1. Mass to light ratio of the Milky Way disc Chris Flynn, Johan Holmberg, Laura Portinari Tuorla Observatory Burkhard Fuchs, Hartmut Jahrei ß Burkhard Fuchs, Hartmut Jahrei ß Astronomisches Rechen-Inst, Heidelberg

2. Local mass and local light Surface density and volume density of mass and light

3. 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

4. Results 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

5. 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.

6. 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.

7. Disc surface density Roughly : 1/3 gas; 1/3 red dwarfs; 1/3 other stars Contributors to the surface density of matter

8. Luminosity of the disc Next step: How much light is generated by the stellar components?

9. 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)

10. 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

11. Rise of the giants B band I band V band Shift from main sequence to giant dominance of luminosity budget

12. Luminosity results band volume luminosity surface luminosity B 0.074 L ⊙ /pc 3 26 L ⊙ /pc 2 V 0.056 L ⊙ /pc 3 24 L ⊙ /pc 2 I 0.063 L ⊙ /pc 3 30 L ⊙ /pc 2 Now, for the stellar component, we have a mass surface density of 36 M ⊙ /pc 2... and... a local mass density of 0.042 M ⊙ /pc 3...

13. Mass-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

14. 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 ~ 3×10 10 L ⊙

15. How many stars? There are ~70 stars per square parsec near the Sun There are ~70 stars per square parsec near the Sun How many in the Galaxy? How many in the Galaxy? Rough total is 65 billion stars (including 30% for binaries) so... Rough total is 65 billion stars (including 30% for binaries) so... 100 billion stars altogether in the disk, bulge and halo 100 billion stars altogether in the disk, bulge and halo

16. Milky Way seen from afar Surface brightness fluctuations are a distance measurement technique Surface brightness fluctuations are a distance measurement technique What does the stellar mix in the solar column look like from a distance? What does the stellar mix in the solar column look like from a distance? Solar neighbourhood is quite blue Solar neighbourhood is quite blue Still, the SBF fluctuation magnitude for local column fits well Still, the SBF fluctuation magnitude for local column fits well Mieske et al 2007 Dunn 2007 (priv. comm.) Local disk of Milky Way lies here

17. 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.

18. Visible baryons are well understood mass and light slot nicely together in the local disc mass and light slot nicely together in the local disc the disc of the galaxy fits well to galaxy evolution models the disc of the galaxy fits well to galaxy evolution models models should reliable for a range of purposes models should reliable for a range of purposes Portinari et al 2006

19. Missing baryons baryons account for 4% of the cosmic energy budget (25% is in both types of matter) 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 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!) but the visible baryons in the galaxy only add up to 0.5% (factor of almost 10 too small!) where are these local baryons? where are these local baryons? Driver et al, 2007

20. Baryonic mass fraction Baryons in structures versus dynamical speed Baryons in structures versus dynamical speed red line is LambdaCDM expectation red line is LambdaCDM expectation Very little scatter in the observed relation Very little scatter in the observed relation McGaugh 2007 Galaxy clusters Milky Way

21. stacked SDSS 1047 edge-on galaxies stacked SDSS 1047 edge-on galaxies seen around blue compact galaxies, other types too seen around blue compact galaxies, other types too very red colour is difficult to fit to ordinary stellar pops 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 if composed of red dwarfs, they could represent a lot of newly found baryons density profile and M/L poorly understood density profile and M/L poorly understood Baryons in red halos Zibetti et al 2004 Bergvall et al 2006

22. No red star excess near us does the Milky Way have a “red halo”? does the Milky Way have a “red halo”? red dwarfs account for over half the nearby mass, but emit little light 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 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 Casagrande/Gardner working on new, huge M dwarf sample

23. Summary Disc M/L is well constrained by Hipparcos/HST. M/L B,V,I = '1 and a bit'. M/L B,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?