Stellar Stream Legacy Survey (SSLS)

• SSLS is a systematic imaging program that surveys faint tidal debris (streams, shells, halos) around thousands of galaxies to trace merger history and dark matter structure.
• It employs multi-telescope observations and advanced processing techniques to reach unprecedented surface brightness limits, enabling detailed morphological classification.
• SSLS findings empirically test ΛCDM predictions by comparing minor merger rates and stellar halo properties with state-of-the-art cosmological simulations.

The Stellar Stream Legacy Survey (SSLS) is a comprehensive, systematic imaging program designed to carry out the first statistical census of faint stellar tidal debris—including streams, shells, and low-surface-brightness (LSB) halos—surrounding several thousand galaxies in the local Universe, with projected distances extending to approximately 100 Mpc. Its primary objective is to empirically constrain minor merger-driven galaxy formation—an essential prediction of the ΛCDM cosmological paradigm—and to probe the distribution and properties of dark matter in galactic halos by mapping the fossil record of satellite accretion across a wide dynamical range of host stellar mass, from Milky Way-mass systems to the dwarf galaxy regime.

1. Scientific Rationale and Survey Objectives

The hierarchical ΛCDM framework predicts that present-day galaxies grow in part by accreting lower mass satellites, leaving behind diffuse stellar streams that encode detailed information about accretion frequency, event timing, orbital parameters, progenitor masses, and the potential granularity of the dark-matter halo (Miro-Carretero et al., 30 Jul 2024, Martinez-Delgado et al., 2021, Sakowska et al., 28 Nov 2025). The SSLS aims to systematically quantify:

• The incidence (fractional occurrence) of observable streams and debris across several thousand galaxies.
• The morphological diversity of tidal features (including great-circle wraps, umbrellas, shells, plumes, and asymmetric halos).
• The photometric properties (surface brightness, colors, structural parameters) of detected streams and debris.
• The dependence of accretion debris observability on host mass, environment, and redshift.
• Direct comparison of these empirical distributions to the outputs of state-of-the-art hydrodynamical and semi-analytical cosmological simulations (e.g., Copernicus Complexio, TNG50, Auriga) to test the validity of ΛCDM predictions for minor merger rates, stream morphology, and detectability (Miro-Carretero et al., 5 Sep 2024).

By moving beyond Local Group studies and compiling a uniform census out to D ≈ 100 Mpc, SSLS seeks to transform constraints on galaxy assembly, stellar-halo growth, the survival and destruction of satellites, and the prevalence of dark-matter substructure.

2. Survey Scope, Dataset Selection, and Depth

SSLS operates by exploiting the DESI Legacy Imaging Surveys—encompassing DECaLS, BASS, and MzLS—alongside public Dark Energy Survey (DES) deep imaging. The footprint covers ~14,000–20,000 deg² in g, r, and z bands, with a catalog parent sample constructed from HyperLeda, SDSS, and NED. Selection criteria for the massive-galaxy component include:

• K-band absolute magnitude $M_K < -19.6$ (implying $M_* \gtrsim 10^{10}\ M_\odot$);
• $2000 < V_\mathrm{LG} < 7000$ km s$^{-1}$ (corresponding to $30 \lesssim D \lesssim 100$ Mpc for $H_0 = 73$ km s$^{-1}$ Mpc$^{-1}$);
• High Galactic latitudes $(|b| > 20^\circ)$ and isolation criteria suppressing confusion with nearby massive neighbors.

This filtering yields $\sim$52,600 galaxies across the sky, with $\sim$3,200 within the Legacy footprint, encompassing $\sim$800 Milky Way analogues (Miro-Carretero et al., 30 Jul 2024, Martinez-Delgado et al., 2021).

For the low-mass/dwarf regime ($M_* < 10^{9.5}\ M_\odot$), selection uses the 50 Mpc Galaxy Catalog, with added constraints on isolation ($D_\mathrm{proj} > 400$ kpc and $|v_\mathrm{sep}| > 250$ km s$^{-1}$ from any $M_* \geq 10^{10}\ M_\odot$ neighbor) and the removal of strongly interacting pairs (Sakowska et al., 28 Nov 2025).

