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Observation Planning with FORCAST: Haro 3 Christian D. Howard SOFIA-USRA, NASA Ames Research Center, MS N211-3, Moffett Field, CA 94035, USA Haro 3 represents a good example of how to plan observations of an extended source using the Sofia Integration Time Estimator (SITE), and to investigate what is feasible for FORCAST. Included is a description of the SITE signal to noise calculations, and its ambiguity when dealing with extended sources. Target Selection and Flux Estimates We turn to observations of an extended source, chosen randomly from the Design Reference Manual (DRM). Haro 3 is a Blue Compact Dwarf (BCD) galaxy has been observed by Spitzer (Figure 1), along with several other infrared observatories (Figure 2), and serves as a good example of how observations of an extended object would be accomplished with FORCAST, as well as how such observations would compliment the currently available data. Fig. 1.— The 4.5 µm ( IRAC2) image (left) and the 8.0 µm (IRAC4) image (right) of Haro 3. Figure and caption courtesy Hunt et al. (2006). Haro 3 is an extended source spanning roughly 40′′ . SED models of this source would benefit from the longer wavelength filter observations from FORCAST. We examine previous observations of Haro 3 in order to estimate a flux for use in the SOFIA Integration Time Estimator (SITE1 ). Because we cannot hope to achieve better results in terms of sensitivity or resolution at the shorter wavelengths of Spitzer, we focus on obtaining observations in the longer wavelength filters of 19.7, 24.2, 31.5, and 37.1 µm. Observations at these wavelengths will also provide data to help further characterize the SED of Haro 3. Using the data from Figure 2, one can determine the expected flux at the FORCAST wavelengths based on the flux as measured from the shorter wavelengths, which one would assume is a power law relationship. As Figure 3 shows, between the 10-100 µm regime, Fλ ∼ λα , where α ∼1.6. 1 https://dcsweb.sofia.usra.edu/proposalDevelopment/SITE/index.jsp Fig. 2.— Table of previous observations of Haro 3 courtesy Hunt et al. (2006), and references therein. Fig. 3.— Plot of previous flux measurements of Haro 3, taken from Hunt et al. (2006), and references therein. The dotted line is a straight line fit to the points between 10 and 100 µm. As expected, the flux follows a power law with Fλ ∼ λ1.6 , which allows an observer to estimate the expected flux at the longer FORCAST wavelength filters. Flux Density Estimates Because Haro 3 is an extended source, one must estimate the curve of growth, i.e. the integrated flux as a function of radius, for the wavelengths we wish to observe. Figure 4 allows us to calculate a flux density (mJy/arcsec2 ) for Spitzer 8 µm observations, and thus estimate the 19.7, 24.2, 31.5, and 37.1 µm flux densities. We begin by taking 5′′ annuli from the Spitzer observations in order to estimate how much of the 40′′ extent of Haro 3 FORCAST would be able to observe. Using the 8 µm data points from Figure 4 yields flux densities of ∼0.8 and ∼0.13 mJy/arcsec2 for radii of 0-5′′ and 5-10′′, respectively. This yields corresponding flux densities at the FORCAST filters of 3.4 and 0.55 mJy/arcsec2 (19.7 µm), 4.7 and 0.77 mJy/arcsec2 (24.2 µm), 7.2 and 1.2 mJy/arcsec2 (31.5 µm), and 9.3 and 1.5 mJy/arcsec2 (37.1 µm). Tracing the flux density of Haro 3 is thus calculated out to its limiting flux value of ∼140 mJy at 8 µm. Fig. 4.— Growth curves for IRAC (left) and MIPS (right) photometry. In both panels, the horizontal dotted line traces the adopted total ux from Haro 3, and the dashed line represents the PRF. In the left panel, IRAC channel 2 is indicated by lled circles, channel 4 by lled squares. In the right panel, MIPS-24 is shown by lled circles, MIPS-70 by open squares, and MIPS-160 by crosses. Haro 3 is extended at shorter wavelengths and possibly at 160 µm but appears virtually pointlike at 24 and 70 µm. Figure and caption courtesy of Hunt et al. (2006). Signal-to-Noise: Point versus Extended Source When using the SITE to calculate exposure times for extended sources, one must note that the signal-to-noise (S/N) ratio is calculated on a per pixel basis, contrary to the point source method of