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Area photometry with a multi-diode array

University of British Columbia

An area photometer using a RETICON RA50x50 multi-diode array as the detector has been developed and used to observe the central regions of stellar systems. Computer programmes have been written to handle the recording of data and the subseguent analysis of images. A seguential image processing language called FIRM was developed for use with an IBM 370 under MTS. A theoretical analysis of the response of diode arrays to Gaussian star images shows that the light from a star can be integrated if the seeing diameter is greater than 2.0 times the diode centre to centre spacing. Aliasing at all freguencies is insignificant for any seeing diameter greater than about 2.5 diode spacings. Technigues for avoiding aliasing are discussed. Images of an area approximately 40" x 40" in the centre of the Sb galaxy NGC 4736(=M94) have been obtained through blue, visual and red filters. Subtraction of a simple, circularly symmetric model with King core radius 6".0 ± 0".5 reveals a small, bar-like central structure about 20" across. This central structure is aligned perpendicularly to the major axis of the galaxy and melds into previously photographed spiral structure. A bright point-like nucleus is also seen, but it does not stand out in the colour maps. ft sharp colour gradient exists at 10" radius. The reddest areas of the central region coincide roughly with patches of negative resi duals after model subtraction. A simple dynamical model is constructed for the inner region of NGC 4736, and it is proposed that two spiral systems exist in this galaxy, one inside the other with the inner system rotating more rapidly if it is a two-armed spiral. This model may also apply to other galaxies. A central mass to blue luminosity ratio of 2.4 to 3.6 is obtained for a range of model parameters. Observations of the X-ray emitting globular cluster NGC 7078 (=1115) reveal a number of red giants within 15" of the centre. A map of b-^-r instrumental colour shows that the nuclear cusp appears to be only a little redder than the main unresolved "background" population of the cluster, but the poor seeing does not allow resolution of the cusp within 2" of the cluster centre. The colour map confirms published spectroscopic results. There is very little radial colour variation when centred circular apertures are simulated.

Text

2010-02-19

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http://hdl.handle.net/2429/20545

Astronomy

University of British Columbia

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