Showing papers in "The Astronomical Journal in 2006"

The Two Micron All Sky Survey (2MASS)

Abstract: Between 1997 June and 2001 February the Two Micron All Sky Survey (2MASS) collected 25.4 Tbytes of raw imagingdatacovering99.998%ofthecelestialsphereinthenear-infraredJ(1.25 � m),H(1.65 � m),andKs(2.16 � m) bandpasses. Observations were conducted from two dedicated 1.3 m diameter telescopes located at Mount Hopkins, Arizona,andCerroTololo,Chile.The7.8sofintegrationtimeaccumulatedforeachpointontheskyandstrictquality control yielded a 10 � point-source detection level of better than 15.8, 15.1, and 14.3 mag at the J, H, and Ks bands, respectively, for virtually the entire sky. Bright source extractions have 1 � photometric uncertainty of <0.03 mag and astrometric accuracy of order 100 mas. Calibration offsets between any two points in the sky are <0.02 mag. The 2MASS All-Sky Data Release includes 4.1 million compressed FITS images covering the entire sky, 471 million source extractions in a Point Source Catalog, and 1.6 million objects identified as extended in an Extended Source Catalog.

The 2.5 m Telescope of the Sloan Digital Sky Survey

Abstract: We describe the design, construction, and performance of the Sloan Digital Sky Survey telescope located at Apache Point Observatory. The telescope is a modified two-corrector Ritchey-Chretien design with a 2.5 m, f/2.25 primary, a 1.08 m secondary, a Gascoigne astigmatism corrector, and one of a pair of interchangeable highly aspheric correctors near the focal plane, one for imaging and the other for spectroscopy. The final focal ratio is f/5. The telescope is instrumented by a wide-area, multiband CCD camera and a pair of fiber-fed double spectrographs. Novel features of the telescope include the following: (1) A 3° diameter (0.65 m) focal plane that has excellent image quality and small geometric distortions over a wide wavelength range (3000-10,600 A) in the imaging mode, and good image quality combined with very small lateral and longitudinal color errors in the spectroscopic mode. The unusual requirement of very low distortion is set by the demands of time-delay-and-integrate (TDI) imaging. (2) Very high precision motion to support open-loop TDI observations. (3) A unique wind baffle/enclosure construction to maximize image quality and minimize construction costs. The telescope had first light in 1998 May and began regular survey operations in 2000.

Constraining the Evolution of the Ionizing Background and the Epoch of Reionization with z~6 Quasars II: A Sample of 19 Quasars

Abstract: We study the evolution of the ionization state of the intergalactic medium (IGM) at the end of the reionization epoch using moderate-resolution spectra of a sample of 19 quasars at 5.74 5.7: the GP optical depth evolution changes from τ ~ (1 + z)4.3 to (1 + z)11, and the average length of dark gaps with τ > 3.5 increases from 80 comoving Mpc. The dispersion of IGM properties along different lines of sight also increases rapidly, implying fluctuations by a factor of 4 in the UV background at z > 6, when the mean free path of UV photons is comparable to the correlation length of the star-forming galaxies that are thought to have caused reionization. The mean length of dark gaps shows the most dramatic increase at z ~ 6, as well as the largest line-of-sight variations. We suggest using dark gap statistics as a powerful probe of the ionization state of the IGM at yet higher redshift. The sizes of H II regions around luminous quasars decrease rapidly toward higher redshift, suggesting that the neutral fraction of the IGM has increased by a factor of 10 from z = 5.7 to 6.4, consistent with the value derived from the GP optical depth. The mass-averaged neutral fraction is 1%-4% at z ~ 6.2 based on the GP optical depth and H II region size measurements. The observations suggest that z ~ 6 is the end of the overlapping stage of reionization and are inconsistent with a mostly neutral IGM at z ~ 6, as indicated by the finite length of the dark absorption gaps.

The sloan digital sky survey quasar survey: Quasar luminosity function from data release 3

Abstract: We determine the number counts and z = 0-5 luminosity function for a well-defined, homogeneous sample of quasars from the Sloan Digital Sky Survey (SDSS). We conservatively define the most uniform statistical sample possible, consisting of 15,343 quasars within an effective area of 1622 deg2 that was derived from a parent sample of 46,420 spectroscopically confirmed broad-line quasars in the 5282 deg2 of imaging data from SDSS Data Release 3. The sample extends from i = 15 to 19.1 at z 3 and to i = 20.2 for z 3. The number counts and luminosity function agree well with the results of the Two Degree Field QSO Redshift Survey (2QZ) at redshifts and luminosities at which the SDSS and 2QZ quasar samples overlap, but the SDSS data probe to much higher redshifts than does the 2QZ sample. The number density of luminous quasars peaks between redshifts 2 and 3, although uncertainties in the selection function in this range do not allow us to determine the peak redshift more precisely. Our best-fit model has a flatter bright-end slope at high redshift than at low redshift. For z < 2.4 the data are best fit by a redshift-independent slope of ? = -3.1 [?(L) ? L?]. Above z = 2.4 the slope flattens with redshift to ? -2.37 at z = 5. This slope change, which is significant at the 5 ? level, must be accounted for in models of the evolution of accretion onto supermassive black holes.

