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DOE OSTI2013

THE FUNDAMENTAL METALLICITY RELATION REDUCES TYPE Ia SN HUBBLE RESIDUALS MORE THAN HOST MASS ALONE

Type Ia supernova Hubble residuals have been shown to correlate with host galaxy mass, imposing a major obstacle for their use in measuring dark energy properties. Here, we calibrate the fundamental metallicity relation (FMR) of Mannucci et al. for host mass and star formation rates measured from broadband colors alone. We apply the FMR to the large number of hosts from the SDSS-II sample of Gupta et al. and find that the scatter in the Hubble residuals is significantly reduced when compared with using only stellar mass (or the mass-metallicity relation) as a fit parameter. Our calibration of the FMR is restricted to only star-forming galaxies and in the Hubble residual calculation we include only hosts with log(SFR) > - 2. Our results strongly suggest that metallicity is the underlying source of the correlation between Hubble residuals and host galaxy mass. Since the FMR is nearly constant between z = 2 and the present, use of the FMR along with light-curve width and color should provide a robust distance measurement method that minimizes systematic errors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TIME-AVERAGE-BASED METHODS FOR MULTI-ANGULAR SCALE ANALYSIS OF COSMIC-RAY DATA

Over the past decade, a number of experiments dealt with the problem of measuring the arrival direction distribution of cosmic rays, looking for information on the propagation mechanisms and the identification of their sources. Any deviation from the isotropy may be regarded to as a signature of unforeseen or unknown phenomena, mostly if well localized in the sky and occurring at low rigidity. It induced experimenters to search for excesses down to angular scales as narrow as 10 Degree-Sign , disclosing the issue of properly filtering contributions from wider structures. A solution commonly envisaged was based on time-average methods to determine the reference value of cosmic-ray flux. Such techniques are nearly insensitive to signals wider than the time window in use, thus allowing us to focus the analysis on medium- and small-scale signals. Nonetheless, the signal often cannot be excluded in the calculation of the reference value, which induces systematic errors. The use of time-average methods recently revealed important discoveries about the medium-scale cosmic-ray anisotropy, present both in the northern and southern hemispheres. It is known that the excess (or deficit) is observed as less intense than in reality and that fake deficit zones are rendered around true excesses because of the absolute lack of knowledge a priori of which signal is true and which is not. This work is an attempt to critically review the use of time-average-based methods for observing extended features in the cosmic-ray arrival distribution pattern.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

PRIMUS: CONSTRAINTS ON STAR FORMATION QUENCHING AND GALAXY MERGING, AND THE EVOLUTION OF THE STELLAR MASS FUNCTION FROM z = 0-1

We measure the evolution of the stellar mass function (SMF) from z = 0-1 using multi-wavelength imaging and spectroscopic redshifts from the PRism MUlti-object Survey (PRIMUS) and the Sloan Digital Sky Survey (SDSS). From PRIMUS we construct an i < 23 flux-limited sample of {approx}40, 000 galaxies at z = 0.2-1.0 over five fields totaling Almost-Equal-To 5.5 deg{sup 2}, and from the SDSS we select {approx}170, 000 galaxies at z = 0.01-0.2 that we analyze consistently with respect to PRIMUS to minimize systematic errors in our evolutionary measurements. We find that the SMF of all galaxies evolves relatively little since z = 1, although we do find evidence for mass assembly downsizing; we measure a Almost-Equal-To 30% increase in the number density of {approx}10{sup 10} M{sub sun} galaxies since z Almost-Equal-To 0.6, and a {approx}< 10% change in the number density of all {approx}> 10{sup 11} M{sub sun} galaxies since z Almost-Equal-To 1. Dividing the sample into star-forming and quiescent using an evolving cut in specific star formation rate, we find that the number density of {approx}10{sup 10} M{sub sun} star-forming galaxies stays relatively constant since z Almost-Equal-To 0.6, whereas the space density of {approx}> 10{sup 11} M{sub sun} star-forming galaxies decreases by Almost-Equal-To 50% between z Almost-Equal-To 1 and z Almost-Equal-To 0. Meanwhile, the number density of {approx}10{sup 10} M{sub sun} quiescent galaxies increases steeply toward low redshift, by a factor of {approx}2-3 since z Almost-Equal-To 0.6, while the number of massive quiescent galaxies remains approximately constant since z Almost-Equal-To 1. These results suggest that the rate at which star-forming galaxies are quenched increases with decreasing stellar mass, but that the bulk of the stellar mass buildup within the quiescent population occurs around {approx}10{sup 10.8} M{sub sun}. In addition, we conclude that mergers do not appear to be a dominant channel for the stellar mass buildup of galaxies at z < 1, even among massive ({approx}> 10{sup 11} M{sub sun}) quiescent galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE MEGAMASER COSMOLOGY PROJECT. IV. A DIRECT MEASUREMENT OF THE HUBBLE CONSTANT FROM UGC 3789

