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Spatially Resolved Stellar Kinematics of the Ultra-diffuse Galaxy Dragonfly 44. I. Observations, Kinematics, and Cold Dark Matter Halo Fits

van Dokkum, Pieter and Wasserman, Asher and Danieli, Shany and Abraham, Roberto and Brodie, Jean and Conroy, Charlie and Forbes, Duncan A. and Martin, Christopher and Matuszewski, Matt and Romanowsky, Aaron J. and Villaume, Alexa (2019) Spatially Resolved Stellar Kinematics of the Ultra-diffuse Galaxy Dragonfly 44. I. Observations, Kinematics, and Cold Dark Matter Halo Fits. Astrophysical Journal, 880 (2). Art. No. 91. ISSN 1538-4357. doi:10.3847/1538-4357/ab2914.

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We present spatially resolved stellar kinematics of the well-studied ultra-diffuse galaxy (UDG) Dragonfly 44, as determined from 25.3 hr of observations with the Keck Cosmic Web Imager. The luminosity-weighted dispersion within the half-light radius is σ_(1/2) = 33^(+3)_(−3) km s^(−1), lower than what we had inferred before from a DEIMOS spectrum in the Hα region. There is no evidence for rotation, with V_(max)/ σ < 0.12 (90% confidence) along the major axis, in possible conflict with models where UDGs are the high-spin tail of the normal dwarf galaxy distribution. The spatially averaged line profile is more peaked than a Gaussian, with Gauss–Hermite coefficient h_4 = 0.13 ± 0.05. The mass-to-light ratio (M/L) within the effective radius is (M_(dyn)/L_I)(<R_e) = 26^(+7)_(-6) M⊙/L ⊙, similar to other UDGs and higher by a factor of six than smaller galaxies of the same luminosity. This difference between UDGs and other galaxies is, however, sensitive to the aperture that is used, and it is much reduced when the M/L ratios are measured within a fixed radius of 10 kpc. Dragonfly 44 has a rising velocity dispersion profile, from σ = 26^(+4)_(-4) km s^(−1) at R = 0.2 kpc to σ = 41^(+8)(-8) km s^(−1) at R = 5.1 kpc. The profile can only be fit with a cuspy Navarro–Frenk–White profile if the orbital distribution has strong tangential anisotropy, with β = -0.8^(+0.4)_(-0.5). An alternative explanation is that the dark matter profile has a core: a Di Cintio et al. density profile with a mass-dependent core provides a very good fit to the kinematics for a halo mass of log(M_(200)/M⊙ = 11.2^(+0.6)_(-0.6) and β = -0.1^(+0.2)_(-0.3), i.e., isotropic orbits. This model predicts a slight positive kurtosis, in qualitative agreement with the measured h_4 parameter. UDGs such as Dragonfly 44 are dark matter dominated even in their centers and can constrain the properties of dark matter in a regime where baryons usually dominate the kinematics: small spatial scales in massive halos. In a companion paper we provide constraints on the axion mass in the context of "fuzzy" dark matter models.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
van Dokkum, Pieter0000-0002-8282-9888
Wasserman, Asher0000-0003-4235-3595
Danieli, Shany0000-0002-1841-2252
Abraham, Roberto0000-0002-4542-921X
Brodie, Jean0000-0002-9658-8763
Conroy, Charlie0000-0002-1590-8551
Martin, Christopher0000-0002-8650-1644
Romanowsky, Aaron J.0000-0003-2473-0369
Villaume, Alexa0000-0003-1887-0621
Additional Information:© 2019 The American Astronomical Society. Received 2019 March 31; revised 2019 May 25; accepted 2019 June 5; published 2019 July 30. This paper is dedicated to Mariella Silvia, a young woman with a passion for astronomy who has inspired us with her fortitude. Support from HST grant HST-GO-14643 and NSF grants AST-1312376, AST-1616710, AST-1518294, and AST-1613582 is gratefully acknowledged. This work was partially supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We are grateful to the staff of Keck Observatory, and in particular Luca Rizzi, for their excellent support and help. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. A.J.R. is a Research Corporation for Science Advancement Cottrell Scholar.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
NASA Hubble FellowshipHST-GO-14643
W. M. Keck FoundationUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Subject Keywords:dark matter – galaxies: evolution – galaxies: halos – galaxies: structure
Issue or Number:2
Record Number:CaltechAUTHORS:20190731-100739159
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Official Citation:Pieter van Dokkum et al 2019 ApJ 880 91
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:97547
Deposited By: Tony Diaz
Deposited On:31 Jul 2019 19:58
Last Modified:16 Nov 2021 17:32

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