The HI content of isolated ultra-diffuse galaxies: A sign of multiple formation mechanisms?

¹ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747AD Groningen, The Netherlands
e-mail: papastergis@astro.rug.nl
² ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7900AA Dwingeloo, The Netherlands
e-mail: adams@astron.nl
³ Department of Physics & Astronomy, San José State University, One Washington Square, San Jose, CA 95192, USA
e-mail: aaron.romanowsky@sjsu.edu

Received: 16 March 2017
Accepted: 26 April 2017

Abstract

We report on the results of radio observations in the 21 cm emission line of atomic hydrogen (HI) of four relatively isolated ultra-diffuse galaxies (UDGs): DGSAT I, R-127-1, M-161-1, and SECCO-dI-2. Our Effelsberg observations resulted in non-detections for the first three UDGs, and a clear detection for the last. DGSAT I, R-127-1, and M-161-1 are quiescent galaxies with gas fractions that are much lower than those of typical field galaxies of the same stellar mass. On the other hand, SECCO-dI-2 is a star forming gas-rich dwarf, similar to two other field UDGs that have literature HI data: SECCO-dI-1 and UGC 2162. This group of three gas-rich UDGs have stellar and gaseous properties that are compatible with a recently proposed theoretical mechanism for the formation of UDGs, based on feedback-driven outflows. In contrast, the physical characteristics of R-127-1 and M-161-1 are puzzling, given their isolated nature. We interpret this dichotomy in the gaseous properties of field UDGs as a sign of the existence of multiple mechanisms for their formation, with the formation of the quiescent gas-poor UDGs remaining a mystery.