The typical $3\sigma$ surface-brightness limit with a 100 arcsec² aperture is $\mu_r \approx 28.8$ mag arcsec$^{-2}$ and $\mu_g \approx 29.2$ mag arcsec$^{-2}$ in the DES pilot, with planned co-adds targeting $\mu_r \gtrsim 29.5$, $\mu_g \gtrsim 30.0$ mag arcsec$^{-2}$, exceeding previous public-imaging surveys (Miro-Carretero et al., 30 Jul 2024). The "Upper Limit Surface Brightness" (ULSB) achieved in practice is typically 0.6–0.8 mag brighter due to correlated noise structure in co-added frames.

3. Data Acquisition, Calibration, and Detection Pipeline

SSLS images are primarily acquired using the Blanco 4m/DECam (South), Mayall 4m, and Bok 2.3m (North) telescopes (Miro-Carretero et al., 30 Jul 2024). The data processing pipeline includes:

• Calibration with Legacypipe to produce flat-fielded, astrometrically registered images.
• Custom background equalization and sky subtraction using a combination of global and local procedures (sigma-clipped median subtraction of image outskirts after masking contaminating sources—stars from Gaia and others).
• Artifact rejection via masking of transient features and careful median stacking onto a common tangent plane.

Stream detection is two-step: expert visual inspection of composite $g+r+z$ images, supported by algorithmic pre-selection using Gnuastro NoiseChisel to flag regions of elevated local background variance along coherent LSB ridges. The Detection Significance Index (DSI)—defined as the candidate’s flux relative to $N=10,000$ random blank-sky apertures—is required to reach %%%%27$^{-1}$28%%%% for candidate selection (Miro-Carretero et al., 30 Jul 2024, Miro-Carretero et al., 5 Sep 2024, Martinez-Delgado et al., 2021). Aperture sizes are chosen to match the projected stream width at the host distance.

4. Empirical Results and Statistical Characterization

Pilot Results for Massive Hosts

• Out of 689 DES pilot galaxies, 63 streams are confirmed ($f_\mathrm{detect} = 9.1\% \pm 1.1\%$), in agreement with simulated $\Lambda$CDM minor-merger frequencies for Milky Way-mass galaxies (Miro-Carretero et al., 30 Jul 2024, Miro-Carretero et al., 5 Sep 2024).
• Surface-brightness measurements for detected streams:
  • $\langle \mu_g \rangle = 28.35 \pm 0.20$,
  • $\langle \mu_r \rangle = 27.81 \pm 0.13$,
  • $\langle \mu_z \rangle = 27.62 \pm 0.09$ mag arcsec$^{-2}$,

following $\mu = m + 2.5\log_{10}(A)$ for aperture area $A$ (arcsec²).

• Most streams reside at distances $40 \lesssim D \lesssim 100$ Mpc, with only 5–14% exhibiting plausible progenitor candidates. Morphological classification encompasses great circles, umbrellas/shells, plumes, and mixed/ambiguous cases (Miro-Carretero et al., 5 Sep 2024).
• The brightest streams are detected 1.1 mag above the noise-defined pixel limit, reflecting the role of correlated noise and the requirement $\Delta \mu < 0.1$ mag arcsec$^{-2}$ for robust photometry (Miro-Carretero et al., 30 Jul 2024).

Dwarf-Galaxy Regime

• In a DESI-LS sample of 730 dwarfs ($M_* < 10^{9.5}\ M_\odot$, $4 < D < 35$ Mpc), only $\leq 5.07\%$ show any accretion feature (all shells or asymmetric halos; no streams in DES footprint), underscoring the SB-limited detectability of minor mergers at low mass and potential enhancement of radial, shell-like debris in this regime (Sakowska et al., 28 Nov 2025).
• Morphological classification for dwarfs is tri-modal: streams (narrow, linear), shells (arc/ripple-like), and asymmetric stellar halos (diffuse, one-sided). Sixteen of twenty accretion systems detected in the DES+DECaLS pilot are new discoveries.

The low detection efficiency for dwarfs is likely a combination of genuinely suppressed minor-merger rates in low-mass systems and selection effects, such as streams being typically fainter (due to steep mass ratio dependencies) and projection effects blending filaments into shells or irregular halos.

