Spectro-photometric analysis of galaxy merger remnants with tidal features

Abstract

We use 21 208 galaxies at redshifts 0.05 < z < 0.45 with images from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) and accompanying spectroscopic observations from the Sloan Digital Sky Survey (SDSS) to study the properties of systems that host features of past mergers: streams and shells. We detect 251 shells and 774 streams through visual classification of the original and filtered galaxy images. We retrieve measurements of stellar mass and stellar population age for 16 936 galaxies from our sample. Galaxies with shells are more massive and older than both featureless galaxies and stream hosts, suggesting that they may be a result of major mergers. We analyse the star formation rates (SFR) for 6738 of our galaxies, derived from reliable measurements of H&alpha; available from the SDSS Legacy Survey. Shell hosts are predominantly among quiescent galaxies while stream hosts span the full range of SFR measurements. We construct diagnostic diagrams for 6778 galaxies to separate star forming galaxies from those that are dominated by Active Galactic Nuclei (AGN). A fraction (10%) of galaxies with streams show evidence of strong AGN activity, which may be a result of material being funnelled into the central region of the host galaxy. In contrast, almost no shell hosts show the presence of strong nuclear activity. We measure the local density in the vicinity of our tidal feature hosts using data from the New York University Value-Added Galaxy Catalog (NYU-VAGC). We find that galaxies with features are, as expected, neither in isolated regions nor in clusters since these environments suppress mergers. We also observe that galaxies with shells tend to be in regions with slightly higher densities than the general population of galaxies. Based on sheer numbers and the distribution of the basic properties of the galaxies, we find that streams can occur in a vast range of galaxy mergers, while the formation of shells needs specific sets of conditions.