Faint Blue Objects on the Hubble Deep Field North and South as Possible Nearby Old Halo White Dwarfs

Abstract

Using data derived from the deepest and finest angular resolution images of the universe yet acquired by astronomers at optical wavelengths using the Hubble Space Telescope (HST) in two postage-stamp sections of the sky (Williams et al. 1996a,b), plus simple geometrical and scaling arguments, we demonstrate that the faint blue population of point-source objects detected on those two fields (Méndez et al. 1996) could actually be ancient halo white dwarfs at distances closer than about 2 kpc from the Sun. This finding has profound implications, as the mass density of the detected objects would account for about half of the missing dark matter in the Milky-Way (Bahcall and Soneira 1980), thus solving one of the most controversial issues of modern astrophysics (Trimble 1987, Ashman 1992.The existence of these faint blue objects points to a very large mass locked into ancient halo white dwarfs. Our estimate indicates that they could account for as much as half of the dark matter in our Galaxy, confirming the suggestions of the MACHO microlensing experiment (Alcock et al. 1997). Because of the importance of this discovery, deep followup observations with HST within the next two years would be needed to determine more accurately the kinematics (tangential motions) for these faint blue old white dwarfs.

A New (Old) Component of the Galaxy as the Origin of the Observed Large Magellanic Cloud Microlensing Events

“…However, a ratio of D2 is roughly consistent with that expected for a Galactic population, since HDF-N looks toward the outer Galaxy while HDF-S is pointed more toward the center of the Galaxy. Mendez & Minniti (1999) also state that these sources could represent of D1 3 È1 2 the dark matter in the Galaxy.…”

“…For a spheroid model we expect a ratio of about 2.4, compared to 1.7 (halo) or 1.5 (shroud). We emphasize again that the details of a particular survey are needed in order to accurately predict this ratio, and note that we have cut on detectable proper motion, as opposed to the work of Mendez & Minniti (1999), which looked only for faint blue point sources.…”

“…Ibata et al (1999) also argue that these moving sources cannot be part of the known disk, thick disk, or spheroid populations and, further, that the number of sources detected is consistent with the number expected for an all white dwarf halo, although this claim is strongly dependent on many factors, especially the assumed initial mass function (IMF) of the white dwarf progenitor population (Richer 1999). Mendez & Minniti (1999) compared faint blue point sources in the northern HDF (HDF-N) and the southern HDF (HDF-S). They Ðnd Ðve such objects in HDF-N and 10 in HDF-S.…”

Evolutionary and pulsational properties of white dwarf stars

“…Most of them have challenged the results of the MACHO experiment -see, for instance, Yoo et al (2004) and references therein. In addition, there have been claims that white dwarfs could be the stellar objects reported in the Hubble Deep Field (Ibata et al 1999;Méndez & Minniti 2000). However, these claims remain inconclusive for lack of spectroscopic identifications.…”

Recent Developments in the Search for Baryonic Dark Matter

“…Three such objects have now been identified spectroscopically (Hodgkin et al 2000, Ibata et al 2000, so this possibility must be taken very seriously. Evidence that the blue population is Galactic comes from the observed asymmetry between the North and South HST fields (Mendez & Minniti 2000); this is expected from dust obscuration. Another interesting signature of the WD scenario would be the generation of background gravitational waves (Hiscock et al 2000).…”