In Papers I and II from the Megamaser Cosmology Project, we reported initial observations of H{sub 2}O masers in an accretion disk of a supermassive black hole at the center of the galaxy UGC 3789, which gave an angular-diameter distance to the galaxy and an estimate of H{sub 0} with 16% uncertainty. We have since conducted more very long baseline interferometric observations of the spatial-velocity structure of these H{sub 2}O masers, as well as continued monitoring of its spectrum to better measure maser accelerations. These more extensive observations, combined with improved modeling of the masers in the accretion disk of the central supermassive black hole, confirm our previous results, but with significantly improved accuracy. We find H{sub 0} = 68.9 {+-} 7.1 km s{sup -1} Mpc{sup -1}; this estimate of H{sub 0} is independent of other methods and is accurate to {+-}10%, including sources of systematic error. This places UGC 3789 at an angular-diameter distance of 49.6 {+-} 5.1 Mpc, with a central supermassive black hole of (1.16 {+-} 0.12) Multiplication-Sign 10{sup 7} M{sub Sun }.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

TESTING CONVERGENCE FOR GLOBAL ACCRETION DISKS

Global disk simulations provide a powerful tool for investigating accretion and the underlying magnetohydrodynamic turbulence driven by magneto-rotational instability (MRI). Using them to accurately predict quantities such as stress, accretion rate, and surface brightness profile requires that purely numerical effects, arising from both resolution and algorithm, be understood and controlled. We use the flux-conservative Athena code to conduct a series of experiments on disks having a variety of magnetic topologies to determine what constitutes adequate resolution. We develop and apply several resolution metrics: (Q{sub z} ) and (Q{sub {phi}}), the ratio of the grid zone size to the characteristic MRI wavelength, {alpha}{sub mag}, the ratio of the Maxwell stress to the magnetic pressure, and / , the ratio of radial to toroidal magnetic field energy. For the initial conditions considered here, adequate resolution is characterized by (Q{sub z} ) {>=} 15, (Q{sub {phi}}) {>=} 20, {alpha}{sub mag} Almost-Equal-To 0.45, and / {approx}0.2. These values are associated with {>=}35 zones per scaleheight H, a result consistent with shearing box simulations. Numerical algorithm is also important. Use of the Harten-Lax-van Leer-Einfeldt flux solver or second-order interpolation can significantly degrade the effective resolution compared to the Harten-Lax-van Leer discontinuities flux solver and third-order interpolation. Resolution at this standard can be achieved only with large numbers of grid zones, arranged in a fashion that matches the symmetries of the problem and the scientific goals of the simulation. Without it, however, quantitative measures important to predictions of observables are subject to large systematic errors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SEARCH FOR SUPERMASSIVE BLACK HOLE BINARIES IN THE SLOAN DIGITAL SKY SURVEY SPECTROSCOPIC SAMPLE

Supermassive black hole (SMBH) binaries are expected in a ΛCDM cosmology given that most (if not all) massive galaxies contain a massive black hole (BH) at their center. So far, however, direct evidence for such binaries has been elusive. We use cross-correlation to search for temporal velocity shifts in the Mg II broad emission lines of 0.36 < z < 2 quasars with multiple observations in the Sloan Digital Sky Survey. For ∼10{sup 9} M{sub ☉} BHs in SMBH binaries, we are sensitive to velocity drifts for binary separations of ∼0.1 pc with orbital periods of ∼100 yr. We find seven candidate sub-parsec-scale binaries with velocity shifts >3.4σ ∼ 280 km s{sup –1}, where σ is our systematic error. Comparing the detectability of SMBH binaries with the number of candidates (N ≤ 7), we can rule out that most 10{sup 9} M{sub ☉} BHs exist in ∼0.03-0.2 pc scale binaries, in a scenario where binaries stall at sub-parsec scales for a Hubble time. We further constrain that ≤16% (one-third) of quasars host SMBH binaries after considering gas-assisted sub-parsec evolution of SMBH binaries, although this result is very sensitive to the assumed size of the broad line region. We estimate the detectability of SMBH binaries with ongoing or next-generation surveys (e.g., Baryon Oscillation Spectroscopic Survey, Subaru Prime Focus Spectrograph), taking into account the evolution of the sub-parsec binary in circumbinary gas disks. These future observations will provide longer time baselines for searches similar to ours and may in turn constrain the evolutionary scenarios of SMBH binaries.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

LIKELIHOOD OF THE POWER SPECTRUM IN COSMOLOGICAL PARAMETER ESTIMATION

The likelihood function is a crucial element of parameter estimation. In analyses of galaxy overdensities and weak lensing shear, one often approximates the likelihood of the power spectrum with a Gaussian distribution. The posterior probability derived from such a likelihood deviates considerably from the exact posterior on the largest scales probed by any survey, where the central limit theorem does not apply. We show that various forms of Gaussian likelihoods can have a significant impact on the estimation of the primordial non-Gaussianity parameter f{sub NL} from the galaxy angular power spectrum. The Gaussian plus log-normal likelihood, which has been applied successfully in analyses of the cosmic microwave background, outperforms the Gaussian likelihoods. Nevertheless, even if the exact likelihood of the power spectrum is used, the estimated parameters may be still biased. As such, the likelihoods and estimators need to be thoroughly examined for potential systematic errors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The coyote universe extended: Precision emulation of the matter power spectrum

