SBF: multi-wavelength data and models
**FULL TITLE**
ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION**
**NAMES OF EDITORS**
SBF: multi-wavelength data and models
M. Cantiello1,2, G. Raimondo2, J.P. Blakeslee1, E. Brocato2, M.
Capaccioli3
Abstract. Recent applications have proved that the Surface Brightness Fluc-
tuations (SBF) technique is a reliable distance indicator in a wide range of dis-
tances, and a promising tool to analyze the physical and chemical properties of
unresolved stellar systems, in terms of their metallicity and age. We present the
preliminary results of a project aimed at studying the evolutionary properties
and distance of the stellar populations in external galaxies based on the SBF
method.
On the observational side, we have succeeded in detecting I-band SBF gra-
dients in six bright ellipticals imaged with the ACS, for these same objects we
are now presenting also B-band SBF data. These B-band data are the first
fluctuations magnitude measurements for galaxies beyond 10 Mpc.
To analyze the properties of stellar populations from the data, accurate
SBF models are essential. As a part of this project, we have evaluated SBF
magnitudes from Simple Stellar Population (SSP) models specifically optimized
for the purpose. A wide range of chemical compositions and ages, as well as
different choices of the photometric system have been investigated. All mod-
els are available at the Teramo-Stellar Populations Tools web site: www.oa-
teramo.inaf.it/SPoT.
We have measured B- and I-band SBF magnitudes for 6 elliptical galax-
ies observed with the ACS camera on board of HST: NGC1407, NGC3258,
NGC3268, NGC4696, NGC5322 and NGC5557. Concerning I-band images,
their high S/N ratio allowed us to obtain SBF measurements in different regions
of the galaxies – 5 concentric annuli (Cantiello et al. 2005). On the contrary,
the B-band images have low S/N (∼1), and SBF amplitudes can be measured
only in one single annulus. The reliability of these B-band measurements has
been verified via numerical simulations, by using a procedure which is able to
reproduce realistic images of elliptical galaxies, including the stellar SBF signal.
The general lack of B-band SBF data hampered up to now a detailed com-
parison with models, our observational data represent the first sample of B-
and I-band SBF measurements for a fair sample of distant galaxies. Figure 1
(left panels) shows the comparison of absolute SBF magnitudes versus (B-I)0
color data with SSP models from the Teramo Stellar Populations Tools group
(SPoT models, Raimondo et al. 2005). SBF and color data appear generally
well reproduced by means of standard SSP models in the M̄I vs. (B-I)0 panel.
However, there is a considerable mismatch between SBF models and data for
Dep. of Physics and Astronomy, Washington State University, Pullman, WA 99164
INAF-Oss. Astronomico di Teramo, Via M. Maggini, 64100, Teramo, Italy
INAF-Oss. Astronomico di Capodimonte, Vicolo Moiariello 16, 80131, Napoli, Italy
http://arxiv.org/abs/0704.1002v1
2 Cantiello M. et al.
some objects in the M̄B vs. (B-I)0 panel. Such disagreement does not depend on
the distance modulus adopted to estimate the absolute SBF magnitudes, in fact
the same mismatch is present also in the distance-free SBF-color vs. color (B-I)0
(Cantiello et al. 2006, ApJ submitted). In addition, adopting other standard
SSP models from literature (e.g. Blakeslee et al. 2001) or also non-standard
SSP models (e.g. alpha enhanced models) the disagreement is not removed.
Figure 1. Left panels: SBF absolute magnitudes vs. the (B-I)0 color derived
from HST data (full dots). Full squares mark the only two other galaxies with
literature data. SSP models are from the SPoT group for the labeled chemical
compositions and 2 Gyr ≤ t ≤ 14 Gyr (symbols of increasing size mark older
ages). Right panels: same data as left panels but compared to CSP models.
One possible solution seems to be the use of Composite Stellar Populations
(CSP). In Figure 1 (right panels) we compare the Blakeslee et al. (2001) CSP
models with the present data. These CSP models are obtained combining SSP
models in such a way to mimic, at least approximately, the evolution of an
elliptical galaxy. With these models the disagreement between SBF data &
models disappears, as it is completely accounted for by CSP with a fraction of
old and metal-poor (t∼ 14 Gyr, [Fe/H]∼-1.3) stars as high as 8%, combined
with a dominant contribution from an old and metal rich stellar component.
In conclusion, our data seem to show that while the integrated properties
of some galaxies might be well interpreted within the scenario of classical SSP
models, there are few objects whose observational properties can only be inter-
preted by means of more complex stellar populations systems. In this view, SBF
and SBF colors, coupled with classical photometric data appear to be a very in-
teresting tool to understand the properties of the unresolved stellar systems in
distant galaxies.
References
Blakeslee, J.P., Vazdekis, A., Ajhar, E. 2001, MNRAS, 320, 193
Cantiello, M. et al. 2005, ApJ, 634, 239
Raimondo, G., et al. 2005, AJ, 130, 2625
