Showing papers in "The Astronomical Journal in 2002"

Detailed structural decomposition of galaxy images

Abstract: We present a two-dimensional fitting algorithm (GALFIT) designed to extract structural components from galaxy images, with emphasis on closely modeling light profiles of spatially well-resolved, nearby galaxies observed with the Hubble Space Telescope. Our algorithm improves on previous techniques in two areas: by being able to simultaneously fit a galaxy with an arbitrary number of components and with optimization in computation speed, suited for working on large galaxy images. We use two-dimensional models such as the "Nuker" law, the Sersic (de Vaucouleurs) profile, an exponential disk, and Gaussian or Moffat functions. The azimuthal shapes are generalized ellipses that can fit disky and boxy components. Some potential applications of our program include: standard modeling of global galaxy profiles; extracting bars, stellar disks, double nuclei, and compact nuclear sources; and measuring absolute dust extinction or surface brightness fluctuations after removing the galaxy model. When examined in detail, we find that even simple looking galaxies generally require at least three components to be modeled accurately, rather than the one or two components more often employed. Many galaxies with complex isophotes, ellipticity changes, and position angle twists can be modeled accurately in two dimensions. We illustrate this by way of 11 case studies, which include regular and barred spiral galaxies, highly disky lenticular galaxies, and elliptical galaxies displaying various levels of complexities. A useful extension of this algorithm is to accurately extract nuclear point sources in galaxies. We compare two-dimensional and one-dimensional extraction techniques on simulated images of galaxies having nuclear slopes with different degrees of cuspiness, and we then illustrate the application of the program to several examples of nearby galaxies with weak nuclei.

Sloan digital sky survey: Early data release

Abstract: The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of ≈106 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux- and wavelength-calibrated, with 4096 pixels from 3800 to 9200 A at R ≈ 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.

Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Main Galaxy Sample

Abstract: We describe the algorithm that selects the main sample of galaxies for spectroscopy in the Sloan Digital Sky Survey (SDSS) from the photometric data obtained by the imaging survey. Galaxy photometric properties are measured using the Petrosian magnitude system, which measures flux in apertures determined by the shape of the surface brightness profile. The metric aperture used is essentially independent of cosmological surface brightness dimming, foreground extinction, sky brightness, and the galaxy central surface brightness. The main galaxy sample consists of galaxies with r-band Petrosian magnitudes r ≤ 17.77 and r-band Petrosian half-light surface brightnesses μ50 ≤ 24.5 mag arcsec-2. These cuts select about 90 galaxy targets per square degree, with a median redshift of 0.104. We carry out a number of tests to show that (1) our star-galaxy separation criterion is effective at eliminating nearly all stellar contamination while removing almost no genuine galaxies, (2) the fraction of galaxies eliminated by our surface brightness cut is very small (~0.1%), (3) the completeness of the sample is high, exceeding 99%, and (4) the reproducibility of target selection based on repeated imaging scans is consistent with the expected random photometric errors. The main cause of incompleteness is blending with saturated stars, which becomes more significant for brighter, larger galaxies. The SDSS spectra are of high enough signal-to-noise ratio (S/N > 4 per pixel) that essentially all targeted galaxies (99.9%) yield a reliable redshift (i.e., with statistical error less than 30 km s-1). About 6% of galaxies that satisfy the selection criteria are not observed because they have a companion closer than the 55'' minimum separation of spectroscopic fibers, but these galaxies can be accounted for in statistical analyses of clustering or galaxy properties. The uniformity and completeness of the galaxy sample make it ideal for studies of large-scale structure and the characteristics of the galaxy population in the local universe.

The u'g'r'i'z' Standard-Star System

Abstract: We present the 158 standard stars that define the u'g'r'i'z' photometric system. These stars form the basis for the photometric calibration of the Sloan Digital Sky Survey. The defining instrument system and filters, the observing process, the reduction techniques, and the software used to create the stellar network are all described. We briefly discuss the history of the star selection process, the derivation of a set of transformation equations for the UBVRCIC system, and plans for future work.

Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Quasar Sample

Abstract: We describe the algorithm for selecting quasar candidates for optical spectroscopy in the Sloan Digital Sky Survey. Quasar candidates are selected via their nonstellar colors in ugriz broadband photometry and by matching unresolved sources to the FIRST radio catalogs. The automated algorithm is sensitive to quasars at all redshifts lower than z ~ 5.8. Extended sources are also targeted as low-redshift quasar candidates in order to investigate the evolution of active galactic nuclei (AGNs) at the faint end of the luminosity function. Nearly 95% of previously known quasars are recovered (based on 1540 quasars in 446 deg2). The overall completeness, estimated from simulated quasars, is expected to be over 90%, whereas the overall efficiency (quasars/quasar candidates) is better than 65%. The selection algorithm targets ultraviolet excess quasars to i* = 19.1 and higher redshift (z 3) quasars to i* = 20.2, yielding approximately 18 candidates deg-2. In addition to selecting "normal" quasars, the design of the algorithm makes it sensitive to atypical AGNs such as broad absorption line quasars and heavily reddened quasars.

