arXiv subjects

G. R. Knapp

Publications and source records attributed to G. R. Knapp.

At least 91 records · Page 5Linked to original sources

arXiv2003

A Catalog of Broad Absorption Line Quasars from the Sloan Digital Sky Survey Early Data Release

We present a catalog of 224 broad absorption line quasars (BALQSOs) from the Sloan Digital Sky Survey's Early Data Release Quasar Catalog, including a relatively complete and homogeneous subsample of 131 BALQSOs. Since the identification of BALQSOs is subject to considerable systematic uncertainties, we attempt to create a complete sample of SDSS BALQSOs by combining the results of two automated selection algorithms and a by-eye classification scheme. One of these automated algorithms finds broad absorption line troughs by comparison with a composite quasar spectrum. We present the details of this algorithm and compare this method to that which uses a power-law fit to the continuum. The BALQSOs in our sample are further classified as high-ionization BALQSOs (HiBALs), low-ionization BALQSOs (LoBALs), and BALQSOs with excited iron absorption features (FeLoBALs); composite spectra of each type are presented. We further present a study of the properties of the BALQSOs in terms of the balnicity distribution, which rises with decreasing balnicity. This distribution of balnicities suggests that the fraction of quasars with intrinsic outflows may be significantly underestimated.

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arXiv2002

Selection of Metal-poor Giant Stars Using the Sloan Digital Sky Survey Photometric System

We present a method for photometric selection of metal-poor halo giants from the imaging data of the Sloan Digital Sky Survey (SDSS). These stars are offset from the stellar locus in the (g-r) vs. (u-g) color-color diagram. Based on a sample of 29 candidates for which spectra were taken, we derive a selection efficiency of the order of 50%, for stars brighter than $r \sim 17^m$. The candidates selected in 400 deg$^2$ of sky from the SDSS Early Data Release trace the known halo structures (tidal streams from the Sagittarius dwarf galaxy, the Draco dwarf spheroidal galaxy), indicating that such a color-selected sample can be used to study the halo structure even without spectroscopic information. This method, and supplemental techniques for selecting halo stars, such as RR Lyrae stars and other blue horizontal branch stars, can produce an unprecedented three-dimensional map of the Galactic halo based on the SDSS imaging survey.

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arXiv2002

Color Confirmation of Asteroid Families

We discuss optical colors of 10,592 asteroids with known orbits selected from a sample of 58,000 moving objects observed by the Sloan Digital Sky Survey (SDSS). This is more than ten times larger sample that includes both orbital parameters and multi-band photometric measurements than previously available. We confirm that asteroid dynamical families, defined as clusters in orbital parameter space, also strongly segregate in color space. In particular, we demonstrate that the three major asteroid families (Eos, Koronis, and Themis), together with the Vesta family, represent four main asteroid color types. Their distinctive optical colors indicate that the variations in chemical composition within a family are much smaller than the compositional differences between families, and strongly support earlier suggestions that asteroids belonging to a particular family have a common origin. We estimate that over 90% of asteroids belong to families.

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arXiv2002

Faint High Latitude Carbon Stars Discovered by the Sloan Digital Sky Survey: Methods and Initial Results

We report the discovery of 39 Faint High Latitude Carbon Stars (FHLCs) from Sloan Digital Sky Survey commissioning data. The objects, each selected photometrically and verified spectroscopically, range over 16.6 < r* < 20.0, and show a diversity of temperatures as judged by both colors and NaD line strengths. At the completion of the Sloan Survey, there will be many hundred homogeneously selected and observed FHLCs in this sample. We present proper motion measures for each object, indicating that the sample is a mixture of extremely distant (>100 kpc) halo giant stars, useful for constraining halo dynamics, plus members of the recently-recognized exotic class of very nearby dwarf carbon (dC) stars. Motions, and thus dC classification, are inferred for 40-50 percent of the sample, depending on the level of statistical significance invoked. The new list of dC stars presented here, although selected from only a small fraction of the final SDSS, doubles the number of such objects found by all previous methods. (Abstract abridged).

