Dynamical Models for the Sculptor Dwarf Spheroidal in a ΛCDM Universe

The 3D motions of stars in small galaxies beyond our own are minute and yet they are crucial for our understanding of the nature of gravity and dark matter 1,2 . Even for the dwarf galaxy Sculptor which is one of the best studied systems and inferred to be strongly dark matter dominated 3,4 , there are conflicting reports 5,6,7 on its mean motion around the Milky Way and the 3D internal motions of its stars have never been measured. Here we report, based on data from the Gaia space mission 8 and the Hubble Space Telescope, a new precise measurement of Sculptor's mean proper motion. From this we deduce that Sculptor is currently at its closest approach to the Milky Way and moving on an elongated highinclination orbit that takes it much farther away than previously thought. For the first time we are also able to measure the internal motions of stars in Sculptor. We find σ R = 11.5 ± 4.3 km s −1 and σ T = 8.5 ± 3.2 km s −1 along the projected radial and tangential directions, implying that the stars in our sample move preferentially on radial orbits as quantified by the anisotropy parameter, which we find to be β ∼ 0.86 +0.12 −0.83 at a location beyond the core radius. Taken at face value such a high radial anisotropy requires abandoning conventional models 9 for the mass distribution in Sculptor. Our sample is dominated by metal-rich stars and for these we find β M R ∼ 0.95−0.27 , a value consistent with multi-component models where Sculptor is embedded in a cuspy dark halo 10 as expected for cold dark matter.To measure the proper motions (PMs) of individual stars in Sculptor we used data taken 12.27 years apart. The first epoch was acquired with the Advanced Camera for Surveys on board HST. The data set consists of two overlapping pointings separated by about 2 ′ (∼ 50 pc, see Fig. 1), each split in several 400 sec exposures in the F775W filter. The overlapping field-of-view has been observed 11 times. We obtained a catalog of positions, instrumental magnitudes and Point Spread Function (PSF) fitting-quality parameters by treating each chip of each exposure independently. Stellar positions were corrected for filter-dependent geometric distortions 11 . We then cross-matched the single catalogs to compute 3σ-clipped * Corresponding author: massariATastro.rug.nl -2 -average positions, magnitudes and corresponding uncertainties. We built the complete HST catalog after excluding all the saturated sources and those that were measured less than 4 times. The second epoch is provided by the Gaia first data release 12 . We extracted from the Gaia archive all sources in the direction of Sculptor.We transformed the HST positions to the equatorial reference frame defined by the Gaia data (right ascension, RA, and declination, DEC), using a six-parameter linear transformation 14 . We found 126 stars in common and their PMs were computed as the difference between the Gaia and HST positions, divided by the temporal baseline. The uncertainties on the PMs were computed as the sum in quadrature between the Gaia and HST positional e...

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“…−0.27 , a value consistent with multi-component models where Sculptor is embedded in a cuspy dark halo 10 as expected for cold dark matter.…”

Section: +004supporting

confidence: 84%

“…Since the metal-rich and metal-poor populations of Sculptor have been clearly separated on the basis of their metallicity 4 , we preferred to work with the iron spectral index rather than with the mean reduced Mg index used in other works 25,10 .…”

mentioning

confidence: 99%

Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era

et al. 2017

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“…As we discussed in the introduction, the dSphs structure is still an open debate due to the difficulty of distinguishing cusps from cores (e.g., Strigari et al 2014). de Bruijne (2012) indicated GAIA as a possible solutions to the riddle, and Takada (2010) suggested the Subaru Hyper-SupremeCamera.…”

Section: Discussionmentioning

confidence: 98%

On the dark matter haloes inner structure and galaxy morphology

Popolo

2016

Astrophys Space Sci

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In the present paper, we extend the study of Del Popolo (Mon. Not. R. Astron. Soc. 408:1808 to determine the slope of the inner density profile of galaxy haloes with different morphologies.We study how galaxy morphology changes the relation between the inner slope of the galaxy halo density profile, α, and the stellar mass, M * , or rotation velocity, V rot . For this, we use the model of Del Popolo (Astrophys. J. 698:293, 2009) in combination with observed data from the Romanowsky and Fall (Astrophys. J. Suppl. Ser. 203:17, 2012) sample of elliptical and spiral galaxies, the Local Group sample compiled by McConnachie (Astrophys. J. 144:4, 2012), and the simulation results by Cloet-Osselaer et al. (Mon. Not. R. Astron. Soc. 442:2909, 2014).We find that the slope α flattens monotonically, from α −1 at V rot 250 km/s, to α 0. After V rot 25 km/s the slope starts to steepen. The steepening happens in the mass range dominated by non-rotationally supported galaxies (e.g., dSphs) and depends on the level of offset in the angular momentum of rotationally and non-rotationally dominated galaxies. The steepening is a consequence of the decrease in baryons content, and angular momentum in spheroidal dwarf galaxies.We finally compare our result to the SPH simulations of Di Cintio. Our result is in qualitative agreement with their simulations, with the main difference that the inner slope α at small stellar masses (M * 10 8 M ) is flatter than that in their simulations.As a result, the claim that finding a core in dwarf galaxies with masses slightly smaller than 10 6 M (as in the Di Cintio, or Governato, supernovae feedback mechanism) would be a problem for the CDM model which should probably be revised.

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“…At least for two of the largest dSphs around the Milky Way, therefore, the data point unambiguously towards cores. Although [504] did construct fits to Sculptor using NFW haloes, this work remains open to the objection that the data are not well matched in the center, where the velocity dispersions of the two populations are too high (a typical failing of all cusped models).…”

Section: Missing Dwarf Galaxies (Author: N Menci)mentioning

confidence: 99%

“…There is debate as to whether the central densities of DM-dominated galaxies are less dense than the haloes that are predicted in DM-only simulations [500,504]. This issue along with the lack of observed satellites around the Milky Way can be ameliorated by considering alternative Dark Matter models.…”

Section: The Kinematics and Formation Of Subhaloes In Warm Darkmentioning

confidence: 99%

A White Paper on keV sterile neutrino Dark Matter

Adhikari

Agostini

et al. 2017

J. Cosmol. Astropart. Phys.

372

316

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Abstract. We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved -cosmology, astrophysics, nuclear, and particle physics -in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

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Dynamical Models for the Sculptor Dwarf Spheroidal in a ΛCDM Universe

Cited by 57 publications

References 39 publications

Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era

Three-dimensional motions in the Sculptor dwarf galaxy as a glimpse of a new era

On the dark matter haloes inner structure and galaxy morphology

A White Paper on keV sterile neutrino Dark Matter

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