The evolution of stellar metallicity gradients of the Milky Way disk from LSS-GAC main sequence turn-off stars: a two-phase disk formation history?

Accurate measurements of stellar metallicity gradients in the radial and vertical directions of the disk and their temporal variations provide important constraints on the formation and evolution of the Milky Way disk. We use 297 042 main sequence turn-off stars selected from the LAMOST Spectroscopic Survey of the Galactic Anticenter（LSS-GAC） to determine the radial and vertical gradients of stellar metallicity,△[Fe/H]/△R and △[Fe/H]/△｜Z ｜ of the Milky Way disk in the direction of the anticenter. We determine ages of those turn-off stars by isochrone fitting and measure the temporal variations of metallicity gradients. We have carried out a detailed analysis of the selection effects resulting from the selection, observation and data reduction of LSS-GAC targets and the potential biases of a magnitude limited sample on the determinations of metallicity gradients. Our results show that the gradients, both in the radial and vertical directions, exhibit significant spatial and temporal variations. The radial gradients yielded by stars with the oldest ages（ 11 Gyr） are essentially zero at all heights from the disk midplane, while those given by younger stars are always negative. The vertical gradients deduced from stars with the oldest ages（ 11 Gyr）are negative and only show very weak variations with Galactocentric distance in the disk plane, R, while those yielded by younger stars show strong variations with R.After being essentially flat at the earliest epochs of disk formation, the radial gradients steepen as age decreases, reaching a maximum（steepest） at age 7–8 Gyr, and then they flatten again. Similar temporal trends are also found for the vertical gradients. We infer that the assembly of the Milky Way disk may have experienced at least two distinct phases. The earlier phase is probably related to a slow, pressure-supported collapse of gas, when the gas settles down to the disk mainly in the vertical direction. In the later phase, there are significant radial flows of gas in the disk, and the rate of gas inflow near the solar neighborhood reaches a maximum around a lookback time of 7–8 Gyr.The transition between the two phases occurs around a lookback time between 8 and11 Gyr. The two phases may be responsible for the formation of the Milky Way＇s thick and thin disks, respectively. Also, as a consequence, we recommend that stellar age is a natural, physical criterion to distinguish stars from the thin and thick disks. From an epoch earlier than 11 Gyr to one between 8 and 11 Gyr, there is an abrupt, significant change in magnitude of both the radial and vertical metallicity gradients, suggesting that stellar radial migration is unlikely to play an important role in the formation of the thick disk.

Stellar Abundance and Galactic Chemical Evolution through LAMOST Spectroscopic Survey

A project of a spectroscopic survey of Galactic structure and evolution with a Large sky Area Multi-Object fiber Spectroscopic Telescope （LAMOST） is presented. The spectroscopic survey consists of two observational modes for various targets in our Galaxy. One is a major survey of the Milky Way aimed at a systematic study of the stellar abundance and Galactic chemical evolution through low resolution （R = 1000 2000） spectroscopy. Another is a follow-up observation with medium resolution （R = 10000） spectrographs aimed at detailed studies of the selected stars with different chemical composition, kinematics and dynamics.

Metallicity and star formation activities of the interacting system Arp 86 from observations with MOS on the Xinglong 2.16m telescope

We present an analysis of the metallicity and star formation activities of H II regions in the interacting system Arp 86, based on the first scientific observations using mulri-object spectroscopy with the 2.16 m telescope at the Xinglong Observing Station. We find that the oxygen abundance gradient in Arp 86 is flatter than that in normal disk galaxies, which confirms that gas inflows caused by tidal forces during encounters can flatten the metallicity distributions in galaxies. The companion galaxy NGC 7752 is currently experiencing a galaxy-wide starburst with a higher star for- marion rate surface density than the main galaxy NGC 7753, which can be explained in that the companion galaxy is more susceptible to the effects of interaction than the primary. We also find that the galaxy 2MASX J23470758＋2926531 has similar abun- dance and star formation properties to NGC 7753, and may be a part of the Arp 86 system.

On the metallicity gradients of the Galactic disk as revealed by LSS-GAC red clump stars

