Investigating the imprints of tidal features on simulated galaxy outskirts in LSST-like mock observations

TL;DR

This work forwards a direct, observation-forward approach by generating LSST-like mock images from four cosmological hydrodynamical simulations to study tidal features around galaxies at $z\sim0$ (NewHorizon near $z\sim0.2$). It couples automated masking and Multi-Gaussian Expansion modelling with radial, elliptical-aperture colour measurements to compare tidal-feature hosts against non-tidal galaxies, revealing that tidal features are more common in blue, star-forming systems and may imprint subtle colour offsets in outskirts. Across simulations, results differ due to subgrid physics implementations, suggesting LSST data will be essential to constrain merger-driven growth and feedback models. The study connects tidal-feature demographics, colour gradients, and sSFR to recent accretion histories, providing testable predictions for LSST to validate or revise current galaxy-formation models.

Abstract

Tidal features provide signatures of recent galaxy mergers, offering insights into the role of mergers in galaxy evolution. The Vera C. Rubin Observatory's upcoming Legacy Survey of Space and Time (LSST) will allow for an unprecedented study of tidal features around millions of galaxies. We use mock images of galaxies at $z\sim0$ ($z\sim0.2$ for \textsc{NewHorizon}) from \textsc{NewHorizon}, \textsc{eagle}, \textsc{IllustrisTNG}, and \textsc{Magneticum Pathfinder} simulations to predict the properties of tidal features in LSST-like images. We find that tidal features are more prevalent around blue galaxies with intrinsic colours $(g-i)\leq0.5$, compared to redder ones, at fixed stellar mass. This trend correlates with elevated specific star formation rates ($\mathrm{sSFR}>10^{-10}\mathrm{\:yr}^{-1}$), suggesting that merger-induced star formation contributes to the bluer colours. Tidal feature hosts in the red sequence appear to exhibit colour profiles offset to bluer colours for galaxies with stellar masses $10^{10}<M_{\star\mathrm{,\:30\:pkpc}}/\mathrm{M}_\odot<10^{11}$, similarly blue cloud tidal feature host galaxies appear to have their colour profiles offset to bluer colours for $10^{9.5}<M_{\star\mathrm{,\:30\:pkpc}}/\mathrm{M}_\odot<10^{10.5}$. However, the differences in colour profiles in either the red sequence or the blue cloud are not statistically robust and larger samples are needed to test if these differences are real. The predictions across the simulations are quantitatively distinct; therefore, LSST observations will allow us to further constrain the differences between different subgrid physics models.

Figures (22)

