Browsing by Author "Hall, PB"

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    2MASS 1315-2649
    (2002) Hall, PB
    In a previous Letter, I reported that 2MASSI J1315309-264951 is an L dwarf with strong Halpha emission. Two spectroscopic epochs appeared to show that the Halpha was variable, decreasing from 121 to 25 Angstrom EW, which I interpreted as a flare during the first observation. Gizis independently discovered this object, and his intermediate spectroscopic epoch shows Halpha with 97 Angstrom EW. A new fourth epoch of spectroscopy again shows a very large Halpha EW (124 A), confirming this object to be a persistent, strong Halpha emitter. Whether the Halpha is steady (like 2MASS 1237+6526) or the result of continuous strong flaring (like PC 0025+0447) remains unclear. Imaging confirms that 2MASS 1315-2649 has a high proper motion (0."71 yr(-1)), corresponding to a transverse velocity of similar to76 km s(-1) at its distance of similar to23 pc. Thus, 2MASS 1315-2649 is consistent with being greater than or similar to2 Gyr old and therefore relatively massive. If that is so, the correlation of Halpha activity with mass found by Gizis et al. would seem to support the continuous strong flaring scenario, although it does not rule out a brown dwarf binary accretion scenario.
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    2MASSI J1315309-264951
    (2002) Hall, PB
    The object 2MASSI J1315309-264951 is an L3 dwarf with strong H emission discovered in the course of a color-selected survey for active galactic nuclei using the Two Micron All Sky Survey (2MASS). The strength of its Halpha emission decreased by about a factor of 2 between two epochs separated by 137 days. This is the first time that variable Halpha emission has been reported in an L dwarf, and it is probably the first observation of an Halpha flare in an L dwarf. The value of (L-Halpha/L-bol)> -4.17 at the discovery epoch is larger than that of any other L dwarf, but it is comparable to that of 2MASSI J1237392-652615, the only reported T dwarf with Halpha emission. The observed variability indicates that the H emission of 2MASSI J1315309-264951 is powered either by magnetic fields or by accretion in a binary system. Spectroscopic or narrowband Halpha monitoring of L and T dwarfs on timescales of hours to days would be the most useful step toward a better understanding of their Halpha emission mechanism(s).
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    A catalog of broad absorption line quasars from the Sloan Digital Sky Survey Early Data Release
    (2003) Reichard, TA; Richards, GT; Schneider, DP; Hall, PB; Tolea, A; Krolik, JH; Tsvetanov, Z; Vanden Berk, DE; York, DG; Knapp, GR; Gunn, JE; Brinkmann, J
    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 comparing with a composite quasar spectrum. We present the details of this algorithm and compare this method with one that 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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    A large, uniform sample of X-ray-emitting AGNs
    (2003) Anderson, SF; Voges, W; Margon, B; Trümper, J; Agüeros, MA; Boller, T; Collinge, MJ; Homer, L; Stinson, G; Strauss, MA; Annis, J; Gómez, P; Hall, PB; Nichol, RC; Richards, GT; Schneider, DP; Vanden Berk, DE; Fan, XH; Ivezic, Z; Munn, JA; Newberg, HJ; Richmond, MW; Weinberg, DH; Yanny, B; Bahcall, NA; Brinkmann, J; Fukugita, M; York, DG
    Many open questions in X-ray astronomy are limited by the relatively small number of objects in uniform optically identified and observed samples, especially when rare subclasses are considered or when subsets are isolated to search for evolution or correlations between wavebands. We describe the initial results of a new program aimed to ultimately yield similar to10(4) fully characterized X-ray source identifications-a sample about an order of magnitude larger than earlier efforts. The technique is detailed and employs X-ray data from the ROSAT All-Sky Survey (RASS) and optical imaging and spectroscopic follow-up from the Sloan Digital Sky Survey (SDSS); these two surveys prove to be serendipitously very well matched in sensitivity. As part of the SDSS software pipelines, optical objects in the SDSS photometric catalogs are automatically positionally cross-correlated with RASS X-ray sources. Then priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned using an algorithm based on the