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Significant variations in stellar populations due to
intervening stream–like structures are detected in the inner
halo. Underlying the many substructures lies a faint,
smooth, and extremely extended halo component,
reaching out to 150 kpc, whose stellar populations are
predominantly metal-poor. They find that the smooth halo
component in M31 has a radially decreasing profile that
can be fitted with a Hernquist model of immense scale
radius ~55 kpc, almost 4 times larger than theoretical
predictions. If it is symmetric, then the total luminosity of
this structure is ~10 9 solar luminosities, again similar to the
stellar halo of the Milky Way.
This vast, smooth, underlying halo is reminiscent of a
classical “monolithic” model and completely unexpected
from modern galaxy formation models. M33 is also found
to have an extended metal-poor halo component, which
can be fitted with a Hernquist model, also of scale radius
~55 kpc. These extended slowly decreasing halos provide
a challenge and strong constraints for further modelling.
ECLIPSING BINARIES AND CEPHEIDS IN
THE ANDROMEDA GALAXY
Cepheids are probably the most studied variable stars.
Their large amplitudes and intrinsic luminosities make
them easily detectable in most photometric variability
surveys. In addition, their well-known period-luminosity (P-
L) relationship has made these variable stars one of the
main cornerstones in deriving extragalactic distances. The
importance of Cepheids for distance determination stands
in contrast with the relative lack of additional information on
the specific characteristics of extragalactic Cepheids and
the possible corrections because of their particular
properties (i.e., metallicity).
A clear example is the Andromeda galaxy (M 31), where
the first identification of Cepheids was already performed
by Hubble in 1929. After the observations of Baade &
Swope in 1965, few efforts have been dedicated to further
analyse the Cepheid population in M 31.
This trend has changed in recent years with the
emergence of new observational capabilities. Several
variability surveys have started to study the stellar content
in M 31 and other Local Group galaxies, obtaining large
samples of Cepheids with accurate photometry. The
detailed study of the observed Cepheids has emphasised
the importance of an issue that was usually overlooked in
most photometric studies, the effect of blending. It has
been proposed that the magnitude of Cepheids may be
affected by the light of unresolved companion stars (i.e.,
blends). The effect of blending is somewhat different from
crowding or confusion noise, since companion stars
appear to be in the same point-like source. Therefore,
even when achieving perfect point-spread function
modelling, blending can still be present. The effect can be
the same as in spectroscopic binaries, where the individual
components cannot usually be resolved from groundbased
images.
With the goal of obtaining accurate distance
determinations to the Andromeda Galaxy, and constraining
the age and evolution of the Universe, astronomers started
a project to use eclipsing binaries as distance indicators to
M 31. Eclipsing binaries have been proved to yield direct
and precise distances that are essentially assumption-free.
To do so, high-quality photometric and spectroscopic data
were needed. As a first step in the project, broad band
photometry (in Johnson B and V) was obtained in a region
(~34'×34') in the north eastern quadrant of the galaxy over
5 years.
The data, containing more than 250 observations per filter,
generated a catalogue with 236,238 objects with photometry
in both B and V passbands. This catalogue is the deepest
(V