THE M31 VELOCITY VECTOR. II. RADIAL ORBIT TOWARD THE MILKY WAY AND IMPLIED LOCAL GROUP MASS

By R. P. van der Marel, Mark A. Fardal, G. Besla, R. Beaton, S. Sohn, Jay Anderson, Tom Brown, P. Guhathakurta. 2012

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We determine the velocity vector of M31 with respect to the Milky Way and use this to constrain the mass of the Local Group, based on Hubble Space Telescope proper-motion measurements of three fields presented in Paper I. We construct N-body models for M31 to correct the measurements for the contributions from stellar motions internal to M31. This yields an unbiased estimate for the M31 center-of-mass motion. We also estimate the center-of-mass motion independently, using the kinematics of satellite galaxies of M31 and the Local Group, following previous work but with an expanded satellite sample. All estimates are mutually consistent, and imply a weighted average M31 heliocentric transverse velocity of (vW, vN) = (− 125.2 ± 30.8, −73.8 ± 28.4) km s−1. We correct for the reflex motion of the Sun using the most recent insights into the solar motion within the Milky Way, which imply a larger azimuthal velocity than previously believed. This implies a radial velocity of M31 with respect to the Milky Way of Vrad, M31 = −109.3 ± 4.4 km s−1, and a tangential velocity of Vtan, M31 = 17.0 km s−1, with a 1σ confidence region of Vtan, M31 ⩽ 34.3 km s−1. Hence, the velocity vector of M31 is statistically consistent with a radial (head-on collision) orbit toward the Milky Way. We revise prior estimates for the Local Group timing mass, including corrections for cosmic bias and scatter, and obtain MLG ≡ MMW, vir + MM31, vir = (4.93 ± 1.63) × 1012 M☉. Summing known estimates for the individual masses of M31 and the Milky Way obtained from other dynamical methods yields smaller uncertainties. Bayesian combination of the different estimates demonstrates that the timing argument has too much (cosmic) scatter to help much in reducing uncertainties on the Local Group mass, but its inclusion does tend to increase other estimates by ∼10%. We derive a final estimate for the Local Group mass from literature and new considerations of MLG = (3.17 ± 0.57) × 1012 M☉. The velocity and mass results at 95% confidence imply that M33 is bound to M31, consistent with expectation from observed tidal deformations.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/1cc9aff78bea25071fd54402b2c70de9b6efbbab

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Citations in the corpus

Hunting for the Dark Matter Wake Induced by the Large Magellanic Cloud (2019, methodology): “…the MW is represented by a Hernquist profile (Hernquist 1990), where the scale length ra was chosen in order to guarantee that the enclosed mass at the virial radius is the same as that of the equivalent NFW profile (for details of this procedure, see the appendix of van der Marel et al. (2012)).”
Evidence for an Intermediate-mass Milky Way from Gaia DR2 Halo Globular Cluster Motions (2018, background): “…et al. 2010, radial anisotropy with Leo I unbound) to as high as 2−3×1012M from abundance-matching studies (e.g. Boylan-Kolchin et al. 2010), the timing argument (Li & White 2008; van der Marel et al. 2012b), or studies of tracers (e.g. Watkins et al. 2010, tangential anisotropy with Leo I bound).”
The APOSTLE project: Local Group kinematic mass constraints and simulation candidate selection (2015, methodology): “We also use the relative tangential velocity of the M31–MW pair derived from M31’s proper motion by van der Marel et al. (2012).”
Too big to fail in the Local Group (2014, background): “…mass must be below ∼7 × 1011 M to eliminate the massive failures (see also BoylanKolchin et al. 2012; Purcell & Zentner 2012; Wang et al. 2012), which is in conflict with large-scale dynamical mass estimates of the MW (van der Marel et al. 2012; Boylan-Kolchin et al. 2013, and references therein).”
Is there a "too big to fail" problem in the field? (2014, background): “This finding weakens the “light” MW argument, because it is unlikely that both the MW and Andromeda are hosted by halos with Mvir < 1× 1012 M⊙ (van der Marel et al. 2012).”
The RAVE survey : the Galactic escape speed and the mass of the Milky Way (2013, background): “…mass and the mass profile of the Galaxy is Article published by EDP Sciences A91, page 1 of 16 crucial for understanding and modeling the dynamic evolution of the MW satellite galaxies (e.g., Kallivayalil et al. 2013, for the Magellanic clouds) and the Local Group (van der Marel et al. 2012b,a).”
Dwarf galaxy planes: the discovery of symmetric structures in the Local Group (2013, methodology): “After correcting the measured PMs for the internal kinematics and averaging over the three fields, van der Marel et al. (2012) arrive at a heliocentric PM measurement for M31 of vW = −162.8 ± 47.0 km s−1 towards the west and vN = −117.2±45.0 km s−1 towards the north, according to their table 3.”
The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, methodology): “Note that if the dark matter can be represented by a Navarro, Frenk & White (NFW) halo, its scale length is rh ≈ 25 kpc, assuming a concentration parameter c ≈ 10 (Figure 1; van der Marel et al. 2012a).”
ELVIS: Exploring the Local Volume in Simulations (2013, methodology): “We further made an effort to include two pairs that had c© 2013 RAS, MNRAS 000, 1–22 very low relative tangential velocities < 15 km s−1 in order to mimic the low relative tangential speed of the MW/M31 pair (van der Marel et al. 2012).”
Galaxy Masses: A Review (2013, background): “If the putative transverse velocity of M31 (van der Marel et al. 2012) is taken into account, the lower bound for the mass of the Milky Way drops from 2.5 × 1012M to 1.5 × 1012M in the 95% confidence region.”
THE SPACE MOTION OF LEO I: THE MASS OF THE MILKY WAY'S DARK MATTER HALO (2012, background): “…value and range are similar to Zaritsky et al. (1989) and lower than, but consistent with, Li & White (2008) and Sohn et al. (2013). van der Marel et al. (2012b) have computed Mvir,MW based on the timing argument for the MW–M31 pair and find an average for Mvir,MW of 1.63 × 1012 M , in…”
A timing constraint on the (total) mass of the Large Magellanic Cloud (2015, background): “Using similar arguments van der Marel et al. (2012b) conclude that neither M33 nor M32 change the barycentre of M31 appreciably.”
THIRD-EPOCH MAGELLANIC CLOUD PROPER MOTIONS. II. THE LARGE MAGELLANIC CLOUD ROTATION FIELD IN THREE DIMENSIONS (2013, background): “More recently it has even become possible to go out as far as M31 (Sohn et al. 2012; van der Marel et al. 2012a,b).”
THE MILKY WAY'S CIRCULAR-VELOCITY CURVE BETWEEN 4 AND 14 kpc FROM APOGEE DATA (2012, methodology): “This measurement leads to a combined mass for the Local Group of ∼2.4× 1012M⊙, which is consistent with the Local-Group-timing-argument within the large cosmic scatter (van der Marel et al. 2012).”
The Local Group on FIRE: dwarf galaxy populations across a suite of hydrodynamic simulations (2018, background): “For a comparison, the MW and M31 are separated by 787 kpc (McConnachie et al. 2005) and are approaching one another with vrad = −109 km s−1 and vtan = 17 ± 17 km s−1 (van der Marel et al. 2012, though see Salomon et al. 2016 and Carlesi et al. 2016).”