The demographics of dwarf galaxy populations have long been in tension with predictions from the Λ cold dark matter (ΛCDM) paradigm1–4. If primordial density fluctuations were scale-free as predicted, dwarf galaxies should themselves host dark-matter subhaloes5, the most massive of which may have undergone star formation resulting in dwarf galaxy groups. Ensembles of dwarf galaxies are observed as satellites of more massive galaxies6–9, and there is observational10 and theoretical11 evidence to suggest that these satellites at redshift z = 0 were captured by the massive host halo as a group. However, the evolution of dwarf galaxies is highly susceptible to environment12–14, making these satellite groups imperfect probes of ΛCDM in the low-mass regime. Here we report one of the clearest examples yet of hierarchical structure formation at low masses: using deep multi-wavelength data, we identify seven isolated, spectroscopically confirmed groups of only dwarf galaxies. Each group hosts three to five known members, has a baryonic mass of ~4.4 × 109 to 2 × 1010 solar masses (M⊙), and requires a mass-to-light ratio of <100 to be gravitationally bound. Such groups are predicted to be rare theoretically and found to be rare observationally at the current epoch, and thus provide a unique window into the possible formation mechanism of more massive, isolated galaxies.
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S.S., S.E.L. and G.C.P. thank S. Veilleux and M. McDonald for the use of their PI instrument, MMTF, and M. McDonald for sharing his advice and wisdom throughout the MMTF observations and data reduction. S.S. acknowledges the L’Oréal USA For Women in Science programme for their grant to conduct this resesarch. S.E.L. acknowledges support from a National Science Foundation (NSF) Graduate Research Fellowship under Grant No. DDGE-1315231. S.E.L. was also partially funded by a Virginia Space Grant Consortium Graduate STEM Research Fellowship and a Clare Boothe Luce Graduate Fellowship. D.R.P. acknowledges a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada which helped to fund this research. G.C.P. was supported by a FONDECYT Postdoctoral Fellowship (No. 3150361). N.K. is supported by the NSF CAREER award 1455260.
These results are based on observations obtained with the APO 3.5-m telescope, which is owned and operated by the Astrophysical Research Consortium. This work has also used catalogues and imaging from the SDSS. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the NSF, the US Department of Energy, NASA, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS website is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory and the University of Washington.
Results are also based on observations obtained at the Gemini Observatory (Program ID: GN-2016A-Q-16) and processed using the Gemini IRAF package, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the NSF (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina) and Ministério da Ciéncia, Tecnologia e Inovacão (Brazil).
The authors declare no competing financial interests.
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Stierwalt, S., Liss, S., Johnson, K. et al. Direct evidence of hierarchical assembly at low masses from isolated dwarf galaxy groups. Nat Astron 1, 0025 (2017). https://doi.org/10.1038/s41550-016-0025
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