FIRST DETECTION OF A SUBKILOPARSEC SCALE MOLECULAR OUTFLOW IN THE STARBURST GALAXY NGC 3628

We successfully detected a molecular outflow with a scale of 370-450 pc in the central region of the starburst galaxy NGC 3628 through deep CO(1-0) observations by using the Nobeyama Millimeter Array (NMA). The mass of the outflowing molecular gas is similar to 2.8 x 10(7) M-circle dot, and the outflow velocity is similar to 90 +/- 10 km s(-1). The expansion timescale of the outflow is 3.3-6.8 Myr, and the molecular gas mass flow rate is 4.1-8.5 M-circle dot yr(-1). It requires mechanical energy of (1.8-2.8) x 10(54) erg to create this sub-kpc scale molecular outflow. In order to understand the evolution of the molecular outflow, we compare the physical properties between the molecular outflow observed from our NMACO(1-0) data and the plasma gas from the soft X-ray emission of the Chandra X-ray Observatory archival data. We found that the distribution between the molecular outflow and the strong plasma outflow seems to be in a similar region. In this region, the ram pressure and the thermal pressure of the plasma outflow are 10(-(8-10)) dyn cm(-2), and the thermal pressure of molecular outflow is 10(-(11-13)) dyn cm(-2). This implies that the molecular outflow is still expanding outward. The molecular gas consumption timescale is estimated as 17-27 Myr, and the total starburst timescale is 20-34 Myr. The evolutionary parameter is 0.11-0.25, suggesting that the starburst activity in NGC 3628 is still in a young stage.