BibSonomy
CiteUlike
Del.icio.us
Digg
Facebook
Mendeley
TwitterA&A 413, 145-157 (2004)
DOI: 10.1051/0004-6361:20031454
Physical limits to the validity of synthesis models
The Lowest Luminosity Limit
M. Cerviño1, 2 and V. Luridiana11 Instituto de Astrofísica de Andalucía (CSIC), Camino bajo de Huétor 24, Apdo. 3004, Granada 18080, Spain
2 Laboratorio de Astrofísica Espacial y Física Fundamental (INTA), Apdo. 50727, Madrid 28080, Spain
(Received 12 February 2003 / Accepted 29 August 2003 )
Abstract
In this paper we establish a necessary condition for the
application of stellar population synthesis models to observed star
clusters. Such a condition is expressed by the requirement that the total
luminosity of the cluster modeled be larger than the contribution of the
most luminous star included in the assumed isochrones, which is referred to
as the Lowest Luminosity Limit (LLL). This limit is independent of the
assumptions on the IMF and almost independent of the star formation
history. We have obtained the Lowest Luminosity Limit for a wide range of
ages (5 Myr to 20 Gyr) and metallicities (
Z=0 to
Z=0.019) from the
Girardi et al. (2002) isochrones. Using the results of evolutionary synthesis
models, we have also obtained the minimal cluster mass associated with the
LLL,
, which is the mass value below which the observed
colors are severely biased with respect to the predictions of synthesis
models. We explore the relationship between
and the
statistical properties of clusters, showing that the magnitudes of clusters
with mass equal to
have a relative dispersion of 32% at
least (i.e., 0.35 mag) in all the photometric bands considered;
analogously, the magnitudes of clusters with mass larger than
have a relative dispersion of 10% at least. The
dispersion is comparatively larger in the near infrared bands: in
particular,
takes values between 10
4 and 10
5
for the
K band, implying that severe sampling effects may
affect the infrared emission of many observed stellar clusters. As an
example of an application to observations, we show that in surveys that
reach the Lowest Luminosity Limit the color distributions will be skewed
toward the color with the smallest number of effective sources, which is
usually the red, and that the skewness is a signature of the cluster mass
distribution in the survey. We also apply our results to a sample of
Globular Clusters, showing that they seem to be affected by sampling
effects, a circumstance that could explain, at least partially, the bias of
the observed colors with respect to the predictions of synthesis models.
Finally, we extensively discuss the advantages and the drawbacks of our
method: it is, on the one hand, a very simple criterion for the detection
of severe sampling problems that bypasses the need for sophisticated
statistical tools; on the other hand, it is not very sensitive, and it does
not identify all the objects in which sampling effects are important and a
statistical analysis is required. As such, it defines a condition necessary
but not sufficient for the application of synthesis models to observed
clusters.
Key words: galaxies: individual: NGC 5128 -- galaxies: star clusters -- galaxies: stellar content -- galaxy: globular cluster: general -- galaxy: open clusters and associations: general
Offprint request: M. Cerviño, mcs@laeff.esa.es
SIMBAD Objects
© ESO 2004