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Register a new userThe wobbly Galaxy: kinematics north and south with RAVE red clump giants
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The RAVE survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using the red clump, we examine the mean velocity components in 3D between an R of 6 and 10 kpc and a Z of -2 to 2 kpc, concentrating on North-South differences. Simple parametric fits to the R, Z trends for VPHI and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, VR, finding that the gradient is more marked below the plane, with a Z gradient also present. The vertical velocity, VZ, also shows clear structure, with indications of a rarefaction-compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the North-South differences in VR and VZ along the line-of-sight, with the VR estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disk, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role.
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We analyse the kinematics and dynamics of a homogeneous sample of red clump stars selected from the second Gaia data release catalogue in the direction of the Galactic poles. The level of completeness of the sample at heights between 0.6 and 3.5 kpc is asserted by comparison with the 2 Micron All Sky Survey catalogue. We show that both the density distribution and velocity dispersion are significantly more perturbed in the North than in the South, in all analysed regions of our Galactic neighbourhoods. We provide a detailed assessment of these North-South asymmetries at large heights. We then proceed to evaluate how such asymmetries could affect determinations of the dynamical matter density under equilibrium assumptions. We find that a Jeans analysis delivers relatively similar vertical forces and integrated dynamical surface densities at large heights above the plane in both hemispheres. At these heights, the densities of stars and gas are very low and the surface density is largely dominated by dark matter, which allows to estimate, separately in the North and South, the local dark matter density derived under equilibrium assumptions. In the presence of vertical perturbations, such values should be considered as an upper limit. This Jeans analysis yields values of the local dark matter density above 2~kpc, $rho_{rm DM} sim 0.013 , {rm M}_odot/{rm pc}^3$ ($ sim 0.509 , {rm GeV/cm}^3$) in the perturbed Northern hemisphere, and $rho_{rm DM} sim 0.010 , {rm M}_odot/{rm pc}^3$ ($ sim 0.374 , {rm GeV/cm}^3$) in the much less perturbed South. As a comparison, we determine the local dark matter density by fitting a global phase-space distribution to the data. We end up with a value in the range of $rho_{rm DM} sim 0.011 - 0.014 , {rm M}_odot/{rm pc}^3$ in global agreement with Jeans analysis.
We have used the AAOMEGA spectrograph to obtain R $sim 1500$ spectra of 714 stars that are members of two red clumps in the Plaut Window Galactic bulge field $(l,b)=0^{circ},-8^{circ}$. We discern no difference between the clump populations based on radial velocities or abundances measured from the Mg$b$ index. The velocity dispersion has a strong trend with Mg$b$-index metallicity, in the sense of a declining velocity dispersion at higher metallicity. We also find a strong trend in mean radial velocity with abundance. Our red clump sample shows distinctly different kinematics for stars with [Fe/H] $<-1$, which may plausibly be attributable to a minority classical bulge or inner halo population. The transition between the two groups is smooth. The chemo-dynamical properties of our sample are reminiscent of those of the Milky Way globular cluster system. If correct, this argues for no bulge/halo dichotomy and a relatively rapid star formation history. Large surveys of the composition and kinematics of the bulge clump and red giant branch are needed to define further these trends.
