Fig. 1:
Up: Image of a single SDSS galaxy which goes into a stack. Below: The
stack of early-type galaxies in the highest mass range. The contours
indicate the ellipticity measured at various distances from the centre.
For the individual galaxy image only the central, high surface
brightness regions are visible, the outer areas are dominated by noise.
While a typical single SDSS image allows one to go to a depth of 26
magnitudes/arcsec2, in our stacked image we can achieve a depth of nearly 32 magnitudes/arcsec2 in the r-band.
Fig 1a - Fig 1b
Fig. 2:
Fraction of accreted stellar light for early-type galaxies as a function of galaxy mass.
Fig. 3:
Same as Fig.2 for late-type galaxies. Here the same trend can be seen,
although the fraction of accreted stellar light is much smaller.
Galaxies contain not only "home-made" stars, but also quite a large fraction of
stars that were accreted from other galaxies, as a new analysis of SDSS images
has recently shown. The scientists at the MPA "stacked" a large number of
individual galaxy images to reveal the faint light of the stellar halo of
distant galaxies. For the most massive, early-type galaxies the fraction of
accreted stars in the stellar halo can be up to 70%.
In recent years, deep photometric observations of the local universe have
revealed that galaxies are surrounded by a "stellar halo", a group of stars
that extend to a distance of up to 100 kpc and more from the centre of the
galaxy. This stellar halo consists of field stars, older globular clusters, and
stars stripped from in-falling satellite galaxies. Too often, this stellar halo
is too faint to observe with present day imaging.
For our own galaxy, the individual stars of the stellar halo have been
resolved with the help of the SDSS survey up to a distance of 50 kpc
from the galactic centre. In addition, deep integrated light studies
have revealed the stellar halo of neighbouring galaxies in the local
universe. However, for large-scale studies of these stellar halos, many
more observations are required. Unfortunately, with present telescopes
and reasonable integration times, we can only detect the stellar halos
of nearby galaxies.
An alternative approach to deep observations of individual galaxies
would be to stack a large number of images of similar galaxies. SDSS
with nearly 60,000 galaxies in its spectroscopic sample, for which
accurate redshifts and stellar masses are available, is just the right
survey for the job. Even though the stellar halos of individual galaxies
cannot be directly observed in the photometric images, by stacking
these images we can increase the signal-to-noise (S/N) ratio, and thus
study the average stellar halos of the galaxies as a function of various
galaxy properties.
The theory of galaxy formation through hierarchical merging predicts not
only that galaxies grow in size and mass through minor mergers, but
also that these minor mergers give rise to the stellar halo. This
implies that the stellar halos of galaxies consist of stars which are
not born in the same galaxy (in-situ stars) but rather of stars which
are born in smaller galaxies that have been accreted (the accreted
component).
Studying the stellar halos of galaxies gives us vital clues to further
constrain the theory of galaxy formation. For example, the fraction of
stellar material accreted from other galaxies is an important physical
constraint in the theory of galaxy formation. In addition, the local
over-densities in the stellar halo of a galaxy often contain vital clues
of the accretion history of the individual galaxy, since the low
density and large relaxation times of stars in the stellar halo
preserves information from the distant past.
A variety of theoretical simulations have been carried out that help us
to better understand the formation of stellar halos. In particular, the
particle-tagging simulations done here at MPA have predicted the stellar
halos of a large range of galaxies and their accreted fractions.
Although the exact properties of the stellar halos of galaxies depend on
the individual accretion history of a galaxy, one can predict the
average stellar halo properties of galaxies as a function of the dark
matter halo mass, of the stellar mass, or as a function of other
properties of the galaxy.
To constrain these theories, we require observations of the stellar
halos of a large number of galaxies, or alternatively stacked images as
mentioned above. In this work, we have used SDSS images of galaxies in
various bins of stellar mass and divided them into late-type and
early-type galaxies. In each stack, we have an average of nearly 3000
galaxies. Before stacking, each image of a galaxy was transformed to a
common redshift (z=0.1) and oriented along the major axis. Other
galaxies and stars in the image are masked out.
The stacked images of the galaxies reveal extra light out to nearly 100
kpc (see Fig. 1) and we find that the amount of extra light in the
stellar halo is a strong function of stellar mass: Larger galaxies have
more extended stellar halos than smaller galaxies. Similarly, early-type
galaxies have larger stellar halos than late-type galaxies. Also, the
ellipticity of the stellar halo is a function of stellar mass and the
haloes of early-type galaxies are more elliptical than those of
late-type galaxies.
To characterize the stellar halo of the galaxy stacks in more detail, we
model the two-dimensional light distribution with a ''Sersic'' profile
(first published in 1963), a general mathematical function to describe
the radial light intensity distribution of a wide variety of galaxy
types. While this provided a good fit in the past, deviations from the
Sersic profile have often been found, as deeper high-resolution data
became available with newer telescopes and instruments.
For our stacked images, we find that the two-dimensional light
distribution cannot be fit by a single Sersic profile, but rather needs
multiple components: a double Sersic profile is needed for early-type
galaxies, and a triple Sersic profile is required for late-type
galaxies.
Each Sersic component traces a different physical and dynamical
component of the galaxy. Under the assumption that the inner Sersic
profile fits the in-situ component, and that the outer Sersic profile
fits the accreted stellar component, we obtain an observational measure
of the average accreted stellar light fraction of the galaxy. For
early-type galaxies, the fraction of accreted stellar light rises from
30% to 70% while for late-type galaxies it is much smaller, increasing
from 2% to 25% over the same mass range (See Figures 2 and 3). This
provides important observational constraints for a range of galaxy
masses and types, which can help differentiate between various theories
of the formation of stellar halos and of the galaxies themselves.
Richard D'Souza
References:
1) R. D'Souza, G. Kauffmann, J. Wang, S. Vegetti, "Parametrizing the Stellar Haloes of Galaxies", submitted to MNRAS2) Cooper, A.P.; D'Souza, R.; Kauffmann, G.; "Galactic accretion and the outer structure of galaxies in the CDM model", MNRAS, 434/4, p.3348-3367, 2013
