Letter to the Editor

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

¹ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía, 18008 Granada, Spain
e-mail: jromanastro@gmail.com
² Instituto de Astrofísica de Canarias, c/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁴ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Rm. N204, Tucson, AZ 85721-0065, USA
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ School of Physics, University of New South Wales, Sydney, NSW 2052, Australia

Received: 6 April 2021
Accepted: 28 April 2021

Abstract

We have explored the properties of a peculiar object detected in deep optical imaging and located at the tip of an H I tail emerging from Hickson Compact Group 16. Using multiband photometry from infrared to ultraviolet, we were able to constrain its stellar age to 58₋₉⁺²² Myr with a rather high metallicity of [Fe/H] = −0.16_−0.41^+0.43 for its stellar mass of M_⋆ = 4.2 × 10⁶ M_⊙, 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 (⟨μ_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 ∼2 Gyr of evolution, even in the deepest current or future optical surveys. Its high H I mass, M(HI) = 3.9 × 10⁸ M_⊙, 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 H I 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.