Holmberg Ix: The Nearest Young Galaxy

TL;DR

Holmberg IX is a young, likely tidal dwarf galaxy with recent star formation, high gas content, and a stellar population primarily younger than 200 million years, possibly formed during a recent galactic interaction.

Contribution

This study provides detailed analysis of Holmberg IX's stellar populations and spatial distribution, supporting its classification as a tidal dwarf galaxy formed from galactic interactions.

Findings

Most stars in Holmberg IX are younger than 200 Myr.

At least 20% of its stellar mass formed in the last 200 Myr.

Older stars are associated with M81 or its tidal debris.

Abstract

Deep images taken with the Wide Field Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope provide the basis for study the resolved stellar population of the M81 companion dwarf irregular galaxy Holmberg IX. Based on color-magnitude diagrams the stellar population toward Holmberg IX contains numerous stars with ages of <~200 Myr as well as older red giant stars. By charting the spatial distribution of the red giant stars and considering their inferred metallicities, we concluded that most of these older stars are associated with M81 or its tidal debris. At least 20% of the stellar mass in Holmberg IX was produced in the last ~200 Myr, giving it the youngest stellar populations if any nearby galaxy. The location of Holmberg IX, its high gas content, and its youthful stellar population suggests that it is a tidal dwarf galaxy, perhaps formed during the last close passage of M82 around M81.