Showing papers in "The Astronomical Journal in 2008"

The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively

Abstract: We measure the star formation efficiency (SFE), the star formation rate (SFR) per unit of gas, in 23 nearby galaxies and compare it with expectations from proposed star formation laws and thresholds. We use H I maps from The H I Nearby Galaxy Survey (THINGS) and derive H2 maps of CO measured by HERA CO-Line Extragalactic Survey and Berkeley-Illinois-Maryland Association Survey of Nearby Galaxies. We estimate the SFR by combining Galaxy Evolution Explorer (GALEX) far-ultraviolet maps and the Spitzer Infrared Nearby Galaxies Survey (SINGS) 24 ?m maps, infer stellar surface density profiles from SINGS 3.6 ?m data, and use kinematics from THINGS. We measure the SFE as a function of the free fall and orbital timescales, midplane gas pressure, stability of the gas disk to collapse (including the effects of stars), the ability of perturbations to grow despite shear, and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at (5.25 ? 2.5) ? 10?10 yr?1 (equivalent to an H2 depletion time of 1.9 ? 109 yr) as a function of all of these variables at our 800 pc resolution. Where the interstellar medium (ISM) is mostly H I, however, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2r 25-0.25r 25. We interpret this decline as a strong dependence of giant molecular cloud (GMC) formation on environment. The ratio of molecular-to-atomic gas appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to H I-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe?phase balance in the H I, H2 formation and destruction, and stellar feedback?governs the formation of GMCs from H I.

The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales

Abstract: We present a comprehensive analysis of the relationship between star formation rate surface density, ΣSFR, and gas surface density, Σgas, at sub-kpc resolution in a sample of 18 nearby galaxies. We use high-resolution H I data from The H I Nearby Galaxy Survey, CO data from HERACLES and the BIMA Survey of Nearby Galaxies, 24 μm data from the Spitzer Space Telescope, and UV data from the Galaxy Evolution Explorer. We target seven spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H2 dominated centers of spiral galaxies, their H I dominated outskirts and the H I rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N = 1.0 ± 0.2 relates ΣSFR and ΣH2 across our sample of spiral galaxies, i.e., that H2 forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ~2 × 109 years. The range of ΣH2 over which we measure this relation is ~3-50 M ☉ pc–2, significantly lower than in starburst environments. We find the same results when performing a pixel-by-pixel analysis, averaging in radial bins, or when varying the star formation tracer used. We interpret the linear relation and constant depletion time as evidence that stars are forming in giant molecular clouds with approximately uniform properties and that ΣH2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas surface density (Σgas) and ΣSFR varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between ΣHI and ΣSFR. As a result, the star formation efficiency (SFE), ΣSFR/Σgas, varies strongly across our sample and within individual galaxies. We show that this variation is systematic and consistent with the SFE being set by local environmental factors: in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. We attribute the similarity to common environments (low density, low metallicity, H I dominated) and argue that shear (which is typically absent in dwarfs) cannot drive the SFE. In addition to a molecular Schmidt law, the other general feature of our sample is a sharp saturation of H I surface densities at ΣHI ≈ 9 M ☉ pc–2 in both the spiral and dwarf galaxies. In the case of the spirals, we observe gas in excess of this limit to be molecular.

Things:the hi nearby galaxy survey

Abstract: We present “The HI Nearby Galaxy Survey (THINGS)”, a high spectral (≤52kms −1 ) and spatial (∼ 6 ′′ ) resolution survey of HI emission in 34 nearby galaxies obtained using the NRAO Very Large Array (VLA) The overarching scientific goal of THINGS is to investigate fundamental characteristics of the interstellar medium (ISM) related to galaxy morphology, star formation and mass distribution across the Hubble sequence Unique characteristics of the THINGS database are the homogeneous sensitivity as well as spatial and velocity resolution of the HI data which is at the limit of what can be achieved with the VLA for a significant number of galaxies A sample of 34 objects at distances 2 < D <15 Mpc (resulting in linear resolutions of ∼100 to 500pc) are targeted in THINGS, covering a wide range of star formation rates (∼ 10 −3 to 6 M⊙ yr −1 ), total HI masses MHI (001 to 14×10 9 M⊙), absolute luminosities MB (–115 to –217mag) and metallicities (75 to 92 in units of 12+log[O/H]) We describe the setup of the VLA observations, the data reduction procedures and the creation of the final THINGS data products We present an atlas of the integrated HI maps, the velocity fields, the second moment (velocity dispersion) maps and individual channel maps of each THINGS galaxy The THINGS data products are made publicly available through a dedicated webpage Accompanying THINGS papers address issues such as the small–scale structure of the ISM, the (dark) matter distribution in THINGS galaxies, and the processes leading to star formation Subject headings: surveys — galaxies: structure — galaxies: ISM — ISM: general — ISM: atoms — radio lines: galaxies

