Aims. We present the first three-dimensional internal motions for individual stars in the Draco dwarf spheroidal galaxy.Methods. By combining first-epoch Hubble Space Telescope observations and second-epoch Gaia Data Release 2 positions, we measured the proper motions of 149 sources in the direction of Draco. We determined the line-of-sight velocities for a sub-sample of 81 red giant branch stars using medium resolution spectra acquired with the DEIMOS spectrograph at the Keck II telescope. Altogether, this resulted in a final sample of 45 Draco members with high-precision and accurate 3D motions, which we present as a table in this paper.Results. Based on this high-quality dataset, we determined the velocity dispersions at a projected distance of similar to 120 pc from the centre of Draco to be sigma(R) = 11.0(1.5)(+2.1) km s(-1), sigma(T) = 9.9(3.1)(+2.3) km s(-1) and sigma(LOS) = 9.0(1.1)(+1.1) km s(-1) in the projected radial, tangential, and line-of-sight directions. This results in a velocity anisotropy beta = 0.25(-1.38)(+0.47) at r greater than or similar to 120 pc. Tighter constraints may be obtained using the spherical Jeans equations and assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also based on the assumption that the 3D velocity dispersion should be lower than approximate to 1/3 of the escape velocity of the system. In this case, we constrain the maximum circular velocity V-max of Draco to be in the range of 10.2-17.0 km s(-1). The corresponding mass range is in good agreement with previous estimates based on line-of-sight velocities only.Conclusions. Our Jeans modelling supports the case for a cuspy dark matter profile in this galaxy. Firmer conclusions may be drawn by applying more sophisticated models to this dataset and with new datasets from upcoming Gaia releases.
Stellar 3D kinematics in the Draco dwarf spheroidal galaxy
L. Spina;
2020
Abstract
Aims. We present the first three-dimensional internal motions for individual stars in the Draco dwarf spheroidal galaxy.Methods. By combining first-epoch Hubble Space Telescope observations and second-epoch Gaia Data Release 2 positions, we measured the proper motions of 149 sources in the direction of Draco. We determined the line-of-sight velocities for a sub-sample of 81 red giant branch stars using medium resolution spectra acquired with the DEIMOS spectrograph at the Keck II telescope. Altogether, this resulted in a final sample of 45 Draco members with high-precision and accurate 3D motions, which we present as a table in this paper.Results. Based on this high-quality dataset, we determined the velocity dispersions at a projected distance of similar to 120 pc from the centre of Draco to be sigma(R) = 11.0(1.5)(+2.1) km s(-1), sigma(T) = 9.9(3.1)(+2.3) km s(-1) and sigma(LOS) = 9.0(1.1)(+1.1) km s(-1) in the projected radial, tangential, and line-of-sight directions. This results in a velocity anisotropy beta = 0.25(-1.38)(+0.47) at r greater than or similar to 120 pc. Tighter constraints may be obtained using the spherical Jeans equations and assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also based on the assumption that the 3D velocity dispersion should be lower than approximate to 1/3 of the escape velocity of the system. In this case, we constrain the maximum circular velocity V-max of Draco to be in the range of 10.2-17.0 km s(-1). The corresponding mass range is in good agreement with previous estimates based on line-of-sight velocities only.Conclusions. Our Jeans modelling supports the case for a cuspy dark matter profile in this galaxy. Firmer conclusions may be drawn by applying more sophisticated models to this dataset and with new datasets from upcoming Gaia releases.
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