Propagating Star Formation in the Collisional Ring Galaxy Arp 10

Propagating star formation in a collisional ring galaxy Arp 10 is investigated by a complex approach , which includes the broad-and narrow-band photometry, long-slit spectroscopy, and scanning Fabry-Perot spectroscopy. The ionized gas velocity field obtained with best spatial resolution to date indicates a non-isotropic expansion of the outer ring with a maximum velocity 110 km s −1. Strong vertical and non-circular motions are also seen in the vicinity of the inner ring. Our kinematic data suggest that Arp 10 has a small inclination i = 22 • and high total (luminous plus dark matter) mass of about 10 12 M ⊙ within a 50 kpc radius. The internal extinction in HII regions and extinction-corrected Hα fluxes are estimated from the emission lines. The abundance of oxygen 12 + log(O/H) in both star-forming rings is about 8.6. The analysis of spectral indices provides an estimate on the propagation velocities of both rings and metallicity of the pre-collision stellar population. A small " knot " near the nucleus of Arp 10, which was previously suspected as a possible candidate for collision, is now unambiguously identified as the " intruder ". The intruder spectrum suggests that it was a spiral galaxy before the collision and its present mass amounts to at least one-fourth of the total mass of Arp 10. A high value of the intruder velocity relative to Arp 10 (≈ 480 km s −1) suggests that the intruder is either gravitationally unbound or marginally bound to the main galaxy. We use a simplified two-dimensional hydrodynamic modeling of galaxy collisions to test a collisional origin of Arp 10. We confirm that the sizes of the inner and outer rings, maximum expansion velocity of the outer ring, and radial profile of the gas circular velocity can be reproduced by a near-central collision with the intruder galaxy, which occurred approximately 85 Myr ago. We acknowledge that an apparent crescent-shaped distribution of Hα emission in the outer ring is caused by a star formation threshold in the gas disk of Arp 10. 1. INTRODUCTION Collisional ring galaxies (Vorontsov-Veliaminov 1976) give us a bright example of star formation induced by radially expanding density waves, which are triggered by a near-axial passage of a companion galaxy through the disk of a target galaxy (Lynds & Toomre 1976; Higdon 1995; Charmandaris & Appleton 1996). The expanding density waves in the gas disk of a target galaxy leave behind …