High-resolution rotation curves and galaxy mass models from THINGS

Abstract: We present rotation curves of 19 galaxies from The H I Nearby Galaxy Survey (THINGS). The high spatial and velocity resolution of THINGS make these the highest quality H I rotation curves available to date for a large sample of nearby galaxies, spanning a wide range of H I masses and luminosities. The high quality of the data allows us to derive the geometric and dynamical parameters using H I data alone. We do not find any declining rotation curves unambiguously associated with a cut-off in the mass distribution out to the last measured point. The rotation curves are combined with 3.6 μm data from the Spitzer Infrared Nearby Galaxies Survey to construct mass models. Our best-fit dynamical disk masses, derived from the rotation curves, are in good agreement with photometric disk masses derived from the 3.6 μm images in combination with stellar population synthesis arguments and two different assumptions for the stellar initial mass function (IMF). We test the cold dark matter (CDM) motivated cusp model, and the observationally motivated central density core model and find that (independent of IMF) for massive, disk-dominated galaxies, all halo models fit apparently equally well; for low-mass galaxies, however, a core-dominated halo is clearly preferred over a cusp-like halo. The empirically derived densities of the dark matter halos of the late-type galaxies in our sample are half of what is predicted by CDM simulations, again independent of the assumed IMF.

Constraining the Age-Activity Relation for Cool Stars: The Sloan Digital Sky Survey Data Release 5 Low-Mass Star Spectroscopic Sample

Abstract: We present a spectroscopic analysis of over 38,000 low-mass stars from the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Analysis of this unprecedentedly large sample confirms the previously detected decrease in the fraction of magnetically active stars (as traced by Hα emission) as a function of the vertical distance from the Galactic plane. The magnitude and slope of this effect vary as a function of spectral type. Using simple 1D dynamical models, we demonstrate that the drop in activity fraction can be explained by thin-disk dynamical heating and a rapid decrease in magnetic activity. The timescale for this rapid activity decrease changes according to the spectral type. By comparing our data to the simulations, we calibrate the age-activity relation at each M dwarf spectral type. We also present evidence for a possible decrease in the metallicity as a function of height above the Galactic plane. In addition to our activity analysis, we provide line measurements, molecular band indices, colors, radial velocities, 3D space motions, and mean properties as a function of spectral type for the SDSS DR5 low-mass star sample.

The segue stellar parameter pipeline. i. description and comparison of individual methods

Abstract: We describe the development and implementation of the Sloan Extension for Galactic Exploration and Understanding (SEGUE) Stellar Parameter Pipeline (SSPP) The SSPP is derived, using multiple techniques, radial velocities, and the fundamental stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) for AFGK-type stars, based on medium-resolution spectroscopy and ugriz photometry obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its Galactic extension (SDSS-II/SEGUE) The SSPP also provides spectral classification for a much wider range of stars, including stars with temperatures outside the window where atmospheric parameters can be estimated with the current approaches This is Paper I in a series of papers on the SSPP; it provides an overview of the SSPP, and tests of its performance using several external data sets Random and systematic errors are critically examined for the current version of the SSPP, which has been used for the sixth public data release of the SDSS (DR-6)

The Second-Generation Guide Star Catalog: Description and Properties

Abstract: The Guide Star Catalog II (GSC-II) is an all-sky database of objects derived from the uncompressed Digitized Sky Surveys that the Space Telescope Science Institute has created from the Palomar and UK Schmidt survey plates and made available to the community. Like its predecessor (GSC-I), the GSC-II was primarily created to provide guide star information and observation planning support for Hubble Space Telescope. This version, however, is already employed at some of the ground-based new-technology telescopes such as GEMINI, VLT, and TNG, and will also be used to provide support for the James Webb Space Telescope (JWST) and GAIA space missions as well as the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, one of the major ongoing scientific projects in China. Two catalogs have already been extracted from the GSC-II database and released to the astronomical community. A magnitude-limited (RF = 18.0) version, GSC2.2, was distributed soon after its production in 2001, while the GSC2.3 release has been available for general access since 2007. The GSC2.3 catalog described in this paper contains astrometry, photometry, and classification for 945,592,683 objects down to the magnitude limit of the plates. Positions are tied to the International Celestial Reference System; for stellar sources, the all-sky average absolute error per coordinate ranges from 02 to 028 depending on magnitude. When dealing with extended objects, astrometric errors are 20% worse in the case of galaxies and approximately a factor of 2 worse for blended images. Stellar photometry is determined to 0.13-0.22 mag as a function of magnitude and photographic passbands (RF , BJ , IN ). Outside of the galactic plane, stellar classification is reliable to at least 90% confidence for magnitudes brighter than RF = 19.5, and the catalog is complete to RF = 20.