known ratios of f(x)/f(opt) for various classes of X-ray emitters at typical RASS fluxes of similar to10(-13) ergs cm(-2) s(-1). SDSS photometric parameters for optical morphology, magnitude, and colors, plus FIRST radio information, serve as proxies for object class. Initial application of this approach to RASS/SDSS data from 1400 deg(2) of sky provides a catalog of more than 1200 spectroscopically confirmed quasars and other AGNs that are probable RASS identifications. Most of these are new identifications, and only a few percent of the AGN counterparts are likely to be random superpositions. The magnitude and redshift ranges of the counterparts are very broad, extending over 15
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    A Lyα-only active galactic nucleus from the Sloan Digital Sky Survey
    (2004) Hall, PB; Hoversten, EA; Tremonti, CA; Vanden Berk, DE; Schneider, DP; Strauss, MA; Knapp, GR; York, DG; Hutsemékers, D; Newman, PR; Brinkmann, J; Frye, B; Fukugita, M; Glazebrook, K; Harvanek, M; Heckman, TM; Ivezic, Z; Kleinman, S; Krzesinski, J; Long, DC; Neilsen, E; Niederste-Ostholt, M; Nitta, A; Schlegel, DJ; Snedden, S
    The Sloan Digital Sky Survey has discovered a z = 2.4917 radio-loud active galactic nucleus (AGN) with a luminous, variable, low-polarization UV continuum, H I two-photon emission, and a moderately broad Lyalpha line (FWHM similar or equal to 1430 km s(-1)) but without obvious metal-line emission. SDSS J113658.36+024220.1 does have associated metal-line absorption in three distinct, narrow systems spanning a velocity range of 2710 km s(-1). Despite certain spectral similarities, SDSS J1136+0242 is not a Lyman break galaxy. Instead, the Lyalpha and two-photon emission can be attributed to an extended, low-metallicity narrow-line region. The unpolarized continuum argues that we see SDSS J1136+0242 very close to the axis of any ionization cone present. We can conceive of two plausible explanations for why we see a strong UV continuum but no broad-line emission in this "face-on radio galaxy'' model for SDSS J1136+0242: the continuum could be relativistically beamed synchrotron emission that swamps the broad-line emission, or more likely, SDSS J1136+0242 could be similar to PG 1407+265, a quasar in which for some unknown reason the high-ionization emission lines are very broad, very weak, and highly blueshifted.
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    A quasar without broad Lyα emission
    (2004) Hall, PB; Snedden, SA; Niederste-Ostholt, N; Eisenstein, DJ; Strauss, MA; York, DG; Schneider, DP
    The z = 3.02 quasar SDSS J095253.83+011421.9 exhibits broad metal-line emission ( C IV FWHM similar or equal to 9000 km s(-1)), but broad Lyalpha emission is not present. Instead, only a narrow Lyalpha line is observed ( FWHM similar or equal to 1140 km s(-1)). The large C IV/Lyalpha ratio in the broad-line region (BLR) emission from this object can be matched most closely by a BLR dominated by gas at very high densities (10(15) cm(-3)), which suppresses the Lyalpha emission, and illuminated by an incident power law extending to similar to200 mum, which yields increased emission from purely collisionally excited coolant lines (such as C IV, N v, and O VI) but not from recombination lines like Lyalpha. However, the strong C III emission predicted by this model is not observed, and the observed broad C III] emission must come from a lower density BLR component and should be accompanied by broad Lyalpha emission, which is not observed. The least unlikely explanation for this spectrum seems to be that any intrinsic broad Lyalpha emission is removed by smooth N v absorption in the red wing of the Lyalpha emission line and by smooth Lyalpha absorption in the blue wing of the Lyalpha emission line. This postulated smooth absorption would be in addition to the strong, associated, narrow absorption seen in numerous ions. Smooth absorption in Lyalpha, N v, and O vi, but not in C IV, would be unusual, but not impossible, although it is suspicious that the postulated absorption must almost exactly cancel the postulated intrinsic broad emission. We conclude that the spectrum of SDSS J0952+0114 appears unique ( among similar or equal to 3600 SDSS spectra of quasars at z > 2.12) because of some combination of unusual parameters, and we discuss possible observations to determine the combination of circumstances responsible for the spectrum.