The Hubble Ultra Deep Field

Abstract: This paper presents the Hubble Ultra Deep Field (HUDF), a 1 million s exposure of an 11 arcmin2 region in the southern sky with the Advanced Camera for Surveys on the Hubble Space Telescope using Director's Discretionary Time. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i 775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB ~ 29 for point sources. The image contains at least 10,000 objects, presented here as a catalog, the vast majority of which are galaxies. Visual inspection of the images shows few if any galaxies at redshifts greater than ~4 that resemble present-day spiral or elliptical galaxies. The image reinforces the conclusion from the original Hubble Deep Field that galaxies evolved strongly during the first few billion years in the infancy of the universe. Using the Lyman break dropout method to derive samples of galaxies at redshifts between 4 and 7, it is possible to study the apparent evolution of the galaxy luminosity function and number density. Examination of the catalog for dropout sources yields 504 B435 dropouts, 204 V 606 dropouts, and 54 i775 dropouts. The i775 dropouts are most likely galaxies at redshifts between 6 and 7. Using these samples, which are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. Assessing the factors needed to derive the luminosity function from the data suggests that there is considerable uncertainty in parameters from samples discovered with different instruments and derived using independent assumptions about the source populations. This assessment calls into question some of the strong conclusions of recently published work on distant galaxies. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6, although this decrease may be the result of faint-end incompleteness in the most distant samples. The highest redshift samples show that star formation was already vigorous at the earliest epochs at which galaxies have been observed, less than 1 billion years after the big bang.

Spitzer Survey of the Large Magellanic Cloud: Surveying the Agents of a Galaxy's Evolution (SAGE). I. Overview and Initial Results

Abstract: We are performing a uniform and unbiased imaging survey of the Large Magellanic Cloud (LMC; ~7° × 7°) using the IRAC (3.6, 4.5, 5.8, and 8 μm) and MIPS (24, 70, and 160 μm) instruments on board the Spitzer Space Telescope in the Surveying the Agents of a Galaxy's Evolution (SAGE) survey, these agents being the interstellar medium (ISM) and stars in the LMC. This paper provides an overview of the SAGE Legacy project, including observing strategy, data processing, and initial results. Three key science goals determined the coverage and depth of the survey. The detection of diffuse ISM with column densities >1.2 × 10^(21) H cm^(-2) permits detailed studies of dust processes in the ISM. SAGE's point-source sensitivity enables a complete census of newly formed stars with masses >3 M_☉ that will determine the current star formation rate in the LMC. SAGE's detection of evolved stars with mass-loss rates >1 × 10^(-8) M_☉ yr^(-1) will quantify the rate at which evolved stars inject mass into the ISM of the LMC. The observing strategy includes two epochs in 2005, separated by 3 months, that both mitigate instrumental artifacts and constrain source variability. The SAGE data are nonproprietary. The data processing includes IRAC and MIPS pipelines and a database for mining the point-source catalogs, which will be released to the community in support of Spitzer proposal cycles 4 and 5. We present initial results on the epoch 1 data for a region near N79 and N83. The MIPS 70 and 160 μm images of the diffuse dust emission of the N79/N83 region reveal a similar distribution to the gas emissions, especially the H I 21 cm emission. The measured point-source sensitivity for the epoch 1 data is consistent with expectations for the survey. The point-source counts are highest for the IRAC 3.6 μm band and decrease dramatically toward longer wavelengths, consistent with the fact that stars dominate the point-source catalogs and the dusty objects detected at the longer wavelengths are rare in comparison. The SAGE epoch 1 point-source catalog has ~4 × 10^6 sources, and more are anticipated when the epoch 1 and 2 data are combined. Using Milky Way (MW) templates as a guide, we adopt a simplified point-source classification to identify three candidate groups—stars without dust, dusty evolved stars, and young stellar objects—that offer a starting point for this work. We outline a strategy for identifying foreground MW stars, which may comprise as much as 18% of the source list, and background galaxies, which may comprise ~12% of the source list.

Empirical Models for Dark Matter Halos. I. Nonparametric Construction of Density Profiles and Comparison with Parametric Models

Abstract: We use techniques from nonparametric function estimation theory to extract the density profiles, and their derivatives, from a set of N-body dark matter halos. We consider halos generated from ΛCDM simulations of gravitational clustering, as well as isolated spherical collapses. The logarithmic density slopes γ ≡ d log ρ/d log r of the ΛCDM halos are found to vary as power laws in radius, reaching values of γ ≈ -1 at the innermost resolved radii, ~10-2rvir. This behavior is significantly different from that of broken-power-law models like the Navarro-Frenk-White (NFW) profile but similar to that of models like de Vaucouleurs's. Accordingly, we compare the N-body density profiles with various parametric models to find which provide the best fit. We consider an NFW-like model with arbitrary inner slope; Dehnen & McLaughlin's anisotropic model; Einasto's model (identical in functional form to Sersic's model but fitted to the space density); and the density model of Prugniel & Simien that was designed to match the deprojected form of Sersic's R1/n law. Overall, the best-fitting model to the ΛCDM halos is Einasto's, although the Prugniel-Simien and Dehnen-McLaughlin models also perform well. With regard to the spherical-collapse halos, both the Prugniel-Simien and Einasto models describe the density profiles well, with an rms scatter some 4 times smaller than that obtained with either the NFW-like model or the three-parameter Dehnen-McLaughlin model. Finally, we confirm recent claims of a systematic variation in profile shape with halo mass.