A Catalog of Extended Green Objects in the GLIMPSE Survey: A New Sample of Massive Young Stellar Object Outflow Candidates

Abstract: Using images from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), we have identified more than 300 extended 4.5 μm sources (Extended Green Objects (EGOs), for the common coding of the [4.5] band as green in three-color composite InfraRed Array Camera images). We present a catalog of these EGOs, including integrated flux density measurements at 3.6, 4.5, 5.8, 8.0, and 24 μm from GLIMPSE and the Multiband Imaging Photometer for Spitzer Galactic Plane Survey. The average angular separation between a source in our sample and the nearest IRAS point source is greater than 1'. The majority of EGOs are associated with infrared dark clouds (IRDCs), and where high-resolution 6.7 GHz CH3OH maser surveys overlap the GLIMPSE coverage, EGOs and 6.7 GHz CH3OH masers are strongly correlated. Extended 4.5 μm emission is thought to trace shocked molecular gas in protostellar outflows; the association of EGOs with IRDCs and 6.7 GHz CH3OH masers suggests that the extended 4.5 μm emission may pinpoint outflows specifically from massive protostars. The mid-IR colors of EGOs lie in regions of color-color space occupied by young protostars still embedded in infalling envelopes.

The sloan digital sky survey-II supernova survey: technical summary

Abstract: The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5° wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for the discovery of new objects. Supernova imaging observations are being acquired between September 1 and November 30 of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation.

Galaxy pairs in the sloan digital sky survey. i. star formation, active galactic nucleus fraction, and the luminosity/mass-metallicity relation

Abstract: We present a sample of 1716 galaxies with companions within Δv <500 km s–1, rp < 80 h–1 70 kpc and stellar mass ratio 0.1 < M 1/M 2 < 10 from the Sloan Digital Sky Survey Data Release 4. The galaxy pairs are selected from the Main Galaxy Sample using stringent and well-understood criteria for redshift, spectral quality, available stellar masses, and metallicities. In agreement with previous studies, we find an enhancement in the star-formation rate (SFR) of galaxy pairs at projected separations <30-40 h–1 70 kpc. In addition, we find that this enhancement is highest (and extends to the greatest separations) for galaxies of approximately equal mass, the so-called major pairs. However, SFR enhancement can still be detected for a sample of galaxy pairs whose masses are within a factor of 10 of each other. Based on these results, we define a sample of close pairs (Δv <500 km s–1, rp < 30 h–1 70 kpc, and 0.1 < M 1/M 2 < 10) which we use to investigate interaction-induced effects in the luminosity-metallicity (LZ) relation. In agreement with the one previous study of the LZ relation in paired galaxies, we find an offset to lower metallicities (by ~0.1 dex) for a given luminosity for galaxies in pairs compared to the control sample. We also present the first mass-metallicity (MZ) relation comparison between paired galaxies and the field and again find an offset to lower metallicities (by ~0.05 dex) for a given mass. The smaller offset in the MZ relation indicates that both higher luminosities and lower metallicities may contribute to the shift of pairs relative to the control in the LZ relation. We show that the offset in the LZ relation depends on galaxy half-light radius, rh . Galaxies with rh 3 h–1 70 kpc and with a close companion show a 0.05-0.1 dex downward offset in metallicity compared to control galaxies of the same size. Larger galaxies do not show this offset and have LZ and MZ relations consistent with the control sample. We investigate the physical impetus behind this empirical dependence on rh and consider the galaxy's dynamical time and bulge fractions as possible causes. We conclude that the former is unlikely to be a fundamental driver of the offset in the LZ relation for paired galaxies, but that bulge fraction may play a role. Finally, we study the active galactic nucleus (AGN) fraction in both the pair and control sample and find that whilst selecting galaxies in different cuts of color and asymmetry yields different AGN fractions, the fraction for pairs and the control sample are consistent for a given set of selection criteria. This indicates that if AGNs are ignited as a result of interactions, this activity begins later than the close pairs stage (i.e. once the merger is complete).

The Acs Survey of Globular Clusters. V. Generating a Comprehensive Star Catalog for each Cluster

Abstract: The ACS Survey of Globular Clusters has used Hubble Space Telescope's Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous data set for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available point-spread function models. We describe here in detail the program's rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we ran a standard set of ~105 tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the World Wide Web.

