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inner rings and pseudo - rings are evident in 50% of all spiral galaxies and in more than 75% of early - type barred spirals ( kormendy 1979 ; de vaucouleurs & buta 1980 ; buta & combes 1996 ) .
these rings are often characterized by intense starbirth activity .
uv imaging neatly isolates such rings from the older disk and bulge components .
uit images of m100 ( ngc 4321 ) , ngc 1317 , and m94 ( ngc 4736 ) provide textbook examples of the starburst ring phenomenon ( see http://trifle.gsfc.nasa.gov/uit/astro1/astro1_pictures.html ) .
as the closest early - type spiral galaxy of low inclination , m94 ( ngc 4736 ) has received concentrated attention from both observers and theorists .
this ( r)sa(r)ab galaxy is noted for its inner ring of ongoing starburst activity ( r @xmath1 1.0 kpc ) , oval stellar distribution at intermediate radius ( r @xmath1 4.9 kpc ) , and outer stellar ring near its holmberg radius ( r @xmath1 7.3 kpc ) .
the uv images obtained by uit and hst reveal hitherto unrecognized patterns of recent star formation , whose presence lends further support to the hypothesis of galaxy evolution via bar - mediated resonances .
the uit s fuv image shows the _ starburst ring _ in high contrast against a mostly dark disk ( waller et al .
1997 ; see also http://trifle.gsfc.nasa.gov/uit/astro2/ngc4736_pr.gif ) .
exterior to the ring are two 500-pc size _ bi - symmetric knots _ on diametrically opposite sides of the nucleus .
the hst s nuv image of the central 20 arcsecs shows a 500-pc long _ mini - bar _ that had been previously inferred from photometric analyses of optical - band images . by comparison ,
a groundbased r - band image shows the underlying bulge and _ oval disk _ made up of cooler and typically older stars .
taken together , the nuclear minibar , starburst ring , bi - symmetric knots , oval disk , and outer pseudo - ring provide compelling evidence for resonant dynamics operating in the disk of m94 .
more specifically , the radii of these various features match those of important orbital resonances , given a pattern speed of 35 km s@xmath2 kpc@xmath2 and our adopted distance and inclination ( see figure  [ fig-1 ] ) .
this pattern of orbital resonances is most likely driven by some combination of the nuclear mini - bar and oval distortion in the disk .
note in figure 1 that our fit places the starburst ring between the two inner lindblad resonances rather than aligned with the outer ilr as modeled by gerin et al .
( 1991 ) and mulder & combes ( 1996 ) .
it is worth noting that at redshifts of 15 , the 2-kpc diameter starburst ring in m94 would subtend angles of only ( 0.7@xmath3  1.0@xmath3)h@xmath4/75 in an einstein - de sitter universe ( q@xmath4 = 1/2 ) and ( 0.3@xmath3  0.2@xmath3)h@xmath4/75 in an open ( milne ) universe ( q@xmath4 = 0 ) .
therefore , many of the `` core - halo '' morphologies that are evident at high - redshift in the restframe fuv ( cf .
giavalisco 1997 ) may , in fact be marginally - resolved representations of galaxies with starburst rings in their centers .
gravitationally - lensed galaxies are fortuitously magnified , enabling resolution of their structure at high s / n .
an important precedent in this regard is the gravitationally - lensed `` pretzel galaxy '' which lies behind the galaxy cluster 0024 + 1654 at an estimated redshift of 1.2  1.8 ( colley et al .
1996 ; tyson et al .
detailed reconstructions of the multiply - lensed galaxy show a clear annular morphology on a scale of several kpc . if m94 and other nearby ringed galaxies can be used as current - epoch analogues , the `` pretzel galaxy '' and perhaps other marginally - resolved `` core - halo '' galaxies at high redshift may represent youthful inner disks evolving under the influence of oval or bar asymmetries .
conversely , if evidence for starburst rings at high redshift proves to be sparse , then massive inner disks featuring ring - bar dynamics have yet to form in most systems , or starbursting bulges are masking their presence .
buta , r. , & combes , f. 1996 , fund .
cosmic phys . , 17 , 95 colley , w. , tyson , j. a. , & turner , e. l. 1996 , , 461 , l83 de vaucouleurs , g. , & buta , r. 1980 , , 44 , 451 giavalisco , m. , et al .
1997 , in the ultraviolet universe at low and high redshift : probing the progress of galaxy evolution , eds .
w. h. waller , m. n. fanelli , j. e. hollis , & a. c. danks ( new york : aip press ) , p. 221 gerin , m. , casoli , f. , & combes , f. 1991 , a&a , 251 , 32 kormendy , j. 1979 , , 227 , 714 mulder , p. s. & van driel , w. 1993 , a&a , 272 , 63 mulder , p. s. & combes , f. 1996 , a&a , 313 , 723 tyson , j. a. , kochanski , g. , & dellantonio , i. 1997 , in the ultraviolet universe at low and high redshift : ... , p. 204
waller , w. h. , et al .
1997 , in the ultraviolet universe at low and high redshift : ... , p. 39
imaging in the restframe ultraviolet has proven to be an effective and vital means of tracing dynamical patterns of star formation in galaxies out to high redshifts . using images from the ultraviolet imaging telescope ( uit ) , hubble space telescope ( hst ) and complementary groundbased telescopes , 
 we have investigated the starburst activity and associated dynamics in nearby early - type disk galaxies . 
 concentrating on the starburst - ring ( r)sa(r)ab galaxy m94 ( ngc 4736 ) , we find compelling evidence for bar - mediated resonances as the primary drivers of evolution at the present epoch . 
 similar ring - bar dynamics may prevail in the centers of early - type disk galaxies at high redshift . 
 the gravitationally - lensed `` pretzel galaxy '' ( 0024 + 1654 ) at a redshift of @xmath01.5 provides an important precedent in this regard .