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Register a new userThe peculiar in-plane velocities in the outer disc of the Milky Way
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We present the peculiar in-plane velocities derived from the LAMOST red clump stars, which are purified and separated by a novel approach into two groups with different ages. The samples are mostly contributed around the Galactic anti-centre direction such that we are able to map the radial profiles of the radial and azimuthal velocities in the outer disc. From the variations of the in-plane velocities with the Galactocentric radius for the younger and older populations, we find that both radial and azimuthal velocities are not axisymmetric at $8<R<14,kpc$. The two red clump populations show that the mean radial velocity is negative within $Rsim9,kpc$ and positive beyond. This is likely because of the perturbation induced by the rotating bar. The cross-zero radius, $Rsim9$, kpc, essentially indicates the rough location of the outer Lindblad resonance (OLR) radius. Given the circular speed of 238,km$rm s^{-1}$, then the pattern speed of the bar can be approximated as $45$,km$rm s^{-1}rm kpc^{-1}$. The young red clump stars show larger mean radial velocity than the old population by about 3$,kmrm s^{-1}$ between $Rsim9$ and 12,kpc. This is possibly because the younger population is more sensitive to the perturbation than the older one. The radial profiles of the mean azimuthal velocity for the two populations show an interesting U-shape, i.e. at $R<10.5,kpc$, the azimuthal velocity declines with $R$ by about 10$,kmrm s^{-1}$, while at $R>10.5$ it increases with $R$ to 240-245$,kmrm s^{-1}$. It is not clear why the mean azimuthal velocity shows the U-shape along the Galactic anti-centre direction. Meanwhile, the azimuthal velocity for the younger population is slightly larger than the older one and the difference moderately declines with $R$. Beyond $Rsim12,kpc$, the azimuthal velocities for the two populations are indistinguishable.
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We use $Gaia$ eDR3 data and legacy spectroscopic surveys to map the Milky Way disc substructure towards the Galactic Anticenter at heliocentric distances $dgeq10,rm{kpc}$. We report the discovery of multiple previously undetected new filaments embedded in the outer disc in highly extincted regions. Stars in these over-densities have distance gradients expected for disc material and move on disc-like orbits with $v_{phi}sim170-230,rm{km,s^{-1}}$, showing small spreads in energy. Such a morphology argues against a quiescently growing Galactic thin disc. Some of these structures are interpreted as excited outer disc material, kicked up by satellite impacts and currently undergoing phase-mixing (feathers). Due to the long timescale in the outer disc regions, these structures can stay coherent in configuration space over several Gyrs. We nevertheless note that some of these structures could also be folds in the perturbed disc seen in projection from the Suns location. A full 6D phase-space characterization and age dating of these structure should help distinguish between the two possible morphologies.
We investigate the kinematic signatures induced by spiral and bar structure in a set of simulations of Milky Way-sized spiral disc galaxies. The set includes test particle simulations that follow a quasi-stationary density wave-like scenario with rigidly rotating spiral arms, and $N$-body simulations that host a bar and transient, co-rotating spiral arms. From a location similar to that of the Sun, we calculate the radial, tangential and line-of-sight peculiar velocity fields of a patch of the disc and quantify the fluctuations by computing the power spectrum from a two-dimensional Fourier transform. We find that the peculiar velocity power spectrum of the simulation with a bar and transient, co-rotating spiral arms fits very well to that of APOGEE red clump star data, while the quasi-stationary density wave spiral model without a bar does not. We determine that the power spectrum is sensitive to the number of spiral arms, spiral arm pitch angle and position with respect to the spiral arm. However, it is necessary to go beyond the line of sight velocity field in order to distinguish fully between the various spiral models with this method. We compute the power spectrum for different regions of the spiral discs, and discuss the application of this analysis technique to external galaxies.