MEASURED METALLICITIES AT THE SITES OF NEARBY BROAD-LINED TYPE Ic SUPERNOVAE AND IMPLICATIONS FOR THE SUPERNOVAE GAMMA-RAY BURST CONNECTION*

Abstract: We compare the chemical abundances at the sites of 12 nearby (z < 0.14) Type Ic supernovae (SN Ic) that showed broad lines, but had no observed gamma-ray burst (GRB), with the chemical abundances in five nearby (z < 0.25) galaxies at the sites of GRBs where broad-lined SN Ic were seen after the fireball had faded. It has previously been noted that GRB hosts are low in luminosity and low in their metal abundances. If low metallicity is sufficient to force the evolution of massive stars to end their lives as GRBs with an accompanying broad-lined SN Ic, then we would expect higher metal abundances for the broad-lined SN Ic that have no detected GRBs. This is what we observe, and this trend is independent of the choice of metallicity calibration we adopt and the mode of SN survey that found the broad-lined SN Ic. A unique feature of this analysis is that we present new spectra of the host galaxies and analyze all measurements of both samples in the same set of methods, using the galaxy emission-line measurements corrected for extinction and stellar absorption, via independent metallicity diagnostics of Kewley & Dopita, McGaugh, and Pettini & Pagel. In our small sample, the boundary between galaxies that have GRBs accompanying their broad-lined SN Ic and those that have broad-lined SN Ic without GRBs lies at an oxygen abundance of 12 + log(O/H)KD02 ~ 8.5, which corresponds to 0.2-0.6 Z☉ depending on the adopted metallicity scale and solar abundance value. Even when we limit the comparison to SN Ic that were found in untargeted supernova surveys, the environment of every broad-lined SN Ic that had no GRB is more metal rich than the site of any broad-lined SN Ic where a GRB was detected.

The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters

Abstract: We validate the accuracy and precision of the current SEGUE (Sloan Extension for Galactic Understanding and Exploration) Stellar Parameter Pipeline (SSPP), which determines stellar atmospheric parameters (effective temperature, surface gravity, and metallicity) and radial velocities (RVs), by comparing these estimates for selected members of three globular clusters (M 13, M 15, and M 2) and two open clusters (NGC 2420 and M 67) to the literature values. Spectroscopic and photometric data obtained during the course of the original Sloan Digital Sky Survey (SDSS-I) and its first extension (SDSS-II/SEGUE) are used to determine atmospheric parameter and RV estimates for stars in these clusters. Based on the scatter in the metallicities derived for the members of each cluster, we quantify the typical uncertainty of the SSPP values, σ ([Fe/H]) = 0.13 dex for stars in the range of –0.3 ≤ g – r ≤ 1.3 and 2.0 ≤ log g ≤ 5.0, at least over the metallicity interval spanned by the clusters studied (–2.3 ≤ [Fe/H] ≤ 0). The surface gravities and effective temperatures derived by the SSPP are also compared with those estimated from the comparison of color-magnitude diagrams with stellar evolution models; we find satisfactory agreement (σ(T eff)< 200 K and σ(log g) ≤ 0.4 dex).

The Structure of Classical Bulges and Pseudobulges: The Link Between Pseudobulges and Sérsic Index

Abstract: In this paper, we study the properties of pseudobulges (bulges that appear similar to disk galaxies) and classical bulges (bulges which appear similar to E-type galaxies) in bulge-disk decompositions. We show that the distribution of bulge Sersic indices, nb , is bimodal, and this bimodality correlates with the morphology of the bulge. Pseudobulges have nb 2 and classical bulges have nb 2 with little to no overlap. Also, pseudobulges do not follow the correlations of Sersic index with structural parameters or the photometric projections of the fundamental plane in the same way that classical bulges and elliptical galaxies do. We find that pseudobulges are systematically flatter than classical bulges and thus more disk-like in both their morphology and shape. We do not find significant differences between different bulge morphologies which we are collectively calling pseudobulges (nuclear spirals, nuclear rings, nuclear bars, and nuclear patchiness); they appear to behave similarly in all parameter correlations. In the Sersic index, flattening, and bulge-to-total ratio, the distinction appears to be between classical bulges and pseudobulges, not between different pseudobulge morphologies. The Sersic index of the pseudobulges does not correlate with B/T, in contrast to classical bulges. Also, the half-light radius of the pseudobulge correlates with the scale length of the disk; this is not the case for classical bulges. The correlation of Sersic index and scale lengths with bulge morphology suggests that secular evolution is creating pseudobulges with low-Sersic index and that other processes (e.g., major mergers) are responsible for the higher Sersic index in classical bulges and elliptical galaxies.

