Letter

Direct evidence of hierarchical assembly at low masses from isolated dwarf galaxy groups

Received:
Accepted:
Published online:

Abstract

The demographics of dwarf galaxy populations have long been in tension with predictions from the Λ cold dark matter (ΛCDM) paradigm 1,​2,​3,​4 . If primordial density fluctuations were scale-free as predicted, dwarf galaxies should themselves host dark-matter subhaloes 5 , 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 galaxies 6,​7,​8,​9 , and there is observational 10 and theoretical 11 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 environment 12,​13,​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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References

  1. 1.

    , & Too big to fail? The puzzling darkness of massive Milky Way subhaloes. Mon. Not. R. Astron. Soc. 415, 40–44 (2011).

  2. 2.

    & The merging history of dark matter haloes in a hierarchical universe. Mon. Not. R. Astron. Soc. 261, 921–928 (1993).

  3. 3.

    , , & Where are the missing galactic satellites? Astrophys. J. 522, 82–92 (1999).

  4. 4.

    et al. Dark matter substructure within galactic halos. Astrophys. J. 524, 19–22 (1999).

  5. 5.

    et al. Sweating the small stuff: Simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites. Mon. Not. R. Astron. Soc. 453, 1305–1316 (2015).

  6. 6.

    , & M31 satellite masses compared to ΛCDM subahloes. Mon. Not. R. Astron. Soc. 440, 3511–3519 (2014).

  7. 7.

    et al. Eight new Milky Way companions discovered in first-year Dark Energy Survey data. Astrophys. J. 807, 50–66 (2015).

  8. 8.

    et al. Leo V: A companion of a companion of the Milky Way galaxy? Astrophys. J. 686, 83–86 (2008).

  9. 9.

    , , & Beasts of the Southern Wild: Discovery of nine ultra faint satellites in the vicinity of the Magellanic Clouds. Astrophys. J. 805, 130–148 (2015).

  10. 10.

    , , & Velocity anti-correlation of diametrically opposed galaxy satellites in the low-redshift Universe. Nature 511, 563–566 (2014).

  11. 11.

    , & Satellite dwarf galaxies in a hierarchical universe: Infall histories, group preprocessing, and reionization. Astrophys. J. 807, 49–61 (2015).

  12. 12.

    , , & A stellar mass threshold for quenching of field galaxies. Astrophys. J. 757, 85–93 (2012).

  13. 13.

    , , & On the assembly of the Milky Way dwarf satellites and their common mass scale. Astrophys. J. 745, 142–155 (2012).

  14. 14.

    , & Formation and evolution of galaxy dark matter halos and their substructure. Astrophys. J. 667, 859–877 (2007).

  15. 15.

    et al., DDO 68: A flea with smaller fleas that on him prey. Astrophys. J. 826, 27–33 (2016).

  16. 16.

    et al. Dwarfs gobbling dwarfs: A stellar tidal stream around NGC 4449 and hierarchical galaxy formation on small scales. Astrophys. J. 748, 24–30 (2012).

  17. 17.

    , & Possible formation scenarios for the giant Hi envelope around the NGC 4490/4485 system. Mon. Not. R. Astron. Soc. 297, 1015–1020 (1998).

  18. 18.

    et al. TiNy Titans: The role of dwarf–dwarf interactions in low-mass galaxy evolution. Astrophys. J. 805, 2–18 (2015).

  19. 19.

    , Dwarf galaxies of the Local Group. Ann. Rev. Astron. Astrophys. 36, 435–506 (1998).

  20. 20.

    , , , & The dearth of neutral hydrogen in galactic dwarf spheroidal galaxies. Astrophys. J. 795, 5–10 (2014).

  21. 21.

    , , & , Satellites and haloes of dwarf galaxies. Mon. Not. R. Astron. Soc. 428, 573–578 (2013).

  22. 22.

    , & Compact groups in theory and practice. I. The spatial properties of compact groups. Mon. Not. R. Astron. Soc. 387, 1281–1290 (2008).

  23. 23.

    , & A photometric catalog of compact groups of galaxies. Astrophys. J. Suppl. 70, 687–698 (1989).

  24. 24.

    Nearby groups of galaxies. II. An all-sky survey within 3000 kilometers per second. Astrophys. J. 321, 280–304 (1987).

  25. 25.

    , , & Squelched galaxies and dark halos. Astrophys. J. 569, 573–581 (2002).

  26. 26.

    et al. Associations of dwarf galaxies. Astron. J. 132, 729–748 (2006).

  27. 27.

    , , & The evolution of HCG 31: Optical and high-resolution Hi study. Astron. Astrophys. 430, 443–464 (2005).

