S⁵ • Southern Stellar Stream Spectroscopic Survey

About Us

Mapping stellar streams of the Milky Way with the Anglo-Australian Telescope.

The Southern Stellar Stream Spectroscopic Survey (S⁵) is a spectroscopic survey of stars in the stellar streams of the Southern sky, with the aim of mapping their kinematics and chemistry. The streams represent material tidally striped from dwarf galaxies and globular clusters, and are essential dynamical probes of the dark matter distribution of the Milky Way, as well as providing a detailed snapshot of its accretion history.

On this page you can find out about the survey, get our data, read our publications, and meet the S⁵ team.

The streams

View on the sky of the observed streams of S⁵. View on sky of the observed streams of S⁵. The green fields are our main targets, and the purple fields are other targets of interesting including dwarf galaxies.

Our focus has been on the streams identified within the footprint of the Dark Energy Survey (DES) and more broadly within Gaia. We have mapped 20 streams including: ATLAS-Aliqa Uma, Elqui, Indus, Jet, Jhelum, Orphan-Chenab, Ophiuchus, Palca, Phoenix, Turranburra, Willka Yaku, 300S, Ravi, Wambelong, and Styx.

From our data we have made the following discoveries:

The ATLAS and Aliqa Uma streams are actually one stream which had an interaction with the Sgr Dwarf Galaxy about 500 million years ago (Ting Li et al. 2021)
The Phoenix stream was formed from a globular cluster below the observed metallicity floor of extant clusters (Zhen Wan et al. 2020); and that the Phoenix stream has the signature of a massive rotating metal-poor star (Andrew Casey et al. 2021).
S⁵-HVS1, the first hypervelocity star that can be definitively associated with coming from the Galactic centre (Sergey Koposov et al. 2021)

The instrumentation

Since 2018, S⁵ has been observing likely stellar members of the streams with 2dF/AAOmega spectrograph on the 3.9-metre Anglo-Australian Telescope at Siding Spring Observatory. The 2 degree field of view and 350 science fibres of the 2dF fibre positioner make it the perfect instrument for our observations. We combine this instrumentation with the precise photometry of the Dark Energy Survey, and the superb proper motions from Gaia allow us to conduct an efficient spectroscopic survey to map these streams. We use about a third of the fibres on each field of stream targets, with the remaining fibres used to two other surveys: a Milky Way halo survey, and a low-redshift galaxy survey.

We also get high-resolution spectral follow-up on stream members with 8-metre class telescopes: MIKE on the Magellan Telescope (see our publication list for Ji et al. 2020 & 2021, Casey et al. 2021, and Hansen et al. 2021); and UVES on the Very Large Telescope (upcoming observations at the end of 2021).

Data and analysis

To determine radial velocities and stellar atmospheric parameters from our AAOmega spectra we use rvspecfit, an automated spectroscopic pipeline developed by Sergey Koposov.

The First Public Data Release of the S⁵ was on April 24, 2021.

Header image credit: James Josephides (Swinburne Astronomy Productions) and the S⁵ Collaboration

The S⁵ Surveys

We are more than just a streams survey.

S⁵-Streams

This is the main survey of streams in the halo of the Milky Way. The goal is to measure radial velocities and metallicities of stream members. S⁵ made the first spectroscopic observations of the DES streams, and has now mapped over 20 streams both inside and outside of Dark Energy Survey footprint. The First Public Data Release of the S⁵ was on April 24, 2021 containing data derived from all observations taken between 2018-2019.

NGC5907 and the faint stellar stream that loops it • Credit: R. Jay GaBany • CC BY-SA 3.0

S⁵-Halo

We use about a third of our fibres for a Milky Way halo survey. This has the aim of observing interesting objects in the halo such as hyper-velocity stars, extremely metal-poor stars, RR Lyraes, and white dwarfs. It was as part of this survey we found S⁵-HVS1, a star travelling 1700 km s⁻¹.

Artist’s impression of Milky Way galaxy and its halo. • Credit: ESO/L. Calçada

S⁵-Lowz

Nearby faint galaxies are difficult to distinguish from the far more numerous background galaxy population via photometry alone. The goal of including low-redshift (low-z) galaxy targets in S⁵ is to increase the number of spectroscopically confirmed low-z galaxies in order to better train photometric selection algorithms, and help build a statistic sample of very low-z galaxies.

