Australian National University

About: Australian National University is a education organization based out in Canberra, Australian Capital Territory, Australia. It is known for research contribution in the topics: Population & Galaxy. The organization has 34419 authors who have published 109261 publications receiving 4315448 citations. The organization is also known as: The Australian National University & ANU.

The 6dF Galaxy Survey: Samples, observational techniques and the first data release

Abstract: The 6dF Galaxy Survey (6dFGS) aims to measure the redshifts of around 150 000 galaxies, and the peculiar velocities of a 15 000-member subsample, over almost the entire southern sky. When complete, it will be the largest redshift survey of the nearby Universe, reaching out to about z similar to 0.15, and more than an order of magnitude larger than any peculiar velocity survey to date. The targets are all galaxies brighter than K-tot = 12.75 in the 2MASS Extended Source Catalog (XSC), supplemented by 2MASS and SuperCOSMOS galaxies that complete the sample to limits of (H, J, r(F), b(J)) = (13.05, 13.75, 15.6, 16.75). Central to the survey is the Six-Degree Field (6dF) multifibre spectrograph, an instrument able to record 150 simultaneous spectra over the 5.7-field of the UK Schmidt Telescope. An adaptive tiling algorithm has been employed to ensure around 95 per cent fibring completeness over the 17 046 deg(2) of the southern sky with \b\ > 10degrees. Spectra are obtained in two observations using separate V and R gratings, that together give R similar to 1000 over at least 4000-7500 Angstrom and signal-to-noise ratio similar to10 per pixel. Redshift measurements are obtained semi-automatically, and are assigned a quality value based on visual inspection. The 6dFGS data base is available at http://www-wfau.roe.ac.uk/6dFGS/, with public data releases occurring after the completion of each third of the survey.

Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from Sloan Digital Sky Survey DR7

Abstract: The structure and kinematics of the recognized stellar components of the Milky Way are explored, based on well-determined atmospheric parameters and kinematic quantities for 32360 "calibration stars" from the Sloan Digital Sky Survey (SDSS) and its first extension, SDSS-II, which included the sub-survey Sloan Extension for Galactic Understanding and Exploration (SEGUE). Full space motions for a sub-sample of 16,920 stars, exploring a local volume within 4 kpc of the Sun, are used to derive velocity ellipsoids for the inner- and outer-halo components of the Galaxy, as well as for the canonical thick-disk and proposed metal-weak thick-disk (MWTD) populations. This new sample of calibration stars represents an increase of 60% relative to the numbers used in a previous analysis. We first examine the question of whether the data require the presence of at least a two-component halo in order to account for the rotational behavior of likely halo stars in the local volume, and whether more than two components are needed. We also address the question of whether the proposed MWTD is kinematically and chemically distinct from the canonical thick disk, and point out that the Galactocentric rotational velocity inferred for the MWTD, as well as its mean metallicity, appear quite similar to the values derived previously for the Monoceros stream, suggesting a possible association between these structures. In addition, we consider the fractions of each component required to understand the nature of the observed kinematic behavior of the stellar populations of the Galaxy as a function of distance from the plane. Scale lengths and scale heights for the thick-disk and MWTD components are determined. Spatial density profiles for the inner- and outer-halo populations are inferred from a Jeans theorem analysis. The full set of calibration stars (including those outside the local volume) is used to test for the expected changes in the observed stellar metallicity distribution function with distance above the Galactic plane in situ, due to the changing contributions from the underlying stellar populations. The above issues are considered, in concert with theoretical and observational constraints from other Milky-Way-like galaxies, in light of modern cold dark matter galaxy formation models.

Galaxy ecology: groups and low-density environments in the SDSS and 2dFGRS

Abstract: We analyse the observed correlation between galaxy environment and Halpha emission-line strength, using volume-limited samples and group catalogues of 24 968 galaxies at 0.05 < z < 0.095, drawn from the 2dF Galaxy Redshift Survey (M-bJ < -19.5) and the Sloan Digital Sky Survey (M-r < -20.6). We characterize the environment by: (1) Sigma(5), the surface number density of galaxies determined by the projected distance to the fifth nearest neighbour; and (2) rho(1.1) and rho(5.5), three-dimensional density estimates obtained by convolving the galaxy distribution with Gaussian kernels of dispersion 1.1 and 5.5 Mpc, respectively. We find that star-forming and quiescent galaxies form two distinct populations, as characterized by their H equivalent width, W-0(Halpha). The relative numbers of star-forming and quiescent galaxies vary strongly and continuously with local density. However, the distribution of W-0(Halpha) amongst the star-forming population is independent of environment. The fraction of star-forming galaxies shows strong sensitivity to the density on large scales, rho(5.5), which is likely independent of the trend with local density, rho(1.1). We use two differently selected group catalogues to demonstrate that the correlation with galaxy density is approximately independent of group velocity dispersion, for sigma = 200-1000 km s(-1). Even in the lowest-density environments, no more than similar to70 per cent of galaxies show significant Halpha emission. Based on these results, we conclude that the present-day correlation between star formation rate and environment is a result of short-time-scale mechanisms that take place preferentially at high redshift, such as starbursts induced by galaxy-galaxy interactions.