The X-Ray-to-Optical Properties of Optically Selected Active Galaxies over Wide Luminosity and Redshift Ranges

Abstract: We present partial-correlation analyses that examine the strengths of the relationships between l2500 A, l2 keV, αOX, and redshift for optically selected active galactic nuclei (AGNs). We extend the work of Strateva and coworkers, which analyzed optically selected AGNs from the Sloan Digital Sky Survey (SDSS), by including 52 moderate-luminosity, optically selected AGNs from the COMBO-17 survey with corresponding deep (≈250 ks to 1 Ms) X-ray observations from the Extended Chandra Deep Field-South. The COMBO-17 survey extends ~3 mag deeper than the SDSS and probes the moderate-luminosity AGNs that numerically dominate the AGN population in the universe. We also include recently published observations of 19 high-redshift, optically selected AGNs and 46 luminous, low-redshift AGNs from the Bright Quasar Survey. The full sample used in our analysis consists of 333 AGNs, extending out to z ~ 6, with 293 (88%) having X-ray detections. The sample spans five decades in UV luminosity and four decades in X-ray luminosity. We confirm that αOX is strongly anticorrelated with l2500 A (13.6 σ), the highest significance found for this relation to date, and find evidence suggesting that the slope of this relation may be dependent on l2500 A. We find that no significant correlation exists between αOX and redshift (1.3 σ) and constrain the maximum evolution of AGN UV-to-X-ray flux ratios to be less than 30% (1 σ) out to z = 5. Using our sample's high X-ray detection fraction, we also find a significant anticorrelation (3.0 σ) between αOX and l2 keV. We make comparisons to earlier studies on this topic and discuss implications for X-ray and optical AGN luminosity functions.

Galaxies in the Hubble Ultra Deep Field. I. Detection, Multiband Photometry, Photometric Redshifts, and Morphology

Abstract: We present aperture-matched point-spread function (PSF)-corrected BVi'z'JH photometry and Bayesian photometric redshifts (BPZ) for objects detected in the Hubble Ultra Deep Field (UDF), 8042 of which are detected at the 10 σ level (e.g., i' < 29.01 or z' < 28.43). Most of our objects are defined identically to those in the public STScI catalogs, enabling a straightforward object-by-object comparison. We have combined detections from i', z', J + H, and B + V + i' + z' images into a single comprehensive segmentation map. Using a new program called SExSeg, we are able to force this segmentation map into SExtractor for photometric analysis. The resulting photometry is corrected for the wider NIC3 PSFs using our ColorPro software. We also correct for the ACS z'-band PSF halo. Offsets are applied to our NIC3 magnitudes, which are found to be too faint relative to the ACS fluxes. Based on BPZ spectral energy distribution (SED) fits to objects of known spectroscopic redshift, we derived corrections of -0.30 ± 0.03 mag in J and -0.18 ± 0.04 mag in H. Our offsets appear to be supported by a recent recalibration of the UDF NIC3 images combined with nonlinearity measured in NICMOS itself. The UDF reveals a large population of faint blue galaxies (presumably young starbursts), bluer than those observed in the original Hubble Deep Fields. To accommodate these galaxies, we have added two new starburst templates to the SED library used in previous BPZ papers. The resulting photometric redshifts are accurate to within 0.04(1 + zspec) out to z < 6. Our BPZ results include a full redshift probability distribution for each galaxy. By adding these distributions, we obtain the redshift probability histogram for galaxies in the UDF. Median redshifts are also provided for different magnitude-limited samples. Finally, we measure galaxy morphology, including Sersic index and asymmetry. Simulations allow us to quantify the reliability of our morphological results. Our full catalog, along with our software packages SExSeg and ColorPro, is available from our ACS Web site.

The Solar Neighborhood. XVII. Parallax Results from the CTIOPI 0.9 m Program: 20 New Members of the RECONS 10 Parsec Sample

Abstract: Astrometric measurements for 25 red dwarf systems are presented, including the first definitive trigonometric parallaxes for 20 systems within 10 pc of the Sun, the horizon of the RECONS sample. The three nearest systems that had no previous trigonometric parallaxes (other than perhaps rough preliminary efforts) are SO 0253+1652 (3.84 ± 0.04 pc, the 23rd nearest system), SCR 1845-6357 AB (3.85 ± 0.02 pc, 24th nearest), and LHS 1723 (5.32 ± 0.04 pc, 56th nearest). In total, seven of the systems reported here rank among the nearest 100 stellar systems. Supporting photometric and spectroscopic observations have been made to provide full characterization of the systems, including complete VRIJHKs photometry and spectral types. A study of the variability of 27 targets reveals six obvious variable stars, including GJ 1207, for which we observed a flare event in the V band that caused it to brighten by 1.7 mag. Improved parallaxes for GJ 54 AB and GJ 1061, both important members of the 10 pc sample, are also reported. Definitive parallaxes for GJ 1001 A, GJ 633, and GJ 2130 ABC, all of which have been reported to be within 10 pc, indicate that they are beyond 10 pc. From the analysis of systems with (previously) high trigonometric parallax errors, we conclude that parallaxes with errors in excess of 10 mas are insufficiently reliable for inclusion in the RECONS sample. The cumulative total of new additions to the 10 pc sample since 2000 is now 34 systems: 28 by the RECONS team and six by other groups. This total represents a net increase of 16% in the number of stellar systems reliably known to be nearer than 10 pc.

