Browsing by Author "Berk, DEV"

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    Broad emission-line shifts in quasars: An orientation measure for radio-quiet quasars?
    (2002) Richards, GT; Berk, DEV; Reichard, TA; Hall, PB; Schneider, DP; SubbaRao, M; Thakar, AR; York, DG
    Using a sample of 3814 quasars from the Early Data Release of the Sloan Digital Sky Survey, we confirm that high-ionization, broad emission lines, such as C IV are significantly blueshifted with respect to low-ionization, broad emission lines, such as Mg II which are thought to be close to the systemic redshift. We examine the velocity shifts of the Mg II and C iv emission lines with respect to [O III and Mg II respectively. C iv emission-line peaks have a range of shifts from a redshift of 500 km s(-1) to blueshifts well in excess of 2000 km s(-1) as compared with Mg II We confirm previous results that suggest an anticorrelation between the shift of the C iv emission-line peak and the rest equivalent width of the C iv emission line. Furthermore, by creating composite quasar spectra as a function of C iv shift, we are able to study in detail the profiles of the line as a function of velocity shift. We find that the apparent shift of the C iv emission-line peak is not a shift so much as it is a lack of flux in the red wing for the composite with the largest apparent shift. This observation should strongly constrain models for the broad emission-line region in quasars. The emission-line blueshift and equivalent width of C iv are also discussed in light of the well-known anticorrelation between the equivalent width of C iv emission and continuum luminosity, otherwise known as the Baldwin effect. We further discuss the C iv emission-line shift as a function of other quasar properties, such as spectral index, radio and X-ray detection. We find a possible correlation between the C iv emission-line shifts and the radio properties of the quasars, which is suggestive of orientation as the cause of the C iv velocity shifts. Finally, we explore whether the C iv emission-line blueshifts correlate with the presence of broad absorption line absorption troughs or with narrow, associated absorption, and how these might be related to orientation.
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    Faint high-latitude carbon stars discovered by the Sloan Digital Sky Survey: Methods and initial results
    (2002) Margon, B; Anderson, SF; Harris, HC; Strauss, MA; Knapp, GR; Fan, XH; Schneider, DP; Berk, DEV; Schlegel, DJ; Deutsch, EW; Ivezic, Z; Hall, PB; Williams, BF; Davidsen, AF; Brinkmann, J; Csabai, I; Hayes, JJE; Hennessy, G; Kinney, EK; Kleinman, SJ; Lamb, DQ; Long, D; Neilsen, EH; Nichol, R; Nitta, A; Snedden, SA; York, DG
    We report the discovery of 39 faint high-latitude carbon stars (FHLCs) from Sloan Digital Sky Survey (SDSS) 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. Although a handful of these stars were previously known, these objects are, in general, too faint and too warm to be effectively identified in other modern surveys such as the Two Micron All Sky Survey, nor are their red/near-IR colors particularly distinctive. The implied surface density of FHLCs in this magnitude range is uncertain at this preliminary stage of the survey because of completeness corrections but is clearly greater than 0.05 deg(-2). 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 (greater than 100 kpc) halo giant stars, useful for constraining halo dynamics, and members of the recently recognized exotic class of very nearby dwarf carbon (dC) stars. The broadband colors of the two populations are indistinguishable. Motions, and thus dC classification, are inferred for 40%-50% 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. The observed kinematics suggest that the dwarfs occupy distinct halo and disk populations. The coolest FHLCs with detectable proper motions in our sample also display multiple CaH bands in their spectra. It may be that CaH is another long-sought, low-resolution, spectroscopic luminosity discriminant between dC's and distant faint giants, at least for the cooler stars.
