The Unusual Tidal Dwarf Candidate in the Merger System NGC 3227/3226: Star Formation in a Tidal Shock?

Abstract

We report the discovery of active star formation in the H I cloudassociated with the interacting Seyfert system NGC 3227/3226 that wasoriginally identified as a candidate tidal dwarf galaxy (TDG) by Mundellet al. and that we name J1023+1952. We present broadband optical B, R, I(from the Isaac Newton Telescope), and ultraviolet images (fromXMM-Newton) that show that the H I cloud is associated with massiveongoing star formation seen as a cluster of blue knots(M_{B}{\lt}~-15.5 mag) surrounded by a diffuse ultraviolet haloand cospatial with a ridge of high neutral hydrogen column density(N_{H}~3.7{\times}10^{21} cm^{-2}) in thesouthern half of the cloud. We also detect H{α} emission from theknots with a flux density ofF_{Halpha}~2.55{\times}10^{-14} ergs s^{-1}cm^{-2} corresponding to a star formation rate ofSFR(H{α})~10.6{\times}10^{-3} M_{solar}yr^{-1}. J1023+1952 lies at the base of the northern tidal tail,and, although it spatially overlaps the edge of the disk of NGC 3227,Mundell et al. showed that the H I cloud is kinematically distinct withan H I mean velocity 150 km s^{-1} higher than that of NGC 3227.Comparison of ionized (H{α}) and neutral (H I) gas kinematics of thecloud shows closely matched recessional velocities, providing strongevidence that the star-forming knots are embedded in J1023+1952 and arenot merely optical knots in the background disk of NGC 3227, thusconfirming J1023+1952 as a gas-rich (M_{H}/L_{B}{\gt}1.5)dwarf galaxy. No star formation is detected in the northern half of thecloud, despite similar H I column densities; instead, our newhigh-resolution H I image shows a ridge of high column densitycoincident with the reddest structures evident in our B-I image. Wesuggest that these structures are caused by the background stellarcontinuum from the disk of NGC 3227 being absorbed by dust intrinsic toJ1023+1952, thus placing J1023+1952 in front of NGC 3227 along the lineof sight. We discuss two scenarios for the origin of J1023+1952: as athird, preexisting dwarf galaxy involved in the interaction with NGC3227 and NGC 3226, or as a newly forming dwarf galaxy condensing out ofthe tidal debris removed from the gaseous disk of NGC 3227. The firstscenario is feasible given that NGC 3227 is the brightest member of agalaxy group, an environment in which preexisting dwarf galaxies areexpected to be common. However, the lack of a detectable old stellarpopulation in J1023+1952 makes a tidal origin more likely. If J1023+1952is a bound object forming from returning gaseous tidal tail material,its unusual location at the base of the northern tail implies adynamically young age similar to its star formation age, and suggests itis in the earliest stages of TDG evolution. Whatever the origin ofJ1023+1952, we suggest that its star formation is shock-triggered bycollapsing tidal debris.