Formation of Dense Molecular Gas and Stars at the Circumnuclear Starburst Ring in the Barred Galaxy NGC 7552

TL;DR

This study investigates the distribution and formation of dense molecular gas and stars in the circumnuclear starburst ring of NGC 7552, revealing spatial correlations and discrepancies with star formation models.

Contribution

It provides detailed multi-wavelength observations and analysis of gas and star formation locations, challenging existing models of star formation in galactic rings.

Findings

Dense gas is concentrated at contact points between dust lanes and the ring.

Star formation occurs downstream of dense gas concentrations, consistent with some models.

The propagation time between dense gas and star formation indicators is shorter than OB star lifetimes.

Abstract

We present millimeter molecular-line complemented by optical observations, along with a reanalysis of archival centimeter HI and continuum data, to infer the global dynamics and determine where dense molecular gas and massive stars preferentially form in the circumnuclear starburst ring of the barred-spiral galaxy NGC 7552. We find diffuse molecular gas in a pair of dust lanes each running along the large-scale galactic bar, as well as in the circumnuclear starburst ring. We do not detect dense molecular gas in the dust lanes, but find such gas concentrated in two knots where the dust lanes make contact with the circumnuclear starburst ring. When convolved to the same angular resolution as the images in dense gas, the radio continuum emission of the circumnuclear starburst ring also exhibits two knots, each lying downstream of an adjacent knot in dense gas. The results agree qualitatively with the idea that massive stars form from dense gas at the contact points, where diffuse gas is channeled into the ring along the dust lanes, and later explode as supernovae downstream of the contact points. Based on the inferred rotation curve, however, the propagation time between the respective pairs of dense gas and centimeter continuum knots is about an order of magnitude shorter than the lifetimes of OB stars. We discuss possible reasons of this discrepancy, and conclude that either the initial mass function is top-heavy or massive stars in the ring do not form exclusively at the contact points where dense molecular gas is concentrated.