Table of contents

This paper describes the Sixth Data Release of the Sloan Digital Sky Survey. With this data release, the imaging of the northern Galactic cap is now complete. The survey contains images and parameters of roughly 287 million objects over 9583 deg², including scans over a large range of Galactic latitudes and longitudes. The survey also includes 1.27 million spectra of stars, galaxies, quasars, and blank sky (for sky subtraction) selected over 7425 deg². This release includes much more stellar spectroscopy than was available in previous data releases and also includes detailed estimates of stellar temperatures, gravities, and metallicities. The results of improved photometric calibration are now available, with uncertainties of roughly 1% in g, r, i, and z, and 2% in u, substantially better than the uncertainties in previous data releases. The spectra in this data release have improved wavelength and flux calibration, especially in the extreme blue and extreme red, leading to the qualitatively better determination of stellar types and radial velocities. The spectrophotometric fluxes are now tied to point-spread function magnitudes of stars rather than fiber magnitudes. This gives more robust results in the presence of seeing variations, but also implies a change in the spectrophotometric scale, which is now brighter by roughly 0.35 mag. Systematic errors in the velocity dispersions of galaxies have been fixed, and the results of two independent codes for determining spectral classifications and redshifts are made available. Additional spectral outputs are made available, including calibrated spectra from individual 15 minute exposures and the sky spectrum subtracted from each exposure. We also quantify a recently recognized underestimation of the brightnesses of galaxies of large angular extent due to poor sky subtraction; the bias can exceed 0.2 mag for galaxies brighter than r = 14 mag.

In 1997 February, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Program (VSOP) mission. Approximately 25% of the mission time was dedicated to the VSOP survey of bright compact active galactic nuclei (AGNs) at 5 GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in the third paper in the series, which contained 102 sources. For most sources, the plots of the (u,v) coverage, the visibility amplitude versus (u,v) distance, and the high-resolution image are presented. Model fit parameters to the major radio components are determined, and the brightness temperature of the core component for each source is calculated. The brightness temperature distributions for all of the sources in the VSOP AGN survey are discussed.

We develop a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies. By combining theoretically well-constrained halo and subhalo mass functions as a function of redshift and environment with empirical halo occupation models, we can estimate where galaxies of given properties live at a particular epoch. This allows us to calculate, in an a priori cosmological manner, where major galaxy-galaxy mergers occur and what kinds of galaxies merge, at all redshifts. We compare this with the observed mass functions, clustering, fractions as a function of halo and galaxy mass, and small-scale environments of mergers, and we show that this approach yields robust estimates in good agreement with observations and can be extended to predict detailed properties of mergers. Making the simple Ansatz that major, gas-rich mergers cause quasar activity (but not strictly assuming they are the only triggering mechanism), we demonstrate that this model naturally reproduces the observed rise and fall of the quasar luminosity density at z = 0–6, as well as quasar luminosity functions, fractions, host galaxy colors, and clustering as a function of redshift and luminosity. The recent observed excess of quasar clustering on small scales at z ∼ 0.2–2.5 is a natural prediction of our model, as mergers will preferentially occur in regions with excess small-scale galaxy overdensities. In fact, we demonstrate that quasar environments at all observed redshifts correspond closely to the empirically determined small group scale, where major mergers of ~L_* gas-rich galaxies will be most efficient. We contrast this with a secular model in which quasar activity is driven by bars or other disk instabilities, and we show that, while these modes of fueling probably dominate the high Eddington ratio population at Seyfert luminosities (significant at z = 0), the constraints from quasar clustering, observed pseudobulge populations, and disk mass functions suggest that they are a small contributor to the z ≳ 1 quasar luminosity density, which is dominated by massive BHs in predominantly classical spheroids formed in mergers. Similarly, low-luminosity Seyferts do not show a clustering excess on small scales, in agreement with the natural prediction of secular models, but bright quasars at all redshifts do so. We also compare recent observations of the colors of quasar host galaxies and show that these correspond to the colors of recent merger remnants, in the transition region between the blue cloud and the red sequence, and are distinct from the colors of systems with observed bars or strong disk instabilities. Even the most extreme secular models, in which all bulge (and therefore BH) formation proceeds via disk instability, are forced to assume that this instability acts before the (dynamically inevitable) mergers, and therefore predict a history for the quasar luminosity density that is shifted to earlier times, in disagreement with observations. Our model provides a powerful means to predict the abundance and nature of mergers and to contrast cosmologically motivated predictions of merger products such as starbursts and active galactic nuclei.

