Constraints on the Minor Merging and Star Formation History of the Wolf-Rayet Galaxy NGC 5430 Through Observations

TL;DR

This study investigates the minor merger history and star formation evolution of NGC 5430 using multi-wavelength molecular gas observations and star formation tracers, revealing a recent off-center starburst triggered by a collision 5-10 million years ago.

Contribution

It provides new insights into the minor merger and star formation history of NGC 5430 through detailed molecular gas analysis and multi-timescale star formation tracers.

Findings

Collision occurred 5-10 Myr ago triggering a Wolf-Rayet starburst.

Current star formation rate has decreased to Galactic levels.

The merger's initial mass ratio is estimated between 7:1 and 20:1.

Abstract

We used multi-wavelength analysis of the newly observed molecular gas (12CO and 13CO (1-0)) with interferometer CARMA and archival star formation tracers to constrain the interaction, merging, and star formation history of an off-center minor merger, a three-spiral barred galaxy NGC 5430 and its satellite embedded in the bar. Morphology of the molecular gas in the bar of NGC 5430 shows minimal signs of recent interactions in our resolution. The apparent morphological remnant of the past galaxy interaction is an asymmetric spiral arm, containing more molecular gas and exhibiting higher star formation rate (SFR) surface density than the two primary arms. Rotation curve analysis suggests that NGC 5430 and its satellite collided several Gyr ago. History of star formation was constrained by using SFRs that trace different timescales (infrared, radio continuum, and H-alpha). The collision occurred 5 - 10 Myr ago, triggering a transient off-center starburst of Wolf-Rayet stars at the eastern bar end. In the past, the global SFR during the Wolf-Rayet starburst peaked at 35 Msun/yr. At present, the merger-driven starburst is rapidly decaying and the current global SFR has decreased to the Galactic value. The SFR will continue to decay as suggested by the present amount of dense gas (traced by HCN (1-0)). Nonetheless, the global SFR is still dominated by the Wolf-Rayet region rather than the circumnuclear region. Compared with other barred galaxies, the circumnuclear region exhibits particularly low dense gas fraction, low star formation activity and high concentration of gas. Physical properties of the molecular gas are inferred by using the large velocity gradient (LVG) calculations. The initial mass ratio of the NGC 5430 and its satellite are suggested to be in the intermediate ratio range of 7:1-20:1.