Interacting system NGC 7805/6 (Arp 112) and its tidal dwarf galaxy candidate

Abstract

We present results from our Giant Metrewave Radio Telescope (GMRT) H I, Himalayan Chandra Telescope (HCT) H_, 1m Sampurnanand Telescope (ST) and 1.3m Devasthal Fast Optical Telescope (DFOT) deep optical observations of the NGC 7805/6 (Arp 112) system to test KUG 2359+311’s tidal dwarf galaxy (TDG) candidacy and explore the properties of the interacting system. Our GMRT H I map shows no H I detection associated with KUG 2359+311, nor any H I tail or bridge-like structure connecting KUG 2359+311 to the NGC 7805/6 system. Our HCT Hα image, on the other hand, displays strong detections in KUG 2359+311, with net SFR~ 0.035± 0.009M⊙ yr^-1. The Hα data constrain the redshift of KUG 2359+311 to 0.00 ≤ z≤ 0.043, compared to the redshift of NGC 7806 of ~0.015. TDGs detected to date have all been H I rich, and displayed H I, ionised gas and stellar tidal debris trails (bridges or tails) linking them to their parent systems. However, neither our H I data nor our optical imaging, while three magnitudes deeper than SDSS, reveals a tidal trail connecting KUG 2359+311 to NGC 7805/6. Lack of H I , presence of an old stellar population, ongoing star formation and reasonably high SFR compared to normal dwarf galaxies suggest that KUG 2359+311 may not be an Arp 112 TDG. It is most likely a case of a regular gas-rich dwarf galaxy undergoing a morphological transformation after having lost its entire gas content to an interaction with the Arp 112 system. Redshift and metallicity from future spectroscopic observations of KUG 2359+311 would help clarify the nature of this enigmatic structure.