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arXiv2002

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

An extensive sample of M, L and T dwarfs identified in the Sloan Digital Sky Survey (SDSS) has been compiled. The sample of 718 dwarfs includes 677 new objects (629 M dwarfs, 48 L dwarfs) together with 41 that have been previously published. All new objects and some of the previously published ones have new optical spectra obtained either with the SDSS spectrographs or with the Apache Point Observatory 3.5m ARC telescope. Spectral types and SDSS colors are available for all objects; approximately 35% also have near-infrared magnitudes measured by 2MASS or on the Mauna Kea system. We use this sample to characterize the color--spectral type and color--color relations of late type dwarfs in the SDSS filters, and to derive spectroscopic and photometric parallax relations for use in future studies of the luminosity and mass functions based on SDSS data. We find that the (i*-z*) and (i*-J) colors provide good spectral type and absolute magnitude (M_i*) estimates for M and L dwarfs. Our distance estimates for the current sample indicate that SDSS is finding early M dwarfs out to about 1.5 kpc, L dwarfs to approximately 100 pc and T dwarfs to near 20 pc. The T dwarf photometric data show large scatter and are therefore less reliable for spectral type and distance estimation.

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arXiv2002

Unusual Broad Absorption Line Quasars from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has confirmed the existence of populations of broad absorption line (BAL) quasars with various unusual properties. We present and discuss twenty-three such objects and consider the implications of their wide range of properties for models of BAL outflows and quasars in general. We have discovered one BAL quasar with a record number of absorption lines. Two other similarly complex objects with many narrow troughs show broad MgII absorption extending longward of their systemic host galaxy redshifts. This can be explained as absorption of an extended continuum source by the rotation-dominated base of a disk wind. Five other objects have absorption which removes an unprecedented ~90% of all flux shortward of MgII. The absorption in one of them has varied across the ultraviolet with an amplitude and rate of change as great as ever seen. This same object may also show broad H-beta absorption. Numerous reddened BAL quasars have been found, including at least one reddened mini-BAL quasar with very strong FeII emission. The five reddest objects have continuum reddenings of E(B-V)~0.5, and in two of them we find strong evidence that the reddening curve is even steeper than that of the SMC. We have found at least one object with absorption from FeIII but not FeII. This may be due to a high column density of moderately high-ionization gas, but the FeIII level populations must also be affected by some sort of resonance. Finally, we have found two luminous, probably reddened high-redshift objects which may be BAL quasars whose troughs partially cover different regions of the continuum source as a function of velocity.

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arXiv2002

The Galactic Distribution of Asymptotic Giant Branch Stars

We study the Galactic distribution of ~10,000 Asymptotic Giant Branch (AGB) stars selected by IRAS colors and variability index. The distance to each star is estimated by assuming a narrow luminosity function and a model-derived bolometric correction. The characteristic AGB star luminosity, L_AGB, is determined from the condition that the highest number density must coincide with the Galactic bulge. Assuming a bulge distance of 8 kpc, we determine L_AGB \~ 3,500 Lo, in close agreement with values obtained for nearby AGB stars using the HIPPARCOS data. We find that there are no statistically significant differences in the Galactic distribution of AGB stars with different IRAS colors, implying a universal density distribution. The direct determination of this distribution shows that it is separable in the radial, R, and vertical, z, directions. Perpendicular to the Galactic plane, the number density of AGB stars is well described by an exponential function with a vertical scale height of 300 pc. In the radial direction the number density of AGB stars is constant up to R ~ 5 kpc, and then it decreases exponentially with a scale length of \~1.6 kpc. This fall-off extends to at least 12 kpc, where the sample becomes too small. The overall normalization implies that there are about 200,000 AGB stars in the Galaxy. We estimate the [25]-[12] color distribution of AGB stars for an unbiased volume-limited sample. By using a model-dependent transformation between the color and mass-loss rate, Mdot, we constrain the time dependence of Mdot. The results suggest that for 10^-6 Mo/yr < Mdot < 10^-5 Mo/yr the mass-loss rate increases exponentially with time. We find only marginal evidence that the mass-loss rate increases with stellar mass.