The Radial Velocity Experiment (RAVE): Fourth Data Release

Abstract: We present the stellar atmospheric parameters (effective temperature, surface gravity, overall metallicity), radial velocities, individual abundances, and distances determined for 425,561 stars, which constitute the fourth public data release of the RAdial Velocity Experiment (RAVE). The stellar atmospheric parameters are computed using a new pipeline, based on the algorithms of MATISSE and DEGAS. The spectral degeneracies and the Two Micron All Sky Survey photometric information are now better taken into consideration, improving the parameter determination compared to the previous RAVE data releases. The individual abundances for six elements (magnesium, aluminum, silicon, titanium, iron, and nickel) are also given, based on a special-purpose pipeline that is also improved compared to that available for the RAVE DR3 and Chemical DR1 data releases. Together with photometric information and proper motions, these data can be retrieved from the RAVE collaboration Web site and the Vizier database.

Calibration of Synthetic Photometry Using DA White Dwarfs

Abstract: We have calibrated four major ground-based photometric systems with respect to the Hubble Space Telescope (HST) absolute flux scale, which is defined by Vega and four fundamental DA white dwarfs. These photometric systems include the Johnson-Kron-Cousins UBVRI, the Stromgren uvby filters, the Two Micron All Sky Survey JHKs, and the Sloan Digital Sky Survey ugriz filters. Synthetic magnitudes are calculated from model white dwarf spectra folded through the published filter response functions; these magnitudes in turn are absolutely calibrated with respect to the HST flux scale. Effective zero-magnitude fluxes and zero-point offsets of each system are determined. In order to verify the external observational consistency, as well as to demonstrate the applicability of these definitions, the synthetic magnitudes are compared with the respective observed magnitudes of larger sets of DA white dwarfs that have well-determined effective temperatures and surface gravities and span a wide range in both of these parameters.

Spitzer Observations of IC 348: The Disk Population at 2-3 Million Years

Abstract: We present near- and mid-infrared photometry obtained with the Spitzer Space Telescope of ~300 known members of the IC 348 cluster. We merge this photometry with existing ground-based optical and near-infrared photometry in order to construct optical-infrared spectral energy distributions (SEDs) for all the cluster members and present a complete atlas of these SEDs. We employ these observations to investigate both the frequency and nature of the circumstellar disk population in the cluster. The Spitzer observations span a wavelength range between 3.6 and 24 μm, corresponding to disk radii of ~0.1-5 AU from the central star. The observations are sufficiently sensitive to enable the first detailed measurement of the disk frequency for very low mass stars at the peak of the stellar initial mass function. Using measurements of infrared excess between 3.6 and 8.0 μm, we find the total frequency of disk-bearing stars in the cluster to be 50% ± 6%. However, only 30% ± 4% of the member stars are surrounded by optically thick, primordial disks, while the remaining disk-bearing stars are surrounded by what appear to be optically thin, anemic disks. Both these values are below previous estimates for this cluster. The disk fraction appears to be a function of spectral type and stellar mass. The fraction of stars with optically thick disks ranges from 11% ± 8% for stars earlier than K6 to 47% ± 12% for K6-M2 stars to 28% ± 5% for M2-M6 stars. The disk longevity and thus conditions for planet formation appear to be most favorable for the K6-M2 stars, which are objects of comparable mass to the Sun for the age of this cluster. The optically thick disks around later type (>M4) stars appear to be less flared than the disks around earlier type stars. This may indicate a greater degree of dust settling and a more advanced evolutionary state for the late M disk population. Finally, we find that the presence of an optically thick dust disk is correlated with gaseous accretion, as measured by the strength of Hα emission. A large fraction of stars classified as classical T Tauri stars possess robust, optically thick disks, and very few such stars are found to be diskless. The majority (64%) of stars classified as weak-lined T Tauri stars are found to be diskless. However, a significant fraction (12%) of these stars are found to be surrounded by thick, primordial disks. These results suggest that it is more likely for dust disks to persist in the absence of active gaseous accretion than for active accretion to persist in the absence of dusty disks.

A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33

Abstract: We present UBVRI photometry obtained from Mosaic images of M31 and M33 using the Kitt Peak National Observatory 4 m telescope. We describe our data reduction and automated photometry techniques in some detail, as we will shortly perform a similar analysis of other Local Group galaxies. The present study covered 2.2 deg2 along the major axis of M31 and 0.8 deg2 on M33, chosen so as to include all of the regions currently active in forming massive stars. We calibrated our data using photometry from the Lowell 1.1 m telescope, and this external method resulted in millimagnitude differences in the photometry of overlapping fields, providing some assurance that our photometry is reliable. The final catalog contains 371,781 and 146,622 stars in M31 and M33, respectively, where every star has a counterpart in (at least) the B, V, and R passbands. Our survey goes deep enough to achieve 1%-2% photometry at 21 mag (corresponding to stars more massive than 20 M⊙) and achieves <10% errors at U ~ B ~ V ~ R ~ I ~ 23 mag. Although our typical seeing was only modest (08-14, with median 10) by some standards, we find excellent correspondence between our catalog sources and those we see in our Hubble Space Telescope ACS data for OB48, a crowded region in M31. We compare our final photometry with that of others and find good agreement with the CCD catalog of M31 stars by Magnier et al., although our study covers twice the area and goes about 2 mag deeper. There is also excellent agreement with the CCD "DIRECT" surveys of M31 and M33. The photographic studies of others fare less well, particularly at the faint end in V, where accurate background subtraction is needed for good photometry. We provide cross-references to the stars confirmed as members by spectroscopy and compare the locations of these to the complete set in color-magnitude diagrams. While follow-up spectroscopy is needed for many projects, we demonstrate the success of our photometry in being able to distinguish M31/M33 members from foreground Galactic stars. Finally, we present the results of a single night of spectroscopy on the WIYN 3.5 m telescope, examining the brightest likely members of M31. The spectra identify 34 newly confirmed members, including B-A supergiants, the earliest O star known in M31, and two new luminous blue variable candidates whose spectra are similar to that of P Cygni.

