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An evolutionary continuum from nucleated dwarf galaxies to star clusters

Nature volume 623, pages 296–300 (2023)Cite this article

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Abstract

Systematic studies1,2,3,4 have revealed hundreds of ultra-compact dwarf galaxies (UCDs5) in the nearby Universe. With half-light radii rh of approximately 10–100 parsecs and stellar masses M* ≈ 106–108 solar masses, UCDs are among the densest known stellar systems6. Although similar in appearance to massive globular clusters7, the detection of extended stellar envelopes4,8,9, complex star formation histories10, elevated mass-to-light ratio11,12 and supermassive black holes13,14,15,16 suggest that some UCDs are remnant nuclear star clusters17 of tidally stripped dwarf galaxies18,19, or even ancient compact galaxies20. However, only a few objects have been found in the transient stage of tidal stripping21,22, and this assumed evolutionary path19 has never been fully traced by observations. Here we show that 106 galaxies in the Virgo cluster have morphologies that are intermediate between normal, nucleated dwarf galaxies and single-component UCDs, revealing a continuum that fully maps this morphological transition and fills the ‘size gap’ between star clusters and galaxies. Their spatial distribution and redder colour are also consistent with stripped satellite galaxies on their first few pericentric passages around massive galaxies23. The ‘ultra-diffuse’ tidal features around several of these galaxies directly show how UCDs are forming through tidal stripping and that this evolutionary path can include an early phase as a nucleated ultra-diffuse galaxy24,25. These UCDs represent substantial visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and more low-mass remnants probably remain to be found.

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Fig. 1: Strongly nucleated dEs, eUCDs and UCDs in the Virgo Cluster core.
Fig. 2: The size–luminosity scaling relationship of Virgo early-type stellar systems.
Fig. 3: Colour–magnitude diagram for eUCDs, strongly nucleated dEs and Virgo early-type galaxies.
Fig. 4: Morphological sequence of tidal stripping.

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Data availability

NGVS data can be accessed via the Canadian Astronomy Data Centre (CADC). ACSVCS data is available via the Mikulski Archive for Space Telescopes (MAST) with the programme ID HST GO-9401. Data products of Burrell Schmidt Deep Virgo Survey can be downloaded from http://astroweb.case.edu/VirgoSurvey/. Gemini/GMOS spectroscopic data of eUCD candidates taken during 2021–2022 can be downloaded through the Gemini Observatory Archive with programme IDs GN-2021A-Q-208, GN-2022A-Q-206 and GN-2022A-Q-307. Source data are provided with this paper.

Code availability

We have made use of standard data analysis tools (GALFIT, Astropy, Photutils, Pypeit, NoiseChisel). All the software and code used in the research are accessible to the public.

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Acknowledgements

K.W. acknowledges support from the National Natural Science Foundation of China, grant nos. 12273001 and 12073002. C.L. acknowledges support from the National Natural Science Foundation of China, grant nos. 12173025 and 11833005, China Manned Space Project, grant no. CMS-CSST-2021-A04, and the MOE Key Lab for Particle Physics, Astrophysics and Cosmology. S.L. acknowledges the support from the Sejong Science Fellowship Program by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (no. NRF-2021R1C1C2006790). T.H.P. acknowledges support through FONDECYT Regular project 1201016 and CONICYT project Basal FB210003. C.S. acknowledges support from ANID/CONICYT through FONDECYT Postdoctoral Fellowship Project no. 3200959. E.T. is thankful for the support given by the NSF-AST- 2206498 grant. This work is based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based on observations made with the Hubble Space Telescope. Support for programme HST GO-9401 was provided through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Spectroscopic data were obtained at the international Gemini Observatory, a programme of NSF’s NOIRLab (Programme ID: GN-2021A-Q-208; GN-2022A-Q-206; GN-2022A-Q-307). The international Gemini Observatory at NOIRLab is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). Observations reported here were obtained in part at the MMT Observatory, a facility operated jointly by the Smithsonian Institution and the University of Arizona. MMT telescope time was granted by NSF’s NOIRLab (Prop. ID: 2010A-0445, PI: E. Peng), through the Telescope System Instrumentation Program (TSIP). TSIP was funded by NSF. This work was enabled by observations made from telescopes (CFHT and Gemini North) located within the Maunakea Science Reserve and adjacent to the summit of Maunakea. We are grateful for the privilege of observing the Universe from a place that is unique in both its astronomical quality and its cultural significance.