Space density of optically selected type 2 quasars

Abstract: Type 2 quasars are luminous active galactic nuclei whose central regions are obscured by large amounts of gas and dust. In this paper, we present a catalog of type 2 quasars from the Sloan Digital Sky Survey, selected based on their optical emission lines. The catalog contains 887 objects with redshifts z< 0.83; this is 6 times larger than the previous version and is by far the largest sample of type 2 quasars in the literature. We derive the [Oiii]5007 luminosity function (LF) for 10 8.3 L�

The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars

Abstract: The authors report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which they compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R {approx} 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/N), they empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km s{sup -1}, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. They estimate random errors for lower S/N spectra based on numerical simulations.

High-Resolution Dark Matter Density Profiles of THINGS Dwarf Galaxies: Correcting for Noncircular Motions

Abstract: We present a new method to remove the impact of random and small-scale noncircular motions from H I velocity fields in (dwarf) galaxies in order to better constrain the dark matter properties for these objects. This method extracts the circularly rotating velocity components from the H I data cube and condenses them into a so-called bulk velocity field. We derive high-resolution (~0.2 kpc) rotation curves of IC 2574 and NGC 2366 based on bulk velocity fields derived from The H I Nearby Galaxy Survey obtained at the Very Large Array. We compare the bulk velocity field rotation curves with those derived from the traditional intensity-weighted mean velocity fields and find significant differences. The bulk velocity field rotation curves are significantly less affected by noncircular motions and constrain the dark matter distribution in our galaxies, allowing us to address the discrepancy between the inferred and predicted dark matter distribution in galaxies (the "cusp/core" problem). Spitzer Infrared Nearby Galaxies Survey 3.6 μm data, which are largely unaffected by dust in these systems, as well as ancillary optical information, are used to separate the contribution of the baryons from the total matter content. Using stellar population synthesis models, assuming various sets of metallicity and star-formation histories, we compute stellar mass-to-light ratios for the 3.6 μm and 4.5 μm bands. Using our predicted value for the 3.6 μm stellar mass-to-light ratio, we find that the observed dark matter distributions of IC 2574 and NGC 2366 are inconsistent with the cusp-like dark matter halo predicted by Λ Cold Dark Matter models, even after corrections for noncircular motions. This result also holds for other assumptions about the stellar mass-to-light ratio. The distribution of dark matter within our sample galaxies is best described by models with a kpc-sized constant-density core.

Meeting the Cool Neighbors. X. Ultracool Dwarfs from the 2MASS All-Sky Data Release

Abstract: Using data from the 2 Micron All Sky Survey All-Sky Point Source Catalogue, we have extended our census of nearby ultracool dwarfs to cover the full celestial sphere above Galactic latitude of 15°. Starting with an initial catalog of 2,139,484 sources, we have winnowed the sample to 467 candidate late-type M or L dwarfs within 20 pc of the Sun. Fifty-four of those sources already have spectroscopic observations confirming them as late-type dwarfs. We present optical spectroscopy of 376 of the remaining 413 sources, and identify 44 as ultracool dwarfs with spectroscopic distances less than 20 pc. Twenty-five of the 37 sources that lack optical data have near-infrared spectroscopy. Combining the present sample with our previous results and data from the literature, we catalog 94 L dwarf systems within 20 pc. We discuss the distribution of activity, as measured by Hα emission, in this volume-limited sample. We have coupled the present ultracool catalog with data for stars in the northern 8 pc sample and recent (incomplete) statistics for T dwarfs to provide a snapshot of the current 20 pc census as a function of spectral type.

New velocimetry and revised cartography of the spiral arms in the milky way—a consistent symbiosis

Abstract: Recent advances in the determinations of the positions (pitch angle, shape, numbers, interarm separation) and velocities (rotation curve) of the spiral arms are evaluated and compared to previous determinations. Based on these results, an average cartographic model is developed that fits the means of basic input data and provides predictions for the locations of the arms in the Milky Way, for each galactic quadrant. For each spiral arm segment in each galactic quadrant, the LSR radial velocities are calculated for the radial distance as well as for its galactic longitude. From our velocimetric model, arm intercepts (between line of sights and spiral arms) are indicated in velocity space and may be used to find the distance and velocity to any arm, in a given longitude range. Velocity comparisons between model predictions and published CO velocity distribution are done for each galactic quadrant, with good results. Our velocimetric model is not hydromagnetic in character, nor is it a particle-simulation scheme, yet it is simple to use for comparisons with the observations and it is in symbiosis and consistent with our cartographic model (itself simple to use for comparisons with observations). A blending in velocity of the Perseus and Cygnus arms is further demonstrated, as well as an apparent longitude-velocity blending of the starting points of the four spiral arms near 4 kpc (not a physical ring). An integrated (distance, velocity) model for the mass in the disk is employed, to yield the total mass of 3.0 × 1011 M Sun within a galactic radius of 28 kpc.