We develop and test a model for the cosmological role of mergers in the formation and quenching of red, early-type galaxies. By combining theoretically well-constrained halo and subhalo mass functions as a function of redshift and environment with empirical halo occupation models, we predict the distribution of mergers as a function of redshift, environment, and physical galaxy properties. Making the simple Ansatz that star formation is quenched after a gas-rich, spheroid-forming major merger, we demonstrate that this naturally predicts the turnover in the efficiency of star formation and baryon fractions in galaxies at ~L_* (without any parameters tuned to this value), as well as the observed mass functions and mass density of red galaxies as a function of redshift, the formation times of early-type galaxies as a function of mass, and the fraction of quenched galaxies as a function of galaxy and halo mass, environment, and redshift. Comparing our model to a variety of semianalytic models in which quenching is primarily driven by halo mass considerations or secular/disk instabilities, we demonstrate that our model makes unique and robust qualitative predictions for a number of observables, including the bivariate red fraction as a function of galaxy and halo mass, the density of passive galaxies at high redshifts, the emergence/evolution of the color-morphology-density relations at high redshift, and the fraction of disky/boxy (or cusp/core) spheroids as a function of mass. In each case, the observations favor a model in which some mechanism quenches future star formation after a major merger builds a massive spheroid. Models where quenching is dominated by a halo mass threshold fail to match the behavior of the bivariate red fractions, predict too low a density of passive galaxies at high redshift, and overpredict by an order of magnitude the mass of the transition from disky to boxy ellipticals. Models driven by secular disk instabilities also qualitatively disagree with the bivariate red fractions, fail to predict the observed evolution in the color-density relations, and predict order-of-magnitude incorrect distributions of kinematic types in early-type galaxies. We make specific predictions for how future observations, for example, quantifying the red fraction as a function of galaxy mass, halo mass, environment, or redshift, can break the degeneracies between a number of different assumptions adopted in present galaxy formation models. We discuss a variety of physical possibilities for this quenching and propose a mixed scenario in which traditional quenching in hot, quasi-static massive halos is supplemented by the strong shocks and feedback energy input associated with a major merger (e.g., tidal shocks, starburst-driven winds, and quasar feedback), which temporarily suppress cooling and establish the conditions of a dynamically hot halo in the central regions of the host, even in low-mass halos (below the traditional threshold for accretion shocks).

We present Hubble Space Telescope WFPC2 Linear Ramp Filter images of high surface brightness emission lines (either [O II], [O III], or H α + [N II]) in 80 3CR radio sources. We overlay the emission-line images on high-resolution VLA radio images (eight of which are new reductions of archival data) in order to examine the spatial relationship between the optical and radio emission. We confirm that the radio and optical emission-line structures are consistent with weak alignment at low redshift (z < 0.6) except in the compact steep-spectrum (CSS) radio galaxies where both the radio source and the emission-line nebulae are on galactic scales and strong alignment is seen at all redshifts. There are weak trends for the aligned emission-line nebulae to be more luminous and for the emission-line nebula size to increase with redshift and/or radio power. The combination of these results suggests that there is a limited but real capacity for the radio source to influence the properties of the emission-line nebulae at these low redshifts (z < 0.6). Our results are consistent with previous suggestions that both mechanical and radiant energy are responsible for generating alignment between the radio source and emission-line gas.

The long-slit spectra obtained along the minor axis, offset major axis, and diagonal axis are presented for 12 E and S0 galaxies of the Coma Cluster drawn from a magnitude-limited sample studied before. The rotation curves, velocity dispersion profiles, and the H₃ and H₄ coefficients of the Hermite decomposition of the line-of-sight velocity distribution are derived. The radial profiles of the Hβ, Mg, and Fe line strength indices are measured too. In addition, the surface photometry of the central regions of a subsample of four galaxies recently obtained with the Hubble Space Telescope is presented. The data will be used to construct dynamical models of the galaxies and study their stellar populations.

