Simulating CCD images of elliptical galaxies

We introduce a procedure developed by the “Teramo Stellar Populations Tools” group (Teramo-SPoT), specifically optimized to obtain realistic simulations of CCD images of elliptical galaxies. Particular attention is devoted to include the Surface Brightness Fluctuation (SBF) signal observed in ellipticals and to simulate the Globular Cluster (GC) system in the galaxy, and the distribution of background galaxies present in real CCD frames. In addition to the physical properties of the simulated objects galaxy distance and brightness profile, luminosity function of GC and background galaxies, etc. the tool presented allows the user to set some of the main instrumental properties FoV, zero point magnitude, exposure time, etc. The light coming from distant galaxies includes a specific SBF signal, essentially correlated with the properties of the host stellar system (Tonry & Schneider 1988). The existence of these luminosity fluctuations is due to the statistical correlation between adjacent galaxy regions (pixels). Since its introduction, the SBF method has been used as a reliable distance indicator for elliptical galaxies (e.g. Tonry et al. 2001) and, more recently, as a tracer of stellar population properties (e.g. Cantiello et al. 2003, 2005; Raimondo et al. 2005, R05). In order to derive SBF magnitudes from CCD images of elliptical galaxies, very high quality CCD data are required. We have developed a tool to simulate CCD images of elliptical galaxies including the SBF signal and other properties of the galaxy surface brightness profile, distance, color profiles, contamination of background galaxies, etc. Due to its statistical nature, a reliable simulation of the SBF signal needs: i) to accurately reproduce the details of the statistics governing the stellar SBF signal, and ii) to take into account the presence of any other source of fluctuations. To include SBF signal in the simulations we use the Teramo-SPoT Single-burst Stellar Populations (SSP) models (R05, visit also the SPoT website www.oateramo.inaf.it/SPoT). These models are provided by computing a number Nsim of independent SSP simulations for a large range of ages and chemical compositions. The latter property of SPoT SSP models is at the base of our simulations of realistic galaxies: we start with a galaxy having an analytic Sersic r1/n profile, then, for each pixel [i,j] at the radius r∗ we substitute the analytic magnitude profile μth(r∗)[i,j] with the integrated magnitudes μsim as evaluated in one of the Nsim independent SSP simulations, having assumed 〈μsim〉 = μth(r∗). In this 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