Large infrared and optical color gradients in the Cartwheel ring galaxy - Evidence for the first epoch of star formation in the wake of an expanding ring

The ring galaxy HRG 54 103: a first study

“…Multiple-band observations were performed by Appleton & Struck-Marcell (1997, and references therein). The prototype RG, the Cartwheel, has been tested observationally to study the ring expansion, star formation in the ring, radial color gradient and stellar ages (see Fosbury & Hawarden 1977;Higdon 1995;Marcum et al 1992;Korchagin et al 2001;Vorobyov & Bizyaev 2001). Color gradients were found in some other RGs (Appleton & Marston 1997), and Marston & Appleton (1995) found that the majority of the star-forming regions are located in the ring.…”

A Ring of X-rays from the Cartwheel Galaxy

“…Most of the activity is in fact in the S-SW quadrant of the ring, where massive and luminous Hi I regions characterized by large Ha luminosities and equivalent widths are found (Higdon 1995). Both dynamical considerations and stellar evolution models suggest an age of 2-4 x 10 8 yr for the starburst; the evidence of very low metallicity, with large deficiency factors in 0 , N and Ne relative to the solar value, and the estimated supernova rate, almost two orders of magnitude higher than in normal galaxies, also support the view that star formation in the ring is a recent phenomenon and that gas currently forming stars was nearly primordial at the time of the impact (Fosbury &;Hawarden 1977;Higdon 1995;Marcum et al 1992). The Cartwheel is located in a tight group (~ 0.2 Mpc size) that includes three nearby companions of comparable size and magnitude, with a redshift within 400 km s _ 1 from one another.…”

“…A small fraction (~ 1/10) of the flux attributed to the Cartwheel might be associated with a star superimposed on the outer ring, well visible in the HST image (NASA Press Release PRC96-36a, Nov. 26, 1996) and already identified as an object with the colors of a G star (Marcum et al 1992). The X-ray to optical flux ratio (estimated at 5 x 10~1 5 erg c m -2 s _ 1 for a 17-18 mag.…”

Observations of Gas in Interacting Galaxies

“…Marcum et al 1992). Horellou et al (1994) have observed the molecular content of a series of ring galaxies, and found that it was also enhanced with respect to normal galaxies.…”