The Search for Tidal Structures around Nearby Large Galaxies

In the hierarchical model of galaxy formation and evolution, interactions and mergers between galaxies play a pivotal role in their mass growth. These processes lead to the formation of low surface brightness tidal structures around galaxies, which are inherently faint. To capture these structures effectively, a combination of long telescope exposure time and specific image processing techniques is necessary. To search for tidal structures around nearby galaxies, the authors selected 76 large galaxy samples from catalogs in the literature and performed image coaddition using g-band and r-band data from ZTF (the Zwicky Transient Facility) time-domain survey. They achieved an average depth (surface brightness limit) of \mu_\mathrmg(3\sigma,10^''\times10^'')=\left(28.86\pm0.27\right)~\mathrmmag/(^'')^2 , \mu_\mathrmr(3\sigma,10^''\times10^'')= \left(28.65\pm0.27\right)~\mathrmmag/(^'')^2 (\sigma is the standard deviation of sky background noise) for the deep images, comparable to the depth of most international deep imaging surveys. Among the 76 sample galaxies, tidal structures were known to exist around 22 galaxies, and the authors successfully detected the structures around 20 galaxies using their image processing methods. What's more, they first discovered tidal structures around one of the sample galaxies. They specifically analysed the morphology of the tidal stream around NGC 5907. The comparison with previous studies well proved the existence of a single circular stream structure. Secondly, they analysed the three newly discovered faint tidal structures around NGC 4725. The structure in the northeast exhibits a blue color (the magnitude difference g-r\sim 0.4−0.6), similar to the companion galaxy NGC 4747 in the same direction, likely resulting from their interaction. The two structures on the east and southeast sides correspond to an overall ring structure both in location and morphology, displaying a reddish color (g-r \sim 0.7−0.9) akin to the eastern outer halo of NGC 4725. These structures may originate from the tidal disruption of an old dwarf spheroid galaxy or the torn-off disk components of NGC 4725 during interaction with NGC 4747. Future work will necessitate further verification of the tidal structures using higher-resolution and deeper WFST (Wide Field Survey Telescope) images, along with data in other wavebands. Besides, numerical simulations will help gain a better understanding of galaxy interaction processes. Finally, in some sample galaxy images, they identified diffuse foreground Galactic cirrus contamination, characterized by rather low surface brightness (fainter than \mu_\mathrmg = 26~\rm mag/(^\prime\prime)^2) and a bluer color (g-r \sim 0.41) compared to extragalactic sources, facilitating their differentiation. The results show that the light reflected by these cirrus affects the detection and analysis of faint tidal structures around galaxies, but is easy to identify.