Molecular Superbubbles in the Starburst Galaxy NGC 253

TL;DR

This study reveals the presence of molecular superbubbles in NGC 253's circumnuclear disk, formed by energetic events like super star clusters or hypernovae, indicating highly disturbed molecular gas dynamics in the starburst region.

Contribution

First detection and analysis of molecular superbubbles in NGC 253, linking their formation to energetic stellar phenomena and providing insights into starburst-driven feedback.

Findings

Discovered two large molecular shells (~100 pc) with high velocity widths.

Estimated each superbubble's age at ~0.5 Myr and energy at ~10^46 J.

Superbubbles likely formed by super star clusters or hypernovae.

Abstract

The central 2x1 kpc of the starburst galaxy NGC 253 has been imaged using the Submillimeter Array at a 60 pc resolution in the J=2-1 transitions of 12CO, 13CO, and C18O as well as in the 1.3 mm continuum. Molecular gas and dust are mainly in the circumnuclear disk of ~500 pc radius, with warm (~40 K) and high area-filling factor gas in its central part. Two gas shells or cavities have been discovered in the circumnuclear disk. They have ~100 pc diameters and have large velocity widths of 80-100 km/s, suggestive of expansion at ~50 km/s. Modeled as an expanding bubble, each shell has an age of ~0.5 Myr and needed kinetic energy of ~1E46 J as well as mean mechanical luminosity of ~1E33 W for its formation. The large energy allows each to be called a superbubble. A ~10^6 Msun super star cluster can provide the luminosity, and could be a building block of the nuclear starburst in NGC 253. Alternatively, a hypernova can also be the main source of energy for each superbubble, not only because it can provide the mechanical energy and luminosity but also because the estimated rate of superbubble formation and that of hypernova explosions are comparable. Our observations indicate that the circumnuclear molecular disk harboring the starburst is highly disturbed on 100 pc or smaller scales, presumably by individual young clusters and stellar explosions, in addition to globally disturbed in the form of the well-known superwind.