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    Red and reddened quasars in the Sloan Digital Sky Survey
    (2003) Richards, GT; Hall, PB; Berk, DEV; Strauss, MA; Schneider, DP; Weinstein, MA; Reichard, TA; York, DG; Knapp, GR; Fan, XH; Ivezic, Z; Brinkmann, J; Budavári, T; Csabai, I; Nichol, RC
    We investigate the overall continuum and emission-line properties of quasars as a function of their optical/ UV spectral energy distributions. Our sample consists of 4576 quasars from the Sloan Digital Sky Survey (SDSS) that were chosen using homogeneous selection criteria. Expanding on our previous work, which demonstrated that the optical/ UV color distribution of quasars is roughly Gaussian but with a red tail, here we distinguish between ( 1) quasars that have intrinsically blue ( optically. at) power-law continua, ( 2) quasars that have intrinsically red ( optically steep) power-law continua, and ( 3) quasars whose colors are inconsistent with a single power-law continuum. We find that 273 (6.0%) of the quasars in our sample fall into the latter category and appear to be redder because of SMC-like dust extinction and reddening rather than because of synchrotron emission. Even though the SDSS quasar survey is optically selected and flux-limited, we demonstrate that it is sensitive to dust reddened quasars with E( B - V) less than or similar to 0.5, assuming a classical SMC extinction curve. The color distribution of our SDSS quasar sample suggests that the population of moderately dust reddened, but otherwise normal (i.e., type 1) quasars is smaller than the population of unobscured quasars: we estimate that a further 10% of the quasar population with M(i) < - 25.61 is missing from the SDSS sample because of extinction, bringing the total fraction of dust-reddened quasars to 15% of broad-line quasars. We also investigate the emission- and absorption-line properties of these quasars as a function of color and comment on how some of these results relate to Boroson-Green- type eigenvectors. Quasars with intrinsically red ( optically steep) power-law continua tend to have narrower Balmer lines and weaker C IV, C III], He II, and 3000 angstrom bump emission as compared with bluer ( optically flatter) quasars. The change in strength of the 3000 angstrom bump appears to be dominated by the Balmer continuum and not by Fe II emission. The dust-reddened quasars have even narrower Balmer lines and weaker 3000 angstrom bumps, in addition to having considerably larger equivalent widths of [O II] and [ O III] emission. The fraction of broad absorption line quasars (BALQSOs) increases from similar to 3.4% for the bluest quasars to perhaps as large as 20% for the dust-reddened quasars, but the intrinsic color distribution will be much bluer if all BALQSOs are affected by dust reddening.
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    Spectroscopic target selection in the Sloan Digital Sky Survey: The quasar sample
    (2002) Richards, GT; Fan, XH; Newberg, HJ; Strauss, MA; Berk, DEV; Schneider, DP; Yanny, B; Boucher, A; Burles, S; Frieman, JA; Gunn, JE; Hall, PB; Ivezic, Z; Kent, S; Loveday, J; Lupton, RH; Rockosi, CM; Schlegel, DJ; Stoughton, C; SubbaRao, M; York, DG
    We describe the algorithm for selecting quasar candidates for optical spectroscopy in the Sloan Digital Sky Survey. Quasar candidates are selected via their nonstellar colors in ugriz broadband photometry and by matching unresolved sources to the FIRST radio catalogs. The automated algorithm is sensitive to quasars at all redshifts lower than z similar to 5.8. Extended sources are also targeted as low-redshift quasar candidates in order to investigate the evolution of active galactic nuclei (AGNs) at the faint end of the luminosity function. Nearly 95% of previously known quasars are recovered (based on 1540 quasars in 446 deg(2)). The overall completeness, estimated from simulated quasars, is expected to be over 90%, whereas the overall efficiency (quasars/quasar candidates) is better than 65%. The selection algorithm targets ultraviolet excess quasars to i* = 19.1 and higher redshift (z greater than or similar to 3) quasars to i* = 20.2, yielding approximately 18 candidates deg(-2). In addition to selecting normal quasars, the design of the algorithm makes it sensitive to atypical AGNs such as broad absorption line quasars and heavily reddened quasars.
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    The ensemble photometric variability of ∼25,000 quasars in the Sloan Digital Sky Survey
    (2004) Berk, DEV; Wilhite, BC; Kron, RG; Anderson, SF; Brunner, RJ; Hall, PB; Ivezic, Z; Richards, GT; Schneider, DP; York, DG; Brinkmann, JV; Lamb, DQ; Nichol, RC; Schlegel, DJ
    Using a sample of over 25,000 spectroscopically confirmed quasars from the Sloan Digital Sky Survey, we show how quasar variability in the rest-frame optical/UV regime depends on rest-frame time lag, luminosity, rest wavelength, redshift, the presence of radio and X-ray emission, and the presence of broad absorption line systems. Imaging photometry is compared with three-band spectrophotometry obtained at later epochs spanning time lags up to about 2 yr. The large sample size and wide range of parameter values allow the dependence of variability to be isolated as a function of many independent parameters. The time dependence of variability (the structure function) is well fitted by a single power law with an index gamma=0.246+/-0.008, on timescales from days to years. There is an anticorrelation of variability amplitude with rest wavelength-e.g., quasars are about twice as variable at 1000 Angstrom as at 6000 Angstrom-and quasars are systematically bluer when brighter at all redshifts. There is a strong anticorrelation of variability with quasar luminosity-variability amplitude decreases by a factor of about 4 when luminosity increases by a factor of 100. There is also a significant positive correlation of variability amplitude with redshift, indicating evolution of the quasar population or the variability mechanism. We parameterize all of these relationships. Quasars with ROSAT All-Sky Survey X-ray detections are significantly more variable (at optical/UV wavelengths) than those without, and radio-loud quasars are marginally more variable than their radio-quiet counterparts. We find no significant difference in the variability of quasars with and without broad absorption line troughs. Currently, no models of quasar variability address more than a few of these relationships. Models involving multiple discrete events or gravitational microlensing are unlikely by themselves to account for the data. So-called accretion disk instability models are promising, but more quantitative predictions are needed.