Astrometry and photometry for cool dwarfs and brown dwarfs

Abstract: Trigonometric parallax determinations are presented for 28 late-type dwarfs and brown dwarfs, including eight M dwarfs with spectral types between M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz*) and/or near-IR wavelengths (JHK) is presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including 10 L and two T dwarfs with parallaxes established by association with bright, usually Hipparcos primaries, this material forms the basis for studying various color-color and color?absolute magnitude relations. The I-J color is a good predictor of absolute magnitude for late M and L dwarfs. MJ becomes monotonically fainter with I-J color and with spectral type through late L dwarfs, then brightens for early T dwarfs. The combination of z*JK colors alone can be used to classify late M, early L, and T dwarfs accurately, as well as to predict their absolute magnitudes, but is less effective at untangling the scatter among mid- and late L dwarfs. The mean tangential velocity of these objects is found to be slightly less than that for dM stars in the solar neighborhood, consistent with a sample with a mean age of several Gyr. Using colors to estimate bolometric corrections and models to estimate stellar radii, effective temperatures are derived. The latest L dwarfs are found to have Teff ~ 1360 K.

The Observed Offset Distribution of Gamma-Ray Bursts from Their Host Galaxies: A Robust Clue to the Nature of the Progenitors* **

Abstract: We present a comprehensive study to measure the locations of gamma-ray bursts (GRBs) relative to their host galaxies. In total, we find the offsets of 20 long-duration GRBs from their apparent host galaxy centers by utilizing ground-based images from Palomar and Keck and space-based images from the Hubble Space Telescope (HST). We discuss in detail how a host galaxy is assigned to an individual GRB and the robustness of the assignment process. The median projected angular (physical) offset is 017 (1.3 kpc). The median offset normalized by the individual host half-light radii is 0.98, suggesting a strong connection of GRB locations with the UV light of their hosts. This provides strong observational evidence for the connection of GRBs to star formation. We further compare the observed offset distribution with the predicted burst locations of leading stellar-mass progenitor models. In particular, we compare the observed offset distribution with an exponential disk, a model for the location of collapsars and promptly bursting binaries (e.g., helium star–black hole binaries). The statistical comparison shows good agreement, given the simplicity of the model, with the Kolmogorov-Smirnov probability that the observed offsets derive from the model distribution of PKS = 0.45. We also compare the observed GRB offsets with the expected offset distribution of delayed merging remnant progenitors (black hole–neutron star and neutron star–neutron star binaries). We find that delayed merging remnant progenitors, insofar as the predicted offset distributions from population synthesis studies are representative, can be ruled out at the 2 × 10-3 level. This is arguably the strongest observational constraint yet against delayed merging remnants as the progenitors of long-duration GRBs. In the course of this study, we have also discovered the putative host galaxies of GRB 990510 and GRB 990308 in archival HST data.

New Understanding of Large Magellanic Cloud Structure, Dynamics, and Orbit from Carbon Star Kinematics

Abstract: We formulate a new and coherent understanding of the structure and dynamics of the Large Magellanic Cloud (LMC) and its orbit around and interaction with the Milky Way. Much of our understanding of these issues hinges on studies of the LMC line-of-sight kinematics. The observed velocity field includes contributions from the LMC rotation curve V(R'), the LMC transverse velocity vector vt, and the rate of inclination change di/dt. All previous studies have assumed di/dt = 0. We show that this is incorrect and that, combined with uncertainties in vt, this has led to incorrect estimates of many important structural parameters of the LMC. We derive general expressions for the velocity field that we fit to kinematic data for 1041 carbon stars. We calculate vt by compiling and improving LMC proper-motion measurements from the literature, and we show that for known vt, all other model parameters are uniquely determined by the data. The position angle of the line of nodes is Θ = 1299 ± 60, consistent with the value determined geometrically by van der Marel & Cioni in 2001. The rate of inclination change is di/dt = -0.37 ± 0.22 mas yr-1 = -103° ± 61° Gyr-1. This is similar in magnitude to predictions from N-body simulations by M. Weinberg, which predict LMC disk precession and nutation due to Milky Way tidal torques. The LMC rotation curve V(R') has amplitude 49.8 ± 15.9 km s-1. This is 40% lower than what has previously (and incorrectly) been inferred from studies of H I, carbon stars, and other tracers. The line-of-sight velocity dispersion has an average value σ = 20.2 ± 0.5 km s-1, with little variation as a function of radius. The dynamical center of the carbon stars is consistent with the center of the bar and the center of the outer isophotes, but it is offset by 12 ± 06 from the kinematic center of the H I. The enclosed mass inside the last data point is MLMC(8.9 kpc) = (8.7 ± 4.3) × 109 M⊙, more than half of which is due to a dark halo. The LMC has a considerable vertical thickness; its V/σ = 2.9 ± 0.9 is less than the value for the Milky Way's thick disk (V/σ ≈ 3.9). Simple arguments for models stratified on spheroids indicate that the (out of plane) axial ratio could be ~0.3 or larger. Isothermal disk models for the observed velocity dispersion profile confirm the finding of Alves & Nelson that the scale height must increase with radius. A substantial thickness to the LMC disk is consistent with the simulations of Weinberg, which predict LMC disk thickening due to Milky Way tidal forces. These affect LMC structure even inside the LMC's tidal radius, which we calculate to be rt = 15.0 ± 4.5 kpc (i.e., 171 ± 51). The new insights into LMC structure need not significantly alter existing predictions for the LMC's self-lensing optical depth, which to lowest order depends only on σ. The compiled proper-motion data imply an LMC transverse velocity vt = 406 km s-1 in the direction of position angle 787 (with errors of ~40 km s-1 in each coordinate). This can be combined with the observed systemic line-of-sight velocity, vsys = 262.2 ± 3.4 km s-1, to calculate the LMC velocity in the Galactocentric rest frame. This yields vLMC = 293 ± 39 km s-1, with radial and tangential components vLMC,rad = 84 ± 7 km s-1 and vLMC,tan = 281 ± 41 km s-1, respectively. This is consistent with the range of velocities that has been predicted by models for the Magellanic Stream. The implied orbit of the LMC has an apocenter-to-pericenter distance ratio ~2.5 : 1, a perigalactic distance ~45 kpc, and a present orbital period around the Milky Way of ~1.5 Gyr. The constraint that the LMC is bound to the Milky Way provides a robust limit on the minimum mass and extent of the Milky Way dark halo: MMW ≥ 4.3 × 1011 M⊙ and rh ≥ 39 kpc (68.3% confidence). Finally, we present predictions for the LMC proper-motion velocity field, and we discuss how measurements of this may lead to kinematic distance estimates for the LMC.