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    An initial survey of white dwarfs in the Sloan Digital Sky Survey
    (2003) Harris, HC; Liebert, J; Kleinman, SJ; Nitta, A; Anderson, SF; Knapp, GR; Krzesinski, J; Schmidt, G; Strauss, MA; Berk, DV; Eisenstein, D; Hawley, S; Margon, B; Munn, JA; Silvestri, NM; Smith, JA; Szkody, P; Collinge, MJ; Dahn, CC; Fan, XH; Hall, PB; Schneider, DP; Brinkmann, J; Burles, S; Gunn, JE; Hennessy, GS; Hindsley, R; Ivezic, Z; Kent, S; Lamb, DQ; Lupton, RH; Nichol, RC; Pier, JR; Schlegel, DJ; SubbaRao, M; Uomoto, A; Yanny, B; York, DG
    An initial assessment is made of white dwarf and hot subdwarf stars observed in the Sloan Digital Sky Survey. In a small area of sky ( 190 square degrees), observed much like the full survey will be, 269 white dwarfs (WDs) and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and five hot subdwarfs were known previously. Most are ordinary DA ( hydrogen atmosphere) and DB ( helium) types. In addition, in the full survey to date, a number of WDs have been found with uncommon spectral types. Among these are blue DQ stars displaying lines of atomic carbon; red DQ stars showing molecular bands of C-2 with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na, and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic field strengths in DA, DB, DQ, and ( probably) DZ spectral types. Photometry alone allows identification of stars hotter than 12,000 K, and the density of these stars for 15 < g < 20 is found to be similar to2.2 deg(-2) at Galactic latitudes of 29degrees - 62degrees. Spectra are obtained for roughly half of these hot stars. The spectra show that for 15 < g < 17, 40% of hot stars are WDs, and the fraction of WDs rises to similar to90% at g = 20. The remainder are hot sdB and sdO stars.
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    An unusual iron Lo-BAL quasar detected by ISOCAM
    (2002) Duc, PA; Hall, PB; Fadda, D; Chanial, P; Elbaz, D; Monaco, P; Pompei, E; Poggianti, BM; Flores, H; Franceschini, A; Biviano, A; Moorwood, A; Cesarsky, C
    We report the discovery of an unusual low-ionization broad absorption line quasar at z = 1.776 which exhibits absorption lines from many excited states of Fe II. This member of the rare class of "FeLoBAL" quasars was serendipitously found in a mid-infrared (MIR) survey of distant clusters carried out with the ISOCAM camera. ISO J005645.1-273816 has a high MIR to UV luminosity ratio, suggesting a strong dust obscuration plus emission from very hot dust. This characteristic makes MIR surveys particularly efficient at detecting LoBAL quasars.
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    Broad absorption line quasars in the Sloan Digital Sky Survey with VLA first radio detections
    (2001) Menou, K; Vanden Berg, DE; Kim, RSJ; Knapp, GR; Richards, GT; Strateva, I; Fan, XH; Gunn, JE; Hall, PB; Heckman, T; Krolik, J; Lupton, RH; Schneider, DP; York, DG; Anderson, SF; Bahcall, NA; Brinkmann, J; Brunner, R; Csabai, I; Fukugita, M; Hennessy, GS; Kunszt, PZ; Lamb, DQ; Munn, JA; Nichol, RC; Szokoly, GP
    We present 13 broad absorption line (BAL) quasars, including 12 new objects, identified in the Sloan Digital Sky Survey (SDSS) and matched within 2 " to sources in the FIRST radio survey catalog. The surface density of this sample of radio-detected BAL quasars is 4.5 +/- 1.2 per 100 deg(2), i.e., approximately 4 times as high as previously found by the shallower FIRST Bright Quasar Survey (FBQS). A majority of these radio-detected BAL quasars are moderately radio-loud objects. The fraction of BAL quasars in the entire radio quasar sample, 4.8% +/- 1.3%, is comparable to the fraction of BAL quasars among the SDSS optical quasar sample (ignoring selection effects). We estimate that the true fraction of BAL quasars (mostly "HiBALs") in the radio sample is 9.2% +/- 2.6%, once selection effects are accounted for. We caution that the absorption troughs of four of the 13 radio-detected quasars considered do not strictly satisfy the standard BALnicity criteria. One or possibly two of the new radio-detected BAL quasars are of the rare "FeLoBAL" type. BAL quasars are generally redder than the median SDSS quasar at the same redshift.
