Unveiling the Halo White Dwarf Controversy: Insights and Implications

1. Halo White Dwarf Controversy Ben R. Oppenheimer UC-Berkeley Nigel Hambly, Andrew Digby University of Edinburgh Simon Hodgkin Cambridge University Didier Saumon Vanderbilt University Painting by Lynette Cook

2. Discovery: Hambly et al. (1997) IR Spectrum: Hodgkin et al. (2000) Oppenheimer et al. (2001) WD 0346+246 Parallax: Hambly et al. (2001) L = 1.82 x 10-5 L MR = 16.06 

3. Discovery: LHS, Harris et al. (1999) Oppenheimer et al. (2001) LHS 3250 L = 3.26 x 10-5 L MR = 15.8 

5. The New Spectra 38 new, featureless WDs At least 3 are cooler or as cool as WD 0346. This is only a sampling of all those spectra, with WD 0346 thrown in as a point of reference.

6. The New Spectra Bizarre: Na? Similar to the strange LHS 3250 and SDSS1337, but with a steeper slope, implying an even lower temperature.

8. Halo Velocity Dispersion 2s 1s Old Disk Velocity Dispersion Biases?

11. The Density of Halo White Dwarfs Science paper: 38 Halo White Dwarfs A statistically meaningful sample over a large fraction of the sky allows one to calculate the space density: 

12. MR < 12m Simulation of Survey

13. 12m < MR < 13m Simulation of Survey

14. 13m < MR < 14m Simulation of Survey

15. 14m < MR < 15m Simulation of Survey

16. 15m < MR < 16m Simulation of Survey

17. MR > 16m Simulation of Survey

18. Retrograde orbits Halo Velocity Dispersion 2s 1s Old Disk Velocity Dispersion Biases?

19. Retrograde orbits Halo Velocity Dispersion 2s 1s Old Disk Velocity Dispersion Thick Disk 3 sig. Biases?

24. Let’s accept only the retrogradestars (4 or 8 stars) • This gives a space density of 2 - 5 x 10-5 pc-3 • Using subdwarf star counts and standard IMF, the standard • stellar halo should have 2 – 2.5 x 10-5 pc-3 • But, what does this really mean? • We have found all halo WDs in the solar neighborhood. • The Galaxy’s halo is less than 10 Gyr old • The luminosity function has an extremely sharp cut off • Thick disk mass becomes 2-10 times more massive with an • IMF peaked above 2 M

25. 3 Cut 7 x 10-5 pc-3, ~ 3 times higher than expected for the halo Either this represents about .5-.8% of the dark matter, or they are simply part of the “standard” stellar halo, which must (1) have formed from a strange IMF, (2) is more massive and (3) probably much younger than previously thought. The thick disk mass is 2-8 times more massive and must have formed with a top-heavy IMF.

26. Three Choices: • Accept our original analysis. The white dwarfs represent >2% of the dark matter. The standard picture of the thick disk is unaffected. Probably need a peaked IMF for the halo (low Z gas). • Accept the Reid et al. argument that only retrograde stars • are certainly halo members. Halo must be very young, the thick disk is far more massive (2-10x) than other • studies show and it must have formed with a weird IMF, while the halo and the thin disk did not. It also must have had a vastly different star formation history than other studies indicate. • 3. Take an intermediate position, i.e. 3, >0.5% of the dark matter density is in white dwarfs in a halo with a peaked IMF. Or the standard halo is only 10 Gyr old and has a weird IMF. The thick disk is more massive possibly with a weird IMF and SF history.

27. Bottom Line(s) • Our interpretation in Science is the simplest and least disruptive of the current, standard observations of galactic structure. • No matter what interpretation you favor, some evolution of the IMF is required. (Our interpretation makes it a probably a function of metallicity. Reid et al.’s would make it non-standard only for the thick disk.) • ~10-20 new white dwarfs below 4000K

28. RPM Diagram RPM depends only on the absolute mag and the transverse velocity, not at all on apparent mag or distance.