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An extended halo around an ancient dwarf galaxy

Nature Astronomy volume 5, pages 392–400 (2021)Cite this article

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Abstract

The Milky Way is surrounded by dozens of ultrafaint (<105 L☉) dwarf satellite galaxies1,2,3. They are the remnants of the earliest galaxies4, as confirmed by their ancient5 and chemically primitive6,7 stars. Simulations8,9,10 suggest that these systems formed within extended dark matter halos and experienced early galaxy mergers and feedback. However, the signatures of these events would lie outside their core regions11, where spectroscopic studies are challenging12. Here we identify members of the Tucana II ultrafaint dwarf galaxy out to nine half-light radii, demonstrating the system to be markedly more spatially extended and chemically primitive than previously found. The distant stars in this galaxy are, on average, extremely metal poor (≲1/1000 of the solar iron abundance), affirming Tucana II as the most metal-poor known galaxy. We observationally establish an extended dark matter halo surrounding an ultrafaint dwarf galaxy out to 1 kpc, with a total mass of >107 M☉, consistent with a generalized Navarro–Frenk–White density profile. The extended nature of Tucana II suggests that it may have undergone strong bursty feedback or been the product of an early galactic merger10,11. We demonstrate that spatially extended stellar populations in ultrafaint dwarf galaxies13,14 are observable, opening up the possibility for detailed studies of the stellar halos of relic galaxies.

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Fig. 1: Spatial, radial velocity and metallicity distributions of Tucana II stars.
Fig. 2: Mass modelling of Tucana II out to 1 kpc.

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Data availability

The velocity and metallicity measurements that support the findings of this study are presented in Supplementary Tables 1, 2 and 3. The individual stellar spectra from which these measurements were derived and any supplementary material (for example line lists) are available from the corresponding author upon reasonable request. The proper motions of the stars analysed in this paper are publicly available from the Gaia DR2 archive (http://gea.esac.esa.int/archive/). Source data are provided with this paper.

Code availability

The stellar synthesis code MOOG can be retrieved from https://github.com/alexji/moog17scat. The other codes used in this analysis are the authors’ implementations of published techniques (for example, calcium ii K and calcium triplet calibrations), and are available from the corresponding author upon reasonable request.

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Acknowledgements

Our data were gathered using the 6.5 m Magellan Baade telescope located at Las Campanas Observatory, Chile. A.C. thanks M. Magg, A. Toomre and T. Slatyer for discussions. A.C. and A.F. acknowledge support from NSF grant AST-1716251. J.D.S. is supported by NSF grant AST-1714873. A.P.J. is supported by NASA through Hubble Fellowship Grant HST-HF2-51393.001, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. H.J. acknowledges support from the Australian Research Council through the Discovery Project DP150100862. This work made use of NASA’s Astrophysics Data System Bibliographic Services, the SIMBAD database, operated at CDS, Strasbourg, France and the open-source Python libraries NumPy, SciPy, Matplotlib and Astropy. This project used public archival data from the DES. Funding for the DES Projects has been provided by the US Department of Energy, the US National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana–Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência, Tecnologia e Inovação, the Deutsche Forschungsgemeinschaft and the Collaborating Institutions in the DES. The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Enérgeticas, Medioambientales y Tecnológicas—Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenössische Technische Hochschule (ETH) Zürich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana–Champaign, the Institut de Ciències de l’Espai (IEEC/CSIC), the Institut de Física d’Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-Maximilians Universität München and the associated Excellence Cluster Universe, the University of Michigan, the National Optical Astronomy Observatory, the University of Nottingham, The Ohio State University, the OzDES Membership Consortium, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex and Texas A&M University. Based in part on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://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. The national facility capability for SkyMapper has been funded through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the ANU, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University and the Australian Astronomical Observatory. SkyMapper is owned and operated by the ANU’s Research School of Astronomy and Astrophysics. This research uses services or data provided by the Astro Data Archive at NSF’s OIR Lab. NSF’s OIR Lab is managed by AURA under a cooperative agreement with the National Science Foundation.

Author information

Authors and Affiliations

  1. Department of Physics & Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA

    Anirudh Chiti & Anna Frebel

  2. Observatories of the Carnegie Institution for Science, Pasadena, CA, USA

    Joshua D. Simon, Lina Necib & Alexander P. Ji

  3. Department of Physics, University of Surrey, Guildford, UK

    Denis Erkal

  4. Department of Physics, Princeton University, Princeton, NJ, USA

    Laura J. Chang

  5. Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australian Capital Territory, Australia

    Helmut Jerjen & John E. Norris

  6. Department of Astronomy, University of California, Berkeley, Berkeley, CA, USA

    Dongwon Kim

Authors
  1. Anirudh Chiti
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  2. Anna Frebel
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  3. Joshua D. Simon
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  4. Denis Erkal
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  6. Lina Necib
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  7. Alexander P. Ji
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Contributions

A.C. selected candidates for the observations, made the observations and led the analysis and paper writing; A.F. assisted with the MagE observations and subsequent analysis and J.D.S. assisted with the IMACS observations and subsequent analysis; H.J., D.K. and J.E.N. provided the SkyMapper images from which targets were selected; D.E. modelled the orbit of Tucana II; L.J.C. and L.N. modelled the extended density profile of Tucana II; A.P.J. contributed to the analysis of the MagE spectra; all authors contributed to the interpretation of the data, and contributed to writing the paper or provided feedback.

Corresponding author

Correspondence to Anirudh Chiti.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Quinn Minor 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.