The VLA Galactic Plane Survey

Abstract: The VLA Galactic Plane Survey (VGPS) is a survey of H I and 21 cm continuum emission in the Galactic plane between longitude 18° and 67° with latitude coverage from |b| < 13 to |b| < 23. The survey area was observed with the Very Large Array in 990 pointings. Short-spacing information for the H I line emission was obtained by additional observations with the Green Bank Telescope. H I spectral line images are presented with a resolution of 1' × 1' × 1.56 km s-1 (FWHM) and an rms noise of 2 K per 0.824 km s-1 channel. Continuum images made from channels without H I line emission have 1' (FWHM) resolution. The VGPS images reveal structures of atomic hydrogen and 21 cm continuum as large as several degrees with unprecedented resolution in this part of the Galaxy. With the completion of the VGPS, it is now possible for the first time to assess the consistency between arcminute-resolution surveys of Galactic H I emission. VGPS images are compared with images from the Canadian Galactic Plane Survey (CGPS) and the Southern Galactic Plane Survey (SGPS). In general, the agreement between these surveys is impressive, considering the differences in instrumentation and image-processing techniques used for each survey. The differences between VGPS and CGPS images are small, 6 K (rms) in channels in which the mean H I brightness temperature in the field exceeds 80 K. A similar degree of consistency is found between the VGPS and SGPS. The agreement we find between arcminute-resolution surveys of the Galactic plane is a crucial step toward combining these surveys into a single uniform data set that covers 90% of the Galactic disk: the International Galactic Plane Survey. The VGPS data will be made available on the World Wide Web through the Canadian Astronomy Data Centre.

MAGPIS: A Multi-Array Galactic Plane Imaging Survey

Abstract: We present the Multi-Array Galactic Plane Imaging Survey (MAGPIS), which maps portions of the first Galactic quadrant with an angular resolution, sensitivity and dynamic range that surpasses existing radio images of the Milky Way by more than an order of magnitude. The source detection threshold at 20 cm is in the range 1-2 mJy over the 85% of the survey region (5{sup o} < l < 32{sup o}, |b| < 0.8{sup o}) not covered by bright extended emission; the angular resolution is {approx} 6''. We catalog over 3000 discrete sources (diameters mostly < 30'') and present an atlas of {approx} 400 diffuse emission regions. New and archival data at 90 cm for the whole survey area are also presented. Comparison of our catalogs and images with the MSX mid-infrared data allow us to provide preliminary discrimination between thermal and non-thermal sources. We identify 49 high-probability supernova remnant candidates, increasing by a factor of seven the number of known remnants with diameters smaller than 50 in the survey region; several are pulsar wind nebula candidates and/or very small diameter remnants (D < 45''). We report the tentative identification of several hundred H II regions based on a comparison with the mid-IR data; they range in size from unresolved ultra-compact sources to large complexes of diffuse emission on scales of half a degree. In several of the latter regions, cospatial nonthermal emission illustrates the interplay between stellar death and birth. We comment briefly on plans for followup observations and our extension of the survey; when complemented by data from ongoing X-ray and mid-IR observations, we expect MAGPIS to provide an important contribution to our understanding of the birth and death of massive stars in the Milky Way.

UBVRI Light Curves of 44 Type Ia Supernovae

Abstract: We present UBVRI photometry of 44 Type Ia supernovae (SNe Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe Ia to date, nearly doubling the number of well-observed, nearby SNe Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SNe Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to the B band.