Intrinsically Red Sources Observed by Spitzer in the Galactic Midplane

Abstract: We present a highly reliable flux-limited census of 18,949 point sources in the Galactic midplane that have intrinsically red mid-infrared colors. These sources were selected from the Spitzer Space Telescope Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) I and II surveys of 274 deg2 of the Galactic midplane, and consist mostly of high- and intermediate-mass young stellar objects (YSOs) and asymptotic giant branch (AGB) stars. The selection criteria were carefully chosen to minimize the effects of position-dependent sensitivity, saturation, and confusion. The distribution of sources on the sky and their location in the Infrared Array Camera and the Multiband Image Photometer for Spitzer 24 μm color-magnitude and color-color space are presented. Using this large sample, we find that YSOs and AGB stars can be mostly separated by simple color-magnitude selection criteria into approximately 50%-70% of YSOs and 30%-50% of AGB stars. Planetary nebulae and background galaxies together represent at most 2%-3% of all the red sources. 1004 red sources in the GLIMPSE II region, mostly AGB stars with high mass-loss rates, show significant (≥0.3 mag) variability at 4.5 and/or 8.0 μm. With over 11,000 likely YSOs and over 7000 likely AGB stars, this is to date the largest uniform census of AGB stars and high- and intermediate-mass YSOs in the Milky Way Galaxy.

The Sloan Digital Sky Survey-II Supernova Survey: Search Algorithm and Follow-up Observations

Abstract: The Sloan Digital Sky Survey-II Supernova Survey has identified a large number of new transient sources in a 300 deg2 region along the celestial equator during its first two seasons of a three-season campaign. Multi-band (ugriz) light curves were measured for most of the sources, which include solar system objects, galactic variable stars, active galactic nuclei, supernovae (SNe), and other astronomical transients. The imaging survey is augmented by an extensive spectroscopic follow-up program to identify SNe, measure their redshifts, and study the physical conditions of the explosions and their environment through spectroscopic diagnostics. During the survey, light curves are rapidly evaluated to provide an initial photometric type of the SNe, and a selected sample of sources are targeted for spectroscopic observations. In the first two seasons, 476 sources were selected for spectroscopic observations, of which 403 were identified as SNe. For the type Ia SNe, the main driver for the survey, our photometric typing and targeting efficiency is 90%. Only 6% of the photometric SN Ia candidates were spectroscopically classified as non-SN Ia instead, and the remaining 4% resulted in low signal-to-noise, unclassified spectra. This paper describes the search algorithm and the software, and the real-time processing of the SDSS imaging data. We also present the details of the supernova candidate selection procedures and strategies for follow-up spectroscopic and imaging observations of the discovered sources.

Pollution of single white dwarfs by accretion of many small asteroids

Abstract: Extrapolating from the solar system's asteroid belt, we propose that externally contaminated white dwarfs without an infrared excess may be experiencing continuous accretion of gas-phase material that is ultimately derived from the tidal destruction of multiple small asteroids. If this scenario is correct, then observations of metal-polluted white dwarfs may lead to determination of the bulk elemental compositions of ensembles of extrasolar minor planets.

The outer disks of early-type galaxies. I. Surface-brightness profiles of barred galaxies

Abstract: We present a study of 66 barred, early-type (S0-Sb) disk galaxies, focused on the disk surface brightness profile outside the bar region, with the aim of throwing light on the nature of Freeman type I and II profiles, their origins, and their possible relation to disk truncations. This paper discusses the data and their reduction, outlines our classification system, and presents R-band profiles for all galaxies in the sample, along with their classifications. In subsequent papers, we will explore the structure of outer disks as revealed by these profiles, and investigate their possible origins. The profiles are derived from a variety of different sources, including the Sloan Digital Sky Survey (Data Release 5). For about half of the galaxies, we have profiles derived from more than one telescope; this allows us to check the stability and repeatability of our profile extraction and classification. The vast majority of the profiles are reliable down to levels of mu(R) approximate to 27 mag arcsec(-2); in exceptional cases, we can trace profiles down to mu(R) > 28. We can typically follow disk profiles out to at least 1.5 times the traditional optical radius R-25; for some galaxies, we find light extending to similar to 3 x R-25. For type I (single-exponential) profiles, this means that we can trace the exponential disk out to 6-7 scale lengths. We classify the profiles into three main groups: type I (single-exponential), type II (down-bending), and type III (up-bending). The frequencies of these types are approximately 27%, 42%, and 24%, respectively, plus another 6% which are combinations of types II and III. We further classify type II profiles by where the break falls in relation to the bar length, and in terms of the postulated mechanisms for breaks at large radii ("classical truncation" of star formation versus the influence of the Outer Lindblad Resonance of the bar). We also classify the type III profiles by the probable morphology of the outer light (disk or spheroid). Illustrations are given for all cases.