Disc truncations are the closest feature to an edge that galaxies have, but the nature of this phenomena is not yet understood. In this paper, we explore the truncations in two nearby (D ~15 Mpc) Milky Way-like galaxies: NGC 4565 and NGC 5907. We cover a wide wavelength range from the NUV and optical, to 3.6 {mu}m. We find that the radius of the truncation (26+/-0.5 kpc) is independent of wavelength. Surprisingly, we identify (at all wavelengths) the truncation at altitudes as high as 3 kpc above the mid-plane, which implies that the thin disc in those outer regions has a width of at least this value. We find the characteristic U-shape radial colour profile associated with a star formation threshold at the location of the truncation. Further supporting such an origin, the stellar mass density at the position of the truncation is ~1-2 M_sun pc^-2, in good agreement with the critical gas density for transforming gas into stars. Beyond the truncation, the stellar mass in the mid-plane of the disc drops to just 0.1-0.2% of the total stellar mass of the galaxies. The detection of the truncation at high altitude in combination with the U shape of the radial colour profile allows us to establish, for the first time, an upper limit to the present-day growth rate of galactic discs. We find that, if the discs of the galaxies are growing inside-out, their growth rate is less than 0.6-0.9 kpc Gyr^-1.
Using a sample of red giant stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16, we infer the conditional distribution $p([alpha/text{Fe}],|,[text{Fe/H}])$ in the Milky Way disk for the $alpha$-elements Mg, O, Si, S, and Ca. In each bin of [Fe/H] and Galactocentric radius $R$, we model $p([alpha/text{Fe}])$ as a sum of two Gaussians, representing low-$alpha$ and high-$alpha$ populations with scale heights $z_1=0.45,text{kpc}$ and $z_2=0.95,text{kpc}$, respectively. By accounting for age-dependent and $z$-dependent selection effects in APOGEE, we infer the [$alpha$/Fe] distributions that would be found for a fair sample of long-lived stars covering all $z$. Near the Solar circle, this distribution is clearly bimodal at sub-solar [Fe/H], with the low-$alpha$ and high-$alpha$ peaks separated by a valley that is $sim 3$ times lower. In agreement with previous results, we find that the high-$alpha$ population is more prominent at smaller $R$, lower [Fe/H], and larger $|z|$, and that the sequence separation is smaller for Si and Ca than for Mg, O, and S. We find significant intrinsic scatter in [$alpha$/Fe] at fixed [Fe/H] for both the low-$alpha$ and high-$alpha$ populations, typically $sim 0.04$-dex. The means, dispersions, and relative amplitudes of this two-Gaussian description, and the dependence of these parameters on $R$, [Fe/H], and $alpha$-element, provide a quantitative target for chemical evolution models and a test for hydrodynamic simulations of disk galaxy formation. We argue that explaining the observed bimodality will probably require one or more sharp transitions in the disks gas accretion, star formation, or outflow history in addition to radial mixing of stellar populations.
We present chemical abundances of 57 metal-poor stars that are likely constituents of the outer stellar halo in the Milky Way. Almost all of the sample stars have an orbit reaching a maximum vertical distance (Z_max) of >5 kpc above and below the Galactic plane. High-resolution, high signal-to-noise spectra for the sample stars obtained with Subaru/HDS are used to derive chemical abundances of Na, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y and Ba with an LTE abundance analysis code. The resulting abundance data are combined with those presented in literature that mostly targeted at smaller Z_max stars, and both data are used to investigate any systematic trends in detailed abundance patterns depending on their kinematics. It is shown that, in the metallicity range of -2<[Fe/H]<-1, the [Mg/Fe] ratios for the stars with Z_max>5 kpc are systematically lower (~0.1 dex) than those with smaller Z_max. This result of the lower [alpha/Fe] for the assumed outer halo stars is consistent with previous studies that found a signature of lower [alpha/Fe] ratios for stars with extreme kinematics. A distribution of the [Mg/Fe] ratios for the outer halo stars partly overlaps with that for stars belonging to the Milky Way dwarf satellites in the metallicity interval of -2<[Fe/H]<-1 and spans a range intermediate between the distributions for the inner halo stars and the stars belonging to the satellites. Our results confirm inhomogeneous nature of chemical abundances within the Milky Way stellar halo depending on kinematic properties of constituent stars as suggested by earlier studies. Possible implications for the formation of the Milky Way halo and its relevance to the suggested dual nature of the halo are discussed.
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