Structural Parameters of Thin and Thick Disks in Edge-on Disk Galaxies

Abstract: We analyze the global structure of 34 late-type, edge-on, undisturbed, disk galaxies spanning a wide range of mass. We measure structural parameters for the galaxies using two-dimensional least-squares fitting to our R-band photometry. The fits require both a thick and a thin disk to adequately fit the data. The thick disks have larger scale heights and longer scale lengths than the embedded thin disks by factors of ~2 and ~1.25, respectively. The observed structural parameters agree well with the properties of thick and thin disks derived from star counts in the Milky Way and from resolved stellar populations in nearby galaxies. We find that massive galaxies' luminosities are dominated by the thin disk. However, in low-mass galaxies (Vc 120 km s-1) thick disk stars contribute nearly half the luminosity and dominate the stellar mass. Thus, although low-mass dwarf galaxies appear blue, the majority of their stars are probably quite old. Our data are most easily explained by a formation scenario in which the thick disk is assembled through direct accretion of stellar material from merging satellites while the thin disk is formed from accreted gas. The baryonic fraction in the thin disk therefore constrains the gas richness of the merging pregalactic fragments. If we include the mass in H I as part of the thin disk, the thick disk contains 10% of the baryons in high-mass galaxies and ~25%–30% of the baryons in low-mass galaxies. Our data, therefore, indicate that the fragments were quite gas rich at the time of merging (fgas = 75%–90%). However, because low-mass galaxies have a smaller fraction of baryons in their thin disks, the pregalactic fragments from which they assembled must have been systematically more gas poor. We believe this trend results from increased outflow due to supernova-driven winds in the lower mass pregalactic fragments. We estimate that ~60% of the total baryonic mass in these systems was lost due to outflows. Pushing the episode of significant winds to early times allows the mass-metallicity relationship for disks to be established early, before the main disk is assembled, and obviates the difficulty in driving winds from diffuse disks with low star formation efficiencies. We discuss other implications of this scenario for solving the G dwarf problem, for predicting abundance trends in thick disks, and for removing discrepancies between semianalytic galaxy formation models and the observed colors of low-mass galaxies.

Seventy-One New L and T Dwarfs from the Sloan Digital Sky Survey

Abstract: We present near-infrared observations of 71 newly discovered L and T dwarfs, selected from imaging data of the Sloan Digital Sky Survey (SDSS) using the i-dropout technique. Sixty-five of these dwarfs have been classified spectroscopically according to the near-infrared L dwarf classification scheme of Geballe et al. and the unified T dwarf classification scheme of Burgasser et al. The spectral types of these dwarfs range from L3 to T7 and include the latest types yet found in the SDSS. Six of the newly identified dwarfs are classified as early to mid-L dwarfs according to their photometric near-infrared colors, and two others are classified photometrically as M dwarfs. We also present new near-infrared spectra for five previously published SDSS L and T dwarfs, and one L dwarf and one T dwarf discovered by Burgasser et al. from the Two Micron All Sky Survey. The new SDSS sample includes 27 T dwarfs and 30 dwarfs with spectral types spanning the complex L-T transition (L7-T3). We continue to see a large (~0.5 mag) spread in J - H for L3-T1 types and a similar spread in H - K for all dwarfs later than L3. This color dispersion is probably due to a range of grain sedimentation properties, metallicity, and gravity. We also find L and T dwarfs with unusual colors and spectral properties that may eventually help to disentangle these effects.

Radio-loud narrow-line type 1 quasars

Abstract: We present the first systematic study of (non-radio-selected) radio-loud narrow-line Seyfert 1 (NLS1) galaxies. Cross-correlation of the Catalogue of Quasars and Active Nuclei with several radio and optical catalogs led to the identification of ~11 radio-loud NLS1 candidates, including four previously known ones. This study almost triples the number of known radio-loud NLS1 galaxies if all candidates are confirmed. Most of the radio-loud NLS1 galaxies are compact, steep-spectrum sources accreting close to or above the Eddington limit. The radio-loud NLS1 galaxies of our sample are remarkable in that they occupy a previously rarely populated regime in NLS1 multiwavelength parameter space. While their [O III]/Hβ and Fe II/Hβ intensity ratios almost cover the whole range observed in NLS1 galaxies, their radio properties extend the range of radio-loud objects to those with small widths of the broad Balmer lines. Their black hole masses are generally at the upper observed end among NLS1 galaxies but are still unusually small in view of the radio loudness of the sources. Among the radio-detected NLS1 galaxies, the radio index R is distributed quite smoothly up to the critical value of R 10 and covers about 4 orders of magnitude in total. Statistics show that ~7% of the NLS1 galaxies are formally radio-loud, while only 2.5% exceed a radio index R > 100. Implications for NLS1 models are discussed. Several mechanisms are considered as explanations for the radio loudness of the NLS1 galaxies and for the lower frequency of radio-loud galaxies among NLS1 galaxies than among quasars. While properties of most sources (with two to three exceptions) generally do not favor relativistic beaming, the combination of accretion mode and spin may explain the observations.

Binary Quasars in the Sloan Digital Sky Survey: Evidence for Excess Clustering on Small Scales

Abstract: We present a sample of 221 new quasar pairs with proper transverse separations Rprop < 1 h-1 Mpc over the redshift range 0.5 < z < 3.0, discovered from an extensive follow-up campaign to find companions around the Sloan Digital Sky Survey and 2dF QSO Redshift Survey quasars. This sample includes 26 new binary quasars with separations Rprop < 50 h-1 kpc (? < 10''), more than doubling the number of such systems known. We define a statistical sample of binaries selected with homogeneous criteria and compute its selection function, taking into account sources of incompleteness. The first measurement of the quasar correlation function on scales 10 h-1 kpc < Rprop < 400 h-1 kpc is presented. For Rprop 40 h-1 kpc, we detect an order of magnitude excess clustering over the expectation from the large-scale (Rprop 3 h-1 Mpc) quasar correlation function, extrapolated down as a power law (? = 1.53) to the separations probed by our binaries. The excess grows to ~30 at Rprop ~ 10 h-1 kpc and provides compelling evidence that the quasar autocorrelation function gets progressively steeper on submegaparsec scales. This small-scale excess can likely be attributed to dissipative interaction events that trigger quasar activity in rich environments. Recent small-scale measurements of galaxy clustering and quasar-galaxy clustering are reviewed and discussed in relation to our measurement of small-scale quasar clustering.