Evolution of the ionizing background and the epoch of reionization from the spectra of z ∼ 6 quasars

Abstract: We study the process of cosmic reionization and estimate the ionizing background in the intergalactic medium (IGM) using the Lyman series absorption in the spectra of the four quasars at 5.7 3) are still mostly neutral, and the comoving mean free path of ionizing photons is shorter than 8 Mpc. Comparison with simulations of cosmological reionization shows that the observed properties of the IGM at z ~ 6 are typical of those in the era at the end of the overlap stage of reionization when the individual H II regions merge. Thus z ~ 6 marks the end of the reionization epoch. The redshift of reionization constrains the small-scale power of the mass-density fluctuations and the star-forming efficiency of the first generation of objects.

Optical and Radio Properties of Extragalactic Sources Observed by the FIRST Survey and the Sloan Digital Sky Survey

Abstract: We discuss the optical and radio properties of ~30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 15 with a Sloan Digital Sky Survey (SDSS) (optical, sensitive to r* ~ 22.2) source in 1230 deg2 of sky. The matched sample represents ~30% of the 108,000 FIRST sources and 0.1% of the 2.5 ? 107 SDSS sources in the studied region. SDSS spectra are available for 4300 galaxies and 1154 quasars from the matched sample and for a control sample of 140,000 galaxies and 20,000 quasars in 1030 deg2 of sky. Here we analyze only core sources, which dominate the sample; the fraction of SDSS-FIRST sources with complex radio morphology is determined to be less than 10%. This large and unbiased catalog of optical identifications provides much firmer statistical footing for existing results and allows several new findings. The majority (83%) of the FIRST sources identified with an SDSS source brighter than r* = 21 are optically resolved; the fraction of resolved objects among the matched sources is a function of the radio flux, increasing from ~50% at the bright end to ~90% at the FIRST faint limit. Nearly all optically unresolved radio sources have nonstellar colors indicative of quasars. We estimate an upper limit of ~5% for the fraction of quasars with broadband optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux?optical flux plane suggests the existence of the quasar radio dichotomy; 8% ? 1% of all quasars with i* 2.22) galaxies, especially those with r* > 17.5. Magnitude- and redshift-limited samples show that radio galaxies have a different optical luminosity distribution than nonradio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. For a given optical luminosity and redshift, the observed optical colors of radio galaxies are indistinguishable from those of all SDSS galaxies selected by identical criteria. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions and resulting selection effects are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. The fraction of radio galaxies whose emission-line ratios indicate an AGN (30%), rather than starburst, origin is 6 times larger than the corresponding fraction for all SDSS galaxies (r* < 17.5). We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies.

The Magellanic Clouds Photometric Survey: The Large Magellanic Cloud Stellar Catalog and Extinction Map

Abstract: We present our catalog of U, B, V, and I stellar photometry of the central 64 deg2 area of the Large Magellanic Cloud. Internal and external astrometric and photometric tests using existing optical photometry (U, B, and V from Massey's bright star catalog and I from the near-infrared sky survey DENIS) are used to confirm our observational uncertainty estimates. We fit stellar atmosphere models to the optical data to check the consistency of the photometry for individual stars across the passbands and to estimate the line-of-sight extinction. Finally, we use the estimated line-of-sight extinctions to produce an extinction map across the Large Magellanic Cloud, confirm the variation of extinction as a function of stellar population, and produce a simple geometric model for the extinction as a function of stellar population.

