---
title: 'STRRINGS: Stellar Streams in Nearby Galaxies'
url: https://www.emergentmind.com/topics/strrings
type: topic
---

# STRRINGS: Stellar Streams in Nearby Galaxies

STRRINGS, short for **STReams in Residual Images of Nearby GalaxieS**, is a residual-image-based survey and stream catalogue built to identify nearby extragalactic stellar streams that are suitable for dynamical modelling. It combines a large visual census of tidal features with a modelling-oriented first release of **35** stream systems selected from **19,387** nearby galaxies in the **Siena Galaxy Atlas 2020** with **\(z \leq 0.02\)**, using original, model, and residual images from the **DESI Legacy Imaging Surveys**. Its stated purpose is not only to inventory tidal debris, but to assemble a sample of **long, narrow streams** that can be compared with simulations to obtain **population-level constraints on halo properties**, especially dark-halo flattening [2508.02154].

## 1. Scope, scientific context, and definition

STRRINGS is explicitly presented as both a **large visual tidal-feature census** of nearby galaxies and the first **released subsample of 35 stellar-stream systems** chosen to be suitable for modelling [2508.02154]. The project is framed within hierarchical galaxy formation in \(\Lambda\)CDM, where mergers and accretion generate distinct tidal structures including **tails**, **streams**, **shells**, **plumes**, and disturbed stellar haloes.

Within that taxonomy, stellar streams are singled out because they arise from tidally disrupted satellites in minor mergers and can trace the progenitor orbit within the host potential. The paper emphasizes that **long, curved streams can constrain the host potential geometrically**, and that populations of many streams can constrain halo properties statistically. It further states that future work aims to infer dark-halo flattening from a sample of order **\(>50\)** streams [2508.02154].

A central novelty is the use of **residual images** from DESI-LS. These are generated by subtracting source models from original survey images, leaving low-surface-brightness asymmetries such as streams more visible. The project therefore targets not merely the detection of tidal features in general, but the construction of a **modelling-ready** stream sample [2508.02154].

## 2. Data basis and parent-galaxy sample

The underlying imaging comes from the **DESI Legacy Imaging Surveys**, specifically **DECaLS**, **BASS**, and **MzLS**, covering about **\(14{,}000\ \mathrm{deg}^2\)** in **\(g,r,z\)** with approximate \(5\sigma\) depths for faint galaxies of **\(g = 24.0\)**, **\(r = 23.4\)**, and **\(z = 22.5\)** AB magnitudes. DESI-LS source modelling uses **The Tractor** through **legacypipe**, and for every image provides the original image, the model image, and the residual image [2508.02154].

Residuals are central because standard source-modelling algorithms are optimized for compact or high-surface-brightness components, whereas diffuse tidal features are often not well modelled and remain in the subtraction products. The paper also notes that segmentation is easier on residual images because the stream is less contaminated by bright host-galaxy light and nearby sources [2508.02154].

Residual-image surface-brightness limits were estimated using **\(3\sigma\), \(10\arcsec \times 10\arcsec\)** boxes placed randomly on masked images, yielding **\(28.5\ \mathrm{mag\ arcsec^{-2}}\)** in \(r\), **\(29.0\ \mathrm{mag\ arcsec^{-2}}\)** in \(g\), and **\(27.4\ \mathrm{mag\ arcsec^{-2}}\)** in \(z\) [2508.02154].

The parent catalogue is the **Siena Galaxy Atlas 2020 (SGA-2020)**. After applying the redshift cut **\(z \leq 0.02\)**, the sample contains **20,077 individual galaxies**. Combining galaxies in close SGA-defined groups into single objects yields **19,387 objects**, which form the visually inspected parent sample [2508.02154]. The paper states that the sample is **23.4%** early-type galaxies, **64.9%** late-type galaxies, and **11.7%** of unknown morphology; **92.3%** are isolated [2508.02154].

Host stellar masses are estimated from Tractor \(g\) and \(r\) fluxes using the Bell et al. prescription
\[
\gamma = 10^{a + (g-r)\times b}
\]
with **\(a=-0.306\)** and **\(b=1.097\)**. The host-galaxy stellar-mass distribution has median **\(10^{9.7}\ M_\odot\)** and 16–84% quantiles **\(10^{8.7}\)–\(10^{10.7}\ M_\odot\)** [2508.02154].

## 3. Visual classification and catalogue construction

The first-stage inspection used pre-made **PNG montages** from the SGA site containing the original \(grz\) colour image, model image, and residual image. The field of view was **3 times the galaxy size**, based on **D25**. For inspection, the residuals were converted to black-and-white and clipped to the **35th–90th percentiles** of the pixel distribution to enhance faint structure [2508.02154].