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Authors and Affiliations

  1. Department of Astronomy, Peking University, Beijing, China

    Kaixiang Wang & Mingcheng Zhu

  2. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China

    Kaixiang Wang & Mingcheng Zhu

  3. NSF’s NOIRLab, Tucson, AZ, USA

    Eric W. Peng & John P. Blakeslee

  4. Department of Astronomy, School of Physics and Astronomy, and Shanghai Key Laboratory for Particle Physics and Cosmology, Shanghai Jiao Tong University, Shanghai, China

    Chengze Liu

  5. Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai, China

    Chengze Liu

  6. Department of Astronomy, Case Western Reserve University, Cleveland, OH, USA

    J. Christopher Mihos

  7. Herzberg Astronomy and Astrophysics Research Centre, National Research Council of Canada, Victoria, British Columbia, Canada

    Patrick Côté, Laura Ferrarese, Stephen Gwyn & Joel Roediger

  8. Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada

    Matthew A. Taylor

  9. AIM Paris Saclay, CNRS/INSU, CEA/Irfu, Université Paris Diderot, Orme des Merisiers, Gif-sur-Yvette Cedex, France

    Jean-Charles Cuillandre

  10. Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, Strasbourg, France

    Pierre-Alain Duc & Ariane Lançon

  11. UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA, USA

    Puragra Guhathakurta

  12. Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-Gu, Daejeon, Republic of Korea

    Youkyung Ko

  13. Department of Astronomy, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, Republic of Korea

    Sungsoon Lim

  14. Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA

    Lauren A. MacArthur

  15. Institute of Astrophysics, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile

    Thomas Puzia & Chelsea Spengler

  16. Department of Physics and Astronomy, University of California, Riverside, CA, USA

    Laura V. Sales

  17. UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh, UK

    Rubén Sánchez-Janssen

  18. Department of Physics and Astronomy, University of the Pacific, Stockton, CA, USA

    Elisa Toloba

  19. School of Astronomy and Space Science, University of Science and Technology of China, Hefei, China

    Hongxin Zhang

  20. CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, China

    Hongxin Zhang

Authors
  1. Kaixiang Wang
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  2. Eric W. Peng
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  3. Chengze Liu
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  4. J. Christopher Mihos
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  15. Sungsoon Lim
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  19. Laura V. Sales
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  22. Elisa Toloba
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  23. Hongxin Zhang
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  24. Mingcheng Zhu
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Contributions

K.W. led the data analysis of the images and spectra of UCDs and nucleated dwarf galaxies, developed the main interpretation of the results and wrote the manuscript. E.W.P. led the project design and management, the main interpretation of the results and contributed to the manuscript. C.L. compiled the Virgo UCD samples and did exploratory work on the morphological sequence of dE,Ns and UCDs. J.C.M. led the Burrell Schmidt Deep Virgo Survey, discovered VLSB-A and VLSB-D, and contributed to the data analysis and the discussion of results. L.F. and P.C. led the NGVS and ACSVCS programme design and management. L.F, P.C. and L.A.M. compiled the NGVS galaxy catalogue. S.G. led the data production pipeline of NGVS data. A.L. and T.P. led the infrared observations of the NGVS. M.A.T. and J.R. contributed to the Gemini spectroscopic observations. P.G. contributed to the observation proposals and the interpretation of the results. Y.K. provided the stacked MMT globular cluster spectra. All co-authors contributed to the discussion of the presented results and the preparation of the manuscript.

Corresponding authors

Correspondence to Kaixiang Wang, Eric W. Peng or Chengze Liu.

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Extended data figures and tables

Extended Data Fig. 1 2D decomposition of eUCDs.

