The combined and respective roles of imaging and stellar kinematics in identifying galaxy merger remnants

Abstract

ABSTRACT One of the central challenges to establishing the role of mergers in galaxy evolution is the selection of pure and complete merger samples in observations. In particular, while large and reasonably pure interacting galaxy pair samples can be obtained with relative ease via spectroscopic criteria, automated selection of post-coalescence merger remnants is restricted to the physical characteristics of remnants alone. Furthermore, such selection has predominantly focused on imaging data – whereas kinematic data may offer a complimentary basis for identifying merger remnants. Therefore, we examine the theoretical utility of both the morphological and kinematic features of merger remnants in distinguishing galaxy merger remnants from other galaxies. Deep classification models are calibrated and evaluated using idealized synthetic images and line-of-sight stellar velocity maps of a heterogeneous population of galaxies and merger remnants from the TNG100 cosmological hydrodynamical simulation. We show that even idealized stellar kinematic data have limited utility compared to imaging and underperforms by $2.1 \pm 0.5 {{\ \rm per\ cent}}$ in completeness and $4.7 \pm 0.4 {{\ \rm per\ cent}}$ in purity for our fiducial model architecture. Combining imaging and stellar kinematics offers a small boost in completeness (by $1.8 \pm 0.4 {{\ \rm per\ cent}}$, compared to $92.7 \pm 0.2 {{\ \rm per\ cent}}$ from imaging alone) but no change in purity ($0.1\pm 0.3 {{\ \rm per\ cent}}$ improvement compared to $92.7 \pm 0.2 {{\ \rm per\ cent}}$, evaluated with equal numbers of merger remnant and non-remnant control galaxies). Classification accuracy of all models is particularly sensitive to physical companions at separations ≲ 40 kpc and to time-since-coalescence. Taken together, our results show that the stellar kinematic data have little to offer in compliment to imaging for merger remnant identification in a heterogeneous galaxy population.