5. Comparison with Cosmological Simulations

SSLS observations are directly compared to mock images constructed from leading cosmological simulations:

• Copernicus Complexio (COCO) employs semi-analytic/particle-tagged models: dominant shell morphologies at given surface-brightness limits, with a rapid rise in detection fraction with SB limit ($\sim$12.5%/mag from $\mu=26$–34, saturating at $f_\mathrm{det} \approx 97\%$ at $\mu_\mathrm{lim}=34$).
• TNG50 (IllustrisTNG project) and Auriga (zoom-ins): hydrodynamical simulations yielding morphological mixes closer to observations, though with some underproduction of great-circle wraps compared to shells. For DECam-like SB limits ($\mu_r \sim 28.6$), observed detection rates ($f_\mathrm{det} \approx 9\%$) match Auriga/TNG50 expectations (Miro-Carretero et al., 5 Sep 2024).
• The surface-brightness threshold for detecting half of streams in simulations is $\mu_{r,\mathrm{lim}}\sim31$, with near-completeness above $\mu_{r,\mathrm{lim}}\sim34$ mag arcsec$^{-2}$.
• Morphological distribution reveals differences between models and observed streams (e.g., overproduction of shells in COCO vs. more loops/circular wraps in SSLS), suggesting possible baryonic dynamical effects (Miro-Carretero et al., 5 Sep 2024).

For robust comparison at the low-mass end, a dedicated suite of $N$-body+hydrodynamical dwarf–dwarf merger simulations, with mock-observed stacks spanning mass ratios and orbital configurations, is needed (Sakowska et al., 28 Nov 2025).

6. Survey Limitations, Sensitivity, and Future Prospects

Current SSLS completeness is set by both surface-brightness limits and the effects of correlated noise, with ULSB lying $0.6$–$0.8$ mag above the extrapolated pixel noise. No artificial stream injection studies are yet available to calibrate completeness variations as a function of stream width, host mass, or distance, though such work is in progress (Miro-Carretero et al., 30 Jul 2024).

Planned improvements include:

• Deeper imaging via full coaddition of Southern and Northern Legacy Surveys, increasing depth by $0.5$–$0.7$ mag ($\mu_r \gtrsim 29.5$, $\mu_g \gtrsim 30.0$).
• Enhanced noise characterization and sky-subtraction, targeting a narrowing of ULSB variance to $\lesssim 0.3$ mag.
• Automation of stream detection using machine-learning-based classifiers (e.g., STREAMFINDER adaptations) calibrated on the DES pilot, facilitating objective recovery and survey scalability (Miro-Carretero et al., 30 Jul 2024).
• Expansion to the full $\sim$3200 galaxy sample, predicting $\sim$300–400 stream systems, which increases statistical power by a factor $\sim$5 over pilot regions.
• For dwarfs, improved classification metrics and modeling, along with theoretical development of mock-observed merger morphologies, are necessary for mapping the observed debris census onto ΛCDM predictions (Sakowska et al., 28 Nov 2025).

The full SSLS is positioned to deliver the first robust, statistically meaningful tests of ΛCDM galaxy assembly predictions—including the minor- and major-merger rates, the spectrum of dark-matter substructure, and the role of mergers in shaping outer galaxy halos—across a wide swath of parameter space not accessible to Local Group-only studies.

7. Scientific Implications and Outlook

The mutual agreement between SSLS stream incidence and ΛCDM simulation predictions at Milky Way-scale masses corroborates hierarchical assembly as the driver of stellar halo growth (Miro-Carretero et al., 5 Sep 2024, Miro-Carretero et al., 30 Jul 2024). The color and photometric distributions of detected streams yield constraints on the star-formation histories, quenching timescales, and infall epochs of progenitors. Morphological statistics, particularly the fraction and properties of shells vs. loops, encode dynamical information on progenitor orbits and host-halo potentials.

Stream width and density structure offer a direct probe of halo granularity and the presence of dark matter subhalos—a crucial input for validating cold, warm, or self-interacting dark-matter scenarios. In the dwarf regime, the rarity of visible debris may constrain merger (and hence satellite disruption) rates and the properties of low-mass dark matter halos, pending advances in both detection depth and theory (Sakowska et al., 28 Nov 2025).

By systematically extending deep, LSB imaging across a full order of magnitude in galaxy mass, and by integrating empirical results with mock observations from cutting-edge cosmological simulations, the SSLS provides a new, quantitative foundation for exploration of minor mergers, halo assembly, and dark matter in the present-day Universe (Miro-Carretero et al., 30 Jul 2024, Miro-Carretero et al., 5 Sep 2024, Sakowska et al., 28 Nov 2025, Martinez-Delgado et al., 2021).