Dynamical history of the Local Group in ΛCDM

Banik, Indranil; Zhao, Hongsheng

doi:10.1093/mnras/stw787

Cited by 26 publications

(72 citation statements)

References 91 publications

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“…The high MW-M31 relative velocity would allow them to gravitationally slingshot any nearby dwarf outwards at high speed, leading to some LG dwarfs having an unusually high radial velocity for their position. We did in fact find some evidence for 5 or 6 high-velocity galaxies (HVGs) like this (Banik & Zhao 2016, a result also confirmed by Peebles (2017b) using his 3D ΛCDM model of the LG. We used a MOND model of the LG to demonstrate that the dwarfs reaching the fastest speeds were likely flung out almost parallel to the motion of the perturber.…”

Section: Introductionsupporting

confidence: 85%

Testing gravity with wide binary stars like α Centauri

Banik

Zhao

2018

Monthly Notices of the Royal Astronomical Society

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111

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We consider the feasibility of testing Newtonian gravity at low accelerations using wide binary (WB) stars separated by 3 kAU. These systems probe the accelerations at which galaxy rotation curves unexpectedly flatline, possibly due to Modified Newtonian Dynamics (MOND). We conduct Newtonian and MOND simulations of WBs covering a grid of model parameters in the system mass, semi-major axis, eccentricity and orbital plane. We self-consistently include the external field (EF) from the rest of the Galaxy on the Solar neighbourhood using an axisymmetric algorithm. For a given projected separation, WB relative velocities reach larger values in MOND. The excess is ≈ 20% adopting its simple interpolating function, as works best with a range of Galactic and extragalactic observations. This causes noticeable MOND effects in accurate observations of ≈ 500 WBs, even without radial velocity measurements.We show that the proposed Theia mission may be able to directly measure the orbital acceleration of Proxima Centauri towards the 13 kAU-distant α Centauri. This requires an astrometric accuracy of ≈ 1 µas over 5 years. We also consider the long-term orbital stability of WBs with different orbital planes. As each system rotates around the Galaxy, it experiences a time-varying EF because this is directed towards the Galactic Centre. We demonstrate approximate conservation of the angular momentum component along this direction, a consequence of the WB orbit adiabatically adjusting to the much slower Galactic orbit. WBs with very little angular momentum in this direction are less stable over Gyr periods. This novel direction-dependent effect might allow for further tests of MOND.

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Section: Introductionsupporting

confidence: 85%

Testing gravity with wide binary stars like α Centauri

Banik

Zhao

2018

Monthly Notices of the Royal Astronomical Society

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111

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“…These results are similar to those obtained by Peebles (2017) using a similar algorithm. Despite using a very different method to Banik & Zhao (2016), the conclusions remain broadly similar.…”

Section: Introductionmentioning

confidence: 76%

“…The integrating factor a (t) accounts for the effect of Hubble drag, which tends to reduce peculiar velocities in the absence of further forcing (as derived in e.g. Banik & Zhao 2016, section 2.1). 1 There is no unique way to solve for gext (t), but neither is it wholly unconstrained.…”

Section: The External Field On the Local Groupmentioning

confidence: 99%

“…Following on from previous spherically symmetric dynamical models (Sandage 1986;Peñarrubia et al 2014), we constructed an axisymmetric model of the LG consistent with the almost radial MW-M31 orbit (van der Marel et al 2012) and the close alignment of Cen A with this line (Ma et al 1998). Treating LG dwarfs as test particles in the gravitational field of these three massive moving objects, we inves-tigated a wide range of model parameters using a full grid search (Banik & Zhao 2016).…”

Section: Introductionmentioning

confidence: 99%

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Origin of the Local Group satellite planes

