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New Light on Dark Matter Halos (1Sep94)
(from NOAO HIGHLIGHTS!, NOAO Newsletter No. 39, 1 September 1994)

It is generally believed that dark matter halos dominate the potential of
spiral galaxies, given the high rotation speeds of the spirals' outer disks.
The nature of this dark matter remains unknown, however, with candidates
ranging from faint low mass stars or brown dwarfs to exotic elementary
particles. To date, no light has been observed from dark matter halos. Penny
Sackett (IAS), Heather Morrison (NOAO/Case), Todd Boroson (NOAO), and Paul
Harding (Steward), however, have discovered a faint luminous halo around the
edge-on spiral galaxy NGC 5907 that appears to match the spatial distribution
of its dark matter halo.

Faint halos of Population II stars are observed around many spiral galaxies
(including our own galaxy as well), but make a very small contribution to the
galactic potential, and are more centrally concentrated than the dark matter
halos in any case. There were a number of attempts in the 1980s to test
whether dark matter halos were made of a separate population of faint, low
mass stars using photographic techniques. However, the limits derived were
not sufficiently strong to rule these out as dark matter candidates.

The advent of large-format CCDs and better computational techniques now makes
much deeper searches for faint halos possible. Morrison, Boroson, and Harding
obtained R band images of NGC 5907 using the KPNO 0.9-m telescope with a 2048
X 2048 CCD (Figure 1). A combination of observing techniques designed to
obtain very good flat fielding and sky subtraction, plus analytical
techniques (including careful error modeling), allowed them to reach reliably
2 magnitudes fainter than previous photographic work. (This work will appear
in the October AJ).


                            [Figure not included]

                     Figure 1: R band image of NGC 5907

Sackett, Morrison, Harding, and Boroson have reported their discovery of a
faint, luminous halo around NGC 5907 in a recent issue of Nature.
This halo is unlike any known luminous component of a spiral galaxy, having
an unusually shallow radial profile. Figure 2 shows the comparison of the
data with a model that provides a good fit to the data (disk plus halo with
an r^-2.26 density law and a 2 kpc core) and a model for disk plus halo with
a power-law slope characteristic of Pop II halos (r^-3.5). The r^-2.26 halo
model provides a much better fit.


                            [Figure not included]

     Figure 2:  Minor axis profile of NGC 5907.  Exponential disk
     model only (dashed), disk plus r^-3.5 halo (dotted), disk plus
     r^-2.26 halo (solid).

The shallow radial profile of NGC 5907's halo suggests that it formed in a
very different way from any other known stellar halo.  It must have suffered
very little of the dynamical evolution (for example, dissipation or violent
relaxation) that produces typical stellar halos.  In addition, the similarity
of its radial profile to that inferred for dark matter halos suggests that
the luminous halo might be connected with NGC 5907's dark matter halo, either
as a tracer or because we are observing some of the "dark" matter itself.

The dynamics of NGC 5907, as measured from its HI rotation curve can be well
fit by taking the space distribution of disk and halo from the model fit to
the surface photometry, and transforming them to mass densities by assigning
mass-to-light ratios (M/L) to disk and halo (Figure 3).  This suggests that
the faint extended light of NGC 5907 may have a radial profile similar to
that of its dark matter halo.  Depending on the assumed disk M/L, the M/LR
for the halo that produces the best fit is in the range 270-540.   This
range is consistent with the M/LR of a metal-weak M dwarf near the
H-burning limit.  However, studies of faint field stars in the
Milky Way  suggest that its massive halo does not consist solely of such
stars.  If the  dark matter halo of NGC 5907 is similar to that of the
Galaxy, it is more  likely that NGC 5907's luminous halo may trace its dark
matter but have  relatively low total mass, itself.


                            [Figure not included]

     Figure 3:  Superposed on the H I kinematics (Sancisi and van
     Albada 1987) is a model rotation curve (heavy line) formed from:
     (1) an H I disk (dotted), (2) an exponential stellar disk (dashed),
     and (3) a halo (thin curve) derived from the faint extended light.

Color observations of this faint halo will provide vital information on its
stellar makeup.  If it is composed of faint low-mass metal-weak stars, its
red color would enable deep I-band photometry to show a strong detection of
the halo.  These data were obtained recently on the Burrell Schmidt.

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