The visual classification used mutually exclusive categories: **No feature**, **Streams**, **Tails/Arms**, **Plumes**, **Shells**, and **Disturbed/asymmetric stellar halo**, with optional **Grand-design spiral** and **Curiosity** flags [2508.02154]. Definitions were morphological in projection.

A total of **nine classifiers** contributed. In the first pass, **two classifiers** inspected all **19,387** objects and flagged **5,120** as showing a feature according to at least one of the two. Those candidates were then classified by the other **seven classifiers** as well [2508.02154].

The voting system first decided whether a galaxy had any feature or no feature; in ties, the galaxy was counted as having a feature. For feature-class ties, the priority order was
\[
\text{Streams} \;>\; \text{Tails} \;>\; \text{Shells} \;>\; \text{Plumes} \;>\; \text{Disturbed halo} \;>\; \text{Grand-design spiral} \;>\; \text{Curiosity},
\]
which the paper explicitly notes **biases results toward streams** [2508.02154]. Confidence was defined as
\[
\text{score} = \frac{\text{number of votes for majority feature}}{\text{total number of votes}}.
\]
No hard score cut was applied in the main catalogue. For stream classifications, **30.9%** have score **\(>0.5\)** and **6.9%** have score **\(>0.75\)**, indicating substantial classifier disagreement [2508.02154].

## 4. Tidal-feature census results

The main census result is that **\(11.9 \pm 0.2\%\)** of the **19,387** inspected objects host a tidal feature, where tidal feature means **streams, tails, plumes, or shells** [2508.02154]. The majority, **\(80.9 \pm 0.3\%\)**, show no feature.

| Category | Fraction |
|---|---:|
| Streams | \(4.4 \pm 0.1\%\) |
| Tails | \(3.3 \pm 0.1\%\) |
| Shells | \(2.4 \pm 0.1\%\) |
| Plumes | \(1.8 \pm 0.1\%\) |
| Disturbed halo | \(1.6 \pm 0.1\%\) |
| Grand-design spiral | \(5.2 \pm 0.2\%\) |
| Curiosities | \(0.4 \pm 0.0\%\) |

Morphology dependence is strong. The fraction hosting any tidal feature is **\(18.9 \pm 0.6\%\)** for early-type galaxies, **\(10.4 \pm 0.3\%\)** for late-type galaxies, and **\(6.3 \pm 0.5\%\)** for unknown morphology [2508.02154]. Streams occur in **\(7.3 \pm 0.4\%\)** of early types and **\(3.9 \pm 0.2\%\)** of late types, while shells are especially concentrated in early types at **\(7.2 \pm 0.4\%\)** versus **\(0.9 \pm 0.1\%\)** in late types [2508.02154].

The incidence also rises strongly with stellar mass, from **\(2.4 \pm 0.4\%\)** around **\(\sim 10^8\ M_\odot\)** to **\(36.5 \pm 1.2\%\)** around **\(\sim 5\times 10^{11}\ M_\odot\)**, with a sharper rise above roughly **\(2\times 10^{10}\ M_\odot\)** [2508.02154]. Environment matters as well: the tidal-feature fraction is **\(31.0 \pm 1.2\%\)** in groups and **\(10.3 \pm 0.2\%\)** in isolated galaxies; the stream fractions are **\(14.0 \pm 0.9\%\)** and **\(3.6 \pm 0.1\%\)** respectively [2508.02154].

These results are accompanied by explicit caveats. The catalogue uses mutually exclusive labels even though real galaxies may host multiple feature types; the voting scheme favors streams; DESI-LS is shallower than dedicated low-surface-brightness surveys; residuals may contain artefacts and oversubtraction; and the **3\(\times\)** galaxy-size inspection field can miss large-radius streams [2508.02154].

## 5. The modelling-ready sample of 35 streams

Among the full census, **861 galaxies** had **streams** as their dominant feature. These were re-examined by **four classifiers** using a focused three-way scheme: **Not streams**, **Stream not selected for modelling**, and **Stream selected for modelling**. The final release includes only objects for which **all four classifiers agreed** on “stream selected for modelling,” producing **35 galaxies/stream systems** [2508.02154].

The selection criteria were tailored to the intended dynamical framework. Desired systems were **relatively thin**, **noticeably curved**, **sufficiently faint**, and around hosts whose **central regions appear undisturbed** [2508.02154]. The stated aim was to isolate streams most consistent with **kinematically simple minor mergers** and a relatively stable host potential.

Segmentation was performed manually with **Jafar**, using DESI-LS residual cutouts in \(g,r,z\) combined by the median of the three bands, clipped to **35–90%** of the pixel distribution, rescaled to **\(0.6\arcsec\)** pixels, and converted to **HiPS** format [2508.02154]. When a stream overlapped or wrapped around itself, it was split into multiple masks assigned to distinct angular regions.

