A diffuse tidal dwarf galaxy destined to fade out as a "dark galaxy"

TL;DR

This study examines a tidal dwarf galaxy with high HI content and diffuse structure, predicting it will fade into an undetectable dark galaxy in about 2 billion years due to stellar aging and fading.

Contribution

It provides observational evidence that HI tidal features can evolve into dark galaxies as their stellar components fade beyond detection limits.

Findings

The object has a stellar age of ~58 Myr and high metallicity.

It has a large effective radius and low surface brightness.

Future fading will render it undetectable in optical surveys within 2 Gyr.

Abstract

We have explored the properties of a peculiar object detected in deep optical imaging and located at the tip of an HI tail emerging from Hickson Compact Group 16. Using multiband photometry from infrared to ultraviolet, we were able to constrain its stellar age to 58$^{+22}_{-9}$ Myr with a rather high metallicity of [Fe/H] = $-$0.16$^{+0.43}_{-0.41}$ for its stellar mass of M$_\star$ = 4.2$\times$10$^6$ M$_\odot$, a typical signature of tidal dwarf galaxies. The structural properties of this object are similar to those of diffuse galaxies, with a round and featureless morphology, a large effective radius (r$_{eff}$ = 1.5 kpc), and a low surface brightness (<$\mu_{g}$>$_{eff}$ = 25.6 mag arcsec$^{-2}$). Assuming that the object is dynamically stable and able to survive in the future, its fading in time via the aging of its stellar component will make it undetectable in optical observations in just $\sim$2 Gyr of evolution, even in the deepest current or future optical surveys. Its high HI mass, M(HI) = 3.9$\times$10$^8$ M$_\odot$, and future undetectable stellar component will make the object match the observational properties of dark galaxies, that is, dark matter halos that failed to turn gas into stars. Our work presents further observational evidence of the feasibility of HI tidal features becoming fake dark galaxies; it also shows the impact of stellar fading, particularly in high metallicity systems such as tidal dwarfs, in hiding aged stellar components beyond detection limits in optical observations.