Received 04th March 1998
Abstract.
The Magellanic Cloud Photometric Survey (PI: Zaritsky) provides a unique and
homogeneous database of UBVI CCD photometry of the Large and the Small
Magellanic Clouds.
It will allow us to explore the star formation history of both Clouds in
unprecedented spatial coverage and detail and provide accurate,
isochrone-based ages for thousands of star clusters in the Clouds.
1. Introduction
The Magellanic Cloud Photometric Survey, carried out with a newly developed
drift-scan camera
(Zaritsky et al. 1996) at the 1 m Swope telescope at Las Campanas
Observatory, will result in a
homogeneous set
of UBVI CCD photometry of the Large and the Small Magellanic Cloud.
This unique resource will allow us to study the detailed star formation
history, star formation processes, and overall evolution of two entire
galaxies in unprecedented spatial coverage and detail.
The survey will cover the central 8° × 8° of the LMC
and the central 3.5° × 4.5° of the SMC with a spatial
scale of 0.7" and a magnitude limit of V ≅ 21 mag
(MV ≅ 2 to 2.5 mag).
The database of transformed UBVI photometry and astrometric positions
will be made public once the data are fully reduced and calibrated.
The data acquisition is almost complete for the SMC and about half finished
for the LMC.
The anticipated time scale for finishing the whole project is five years.
It may be extended to cover halo and inter-Cloud regions.
The survey is complemented by parallel emission-line (Smith et al. 1996),
CO (Fukui et al. in prep.), H I (Kim et al. 1997;
Staveley-Smith et al. 1997),
and X-ray surveys (e.g., Snowden & Petre 1994).
2. Projects
The Magellanic Cloud Photometric Survey will be used in a number of projects,
including
- Field star populations of the Magellanic Clouds:
- The detailed star formation history of young and intermediate-age
populations;
- Census of field stars as a function of magnitude, color, spatial
distribution;
- Census of OB associations, their distribution, sizes, age structure,
ages, etc.;
- Large-scale structure, shells, supershells, stellar content, and
age structure;
- Star clusters of the Magellanic Clouds:
- Census of star clusters as a function of brightness, color, age,
spatial distribution; detection of new star clusters (e.g.,
Zaritsky et al. 1997);
- Color-magnitude-diagram-based ages for all clusters in the
Magellanic Clouds; new age calibration of integrated colors;
- Global star formation history using star clusters as tracers;
- Structural parameters for all clusters as a function of size,
number of member stars, and age; cluster dynamical evolution and
disruption time scales;
- Extinction of the Magellanic Clouds:
- Detailed extinction maps (e.g., Harris et al. 1997); mean extinction
vs. dust extinction;
- Background galaxies behind the Magellanic Clouds:
- Number counts and galaxy density;
- probes for the intervening ISM;
- Astrometric reference points.
3. First results for star clusters
More than 2000 clusters in the LMC and almost 1000 in the SMC have been
identified through photographic surveys.
We estimate that we may at least double the number of known clusters with
the Magellanic Cloud Photometric Survey.
We use an automated cluster-finding algorithm that we run on magnitude- or
color-selected subsets of the full data set.
Since clusters in the Magellanic Clouds cover all ages and range from loose
to compact, from large to small, a range of detection criteria is needed.
We require that objects be identified independently by at least two different
criteria to qualify as candidate clusters.
In the 2° × 1.5° area analyzed so far we found
174 clusters that were detected in at least two or more data subsets,
54 of which are listed in the catalog by Kontizas et al. (1990).
The new detections are mostly small, poor, or loose clusters.
Some of the clusters may have formed that way; others may be in varying states
of dissolution.
Our survey helps to complete the cluster census toward the faint end of the
cluster luminosity function.
We find our CMD-based ages deviate in part significantly from ages found
from integrated photometry and the SWB-type age calibration of integrated
colors by Girardi et al. (1995).
After deriving isochrone-based ages for all detected clusters we can
then age-calibrate their integrated colors and extend this calibration to
include I as well.
The results will prove valuable for interpreting the integrated colors of
unresolved clusters in distant galaxies and as input for population synthesis
models.
The positions of star clusters as a function of age will allow us to trace
the star formation history of the Magellanic Clouds.
The age distribution function will be used to explore the presence of gaps
and peaks in cluster formation in both Clouds.
Ages in correlation with structural parameters such as tidal radii and
richness will allow us to study cluster evolution and disruption as well
as the evolution of the cluster luminosity function as a function of time.
Acknowledgments.
Financial support for the survey is provided by NSF AST-9619576,
NASA NAG-5-3501, the David & Lucille Packard Foundation and the
Alfred P. Sloan Foundation.
References
- Girardi L., Chiosi C., Bertelli G., Bressan A., 1995, A&A 198, 87
- Harris J., Zaritsky D., Thompson I., 1997, AJ 114, 1933
- Kim S., Staveley-Smith L., Sault R.J., et al.,
1997, PASA 14, 119
- Kontizas M., Morgan D.H., Hatzidimitriou D., Kontizas E., 1990,
A&AS 84, 527
- Smith R.C., Bregman J., Chu Y.-H., et al.,
1996, BAAS 188, 5101
- Snowden S.L., Petre R. 1994, ApJ 436, L123
- Staveley-Smith L., Sault R.J., Hatzidimitriou D., Kesteven M.J.,
McConnell D., 1997, MNRAS 289, 225
- Zaritsky D., Harris J., Thompson I., 1997, AJ 114, 1002
- Zaritsky D., Schectman S.A., Bredthauer G., 1996, PASP 108, 104
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| First version: | 18th | March, | 1998
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| Last update: | 08th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)