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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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    Composite quasar spectra from the Sloan Digital Sky Survey
    (2001) Vanden Berk, DE; Richards, GT; Bauer, A; Strauss, MA; Schneider, DP; Heckman, TM; York, DG; Hall, PB; Fan, XH; Knapp, GR; Anderson, SF; Annis, J; Bahcall, NA; Bernardi, M; Briggs, JW; Brinkmann, J; Brunner, R; Burles, S; Carey, L; Castander, FJ; Connolly, AJ; Crocker, JH; Csabai, I; Doi, M; Finkbeiner, D; Friedman, S; Frieman, JA; Fukugita, M; Gunn, JE; Hennessy, GS; Ivezic, Z; Kent, S; Kunszt, PZ; Lamb, DQ; Leger, RF; Long, DC; Loveday, J; Lupton, RH; Meiksin, A; Pier, JR; Pope, A; Rockosi, CM; Schlegel, DJ; Siegmund, WA; Smee, S; Snir, Y; Stoughton, C; Stubbs, C; SubbaRao, M; Szalay, AS; Szokoly, GP; Tremonti, C; Uomoto, A; Waddell, P; Yanny, B; Zheng, W
    We have created a variety of composite quasar spectra using a homogeneous data set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). The quasar sample spans a redshift range of 0.044 less than or equal to z less than or equal to 4.789 and an absolute r' magnitude range of -18.0 to -26.5. The input spectra cover an observed wavelength range of 3800-9200 Angstrom at a resolution of 1800. The median composite covers a rest-wavelength range from 800 to 8555 Angstrom and reaches a peak signal-to-noise ratio of over 300 per 1 Angstrom resolution element in the rest frame. We have identified over 80 emission-line features in the spectrum. Emission-line shifts relative to nominal laboratory wavelengths are seen for many of the ionic species. Peak shifts of the broad permitted and semiforbidden lines are strongly correlated with ionization energy, as previously suggested, but we find that the narrow forbidden lines are also shifted by amounts that are strongly correlated with ionization energy. The magnitude of the forbidden line shifts is less than or similar to 100 km s(-1), compared with shifts of up to 550 km s(-1) for some of the permitted and semiforbidden lines. At wavelengths longer than the Ly alpha emission, the continuum of the geometric mean composite is well fitted by two power laws, with a break at approximate to 5000 Angstrom. The frequency power-law index, alpha (v), is -0.44 from approximate to 1300 to 5000 and -2.45 redward of approximate to 5000 The abrupt change in slope can be accounted for partly by host-galaxy contamination at low redshift. Stellar absorption lines, including higher order Balmer lines, seen in the composites suggest that young or intermediate-age stars make a significant contribution to the light of the host galaxies. Most of the spectrum is populated by blended emission lines, especially in the range 1500-3500 Angstrom, which can make the estimation of quasar continua highly uncertain unless large ranges in wavelength are observed. An electronic table of the median quasar template is available.
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    Continuum and emission-line properties of broad absorption line quasars
    (2003) Reichard, TA; Richards, GT; Hall, PB; Schneider, DP; Vanden Berk, DE; Fan, XH; York, DG; Knapp, GR; Brinkmann, J
    We investigate the continuum and emission-line properties of 224 broad absorption line quasars (BALQSOs) with 0.9less than or similar tozless than or similar to4.4 drawn from the Sloan Digital Sky Survey Early Data Release, which contains 3814 bona. de quasars. We find that low-ionization BALQSOs (LoBALs) are significantly reddened as compared with normal quasars, in agreement with previous work. High-ionization BALQSOs (HiBALs) are also more reddened than the average non-BALQSO. Assuming SMC-like dust reddening at the quasar redshift, the amount of reddening needed to explain HiBALs is E(B-V)similar to0.023 and LoBALs is E( B-V)similar to0.077 (compared with the ensemble average of the entire quasar sample). We find that there are differences in the emission-line properties between the average HiBAL, LoBAL, and non-BAL quasar. These differences, along with differences in the absorption-line troughs, may be related to intrinsic quasar properties such as the slope of the intrinsic (unreddened) continuum; more extreme absorption properties are correlated with bluer intrinsic continua. Despite the differences among BALQSO subtypes and non-BALQSOs, BALQSOs appear to be drawn from the same parent population as non-BALQSOs when both are selected by their UV/optical properties. We find that the overall fraction of traditionally defined BALQSOs, after correcting for color-dependent selection effects due to different SEDs of BALQSOs and non-BALQSOs, is 13.4%+/-1.2% and shows no significant redshift dependence for 1.7less than or equal tozless than or equal to3.45. After a rough completeness correction for the effects of dust extinction, we find that approximately one in every six quasars is a BALQSO.
