Uwe Herbstmeier

The Small Magellanic Cloud in the far infrared - I. ISO's 170 μm map and revisit of the IRAS 12–100 μm data

The ISOPHOT experiment onboard the ISO satellite generated a complete view of the Small Magellanic Cloud (SMC) at 170 µm with 1.5 arcmin resolution. The map is analysed using an automated photometry program enabling accurate photometric characterization of the far infrared (FIR) emitting regions. An integrated FIR luminosity of 8 .5×10 7 Lis obtained, leading to a star formation rate of SFRFIR = 0.015 M� /yr. With an average dust temperature of , the total dust mass follows to MD = 3.7 × 10 5 M� . In this paper, the sources detected at 170 µm are compared with those obtainable from the IRAS satellite data. For this purpose, the 12 µm, 25 µm, 60 µm, and 100 µm IRAS high resolution (HiRes) maps of the SMC are re-examined using the same method. In contrast to former studies, this provides an all-band ISO/IRAS source catalog which is no longer based on eyeball classification, but relies on an algorithm which is capable of automated, repeatable photometry, even for irregular sources. In the mid infrared IRAS bands numerous bright FIR emitting regions in the SMC are detected and classified: 73 sources are found at 12 µm, 135 at 25 µm (most of them with Fν 30 K) for the first time. A comparison with earlier IRAS results suggests that many source flux densities in those studies have been under- or overestimated because of non-standardized fitting methods. Many sources with flux densities up to 40 Jy listed in former catalogs cannot be identified in our data.

Small-scale structure of the galactic cirrus emission

We examined the Fourier power spectrum characteristics of cirrus structures in 13 sky fields with faint to bright cirrus emission observed with ISOPHOT in the 90-200 µm wavelength range in order to study variations of the spectral index α .W e found that α varies from field to field with -5.3 ≤ α ≤-2.1. It depends on the absolute surface brightness and on the hydrogen column density. We also found different spectral indices for the same sky region at different wavelengths. Longer wavelength measurements show steeper power spectra. This can be explained by the presence of dust at various temperatures, in particular of a cold extended component. For the faintest areas of the far-infrared sky we derive a wavelength-independent spectral index of α = -2.3 ± 0.6 for the cirrus power spectrum. The application of the correct spectral index is a precondition for the proper disentanglement of the cirrus foreground component of the Cosmic Far-Infrared Background and its fluctuations.

ISOPHOT far-infrared serendipity sky survey

The ISOPHOT Serendipity Survey utilizes the slew time between ISOs pointed observations with strip scanning measurements of the sky in the far-IR at 170 micrometers . The slews contain information about two fundamentally different types of objects, namely unresolved galactic and extragalactic far-IR sources as well as extended regions of galactic cirrus emission. Since the structure of the obtained data is almost unique, the development of dedicated software to extract astrophysically interesting parameters for the crossed sources is mandatory. Data analysis is currently in its early stages and concentrates on the detection of point sources. First results from an investigation of a high galactic latitude field near the North Galactic Pole indicate that the detection completeness with respect to previously known IRAS sources will be almost 100 percent for sources with f(subscript 100micrometers > 2 Jy, dropping below approximately equals 50 percent for f(subscript 100micrometers < 1.5 Jy. Nevertheless, even faint sources down to a level of f(subscript 170micrometers approximately equals 1 Jy can be detected. Since the majority of the detected point sources are galaxies, the Serendipity Survey will result in a large database of approximately equals 2000 galaxies.

ISOPHOT: In-flight performance report

The imaging photopolarimeter ISOPHOT on-board the European satellite ISO houses 144 background detectors of Si:Ga, Si:P, Ge:Ga and stressed Ge:Ga, all sampled by newly developed cold read-out electronics. There is large temporal radiation damage to most of these detectors on the daily passage through the earths radiation belts. In addition the Ge:Ga detectors exhibit a continuous responsivity increase caused by the cosmic radiation far off the earth. Effective curing procedure shave been developed to heat out these effects. The in-flight sensitivities achieved are close to the pre-flight predictions for most channels. At 100-200 micrometers cirrus confusion is a serious limit for the detection of faint objects on large parts of the sky. The cold filter wheel carrying 56 optical elements, such as filters, apertures and polarizers, as well as the focal plane chopper, operate with high precision and very low power consumption. Due to an effective cold internal baffle system the measured near-field straylight was close to the pre- flight theoretical prediction based on APART simulations. THe sun and moon straylight at 25 and 175 micrometers was measured during several solar eclipses. Drift and transients of the detectors, non-linearities of the preamplifiers, ionizing radiation effects and a complex optical path make the photometric calibration of this instrument challenging. Because most of these effects are reproducible, a calibration accuracy of < 30 percent is already available for most photometric modes. Examples of observations, including the 175 micrometers Serendipitous Sky Survey, will highlight the capabilities of the instrument.

The ISO Serendipity Survey

The ISO Serendipity Sky Survey has been set up to fill the otherwise useless slew time in-between ISO’s fine pointings with measurements in a yet unexplored wavelength regime near 200 μm. During slews longer than about 30 seconds or 0.5 degrees, respectively, ISOPHOT’s C200 array camera is switched on to measure 3 arcmin wide strips of the sky and detect point sources and extended emission along the slew. The fixed read-out rate of the camera is set up to cope with the maximum slew speed of 7 arcmin sec-1 and to match its dynamic range to the variety of expected sky surface brightness in the most optimal way.