NGC1309 from the DECaLS DR5 • Credit: DECam Legacy Survey

S⁵-Hires

We are also using Magellan/MIKE and ESO/UVES to obtain high-resolution spectra of bright stream members in order to derive stellar parameters and precise elemental abundances. These will enable us to understand the stream progenitors and chemical evolution. See our publication list for Ji et al. 2020 & 2021, Casey et al. 2021, and Hansen et al. 2021.

A high-resolution version of the spectrum of our Sun • Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

Get the data

Data Release 1

The First Public Data Release of the Southern Stellar Stream Spectroscopic Survey was on April 24, 2021.

S⁵ DR1 contains data derived from all observations taken between 2018-2019. For details on target selection, data reduction, survey validation see Li et al. (2019) and this PDF for documenation and column descriptions of S⁵ DR1.

DR1 can be downloaded from zenodo. There are two versions of the DR1: s5_pdr1.fits which contains all columns; and s5_pdr1_light.fits which only contains the small subset with the most frequently used columns. Spectra and the best-fit models can be viewed via the S5 Spectral Visualization Tool.

If any data from this release is used for publication, please cite Li et al. (2019) for reference and include the following text in your acknowledgments:

Based on data acquired at the Anglo-Australian Telescope. We acknowledge the traditional owners of the land on which the AAT stands, the Gamilaraay people, and pay our respects to elders past and present.

Other large data tables

If you are interested in the S⁵ reduced spectra, please contact Ting Li

We provide machine-readable versions of large tables published in our papers:

T. S. Li et al (2019) • The Overview paper:

Table 2: S⁵ fields observed with the AAT as of June 2019.
Table B2: A list of 674 QSOs identified by S⁵ with robust redshift measurements.

Ji et al (2020) • Chemical Abundances of Seven Stellar Streams:

Table 3: Atomic Data
Table 5: Line Measurements
Table 6: Stellar Abundances
(If other formats are desired please contact Alex Ji at alexji@uchicago.edu)

T. S. Li et al (2021) • Broken into Pieces: ATLAS and Aliqa Uma as One Single Stream.

There is a GitHub repository of data for this paper:

Table 1: Radial velocites, metallicities of member stars
Table 2: Distance moduli of blue horizontal branch stars and RR Lyrae stars
The STAN model used to model the 2-D density distribution of stars with splines
The best-fit stream and its associated orbit

Publications

Peer-reviewed and submitted publications from the S⁵ collaboration.

Press Coverage

S⁵ in the news.

One Dozen Streams:
Twelve for dinner

The Milky Way's feeding habits shine a light on dark matter

Phoenix stream:
Ancient stars that time forgot

First evidence of vanished star clusters

S⁵-HVS1:
Discovery of a star ejected from the Galaxy by Sgr A*

A nearby star travelling 1700 km s⁻¹

The Humans of S⁵

The S⁵ Collaboration consists of roughly 30 members spread across many institutions.

S⁵ is a collaboration between the members of the DES Milky Way Working Group and a group of Australian astronomers. If you are interested in getting involved in S⁵, please contact Ting Li or other members in the S⁵ leadership team.

S⁵ Leadership

S⁵ leadership team helps to coordinate the operational and scientific efforts of the collaboration.

Kyler Kuehn

Lowell Observatory

Geraint Lewis

The University of Sydney

Ting Li

University of Toronto

Daniel Zucker

Macquarie University

Other team members

Joss Bland-Hawthorn

The University of Sydney

Denis Erkal

University of Surrey
S⁵ Dynamical modeling coordinator

Alex Ji

University of Chicago
S⁵ HIRES coordinator

Sergey Koposov

University of Edinburgh
S⁵ Data release coordinator

Yao-Yuan Mao

Rutgers
S⁵ Low-Z Coordinator

Jeremy Mould

Swinburne University of Technology

Andrew Pace

Carnegie Mellon University

Nora Shipp

University of Chicago
S⁵ Proper motion coordinator

Jeffrey Simpson

S⁵ Website coordinator

And

Sahar Allam, Eduardo Balbinot, Keith Bechtol, Vasily Belokurov, Andrew Casey, Lara Cullinane, Gary Da Costa, Gayandhi De Silva, Alex Drlica-Wagner (), Marla Geha, Terese Hansen (), Sophia Lilleengen (), Dougal Mackey, Sarah Martell (, ), Sanjib Sharma, Josh Simon, Kiyan Tavangar, Douglas Tucker, Kathy Vivas, Zhen Wan, Risa Wechsler (), Brian Yanny