First column: Radial surface brightness profile of the UCD and best-fit result of the two-component fitting. All eUCDs can be well-fit with an inner King model and an outer Sérsic model. The total, King, and Sérsic model profiles are shown with red, magenta and green dashed lines, respectively. The grey dashed line is the PSF profile. Error bars represent 1σ uncertainties. The fitting residuals are shown below the source profile. Second column: NGVS u*g′i′ colour composite image. Third column: The g′-band image of UCD, with contours showing the surface brightness at µg′ = 24 to 27 mag arcsec−2. Last column: (data−model) residual map.

Extended Data Fig. 2 The best-fit three-component model of VCC 1672.

The right panels display, from top to bottom, images of the data overlapped with isophotes, the best-fit model, and the residuals. North is up and east to the left. The left panels display the isophotal analysis of the 2D images and model fitting. From top to bottom, the panels show radial profiles of the axis ratio (q), the position angle (PA), the g′-band surface brightness, and the fitting residuals. For an illustration of the breaks in position angle and axis ratio, the effective radius of envelope components and outer components are plotted as vertical dot-dashed lines. Ellipses represent the effective radius of each individual component shown in the model image, which uses the same colour legend as in the surface brightness plot. We note that the shifts in both q and PA roughly correspond to the radius at which the outer component begins to dominate over the central envelope, at Re ∼ 500 pc. Error bars represent 1σ uncertainties.

Extended Data Fig. 3 Examples of Gemini/GMOS spectra of a bona fide eUCD and a background galaxy.

The Gemini/GMOS optical spectrum (black), the best-fitting template (red) and fitting residuals (gray) are plotted in the left panels. Right panels show the g′-band NGVS imaging and contours for each objects, both of which show extended envelopes. The strong Hα absorption is the main diagnostic for bona fide eUCDs. In contrast, the measured redshift of NGVS-UCD147 is z ∼ 0.133, a background galaxy composed of bulge+disk.

Extended Data Fig. 4 A comparison between NGVS and HST imaging for NGVS-UCD330.

The upper and lower row are NGVS and HST data, respectively. Contours in the left column represent the surface brightness at µg′ = 24 − 27 mag arcsec−2 in NGVS and µF475W = 23 − 25 mag arcsec−2 for HST. In the right column is the one-dimensional surface brightness profile of the UCD. NGVS-UCD330 shows marginal two components in NGVS imaging, and can be much clearly decomposed into two components in HST imaging. The total, King and Sérsic model profile are shown with red, magenta and green dashed line. The grey dashed line is the PSF profile. Error bars represent 1σ uncertainties.

Extended Data Fig. 5 A comparison between NGVS and HST imaging for NGVS-UCD167.

NGVS-UCD167 shows only one component in NGVS imaging, but can be resolved to two components in HST imaging.

Extended Data Fig. 6 Spatial distribution of compact stellar systems in the Virgo cluster.

Yellow and dark red circles represent UCDs and eUCDs, respectively. Blue circles and squares are strongly-nucleated dEs with and without velocity measurements. Gray circles represent all UCD candidates without velocity measurements4. Disrupting dEs21,28,30 and eUCDs that show tidal features are labelled with green stars. The Gaussian-smoothed distribution of Virgo dE,Ns identified in NGVS is shown in the background. While the density peak of all dE,Ns is located around M84 & M86 (northwest of M87), most eUCDs and strongly-nucleated dEs are concentrated around M87. Massive Virgo galaxies (Mg′ < −20) that could be primary contributors of tidal disruption are shown in white dashed circles, with circle sizes corresponding to their total luminosity. Most strongly-nucleated dEs and eUCDs have clear associations with larger galaxies, or form coherent substructures.

Source Data

Extended Data Fig. 7 Nuclear star cluster colour–magnitude diagram for eUCDs, strongly-nucleated dEs, and Virgo early-type galaxies.

Unlike the colours of the stellar envelopes, which are offset from the main galaxy colour–magnitude relation, the nuclear star clusters in UCDs have colours that are very similar to UCDs without envelopes, and the NSCs of normal dE,Ns. 1σ error bar for the colour and magnitude is shown for each object.

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Wang, K., Peng, E.W., Liu, C. et al. An evolutionary continuum from nucleated dwarf galaxies to star clusters. Nature 623, 296–300 (2023). https://doi.org/10.1038/s41586-023-06650-z

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