arXiv subjects

A. D. Myers

Publications and source records attributed to A. D. Myers.

87 records · Page 5Linked to original sources

arXiv2013

Broad Absorption Line Quasars with Redshifted Troughs: High-Velocity Infall or Rotationally Dominated Outflows?

We report the discovery in the Sloan Digital Sky Survey and the SDSS-III Baryon Oscillation Spectroscopic Survey of seventeen broad absorption line (BAL) quasars with high-ionization troughs that include absorption redshifted relative to the quasar rest frame. The redshifted troughs extend to velocities up to v=12,000 km/s and the trough widths exceed 3000 km/s in all but one case. Approximately 1 in 1000 BAL quasars with blueshifted C IV absorption also has redshifted C IV absorption; objects with C IV absorption present only at redshifted velocities are roughly four times rarer. In more than half of our objects, redshifted absorption is seen in C II or Al III as well as C IV, making low-ionization absorption at least ten times more common among BAL quasars with redshifted troughs than among standard BAL quasars. However, the C IV absorption equivalent widths in our objects are on average smaller than those of standard BAL quasars with low-ionization absorption. We consider several possible ways of generating redshifted absorption. The two most likely possibilities may be at work simultaneously, in the same objects or in different ones. Rotationally dominated outflows seen against a quasar's extended continuum source can produce redshifted and blueshifted absorption, but variability consistent with this scenario is seen in only one of the four objects with multiple spectra. The infall of relatively dense and low-ionization gas to radii as small as 400 Schwarzschild radii can in principle explain the observed range of trough profiles, but current models do not easily explain the origin and survival of such gas. Whatever the origin(s) of the absorbing gas in these objects, it must be located at small radii to explain its large redshifted velocities, and thus offers a novel probe of the inner regions of quasars.

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arXiv2013

Luminosity Function from dedicated SDSS-III and MMT data of quasars in 0.7<z<4.0 selected with a new approach

We present a measurement of the quasar luminosity function in the range 0.68 2.15. Using pure luminosity evolution models, we fitted our LF measurements, and predicted quasar number counts as a function of redshift and observed magnitude. These predictions are useful inputs for future cosmology surveys such as those relying on the observation of quasars to measure baryon acoustic oscillations.

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arXiv2012

An Infrared Excess Identified in Radio-Loud Broad Absorption Line Quasars

If broad absorption line (BAL) quasars represent a high covering fraction evolutionary state (even if this is not the sole factor governing the presence of BALs), it is expected that they should show an excess of mid-infrared radiation compared to normal quasars. Some previous studies have suggested that this is not the case. We perform the first analysis of the IR properties of radio-loud BAL quasars, using IR data from WISE and optical (rest-frame ultraviolet) data from SDSS, and compare the BAL quasar sample with a well-matched sample of unabsorbed quasars. We find a statistically significant excess in the mid- to near-infrared luminosities of BAL quasars, particularly at rest-frame wavelengths of 1.5 and 4 microns. Our sample was previously used to show that BALs are observed along many lines of sight towards quasars, but with an overabundance of more edge-on sources, suggesting that orientation factors into the appearance of BALs. The evidence here---of a difference in IR luminosities between BAL quasars and unabsorbed quasars---may be ascribed to evolution. This suggests that a merging of the current BAL paradigms is needed to fully describe the class.

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arXiv2012

Column density distribution and cosmological mass density of neutral gas: Sloan Digital Sky Survey-III Data Release 9

We present the first results from an ongoing survey for Damped Lyman-alpha systems (DLAs) in the spectra of z>2 quasars observed in the course of the Baryon Oscillation Spectroscopic Survey (BOSS), which is part of the Sloan Digital Sky Survey (SDSS) III. Our full (non-statistical) sample, based on Data Release 9, comprises 12,081 systems with log N(HI)>=20, out of which 6,839 have log N(HI)>=20.3. This is the largest DLA sample ever compiled, superseding that from SDSS-II by a factor of seven. Using a statistical sub-sample and estimating systematics from realistic mock data, we probe the N(HI) distribution at = 2.5. Contrary to what is generally believed, the distribution extends beyond 10^22 cm^-2 with a moderate slope of index\approx-3.5. This result matches surprisingly well the opacity-corrected distribution observed at z = 0. The cosmological mass density of neutral gas in DLAs is found to be Omega_g_DLA~10^-3, evolving only mildly over the past 12 billion years.