Extended data

Extended Data Fig. 1 Identification of candidate members of Tucana II.

a. Locations of candidate members (blue data points) with g < 19.5. Candidates were selected by identifying metal-poor giants with SkyMapper photometry (photometric [Fe/H] < − 1.0 and photometric log g < 3.0 ref. 21), and then only including stars with proper motions around the systemic proper motion of Tucana II (0.2 mas/yr < \({\mu_{\alpha}}\cos\delta\)< 1.4 mas/yr and −1.7 mas/yr < \(\mu_\delta\) < −0.5 mas/yr). All stars confirmed as members of Tucana II in this work or prior work15,17 are highlighted in orange. Confirmed non-members of Tucana II that were observed in this work are marked in magenta. b. Proper motions of candidate members with g < 19.5. The notation of the data points is equivalent to that in panel a. The majority of stars with proper motions near the systemic proper motion of Tucana II are members. This results from our exclusion of stars that are not metal-poor giants using log g to cut out foreground stars. Milky Way foreground stars outside our proper motion selection criteria are shown as small black points. The error bars on the proper motions correspond to 1sigma uncertainties in the Gaia DR2 catalog.

Source data

Extended Data Fig. 2 SkyMapper photometry of Tucana II members observed with MagE.

a. A metallicity-sensitive SkyMapper color-color plot of every star within a degree of Tucana II. The Tucana II members observed with MagE in this study are shown as red stars, and all have photometric [Fe/H] < -1.0. Photometric metallicities were derived following ref. 21, and are indicated by the color scale. The magnitudes are in the AB magnitude system. b. A surface gravity-sensitive SkyMapper color-color plot of every star within a degree of Tucana II. Similarly to the metallicity-sensitive plot, the Tucana II members observed with MagE separate from the foreground population due to their low surface gravities. The surface gravities are indicated by the color scale. Magnitudes are in the AB magnitude system.

Source data

Extended Data Fig. 3 Color-magnitude diagram of Tucana II and sample spectra.

a. Color-magnitude diagram of the MagE and IMACS Tucana II members with DES photometry. A 10 Gyr, [Fe/H] = -2.2 MIST isochrone104,105,106,107,108 at the distance modulus of Tucana II18 is overplotted for reference. The horizontal branch from a PARSEC isochrone109,110,111,112,113,114 with the same parameters is also shown. Members and non-members are indicated in blue and orange, respectively. The two most distant members are outlined in pink. Magnitudes are in the AB magnitude system. b–d. MagE spectra of the magnesium region Tuc2-319, Tuc2-318, and Tuc2-305. The absorption lines in the region become noticeably weaker at lower metallicities. A dashed horizontal line is drawn at the continuum level to guide the eye.

Source data

Extended Data Fig. 4 Comparison of Tucana II radial velocities and metallicities to simulated radial velocities and metallicities of foreground stars.

a. A histogram of MagE and IMACS radial velocities of stars determined to be non-members of Tucana II is shown in orange. In blue, we plot a scaled histogram of radial velocities of stars in the field of Tucana II, as generated from the Besancon model of stellar populations in the galaxy72 after replicating our target selection cuts (blue). The vertical red line marks the systemic velocity of Tucana II15 and the green shaded region corresponds to our Tucana II velocity membership criteria (−141 km/s < HRV < − 110 km/s; [Fe/H] < − 2.0), which is well separated from the foreground velocity distribution. b. Scaled histogram of metallicities of stars generated from the Besancon model following those in panel a. The green shaded region ([Fe/H] < − 2.0) corresponds to the metallicities of the newly detected Tucana II members. Only 0.4% of simulated foreground stars satisfy our Tucana II velocity and metallicity membership criteria (−141 km/s < HRV < − 110 km/s; [Fe/H] < − 2.0).

Supplementary information

Supplementary Information

Supplementary Table 1.

Supplementary Table 2

All velocity measurements and uncertainties. Summary of velocity measurements of stars in our sample. The MJD column lists the modified Julian date of each observation. The Right Ascension (RA) and Declination (DEC) columns indicate the coordinates. The following column lists DES g magnitudes for stars observed with IMACS, and SkyMapper g magnitudes for stars observed with MagE. The subsequent column indicates the instrument with which each velocity was determined. The S/N column indicates the signal-to-noise ratio at 850nm. The rv and e_rv columns indicate the velocity measurements and uncertainties. The last two columns list membership status and comments, where W16 refers to ref. 1.

Supplementary Table 3

Comprehensive list of metallicities and uncertainties of Tucana II members. Summary of metallicities of Tucana II member stars. The Right Ascension (RA) and Declination (DEC) indicate the coordinates of each star. [Fe/H]_mg and e_[Fe/H]_mg indicate the metallicities and uncertainties as derived from the magnesium b region at ~515nm. [Fe/H]_CaT and e_[Fe/H]_CaT indicate the metallicities and uncertainties as derived from the calcium triplet region at ~850nm. The instrument with which each metallicity was determined is listed as the next-to-last column. Comments are listed in the last column, where W16 refers to ref. 1.

Source data

Source Data Extended Data Fig. 1

Coordinates (RA J2000 and dec. J2000), Gaia DR2 proper motions and uncertainties, and Tucana II membership status of stars.

Source Data Extended Data Fig. 2

SkyMapper g, i, v, u photometry, along with photometric surface gravities and metallicities. Confirmed Tucana II members that were observed with MagE are indicated.

Source Data Extended Data Fig. 3

Coordinates (RA J2000 and dec. J2000), DES g, i photometry and Tucana II membership status.

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Chiti, A., Frebel, A., Simon, J.D. et al. An extended halo around an ancient dwarf galaxy. Nat Astron 5, 392–400 (2021). https://doi.org/10.1038/s41550-020-01285-w

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