Issue |
A&A
Volume 451, Number 1, May III 2006
|
|
---|---|---|
Page(s) | 375 - 375 | |
Section | Instruments, observational techniques, and data processing | |
DOI | https://doi.org/10.1051/0004-6361:20054175 | |
Published online | 25 April 2006 |
Accurate photometry of extended spherically symmetric sources
1
Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: panders@astro.physik.uni-goettingen.de
2
Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands e-mail: M.Gieles@astro.uu.nl
3
Department of Physics & Astronomy, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield, S3 7R, UK e-mail: R.DeGrijs@sheffield.ac.uk
Received:
8
September
2005
Accepted:
2
February
2006
We present a new method to derive reliable photometry of extended
spherically symmetric sources from HST images (WFPC2, ACS/WFC and NICMOS/NIC2
cameras), extending existing studies of point sources and marginally resolved
sources. We develop a new approach to accurately determine intrinsic
sizes of extended spherically symmetric sources, such as star clusters in galaxies beyond
the Local Group (at distances 20 Mpc), and provide a
detailed cookbook to perform aperture photometry on such sources, by
determining size-dependent aperture corrections (ACs) and taking sky
oversubtraction as a function of source size into account.
In an extensive Appendix, we provide the parameters of polynomial
relations between the FWHM of various input profiles and those
obtained by fitting a Gaussian profile (which we have used for reasons
of computational robustness, although the exact model profile used is
irrelevant), and between the intrinsic and measured FWHM of the
cluster and the derived AC. Both relations are given for a number of
physically relevant cluster light profiles, intrinsic and
observational parameters. AC relations are provided for a wide range
of apertures. Depending on the size of the source and the annuli used
for the photometry, the absolute magnitude of such extended objects can be
underestimated by up to 3 mag, corresponding to an error in mass of a
factor of 15.
We carefully compare our results to those from the more widely used
DeltaMag method, and find an improvement of a factor of 3-40 in both
the size determination and the AC.
Key words: Galaxy: globular clusters: general / Galaxy: open clusters and associations: general / galaxies: star clusters / methods: data analysis
© ESO, 2006
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