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arXiv2002

Optical and Radio Properties of Extragalactic Sources Observed by the FIRST and SDSS Surveys

We discuss the optical and radio properties of 30,000 FIRST sources positionally associated with an SDSS source in 1230 deg$^2$ of sky. The majority (83%) of the FIRST sources identified with an SDSS source brighter than r=21 are optically resolved. We estimate an upper limit of 5% for the fraction of quasars with broad-band optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux -- optical flux plane supports the existence of the "quasar radio-dichotomy"; 8% of all quasars with i<18.5 are radio-loud and this fraction seems independent of redshift and optical luminosity. The radio-loud quasars have a redder median color by 0.08 mag, and a 3 times larger fraction of objects with red colors. FIRST galaxies represent 5% of all SDSS galaxies with r<17.5, and 1% for r<20, and are dominated by red galaxies. Magnitude and redshift limited samples show that radio galaxies have a different optical luminosity distribution than non-radio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions, and resulting selection effects, are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux, and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies (abridged).

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arXiv2001

Towards Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis

We present 0.6-2.5um, R~400 spectra of twenty-seven cool, low luminosity stars and substellar objects. Based on these and previously published spectra we develop a preliminary spectral classification system for L and T dwarfs. For late L and T types the classification system is based entirely on four spectral indices in the 1-2.5um interval. Two of these indices are derived from water absorption bands at 1.15um and 1.4um, the latter of which shows a smooth increase in depth through the L and T sequences and can be used to classify both spectral types. The other two indices make use of methane absorption features in the H and K bands, with the K band index also applicable to mid to late L dwarfs. Continuum indices shortward of 1um used by previous authors to classify L dwarfs are found to be useful only through mid L subclasses. We employ the 1.5um water index and the 2.2um methane index to complete the L classification through L9.5 and to link the new system with a modified version of the 2MASS ``Color-d'' index. By correlating the depths of the methane and water absorption features, we establish a T spectral sequence from types T0 to T8, based on all four indices, which is a smooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one SDSS object as L9. In the proposed system methane absorption appears in the K band approximately at L8, two subclasses earlier than its appearance in the H band. The L and T spectral classes are distinguished by the absence and presence, respectively, of H band methane absorption.

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arXiv2001

The Optical, Infrared and Radio Properties of Extragalactic Sources Observed by SDSS, 2MASS and FIRST Surveys

We positionally match sources observed by the Sloan Digital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), and the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey. Practically all 2MASS sources are matched to an SDSS source within 2 arcsec; ~11% of them are optically resolved galaxies and the rest are dominated by stars. About 1/3 of FIRST sources are matched to an SDSS source within 2 arcsec; ~80% of these are galaxies and the rest are dominated by quasars. Based on these results, we project that by the completion of these surveys the matched samples will include about 10^7 stars and 10^6 galaxies observed by both SDSS and 2MASS, and about 250,000 galaxies and 50,000 quasars observed by both SDSS and FIRST. Here we present a preliminary analysis of the optical, infrared and radio properties for the extragalactic sources from the matched samples. In particular, we find that the fraction of quasars with stellar colors missed by the SDSS spectroscopic survey is probably not larger than ~10%, and that the optical colors of radio-loud quasars are ~0.05 mag. redder (with 4-sigma significance) than the colors of radio-quiet quasars.

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arXiv2001

Early-type galaxies in the SDSS

A sample of nearly 9000 early-type galaxies, in the redshift range 0.01<z<0.3, was selected from the Sloan Digital Sky Survey using morphological and spectral criteria. The sample was used to study how early-type galaxy observables, including luminosity L, effective radius R, surface brightness I, color, and velocity dispersion V, are correlated with one another. Measurement biases are understood with mock catalogs which reproduce all of the observed scaling relations. At any given redshift, the intrinsic distribution of luminosities, sizes and velocity dispersions in our sample are all approximately Gaussian. In the r* band L ~ V^3.91, L ~ R^1.58, R ~ I^(-0.75), and the Fundamental Plane relation is R ~ V^(1.49) I^(-0.75). These relations are approximately the same in the g*, i* and z* bands. At fixed luminosity, the mass-to-light ratio scales as M/L ~ L^0.14. The g*-r* color scales as V^0.25. Color also correlates with magnitude and size, but these correlations are entirely due to the L-V and R-V relations. Chemical evolution and star formation histories are investigated using co-added spectra of similar objects in our sample. Chemical abundances correlate primarily with velocity dispersion. At fixed V, the higher redshift population is bluer, is weaker in Mg2, and is stronger in Hbeta than the population nearby. In addition, the population at higher redshifts is slightly more luminous. These differences are consistent with that of a passively evolving population which formed the bulk of its stars about 9 Gyrs ago. The Fundamental Plane suggests that galaxies in dense regions are slightly different from those in less dense regions, but the co-added spectra and color--magnitude relations show no statistically significant dependence on environment.

