Comparing the Structural Parameters of the Milky Way to Other Spiral Galaxies

The structural parameters of a galaxy can be used to gain insight into its formation and evolution history.In this paper,we strive to compare the Milky Way’s structural parameters to other,primarily edge-on,spiral galaxies in order to determine how our Galaxy measures up to the Local Universe.For our comparison,we use the galaxy structural parameters gathered from a variety of literature sources in the optical and near-infrared wave bands.We compare the scale length,scale height,and disk flatness for both the thin and thick disks,the thick-to-thin disk mass ratio,the bulge-to-total luminosity ratio,and the mean pitch angle of the Milky Way’s spiral arms to those in other galaxies.We conclude that many of the Milky Way’s structural parameters are largely ordinary and typical of spiral galaxies in the Local Universe,though the Galaxy’s thick disk appears to be appreciably thinner and less extended than expected from zoom-in cosmological simulations of Milky Way-mass galaxies with a significant contribution of galaxy mergers involving satellite galaxies.

Modeling the Vertical Distribution of the Milky Way’s Flat Subsystem Objects

This paper is an initial stage of consideration of the general problem of joint modeling of the vertical structure of a Galactic flat subsystem and the average surface of the disk of the Galaxy,taking into account the natural and measurement dispersions.We approximate the average surface of the Galactic disk in the region covered by the data with a general(polynomial)model and determine its parameters by minimizing the squared deviations of objects along the normal to the model surface.The smoothness of the model,i.e.,its order n,is optimized.An outlier elimination algorithm is applied.The developed method allows us to simultaneously identify significant details of the Galactic warping and estimate the offset z_(⊙) of the Sun relative to the average(in general,non-flat)surface of the Galactic disk and the vertical scale of the object system under consideration for an arbitrary area of the disk covered by data.The method is applied to data on classical Cepheids.Significant local extremes of the average disk surface model were found based on Cepheid data:the minimum in the first Galactic quadrant and the maximum in the second.A well-known warp(lowering of the disk surface)in the third quadrant has been confirmed.The optimal order of the model describing all these warping details was found to be n_(o)=4.The local(for a small neighborhood of the Sun,n_(o)=0)estimate of z_(⊙)=28.1±6.1|_(stat).±1.3|_(cal).pc is close to the non-local(taking into account warping,n_(o)=4)z_(⊙)=27.1±8.8|_(stat.-1.2)^(+1.3)|_(cal).pc(statistical and calibration uncertainties are indicated),which suggests that the proposed modeling method eliminates the influence of warping on the z_(⊙) estimate.However,the non-local estimate of the vertical standard deviation of Cepheids σ_(p)=132.0±3.7|_(stat.-5.9)^(+6.3)|_(cal).pc differs significantly from the local σ_(ρ)=76.5±4.4|_(stat.-3.4)^(+3.6)|_(cal).pc,which implies the need to introduce more complex models for the vertical distribution outside the Sun’s vicinity.

Galactic disk bulk motions as revealed by the LSS-GAC DR2

We report a detailed investigation of the bulk motions of the nearby Galactic stellar disk, based on three samples selected from the LSS-GAC DR2： a global sample containing 0.57 million FGK dwarfs out to ～2 kpc, a local subset of the global sample consisting of ～5400 stars within 150 pc, and an anti-center sample containing ～4400AFGK dwarfs and red clump stars within windows a few degrees wide centered on the Galactic Anti-center. The global sample is used to construct a three-dimensional map of bulk motions of the Galactic disk from the solar vicinity out to ～2 kpc with a spatial resolution of ～250 pc. Typical values of the radial and vertical components of bulk motion range from-15 km s-1to 15 km s-1; in contrast, the lag behind the circular motion dominates the azimuthal component by up to ～15 km s-1. The map reveals spatially coherent, kpc-scale stellar flows in the disk, with typical velocities of a few tens of km s-1. Bending- and breathing-mode perturbations are clearly visible,and vary smoothly across the disk plane. Our data also reveal higher-order perturbations, such as breaks and ripples, in the profiles of vertical motion versus height. From the local sample, we find that stars from different populations exhibit very different patterns of bulk motion. Finally, the anti-center sample reveals a number of peaks in stellar number density in the line-of-sight velocity versus distance distribution, with the nearer ones apparently related to the known moving groups. The ＂velocity bifurcation＂ reported by Liu et al. at Galactocentric radii 10–11 kpc is confirmed. However,just beyond this distance, our data also reveal a new triple-peaked structure.