Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userExcitation of coupled stellar motions in the Galactic Disk by orbiting satellites
54
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We use a set of high-resolution N-body simulations of the Galactic disk to study its interactions with the population of satellites predicted cosmologically. One simulation illustrates that multiple passages of massive satellites with different velocities through the disk generate a wobble, having the appearance of rings in face-on projections of the stellar disk. They also produce flares in the disk outer parts and gradually heat the disk through bending waves. A different numerical experiment shows that an individual satellite as massive as the Sagittarius dwarf galaxy passing through the disk will drive coupled horizontal and vertical oscillations of stars in underdense regions, with small significant associated heating. This experiment shows that vertical excursions of stars in these low-density regions can exceed 1 kpc in the Solar neighborhood, resembling the coherent vertical oscillations recently detected locally. They can also induce non-zero vertical streaming motions as large as 10-20 km s$^{-1}$, consistent with recent observations in the Galactic disk. This phenomenon appears as a local ring, with modest associated disk heating.
rate research
Read More
We use idealized N-body simulations of equilibrium stellar disks embedded within course-grained dark matter haloes to study the effects of spurious collisional heating on disk structure and kinematics. Collisional heating drives a systematic increase in both the vertical and radial velocity dispersions of disk stars, and leads to an artificial increase in the thickness and size of disks; the effects are felt at all galacto-centric radii, and are not limited to the central regions of galaxies. We demonstrate that relaxation is driven primarily by the coarse-grained nature of simulated dark matter haloes, with bulges, stellar haloes and disk stars contributing little to disk heating. The integrated effects of collisional heating are determined primarily by the mass of dark matter particles (or equivalently by the number of particles at fixed halo mass), their local density and characteristic velocity, but are largely insensitive to the masses of stellar particles. This suggests that the effects of numerical relaxation on simulated galaxies can be reduced by increasing the mass resolution of the dark matter in cosmological simulations, with limited benefits from increasing the baryonic (or stellar) mass resolution. We provide a simple empirical model that accurately captures the effects of collisional heating on the vertical and radial velocity dispersions of disk stars, as well as on their scale heights. We use the model to assess the extent to which spurious collisional relaxation may have affected the structure of simulated galaxy disks. For example, we find that dark matter haloes resolved with fewer than $approx 10^6$ particles will collisionally heat stars near the stellar half-mass radius such that their vertical velocity dispersion increases by more than 10 per cent of the halos virial velocity in approximately one Hubble time.
The spatial structure of stellar populations with different chemical abundances in the Milky Way contains a wealth of information on Galactic evolution over cosmic time. We use data on 14,699 red-clump stars from the APOGEE survey, covering 4 kpc <~ R <~ 15 kpc, to determine the structure of mono-abundance populations (MAPs)---stars in narrow bins in [a/Fe] and [Fe/H]---accounting for the complex effects of the APOGEE selection function and the spatially-variable dust obscuration. We determine that all MAPs with enhanced [a/Fe] are centrally concentrated and are well-described as exponentials with a scale length of 2.2+/-0.2 kpc over the whole radial range of the disk. We discover that the surface-density profiles of low-[a/Fe] MAPs are complex: they do not monotonically decrease outwards, but rather display a peak radius ranging from ~5 kpc to ~13 kpc at low [Fe/H]. The extensive radial coverage of the data allows us to measure radial trends in the thickness of each MAP. While high-[a/Fe] MAPs have constant scale heights, low-[a/Fe] MAPs flare. We confirm, now with high-precision abundances, previous results that each MAP contains only a single vertical scale height and that low-[Fe/H], low-[a/Fe] and high-[Fe/H], high-[a/Fe] MAPs have intermediate (h_Z~300 to 600 pc) scale heights that smoothly bridge the traditional thin- and thick-disk divide. That the high-[a/Fe], thick disk components do not flare is strong evidence against their thickness being caused by radial migration. The correspondence between the radial structure and chemical-enrichment age of stellar populations is clear confirmation of the inside-out growth of galactic disks. The details of these relations will constrain the variety of physical conditions under which stars form throughout the MW disk.
