Pre-existing dwarfs, tidal knots and a tidal dwarf galaxy: an unbiased HI study of the gas-rich interacting galaxy group NGC 3166/9

TL;DR

This study uses ALFALFA and GMRT HI observations to identify and analyze gas-rich dwarf companions and a potential tidal dwarf galaxy in the NGC 3166/9 group, providing insights into their origins and properties.

Contribution

It presents one of the first unbiased physical studies of gas-rich dwarf companions and tidal dwarf galaxy candidates in a nearby galaxy group using combined HI data.

Findings

Identified eight low-mass HI-rich objects in the group.

Detected a candidate tidal dwarf galaxy, AGC 208457.

Found that ~60% of HI is smoothly distributed on large scales.

Abstract

We present Arecibo Legacy Fast ALFA (ALFALFA) and follow-up Giant Metrewave Radio Telescope (GMRT) HI observations of the gas-rich interacting group NGC 3166/9. The sensitive ALFALFA data provide a complete census of HI-bearing systems in the group while the high-resolution GMRT data elucidate their origin, enabling one of the first unbiased physical studies of gas-rich dwarf companions and the subsequent identification of second generation, tidal dwarf galaxies in a nearby group. The ALFALFA maps reveal an extended HI envelope around the NGC 3166/9 group core, which we mosaic at higher resolution using six GMRT pointings spanning ~1 square degree. A thorough search of the GMRT datacube reveals eight low-mass objects with gas masses ranging from 4x10^7 to 3x10^8 M_sol and total dynamical masses up to 1.4x10^9 M_sol. A comparison of the HI fluxes measured from the GMRT data to those measured in the ALFALFA data suggests that a significant fraction (~60%) of the HI is smoothly distributed on scales greater than an arcminute (~7 kpc at the NGC 3166/9 distance). We compute stellar masses and star formation rates for the eight low-mass GMRT detections, using ancillary SDSS and GALEX data, and use these values to constrain their origin. Most of the detections are likely to be either pre-existing dwarf irregular galaxies or short-lived, tidally formed knots; however, one candidate, AGC 208457, is clearly associated with a tidal tail extending below NGC 3166, exhibits a dynamical to gas mass ratio close to unity and has a stellar content and star formation rate that are broadly consistent with both simulated as well as candidate tidal dwarf galaxies from the literature. Our observations therefore strongly suggest that AGC 208457 is a tidal dwarf galaxy.