Ultraviolet Signposts of Resonant Dynamics in the Starburst-ringed S[CLC]ab[/CLC] Galaxy M94 (NGC 4736)

William H. Waller, Michael N. Fanelli, William C. Keel, Ralph Bohlin, Nicholas R. Collins, Barry F. Madore, Pamela M. Marcum, Susan G. Neff, Robert W. O'Connell, Joel D. Offenberg, Morton S. Roberts, Andrew M. Smith (+1 others)
2001 Astronomical Journal  
M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV-band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (H-alpha, R, and I bands), and Palomar 5-m telescope (B-band), along with spectra from the International Ultraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUV emission from (a) an elongated nucleus, (b) a diffuse inner disk, where H-alpha is observed in absorption, (c) a bright inner ring of H II regions at the
more » ... regions at the perimeter of the inner disk (R = 48 arcsec. = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring on diametrically opposite sides of the nucleus (R= 130 arcsec. = 2.9 kpc). The HST/FOC image resolves the NUV emission from the nuclear region into a bright core and a faint 20 arcsec. long "mini-bar" at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk indicates an approximately 10^7 or 10^8 yr-old stellar population from low-level starbirth activity blended with some LINER activity. Analysis of the H-alpha, FUV, NUV, B, R, and I-band emission along with other observed tracers of stars and gas in M94 indicates that most of the star formation is being orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi-symmetric knots at intermediate radius, in particular, argue for bar-mediated resonances as the primary drivers of evolution in M94 at the present epoch. Similar processes may be governing the evolution of the "core-dominated" galaxies that have been observed at high redshift. The gravitationally-lensed "Pretzel Galaxy" (0024+1654) at a redshift of approximately 1.5 provides an important precedent in this regard.
doi:10.1086/319384 fatcat:g7266uei4rb35e2qjfhymbtgxy