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Register a new userStellar mass distribution and star formation history of the Galactic disk revealed by mono-age stellar populations from LAMOST
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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.
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Using a sample of 96,201 primary red clump (RC) stars selected from the LAMOST and Gaia surveys, we investigate the stellar structure of the Galactic disk. The sample stars show two separated sequences of high-[{alpha}/Fe] and low-[{alpha}/Fe] in the [{alpha}/Fe]-[Fe/H] plane. We divide the sample stars into five mono-abundance populations (MAPs) with different ranges of [{alpha}/Fe] and [Fe/H], named as the high-[{alpha}/Fe], high-[{alpha}/Fe] & high-[Fe/H], low-[Fe/H], solar, high-[Fe/H] MAPs respectively. We present the stellar number density distributions in the R R Z plane, and the scale heights and scale lengths of the individual MAPs by fitting their vertical and radial density profiles. The vertical profiles, the variation trend of scale height with the Galactocentric radius, indicate that there is a clear disk flare in the outer disk both for the low-[{alpha}/Fe] and the high-[{alpha}/Fe] MAPs. While the radial surface density profiles show a peak radius of 7 kpc and 8 kpc for the high-[{alpha}/Fe] and low-[{alpha}/Fe] MAPs, respectively. We also investigate the correlation between the mean rotation velocity and metallicity of the individual MAPs, and find that the mean rotation velocities are well separated and show different trends between the high-[{alpha}/Fe] and the low-[{alpha}/Fe] MAPs. At last, we discuss the character of the high-[{alpha}/Fe] & high-[Fe/H] MAP and find that it is more similar to the high-[{alpha}/Fe] MAP either in the radial and vertical density profiles or in the rotation velocity.
Studying the Milky Way disk structure using stars in narrow bins of [Fe/H] and [alpha/Fe] has recently been proposed as a powerful method to understand the Galactic thick and thin disk formation. It has been assumed so far that these mono-abundance populations (MAPs) are also coeval, or mono-age, populations. Here we study this relationship for a Milky Way chemo-dynamical model and show that equivalence between MAPs and mono-age populations exists only for the high-[alpha/Fe] tail, where the chemical evolution curves of different Galactic radii are far apart. At lower [alpha/Fe]-values a MAP is composed of stars with a range in ages, even for small observational uncertainties and a small MAP bin size. Due to the disk inside-out formation, for these MAPs younger stars are typically located at larger radii, which results in negative radial age gradients that can be as large as 2 Gyr/kpc. Positive radial age gradients can result for MAPs at the lowest [alpha/Fe] and highest [Fe/H] end. Such variations with age prevent the simple interpretation of observations for which accurate ages are not available. Studying the variation with radius of the stellar surface density and scale-height in our model, we find good agreement to recent analyses of the APOGEE red-clump (RC) sample when 1-4 Gyr old stars dominate (as expected for the RC). Our results suggest that the APOGEE data are consistent with a Milky Way model for which mono-age populations flare for all ages. We propose observational tests for the validity of our predictions and argue that using accurate age measurements, such as from asteroseismology, is crucial for putting constraints on the Galactic formation and evolution.
Recent determinations of the radial distributions of mono-metallicity populations (MMPs, i.e., stars in narrow bins in [Fe/H] within wider [$alpha$/Fe] ranges) by the SDSS-III/APOGEE DR12 survey cast doubts on the classical thin - thick disk dichotomy. The analysis of these observations lead to the non-$[alpha$/Fe] enhanced populations splitting into MMPs with different surface densities according to their [Fe/H]. By contrast, $[alpha$/Fe] enhanced (i.e., old) populations show an homogeneous behaviour. We analyze these results in the wider context of disk formation within non-isolated halos embedded in the Cosmic Web, resulting in a two-phase mass assembly. By performing hydrodynamical simulations in the context of the $rm Lambda CDM$ model, we have found that the two phases of halo mass assembly (an early, fast phase, followed by a slow one, with low mass assembly rates) are very relevant to determine the radial structure of MMP distributions, while radial mixing has only a secondary role, depending on the coeval dynamical and/or destabilizing events. Indeed, while the frequent dynamical violent events occuring at high redshift remove metallicity gradients, and imply efficient stellar mixing, the relatively quiescent dynamics after the transition keeps [Fe/H] gaseous gradients and prevents newly formed stars to suffer from strong radial mixing. By linking the two-component disk concept with the two-phase halo mass assembly scenario, our results set halo virialization (the event marking the transition from the fast to the slow phases) as the separating event marking periods characterized by different physical conditions under which thick and thin disk stars were born.
We present the star formation history of the extremely metal-poor dwarf galaxy DDO 68, based on our photometry with the Advanced Camera for Surveys. With a metallicity of only $12+log(O/H)=7.15$ and a very isolated location, DDO 68 is one of the most metal-poor galaxies known. It has been argued that DDO 68 is a young system that started forming stars only $sim 0.15$ Gyr ago. Our data provide a deep and uncontaminated optical color-magnitude diagram that allows us to disprove this hypothesis, since we find a population of at least $sim 1$ Gyr old stars. The star formation activity has been fairly continuous over all the look-back time. The current rate is quite low, and the highest activity occurred between 10 and 100 Myr ago. The average star formation rate over the whole Hubble time is $simeq 0.01$ M$_{odot}$ yr$^{-1}$, corresponding to a total astrated mass of $simeq 1.3 times 10^8$ M$_{odot}$. Our photometry allows us to infer the distance from the tip of the red giant branch, $D = 12.08 pm 0.67$ Mpc; however, to let our synthetic color-magnitude diagram reproduce the observed ones we need a slightly higher distance, $D=12.65$ Mpc, or $(m-M)_0 = 30.51$, still inside the errors of the previous determination, and we adopt the latter. DDO 68 shows a very interesting and complex history, with its quite disturbed shape and a long Tail probably due to tidal interactions. The star formation history of the Tail differs from that of the main body mainly for an enhanced activity at recent epochs, likely triggered by the interaction.
We perform analysis of the three-dimensional kinematics of Milky Way disk stars in mono-age populations. We focus on stars between Galactocentric distances of $R=6$ and 14 ,kpc, selected from the combined LAMOST DR4 red clump giant stars and Gaia DR2 proper motion catalogue. We confirm the 3D asymmetrical motions of recent works and provide time tagging of the Galactic outer disk asymmetrical motions near the anticenter direction out to Galactocentric distances of 14,kpc. Radial Galactocentric motions reach values up to 10 km s$^{-1}$, depending on the age of the population, and present a north-south asymmetry in the region corresponding to density and velocity substructures that were sensitive to the perturbations in the early 6 ,Gyr. After that time, the disk stars in this asymmetrical structure have become kinematically hotter, and are thus not sensitive to perturbations, and we find the structure is a relatively younger population. With quantitative analysis, we find stars both above and below the plane at $Rgtrsim 9$ kpc that exhibit bending mode motions of which the sensitive duration is around 8 ,Gyr. We speculate that the in-plane asymmetries might not be mainly caused by a fast rotating bar, intrinsically elliptical outer disk, secular expansion of the disk, or streams. Spiral arm dynamics, out-of-equilibrium models, minor mergers or others are important contributors. Vertical motions might be dominated by bending and breathing modes induced by complicated inner or external perturbers. It is likely that many of these mechanisms are coupled together.
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