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    Double-peaked low-ionization emission lines in active galactic nuclei
    (2003) Strateva, IV; Strauss, MA; Hao, L; Schlegel, DJ; Hall, PB; Gunn, JE; Li, LX; Ivezic, Z; Richards, GT; Zakamska, NL; Voges, W; Anderson, SF; Lupton, RH; Schneider, DP; Brinkmann, J; Nichol, RC
    We present a new sample of 116 double-peaked Balmer line active galactic nuclei (AGNs) selected from the Sloan Digital Sky Survey. Double-peaked emission lines are believed to originate in the accretion disks of AGNs, a few hundred gravitational radii (R-G) from the supermassive black hole. We investigate the properties of the candidate disk emitters with respect to the full sample of AGNs over the same redshifts, focusing on optical, radio, and X-ray flux, broad-line shapes and narrow-line equivalent widths, and line flux ratios. We find that the disk emitters have medium luminosities (similar to10(44) ergs s(-1)) and FWHM on average 6 times broader than the AGNs in the parent sample. The double-peaked AGNs are 1.6 times more likely to be radio sources and are predominantly (76%) radio-quiet, with about 12% of the objects classified as LINERs. Statistical comparison of the observed double-peaked line profiles with those produced by axisymmetric and nonaxisymmetric accretion disk models allows us to impose constraints on accretion disk parameters. The observed Halpha line profiles are consistent with accretion disks with inclinations smaller than 50degrees, surface emissivity slopes of 1.0-2.5, outer radii larger than similar to2000R(G), inner radii of (200-800) R-G, and local turbulent broadening of 780-1800 km s(-1). The comparison suggests that 60% of accretion disks require some form of asymmetry ( e. g., elliptical disks, warps, spiral shocks, or hot spots).
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    Dynamically close galaxy pairs and merger rate evolution in the CNOC2 redshift survey
    (2002) Patton, DR; Pritchet, CJ; Carlberg, RG; Marzke, RO; Yee, HKC; Hall, PB; Lin, H; Morris, SL; Sawicki, M; Shepherd, CW; Wirth, GD
    We investigate redshift evolution in the galaxy merger and accretion rates, using a well-defined sample of 4184 galaxies with 0.12 less than or equal toz 0.55 and R-C less than or equal to 21.5. We identify 88 galaxies in close (5 less than or equal to r(p) less than or equal to 20 h(-1) kpc) dynamical (Deltav less than or equal to 1500 km s(-1)) pairs. These galaxies are used to compute global pair statistics, after accounting for selection effects resulting from the flux limit, k-corrections, luminosity evolution, and spectroscopic incompleteness. We find that the number of companions per galaxy (for -21 less than or equal to M-B(k,e) less than or equal to -18) is N-c = 0.0321 +/- 0.0077 at z = 0.3. The luminosity in companions, per galaxy, is L-c = 0.0294 +/- 0.0084 x 10(10) h(2) L-circle dot. We assume that is proportional to the galaxy merger rate, while L-c is directly related to the mass accretion rate. After increasing the maximum pair separation to 50 h(-1) kpc and comparing with the low-redshift SSRS2 pair sample, we infer evolution in the galaxy merger and accretion rates of (1+z)(2.3+/-0.7) and (1+z)(2.3+/-0.9), respectively. These are the first such estimates to be made using only confirmed dynamical pairs. When combined with several additional assumptions, this implies that approximately 15% of present epoch galaxies with -21 less than or equal to M-B less than or equal to -18 have undergone a major merger since z = 1.