  28. 28.

    et al. Hierarchical structure formation and modes of star formation in Hickson Compact Group 31. Astron. J. 139, 545–564 (2010).

  29. 29.

    , , , & A catalog of bulge, disk, and total stellar mass estimates for the Sloan Digital Sky Survey. Astrophys. J. Suppl. 210, 3–24 (2014).

  30. 30.

    Evolution of compact groups and the formation of elliptical galaxies. Nature 338, 123–126 (1989).

  31. 31.

    et al. MMTF: The Maryland-Magellan Tunable Filter. Astron. J. 139, 145–157 (2010).

  32. 32.

    Faint spectrophotometric standard stars. Astron. J. 99, 1621–1631 (1990).

  33. 33.

    et al. The Gemini-North Multi-Object Spectrograph: Performance in imaging, long-slit, and multi-object spectroscopic modes. Publ. Astron. Soc. Pacif. 116, 425–440 (2004).

  34. 34.

    , & Maps of dust infrared emission for use in estimation of reddening and cosmic microwave background radiation foregrounds. Astrophys. J. 500, 525–553 (1998).

  35. 35.

    & K-corrections and filter transformations in the ultraviolet, optical, and near-infrared. Astron. J. 133, 734–754 (2007).

  36. 36.

    Galactic stellar and substellar initial mass function. Publ. Astron. Soc. Pacif. 115, 763–795 (2003).

  37. 37.

    , & Estimating the masses of galaxy groups — alternatives to the virial theorem. Astrophys. J. 298, 8–17 (1985).

  38. 38.

    , , & Dwarfs walking in a row. The filamentary nature of the NGC3109 association. Astron. Astrophys. 559, L11 (2013).

  39. 39.

    et al. Antlia B: A faint dwarf galaxy member of the NGC 3109 association. Astrophys. J. 812, 13–19 (2015).

  40. 40.

    , , & The optical and near-infrared properties of galaxies. I. Luminosity and stellar mass functions. Astrophys. J. Suppl. 149, 289–312 (2003).

  41. 41.

    et al. The seventh data release of the Sloan Digital Sky Survey. Astrophys. J. Suppl. 182, 543–558 (2009).

  42. 42.

    , , , & A catalog of bulge+disk decompositions and updated photometry for 1.12 million galaxies in the Sloan Digital Sky Survey. Astrophys. J. Suppl. 196, 11–31 (2011).

  43. 43.

    & The luminosity dependence of the galaxy merger rate. Astrophys. J. 685, 235 (2008).

  44. 44.

    et al. New techniques for relating dynamically close galaxy pairs to merger and accretion rates: Application to the Second Southern Sky Redshift Survey. Astrophys. J. 536, 153–172 (2000).

  45. 45.

    et al. An efficient targeting strategy for multiobject spectrograph surveys: the Sloan Digital Sky Survey ‘tiling’ algorithm. Astron. J. 125, 2276–2286 (2003).

  46. 46.

    et al. Dynamically close galaxy pairs and merger rate evolution in the CNOC2 Redshift Survey. Astrophys. J. 565, 208–222 (2002).

  47. 47.

    et al. Galaxy pairs in the Sloan Digital Sky Survey — XI. A new method for measuring the influence of the closest companion out to wide separations. Mon. Not. R. Astron. Soc. 461, 2589–2604 (2016).

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Author information

Affiliations

  1. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22904, USA

    • S. Stierwalt
  2. Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, Virginia 22904, USA

    • S. Stierwalt
    • , S. E. Liss
    • , K. E. Johnson
    •  & N. Kallivayalil
  3. Department of Physics & Astronomy, Trent University, 1600 West Bank Drive, Peterborough, Ontario, K9L 0G2, Canada

    • D. R. Patton
  4. Instituto de Astrofísica, Facultad de Física, Pontifica Universidad Católica de Chile, Casilla 306, Santiago 22, Chile

    • G. C. Privon
  5. Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, Arizona 85719, USA

    • G. Besla
  6. Department of Astronomy, Columbia University, Mail Code 5246, 550 West 120th Street, New York, New York 10027, USA

    • M. Putman

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Contributions

S.S. identified the group candidates, led the Magellan proposal and reduced the APO data. S.E.L. led the Gemini and APO proposals and led the Magellan and Gemini data reduction. S.S. and K.E.J. coordinated the analysis, interpretation and writing of the paper. S.S., S.E.L. and G.C.P. conducted the Magellan and APO observations. D.R.P. led the SDSS-based analysis including identifying the original pairs and calculating the isolation fraction. All authors discussed the results, their interpretation and the presentation of the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to S. Stierwalt.