The Nucleus of the Sagittarius dSph Galaxy and M54: A Window on the Process of Galaxy Nucleation

Abstract: We present the results of a thorough study of the nucleus of the Sgr dwarf spheroidal galaxy (Sgr dSph) and of the bright globular cluster M54 (NGC 6715) that resides within the same nucleus (Sgr,N). We have obtained accurate radial velocities and metallicity estimates for 1152 candidate Red Giant Branch stars of Sgr and M54 lying within ∼ 9 ′ from the center of the galaxy, from Keck/DEIMOS and VLT/FLAMES spectra of the infrared Calcium II triplet. Using both velocity and metallicity information we selected two samples of 425 and 321 very-likely members of M54 and of Sgr,N, respectively. The two considered systems display significantly different velocity dispersion profiles: M54 has a steeply decreasing profile from r = 0 ′ , where σ ≃ 14.2 km/s, to r ≃ 3 ′ where it reaches σ ≃ 5.3 km/s, then it appears to rise again to σ ≃ 10 km/s at r ∼ 7 ′ . In contrast Sgr,N has a uniformly flat profile at σ ≃ 9.6 km/s over the whole 0 ′ ≤ r ≤ 9 ′ range. Using data from the literature we show that the velocity dispersion of Sgr remains constant at least out to r ∼ 100 ′ and there is no sign of the transition between the outer flat-luminosity-profile core and the inner nucleus in the velocity profile. These results - together with a re-analysis of the Surface Brightness profile of Sgr,N and a suite of dedicated N-body simulations - provide very strong support for the hypothesis that the nucleus of Sgr formed independently of M54, which probably plunged to its present position, coincident with Sgr,N, because of significant decay of the original orbit due to dynamical friction. Subject headings: galaxies: dwarf — globular clusters: individual(NGC 6715) — stars: kinematics — galaxies: nuclei — galaxies: individual (Sgr dSph)

Rotational and radial velocities for a sample of 761 hipparcos giants and the role of binarity

Abstract: We present rotational and radial velocities for a sample of 761 giants selected from the Hipparcos Catalogue to lie within 100 pc of the Sun. Our original goal was to examine stellar rotation in field giants using spectroscopic line broadening to look for evidence of excess rotation that could be attributed to planets that were engulfed as the parent stars expanded. Thus we were obliged to investigate other sources of line broadening, including tidal coupling in close binaries and macroturbulence. For all the binaries in our sample with periods shorter than 20 days the orbits have been circularized, while about half the orbits with periods in the range 20–100 days still show significant eccentricity. All our primaries in orbits shorter than 30 days show line broadening consistent with synchronized rotation, while about half the primaries with periods in the range 30–120 days are synchronized. To study the dependence of rotation on stellar evolution when tidal effects are not important, we used a subsample of single stars and members in wide binaries. We found evidence to suggest that the first dredge-up may play a role in speeding up the rotation of the observable outer layers of giants and that the rotational velocity of horizontal branch stars is larger by a few km s −1 than that of first-ascent giants with similar mass, effective temperature, and radius. Finally, we found three giants that rotate more rapidly than expected. We conjecture that they acquired their excess angular momentum by ingesting planets.

Age determination of six intermediate-age small magellanic cloud star clusters with HST/ACS

Abstract: We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color-magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5 ± 0.5 Gyr (Lindsay 1), 6.5 ± 0.5 Gyr (Kron 3), 6 ± 0.5 Gyr (NGC 339), 6 ± 0.5 Gyr (NGC 416), and 6.5 ± 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turnoffs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud and the Milky Way. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally-concentrated blue straggler star candidates in NGC 416, while these are not present for the other clusters. Using the red clump magnitudes, we find that the closest cluster, NGC 419 (~50 kpc), and the farthest cluster, Lindsay 38 (~67 kpc), have a relative distance of ~17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, and Kron 3) lie in the northwestern part of the SMC, while the youngest (NGC 419) is located near the SMC main body.