• Figure 1: Illustration of the process we followed for measuring galaxy size and colour. The middle column shows the MGE fit (red contours) to the data (black contours), and the yellow shows the masked regions. The right-hand column shows a masked grey-scale gri coadded image with the apertures used to measure our colours and colour profiles overplotted. The left-hand column shows the gri colour images that were used in the process of classification, the white rectangle shows the box used for the final colour measurements. All the galaxies in our sample are placed at $\sim109\mathrm{\:cMpc}$ away from the observer. From top to bottom, we illustrate fits on a galaxy without any tidal features, a galaxy hosting a stream/tail, a galaxy hosting shells, a galaxy hosting an asymmetric halo and a galaxy hosting a double nucleus.
• Figure 2: The size-stellar mass distribution for galaxies in each simulation. The full sample of galaxies for which we have reliable measurements out to 1 $R_{e}^\mathrm{maj}$ is indicated here. The sizes are given in proper kpc, and the stellar masses are provided in $\log_{10}(M_{\star\text{, 30 pkpc}}/\mathrm{M}_\odot)$. We see that qualitatively the distributions are similar, with the distribution for Magneticum having less scatter than EAGLE or TNG. The black dashed line is the median size-stellar mass relation for each simulation, binned in $\log_{10}(M_{\star\mathrm{,\:30\:pkpc}}/$M$_{\odot})$ with 0.5 dex wide bins ranging from 9.5 to 12, plotted only for bins with at least 10 galaxies. The blue and red dashed lines are empirical fit relations from the HSC-SSP survey kawinwanichakijHyperSuprimeCamSubaru2021 for star-forming and quiescent galaxies, respectively. Our size-mass relations are in reasonable agreement with those from HSC-SSP.
• Figure 3: (Top panel) The stellar mass distributions for our samples from each simulation, measured in $\log_{10}(M_{\star\text{, 30 pkpc}}/\mathrm{M}_\odot)$. (Bottom panel) The halo mass distribution for our samples from each simulation, measured in $\log_{10}(M_{\text{200, crit}}/\mathrm{M}_\odot)$. We display the distributions in terms of the fraction of the total parent catalogue in each bin, i.e. counts in bin/total parent. We show the parent distribution of all galaxies in the tidal feature catalogue in grey. The solid lines show the distributions for the sample where our measurements of galaxy colour within $1$$R_e^{\mathrm{maj}}$ meet the minimum size ($\geq10$ pixels) and maximum masking within aperture (50 per cent) criteria, and the dashed lines show the distribution for the sample where the same criteria have been met out to the aperture between $4$ and $5$$R_e^{\mathrm{maj}}$. There are no significant systematic differences between any of the distributions.
• Figure 4: (Top panel) The $(g-i)_{R_e^\mathrm{maj}}$ colour and spherical aperture stellar mass contours for our simulations in green and the Galactic extinction corrected model $g-i$ colour and stellar masses from SDSS observations of galaxies at $z\leq0.035$ in orange, the lines show the 5 per cent, 20 per cent, 40 per cent, 60 per cent, 80 per cent and 95 per cent contours. We also show the galaxies with $M_{\star\mathrm{,\:30\:pkpc}}\geq10^{10.5}\:\mathrm{M}_\odot$ and $(g-i)_{R_e^\mathrm{maj}}<0.5$ as light blue points, as while this region lies outside the $95\%$ contours, these points do contribute to the tidal feature fractions shown. The solid grey line shows the linear fit to the red sequence for simulated galaxies, and the dashed grey lines show the selection of red sequence galaxies ($\pm 3\times$ the median distance of all non-blue cloud galaxies from the red sequence fit). Each simulation resolves a red sequence and a blue cloud of galaxies. There are quantitative differences in the shape and scatter of the relations across simulations and when compared to observations. The simulated blue clouds all scatter to bluer colours relative to observations, and the slopes of the red sequence in all simulations are all flatter than the slope for SDSS. (Bottom panel) Tidal feature fractions as a function of stellar mass for 4 stellar mass bins (bin edges $\log_{10}(M_{\star\mathrm{,\:30\:pkpc}}/$M$_{\odot})=[9.5,\:10,\:10.5,\:11,\:12]$) and 3 colour bins (bin edges $(g-i)_{R_e^\mathrm{maj}}=[-0.02,\: 0.5,\: 1,\: 1.3]$), the blue points correspond to the bluest bin, the light pink points correspond to the middle colour bin and the red points correspond to the reddest bin. The error bars on the tidal feature fractions indicate the $1\sigma$ binomial uncertainties. The error bars in the stellar mass axis show the sizes of the bins. Tidal feature fractions increase with increasing stellar mass at all colours. The red sequence galaxies generally have lower tidal feature fractions than the blue cloud at all stellar masses.
• Figure 5: The tidal feature fraction as a function of spherical aperture stellar mass binned by colour for each simulation and tidal feature morphology. Top to bottom, the panels show fractions for: streams/tails, shells, asymmetric halos and double nuclei. The binning is identical to Fig. \ref{['fig:col_mag_total']} and illustrated in the x-error bars. The y-error bars illustrate the $1\sigma$ binomial uncertainties on the tidal feature fractions. The trend for bluer galaxies to have a greater tidal feature fraction seen in Fig. \ref{['fig:col_mag_total']} is driven primarily by the asymmetric halos.