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.
Subscribe to Journal
Get full journal access for 1 year
only $8.67 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
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.
White, S. D. M. & Rees, M. J. Core condensation in heavy halos: a two-stage theory for galaxy formation and clustering. Mon. Not. R. Astron. Soc. 183, 341–358 (1978).
Johnston, K. V., Hernquist, L. & Bolte, M. Fossil signatures of ancient accretion events in the halo. Astrophys. J. 465, 278–287 (1996).
Newberg, H. J. & Carlin, J. L. (eds) Tidal Streams in the Local Group and Beyond (Springer, 2016).
Quinn, P. J. & Goodman, J. Sinking satellites of spiral systems. Astrophys. J. 309, 472–495 (1986).
Walker, I. R., Mihos, J. C. & Hernquist, L. Quantifying the fragility of Galactic Disks in minor mergers. Astrophys. J. 460, 121–135 (1996).
Read, J. I., Lake, G., Agertz, O. & Debattista, V. P. Thin, thick and dark discs in Λ CDM. Mon. Not. R. Astron. Soc. 389, 1041–1057 (2008).
Hopkins, P. F., Cox, T. J., Younger, J. D. & Hernquist, L. How do disks survive mergers? Astrophys. J. 691, 1168–1201 (2009).
Read, J. I., Mayer, L., Brooks, A. M., Governato, F. & Lake, G. A dark matter disc in three cosmological simulations of Milky Way mass galaxies. Mon. Not. R. Astron. Soc. 397, 44–51 (2009).
Purcell, C. W., Bullock, J. S. & Kaplinghat, M. The dark disk of the Milky Way. Astrophys. J. 703, 2275–2284 (2009).
Ling, F. S., Nezri, E., Athanassoula, E. & Teyssier, R. Dark matter direct detection signals inferred from a cosmological N-body simulation with baryons. J. Cosmol. Astropart. Phys. 2010, 012 (2010).
Pillepich, A., Kuhlen, M., Guedes, J. & Madau, P. The distribution of dark matter in the Milky Way’s disk. Astrophys. J. 784, 161 (2014).
Gómez, F. A. et al. Lessons from the Auriga discs: the hunt for the Milky Way as ex situ disc is not yet over. Mon. Not. R. Astron. Soc. 472, 3722–3733 (2017).
Ruchti, G. R. et al. The Gaia-ESO survey: a quiescent Milky Way with no significant dark/stellar accreted disc. Mon. Not. R. Astron. Soc. 450, 2874–2887 (2015).
Ostdiek, B. et al. Cataloging accreted stars within Gaia DR2 using deep learning. Astron. Astrophys. 636, A75 (2020).
Brown, A. G. A. et al. Gaia data release 2. Summary of the contents and survey properties. Astron. Astrophys. 616, A1 (2018).
Sanderson, R. E. et al. Synthetic Gaia surveys from the FIRE cosmological simulations of Milky Way-mass galaxies. Astrophys. J. Suppl. 246, 6 (2020).
Wetzel, A. R. et al. Reconciling dwarf galaxies with Λ CDM cosmology: simulating a realistic population of satellites around a Milky Way-mass galaxy. Astrophys. J. 827, L23 (2016).
Hopkins, P. F., Narayanan, D. & Murray, N. The meaning and consequences of star formation criteria in galaxy models with resolved stellar feedback. Mon. Not. R. Astron. Soc. 432, 2647–2653 (2013).
Hopkins, P. F. et al. FIRE-2 simulations: physics versus numerics in galaxy formation. Mon. Not. R. Astron. Soc. 480, 800–863 (2018).
Necib, L. et al. Under the FIRElight: stellar tracers of the local dark matter velocity distribution in the Milky Way. Astrophys. J. 883, 27 (2019).
Kunder, A. et al. The Radial Velocity Experiment (RAVE): fifth data release. Astron. J. 153, 75 (2017).
Necib, L. et al. Chasing accreted structures within Gaia DR2 using deep learning. Preprint at https://arxiv.org/abs/1907.07681 (2019).
Antoja, T., Figueras, F., Torra, J., Valenzuela, O. & Pichardo, B. in Lecture Notes and Essays in Astrophysics Vol. 4 (eds Ulla, A. & Manteiga, M.) 13–31 (2010).
Mateu, C., Read, J. I. & Kawata, D. Fourteen candidate RR Lyrae star streams in the inner galaxy. Mon. Not. R. Astron. Soc. 474, 4112–4129 (2018).