Spitzer SAGE survey of the Large Magellanic Cloud III: star formation and ~1000 new candidate young stellar objects

Abstract: We present ~1000 new candidate Young Stellar Objects (YSOs) in the Large Magellanic Cloud selected from Spitzer Space Telescope data, as part of the Surveying the Agents of a Galaxy's Evolution (SAGE) Legacy program. The YSOs, detected by their excess infrared (IR) emission, represent early stages of evolution, still surrounded by disks and/or infalling envelopes. Previously, fewer than 20 such YSOs were known. The candidate YSOs were selected from the SAGE Point Source Catalog from regions of color-magnitude space least confused with other IR-bright populations. The YSOs are biased toward intermediate- to high-mass and young evolutionary stages, because these overlap less with galaxies and evolved stars in color-magnitude space. The YSOs are highly correlated spatially with atomic and molecular gas, and are preferentially located in the shells and bubbles created by massive stars inside. They are more clustered than generic point sources, as expected if star formation occurs in filamentary clouds or shells. We applied a more stringent color-magnitude selection to produce a subset of "high-probability" YSO candidates. We fitted the spectral-energy distributions (SEDs) of this subset and derived physical properties for those that were well fitted. The total mass of these well-fitted YSOs is ~2900 M_☉ and the total luminosity is ~2.1 × 10^6 L_☉ . By extrapolating the mass function with a standard initial mass function and integrating, we calculate a current star-formation rate of ~0.06 M_☉ yr^(–1), which is at the low end of estimates based on total ultraviolet and IR flux from the galaxy (~0.05 – 0.25 M_☉ yr^(–1)), consistent with the expectation that our current YSO list is incomplete. Follow-up spectroscopy and further data mining will better separate the different IR-bright populations and likely increase the estimated number of YSOs. The full YSO list is available as electronic tables, and the SEDs are available as an electronic figure for further use by the scientific community.

The Sloan Digital Sky Survey-II Photometry and Supernova IA Light Curves from the 2005 Data

Abstract: We present ugriz light curves for 146 spectroscopically-confirmed or spectroscopically-probable Type Ia supernovae (SNe) from the 2005 season of the Sloan Digital Sky Survey-II Supernova (SN) survey. The light curves have been constructed using a photometric technique that we call scene modeling, which is described in detail here; the major feature is that SN brightnesses are extracted from a stack of images without spatial resampling or convolution of the image data. This procedure produces accurate photometry along with accurate estimates of the statistical uncertainty, and can be used to derive photometry taken with multiple telescopes. We discuss various tests of this technique that demonstrate its capabilities. We also describe the methodology used for the calibration of the photometry, and present calibrated magnitudes and fluxes for all of the spectroscopic SNe Ia from the 2005 season.

A SURVEY OF z ∼ 6 QUASARS IN THE SLOAN DIGITAL SKY SURVEY DEEP STRIPE. I. A FLUX-LIMITED SAMPLE AT zAB < 21*

Abstract: W e present the discovery ofve quasars at z � 6 selected from 260 deg 2 ofthe Sloan Digital Sky Survey (SDSS)southern survey,a deep im aging survey obtained by repeatedly scanning a stripe along the CelestialEquator. Theve quasarswith 20 < zA B < 21 are 1{2 m agnitudesfainterthan the lum inous z � 6 quasarsdiscovered in the SDSS m ain survey. One ofthem was independently discovered by the UKIRT Infrared Deep Sky Survey. These quasars,com bined with another z � 6 quasarknown in thisregion,m ake a com plete ux-lim ited quasarsam ple at zA B < 21. The sam ple spanstheredshiftrange5:85 � z � 6:12 and the lum inosity range 26:5 � M 1450 � 25:4 (H0 = 70 km s 1 M pc 1 ,m = 0:3,and � = 0:7).W eusethe 1=Va m ethod to determ ine thatthe com oving quasarspatialdensity at hzi = 6:0 and hM 1450i = 25:8 is (5:0 � 2:1) � 10 9 M pc 3 m ag 1 . W e m odelthe bright-end quasarlum inosity function (QLF)at z � 6 asa powerlaw �(L1450) / L � . The slopecalculated from a com bination ofoursam ple and the lum inousSDSS quasarsam ple is 3:1 � 0:4,signicantly steeper than the slope ofthe QLF at z � 4. Based on the derived QLF, wend thatthe quasar/AGN population cannotprovide enough photonsto ionize the intergalactic m edium (IGM )at z � 6 unlessthe IGM isvery hom ogeneousand the lum inosity (L � )atwhich theQLF powerlaw breaksisvery low. Subjectheadings: galaxies:active| quasars:em ission lines| quasars:general