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arXiv2012

Broad Absorption Line Disappearance on Multi-Year Timescales in a Large Quasar Sample

We present 21 examples of C IV Broad Absorption Line (BAL) trough disappearance in 19 quasars selected from systematic multi-epoch observations of 582 bright BAL quasars (1.9 < z < 4.5) by the Sloan Digital Sky Survey-I/II (SDSS-I/II) and SDSS-III. The observations span 1.1-3.9 yr rest-frame timescales, longer than have been sampled in many previous BAL variability studies. On these timescales, ~2.3% of C IV BAL troughs disappear and ~3.3% of BAL quasars show a disappearing trough. These observed frequencies suggest that many C IV BAL absorbers spend on average at most a century along our line of sight to their quasar. Ten of the 19 BAL quasars showing C IV BAL disappearance have apparently transformed from BAL to non-BAL quasars; these are the first reported examples of such transformations. The BAL troughs that disappear tend to be those with small-to-moderate equivalent widths, relatively shallow depths, and high outflow velocities. Other non-disappearing C IV BALs in those nine objects having multiple troughs tend to weaken when one of them disappears, indicating a connection between the disappearing and non-disappearing troughs, even for velocity separations as large as 10000-15000 km/s. We discuss possible origins of this connection including disk-wind rotation and changes in shielding gas.

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arXiv2011

The LABOCA Survey of the Extended Chandra Deep Field South: Clustering of submillimetre galaxies

We present a measurement of the spatial clustering of submillimetre galaxies (SMGs) at z = 1-3. Using data from the 870 micron LESS survey, we employ a novel technique to measure the cross-correlation between SMGs and galaxies, accounting for the full probability distributions for photometric redshifts of the galaxies. From the observed projected two-point cross-correlation function we derive the linear bias and characteristic dark matter (DM) halo masses for the SMGs. We detect clustering in the cross-correlation between SMGs and galaxies at the > 4 sigma level. For the SMG autocorrelation we obtain r_0 = 7.7 (+1.8,-2.3) h^-1 Mpc, and derive a corresponding DM halo mass of log(M_halo [h^-1 M_sun]) = 12.8 (+0.3,-0.5). Based on the evolution of DM haloes derived from simulations, we show that that the z = 0 descendants of SMGs are typically massive (~2-3 L*) elliptical galaxies residing in moderate- to high-mass groups (log(M_halo [h^-1 M_sun]) = 13.3 (+0.3,-0.5). From the observed clustering we estimate an SMG lifetime of ~100 Myr, consistent with lifetimes derived from gas consumption times and star-formation timescales, although with considerable uncertainties. The clustering of SMGs at z ~ 2 is consistent with measurements for optically-selected quasi-stellar objects (QSOs), supporting evolutionary scenarios linking starbursts and QSOs. Given that SMGs reside in haloes of characteristic mass ~ 6 x 10^12 h^-1 M_sun, we demonstrate that the redshift distribution of SMGs can be described remarkably well by the combination of two effects: the cosmological growth of structure and the evolution of the molecular gas fraction in galaxies. We conclude that the powerful starbursts in SMGs likely represent a short-lived but universal phase in massive galaxy evolution, associated with the transition between cold gas-rich, star-forming galaxies and passively evolving systems. [Abridged]

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arXiv2011

Variability selected high-redshift quasars on SDSS Stripe 82

The SDSS-III BOSS Quasar survey will attempt to observe z>2.15 quasars at a density of at least 15 per square degree to yield the first measurement of the Baryon Acoustic Oscillations in the Ly-alpha forest. To help reaching this goal, we have developed a method to identify quasars based on their variability in the u g r i z optical bands. The method has been applied to the selection of quasar targets in the SDSS region known as Stripe 82 (the Southern equatorial stripe), where numerous photometric observations are available over a 10-year baseline. This area was observed by BOSS during September and October 2010. Only 8% of the objects selected via variability are not quasars, while 90% of the previously identified high-redshift quasar population is recovered. The method allows for a significant increase in the z>2.15 quasar density over previous strategies based on optical (ugriz) colors, achieving a density of 24.0 deg^{-2} on average down to g~22 over the 220 deg^2 area of Stripe 82. We applied this method to simulated data from the Palomar Transient Factory and from Pan-STARRS, and showed that even with data that have sparser time sampling than what is available in Stripe 82, including variability in future quasar selection strategies would lead to increased target selection efficiency in the z>2.15 redshift range. We also found that Broad Absorption Line quasars are preferentially present in a variability than in a color selection.