Shapes and Shears, Stars and Smears: Optimal Measurements for Weak Lensing

Abstract: We present the theoretical and analytical bases of optimal techniques to measure weak gravitational shear from images of galaxies. We first characterize the geometric space of shears and ellipticity and then use this geometric interpretation to analyze images. The steps of this analysis include measurement of object shapes on images, combining measurements of a given galaxy on different images, estimating the underlying shear from an ensemble of galaxy shapes, and compensating for the systematic effects of image distortion, bias from point-spread function (PSF) asymmetries, and "dilution" of the signal by the seeing. These methods minimize the ellipticity measurement noise, provide calculable shear uncertainty estimates, and allow removal of systematic contamination by PSF effects to arbitrary precision. Galaxy images and PSFs are expressed as "Laguerre expansions," a two-dimensional generalization of the Edgeworth expansion, making the PSF correction and shape measurement relatively straightforward and computationally efficient. We also discuss sources of noise-induced bias in weak-lensing measurements—selection biases, and "centroid" biases arising from noise rectification—and provide a solution for these and previously identified biases.

Characterization of M, L, and T Dwarfs in the Sloan Digital Sky Survey

Abstract: An extensive sample of M, L, and T dwarfs identified in the Sloan Digital Sky Survey (SDSS) has been compiled The sample of 718 dwarfs includes 677 new objects (629 M dwarfs and 48 L dwarfs), together with 41 that have been previously published All new objects and some of the previously published ones have new optical spectra obtained either with the SDSS spectrographs or with the Apache Point Observatory 35 m ARC telescope Spectral types and SDSS colors are available for all objects; approximately 35% also have near-infrared magnitudes measured by 2MASS (Two Micron All Sky Survey) or on the Mauna Kea system We use this sample to characterize the color–spectral type and color-color relations of late-type dwarfs in the SDSS filters and to derive spectroscopic and photometric parallax relations for use in future studies of the luminosity and mass functions based on SDSS data We find that the i* - z* and i* - J colors provide good spectral type and absolute magnitude (Mi*) estimates for M and L dwarfs Our distance estimates for the current sample indicate that SDSS is finding early M dwarfs out to ~15 kpc, L dwarfs to ~100 pc, and T dwarfs to ~20 pc The T dwarf photometric data show large scatter and are therefore less reliable for spectral type and distance estimation

Radio Sources and Star Formation in the Local Universe

Abstract: Galaxies from the entire Uppsala Galaxy Catalog (UGC) have been identified with 4583 radio sources stronger than 2.5 mJy at 1.4 GHz from the NRAO VLA Sky Survey (NVSS). The complete sample of 3398 galaxies brighter than mp = 14.5 in the area defined by δ > -2°30' and |b| > 20° yielded the UGC/NVSS sample of 1966 radio sources. Their dominant energy sources were classified as stars (85%) or active galactic nuclei (15%). The luminosity function of star-forming galaxies agrees well with the far-infrared (FIR) luminosity function converted to 1.4 GHz by the FIR/radio correlation. The spectral power density of star-forming galaxies is USF = (1.53 ± 0.07) × 1019 W Hz-1 Mpc-3 (statistical errors only) if H0 = 70 km s-1 Mpc-1. We used a model consistent with the observed FIR/radio correlation to estimate the corresponding star formation rate density within the past τ ~ 3 × 108 yr; it is ρSF(M > 0.1 M⊙) ≈ 0.018 M⊙ yr-1 Mpc-3. The radio sources in star-forming galaxies may be evolving even at moderately low redshifts (z ~ 0.1).

Formation of the Galilean Satellites: Conditions of Accretion

Abstract: We examine formation conditions for the Galilean satellites in the context of models of late-stage giant planet accretion and satellite-disk interactions. We first reevaluate the current standard, in which the satellites form from a ‘‘ minimum mass subnebula ’’ disk, obtained by augmenting the mass of the current satellites to solar abundance and resulting in a disk mass containing about 2% of Jupiter’s mass. Conditions in such a massive and gas-rich disk are difficult to reconcile with both the icy compositions of Ganymede and Callisto and the protracted formation time needed to explain Callisto’s apparent incomplete differentiation. In addition, we argue that disk torques in such a gas-rich disk would cause large satellites to be lost to inward decay onto the planet. These issues have prevented us from identifying a self-consistent scenario for the formation and survival of the Galilean satellites using the standard model. We then consider an alternative, in which the satellites form in a circumplanetary accretion disk produced during the very end stages of gas accretion onto Jupiter. In this case, an amount of gas and solids of at least � 0.02 Jovian masses must still be processed through the disk during the satellite formation era, but this amount need not have been present all at once. We find that an accretion disk produced by a slow inflow of gas and solids, e.g., 2 � 10 � 7 Jovian masses per year, is most consistent with conditions needed to form the Galilean satellites, including disk temperatures low enough for ices and protracted satellite accretion times of � 10 5 yr. Such a ‘‘ gas-starved ’’ disk has an orders-of-magnitude lower gas surface density than the minimum mass subnebula (and for many cases is optically thin). Solids delivered to the disk build up over many disk viscous cycles, resulting in a greatly reduced gas-to-solids ratio during the final stages of satellite accretion. This allows for the survival of Galilean-sized satellites against disk torques over a wide range of plausible conditions.