Using a sample of over 70 000 red clump（RC） stars with 5%–10% distance accuracy selected from the LAMOST Spectroscopic Survey of the Galactic Anti-center（LSS-GAC）, we study the radial and vertical gradients of the Galactic disk（s） mainly in the anti-center direction, covering a significant volume of the disk in the range of projected Galactocentric radius 7 ≤ RGC ≤ 14 kpc and height from the Galactic midplane 0 ≤ ｜Z ｜ ≤ 3 kpc. Our analysis shows that both the radial and vertical metallicity gradients are negative across much of the volume of the disk that is probed, and they exhibit significant spatial variations. Near the solar circle（7 ≤ RGC ≤ 11.5 kpc）, the radial gradient has a moderately steep, negative slope of-0.08 dex kpc-1near the midplane（｜Z ｜ 〈 0.1 kpc）, and the slope flattens with increasing ｜Z ｜. In the outer disk（11.5 〈 RGC ≤ 14 kpc）, the radial gradients have an essentially constant, much less steep slope of-0.01 dex kpc-1at all heights above the plane, suggesting that the outer disk may have experienced an evolutionary path different from that of the inner disk. The vertical gradients are found to flatten largely with increasing RGC. However, the vertical gradient of the lower disk（0 ≤ ｜Z ｜ ≤ 1 kpc）is found to flatten with RGC quicker than that of the upper disk（1 〈 ｜Z ｜ ≤ 3 kpc）.Our results should provide strong constraints on the theory of disk formation and evolution, as well as the underlying physical processes that shape the disk（e.g. gas flows,radial migration, and internal and external perturbations）.

Numerical simulation of a possible origin of the positive radial metallicity gradient of the thick disk

We analyze the radial and vertical metallicity and [α/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemo- dynamical galaxy evolution code GCD＋. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity （[α/Fe]） gra- dient becomes more positive （negative） with increasing height, changing sign around 1.5 kpc above the plane. At the largest vertical height （2 〈 丨z丨 〈 3 kpc）, our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity gradient is caused by the age-metallicity and age-velocity dispersion relation, where younger stars have higher metallicity and lower velocity dispersion. Due to the age- velocity dispersion relation, a greater fraction of younger stars reaches 丨z丨 〉 2 kpc at the outer region, because of the lower gravitational restoring force of the disk, i.e. flaring. As a result, the fraction of younger stars with higher metallicity due to the age-metallicity relation becomes higher at the outer radii, which makes the median metallicity higher at the outer radii. Combining this result with the recently observed age-metallicity and age-velocity dispersion relation for the Milky Way thick disk stars suggested by Haywood et al., we argue that the observed （small） positive radial metal- licity gradient at large heights of the Milky Way disk stars can be explained by flaring of the younger thick and/or thin disk stars.

LAMOST spectral survey——An overview

LAMOST （Large sky Area Multi-Object fiber Spectroscopic Telescope） is a Chinese national scientific research facility operated by National Astronomical Observatories, Chinese Academy of Sciences （NAOC）. After two years of commis- sioning beginning in 2009, the telescope, instruments, software systems and opera- tions are nearly ready to begin the main science survey. Through a spectral survey of millions of objects in much of the northern sky, LAMOST will enable research in a number of contemporary cutting edge topics in astrophysics, such as discovery of the first generation stars in the Galaxy, pinning down the formation and evolution history of galaxies - especially the Milky Way and its central massive black hole, and look- ing for signatures of the distribution of dark matter and possible sub-structures in the Milky Way halo. To maximize the scientific potential of the facility, wide national par- ticipation and international collaboration have been emphasized. The survey has two major components： the LAMOST ExtraGAlactic Survey （LEGAS） and the LAMOST Experiment for Galactic Understanding and Exploration （LEGUE）. Until LAMOST reaches its full capability, the LEGUE portion of the survey will use the available ob- serving time, starting in 2012. An overview of the LAMOST project and the survey that will be carried out in the next five to six years is presented in this paper. The sci- ence plan for the whole LEGUE survey, instrumental specifications, site conditions, and the descriptions of the current on-going pilot survey, including its footprints and target selection algorithm, will be presented as separate papers in this volume.

LAMOST Experiment for Galactic Understanding and Exploration (LEGUE)——The survey's science plan

We describe the current plans for a spectroscopic survey of millions of stars in the Milky Way galaxy using the Guo Shou Jing Telescope （GSJT, formerly calledthe Large sky Area Multi-Object fiber Spectroscopic Telescope -- LAMOST）. The survey will obtain spectra for 2.5 million stars brighter than r 〈 19 during dark/grey time, and 5 million stars brighter than r 〈 17 or J 〈 16 on nights that are moonlit or have low transparency. The survey will begin in the fall of 2012, and will run for at least four years. The telescope＇s design constrains the optimal declination range for observations to 10~ 〈 di 〈 50~, and site conditions lead to an emphasis on stars in the direction of the Galactic anticenter. The survey is divided into three parts with different target selection strategies： disk, anticenter, and spheroid. The resulting dataset will be used to study the merger history of the Milky Way, the substructure and evolution of the disks, the nature of the first generation of stars through identification of the lowest metallicity stars, and star formation through study of open clusters and OB associations. Detailed design of the LAMOST Experiment for Galactic Understanding and Exploration （LEGUE） survey will be completed in summer 2012, after a review of the results of the pilot survey.