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arXiv2001

Cataclysmic Variables from SDSS I. The First Results

The commissioning year of the Sloan Digital Sky Survey has demonstrated that many cataclysmic variables have been missed in previous surveys with brighter limits. We report the identification of 22 cataclysmic variables, of which 19 are new discoveries and 3 are known systems (SW UMa, BH Lyn and Vir4). A compendium of positions, colors and characteristics of these systems obtained from the SDSS photometry and spectroscopy is presented along with data obtained during follow-up studies with the Apache Point Observatory (APO) and Manastash Ridge Observatory (MRO) telescopes. We have determined orbital periods for 3 of the new systems: two show dwarf nova outbursts, and the third is a likely magnetic system with eclipses of its region of line emission. Based on these results, we expect the completed survey to locate at least 400 new CVs. Most of these will be faint systems with low accretion rates that will provide new constraints on binary evolution models.

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arXiv2001

Infrared Classification of L and T Dwarfs

We use new and published medium resolution 0.6--2.5 $μ$m spectra of L and T dwarfs to develop a unified classification system for both of these new spectral classes. Two indices of the system at 1.2 $μ$m and 1.5 $μ$m are based on nearby absorption bands of water vapor and two are associated with methane bands near 1.6 $μ$m and 2.2 $μ$m. The 1.5 $μ$m index is monotonic through the L and T sequences, and forms the backbone of the system; the indices for the other bands provide extensive, but only partial, coverage. We correlate the 1.5 $μ$m index with continuum indices shortward of 1 $μ$m devised by others for classifying L dwarfs, in order to obtain a tight link between optical and infrared classifications. Our proposed system defines ten spectral subclasses for L (L0--L9) and nine for T (T0--T8). The boundary between L and T is defined to be the onset of absorption by methane in the H band. Methane absorption in the K band near 2.2 $μ$m is found to begin approximately at L8.

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arXiv2001

Infrared Colors of L and T Dwarfs

We discuss the behaviour of the JHKL'M' colors of L and T dwarfs, based on photometric and spectroscopic data obtained at the United Kingdom Infrared Telescope in Hawaii. We have measured the first accurate M' photometry for L and T dwarfs. The K-M' colors of T dwarfs are much bluer than predicted by published models, suggesting that CO may be more abundant than expected, as has been found spectroscopically for the T6 dwarf Gl 229B. We also find that K-L' increases monotonically through most of the M, L, and T subclasses, but it is approximately constant between types L6 and T5, due to the onset of methane absorption at the blue edge of the L' bandpass. The JHK colors of L dwarfs show significant scatter, suggesting variations in the amount and properties of photospheric dust, and indicating that it may not be possible to associate a unique effective temperature with a given L spectral type. The H-K colors of the later T dwarfs also show some scatter which we suggest is due to variations in pressure-induced molecular hydrogen opacity, which is sensitive to gravity (or age for a brown dwarf) and metallicity.

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arXiv2001

Evidence for Reionization at z ~ 6: Detection of a Gunn-Peterson Trough in a z=6.28 Quasar