The Supplemental IRAS Minor Planet Survey

Abstract: We present additional and revised IRAS diameters and albedos for the 1992 IRAS Minor Planet Survey (IMPS). Using orbital elements for 26,791 numbered asteroids, we found 2228 different multiply observed asteroids associated with IRAS sources, an increase of 432 (24%) over IMPS. The IRAS sample of small asteroids, diameters D < 20.0 km, has increased by 72% (from 306 to 526), the sample of Jupiter Trojan asteroids by 77% (from 39 to 69), and the sample of small Trojan asteroids (D < 80 km) by nearly a factor of 3 (from nine to 26). We present the entire Supplemental IRAS Minor Planet Survey data set, describe how it was created, compare it with the IMPS data set, and estimate how many more asteroids remain to be found in the IRAS data.

Broad Emission-Line Shifts in Quasars: An Orientation Measure for Radio-Quiet Quasars?

Abstract: Using a sample of 3814 quasars from the Early Data Release of the Sloan Digital Sky Survey, we confirm that high-ionization, broad emission lines, such as C IV, are significantly blueshifted with respect to low-ionization, broad emission lines, such as Mg II, which are thought to be close to the systemic redshift. We examine the velocity shifts of the Mg II and C IV emission lines with respect to [O III] and Mg II, respectively. C IV emission-line peaks have a range of shifts from a redshift of 500 km s-1 to blueshifts well in excess of 2000 km s-1 as compared with Mg II. We confirm previous results that suggest an anticorrelation between the shift of the C IV emission-line peak and the rest equivalent width of the C IV emission line. Furthermore, by creating composite quasar spectra as a function of C IV shift, we are able to study in detail the profiles of the line as a function of velocity shift. We find that the apparent shift of the C IV emission-line peak is not a shift so much as it is a lack of flux in the red wing for the composite with the largest apparent shift. This observation should strongly constrain models for the broad emission-line region in quasars. The emission-line blueshift and equivalent width of C IV are also discussed in light of the well-known anticorrelation between the equivalent width of C IV emission and continuum luminosity, otherwise known as the Baldwin effect. We further discuss the C IV emission-line shift as a function of other quasar properties, such as spectral index, radio and X-ray detection. We find a possible correlation between the C IV emission-line shifts and the radio properties of the quasars, which is suggestive of orientation as the cause of the C IV velocity shifts. Finally, we explore whether the C IV emission-line blueshifts correlate with the presence of broad absorption line absorption troughs or with narrow, associated absorption, and how these might be related to orientation.

Evidence for Stellar Substructure in the Halo and Outer Disk of M31

Abstract: We report the discovery of significant stellar substructure in the halo and outer disk of our nearest large galactic neighbor, M31. Our deep panoramic survey with the Isaac Newton Telescope Wide Field Camera currently maps out an area of ≈25 deg2 around M31, extending along the semimajor axis to 55 kpc and is the first to allow an uninterrupted study of the density and color distribution of individual red giant branch stars across a large fraction of the halo of an external spiral galaxy. We find evidence for both spatial density and metallicity (as inferred from color information) variations, which are often, but not always, correlated. In addition to the previously reported giant stellar stream, the data reveal the presence of significant stellar overdensities at large radii close to the southwestern major axis, in the proximity of the very luminous globular cluster G1, and near the northeastern major axis, coinciding with and extending beyond the previously known northern spur. The most prominent metallicity variations are found in the southern half of the halo, where two large structures with above average metallicites are apparent; one of these coincides with the giant stellar stream while the other corresponds to a much lower level stellar enhancement. Our findings contrast with, but do not conflict with, past studies of the M31 halo and outer disk that have suggested a rather homogeneous stellar population at large radius: the bulk of our newly detected substructure lies in the previously uncharted far outer regions of the galaxy. We discuss the possible origin of the substructure observed and the implications it has for constraining the galaxy assembly process.

A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries*

Abstract: We report 25,563 radial velocity measurements for 1359 single-lined stars in the Carney-Latham sample of 1464 stars selected for high proper motion. For 171 of these, we present spectroscopic orbital solutions. We find no obvious difference between the binary characteristics in the halo and the disk populations. The observed frequency is the same, and the period distributions are consistent with the hypothesis that the two sets of binaries were drawn from the same parent population. This suggests that metallicity in general, and radiative opacities in particular, have little influence over the fragmentation process that leads to short-period binaries. All the binaries with periods shorter than 10 days have nearly circular orbits, while the binaries with periods longer than 20 days exhibit a wide range of eccentricities and a median value of 0.37. For the metal-poor high-velocity halo binaries in our sample, the transition from circular to eccentric orbits appears to occur at about 20 days, supporting the conclusion that tidal circularization on the main sequence is important for the oldest binaries in the Galaxy.