We present a study of the luminosity density distribution of the Galactic bar using number counts of red clump giants (RCGs) from the OGLE-III survey. The data were recently published by Nataf et al. (2013) for 9019 fields towards the bulge and have $2.94times 10^6$ RC stars over a viewing area of $90.25 ,textrm{deg}^2$. The data include the number counts, mean distance modulus ($mu$), dispersion in $mu$ and full error matrix, from which we fit the data with several tri-axial parametric models. We use the Markov Chain Monte Carlo (MCMC) method to explore the parameter space and find that the best-fit model is the $E_3$ model, with the distance to the GC is 8.13 kpc, the ratio of semi-major and semi-minor bar axis scale lengths in the Galactic plane $x_{0},y_{0}$, and vertical bar scale length $z_0$, is $x_0:y_0:z_0 approx 1.00:0.43:0.40$ (close to being prolate). The scale length of the stellar density profile along the bars major axis is $sim$ 0.67 kpc and has an angle of $29.4^circ$, slightly larger than the value obtained from a similar study based on OGLE-II data. The number of estimated RC stars within the field of view is $2.78 times 10^6$, which is systematically lower than the observed value. We subtract the smooth parametric model from the observed counts and find that the residuals are consistent with the presence of an X-shaped structure in the Galactic centre, the excess to the estimated mass content is $sim 5.8%$. We estimate the total mass of the bar is $sim 1.8 times 10^{10} M_odot$. Our results can be used as a key ingredient to construct new density models of the Milky Way and will have implications on the predictions of the optical depth to gravitational microlensing and the patterns of hydrodynamical gas flow in the Milky Way.
If the Galaxy is axisymmetric and in dynamical equilibrium, we expect negligible fluctuations in the residual line-of-sight velocity field. Recent results using the apg{} survey find significant fluctuations in velocity for stars in the midplane ($|z|<$0.25 kpc) out to 5 kpc, suggesting that the dynamical influence of non-axisymmetric features i.e., the Milky Ways bar, spiral arms and merger events extends out to the Solar neighborhood. Their measured power spectrum has a characteristic amplitude of 11 kms{} on a scale of 2.5 kpc. The existence of such large-scale streaming motions has important implications for determining the Suns motion about the Galactic Centre. Using Red Clump stars from glh{} and apg{}, we map the line-of-sight velocities around the Sun (d$<$5 kpc), and $|z|<$1.25 kpc from the midplane. By subtracting a smooth axisymmetric model for the velocity field, we study the residual fluctuations and compare our findings with mock survey generated by glx{}. We find negligible large-scale fluctuations away from the plane. In the mid-plane, we reproduce the earlier apg{} power spectrum but with 20% smaller amplitude (9.3 kms{}) after taking into account a few systematics (e.g., volume completeness). Using a flexible axisymmetric model the power-amplitude is further reduced to 6.3 kms{}. Additionally, our simulations show that, in the plane, distances are underestimated for high-mass Red Clump stars which can lead to spurious power-amplitude of about 5.2 kms{}. Taking this into account, we estimate the amplitude of real fluctuations to be $<$4.6 kms{}, about a factor of three less than the apg{} result.
We present results from an extensive spectroscopic survey of field stars in the Small Magellanic Cloud (SMC). 3037 sources, predominantly first-ascent red giants, spread across roughly 37.5 sq. deg, are analysed. The line of sight velocity field is dominated by the projection of the orbital motion of the SMC around the LMC/Milky Way. The residuals are inconsistent with both a non-rotating spheroid and a nearly face on disk system. The current sample and previous stellar and HI kinematics can be reconciled by rotating disk models with line of nodes position angle, theta, ~ 120-130 deg., moderate inclination (i ~ 25-70 deg.), and rotation curves rising at 20-40 km/s/kpc. The metal-poor stars exhibit a lower velocity gradient and higher velocity dispersion than the metal-rich stars. If our interpretation of the velocity patterns as bulk rotation is appropriate, then some revision to simulations of the SMC orbit is required since these are generally tuned to the SMC disk line-of-nodes lying in a NE-SW direction. Residuals show strong spatial structure indicative of non-circular motions that increase in importance with increasing distance from the SMC centre. Kinematic substructure in the north-west part of our survey area is associated with the tidal tail or Counter-Bridge predicted by simulations. Lower line-of-sight velocities towards the Wing and the larger velocities just beyond the SW end of the SMC Bar are probably associated with stellar components of the Magellanic Bridge and Counter-Bridge, respectively. Our results reinforce the notion that the intermediate-age stellar population of the SMC is subject to substantial stripping by external forces.
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