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arXiv2011

The Discovery and Nature of Optical Transient CSS100217:102913+404220

We report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations show this transient was coincident with a galaxy at redshift z=0.147, and reached an apparent magnitude of V ~ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be M_V =-22.7 approximately 45 days after maximum light. Based on our unfiltered optical photometry the peak optical emission was L = 1.3 x 10^45 erg s^-1, and over a period of 287 rest-frame days had an integrated bolometric luminosity of 1.2 x 10^52 erg. Analysis of the pre-outburst SDSS spectrum of the source shows features consistent with a Narrow-line Seyfert1 (NLS1) galaxy. High-resolution HST and Keck followup observations show the event occurred within 150pc of nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation. We use extensive follow-up of the event along with archival CSS and SDSS data to investigate the three most likely sources of such an event; 1) an extremely luminous supernova; 2) the tidal disruption of a star by the massive nuclear black hole; 3) variability of the central AGN. We find that CSS100217 was likely an extremely luminous type IIn supernova that occurred within range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity.

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arXiv2008

Constraining the quasar population with the broad-line width distribution

We measure the width of the MgII $\lambda2799$ line in quasar spectra from the SDSS, 2QZ and 2SLAQ surveys and, by invoking an unnormalised virial mass estimator, relate the scatter in line width to the scatter in mass in the underlying black hole population. We find conclusive evidence for a trend such that there is less scatter in line width, and hence black hole mass, in more luminous objects. However, the most luminous objects in our sample show such a low degree of scatter in line width that, when combined with measures for the intrinsic scatter in the radius-luminosity relation for the broad-line region in active galaxies, an inconsistency arises in the virial technique for estimating black hole masses. This analysis implies that, at least for the most luminous quasars, either there is little-to-no intrinsic scatter in the radius-luminosity relation or the MgII broad emission line region is not totally dominated by virial velocities. Finally we exploit the measured scatter in line widths to constrain models for the velocity field of the broad-line region. We show that the lack of scatter in broad line-widths for luminous quasars is inconsistent with a pure planar/disk-like geometry for the broad-line region... (abridged)

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arXiv2008

QSO-LRG 2-Point Cross-Correlation Function and Redshift-Space Distorions

We have measured the bias of QSOs as a function of QSO luminosity at fixed redshift (z<1) by cross-correlating them with LRGs in the same spatial volume, hence breaking the degeneracy between QSO luminosity and redshift. We use three QSO samples from 2SLAQ, 2QZ and SDSS covering a QSO absolute magnitude range, -24.5<M_{b_J}<-21.5, and cross-correlate them with 2SLAQ (z~0.5) and AAOmega (z~0.7) photometric and spectroscopic LRGs in the same redshift ranges. The 2-D and 3-D cross-clustering measurements are generally in good agreement. Our (2SLAQ) QSO-LRG clustering amplitude (r_0=6.8_{-0.3}^{+0.1}h^{-1}Mpc) as measured from the semi-projected cross-correlation function appears similar to the (2SLAQ) LRG-LRG auto-correlation amplitude (r_0=7.45\pm0.35h^{-1}Mpc) and both are higher than the (2QZ+2SLAQ) QSO-QSO amplitude (r_0\simeq5.0h^{-1}Mpc). Our measurements show remarkably little QSO-LRG cross-clustering dependence on QSO luminosity. If anything, the results imply that brighter QSOs may be less highly biased than faint QSOs, the opposite direction expected from simple high peaks biasing models. Assuming a standard LCDM model and values for b_{LRG} measured from LRG autocorrelation analyses, we find b_Q=1.45\pm0.11 at M_{b_J}\approx-24 and b_Q=1.90\pm0.16 at M_{b_J}~-22. We also find consistent results for the QSO bias from a z-space distortion analysis of the QSO-LRG cross-clustering at z~0.55. The dynamical infall results give β_Q=0.55\pm0.10, implying b_Q=1.4\pm0.2. Thus both the z-space distortion and the amplitude analyses yield b_Q~1.5 at M_{b_J}~-23. The implied DM halo mass inhabited by QSOs at z~0.55 is \sim10^{13}h^{-1}M_{\sun}, again approximately independent of QSO luminosity.

