MEGACAM and the Besançon model

M. Schultheis & A. Robin  (Observatoire de Besançon)

MEGACAM is a wide field imager at CFHT which consists of 36  2048 x 4612 pixel CCDs. The pixel scale is 0.185''  which leads to a total field of view of ~ 1 square degree. We refer for a detailed description of MEGACAM to the corresponding webpages (MEGACAM).

We have implemented in the Besançon model the set of filters used at the CFHT with the MEGACAM instrument in order to be used for star count simulations. We also have considered a new set of libraries in order to compute more accurately the synthetic photometry in these filters. We here describe this implementation.

Megacam filter set

Figure 1 shows the filter transmission curves of  MEGACAM compared with the SDSS filters. Note that the MEGACAM transmissions are higher in u*, g' and z'. Filters  u* and g' are slightly redder than  SDSS comparable filters.

Fig.1 : Megacam and SDSS filter sets.

Stellar atmospheres

In order to produce synthetic photometry of stars in the MEGACAM filters, we have used 2 spectral libraries: Basel 3.1 and NextGen. The Basel 3.1 library is a semi-empirical library, based on previous Basel 2.2 (Lejeune et al. 1997), extended to non-solar metallicities (Westera et al., 2002). As we show below, the Basel library gives good colour estimates at Teff > 4000 K but does not give realistic colours for cool dwarfs. Hence we have used complementary NextGen models from PHOENIX stellar atmosphere programs (Hauschildt et al., in preparation and Hauschildt et al, 2002) for stars of Teff < 4000 K.  As they use a direct opacity sampling including over 500 million lines they give a more realistic description of the M dwarf population. Conversely, the NextGen library seems to give systematic deviations from data colours in  u*-g' and for substellar metallicities. Hence, we combine both libraries to get good colour estimates at all temperatures and metallicities.

Fig 2: Stellar atmospheres: Colour-colour diagram  of stars in the CFHTLS W3 field (see below) compared to the synthetic colour-colour diagram from Basel 3.1 stellar libraries(left panel) and NextGen (right panel) for different metallicities.  The red lines are for [Fe/H]=0.0,  the green lines  [Fe/H]=-1.0 and the blue lines [Fe/H]=-2.0. We use only stars with a photometric error smaller than 0.01 mag in each filter for a more accurate stellar locus. Approximate temperatures are also indicated.

Figure 2 shows g'-r' versus r'-i' diagrams observed with Megacam, superimposed with the synthetic colours of dwarf stars using the Basel3.1 stellar library for solar metallicity, [Fe/H]=-1.0  and [Fe/H]=-2.0 (left panel) and the NextGen library for [Fe/H]=0.0 and [Fe/H]=-1.0  (right panel). We used here only stars which have a photometric error smaller than 0.01mag in each filter. The diagram illustrates the sensitivity of the  colours in the MEGACAM system  to metallicity and also the differences in the stellar libraries. Especially at temperatures below 3500 K which corresponds to K/M dwarfs, the Basel3.1 library does not give realistic colours. The r'-i' colour is sensitive to effective temperature while g'-r' is only sensitive to it at Teff > 4000 K.  Note the striking difference in the atmosphere models in g'-r' at low temperatures. For late type dwarfs the most sensitive metallicity indicator seems to be the g'-r'colour . For objects with g'-r' > 1.0 one can distinguish, assuming a very accurate photometry, between stars with solar metallicity and [Fe/H]=-1.0. The comparison between both libraries and MEGACAM data lead us to adopt  Basel3.1 colours as a base and complementary NextGen colours for Teff < 4000 K.

The Besançon model with the MEGACAM filter system

In the case of the MEGACAM photometric system, we have used the ccd+filter definition of the passbands, and applied these bandpasses to the spectral libraries. The model is otherwise the same as the standard version (Robin et  al, 2003).

Comparison between CFHTLS data and the Besançon model simulations

The CFHT Legacy Survey project is a large observationnal project with the CFH Telescope equiped with the new primary focus Megaprime and the Megacam instrument. It consists of three different surveys. The Deep is a survey of 4 patches  (D1,D2,D3,D4)  of 1 square degree in the filters u*,g',r',i'  and z', complete to r' ~ 28 mag; the Wide survey covers 3 patchs (W1,W2,W3) of ~ 7 deg2, each observed to r' ~ 25 mag, and the Very Wide is dedicated to the ecliptic and covers 1300 deg2 to r'~23. We refer for a detailed description to the corresponding web pages (CFHTLS). All the observations are reduced by the TERAPIX pipeline at the Institut d'Astrophysique de Paris.

As a test for the implementation of the Megacam filter set, we here present a simulation from the Besançon model in the MEGACAM filters to be compared with preliminary data of a subset of
4 deg2 of the W3 field located at l = 98.85 and b = 58.4. The cut-off limit in i' is 20.5 imposed by the star/galaxy separation.


Fig 3: g'-r' vs. r'-i' diagram. The bottom panel shows the data of the W3 field. The top panel shows the synthetic colours predicted by the Besançon model. The red points are thin disc stars, the green points are for the thick disc and magenta for the spheroid.

The blue part of the observed colour-colour diagrams is populated by a population which has a larger spread than the predicted stellar locus. We suspect these objects to be compact faint galaxies misclassified as stars from the morphological criterium used here. Additional proper motion studies and/or spectroscopy would help to make a cleaner separation between stars and galaxies.

  •  The colour-magnitude diagram:  Figure 4 shows the colour-magnitude (CMD) diagrams of a subset of the W3 field. As for the colour-colour diagram, the model predictions reproduce well the observations.The overpopulation of blue objects in the observations is due to compact galaxies and quasars, as already seen in the colour-colour diagram. We indicate the location of white dwarfs (open circles)  which are characterized  by their blue  r'-i' colour.

Fig 4: Colour-magnitude diagram of a subset of data in the W3 field. The left panel shows the W3 data and the right panel the synthetic colour-magnitude diagram derived from the Besançon model. The symbols are the same as in Fig.3. The open circles denote the location of the white dwarf populations.

  •  The colour histogram:  Figure 5 shows the colour-distribution of the W3 field. Note the good agreement between the numbers expected from the Galaxy model (dashed line) and the observation (black solid line) for the three galactic components, although some colour shift at high temperature and/or low metallicity (blue side of the histogram) might be present.
Fig. 5: Histograms of the g'-r' colours. The black solid line is the distribution in colours of the stars in a field of 4 sqaure degrees, a subset of the W3 field of CFHTLS. The dashed black line the colour distribution predicted by the Besançon model. The red line is the thin disc population, the green line the thick disc and in magenta the spheroid population.

e-mail: mathias at,  annie at