Age of the Disk and Halo

The remnants of the earliest star formation events in the Galaxy are currently observable as the coolest white dwarfs. Since the cooling rate of white dwarfs is fairly well-known (e.g., Saumon & Jacobson 1999, Hansen 1999), they can be age-dated based on their observed luminosities. The luminosity function for white dwarfs has consequently been argued as a robust and independent measure of the age of the Galaxy and its star formation history (e.g., Winget et al 1987, Leggett et al. 1998).

The extreme faintness of the oldest white dwarfs (M_V>~16; fig. 11) restricts the volume in which they will be detected to the solar neighborhood. While recent work has led to preliminary determinations of the age of the Galactic disk by this method (e.g., Leggett et al. 1998) larger area and deeper searches would firmly establish the age of the disk as well as enable the first studies of halo white dwarfs. Ages for the rare halo white dwarfs (~3-10% of disk white dwarfs by number) would place unique constraints on the age of the halo and consequently the age of the Universe.

 

Figure 11: Because their cooling rates are well-understood, cool white dwarfs (M_V > 17) offer a robust and independent measure of the age of the Galaxy and a constraint on the age of the Universe (e.g., Saumon & Jacobson 1999). Spectroscopy is required for detection since significant contamination remains even after photometric selection criteria have been applied.

Since both low metallicity stars and distant galaxies have colors similar to those of cool white dwarfs, the detection of cool white dwarfs requires spectroscopy because of the significant contamination that remains even after photometric selection criteria have been applied (fig. 12). New photometric selection techniques (Claver 1995) are currently able to weed out many of the potential contaminants, leaving roughly 10 contaminants for every bona fide white dwarf. Of these, only a small fraction have the low luminosities (M_V>17; log(L/L_sun)<-4.5) that are capable of determining the ages of the disk and halo.

  

Figure 12: Searching for a needle-in-a-haystack. In this shallow 45' field, photometric selection criteria identify ~30 white dwarf candidates to V=18.5. Follow-up spectroscopy revealed that, of these, 3 are bona fide white dwarfs, but all are 4-5 Gyr old, much younger than of interest. Deep spectroscopy over a wider field of view is needed to detect old disk white dwarfs and the even rarer halo white dwarfs.

A Representative Project: Cool White Dwarfs in the Halo and Disk
A magnitude limited spectroscopic survey provides a kinematically unbiased method for culling out the coolest white dwarfs in the solar neighborhood. Results from this survey will be critical toward quantifying the kinematic selection biases of future large-area (~10,000sq.deg.) proper motion surveys of the halo white dwarf population. A reasonable compromise in survey area and depth is a survey for white dwarfs over 100 sq.deg. to a limit of V=25 in order to probe white dwarfs with ages >10Gyr. At this magnitude limit, photometric selection will whittle down the number of white dwarf candidates to a manageable ~2000/sq.deg. Of these, ~200 will be bona fide white dwarfs and ~1 will be in the critical range log(L/L_sun) < -4.5. Currently known disk white dwarfs in this luminosity range number between 1 and 5. Therefore, in establishing accurate statistics at log(L/L_sun) < -4.5, a survey of 100^ will definitively establish the age of the disk. Of the 100 cool white dwarfs that will be detected in the survey, 3-10% will belong to the halo population, enabling an initial estimate for the age of the halo. The survey will thereby provide a self-consistent estimate for the relative ages of the disk and halo, both of which have been obtained from the same method and models.