Living the stream: Properties and progenitors of tidal shells and streams around galaxies from Magneticum

TL;DR

Using the Magneticum Pathfinder cosmological hydrodynamical simulations, this study analyzes tidal shells and streams around galaxies, linking their velocity dispersion, ages, and metallicities to the properties of their progenitors. The authors create mock observations to compare with real data, finding depressed velocity dispersion in shells and streams and that shells are typically more metal-rich while streams are generally younger. Shells form mainly from radial major mergers but can arise from minor mergers, whereas streams arise from minor mergers on circular orbits; widths of streams do not trace progenitor sizes, though progenitors follow the mass–metallicity relation. A novel in-situ class of young streams forms directly from host gas rings, triggered by encounters, illustrating the diverse origins of tidal features and enabling progenitor reconstruction from stellar populations.

Abstract

Stellar shells and streams are remnants of satellite galaxies visible around galaxies. Advances in low-surface-brightness observations and increasing resolution of cosmological simulations now allow investigating the properties and origin of these features. The metallicity, age, and velocity dispersion of shells and streams are investigated to infer their progenitor galaxies properties. We employed the hydrodynamical cosmological simulations Magneticum Pathfinder to extract these properties and identify the progenitors of the shells and streams. We compared to observational results from surveys and individual galaxies, matching and testing the methodology used in observations. Mock observations of shells and streams agree well with observational data regarding their morphology and spatial distribution. We find that both types of features are associated with localized depressions in stellar velocity dispersion compared to the surrounding regions. They are not as clearly distinct in metallicity and ages, though overall shells and more metal rich and streams are younger. We confirm results from idealized models that shells form commonly from radial major mergers but also through minor mergers, while streams usually form from minor mergers on circular orbits. We do not find the widths of streams to correlate with the half-mass radii of their progenitors, but the progenitors follow the mass-metallicity relation. On average, the masses measured for shells and streams approximately corresponds to 20% of the progenitor mass. We introduce a class of star-forming streams, which originate from in-situ star formation rather than the disruption of a satellite galaxy. Measuring stellar population properties of shells and streams provides the means to reconstruct the progenitor properties, and especially distinguish those streams that are not made through the disruption of a galaxy but formed in-situ.

Figures (18)

• Figure 1: Top row: Stellar mock surface brightness map, 2D-binned stellar surface density map, luminosity-weighted Voronoi-binned maps of velocity dispersion $\langle \mathrm{los}\sigma \rangle_\mathrm{lw}$, stellar age $\langle t_\star \rangle_\mathrm{lw}$ and stellar metallicity $\langle [Z/H] \rangle_\mathrm{lw}$. All are in the face-on projection and have an extent of $14r_{1/2} \times 14 r_{1/2}$. Here $r_{1/2}$ is the stellar half-mass radius. Overplotted are the contours of the slice and the radial range of the shell as cyan lines, and the shape ellipses at $1r_{1/2}$ and $3r_{1/2}$ in blue. Middle row: Radial velocity-radius phase space distribution of stellar particles colored by a Gaussian kernel density estimation, stellar surface density profile within the slice (blue solid line), for the global galaxy (magenta dash-dotted line), and a fit of a Sérsic profile to the global profile (black solid line). The remaining three panels show the radial profiles of the velocity dispersion $\mathrm{los}\sigma$, stellar age $\langle t_\star \rangle_\mathrm{lw}$, and metallicity $\langle [Z/H] \rangle_\mathrm{lw}$. In each panel, the black dots represent individual Voronoi-bins, and the blue line represents median values within radial bins, which each contain 50 Voronoi-bins, within the slice. The cyan contours stretch from the 0.32 to 0.67-quantile within these radial bins. The magenta dash-dotted line represents the global radial profile, calculated in the radial bins defined by the slice profile. The orange horizontal lines depict the extremum (minimum for $\langle \mathrm{los}\sigma \rangle$ and age, maximum for metallicity) of the median radial profile (blue line) within the radial range of the shell (cyan vertical lines) and the opposite extremum (maximum for $\langle \mathrm{los}\sigma \rangle$ and age, minimum for metallicity) within the radial ranges of width $w_\mathrm{shell}$ adjacent to the shell, together with their 0.32 and 0.67 quantiles. Bottom row: The difference between the profile of each property within the slice and the global profile (or a fit to it in the case of the surface density) is shown. (UID 13500)
• Figure 2: Overview of the shapes of shells (orange) and streams (blue). The black ellipses in the background represent the host galaxy's shapes at $1r_{1/2}$ and $3r_{1/2}$. The gray horizontal line is a scale bar of $15k\parsec$. All galaxies are rotated face-on. Inspired by sola+22 (their Fig. 7).
• Figure 3: Top row: Distribution of the mean radii $\langle r \rangle$ and widths $w$ of shells (orange dashed lines) and streams (blue solid lines). The magenta distribution in the left panel represents shell radii measured by sola+25. Also included are the shell radii of NGC 474 (gray) as measured by bilek+22 and shell radii of NGC 3923 (black) as measured by bilek+15. In the right panel, the widths (annotated) of a sample of streams (cyan solid line) and a sample of streams combined with tails (cyan dash-dotted line) measured by sola+25 is shown, as well as measurements of the widths (Gaussian fit) from residual images provided by the STRRINGS project sola+25b. Additionally, the widths (FWHM of a Gaussian fit; described in \ref{['subsec:streampy']}) of a sample of 9 streams (gray solid line) and a sample of the streams combined with 4 tails (gray dash-dotted line) as measured by pippert+25. Bottom row: Distributions of the mean angular position $\langle \theta \rangle$ (i.e., angle between the center of the feature and the major axis), angular extent $\langle \Delta \theta \rangle$ (i.e., opening angle of the shell-defining slice or the maximal pairwise angular separation of the points of the stream enclosing polygon), and the projected stellar mass $M_\star$ of shells and streams. The short vertical lines represent the respective medians of each distribution. A Gaussian kernel was used to estimate the density of each distribution, using the KernelDensity.jl packages from the https://juliastats.org project.
• Figure 4: Top row: The respective quantity within the surroundings (sur) of shells (orange circles) and streams (blue squares) as a function of the quantity within the feature (feat). The solid black line is the one-to-one relation. From left to right: The luminosity-weighted mean stellar line-of-sight velocity dispersion $\langle \mathrm{los}\sigma \rangle_\mathrm{lw}$, the mean stellar age $\langle t_\star \rangle_\mathrm{lw}$, and the mean stellar metallicity $\langle [Z/H] \rangle_\mathrm{lw}$. Bottom row: The ratio of the respective quantity within the feature and within its surroundings as a function of the quantity within the feature. The error bars indicate the $1\sigma$ scatter within each region.
• Figure 5: Leftmost panel: Stellar surface density map of an illustrative galaxy exhibiting shells (UID 13500). Second panel on the left: Stellar surface density map of the particles that used to belong to a common subhalo, that was identified to be the progenitor of the shells, traced forward to $z=0.07$. Right panels: Same as left panels but for an illustrative stream progenitor (UID 7902).