Dynamics of Starbursting Dwarf Galaxies: I Zw 18

TL;DR

This study reveals that I Zw 18, a prototypical blue compact dwarf galaxy, hosts a compact, fast-rotating HI disk with high column densities, suggesting it resembles a miniature high-surface-brightness disk galaxy and likely results from a dwarf-dwarf merger.

Contribution

The paper provides detailed dynamical analysis of I Zw 18's HI gas, showing it differs from typical dwarf irregulars and may be a high-surface-brightness disk galaxy formed by a merger.

Findings

HI forms a compact, fast-rotating disk with high column densities.

Mass models indicate baryons may dominate inner gravitational potential.

Extended HI emission suggests recent dwarf-dwarf interaction.

Abstract

I Zw 18 is a prototype Blue Compact Dwarf (BCD), characterized by a strong starburst and extremely low metallicity (Z ~ 0.02 Zsun). It has long been considered a candidate young galaxy in the Local Universe, but recent studies indicate the presence of old stars. We analysed archival VLA observations of the 21 cm line and found that the HI associated to the starburst region forms a compact fast-rotating disk. The HI column densities are very high, up to ~50-100 Msun/pc^2 (~0.6-1.2 x 10^22 atoms/cm^2). The rotation curve is flat with a steep rise in the inner parts, indicating the presence of a strong central concentration of mass. Mass models with a dark matter halo show that baryons may dominate the gravitational potential in the inner regions. A radial inflow/outflow motion of ~15 km/s is also present. I Zw 18 appears structurally different from typical dwarf irregulars in terms of gas distribution, stellar distribution and dynamics. It may be considered as a "miniature" high-surface-brightness disk galaxy. These dynamical properties must be tightly related to the starburst. They also shed new light on the question of the descendants of BCDs. There is also extended HI emission towards the outlying stellar complex I Zw 18 C and a ~13.5 kpc HI tail. An interaction/merger between gas-rich dwarfs is the most likely explanation for the starburst.