7.1 HST WFPC2 Image

The HST WFPC2 image on which the simulations presented in this Chapter are based is a Planetary Camera (PC, see Section 6.1) image of the central regions of the M100 spiral galaxy, obtained with a 900 s exposure with the F555W filter. This image was chosen because it contains most interesting features one would like to observe in bright galaxies: a conspicuous core, large surface brightness variations on short space scales, spiral arms and HII regions^7.1. The central part of this image, namely a square of about 16 arcsec side whose flux map was reconstructed from the simulated observations, is shown in Figure 7.1.

Figure 7.1: HST WFPC2 PC image of M100. Central part of a WFPC2 PC image of the spiral galaxy M100, obtained with a 900 s exposure with the F555W filter, similar to $V$. Encircled on the right image is the position of the eight zones used to calculate the median surface brightness in Table 7.1 (letters $a$-$h$) and of the five HII regions of which aperture photometry is carried out in Section 7.7 (numbers 1-5). The image side is about 16 arcsec and the circles have a diameter of about 0.7 arcsec.

The median surface brightness in $V$ inside the eight circles marked by letters is given in Table 7.1. These values were calculated following the WFPC2 photometric calibration obtained by [Holtzman et al. 1995b], and indicate the surface brightness range spanned by the image. Note that the median surface brightness of the whole image is $\mu_V=19.42$ mag/arcsec$^2$.

Table 7.1: Characteristic values of $V$-band surface brightness of the HST image of M100. Median $V$-band surface brightness within the zones indicated in Figure 7.1, expressed in mag/arcsec$^2$.

Zone	$a$	$b$	$c$	$d$	$e$	$f$	$g$	$h$
Median $\mu_V$	19.10	20.65	20.71	18.96	17.83	18.35	17.59	16.80

M100 is classified as Sc(s)I in [Sandage and Bedke 1994] and the [de Vaucouleurs et al. 1991] report a photoelectric total magnitude $B_T=10.05$ and an effective radius of about 104 arcsec, meaning that the sky region shown in Figure 7.1 covers its very central parts only.