Evidence for a vast prograde stellar stream in the solar vicinity


Massive dwarf galaxies that merge with the Milky Way on prograde orbits can be dragged into the disk plane before being completely disrupted. Such mergers can contribute to an accreted stellar disk and a dark matter disk. Here we present Nyx, a vast stellar stream in the vicinity of the Sun, which provides the first indication that such an event occurred in the Milky Way. We identify about 200 stars that have coherent radial and prograde motion in this stream using a catalogue of accreted stars built by applying deep learning methods to the Gaia data. Taken together with chemical abundance and orbital information, these results strongly favour the interpretation that Nyx is the remnant of a disrupted dwarf galaxy. Further justified by FIRE hydrodynamic simulations, we demonstrate that prograde streams like Nyx can be found in the disk plane of galaxies and identified using our methods.

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Fig. 1: Dynamics of the Nyx stars.
Fig. 2: Chemical abundances of the Nyx stars.
Fig. 3: Colour–magnitude diagram of the Nyx stars.
Fig. 4: Example prograde stellar stream in a simulated Milky Way galaxy.

Data availability

The accreted star catalogue used for this analysis is available at https://doi.org/10.5281/zenodo.3579379. The simulation m12f is available at https://fire.northwestern.edu/ananke/. The IDs of the Nyx stars are available as a Supplementary Data file.

Code availability

This analysis makes use of emcee and the extreme deconvolution for the Gaussian mixture model. The Python Markov chain Monte Carlo code emcee is freely available and documented at http://dfm.io/emcee/current/. Extreme deconvolution is freely available at https://github.com/jobovy/extreme-deconvolution. Details regarding the application of these two public codes are provided in ref. 22.


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We thank A. Helmi, J. Johnson, E. Kirby, N. Laracy, J. Read, N. Shipp and J. Wojno for helpful discussions. This work was performed in part at Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607611. This research was supported by the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG cluster of excellence ‘Origin and Structure of the Universe’. This research was supported in part by the National Science Foundation under Grant No. NSF PHY-1748958. L.N. is supported by the DOE under award number DESC0011632, and the Sherman Fairchild fellowship. M.L. is supported by the DOE under contract DESC0007968 and the Cottrell Scholar Program through the Research Corporation for Science Advancement. B.O. and T.C. are supported by the US Department of Energy under grant number DE-SC0011640. M.F. is supported by the Zuckerman STEM Leadership Program and in part by the DOE under grant number DE-SC0011640. S.G.-K. and P.F.H are supported by an Alfred P. Sloan Research Fellowship, NSF Collaborative Research grant number 1715847 and CAREER grant number 1455342, and NASA grants NNX15AT06G, JPL 1589742 and 17-ATP17-0214. A.W. is supported by NASA, through ATP grant 80NSSC18K1097 and HST grants GO-14734 and AR-15057 from STScI, and a Hellman Fellowship from UC Davis. This work utilized the University of Oregon Talapas high-performance computing cluster. Numerical simulations were run on the Caltech compute cluster ‘Wheeler’, allocations from XSEDE TG-AST130039 and PRAC NSF.1713353 supported by the NSF, and NASA HEC SMD-16-7592. R.S. thanks N. Carriero, I. Fisk and D. Simon of the Scientific Computing Core at the Flatiron Institute for their support of the infrastructure housing the synthetic surveys and simulations used for this work. This work has made use of data from the European Space Agency (ESA) mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Funding for RAVE has been provided by: the Australian Astronomical Observatory; the Leibniz-Institut für Astrophysik Potsdam (AIP); the Australian National University; the Australian Research Council; the French National Research Agency; the German Research Foundation (SPP 1177 and SFB 881); the European Research Council (ERC-StG 240271 Galactica); the Istituto Nazionale di Astrofisica at Padova; The Johns Hopkins University; the National Science Foundation of the USA (AST-0908326); the W. M. Keck foundation; the Macquarie University; the Netherlands Research School for Astronomy; the Natural Sciences and Engineering Research Council of Canada; the Slovenian Research Agency; the Swiss National Science Foundation; the Science and Technology Facilities Council of the UK; Opticon; Strasbourg Observatory; and the Universities of Groningen, Heidelberg and Sydney. The RAVE website is at https://www.rave-survey.org.

Author information




All authors discussed the results and commented on the manuscript. M.L. and T.C. conceived the project. L.N. built the data analysis pipeline. B.O., T.C. and M.F. conceptualized the machine learning algorithms. B.O. built the deep neural network and produced the accreted stellar catalogue. Interpretation of the results and writing of the original manuscript were done by L.N. and M.L. The FIRE-2 simulation code was built by P.F.H., and run by P.F.H., S.G.-K. and A.W. S.G.-K. and A.W. ran the halo finding algorithm on the simulation, and L.N. identified the mergers as a function of redshift. R.S. built the mock catalogues used in the training of the neural network.

Corresponding author

Correspondence to Lina Necib.

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Peer review information Nature Astronomy thanks Jeffrey Carlin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–7 and references.

Supplementary Data 1

List of the 232 stars with a probability larger than 0.95 of belonging to Nyx. It includes their Gaia source IDs, their positions in the sky (right ascension, declination) and their magnitudes in the G band.

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Necib, L., Ostdiek, B., Lisanti, M. et al. Evidence for a vast prograde stellar stream in the solar vicinity. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1131-2

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