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arXiv2006

The 2dF-SDSS LRG and QSO Survey: QSO clustering and the L-z degeneracy

We combine the QSO samples from the 2dF QSO Redshift Survey (2QZ) and the 2dF-SDSS LRG and QSO Survey (2SLAQ) in order to investigate the clustering of z~1.4 QSOs and measure the correlation function. The clustering signal in z-space, projected along the sky direction, is similar to that previously obtained from 2QZ alone. By fitting the z-space correlation function and lifting the degeneracy between beta and Omega_m_0 by using linear theory predictions, we obtain beta(z=1.4) = 0.60+-0.12 and Omega_m_0=0.25+-0.08, implying a value for the QSO bias, b(z=1.4)=1.5+-0.2. We further find that QSO clustering does not depend strongly on luminosity at fixed redshift. This result is inconsistent with the expectation of simple `high peaks' biasing models where more luminous, rare QSOs are assumed to inhabit higher mass haloes. The data are more consistent with models which predict that QSOs of different luminosities reside in haloes of similar mass. We find that halo mass does not evolve strongly with redshift nor depend on QSO luminosity. We finally investigate how black hole mass correlates with luminosity and redshift and ascertain the relation between Eddington efficiency and black hole mass. Our results suggest that QSOs of different luminosities may contain black holes of similar mass.

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arXiv2005

On Statistical Lensing and the Anti-Correlation Between 2dF QSOs and Foreground Galaxies

We cross-correlate APM and SDSS galaxies with background QSOs from the 2dF QSO Redshift Survey, and detect a significant (2.8sigma) anti-correlation. The lack of a signal between 2dF stars and our galaxy samples suggests the anti-correlation is not due to a systematic error. The possibility that dust in the foreground galaxies could produce the anti-correlation is marginally rejected, at the 2sigma level through consideration of QSO colours. It is possible that dust that obscures QSOs without reddening them, or preferentially discards reddened QSOs from the 2QZ sample, could produce such an anti-correlation, however, such models are at odds with the positive QSO-galaxy correlations found at bright magnitudes by other authors. Our detection of a galaxy-QSO anti-correlation is consistent with statistical lensing theory. When combined with earlier results that have reported a positive galaxy-QSO correlation, a consistent, compelling picture emerges that spans faint and bright QSO samples showing positive or negative correlations according to the QSO N(m) slope. We find that galaxies are highly anti-biased on small scales. We consider two models that use different descriptions of the lensing matter and find they yield consistent predictions for the strength of galaxy bias on 0.1Mpc/h scales of b~0.1 (for LCDM). Whilst the slope of our power-law fit to the QSO-galaxy cross-correlation does not rule out linear bias, when we compare our measurement of b on 100 kpc/h scales to independent methods that determine b~1 on Mpc/h scales, we conclude that bias, on these small scales, is scale-dependent. These results indicate more mass, at least on the 100 kpc/h scales probed, than predicted by simple LCDM biasing prescriptions, and can thus constrain halo occupation models of the galaxy distribution.

