A CO map of the inner spiral arms of NGC 628

Abstract

The Berkeley-Illinois-Maryland Association Millimeter Array was used to study the 12 CO 7 = 1-0 emission in the central 2' (6 kpc) of the nearby (7-10 Mpc), face-on Sc galaxy NGC 628. The angular resolution of 7'.1X11'.'6 corresponds to a linear resolution of 340X560 pc, comparable to the width of the inner spiral arms. Comparison with single-dish CO observations of the central region shows that about 45% of the single-dish flux is recovered. The CO appears to be associated with the optical spiral arms, but does not show a consistent trend of being aligned with either the dust lanes or star forming regions. It is clumpy in appearance with gaps between the major CO associations making up the spiral arms. These results suggest the presence of a spiral density wave, but indicate that local processes such as star formation and cloud-cloud interactions must also play a role in determining the distribution of gas in the inner galaxy. The lack of consistent alignment between the CO distribution and the optical dust lanes precludes us from determining the strength of the shock in the disk of NGC 628. While there is a good deal of CO in the central part of the galaxy, our map shows that there is no concentration of CO in the very center, similar to the situation in M51. As single-dish data often appear to suggest the presence of nuclear CO, this highlights the need for high spatial resolution to study star formation in external galaxies.