The Velocity Dispersion Profile of the Remote Dwarf Spheroidal Galaxy Leo I: A Tidal Hit and Run?

By M. Mateo, E. Olszewski, M. Walker. 2007

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We present new kinematic results for 387 stars near the Milky Way satellite dwarf spheroidal galaxy Leo I. Spectra were obtained with the Hectochelle multiobject echelle spectrograph on the MMT, centered in the optical near 5200 Å. From 297 repeat measurements of 108 stars, we estimate the mean velocity error (1 σ) of our sample to be 2.4 km s−1, with a systematic precision of ≤1 km s−1. The final sample of 328 Leo I members gives a mean heliocentric velocity of 282.9 ± 0.5 km s−1 and a dispersion of 9.2 ± 0.4 km s−1. The dispersion profile of Leo I is flat to beyond its classical “tidal” radius. We fit the profile to various equilibrium dynamical models. We strongly rule out all models where mass follows light. Anisotropic Sérsic+NFW models fit the dispersion profile well, but isotropic models are ruled out at a 95% confidence level. Inside a projected radius of ~1040 pc, the mass and V-band mass-to-light ratio of Leo I from equilibrium models are in the ranges (5–7) × 107 M☉ and 9-14 (solar units), respectively. Leo I members outside a “break radius” of Rb ∼ 400″ (500 pc) exhibit significant velocity anisotropy, whereas stars interior to this radius are consistent with an isotropic velocity distribution. We interpret the break radius as the tidal radius of Leo I at perigalacticon some 1-2 Gyr ago. This interpretation accounts for the complex star formation history of Leo I, population segregation within the galaxy, and Leo I's large outward galactocentric velocity. The lack of evident tidal arms in Leo I suggests that the galaxy may have been injected into its present highly elliptical orbit by a third body a few Gyr before its last perigalacticon. This scenario is plausible within current hierarchical structure formation models.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/e8ff704091a4d9d0cf81f0fa273830fe87291309

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Citations in the corpus

THE M31 VELOCITY VECTOR. II. RADIAL ORBIT TOWARD THE MILKY WAY AND IMPLIED LOCAL GROUP MASS (2012, background): “…of reasons, especially when the goal is to estimate galaxy masses: the satellite system may not be virialized, with continuing orbital evolution (Mateo et al. 2008) or infall (e.g., Chapman et al. 2007; Majewski et al. 2007); the distribution of satellite orbits may not be isotropic (e.g.,…”
The discovery of Segue 2: a prototype of the population of satellites of satellites (2009, methodology): “We extracted and calibrated spectra following the procedure described by Mateo et al. (2008).”
A UNIVERSAL MASS PROFILE FOR DWARF SPHEROIDAL GALAXIES? (2009, methodology): “Among the remaining dSphs in our sample, there exists kinematic evidence of tidal streaming motion in the outer parts of Carina (Muñoz et al. 2006) and Leo I (Sohn et al. 2007; Mateo et al. 2008).”
Core formation in dwarf haloes with self-interacting dark matter: no fine-tuning necessary (2014, methodology): “The points are taken from Wolf et al. (2010), who used data from Muñoz et al. (2005), Koch et al. (2007), Simon & Geha (2007), Mateo et al. (2008), and Walker et al. (2009).”
Gaia DR2 proper motions of dwarf galaxies within 420 kpc (2018, methodology): “For Leo I we use the catalog of Mateo et al. (2008), which does not contain membership probabilities nor gravities.”
THE SPACE MOTION OF LEO I: THE MASS OF THE MILKY WAY'S DARK MATTER HALO (2012, background): “…argument as applied to Leo I will almost certainly overestimate Mvir,MW. Leo I’s extreme kinematic properties have therefore led to substantial debate about its status as a bound satellite of the MW (Zaritsky et al. 1989; Byrd et al. 1994; Sales et al. 2007; Sohn et al. 2007; Mateo et al. 2008).”
Can feedback solve the too-big-to-fail problem? (2013, methodology): “…values of the mass enclosed within the deprojected half-light radius of each of the bright dSphs around the Milky Way (computed by Wolf et al. 2010, who used data from Walker et al. 2009 in addition to data from Muñoz et al. 2005; Koch et al. 2007; Simon & Geha 2007 and Mateo et al. 2008).”
DWARF GALAXY ANNIHILATION AND DECAY EMISSION PROFILES FOR DARK MATTER EXPERIMENTS (2014, methodology): “We use the stellar-kinematic data published by Mateo et al. (2008) for Leo I, and by Walker, Mateo & Olszewski (2009) for Carina, Fornax, Sculptor and Sextans.”
STELLAR VELOCITIES IN THE CARINA, FORNAX, SCULPTOR, AND SEXTANS dSph GALAXIES: DATA FROM THE MAGELLAN/MMFS SURVEY (2008, background): “…of stars per dSph, velocity dispersion profiles are now available for all of the brighter MW dSphs (Kleyna et al. 2002, 2004; Wilkinson et al. 2004; Muñoz et al. 2005, 2006; Sohn et al. 2007; Walker et al. 2006a, 2006b, 2007b; Battaglia et al. 2006; Mateo et al. 2008; Koch et al. 2007b, 2007a).”
Formation of isolated dwarf galaxies with feedback (2009, background): “Their dynamics appear to be largely dark matter dominated, and measurements of stellar velocity dispersions (e.g. Koch et al. 2007; Walker et al. 2007; Mateo et al. 2008; Walker et al. 2009) indicate that they possess the highest mass-to-light ratios of any known galactic systems.”
Accurate masses for dispersion-supported galaxies (2009, methodology): “…mass likelihoods, we analyze the kinematics of individual stars (Muñoz et al. 2005, 2006; Koch et al. 2007; Martin et al. 2007; Simon & Geha 2007; Mateo et al. 2008; Walker et al. 2009a; Geha et al. 2009, Willman et al., in preparation)16, in conjunction with the distances and stellar surface…”
MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. III. METALLICITY DISTRIBUTIONS OF MILKY WAY DWARF SATELLITE GALAXIES (2010, background): “Its highly elliptical orbit (Sohn et al. 2007; Mateo et al. 2008) allows Leo I to spend most of its time far from the MW center, allowing little time for strong tidal interactions.”
One Law to Rule Them All: The Radial Acceleration Relation of Galaxies (2016, unknown): “…(1995), 3: Walker et al. (2009c), 4: Bonanos et al. (2004), 5: Martin et al. (2008), 6: Walker et al. (2007), 7: Bellazzini et al. (2004), 8: Mateo et al. (2008), 9: Bellazzini et al. (2005), 10: Koch et al. (2007), 11: Pietrzyński et al. (2008), 12: Lee et al. (2009), 13: Carrera et al.…”