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HST-UV imaging: results and interpretation

Below we provide a detailed description of the UV morphology of the galaxies. Some quantitative photometric properties extracted from the images are listed in Table 2. These include the effective (or half-light) radius re of the central structure, the corresponding effective surface brightness 22#22, the total flux within the 23#23 FOC image after sky subtraction and the total F210M flux measured from the IUE spectra as measured with the IRAF/STSDAS SYNPHOT package. Note that the IUE flux is typically higher than the FOC flux often by a considerable factor (e.g. IC3639). This is because the total fluxes in our images are very sensitive to the sky level which is difficult to accurately determine. Typically the 124#24 uncertainty in the sky measurement integrated over the FOC area amounts to about half the missing flux. Sky is measured on the images far from the obvious sources of UV emission, but low surface brightness extended emission may have been subtracted away with the sky. In addition since the dimensions of the IUE are comparable or larger than the FOC image, some of the missing flux may be in sources outside of the field of view of the images. This uncertainty in the total flux gives rise to a corresponding uncertainty in the ``true'' re and 22#22 of the total UV light distribution. Instead we measure the re and 22#22 of an inner high surface brightness component of the images which we associate with the starburst plus Seyfert 2 nucleus. The extent of this component is easily discernable as an inflection point in radial growth curves. The inflection point is the result of a change in slope of the surface brightness profile. Surface brightness profiles of the three galaxies are shown in Figure 2 illustrating that the inflection point occurs typically at 25#25. We take re to be the radius enclosing half of the flux within this inflection point, and 22#22 to be the mean surface brightness within re.



 
next up previous
Next: NGC 7130 Up: Ultraviolet-Optical observations of the Previous: HST optical imaging results
Rosa Gonzalez
1998-06-20