Banik

O’Ryan

Zhao

2018

Monthly Notices of the Royal Astronomical Society

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We attempt to understand the planes of satellite galaxies orbiting the Milky Way (MW) and M31 in the context of Modified Newtonian Dynamics (MOND), which implies a close MW-M31 flyby occurred ≈ 8 Gyr ago. Using the timing argument, we obtain MW-M31 trajectories consistent with cosmological initial conditions and present observations. We adjust the present M31 proper motion within its uncertainty in order to simulate a range of orbital geometries and closest approach distances. Treating the MW and M31 as point masses, we follow the trajectories of surrounding test particle disks, thereby mapping out the tidal debris distribution.Around each galaxy, the resulting tidal debris tends to cluster around a particular orbital pole. We find some models in which these preferred spin vectors align fairly well with those of the corresponding observed satellite planes. The radial distributions of material in the simulated satellite planes are similar to what we observe. Around the MW, our best-fitting model yields a significant fraction (0.22) of counter-rotating material, perhaps explaining why Sculptor counter-rotates within the MW satellite plane. In contrast, our model yields no counter-rotating material around M31. This is testable with proper motions of M31 satellites.In our best model, the MW disk is thickened by the flyby 7.65 Gyr ago to a root mean square height of 0.75 kpc. This is similar to the observed age and thickness of the Galactic thick disk. Thus, the MW thick disk may have formed together with the MW and M31 satellite planes during a past MW-M31 flyby.

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“…Since Pal 4 is a young halo globular cluster (YHGC) it most probably was formed in a tidal tail possibly drawn out from the young Milky Way in an early galaxy-galaxy encounter (Zhao et al 2013;Banik & Zhao 2016) which also spawned the disk-of-satellites (DoS, Kroupa, Theis, & Boily 2005;Metz, Kroupa, & Jerjen 2009). Together with the satellite galaxies and tidal streams, the YHGCs form the vast polar structure around the Milky Way (VPOS, Pawlowski, Kroupa, & de Boer 2011;Pawlowski & Kroupa 2013) show that the satellite galaxies which have porper motion measurements have Galactocentric tangential velocities which are typically larger than their Galactocentric radial velocities, implying orbital eccentricities which are small.…”

Section: Discussionmentioning

confidence: 99%

Direct N-body simulations of globular clusters – III. Palomar 4 on an eccentric orbit

Zonoozi

Haghi

Kroupa

et al. 2017

Mon. Not. R. Astron. Soc.

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Palomar 4 is a low-density globular cluster with a current mass ≈ 30000 M ⊙ in the outer halo of the Milky Way with a two-body relaxation time of the order of a Hubble time. Yet, it is strongly mass segregated and contains a stellar mass function depleted of low-mass stars. Pal 4 was either born this way or it is a result of extraordinary dynamical evolution. Since two-body relaxation cannot explain these signatures alone, enhanced mass loss through tidal shocking may have had a strong influence on Pal 4. Here, we compute a grid of direct N-body simulations to model Pal 4 on various eccentric orbits within the Milky Way potential to find likely initial conditions that reproduce its observed mass, half-light radius, stellar MF-slope and line-of-sight velocity dispersion. We find that Pal 4 is most likely orbiting on an eccentric orbit with an eccentricity of e ≈ 0.9 and pericentric distance of R p ≈ 5 kpc. In this scenario, the required 3D half-mass radius at birth is similar to the average sizes of typical GCs (R h ≈ 4−5 pc), while its birth mass is about M 0 ≈ 10 5 M ⊙ . We also find a high degree of primordial mass segregation among the cluster stars, which seems to be necessary in every scenario we considered. Thus, using the tidal effect to constrain the perigalactic distance of the orbit of Pal 4, we predict that the proper motion of Pal 4 should be in the range −0.52 µ δ −0.38 mas yr −1 and −0.30 µ α cos δ −0.15 mas yr −1 .

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Dynamical history of the Local Group in ΛCDM

Cited by 26 publications

References 91 publications

Testing gravity with wide binary stars like α Centauri

Testing gravity with wide binary stars like α Centauri

Origin of the Local Group satellite planes

Direct N-body simulations of globular clusters – III. Palomar 4 on an eccentric orbit

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