Letter to the Editor

Formation of an ultra-diffuse galaxy in the stellar filaments of NGC 3314A: Caught in the act?

¹ INAF-Astronomical Observatory of Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
e-mail: enrichetta.iodice@inaf.it
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany
³ INAF-Astronomical Observatory of Abruzzo, Via Maggini, 64100 Teramo, Italy
⁴ Instituto de Astrofísica, Facultad de Fisica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁵ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁷ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
⁸ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁹ University of Naples “Federico II”, C.U. Monte Sant’Angelo, Via Cinthia, 80126 Naples, Italy

Received: 14 April 2021
Accepted: 9 July 2021

Abstract

The VEGAS imaging survey of the Hydra I cluster has revealed an extended network of stellar filaments to the south-west of the spiral galaxy NGC 3314A. Within these filaments, at a projected distance of ∼40 kpc from the galaxy, we discover an ultra-diffuse galaxy (UDG) with a central surface brightness of μ_0, g ∼ 26 mag arcsec⁻² and effective radius R_e ∼ 3.8 kpc. This UDG, named UDG 32, is one of the faintest and most diffuse low-surface-brightness galaxies in the Hydra I cluster. Based on the available data, we cannot exclude that this object is just seen in projection on top of the stellar filaments and is thus instead a foreground or background UDG in the cluster. However, the clear spatial coincidence of UDG 32 with the stellar filaments of NGC 3314A suggests that it might have formed from the material in the filaments, becoming a detached, gravitationally bound system. In this scenario, the origin of UDG 32 depends on the nature of the stellar filaments in NGC 3314A, which is still unknown. The stellar filaments could result from ram-pressure stripping or have a tidal origin. In this letter we focus on the comparison of the observed properties of the stellar filaments and of UDG 32 and speculate on their possible origin. The relatively red colour (g − r = 0.54 ± 0.14 mag) of the UDG, similar to that of the disk in NGC 3314A, combined with an age older than 1 Gyr and the possible presence of a few compact stellar systems, points towards a tidal formation scenario.