calculating S/N per resolution element2 . Thus, one must take into consid2 As SITE currently stands, the S/N calculation method of per resolution element for point sources and per pixel for extended sources is not documented. A rough way to verify this is to calculate an exposure time for a 25 mJy (at 19.7 µm) point source and compare that time to an extended source of 1 mJy/arcsec2. At 19.7 µm, a point source will have an apparent size of 3.8′′ , or ∼5.1x5.1 pixels. Both point and extended sources will have the same flux per resolution element, but the estimated integration times will differ significantly for a given S/N ratio. However, if one species the required S/N ratio for the extended source to be 5/5.1∼1, the integration times become equivalent. eration the source type (extended versus point source) when specifying a desired S/N ratio. For example, for our point source example of RNO 1B, we required a S/N of 5. At 19.7 µm, the apparent source size (FWHM) is 3.8′′ (5.1 x 5.1 pixels), so the S/N is calculated p by summing over every pixel constituting the source size. Since S/N scales as Npix , for an extended source one can loosen the S/N criteria by dividing the desired overall S/N per resolution element by the width of the resolution element in pixels. The FWHM is 3.8′′ , 4.0′′ , 4.3′′ , and 4.7′′ for the 19.7, 24.2, 31.5, and 38 µm filters, respectively (with 0.75′′ /pixel). Another issue to consider is the fact that because due to the small size of the extended emission of Haro 3 (∼40′′ ), chopping will be on-chip and thus will reduce the integration time reported by the SITE by a factor3 of 2, or equivalently, reduces the S/N by a factor √ of 2. Thus, for a desired total S/N of 5 per beam, each of our four chosen wavelengths will require a S/N (per pixel) of 0.71 (19.7 µm), 0.66 (24.2 µm), 0.62 (31.5 µm), and 0.57 (37.1 µm). Because the flux density estimated above is quite faint beyond 0-5′′ , we choose to observe without using the beamsplitter, thus minimizing our proposed integration times. Exposure Time Estimates We start by estimating the integration times for the 5-10′′ annulus, since our flux estimates at those radii are quite low, and we wish to get an idea of how faint we might hope to observe. Using the above values for the S/N per pixel for on-chip chopping, the SITE reports estimated integration times (including overhead and specifying single wavelength observations) of 3.4 hours, 4 hours, 6 hours, and 9 hours for the 19.7, 24.2, 31.5, and 37.1 µm filters. As in the point source example of RNO 1B, we assume observing conditions with 27 µm water vapor overburden and 20◦ elevation angle. As can be seen from these exposure time estimates, Haro 3 will be detectable in the 5-10” annulus at 31.5 and 37.1 µm only with significant integrations, although will be easily detectable in the 0-5 ′′ annulus. Because the flux density drops as one moves to larger radii, it becomes clear that the extended emission of Haro 3 will be undetectable by FORCAST beyond 10′′ , and will be resolved at the longer wavelengths only with significant integration times. However, the methods described are a valid example of how to estimate the viability of observing an extended source with FORCAST. Haro 3, however, is still a viable target for point source observations, as it exhibits evidence of ongoing star formation (Figure 5). Planning observations of Haro 3 with FORCAST would follow the same strategy covered in the point source example observation of RNO 1B. 3 https://dcsweb.sofia.usra.edu/proposalDevelopment/SITE/itcHelp/ITChelpFORCASTProperties.html#optical Fig. 5.— left: VLA 3.6 cm contours of Haro 3 superposed on an HST V-band gray-scale image. The contour levels are 3, 3, 4, 5, 7, 10, and 15 σ (0.023 mJy beam−1 ). Right: An enlargement of region A. Figure and caption courtesy of Johnson et al. (2004). Although observing the extended emission of Haro 3 is not feasible with FORCAST, it exhibits knots of apparent star formation that would benefit from SOFIA observations. REFERENCES Hunt, L. K., Thuan, T. X., Sauvage, M., & Izotov, Y. I. 2006, ApJ, 653, 222 Johnson, K. E., Indebetouw, R., Watson, C., & Kobulnicky, H. A. 2004, AJ, 128, 610 This preprint was prepared with the AAS LATEX macros v5.2.