Formation of emission line dots and extremely metal-deficient dwarfs from almost dark galaxies

TL;DR

This paper explores how interactions and mergers of gas-rich, faint dwarf galaxies can trigger star formation, leading to observable emission line dots and extremely metal-deficient dwarfs, revealing hidden populations of faint galaxies.

Contribution

It demonstrates that almost dark dwarf galaxies can become observable as ELdots or XMD dwarfs through interactions, providing new insights into their evolution and detectability.

Findings

Star formation rates increase significantly during dwarf interactions.

Interacting dwarfs can be identified as emission line dots (ELdots).

Remnants can evolve into extremely metal-deficient dwarfs.

Abstract

Recent observations have discovered a number of extremely gas-rich very faint dwarf galaxies possibly embedded in low-mass dark matter halos. We investigate star formation histories of these gas-rich dwarf ("almost dark") galaxies both for isolated and interacting/merging cases. We find that although star formation rates (SFRs) are very low (<10^-5 M_sun/yr) in the simulated dwarfs in isolation for the total halo masses (M_h) of 10^8-10^9 M_sun, they can be dramatically increased to be ~ 10^{-4} M_sun/yr when they interact or merge with other dwarfs. These interacting faint dwarfs with central compact HII regions can be identified as isolated emission line dots ("ELdots") owing to their very low surface brightness envelopes of old stars. The remnant of these interacting and merging dwarfs can finally develop central compact stellar systems with very low metallicities (Z/Z_sun<0.1), which can be identified as extremely metal-deficient ("XMD") dwarfs. These results imply that although there would exist many faint dwarfs that can be hardly detected in the current optical observations, they can be detected as isolated ELdots or XMD dwarfs, when they interact with other galaxies and their host environments. We predict that nucleated ultra-faint dwarfs formed from merging of almost dark dwarfs can be identified as globular clusters owing to the very low surface brightness stellar envelopes, if they are distant objects.