The massive relic galaxy NGC 1277 is dark matter deficient

From dynamical models of integral-field stellar kinematics out to five effective radii^⋆

¹ Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
e-mail: lsebasti@ull.edu.es
² Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
³ Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁴ Departamento de Física Teórica, Atómica y Óptica, Universidad de Valladolid, 47011 Valladolid, Spain
⁵ Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisbon, Portugal
⁶ Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Luis Enrique Erro No. 1, Tonantzintla, Puebla CP 72840, Mexico
⁷ Consejo Nacional de Ciencia y Tecnología, Av. Insurgentes Sur 1582, 03940 México City, Mexico
⁸ Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679 Munich, Germany
⁹ Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St, 03680 Kyiv, Ukraine
¹⁰ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹¹ Instituto de Física de Partículas y del Cosmos IPARCOS, Fac. de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 1 March 2023
Accepted: 6 June 2023

Abstract

According to the Λ cold dark matter (ΛCDM) cosmology, present-day galaxies with stellar masses M_⋆ > 10¹¹ M_⊙ should contain a sizable fraction of dark matter within their stellar body. Models indicate that in massive early-type galaxies (ETGs) with M_⋆ ≈ 1.5 × 10¹¹ M_⊙, dark matter should account for ∼15% of the dynamical mass within one effective radius (1 R_e) and for ∼60% within 5 R_e. Most massive ETGs have been shaped through a two-phase process: the rapid growth of a compact core was followed by the accretion of an extended envelope through mergers. The exceedingly rare galaxies that have avoided the second phase, the so-called relic galaxies, are thought to be the frozen remains of the massive ETG population at z ≳ 2. The best relic galaxy candidate discovered to date is NGC 1277, in the Perseus cluster. We used deep integral field George and Cynthia Mitchel Spectrograph (GCMS) data to revisit NGC 1277 out to an unprecedented radius of 6 kpc (corresponding to 5 R_e). By using Jeans anisotropic modelling, we find a negligible dark matter fraction within 5 R_e (f_DM(5 R_e) < 0.05; two-sigma confidence level), which is in tension with the ΛCDM expectation. Since the lack of an extended envelope would reduce dynamical friction and prevent the accretion of an envelope, we propose that NGC 1277 lost its dark matter very early or that it was dark matter deficient ab initio. We discuss our discovery in the framework of recent proposals, suggesting that some relic galaxies may result from dark matter stripping as they fell in and interacted within galaxy clusters. Alternatively, NGC 1277 might have been born in a high-velocity collision of gas-rich proto-galactic fragments, where dark matter left behind a disc of dissipative baryons. We speculate that the relative velocities of ≈2000 km s⁻¹ required for the latter process to happen were possible in the progenitors of the present-day rich galaxy clusters.