches Rechen-Inst, HeidelbergAstronomisches Rechen-Inst, Heidelberg
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Local mass and local lightLocal mass and local light
Surface density and volume density of mass and light
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First measure the massFirst 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
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ResultsResults
Bahcall, Flynn and Gould (1992) = 80 +/- 20 M⊙ / pc²
Flynn and Fuchs (1994) = 50 +/- 10 M⊙ / pc²
Kuijken and Gilmore (1989) = 46 +/- 9 M⊙ / pc²
Creze et al (1989) ~ 50 M⊙ / pc²
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Hipparcos and HSTHipparcos and HSTIn the 1990s:In the 1990s:
Hipparcos provided superb parallaxes...Hipparcos provided superb parallaxes...
... and ... HST nailed down the number of faint ... and ... HST nailed down the number of faint stars in the Galaxystars in the Galaxy
Holmberg and Flynn (2004) = 56 +/- 6 M⊙ / pc²
--- using A stars, F stars, K giants.
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Mass model of discMass model of disc
The total local disc mass is measured via The total local disc mass is measured via dynamics... dynamics...
... and can be compared to the total amount ... and can be compared to the total amount of visible mass.of visible mass.
Result: Result: The two agree. No significant The two agree. No significant evidence for missing disc matter.evidence for missing disc matter.
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Disc surface densityDisc surface density
Roughly : 1/3 gas; 1/3 red dwarfs; 1/3 other stars
Contributorsto the surface density of matter
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Luminosity of the discLuminosity of the disc
Next step: How much light is generated by Next step: How much light is generated by the stellar components?the stellar components?
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The sources of nearby V band light 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)
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Going upGoing upWe now integrate the We now integrate the
luminosity sources luminosity sources vertically (via the disc vertically (via the disc mass model and/or mass model and/or velocity dispersions velocity dispersions locally) to obtain surface locally) to obtain surface luminositiesluminosities
Giants have a higher scale Giants have a higher scale height than turn-off stars, height than turn-off stars, and contribute much and contribute much more to the surface more to the surface luminosityluminosity
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Rise of the giantsRise of the giantsB band I bandV band
Shift from main sequence to giant dominance of luminosity budget
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Luminosity resultsLuminosity 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 ...
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Mass-to-light : the resultsMass-to-light : the results
band volume M/L surface M/L 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
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Boldly go... Boldly go...
We can extrapolate from We can extrapolate from
the Solar Neighbourhood and the Solar Neighbourhood and
compute the disc mass and compute the disc mass and
luminosity...luminosity...
Turns out to be amazingly Turns out to be amazingly
insensitive to the disc's insensitive to the disc's
scalelength... scalelength...
(Sommer-Larsen and Dolgov, 2001)(Sommer-Larsen and Dolgov, 2001)
Disc I band luminosity ~ 3×1010 L⊙
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How many stars?How many stars?
There are ~70 stars There are ~70 stars per square parsec near per square parsec near the Sunthe Sun
How many in the How many in the Galaxy? Galaxy?
Rough total is 65 billion Rough total is 65 billion stars (including 30% stars (including 30% for binaries) so...for binaries) so...
100 billion stars 100 billion stars altogether in the disk, altogether in the disk, bulge and halobulge and halo
Stars per square parsec
Main Sequence
Giants
White dwarfs
Red dwarfs
Sun-like
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Milky Way seen from afarMilky Way seen from afar
Surface brightness Surface brightness fluctuations are a fluctuations are a distance measurement distance measurement technique technique
What does the stellar mix What does the stellar mix in the solar column look in the solar column look like from a distance?like from a distance?
Solar neighbourhood is Solar neighbourhood is quite bluequite blue
Still, the SBF fluctuation Still, the SBF fluctuation magnitude for local magnitude for local column fits wellcolumn fits well Mieske et al 2007
Dunn 2007 (priv. comm.)
Local disk of Milky Way lies here
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Milky Way compared to External discsMilky Way compared to External discs
Where does the Where does the
Milky Way lie on theMilky Way lie on the
Tully-Fisher relation?Tully-Fisher relation?
Pretty much within the (1-sigma) Pretty much within the (1-sigma) scatter.scatter.
It's a bit under-luminous for its It's a bit under-luminous for its rotation raterotation rate
Copernicus wouldn't mind too Copernicus wouldn't mind too much, presumably.much, presumably.
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Visible baryons are well Visible baryons are well understoodunderstood
mass and light slot nicely mass and light slot nicely together in the local disctogether in the local disc
the disc of the galaxy fits well the disc of the galaxy fits well to galaxy evolution modelsto galaxy evolution models
models should reliable for a models should reliable for a range of purposesrange of purposes
Portinari et al 2006
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Missing baryonsMissing baryons
baryons account for 4% of the baryons account for 4% of the cosmic energy budget (25% is cosmic energy budget (25% is in both types of matter)in both types of matter)
much of the baryons should much of the baryons should be in associated with galaxiesbe in associated with galaxies
but the visible baryons in the but the visible baryons in the galaxy only add up to 0.5% galaxy only add up to 0.5% (factor of almost 10 too (factor of almost 10 too small!)small!)
where are these local where are these local baryons?baryons?
Driver et al , 2007
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Baryonic mass fractionBaryonic mass fraction
Baryons in Baryons in structures versus structures versus dynamical speeddynamical speed
red line is red line is LambdaCDM LambdaCDM expectationexpectation
Very little scatter in Very little scatter in the observed the observed relationrelation
McGaugh 2007
Galaxy clusters
Milky Way
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stacked SDSS 1047 edge-on stacked SDSS 1047 edge-on galaxiesgalaxies
seen around blue compact seen around blue compact galaxies, other types toogalaxies, other types too
very red colour is difficult to fit to very red colour is difficult to fit to ordinary stellar popsordinary stellar pops
if composed of red dwarfs, they if composed of red dwarfs, they could represent a lot of newly could represent a lot of newly found baryonsfound baryons
density profile and M/L poorly density profile and M/L poorly understoodunderstood
Baryons in red Baryons in red haloshalos
Zibetti et al 2004
Bergvall et al 2006
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No red star excess near usNo red star excess near us does the Milky Way have a does the Milky Way have a
“red halo”?“red halo”? red dwarfs account for over red dwarfs account for over
half the nearby mass, but half the nearby mass, but emit little lightemit little light
star counts with star counts with Hipparcos/Tycho/HST were Hipparcos/Tycho/HST were used, and rule out a red star used, and rule out a red star excess near the Sunexcess near the Sun
Casagrande/Gardner Casagrande/Gardner working on new, huge M working on new, huge M dwarf sampledwarf sample
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Summary Summary
Disc M/L is well constrained by Hipparcos/HST. Disc M/L is well constrained by Hipparcos/HST.
M/LM/LB,V,IB,V,I = '1 and a bit'. = '1 and a bit'.
Next comes infra-red (J, H, K) from 2MASS. Work in progress.Next comes infra-red (J, H, K) from 2MASS. Work in progress.
Galactic chemical evolutionary models of Solar neighbourhood Galactic chemical evolutionary models of Solar neighbourhood fit the data very well. Models gain street credit!fit the data very well. Models gain street credit!
Are there still at least as many baryons to be found in the Milky Are there still at least as many baryons to be found in the Milky Way?Way?