Track reconstruction followed a procedure in which the stream is segmented into non-overlapping masks, the image is divided into angular bins, radial residual-light profiles are measured in each bin, and one or more **Gaussians** are fitted to those profiles to recover the ordered stream track [2508.02154]. This provides the basis for geometric measurements such as length, width, angular extent, and galactocentric radius.

## 6. Measured stream properties and candidate progenitors

For the 35 selected systems, photometry used DESI-LS viewer residual cutouts in \(g,r,z\) at native pixel scale **\(0.262\arcsec\)**. Remaining sources were masked with **SEP**, and flux, surface brightness, and colour were measured inside the stream mask. Because residual images can contain negative values from oversubtraction, the flux distribution was **truncated to keep only positive values** when measuring stream photometry [2508.02154].

The reported median stream surface brightnesses are **\(26.8\ \mathrm{mag\ arcsec^{-2}}\)** in \(g\), **\(26.4\ \mathrm{mag\ arcsec^{-2}}\)** in \(r\), and **\(25.1\ \mathrm{mag\ arcsec^{-2}}\)** in \(z\). The faintest detected stream reaches **\(27.5\ \mathrm{mag\ arcsec^{-2}}\)** in \(g\), **\(27.4\ \mathrm{mag\ arcsec^{-2}}\)** in \(r\), and **\(25.4\ \mathrm{mag\ arcsec^{-2}}\)** in \(z\) [2508.02154]. The typical median **\(g-r\)** colour is **0.5**.

The sample is deliberately long by construction. The **median length** is **124 kpc**, the **mean length** is **224 kpc**, and the **5–95% range** is **24–1151 kpc**. Widths are more conventional: median and mean **5.6 kpc**, with a **5–95% range** of **2.3–9.9 kpc** [2508.02154]. Most streams lie within projected radius **\(\lesssim 30\) kpc**, with a few extending to **40–60 kpc**, so the sample predominantly probes the inner dark-matter distribution [2508.02154].

The median stream stellar mass is **\(6.2\times 10^8\ M_\odot\)**. Stream-to-host stellar mass ratios have median **0.007**, mean **0.02**, and only **4 streams** exceed **0.05**, which the paper states is consistent with **minor mergers**, roughly in the range **\(1:100\) to \(1:10\)** [2508.02154].

Several reported trends organize the sample. Stream stellar mass correlates positively with stream length; there is a hint that more massive streams are redder; more massive streams are somewhat wider; and physically wider streams tend to lie at larger galactocentric radius [2508.02154]. For the angular-width ratio
\[
\frac{\text{width}}{\text{radius}},
\]
the paper gives linear fits
\[
y=-0.0021x + 0.29,\qquad R^2=0.13
\]
for median values and
\[
y=-0.0028x + 0.33,\qquad R^2=0.07
\]
for all individual measurements, with the former having **p-value = 0.03** from Wald’s test [2508.02154].

The segmentation stage identified **six** streams containing a compact candidate dwarf progenitor: **ESO356-012**, **PGC1092512**, **NGC0804**, **PGC430221**, **NGC5513_GROUP**, and **UGC09239**. The paper concludes that **five** are likely genuine progenitors, while the **NGC5513_GROUP** case is more doubtful [2508.02154].

## 7. Interpretation, limitations, and projected development

STRRINGS is presented as a proof-of-concept sample for extragalactic stream modelling at scale. Its intended scientific roles are to trace recent minor accretion, to characterize merger demographics as a function of stellar mass, morphology, and environment, and to constrain the host gravitational potential and dark-matter halo structure. The paper explicitly states that the catalogue will be used in future work to constrain the **flattening of dark matter haloes at the population level** [2508.02154].

The project is also explicit about its limitations. DESI-LS is relatively shallow compared with dedicated low-surface-brightness surveys; residual images can contain artefacts and oversubtraction; stream photometry on residuals likely underestimates total stream light; stream-to-host mass ratios are therefore **lower bounds**; and manual segmentation introduces subjectivity [2508.02154]. Most systems probe only **inner halo radii**, not extreme outskirts.

A plausible implication is that the catalogue’s strongest present value lies less in completeness than in curation: the sample is intentionally restricted to streams that satisfy the assumptions of a particular modelling framework. This is consistent with its finding that only **35** of **861** stream-dominant systems were retained for first-release modelling [2508.02154].

Planned extensions include expanding beyond **\(z \leq 0.02\)**, using residual images for automated detection and segmentation, fine-tuning tools such as **Zoobot**, and scaling to future surveys such as **Rubin/LSST** and **Euclid** [2508.02154]. In that sense, STRRINGS is both a catalogue and a methodological template for turning residual-image tidal-feature searches into a modelling-oriented program for constraining halo structure in external galaxies [2508.02154].

Source: https://www.emergentmind.com/topics/strrings