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    Environment and galaxy evolution at intermediate redshift in the CNOC2 survey
    (2001) Carlberg, RG; Yee, HKC; Morris, SL; Lin, H; Hall, PB; Patton, DR; Sawicki, M; Shepherd, CW
    The systematic variation of galaxy colors and types with clustering environment could either be the result of local conditions at formation or subsequent environmental effects as larger scale structures draw together galaxies whose stellar mass is largely in place. Below redshift 0.7 galaxy luminosities (k-corrected and evolution compensated) are relatively invariant, whereas galaxy star formation rates, as reflected in their colors, are a "transient" property that have a wide range for a given luminosity. The relations between these galaxy properties and the clustering properties are key statistics for understanding the forces driving late-time galaxy evolution. At z similar to 0.4 the comoving galaxy correlation length, r(o), measured in the CNOC2 sample is strongly color dependent, rising from 2 h(-1) Mpc to nearly 10 h(-1) Mpc as the volume-limited subsamples range from blue to red. The luminosity dependence of r(o) at z similar to 0.4 is weak below L-* in the R band, although there is an upturn at high luminosity, where its interpretation depends on separating it from the r(o)-color relation. In the B band there is a slow, smooth increase of r(o) with luminosity, at least partially related to the color dependence. Study of the evolution of galaxies within groups, which create much of the strongly nonlinear correlation signal, allows a physical investigation of the source of these relations. The dominant effect of the group environment on star formation is seen in the radial gradient of the mean galaxy colors, which on the average become redder than the field toward the group centers. The color differentiation begins around the dynamical radius of virialization of the groups. The redder-than-field trend applies to groups with a line-of-sight velocity dispersion, sigma (1) > 150 km s(-1). There is an indication, somewhat statistically insecure, that the high-luminosity galaxies in groups with sigma (1) < 125 km s(-1) become bluer toward the group center. Monte Carlo orbit integrations initiated at the measured positions and velocities show that the rate of galaxy merging in the (1) > 150 km s(-1) groups is very low, whereas for sigma (1) < 150 km s(-1) about 25% of the galaxies will merge in 0.5 Gyr. We conclude that the higher velocity dispersion groups largely act to suppress star formation relative to the less clustered field, leading to "embalmed" galaxies. On the other hand, the low velocity dispersion groups are prime sites of both strong merging and enhanced star formation that leads to the formation of some new massive galaxies at intermediate redshifts. The tidal fields within the groups appear to be a strong candidate for the physical source of the reduction of star formation in group galaxies relative to field. Tides operate effectively at all velocity dispersions to remove gas-rich companions and low-density gas in galactic halos. We find a close resemblance of the color-dependent galaxy luminosity function evolution in the field and groups, suggesting that the clustering-dependent star formation reduction mechanism is important for the evolution of field galaxies as a whole.
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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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    Galaxy groups at intermediate redshift
    (2001) Carlberg, RG; Yee, HKC; Morris, SL; Lin, H; Hall, PB; Patton, DR; Sawicki, M; Shepherd, CW
    Galaxy groups likely to be virialized are identified within the CNOC2 intermediate-redshift galaxy survey. The resulting groups have a median velocity dispersion, sigma (1) similar or equal to 200 km s(-1). The virial mass-to-light ratios, using k-corrected and evolution-compensated luminosities, have medians in the range of 150-250 h M./L., depending on group definition details. The number-velocity dispersion relation at sigma (1) greater than or similar to 200 km s(-1) is in agreement with the low-mass extrapolation of the cluster-normalized Press-Schechter model. Lower velocity dispersion groups are deficient relative to the Press-Schechter model, The two-point group-group autocorrelation function has r(0) = 6.8 +/- 0.3 h(-1) Mpc, which is much larger than the correlations of individual galaxies, but about as expected from biased clustering. The mean number density of galaxies around group centers falls nearly as a power law with r(-2.5) and has no well-defined core. The projected velocity dispersion of galaxies around group centers is either hat or slowly rising outward. The combination of a steeper than isothermal density profile and the outward rising velocity dispersion implies that the mass-to-light ratio of groups rises with radius if the velocity ellipsoid is isotropic but could be nearly constant if the galaxy orbits are nearly circular. Such strong tangential anisotropy is not supported by other evidence. Although the implication of a rising M/L must be viewed with caution, it could naturally arise through dynamical friction acting on the galaxies in a background of "classical" collisionless dark matter.
