Simulating CCD images of elliptical galaxies
**FULL TITLE**
ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION**
**NAMES OF EDITORS**
Simulating CCD images of elliptical galaxies
M. Cantiello1,2, G. Raimondo2, E. Brocato2, J.P. Blakeslee1, M.
Capaccioli3
Abstract. We introduce a procedure developed by the “Teramo Stellar Pop-
ulations 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 Fluctua-
tion (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, essen-
tially correlated with the properties of the host stellar system (Tonry & Schnei-
der 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 sig-
nal, 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.oa-
teramo.inaf.it/SPoT). These models are provided by computing a number Nsim
of independent SSP simulations for a large range of ages and chemical composi-
tions. 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
http://arxiv.org/abs/0704.1004v1
2 Cantiello et al.
way the poissonian correlation between adjacent pixels is introduced, preserving
the galaxy brightness profile.
To be realistic, the simulation must also include the presence of GC and
background galaxies - which, in addition, can strongly affect the fluctuations
signal derived from the CCD. These sources are indeed included into our simu-
lations according to their typical luminosity functions, i.e., the total luminosity
function is assumed to be the sum of a power law for galaxies, and a gaus-
sian distribution for the GC component. The characteristic parameters of these
functions can be arbitrarily set by the user. Once the luminosity functions are
randomly populated all the background galaxies are randomly distributed in
the frame, while the GC spatial distribution is additionally convolved with an
inverse power law centered on the galaxy.
Finally, a uniform sky value is included, and the detector noise is added
according to the readout-noise and gain values of the selected instrument.
After the galaxy profile - including SBF -, the GC system, the background
galaxies, and the detector noise properties have been properly chosen, the simu-
lation can be carried out. The panels of Figure 1 show the frames associated to
some of the steps described above, the final frame simulated, and the luminosity
functions of GC and background galaxies.
Figure 1. The first three panels of the figure (left to right) show the profile
of the galaxy simulated, the frame of GC and background galaxies, and the
sum of the previous two frames plus sky and noise, respectively. ACS camera
properties are adopted for the instrumental characteristics. The last plot
shows the luminosity functions adopted of the GC (short dashed line) and
the background galaxies (long dashed line), and their sum (solid line).
The capabilities of the procedure here briefly described makes it useful to
simulate astronomical data for a wide range of applications. As a specific case
we mention the use of this tool to simulate realistic runs at defined telescopes
with the aim of measuring SBF. For example, we have applied this technique to
simulate ISAAC@VLT Ks-band, and ACS@HST F814W-band (reported Figure
1) images of given ellipticals in order to evaluate the proper exposure times
required to reach a defined S/N ratio for objects at different distances.
References
Cantiello, M., Raimondo, G., Brocato, E., Capaccioli, M. 2003, AJ, 611, 670
Cantiello, M., et al. 2005, ApJ, 634, 239
Raimondo, G., et al. 2005, AJ, 130, 2625
Simulating images of elliptical galaxies 3
Tonry, J. & Schneider, D.P. 1988, AJ, 96, 807
Tonry, J. et al. 2001, AJ, 546, 681