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arXiv2004

Evidence for an Extended SZ Effect in WMAP Data

We have cross-correlated the WMAP data with several surveys of extragalactic sources and find evidence for temperature decrements associated with galaxy clusters and groups detected in the APM Galaxy Survey survey and the ACO catalogue. We interpret this as evidence for the thermal Sunyaev-Zeldovich (SZ) effect from the clusters. Most interestingly, the signal may extend to ~1 deg around both groups and clusters and we suggest that this may be due to hot `supercluster' gas. We have further cross-correlated the WMAP data with clusters identified in the 2MASS galaxy catalogue and also find evidence for temperature decrements there. From the APM group data we estimate the mean Compton parameter as y(z<0.2)=7x10^(-7). We have further estimated the gas mass associated with the galaxy group and cluster haloes. Assuming temperatures of 5 keV for ACO clusters and 1 keV for APM groups and clusters, we derive average gas masses. Using the space density of APM groups we then estimate Omega_gas. For an SZ extent of theta_max=20', kT=1 keV and h=0.7, this value of Omega_gas=0.04 is consistent with the standard value of Omega_baryon=0.044 but if the indications we have found for a more extended SZ effect out to theta=60' are confirmed, then higher values of Omega_gas will be implied. Finally, the contribution to the WMAP temperature power spectrum from the extended SZ effect around the z<0.2 APM+ACO groups and clusters is 1-2 orders of magnitude lower than the l=220 first acoustic peak. But if a similar SZ effect arises from more distant clusters then this contribution could increase by a factor >10 and then could seriously affect the WMAP cosmological fits.

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arXiv2003

The 2dF QSO Redshift Survey - X. Lensing of Background QSOs by Galaxy Groups

We cross-correlate QSOs from the 2dF Survey with galaxy groups. The galaxy samples are limited to B < 20.5. We use an objective algorithm to detect galaxy groups. A 3sigma anti-correlation is observed between QSOs and galaxy groups. This paucity of faint QSOs around groups is neither a selection effect nor due to restrictions on the placement of 2dF fibres. By observing the colours of QSOs on the scales of the anti-correlation, we limit dust in galaxy groups, finding a maximum reddening of E(b_j-r) < 0.012 at the 95% level. The small amount of dust thus inferred is insufficient to cause the anti-correlation, supporting the suggestion by Croom & Shanks that the signal is due to gravitational lensing. The possibility remains that tailored dust models, such as grey dust, heavy patches of dust or a combination of dust and lensing, could explain the anti-correlation. Assuming the signal is caused by lensing rather than dust, we measure the average velocity dispersion of a Singular Isothermal Sphere that would cause the anti-correlation as around 1150 km/s. Simulations reject 600 km/s at the 5% significance level. We also model foreground lenses as NFW haloes and measure the typical mass within 1.5 Mpc/h of the halo centre as M_{1.5} = (1.2 +/- 0.9) x 10^{15} solarmasses/h. Regardless of whether we utilise a SIS or NFW dark matter profile, our model favours more mass in groups than accounted for in a universe with density parameter Omega_m = 0.3. Detailed simulations and galaxy group redshifts will significantly reduce the current systematic uncertainties in these $Ω_m$ estimates. Reducing the remaining uncertainty will require larger QSO and galaxy group surveys (abridged).

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arXiv2003

The 2dF QSO Redshift Survey - XI. The QSO Power Spectrum

We present a power spectrum analysis of the final 2dF QSO Redshift Survey catalogue containing 22652 QSOs. Utilising the huge volume probed by the QSOs, we can accurately measure power out to scales of ~500Mpc and derive new constraints, at z~1.4, on the matter and baryonic contents of the Universe. Importantly, these new cosmological constraints are derived at an intermediate epoch between the CMB observations at z~1000, and local (z~0) studies of large-scale structure; the average QSO redshift corresponds to a look-back time of approximately two-thirds of the age of the Universe. We find that the amplitude of clustering of the QSOs at z~1.4 is similar to that of present day galaxies. The power spectra of the QSOs at high and low redshift are compared and we find little evidence for any evolution in the amplitude. Assuming a lambda cosmology to derive the comoving distances, r(z), to the QSOs, the power spectrum derived can be well described by a model with shape parameter Gamma=0.13+-0.02. If an Einstein-de Sitter model r(z) is instead assumed, a slightly higher value of Gamma=0.16+-0.03 is obtained. A comparison with the Hubble Volume LCDM simulation shows very good agreement over the whole range of scales considered. A standard (Omega_m=1) CDM model, however, predicts a much higher value of Gamma than is observed, and it is difficult to reconcile such a model with these data. We fit CDM model power spectra (assuming scale-invariant initial fluctuations), convolved with the survey window function, and corrected for redshift space distortions, and find that models with baryon oscillations are slightly preferred, with the baryon fraction Omega_b/Omega_m=0.18+-0.10. The overall shape of the power spectrum provides a strong constraint on Omega_m*h (where h is the Hubble parameter), with Omega_m*h=0.19+-0.05.

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