The COMPLETE Survey of star-forming regions: Phase I data

Abstract: We present an overview of data available for the Ophiuchus and Perseus molecular clouds from Phase I of the COMPLETE Survey of Star-Forming Regions. This survey provides a range of data complementary to the Spitzer Legacy Program "From Molecular Cores to Planet Forming Disks." Phase I includes the following: extinction maps derived from the Two Micron All Sky Survey (2MASS) near-infrared data using the NICER algorithm; extinction and temperature maps derived from IRAS 60 and 100 µm emission; H I maps of atomic gas; 12CO and 13CO maps of molecular gas; and submillimeter continuum images of emission from dust in dense cores. Not unexpectedly, the morphology of the regions appears quite different depending on the column density tracer that is used, with IRAS tracing mainly warmer dust and CO being biased by chemical, excitation, and optical depth effects. Histograms of column density distribution are presented, showing that extinction as derived from 2MASS NICER gives the closest match to a lognormal distribution, as is predicted by numerical simulations. All the data presented in this paper, and links to more detailed publications on their implications, are publicly available at the COMPLETE Web site.

A spitzer space telescope infrared survey of supernova remnants in the inner galaxy

Abstract: Using Infrared Array Camera (IRAC) images at 3.6, 4.5, 5.8, and 8 ?m from the GLIMPSE Legacy science program on the Spitzer Space Telescope, we searched for infrared counterparts to the 95 known supernova remnants that are located within Galactic longitudes 65? > |l| > 10? and latitudes |b| < 1?. Eighteen infrared counterparts were detected. Many other supernova remnants could have significant infrared emission but are in portions of the Milky Way too confused to allow the separation of bright H II regions and pervasive mid-infrared emission from atomic and molecular clouds along the line of sight. Infrared emission from supernova remnants originates from synchrotron emission, shock-heated dust, atomic fine-structure lines, and molecular lines. The detected remnants are G11.2-0.3, Kes?69, G22.7-0.2, 3C?391, W44, 3C?396, 3C?397, W49B, G54.4-0.3, Kes?17, Kes?20A, RCW 103, G344.7-0.1, G346.6-0.2, CTB?37A, G348.5-0.0, and G349.7+0.2. The infrared colors suggest emission from molecular lines (nine remnants), fine-structure lines (three remnants), polycyclic aromatic hydrocarbons (four remnants), or a combination; some remnants feature multiple colors in different regions. None of the remnants are dominated by synchrotron radiation at mid-infrared wavelengths. The IRAC-detected sample emphasizes remnants interacting with relatively dense gas, for which most of the shock cooling occurs through molecular or ionic lines in the mid-infrared.

Contact Binaries with Additional Components. I. The Extant Data

Abstract: We have attempted to establish observational evidence for the presence of distant companions that may have acquired and/or absorbed angular momentum during the evolution of multiple systems, thus facilitating or enabling the formation of contact binaries. In this preliminary investigation we use several techniques (some of them distance-independent) and mostly disregard the detection biases of individual techniques in an attempt to establish a lower limit to the frequency of triple systems. While the whole sample of 151 contact binary stars brighter than Vmax = 10 mag gives a firm lower limit of 42% ± 5%, the corresponding number for the much better observed northern-sky subsample is 59% ± 8%. These estimates indicate that most contact binary stars exist in multiple systems.

The gravity field of the saturnian system from satellite observations and spacecraft tracking data

Abstract: We present values for the masses of Saturn and its major satellites, the zonal harmonics in the spherical harmonic expansion of Saturn's gravitational potential, and the orientation of the pole of Saturn. We determined these values using an extensive data set: satellite astrometry from Earth-based observatories and the Hubble Space Telescope; Earth-based, Voyager 1, and Voyager 2 ring occultation measurements; Doppler tracking data from Pioneer 11; and Doppler tracking, radiometric range, and imaging data from Voyager 1, Voyager 2, and Cassini.

CIRS: Cluster Infall Regions in the Sloan Digital Sky Survey. I. Infall Patterns and Mass Profiles

Abstract: We use the Fourth Data Release of the Sloan Digital Sky Survey (SDSS) to test the ubiquity of infall patterns around galaxy clusters and measure cluster mass profiles to large radii. The Cluster and Infall Region Nearby Survey (CAIRNS) found infall patterns in nine clusters, but the cluster sample was incomplete. Here we match X-ray cluster catalogs with SDSS, search for infall patterns, and compute mass profiles for a complete sample of X-ray-selected clusters. Very clean infall patterns are apparent in most of the clusters, with the fraction decreasing with increasing redshift due to shallower sampling. All 72 clusters in a well-defined sample limited by redshift (ensuring good sampling) and X-ray flux (excluding superpositions) show infall patterns sufficient to apply the caustic technique. This sample is by far the largest sample of cluster mass profiles extending to large radii to date. Similar to CAIRNS, cluster infall patterns are better defined in observations than in simulations. Further work is needed to determine the source of this difference. We use the infall patterns to compute mass profiles for 72 clusters and compare them to model profiles. Cluster scaling relations using caustic masses agree well with those using X-ray or virial mass estimates, confirming the reliability of the caustic technique. We confirm the conclusion of CAIRNS that cluster infall regions are well fitted by Navarro-Frenk-White (NFW) and Hernquist profiles and poorly fitted by singular isothermal spheres. This much larger sample enables new comparisons of cluster properties with those in simulations. The shapes (specifically NFW concentrations) of the mass profiles agree well with the predictions of simulations. The mass in the infall region is typically comparable to or larger than that in the virial region. Specifically, the mass inside the turnaround radius is on average 2.19 ± 0.18 times that within the virial radius. This ratio agrees well with recent predictions from simulations of the final masses of dark matter halos.

