3 Observations and photometric results

All galaxies were observed with two broadband filters, LW3 (12-18$\mu$m) and LW2 (5-8.5$\mu$m), that we shall hereafter designate by their central wavelength, respectively 15 and 7$\mu$m. This was expected to provide F₁₅/F₇ colors directly linked with star formation intensity, since the LW2 filter covers the emission from a family of bands (see Sect. 4), which are ubiquitous in the interstellar medium, and LW3 was supposed to cover mainly a thermal continuum observed to rise faster than the emission bands in star-forming regions, for instance from the IRAS F₂₅/F₁₂ ratio (Helou 1986); however, we will see that the picture is more complicated. Maps covering the whole infrared-emitting disk were constructed in raster mode. In all cases, the field of view is large enough to obtain a reliable determination of the background level, except for NGC4736 and 6744. The pixel size is either $3\hbox {$^{\prime \prime }$ }$ or $6\hbox {$^{\prime \prime }$ }$, depending on the galaxy size. The half-power/half-maximum diameters of the point spread function are respectively $6.8\hbox{$^{\prime\prime}$ }$/$\simeq$ $3.1\hbox{$^{\prime\prime}$ }$ at 7$\mu$m with a $3\hbox {$^{\prime \prime }$ }$ pixel size, $9.5\hbox{$^{\prime\prime}$ }$/ $5.7\hbox{$^{\prime\prime}$ }$ at 7$\mu$m with a $6\hbox {$^{\prime \prime }$ }$ pixel size, $9.6\hbox{$^{\prime\prime}$ }$/ $3.5\hbox{$^{\prime\prime}$ }$ at 15$\mu$m with a $3\hbox {$^{\prime \prime }$ }$ pixel size and $14.2\hbox{$^{\prime\prime}$ }$/ $6.1\hbox{$^{\prime\prime}$ }$ at 15$\mu$m with a $6\hbox {$^{\prime \prime }$ }$ pixel size. The data reduction is described in the Atlas.

Since the emission from various dust species and atomic lines is mixed in the broadband filters (see Sect. 4), it is essential to complement our maps with spectro-imaging data. These allow an estimate of the relative importance of all species as a function of the location inside a galaxy. We have thus obtained spectra between 5 and 16$\mu$m of the inner disks ( $3\hbox{$^\prime$ }\times 3\hbox{$^\prime$ }$ or $1.5\hbox{$^\prime$ }\times 1.5\hbox{$^\prime$ }$) of five bright galaxies: NGC613, 1097, 1365, 5194 and 5236 (Fig. 1). Spectra averaged over a few central pixels covering approximately the extent of the circumnuclear region (left column) are compared with spectra averaged over the inner disk, excluding the central part and a possible ghost image (middle column). The right column shows the observed spectrum of the faintest pixels, consisting of the zodiacal spectrum contaminated by emission features from the target galaxy, because the field of view never extends beyond the galactic disk. For this reason, we cannot measure exactly the level of the zodiacal foreground to remove. Instead, as explained in the Atlas, we first fit a reference zodiacal spectrum to the average spectrum of the faintest pixels (excluding the spectral regions where emission features appear). The upper limit to the zodiacal foreground is set by offsetting the fitted spectrum within the dispersion range, with the additional constraint that the corrected disk spectrum remains positive; the lower limit is symmetric to the upper limit with respect to the fit. This makes little difference for the nuclear spectra but it does for the disk spectra, although it does not affect the spectral shape. Note that due to the configuration of the instrument, two different filters are used for the short and long wavelength parts of the spectra, and that a small offset can result at the junction of these filters, around 9.2$\mu$m.

The mid-infrared maps generally show an intense circumnuclear source. Decomposing surface brightness profiles into a central condensation and a disk (see details in the Atlas), we define a radius for this circumnuclear region, $R_{\rm CNR}$. Total fluxes and fluxes inside $R_{\rm CNR}$are listed in Table 2 with the background level for each broadband filter. Explanations about the method employed for photometry and the estimation and meaning of errors can be found in the Atlas. The dominant uncertainty arises from memory effects for relatively bright galaxies, and from other sources of error (essentially the readout and photon noise) for faint galaxies, especially at 15$\mu$m. For galaxies drawn from the Sf-glx project, the number of exposures per sky position is very small ($\simeq$10) and does not allow a proper estimate of memory effects: their photometric errors are thus especially ill-determined. Typical errors are $\approx$10% at 7$\mu$m and 18% at 15$\mu$m. Note that flux density calibration uncertainties, which are of the order of 5 to 10%, are not included. However, this is a systematic effect, hence not affecting relative fluxes.