Myeong, G. C., Evans, N. W., Belokurov, V., Amorisco, N. C. & Koposov, S. E. Halo substructure in the SDSS-Gaia catalogue: streams and clumps. Mon. Not. R. Astron. Soc. 475, 1537–1548 (2018).
Koppelman, H., Helmi, A. & Veljanoski, J. One large blob and many streams frosting the nearby stellar halo in Gaia DR2. Astrophys. J. 860, L11 (2018).
Necib, L., Lisanti, M. & Belokurov, V. Inferred evidence for dark matter kinematic substructure with SDSS-Gaia. Astrophys. J. 874, 3 (2019).
Casey, A. R. et al. The RAVE-on catalog of stellar atmospheric parameters and chemical abundances for chemo-dynamic studies in the Gaia era. Astrophys. J. 840, 59 (2017).
Venn, K. A. et al. Stellar chemical signatures and hierarchical galaxy formation. Astron. J. 128, 1177–1195 (2004).
Tolstoy, E., Hill, V. & Tosi, M. Star-formation histories, abundances, and kinematics of dwarf galaxies in the Local Group. Annu. Rev. Astron. Astrophys. 47, 371–425 (2009).
Ruchti, G. R., Read, J. I., Feltzing, S., Pipino, A. & Bensby, T. The hunt for the Milky Way as accreted disc. Mon. Not. R. Astron. Soc. 444, 515–526 (2014).
Andrae, R. et al. Gaia data release 2. First stellar parameters from Apsis. Astron. Astrophys. 616, A8 (2018).
Babusiaux, C. et al. Gaia data release 2. Observational Hertzsprung–Russell diagrams. Astron. Astrophys. 616, A10 (2018).
Dotter, A. MESA Isochrones and Stellar Tracks (MIST) 0: methods for the construction of stellar isochrones. Astrophys. J. Suppl. 222, 8 (2016).
Choi, J. et al. Mesa Isochrones and Stellar Tracks (MIST). I. Solar-scaled models. Astrophys. J. 823, 102 (2016).
Martig, M., Minchev, I., Ness, M., Fouesneau, M. & Rix, H.-W. A radial age gradient in the geometrically thick disk of the Milky Way. Astrophys. J. 831, 139 (2016).
Price-Whelan, A. M. Gala: a Python package for galactic dynamics. J. Open Source Softw. 2, https://doi.org/10.21105/joss.00388 (2017).
Bovy, J. galpy: a Python library for Galactic dynamics. Astrophys. J. Suppl. 216, 29 (2015).
Li, C. & Zhao, G. The evolution of the Galactic thick disk with the LAMOST survey. Astrophys. J. 850, 25 (2017).
Dehnen, W. The effect of the outer Lindblad resonance of the Galactic bar on the local stellar velocity distribution. Astron. J. 119, 800–812 (2000).
Fux, R. Order and chaos in the local disc stellar kinematics induced by the Galactic bar. Astron. Astrophys. 373, 511–535 (2001).
De Simone, R., Wu, X. & Tremaine, S. The stellar velocity distribution in the solar neighbourhood. Mon. Not. R. Astron. Soc. 350, 627–643 (2004).
Quillen, A. C. & Minchev, I. The effect of spiral structure on the stellar velocity distribution in the solar neighborhood. Astron. J. 130, 576–585 (2005).
Minchev, I. et al. Is the Milky Way ringing? The hunt for high-velocity streams. Mon. Not. R. Astron. Soc. 396, L56–L60 (2009).
Quillen, A. C., Minchev, I., Bland-Hawthorn, J. & Haywood, M. Radial mixing in the outer Milky Way disc caused by an orbiting satellite. Mon. Not. R. Astron. Soc. 397, 1599–1606 (2009).
Katz, D. et al. Gaia data release 2. Mapping the Milky Way disc kinematics. Astron. Astrophys. 616, A11 (2018).
Helmi, A. et al. Pieces of the puzzle: ancient substructure in the Galactic Disk. Mon. Not. R. Astron. Soc. 365, 1309–1323 (2006).
Kirby, E. N. et al. The universal stellar mass–stellar metallicity relation for dwarf galaxies. Astrophys. J. 779, 102 (2013).
Sanderson, R. E. et al. Reconciling observed and simulated stellar halo masses. Astrophys. J. 869, 12 (2018).
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.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Jeffrey Carlin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
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