We present moderate resolution Keck spectroscopy of quasars at z=5.82, 5.99 and 6.28, discovered by the Sloan Digital Sky Survey (SDSS). We find that the Ly Alpha absorption in the spectra of these quasars evolves strongly with redshift. To z~5.7, the Ly Alpha absorption evolves as expected from an extrapolation from lower redshifts. However, in the highest redshift object, SDSSp J103027.10+052455.0 (z=6.28), the average transmitted flux is 0.0038+-0.0026 times that of the continuum level over 8450 A < lambda < 8710 A (5.95 150, and is consistent with zero flux in the Ly Alpha forest region immediately blueward of the Ly Alpha emission line, compared with a drop by a factor of ~10 at z(abs)~5.3. A similar break is seen at Ly Beta; because of the decreased oscillator strength of this transition, this allows us to put a considerably stronger limit, tau(eff) > 20, on the optical depth to Ly Alpha absorption at z=6. This is a clear detection of a complete Gunn-Peterson trough, caused by neutral hydrogen in the intergalactic medium. Even a small neutral hydrogen fraction in the intergalactic medium would result in an undetectable flux in the Ly Alpha forest region. Therefore, the existence of the Gunn-Peterson trough by itself does not indicate that the quasar is observed prior to the reionization epoch. However, the fast evolution of the mean absorption in these high-redshift quasars suggests that the mean ionizing background along the line of sight to this quasar has declined significantly from z~5 to 6, and the universe is approaching the reionization epoch at z~6.

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arXiv2001

Infrared Photometry of Late-M, L, and T Dwarfs

We present ZJHKL'M' photometry of a sample of 58 late-M, L, and T dwarfs, most of which are identified from the Sloan Digital Sky Survey and the Two Micron All-Sky Survey. Near-infrared spectra and spectral classifications for most of this sample are presented in a companion paper by Geballe et al. We derive the luminosities of 18 dwarfs in the sample and the results imply that the effective temperature range for the L dwarfs in our sample is approximately 2200-1300 K and for the T dwarfs 1300-800 K. We obtained new photometric data at the United Kingdom Infrared Telescope for: 42 dwarfs at Z, 34 dwarfs at JHK, 21 dwarfs at L', as well as M' data for two L dwarfs and two T dwarfs. The M' data provide the first accurate photometry for L and T dwarfs in this bandpass - for a T2 and a T5 dwarf, we find K-M'=1.2 and 1.6, respectively. These colors are much bluer than predicted by models suggesting that CO may be more abundant in these objects than expected, as has been found for the T6 dwarf Gl 229B. We also find that K-L' increases monotonically through most of the M, L, and T subclasses, but it is almost constant between types L6 and T5. The degeneracy is probably due to the onset of methane absorption at the blue edge of the L' bandpass. The JHK colors of L dwarfs show significant scatter, suggesting that the fluxes in these bandpasses are sensitive to variations in photospheric dust properties. The H-K colors of the later T dwarfs also show some scatter which we suggest is due to variations in pressure-induced molecular hydrogen opacity, which is sensitive to gravity and metallicity.

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arXiv2001

Spectroscopic Target Selection for the Sloan Digital Sky Survey: The Luminous Red Galaxy Sample

We describe the target selection and resulting properties of a spectroscopic sample of luminous, red galaxies (LRG) from the imaging data of the Sloan Digital Sky Survey (SDSS). These galaxies are selected on the basis of color and magnitude to yield a sample of luminous, intrinsically red galaxies that extends fainter and further than the main flux-limited portion of the SDSS galaxy spectroscopic sample. The sample is designed to impose a passively-evolving luminosity and rest-frame color cut to a redshift of 0.38. Additional, yet more luminous, red galaxies are included to a redshift of 0.5. Approximately 12 of these galaxies per square degree are targeted for spectroscopy, so the sample will number over 100,000 with the full survey. SDSS commissioning data indicate that the algorithm efficiently selects luminous (M_g=-21.4), red galaxies, that the spectroscopic success rate is very high, and that the resulting set of galaxies is approximately volume-limited out to z=0.38. When the SDSS is complete, the LRG spectroscopic sample will fill over 1h^-3 Gpc^3 with an approximately homogeneous population of galaxies and will therefore be well suited to studies of large-scale structure and clusters out to z=0.5.

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arXiv2001

Extreme BAL Quasars from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has discovered a population of broad absorption line quasars with various extreme properties. Many show absorption from metastable states of FeII with varying excitations; several objects are almost completely absorbed bluewards of MgII; at least one shows stronger absorption from FeIII than FeII, indicating temperatures T>35000 K in the absorbing region; and one object even seems to have broad H-beta absorption. Many of these extreme BALs are also heavily reddened, though `normal' BALs (particularly LoBALs) from SDSS also show evidence for internal reddening.

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