Tidal features as tracers of galaxy merger histories: the colours of tidal features in HSC-SSP

TL;DR

This study uses radial $g-i$ colour profiles for ~32,000 HSC-SSP galaxies, including 1415 with tidal features, to link tidal morphologies to merger histories. A rigorous automated pipeline—encompassing masking, background and scattered-light subtraction, PSF handling, and MGE-based size measurements—yields reliable colours out to $5R_e^{ m{maj}}$. The authors find that red-sequence galaxies with tidal features have redder outskirts beyond $2R_e^{ m{maj}}$, consistent with accretion from gas-poor minor mergers, while blue-cloud galaxies show weaker or mass-dependent effects; shells are the reddest feature class and typically the most massive, streams are less massive, and tails reflect material drawn from the host. These results align with observational and simulation-based studies, validating the approach and highlighting tidal-feature colours as a diagnostic of galaxy assembly histories across environments and mass scales.

Abstract

We measure the radial $g-i$ colour profiles of $\sim$32,000 galaxies drawn from the Hyper Suprime-Cam Subaru Strategic Program optical imaging survey, including 1415 exhibiting tidal features. We compare the colour profiles of galaxies with and without tidal features to extract information about the properties of the mergers that created these features. We find negative colour gradients for both galaxies with and without tidal features and find that tidal feature-hosting red sequence galaxies have redder outskirts than their non-tidal feature hosting counterparts, consistent with the outskirts of these galaxies being dominated by stars accreted from gas-poor minor mergers. We find decreasing mass ratios of tidal features-to-host galaxy with increasing galaxy stellar mass, suggesting that less massive galaxies undergo mergers with companions closer in mass than more massive galaxies. Galaxies exhibiting streams have bluer outskirts than those hosting shells, and shells around red sequence galaxies tend to be more massive and have higher mass ratios to their hosts than streams, consistent with streams being formed from mergers with satellites less massive than those responsible for shells. The agreement between our findings and those of other observational and simulation-based works confirms the validity of our methodology and highlights the value of tidal features colours as a probe into the process through which galaxies evolve.

Figures (12)

• Figure 1: Example of the masks generated using our cold and hot mask approach on a galaxy from our sample. Top left: the $gi$ coadded image of the galaxy. Top right: the cold mask before being expanded. Bottom left: the hot mask before being expanded. Bottom right: the final mask with the central galaxy unmasked, after expanding the cold and hot masks by convolving them with Gaussian kernels.
• Figure 2: Example of our scattered light subtraction process for a galaxy with a nearby star. The top and bottom rows show the same process in $g-$ and $i-$band, respectively. The left panels show the $256\times256$ pixel cutout of the galaxy, and the middle-left panels show a 1000 pixel$^2$ cutout (cropped from the original 3400 pixel$^2$ cutout for visibility) centred on the star to be subtracted. The middle-right panels show the profile of the final scaled PSF (dashed lines) compared to the profile of the star (solid lines). The grey shaded region in these panels show the radial range used when calculating the PSF scale factor, and the dashed vertical lines show the distance of the galaxy relative to the star. The right panels show the final $256\times256$ pixel cutout once the scattered light from the star has been subtracted.
• Figure 3: Illustration of the method used to measure galaxy size and colour. Left column: arcsinh stretched $gri$ false colour image showing examples of each type of tidal feature in the Desmons2025HSC_TF classification scheme. From top to bottom: shell, stream, tail, plume, and double nucleus. Middle column: MGE fit (red contours) to the data (black contours), with the yellow showing masked regions. The fits are performed on $gi$ coadded images of the galaxies in the left column. Right column: masked grey-scale $gi$ coadded images and the apertures at $1R_e^{\rm{maj}}$ (solid red) and $2, 3,4,\mathrm{and}~5R_e^{\rm{maj}}$ (dashed red) used to measure the colour profiles of the galaxies. The apertures in this column are based on the average ellipticity and position angle of the galaxy, as given by find_galaxy.
• Figure 4: The first two panels show the distribution of stellar mass and photometric redshift for the parent sample of 34,331 galaxies (grey) and galaxies with reliable size and colour measurements out to $5R_e^{\rm{maj}}$ (blue). The last two panels show the distribution of measured $g-i$ colour within $1R_e^{\rm{maj}}$ and effective radius for galaxies with reliable size and colour measurements out to $5R_e^{\rm{maj}}$.
• Figure 5: Left: distribution of $(g-i)_{R_{e}^{\mathrm{maj}}}$ colours in our sample, with the dashed line at $(g-i)_{R_{e}^{\mathrm{maj}}}=0.975$ showing our preliminary threshold between red and blue galaxies. Right: the red sequence and blue cloud populations in our sample, with the dashed line showing our final red vs. blue cut-off as defined by Equation \ref{['eq:col_cut']}.