  

Table 2: Photometric results at 15 and 7$\mu$m, obtained as described in the Atlas (total fluxes, diameter aperture used for central regions, fluxes inside this aperture and background levels). We warn the reader that the uncertainties can only be taken as order-of-magnitude values (see the Atlas), especially for galaxies of the third subsample belonging to the Sf_glx project, with a very low number of exposures per sky position. Galaxies with no reported central fluxes have no identifiable central concentration: the radial surface brightness profile is consistent with a disk alone at our angular resolution (NGC4580 rather shows a smooth central plateau and NGC4634 is seen edge-on). For NGC7552, we used only the maps with a $3\hbox {$^{\prime \prime }$ }$ pixel size, because those at $6\hbox {$^{\prime \prime }$ }$ are strongly saturated in both filters; for the other galaxies mapped with both pixel sizes, we used the $6\hbox {$^{\prime \prime }$ }$ sampling because of the higher signal to noise ratio and the more reasonable field of view.

$$\begin{tabular}{\vert lr@{$\,$}lr@{$\,$}lrrr@{$\,$}lr@{$\,$}l... ... ~ & 796. $\pm$ & 3. & 132. $\pm$ & 2. \\ \hline \end{tabular}$$

 

Table 2: continued.

$${ \begin{tabular}{\vert lr@{$\,$}lr@{$\,$}lrrr@{$\,$}lr@{$\,$... ... & 519. $\pm$ & 5. & 112. $\pm$ & 3. \\ \hline \end{tabular}}$$

^a The conversion from flux densities to fluxes is: $F {\rm (W\,m}^{-2}) = 10^{-14} F_{\lambda} {\rm (Jy)} \times \Delta_{\nu}(\lambda) {\rm (THz)}$, with the filter widths $\Delta_{\nu}(15) = 6.75\, {\rm THz}$ and $\Delta_{\nu}(7) = 16.18\, {\rm THz}$.
#: Nucleus saturated (for NGC5236: both at 15 and 7$\mu$m, but more severely at 15$\mu$m since the same gain and integration time were used for both filters and since F₁₅/F₇ is above 1 in electronic units; for NGC7552: slightly at 15$\mu$m, but not at 7$\mu$m; for the central pixel of NGC4388: at 15$\mu$m, but not at 7$\mu$m; for NGC3620: at 7$\mu$m but not at 15$\mu$m, which is possible because the integration time was respectively 5s and 2s; for NGC4102 and 6946: both at 7 and 15$\mu$m, but more severely at 7$\mu$m, with the same configuration as NGC3620). Thus, F₁₅/F₇ colors in central regions are respectively lower limits for NGC5236, 7552 and 4388 and upper limits for NGC3620, 4102 and 6946.
(-): The field of view is too small to allow a precise determination of the backgroud level and total fluxes are lower limits. The error bars are only formal. The comparison of our measurements with those of Rice et al. (1988) at 12$\mu$m, inside the IRAS band 8-15$\mu$m which overlaps with our 5-8.5$\mu$m and 12-18$\mu$m bands, indicates that we miss of the order of 15% of total fluxes for NGC4736 and between 15 and 45% for NGC6744, provided IRAS fluxes are not overestimated as this is often the case for co-added observations.
(+): From their spectral energy distributions shown by Boselli et al. (1998), these galaxies probably have a non-negligible contribution from the Rayleigh-Jeans tail of cold stars to their 7$\mu$m emission. We did not attempt to remove this contribution, because it would require a careful modelling of stellar populations.
: The disks of these galaxies slightly overlap in projection. We attempted to separate them by the means of a mask defined visually, but the disk fluxes are much more uncertain than estimated.