Metallicities of Old Open Clusters

Abstract: We present radial velocities and metallicities for a sample of 39 open clusters with ages greater than about 700 million years. For 24 clusters new moderate-resolution spectroscopic data obtained with multiobject spectrographs on the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory 4 m telescopes are used to determine radial velocities and mean cluster metallicities. These new results are combined with data published previously by Friel & Janes to provide a sample of 459 giants in 39 old open clusters, which are used to investigate radial abundance gradients in the Galactic disk. Based on an updated abundance calibration of spectroscopic indices measuring Fe and Fe-peak element blends, this larger sample yields an abundance gradient of -0.06 ± 0.01 dex kpc-1 over a range in Galactocentric radius of 7 to 16 kpc. There is a slight suggestion of a steepening of the abundance gradient with increasing cluster age in this sample, but the significance of the result is limited by the restricted distance range for the youngest clusters. The clusters show no correlation of metallicity with age in the solar neighborhood. Consistent with the evidence for a steepening of the gradient with age, the clusters in the outer disk beyond 10 kpc show a suggestion at the 1.5 σ level of a dependence of metallicity on age.

Exploring the Full Stellar Population of the Upper Scorpius OB Association

Abstract: We investigate the stellar population and star formation history of the Upper Scorpius OB association, the most nearby region of recent massive star formation, over the full stellar mass range from 0.1 to 20 M?. The first part of this paper describes an extension of our large spectroscopic survey (Preibisch et al., published in 2001) for low-mass pre?main-sequence (PMS) stars in Upper Scorpius. Using the multiobject spectrograph 2dF at the Anglo-Australian Telescope, we obtained spectra of 469 stars with magnitudes R = 12.5?18.0 in a 6 deg2 area. Among these, we find 68 new PMS stars, nearly all of them M-type stars, by their strong lithium absorption lines. The total area covered by our 2dF survey is now 9 deg2 and contains 166 new PMS stars. Combining these results with our earlier investigation (Preibisch & Zinnecker) yields a sample of 250 PMS stars in the mass range ~0.1 to ~2 M?. The location of these stars in the HR diagram suggests a mean age of 5 Myr without a significant age spread. In the second part of this paper, we also consider the population of 114 high-mass members identified in detailed Hipparcos studies. We construct a combined HR diagram for the 364 high- and low-mass members and find that the whole stellar population is very well characterized by a very narrow age distribution around 5 Myr. We estimate individual masses for all members and construct an empirical mass function covering the mass range from 0.1 up to 20 M?. A power-law fit to the mass function gives a slope of ? ~ -2.6 above ~2 M? and a much flatter slope (? ~ -0.9) below ~0.6 M?. The initial mass function of Upper Sco is not identical, but within the errors consistent with recent determinations of the field initial mass function. There is certainly no deficit of low-mass stars in the Upper Sco OB association, but rather a small excess of low-mass stars. Our results on the stellar age distribution confirm earlier indications that the star formation process in Upper Sco was triggered and support previous conjectures that the triggering event was a supernova shock wave originating from the nearby Upper Centaurus?Lupus association.

A Hubble Space Telescope Census of Nuclear Star Clusters in Late-Type Spiral Galaxies. I. Observations and Image Analysis*

Abstract: We present new Hubble Space Telescope I-band images of a sample of 77 nearby late-type spiral galaxies with low inclination. The main purpose of this catalog is to study the frequency and properties of nuclear star clusters. In 59 galaxies of our sample, we have identified a distinct, compact (but resolved), and dominant source at or very close to the photocenter. In many cases, these clusters are the only prominent source within a few kiloparsecs from the galaxy nucleus. We present surface brightness profiles, derived from elliptical isophote fits, of all galaxies for which the fit was successful. We use the fitted isophotes at radii larger than 2'' to check whether the location of the cluster coincides with the photocenter of the galaxy and confirm that in nearly all cases, we are truly dealing with "nuclear" star clusters. From analytical fits to the surface brightness profiles, we derive the cluster luminosities after subtraction of the light contribution from the underlying galaxy disk and/or bulge.

Fourier Techniques for Very Long Astrophysical Time-Series Analysis

Abstract: We present an assortment of both standard and advanced Fourier techniques that are useful in the analysis of astrophysical time series of very long duration—where the observation time is much greater than the time resolution of the individual data points. We begin by reviewing the operational characteristics of Fourier transforms of time-series data, including power-spectral statistics, discussing some of the differences between analyses of binned data, sampled data, and event data, and we briefly discuss algorithms for calculating discrete Fourier transforms (DFTs) of very long time series. We then discuss the response of DFTs to periodic signals and present techniques to recover Fourier amplitude "lost" during simple traditional analyses if the periodicities change frequency during the observation. These techniques include Fourier interpolation, which allows us to correct the response for signals that occur between Fourier frequency bins. We then present techniques for estimating additional signal properties such as the signal's centroid and duration in time, the first and second derivatives of the frequency, the pulsed fraction, and an overall estimate of the significance of a detection. Finally, we present a recipe for a basic but thorough Fourier analysis of a time series for well-behaved pulsations.

