4.4 Background Estimate

As with all observations, we will need to determine the IR background at the location of the targets. In general, the IR background contribution is a combination of the zodiacal light, interstellar medium, and cosmic background radiation. At IRAC wavelengths, the background is dominated by zodiacal light, so a general rule-of-thumb is that the background is ``low'' near the ecliptic poles (absolute ecliptic latitude greater than about $60^\circ$), ``high'' if it is in the ecliptic plane (absolute ecliptic latitude less than $30^\circ$), and ``medium'' for all other ecliptic latitudes.

We obtain a more quantitative estimate through the Spot Target entry dialog. Within the target entry/modification window, clicking on ``Background'' (see Figure 4.1), brings up the ``Background Dialog'' window, which we display in Figure 4.3.

Figure 4.3: The background dialog entry dialog.

We enter 8.0 microns as the wavelength to do the estimate, leave the ``Calculate Range When Visible'' button selected, and click ``Do Estimate.'' A screen capture of the results for Abell 2218 is shown in Figure 4.4, and the results for all of the targets are given in Table 4.2. The 8 micron maximum backgrounds vary by a factor of three (5.678 MJy/sr for Abell 2218, to 16.372 for Abell 1689), and are classified by the SSC as ``low'' to ``high'' backgrounds respectively^4.2.

Figure 4.4: The 8.0 micron background at the Abell 2218 central position.

Table 4.2: The 8.0 micron background at the target positions.

Target	Maximum Background [MJy/sr]	Background Level
Abell 2218	5.678	Low
Abell 1689	16.372	High
AC 114	11.620	Medium
Abell 665	8.591	Medium
cl0024+16	13.982	High

It is a good idea to verify the background estimate provided by Spot with a visual check of observations provided by IRAS. To do this, we first retrieve the IRAS Sky Survey Atlas (ISSA) image of the target field (in this case, we do it for Abell 2218). To do this, select ``ISSA image...'' from the ``Images'' pulldown menu. The ISSA image retrieval pop-up window, shown in Figure 4.5, will appear.

Figure 4.5: The ISSA image retrieval entry dialog.

We select all wavelengths (12, 25, 60 and 100 micron), choose a 1 degree FOV, put all plots in a current frame, no zoom, and hit ``OK.'' When Spot has retrieved the images, you will note that there are now large black arrows and a slider bar in the upper right; see Figure 4.6. This is how you can flip between images loaded into the same window. If you load fewer than four images into the same window, then the slider only includes those images as options.

Figure 4.6: Flipping between $12$, $25$, $60$, and $100 \; \mu {\rm m}$ images.

Putting the slider on 12 microns (closest to the IRAC bands), and increasing the zoom (by hitting the magnifying glass icon with the ``+'' sign on the left hand side of the main Spot window), allows us to examine the structure of the IR emission in the target field. Now the ISSA pixels are $90^{\prime\prime}$, so the resolution is only a few pixels per IRAC FOV. Nevertheless, we see no evidence of very bright spots in the emission, and the flux is $\simeq 1 \; {\rm MJy/sr}$ at 12 microns, somewhat below the estimates at 8 microns returned by Spot.

Why is there a difference between Spot's estimates and that seen in the ISSA maps? Ignoring the slight difference in wavelength, there are some differences in how the background is estimated with Spot that may be important for your science. For a detailed discussion, see §9.5.1.