S BF A nalysis P ipeline for the A stronomical C ommunity

• SAPAC homepage
• SAPAC manual
• Exposure Time calculator

Status Update

• 12/05/07 SAPAC version 1.1.0 has been released.Downloard HERE (You will need a valid username and password. Please contact me for more information.
• 19/06/06 SAPAC version 1.0.5 has been released.
• 06/06/06 Paper desribing SAPAC and initial results accepted for publication in AJ( 2006AJ....132.1384D)
• 25/10/05 SAPAC manual has been updated. Information on all 5 steps is now available
• 20/10/05 SAPAC is compiled and ready to be tested by some very kind 'guinea pigs'.
• 19/09/05 SAPAC is ready to be tested. We have a few problems in compiling it into an executable but hopefully these issues should be addressed shortly.

INTRODUCTION:

Large volumes of CCD imaging data that will become available from wide-field cameras at telescopes such as the CFHT, SUBARU, VST, or VISTA in the near future are highly suitable for systematic distance surveys of early-type galaxiesin the local Universe using the Surface Brightness Fluctuation (SBF) method. For the efficient processing of such large data sets, we are developing the first semi-automatic SBF analysis pipeline named SAPAC.

ANALYSIS PREREQUISITES:

The minimal requirements for the SBF analysis of an early-type galaxy are:

• Galaxy morphology: the light distribution of the stellar system must be radially symmetric and have minimal structure. An overall elliptical shape of the galaxy is crucial as this is modelled and subtracted as part of the SBF analysis.
• Photometry: calibrated CCD images are required in two photometric bands, e.g. (B, R) or (g₄₇₅, z₈₅₀), as the fluctuation magnitude shows a colour dependency.
• Image quality: , where r_eff is the half-light radius of the galaxy.
• Integration time: , where μ_gal is the mean surface brightness of the galaxy, μ_sky the surface brightness of the sky background, DM the estimates distance modulus of the galaxy, M the fluctuation luminosity of the underlying stellar population, and m₁ the magnitude of a star providing 1 count/sec on the CCD detector at the telescope.

To give a general idea of these constraints: to obtain a galaxy power spectrum suitable for a distance measurement, the minimum exposure time required for a Virgo cluster dE with a mean surface brightness of 25 mag arcsec² is 1600s at the VLT+FORS1/2. It is interesting to note that this exposure time is by a factor of 20 shorter than the 32,000s of HST time spent by Harris et al. (1998) to measure the TRGB distance of a dwarf elliptical at a similar distance.

To determine the exposure time required for your galaxy, or to determine if your image has the enough time, please use the Exposure Time Calculator

SAPAC Outline

Previous SBF work has entailed individuals hand selecting regions in galaxy images for the analysis. To make the results as impartial as possible and data reduction more efficient we developed a rapid, semi-automatic SBF analysis package that can process large numbers of galaxies.

SAPAC is a software package that carries out a semi-automatic SBF analysis of any early-type galaxy for which CCD data meets the requirements as discussed above. For a detailed description of the fluctuation magnitude calibration and the individual reduction steps such as the modelling of the galaxy, foreground star removal, selection of SBF fields etc. we refer the reader to Jerjen (2003).

SAPAC consists of Perl scripts using IRAF modules and a sophisticated graphical user interface, also written in Perl. The average processing time for 10 SBF fields in a single galaxy and measuring its distance is approximately 20 minutes.

Initially we concentrated on processing B & R images in the pipeline but we have recently implemented two new sets of filters. These filter sets are both for HST data, the first being F850LP and F475W (equivalent to SDSS z' and g') and the second is F814W and F435W (equivalent to I and V). More information on the calibrations can be found in the SAPAC manual . Other filters sets may be implemented in the future, depending on calibrations available.

SAPAC is a freely available. If you have suitable data and are interested in using SAPAC please contact me so I can give you acces to the pipeline for a trial.

For more information please see the proceedings for IAUC 198, at which I presented a poster. The report of this poster can be seen here.

More detailed information can be found in the paper: First Results From SAPAC: Toward A Three-Dimensional Picture Of The Fornax Cluster Core which was accepted for publication in June and can be found here.

This is the interface for

SAPAC.

It simply requires the names of the two fits files and a selection for the photometric band and size of the image and the boxes for the appropriate steps to be checked.