The Palomar/MSU Nearby Star Spectroscopic Survey. III. Chromospheric Activity, M Dwarf Ages, and the Local Star Formation History

Abstract: We present high-resolution echelle spectroscopy of 676 nearby M dwarfs. Our measurements include radial velocities, equivalent widths of important chromospheric emission lines, and rotational velocities for rapidly rotating stars. We identify several distinct groups by their Hα properties and investigate variations in chromospheric activity among early (M0–M2.5) and mid (M3–M6) dwarfs. Using a volume-limited sample together with a relationship between age and chromospheric activity, we show that the rate of star formation in the immediate solar neighborhood has been relatively constant over the last 4 Gyr. In particular, our results are inconsistent with recent large bursts of star formation. We use the correlation between Hα activity and age as a function of color to set constraints on the properties of L and T dwarf secondary components in binary systems. We also identify a number of interesting stars, including rapid rotators, radial velocity variables, and spectroscopic binaries.

Post-T Tauri Stars in the Nearest OB Association

Abstract: We present results of a spectroscopic survey of X-ray– and proper-motion–selected samples of late-type stars in the Lower Centaurus–Crux (LCC) and Upper Centaurus–Lupus (UCL) subgroups of the nearest OB association: Scorpius-Centaurus. The primary goals of the survey are to determine the star formation history of the OB subgroups and to assess the frequency of accreting stars in a sample dominated by "post–T Tauri" pre–main-sequence (PMS) stars. We investigate two samples: (1) proper-motion candidates from the ACT Catalog and Tycho Reference Catalog (TRC) with X-ray counterparts in the ROSAT All-Sky Survey (RASS) Bright Source Catalog and (2) G- and K-type stars in the Hipparcos catalog found to be candidate members by de Zeeuw et al. We obtained optical spectra of 130 candidates with the Siding Spring 2.3 m dual-beam spectrograph. PMS stars were identified by (1) strong Li λ6707 absorption, (2) subgiant surface gravities, (3) proper motions consistent with Sco-Cen membership, and (4) H-R diagram positions consistent with being PMS. We find 93% of the RASS-ACT/TRC stars to be probable PMS members, compared with 73% of the Hipparcos candidates. We demonstrate that measuring the gravity-sensitive band ratio of Sr II λ4077 to Fe I λ4071 is a valuable means of discriminating PMS and zero-age main-sequence (ZAMS) stars. Using secular parallaxes and Hipparcos, Tycho-2, and Two Micron All Sky Survey photometry, we construct an H-R diagram. Depending on the choice of published evolutionary tracks, we find the mean ages of the PMS populations to range between 17 and 23 Myr for LCC and 15 and 22 Myr for UCL. Taking into account observational errors, it appears that 95% of the low-mass star formation in each subgroup must have occurred in less than 8 Myr (LCC) and 12 Myr (UCL). Using the Bertelli et al. tracks, we find main-sequence turnoff ages for Hipparcos B-type members to be 16 ± 1 Myr for LCC and 17 ± 1 Myr for UCL. Contrary to previous findings, it appears that LCC is coeval with, or slightly older than, UCL. The secular parallaxes of the Sco-Cen PMS stars yield distances of 85–215 pc, with 12 of the LCC members lying within 100 pc of the Sun. Only one out of 110 (0.9%; 1 σ) PMS solar-type stars in the sample with ages of 13 ± 1 (s.e.) ± 6 (1 σ) Myr and masses of 1.3 ± 0.2 (1 σ) M⊙ shows both enhanced Hα emission and a K-band excess indicative of accretion from a truncated circumstellar disk: the nearby (d 86 pc) classical T Tauri star PDS 66.

The Palomar/MSU Nearby Star Spectroscopic Survey. IV. The Luminosity Function in the Solar Neighborhood and M Dwarf Kinematics*

Abstract: We have used new astrometric and spectroscopic observations to refine the volume-complete sample of M dwarfs defined in previous papers in this series. With the addition of Hipparcos astrometry, our revised VC2 sample includes 558 main-sequence stars in 448 systems. Analysis of that data set shows no evidence of any systematic kinematic bias. Combining those data with a Hipparcos-based sample of AFGK dwarfs within 25 pc of the Sun, we have derived the solar neighborhood luminosity function, ?(MV), for stars with absolute magnitudes between -1 and +17. Using empirical and semiempirical mass-MV relations, we transform ?(MV) to the present-day mass function, ?(M) (=dN/dM). Depending on the mass-luminosity calibration adopted, ?(M) can be represented by either a two-component or a three-component power law. In either case, the power-law index ? has a value of ~1.3 at low masses (0.1 M? 4) stars in the Hipparcos 25 pc sample are well represented by two-component Gaussian distributions, with ~10% of the stars in the higher velocity dispersion component. We suggest that the latter component is the thick disk, and we offer a possible explanation for the relatively low velocity dispersions shown by ultracool dwarfs.

