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arXiv · 2604.02831

Holmberg IX: A Unique, Infant but Inactive Galaxy as Revealed via a Multiwavelength Approach

Ye-Wei Mao·Luis C. Ho·Alexei V. Moiseev·Oleg V. Egorov·Andrej M. Sobolev
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Abstract

In this Letter, we report a novel discovery of unique characteristics for the tidal dwarf galaxy (candidate) Holmberg IX via a multiwavelength investigation. New observations are taken for deeply mapping Hα emission and combined with archival/published data for comprehensively probing dust, gas, and stellar populations in this galaxy. We find in Holmberg IX a dearth of dust incompatible with its rich gas and metal; globally young stellar populations with prominent far-ultraviolet but deficient and marginal Hα emissions, distinct from other tidal dwarf galaxies ever known. By assuming a normal initial mass function (IMF), Holmberg IX is suggested to be born ~130 Myr ago from a bursty star formation event, which then rapidly ceased, with very few stars formed in the past ~80 Myr that demarcates a lower age limit for the galactic mainbody; current star formation occurs only in outskirts, bringing a conundrum about the reason for the recent quenching in such a gas-rich environment. Contradicting the general expectation for tidal dwarf galaxies hosting continuous star formation, the present quiescence implies Holmberg IX currently staying in a rarely-seen transient period. Without star formation continuing, Holmberg IX is likely transforming into a dwarf spheroidal galaxy, or oppositely into a(n) (ultra-)diffuse system which will probably dissolve in the end. Instead, if Holmberg IX possesses peculiar IMF and hosts low-mass, weak-Hα star formation, it is able to maintain long-term survival in its current status. On whichever evolutionary pathway in reality, Holmberg IX appears as a special case updating conventional understandings of tidal dwarf galaxies and hinting potential existence of similar analogs in the Universe.

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Ye-Wei Mao, Luis C. Ho, Alexei V. Moiseev, Oleg V. Egorov, Andrej M. Sobolev. 2026-04-16. Holmberg IX: A Unique, Infant but Inactive Galaxy as Revealed via a Multiwavelength Approach. https://doi.org/10.3847/2041-8213%2Fae5a8b

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