Radially truncated galactic discs

We present the first results of a systematic analysis of radially truncatedexponential discs for four galaxies of a sample of disc-dominated edge-onspiral galaxies. Edge-on galaxies are very useful for the study of truncatedgalactic discs, since we can follow their light distributions out to largerradii than in less highly inclined galaxies. The origin of these truncationsand their asymmetry and sharpness are helpful to better constrain theories ofgalaxy formation. In general, the discs of our sample galaxies are truncated at similar radiion either side of their centres. With the exception of the disc of ESO 416-G25,it appears that our sample galaxies are closely symmetric, in terms of both thesharpness of their disc truncations and the truncation length. However, thetruncations occur over a larger region and not as abruptly as found by van derKruit & Searle (KS1-4). We show that the truncated luminosity distributions of our sample galaxies,if also present in the mass distributions, comfortably meet the requirementsfor longevity. Stability requirements for galactic discs may play an importantrole in the formation and maintenance of disc truncations. A scenario in whichgalactic discs are truncated at the radii where the gas density becomessubcritical for star formation to occur, combined with slowly growing discs, ispossibly what we observe in our sample galaxies. This hypothesis is supportedby the radial (B-I) colour profiles, showing that, on average, the disc colourtends to get bluer in the truncation region compared to the colours in theexponential disc. Submitter: rd7a@solstice.astro.virginia.ed