Luminosity Functions of 10 Nearby Clusters of Galaxies. I. Data

Abstract: Wide field CCD imaging was carried out in the RC band for 10 nearby clusters of galaxies and four control fields to derive the total and type-specific luminosity functions. Observation and data reduction procedures are described. We extract galaxies down to RC ~ 20 mag and classify them into two broad types, r1/4-like and exponential-like, on the basis of the bulge-to-total luminosity ratio B/T estimated from Petrosian quantities. We describe our classification scheme in detail. We apply a single classification scheme to both giant and dwarf galaxies. The consistency of our classification is verified for giant galaxies using both simulated images and real data in the literature. We set the boundary of our two types at B/T = 0.35. This boundary gives 70% completeness to both the r1/4-like sample (for E/S0 galaxies) and the exponential-like sample (for Sa–Irr galaxies). Our classification for dwarf galaxies is investigated using higher resolution images of some 20 dwarf galaxies in the Virgo Cluster. Galaxy catalogs are constructed, which include position, magnitude, and B/T. The projected sky distribution of each type of galaxies is shown for the clusters and control fields.

Optical Photometry of the Type Ia Supernova 1999ee and the Type Ib/c Supernova 1999ex in IC 5179

Abstract: We present UBVRIz light curves of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex, both located in the galaxy IC 5179. SN 1999ee has an extremely well-sampled light curve spanning from 10 days before Bmax through 53 days after peak. Near maximum, we find systematic differences of ~0.05 mag in photometry measured with two different telescopes, even though the photometry is reduced to the same local standards around the supernova using the specific color terms for each instrumental system. We use models for our bandpasses and spectrophotometry of SN 1999ee to derive magnitude corrections (S-corrections) and remedy this problem. This exercise demonstrates the need of accurately characterizing the instrumental system before great photometric accuracies of Type Ia supernovae can be claimed. It also shows that this effect can have important astrophysical consequences, since a small systematic shift of 0.02 mag in the B-V color can introduce a 0.08 mag error in the extinction-corrected peak B magnitude of a supernova and thus lead to biased cosmological parameters. The data for the Type Ib/c SN 1999ex present us with the first ever observed shock breakout of a supernova of this class. These observations show that shock breakout occurred 18 days before Bmax and support the idea that Type Ib/c supernovae are due to the core collapse of massive stars rather than thermonuclear disruption of white dwarfs.

Large-amplitude X-ray outbursts from galactic nuclei: A systematic survey using ROSAT archival data

Abstract: In recent years, luminous X-ray outbursts with variability amplitudes as high as ≈400 have been serendipitously detected from a small number of active and inactive galaxies. These outbursts may result from the tidal disruptions of stars by supermassive black holes, as well as accretion disk instabilities. In order to place the first reliable constraints on the rate of such outbursts in the universe and to test the stellar tidal disruption hypothesis, we have performed a systematic and complete survey for them by cross-correlating ROSAT All-Sky Survey (RASS) and pointed Position Sensitive Proportional Counter data. We have detected five galaxies that were in outburst during the RASS, three of which show no signs of nuclear activity; these objects had been reported on individually in previous studies. After making reasonable corrections for the complicated selection effects, we conclude that the rate of large-amplitude X-ray outbursts from inactive galaxies in the local universe is ≈9.1 × 10-6 galaxy-1 yr-1. This rate is consistent with the predicted rate of stellar tidal disruption events in such galaxies. When only the two active galaxies are considered, we find a rate for active galaxies of ≈8.5 × 10-4 galaxy-1 yr-1. In order to place tighter constraints on these rates, additional outbursts must be detected.

A New Spectral Classification System for the Earliest O Stars: Definition of Type O2

Abstract: High-quality, blue-violet spectroscopic data are collected for 24 stars that have been classified as type O3 and that display the hallmark N IV and N V lines. A new member of the class is presented; it is the second known in the Cyg OB2 association, and only the second in the northern hemisphere. New digital data are also presented for several of the other stars. Although the data are inhomogeneous, the uniform plots by subcategory reveal some interesting new relationships. Several issues concerning the classification of the hottest O-type spectra are discussed, and new digital data are presented for the five original O3 dwarfs in the Carina Nebula, in which the N IV, N V features are very weak or absent. New spectral types O2 and O3.5 are introduced here as steps toward resolving these issues. The relationship between the derived absolute visual magnitudes and the spectroscopic luminosity classes of the O2–O3 stars shows more scatter than at later O types, at least partly because some overluminous dwarfs are unresolved multiple systems, and some close binary systems of relatively low luminosity and mass emulate O3 supergiant spectra. However, it also appears that the behavior of He II λ4686, the primary luminosity criterion at later O types, responds to other phenomena in addition to luminosity at spectral types O2–O3. There is evidence that these spectral types may correspond to an immediate pre-WN phase, with a correspondingly large range of luminosities and masses. A complete census of spectra classified into the original O3 subcategories considered here (not including intermediate O3/WN types or O3 dwarfs without N IV, N V features) totals 45 stars; 34 of them belong to the Large Magellanic Cloud and 20 of the latter to 30 Doradus.