We present a detailed determination and analysis of 3D stellar mass distribution of the Galactic disk for mono-age populations using a sample of 0.93 million main-sequence turn-off and subgiant stars from the LAMOST Galactic Surveys. Our results show (1) all stellar populations younger than 10,Gyr exhibit strong disk flaring, which is accompanied with a dumpy vertical density profile that is best described by a $sech^n$ function with index depending on both radius and age; (2) Asymmetries and wave-like oscillations are presented in both the radial and vertical direction, with strength varying with stellar populations; (3) As a contribution by the Local spiral arm, the mid-plane stellar mass density at solar radius but 400--800,pc (3--6$^circ$) away from the Sun in the azimuthal direction has a value of $0.0594pm0.0008$,$M_odot$/pc$^3$, which is 0.0164,$M_odot$/pc$^3$ higher than previous estimates at the solar neighborhood. The result causes doubts on the current estimate of local dark matter density; (4) The radial distribution of surface mass density yields a disk scale length evolving from $sim$4,kpc for the young to $sim$2,kpc for the old populations. The overall population exhibits a disk scale length of $2.48pm0.05$,kpc, and a total stellar mass of $3.6(pm0.1)times10^{10}$,$M_odot$ assuming $R_{odot}=8.0$,kpc, and the value becomes $4.1(pm0.1)times10^{10}$,$M_odot$ if $R_{odot}=8.3$,kpc; (5) The disk has a peak star formation rate ({rm SFR}) changing from 6--8,Gyr at the inner to 4--6,Gyr ago at the outer part, indicating an inside-out assemblage history. The 0--1,Gyr population yields a recent disk total {rm SFR} of $1.96pm0.12$,$M_odot$/yr.
With the release of Gaia DR2, it is now possible to measure the proper motions (PMs) of the lowest mass, ultra-faint satellite galaxies in the Milky Ways (MW) halo for the first time. Many of these faint satellites are posited to have been accreted as satellites of the Magellanic Clouds (MCs). Using their 6-dimensional phase space information, we calculate the orbital histories of 13 ultra-faint satellites and five classical dwarf spheroidals in a combined MW+LMC+SMC potential to determine which galaxies are dynamically associated with the MCs. These 18 galaxies are separated into four classes: i.) long-term Magellanic satellites that have been bound to the MCs for at least the last two consecutive orbits around the MCs (Carina 2, Carina 3, Horologium 1, Hydrus 1); ii.) Magellanic satellites that were recently captured by the MCs $<$ 1 Gyr ago (Reticulum 2, Phoenix 2); iii.) MW satellites that have interacted with the MCs (Sculptor 1, Tucana 3, Segue 1); and iv.) MW satellites (Aquarius 2, Canes Venatici 2, Crater 2, Draco 1, Draco 2, Hydra 2, Carina, Fornax, Ursa Minor). Results are reported for a range of MW and LMC masses. Contrary to previous work, we find no dynamical association between Carina, Fornax, and the MCs. Finally, we determine that the addition of the SMCs gravitational potential affects the longevity of satellites as members of the Magellanic system (long-term versus recently captured), but it does not change the total number of Magellanic satellites.
We study the formation of planes of dwarf galaxies around Milky Way (MW)-mass haloes in the EAGLE galaxy formation simulation. We focus on satellite systems similar to the one in the MW: spatially thin or with a large fraction of members orbiting in the same plane. To characterise the latter, we introduce a robust method to identify the subsets of satellites that have the most co-planar orbits. Out of the 11 MW classical dwarf satellites, 8 have highly clustered orbital planes whose poles are contained within a $22^circ$ opening angle centred around $(l,b)=(182^circ,-2^circ)$. This configuration stands out when compared to both isotropic and typical $Lambda$CDM satellite distributions. Purely flattened satellite systems are short-lived chance associations and persist for less than $1~rm{Gyr}$. In contrast, satellite subsets that share roughly the same orbital plane are longer lived, with half of the MW-like systems being at least $4~rm{Gyrs}$ old. On average, satellite systems were flatter in the past, with a minimum in their minor-to-major axes ratio about $9~rm{Gyrs}$ ago, which is the typical infall time of the classical satellites. MW-like satellite distributions have on average always been flatter than the overall population of satellites in MW-mass haloes and, in particular, they correspond to systems with a high degree of anisotropic accretion of satellites. We also show that torques induced by the aspherical mass distribution of the host halo channel some satellite orbits into the hosts equatorial plane, enhancing the fraction of satellites with co-planar orbits. In fact, the orbital poles of co-planar satellites are tightly aligned with the minor axis of the host halo.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
