Kinematics of dwarf galaxies in gas-rich groups, and the survival and detectability of tidal dwarf galaxies

TL;DR

This study investigates the kinematics of dwarf galaxies in gas-rich groups, highlighting the challenges in detecting tidal dwarf galaxies due to their faint signals and complex rotation curves.

Contribution

It provides the first detailed kinematic analysis of dwarf galaxies in gas-rich groups and assesses the detectability of tidal dwarf galaxies using rotation curves.

Findings

Two galaxies identified as strong tidal dwarf galaxy candidates.

Falling rotation curves are difficult to detect in tidal dwarfs due to observational limitations.

Only 14% of the sample had detectable Hα emission suitable for kinematic analysis.

Abstract

We present DEIMOS multi-object spectroscopy (MOS) of 22 star-forming dwarf galaxies located in four gas-rich groups, including six newly-discovered dwarfs. Two of the galaxies are strong tidal dwarf galaxy (TDG) candidates based on our luminosity-metallicity relation definition. We model the rotation curves of these galaxies. Our sample shows low mass-to-light ratios (M/L=0.73$\pm0.39M_\odot/L_\odot$) as expected for young, star-forming dwarfs. One of the galaxies in our sample has an apparently strongly-falling rotation curve, reaching zero rotational velocity outside the turnover radius of $r_{turn}=1.2r_e$. This may be 1) a polar ring galaxy, with a tilted bar within a face-on disk; 2) a kinematic warp. These scenarios are indistinguishable with our current data due to limitations of slit alignment inherent to MOS-mode observations. We consider whether TDGs can be detected based on their tidal radius, beyond which tidal stripping removes kinematic tracers such as H$\alpha$ emission. When the tidal radius is less than about twice the turnover radius, the expected falling rotation curve cannot be reliably measured. This is problematic for as much as half of our sample, and indeed more generally, galaxies in groups like these. Further to this, the H$\alpha$ light that remains must be sufficiently bright to be detected; this is only the case for three (14%) galaxies in our sample. We conclude that the falling rotation curves expected of tidal dwarf galaxies are intrinsically difficult to detect.