We have carried out submillimeter ¹²CO(J = 3–2) observations of six giant molecular clouds (GMCs) in the Large Magellanic Cloud (LMC) with the ASTE 10 m submillimeter telescope at a spatial resolution of 5 pc and very high sensitivity. We have identified 32 molecular clumps in the GMCs and revealed significant details of the warm and dense molecular gas with n(H₂) ∼ 10³–10⁵ cm⁻³ and T_kin ∼ 60 K. These data are combined with ¹²CO(J = 1–0) and ¹³CO(J = 1–0) results and compared with LVG calculations. The results indicate that clumps that we detected are distributed continuously from cool (~10-30 K) to warm (≳30-200 K), and warm clumps are distributed from less dense (~10³ cm⁻³) to dense (~10^3.5-10⁵ cm⁻³). We found that the ratio of ¹²CO(J = 3–2) to ¹²CO(J = 1–0) emission is sensitive to and is well correlated with the local Hα flux. We infer that differences of clump properties represent an evolutionary sequence of GMCs in terms of density increase leading to star formation. Type I and II GMCs (starless GMCs and GMCs with H II regions only, respectively) are at the young phase of star formation where density does not yet become high enough to show active star formation, and Type III GMCs (GMCs with H II regions and young star clusters) represent the later phase where the average density is increased and the GMCs are forming massive stars. The high kinetic temperature correlated with Hα flux suggests that FUV heating is dominant in the molecular gas of the LMC.

We present ammonia observations of 193 dense cores and core candidates in the Perseus molecular cloud made using the Robert F. Byrd Green Bank Telescope. We simultaneously observed the NH₃(1,1), NH₃(2,2), C₂S (2₁→ 1₀), and C³⁴₂S(2₁→ 1₀) transitions near ν = 23 GHz for each of the targets with a spectral resolution of δ v ≈ 0.024 km s⁻¹. We find ammonia emission associated with nearly all of the (sub)millimeter sources, as well as at several positions with no associated continuum emission. For each detection, we have measured physical properties by fitting a simple model to every spectral line simultaneously. Where appropriate, we have refined the model by accounting for low optical depths, multiple components along the line of sight, and imperfect coupling to the GBT beam. For the cores in Perseus, we find a typical kinetic temperature of T_k = 11 K, a typical column density of N_NH3 ≈ 10^14.5 cm ⁻², and velocity dispersions ranging from σ_v = 0.07 to 0.7 km s⁻¹. However, many cores with σ_v > 0.2 km s⁻¹ show evidence for multiple velocity components along the line of sight.

We present a catalog of 535 planetary nebulae (PNs) discovered in the flattened elliptical galaxy NGC 4697, using the FORS1 Cassegrain spectrograph of the Very Large Telescope of the European Southern Observatory at Cerro Paranal, Chile. The catalog provides positions [(x,y)-coordinates relative to the center of light of NGC 4697, as well as α,δ] and, for almost all PNs, the magnitude m(5007) and the heliocentric radial velocity in km s⁻¹.

We present a new version of the fully three-dimensional photoionization and dust radiative transfer code, MOCASSIN, that uses a Monte Carlo approach for the transfer of radiation. The X-ray enabled MOCASSIN allows a fully geometry-independent description of low-density gaseous environments strongly photoionized by a radiation field extending from radio to gamma rays. The code has been thoroughly benchmarked against other established codes routinely used in the literature, using simple plane-parallel models designed to test performance under standard conditions. We show the results of our benchmarking exercise and discuss the applicability and limitations of the new code, which should be of guidance for future astrophysical studies with MOCASSIN.

We present recent measurements of absolute electron-impact ionization cross sections for Be-like C III, N IV, and O V forming Li-like C IV, N V, and O VI. The measurements were taken using the crossed-beams apparatus at Oak Ridge National Laboratory. A gas cell beam attenuation method was used to independently measure the metastable fractions present in the ion beams. The measured ionization cross sections were compared with calculations using the R-matrix with pseudostates and distorted-wave theoretical methods. Best agreement is found with the R-matrix with pseudostates cross sections results that account for the metastable fractions inferred from the gas attenuation measurements. We present a set of recommended rate coefficients for electron-impact single ionization from the ground state and metastable term of each ion.