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

Local mass and local lightLocal mass and local light

Surface density and volume density of mass and light

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

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

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.

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.

Disc surface densityDisc surface density

Roughly : 1/3 gas; 1/3 red dwarfs; 1/3 other stars

Contributorsto the surface density of matter

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?

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)

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

Rise of the giantsRise of the giantsB band I bandV band

Shift from main sequence to giant dominance of luminosity budget

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 ...

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

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⊙

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

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

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.

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

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

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

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

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

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?