Identification of new M dwarfs in the solar neighborhood

Abstract: We present the results from a spectroscopic study of 1080 nearby active M dwarfs, selected by correlating the Two Micron All Sky Survey and ROSAT catalogs. We have derived the spectral types and estimated distances for all of our stars. The spectral types range between K5 and M6. Nearly half of our stars lie within 50 pc. We have measured the equivalent width of the Hα emission line. Our targets show an increase in chromospheric activity from early to midspectral types, with a peak in activity around M5. Using the count rate and hardness ratios obtained from the ROSAT catalog, we have derived the X-ray luminosities. Our stars display a "saturation-type" relation between the chromospheric and coronal activity. The relation is such that log LX/Lbol remains "saturated" at a value of approximately -3 for varying Hα equivalent width. We have found 568 matches in the USNO-B catalog and have derived the tangential velocities for these stars. There is a slight trend of decreasing chromospheric activity with age, such that the stars with higher vtan have lower Hα equivalent widths. The coronal emission, however, remains saturated at a value of log LX/Lbol ~ -3 for varying tangential velocities, suggesting that the coronal activity remains saturated with age. We do not find any break in the saturation-type relation at the spectral type at which stars become fully convective (~M3.5). Most of the stars in our sample show more coronal emission than the dMe stars in the Hyades and Praesepe and have vtan < 40 km s-1, suggesting a young population.

Internal Kinematics of the Fornax Dwarf Spheroidal Galaxy

Abstract: We present new radial velocity results for 176 stars in the Fornax dwarf spheroidal galaxy, of which at least 156 are probable Fornax members We combine with previously published data to obtain a radial velocity sample with 206 stars, of which at least 176 are probable Fornax members We detect the hint of rotation about an axis near Fornax's morphological minor axis, although the significance of the rotation signal in the galactic rest frame is sensitive to the adopted value of Fornax's proper motion Regardless, the observed stellar kinematics is dominated by random motions, and we do not find kinematic evidence of tidal disruption The projected velocity dispersion profile of the binned data set remains flat over the sampled region, which reaches a maximum angular radius of 65' Single-component King models in which mass follows light fail to reproduce the observed flatness of the velocity dispersion profile Two-component (luminous plus dark matter) models can reproduce the data, provided that the dark component extends sufficiently beyond the luminous component and the central dark matter density is of the same order as the central luminous density These requirements suggest a more massive, darker Fornax than standard core-fitting analyses have previously concluded, with M/LV over the sampled region reaching 10-40 times the M/LV of the luminous component We also apply a nonparametric mass estimation technique, introduced in a companion paper Although it is designed to operate on data sets containing velocities for >1000 stars, the estimation yields preliminary results suggesting M/LV ~ 15 inside r < 15 kpc

Metallicity and nuclear star formation in nearby galaxy pairs: evidence for tidally induced gas flows

Abstract: We derive the first luminosity-metallicity relation for a large objectively selected sample of local galaxy pairs and we compare the pairs LZ relation with the relation for the Nearby Field Galaxy Survey (NFGS). Galaxy pair members with small projected separations (s < 20 kpc/h) have systematically lower metallicities (� 0.2 dex on average) than the field galaxies, or than more widely separated pairs at the same luminosity. There is a strong correlation between metallicity and central burst strength in the galaxy pairs. All five galaxies in the pairs sample with strong central bursts have close companions and metallicities lower than the comparable field galaxies. Our results provide strong observational evidence for a merger scenario where galaxy interactions cause gas flows towards the central regions, carrying less enriched gas from the outskirts of the galaxy into the central regions. The less enriched gas dilutes the pre-existing nuclear gas to produce a lower metallicity than would be obtained prior to the interaction. These gas flows trigger central bursts of star formation, causing strong central burst strengths, and possibly aiding the formation of blue bulges. We show that the timescale and central gas dilution required by this scenario are consistent with predictions from hydrodynamic merger models. Subject headings: galaxies:starburst–galaxies:abundances–galaxies:fundamental parameters– galaxies:interactions

Terrestrial planet formation. i. the transition from oligarchic growth to chaotic growth

Abstract: We use a hybrid, multiannulus, n-body-coagulation code to investigate the growth of kilometer-sized planetesimals at 0.4-2 AU around a solar-type star. After a short runaway growth phase, protoplanets with masses of ~1026 g and larger form throughout the grid. When (1) the mass in these oligarchs is roughly comparable to the mass in planetesimals and (2) the surface density in oligarchs exceeds 2-3 g cm-2 at 1 AU, strong dynamical interactions among oligarchs produce a high merger rate, which leads to the formation of several terrestrial planets. In disks with lower surface density, milder interactions produce several lower-mass planets. In all disks, the planet formation timescale is ~10-100 Myr, similar to estimates derived from the cratering record and radiometric data.