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    Gemini-South plus FLAMINGOS demonstration science
    (2003) Stern, D; Hall, PB; Barrientos, LF; Bunker, AJ; Elston, R; Ledlow, MJ; Raines, SN; Willis, J
    We report an infrared 1-1.8 mum (J+H-band), low-resolution (R=450) spectrogram of the highest redshift radio-loud quasar currently known, SDSS J083643.85+005453.3, obtained during the spectroscopic commissioning run of the FLAMINGOS multiobject, near-IR spectrograph at the 8 m Gemini-South Observatory. These data show broad emission from both C IV lambda1549 and C III] lambda1909, with strengths comparable to lower redshift quasar composite spectra. The implication is that there is substantial enrichment of the quasar environment, even at times less than a billion years after the big bang. The redshift derived from these features is z=5.774+/-0.0003, more accurate and slightly lower than the z=5.82 reported in the discovery paper based on the partially absorbed Lyalpha emission line. The infrared continuum is significantly redder than lower redshift quasar composites. Fitting the spectrum from 1.0 to 1.7 mum with a power law f(v) proportional to nu(-alpha), the derived power-law index is alpha=1.55 compared to the average continuum spectral index =0.44 derived from the first Sloan Digital Sky Survey composite quasar. Assuming an SMC-like extinction curve, we infer a color excess of E(B-V)=0.09+/-0.01. Only approximate to6% of quasars in the optically selected Sloan Digital Sky Survey show comparable levels of dust reddening.
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    Magnetic white dwarfs from the Sloan Digital Sky Survey: The first data release
    (2003) Schmidt, GD; Harris, HC; Liebert, J; Eisenstein, DJ; Anderson, SF; Brinkmann, J; Hall, PB; Harvanek, M; Hawley, S; Kleinman, SJ; Knapp, GR; Krzesinski, J; Lamb, DQ; Long, D; Munn, JA; Neilsen, EH; Newman, PR; Nitta, A; Schlegel, DJ; Schneider, DP; Silvestri, NM; Smith, JA; Snedden, SA; Szkody, P; Berk, DV
    Beyond its goals related to the extragalactic universe, the Sloan Digital Sky Survey (SDSS) is an effective tool for identifying stellar objects with unusual spectral energy distributions. Here we report on the 53 new magnetic white dwarfs discovered during the first two years of the survey, including 38 whose data are made public in the 1500 deg(2) First Data Release. Discoveries span the magnitude range 16.3 less than or equal to g less than or equal to 20.5, and based on the recovery rate for previously known magnetic white dwarfs, the completeness of the SDSS appears to be high for reasonably hot stars with B greater than or similar to 3 MG and g greater than or similar to 15. The new objects nearly double the total number of known magnetic white dwarfs and include examples with polar field strengths B-p > 500 MG, as well as several with exotic atmospheric compositions. The improved sample statistics and uniformity indicate that the distribution of magnetic white dwarfs has a broad peak in the range similar to5-30 MG and a tail extending to nearly 10(9) G. Degenerates with polar fields B-p greater than or similar to 50 MG are consistent with being descendents of magnetic Ap/Bp main-sequence stars, but low- and moderate-field magnetic white dwarfs appear to imply another origin. Yet-undetected magnetic F-type stars with convective envelopes that destroy the ordered underlying field are attractive candidates.
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    Mass and dust in the disk of a spiral lens galaxy
    (2003) Winn, JN; Hall, PB; Schechter, PL
    Gravitational lensing is a potentially important probe of spiral galaxy structure, but only a few cases of lensing by spiral galaxies are known. We present Hubble Space Telescope and Magellan observations of the two-image quasar PMN J2004-1349, revealing that the lens galaxy is a spiral galaxy. One of the quasar images passes through a spiral arm of the galaxy and suffers 3 mag of V-band extinction. Using simple lens models, we show that the mass quadrupole is well aligned with the observed galaxy disk. A more detailed model with components representing the bulge and disk gives a bulge-to-disk mass ratio of 0.16+/-0.05. The addition of a spherical dark halo, tailored to produce an overall. at rotation curve, does not change this conclusion.