High-resolution infrared maps from IRAS observations of large galaxies

Abstract

High angular resolution infrared maps of 18 galaxies have been generated using the infrared Astronomical Satellite(IRAS) measurements. The sample of galaxies has been chosen from the Large Galaxy Catalog (LGC) of Rice et al.(1988), which contains galaxies with optical size(D₂₅) greater than 8'. The two-dimensional grids generated by the mission from the IRAS pointed observations using the survey detectors(POSD) at 12,25,60, and 100μm and from the IRAS Chopped Photometric Channel observations (CPC) at 50 and 100 μm have been used. Our sample of galaxies is restricted by the availability of POSD observations (in the LGC) usable for map deconvolution. In order to generate high angular resolution intensity maps, a maximum-emtropy method (MEM) has been used (for both POSD and CPC). A self-adaptive procedure has been developed and successfully used here which drives the MEM iterations guided by several man diagnostics. The error (or uncertainty) in the brightness measurement of each picture element has been estimated taking into account contributions from various instrumental as well as computational aspects. Integrated flux densities and angular sizes have been obtained from the intensity maps. The flux densities and angular sizes from POSD and CPC maps compare quite well. It has been demonstrated that the CPC maps do give new reliable structural information, although for only sufficiently strong sources. The POSD intensity maps have futher been used to obtain the maps of interband (in the neighboring bands) temperature (assuming an emissivity ελ œλ^-1) and optical depth. The structural properties of intensity, temperature, and optical depth distribution have been compared and commented upon. The ratios of infrared (12-100 μm) to optical sizes(D₂₅) are quite large(0.5-0.7), implying sufficient depth of the infrared maps. Whereas the T(25/60) isotherm structures of galaxies show that a large fraction of galaxies have their nuclei reatively cooler than the surroundings, the same structures for T(60/100) often show negligible gradients near the galactic nuclei. These can be understood in terms of higher dust optical depth in star-forming regions close to the nucleus. Further, these nuclear (temperature) gradients along the major axis show no correlation with either the inclination angle of the galaxy or the far-infrared luminosity. From the optical depth maps at 25,60, and 100μm, it is found that for most of them the peaks are close to the galactic nuclei. It is also seen T₆₀ map has mostly a single peak, whereas T₂₅ and T₁₀₀ maps often show multiple peaks.