Modern sky surveys are returning precision measurements of cosmological statistics such as weak lensing shear correlations, the distribution of galaxies, and cluster abundance. To fully exploit these observations, theorists must provide predictions that are at least as accurate as the measurements, as well as robust estimates of systematic errors that are inherent to the modeling process. In the nonlinear regime of structure formation, this challenge can only be overcome by developing a large-scale, multi-physics simulation capability covering a range of cosmological models and astrophysical processes. As a first step to achieving this goal, we have recently developed a prediction scheme for the matter power spectrum (a so-called emulator), accurate at the 1% level out to k ∼ 1 Mpc{sup –1} and z = 1 for wCDM cosmologies based on a set of high-accuracy N-body simulations. It is highly desirable to increase the range in both redshift and wavenumber and to extend the reach in cosmological parameter space. To make progress in this direction, while minimizing computational cost, we present a strategy that maximally reuses the original simulations. We demonstrate improvement over the original spatial dynamic range by an order of magnitude, reaching k ∼ 10 h Mpc{sup –1}, a four-fold increase in redshift coverage, to z = 4, and now include the Hubble parameter as a new independent variable. To further the range in k and z, a new set of nested simulations run at modest cost is added to the original set. The extension in h is performed by including perturbation theory results within a multi-scale procedure for building the emulator. This economical methodology still gives excellent error control, ∼5% near the edges of the domain of applicability of the emulator. A public domain code for the new emulator is released as part of the work presented in this paper.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

How good a clock is rotation? The stellar rotation-mass-age relationship for old field stars

The rotation-mass-age relationship offers a promising avenue for measuring the ages of field stars, assuming the attendant uncertainties to this technique can be well characterized. We model stellar angular momentum evolution starting with a rotation distribution from open cluster M37. Our predicted rotation-mass-age relationship shows significant zero-point offsets compared to an alternative angular momentum loss law and published gyrochronology relations. Systematic errors at the 30% level are permitted by current data, highlighting the need for empirical guidance. We identify two fundamental sources of uncertainty that limit the precision of rotation-based ages and quantify their impact. Stars are born with a range of rotation rates, which leads to an age range at fixed rotation period. We find that the inherent ambiguity from the initial conditions is important for all young stars, and remains large for old stars below 0.6 M {sub ☉}. Latitudinal surface differential rotation also introduces a minimum uncertainty into rotation period measurements and, by extension, rotation-based ages. Both models and the data from binary star systems 61 Cyg and α Cen demonstrate that latitudinal differential rotation is the limiting factor for rotation-based age precision among old field stars, inducing uncertainties at the ∼2 Gyr level. We also examine the relationship between variability amplitude, rotation period, and age. Existing ground-based surveys can detect field populations with ages as old as 1-2 Gyr, while space missions can detect stars as old as the Galactic disk. In comparison with other techniques for measuring the ages of lower main sequence stars, including geometric parallax and asteroseismology, rotation-based ages have the potential to be the most precise chronometer for 0.6-1.0 M {sub ☉} stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Massive star-forming host galaxies of quasars on Sloan digital sky survey stripe 82

The stellar properties of about 800 galaxies hosting optically luminous, unobscured quasars at z < 0.6 are analyzed. Deep co-added Sloan Digital Sky Survey (SDSS) images of the quasars on Stripe 82 are decomposed into nucleus and host galaxy using point spread function and Sérsic models. The systematic errors in the measured galaxy absolute magnitudes and colors are estimated to be less than 0.5 mag and 0.1 mag, respectively, with simulated quasar images. The effect of quasar light scattered by the interstellar medium is also carefully addressed. The measured quasar-to-galaxy ratio in total flux decreases toward longer wavelengths, from ∼8 in the u band to ∼1 in the i and z bands. We find that the SDSS quasars are hosted exclusively by massive galaxies (stellar mass M {sub star} > 10{sup 10} M {sub ☉}), which is consistent with previous results for less luminous narrow-line (obscured) active galactic nuclei (AGNs). The quasar hosts are very blue and almost absent on the red sequence, showing stark contrast to the color-magnitude distribution of normal galaxies. The fact that more powerful AGNs reside in galaxies with higher star-formation efficiency may indicate that negative AGN feedback, if it exists, is not concurrent with the most luminous phase of AGNs. We also find positive correlation between the mass of supermassive black holes (SMBHs; M {sub BH}) and host stellar mass, but the M {sub BH}-M {sub star} relation is offset toward large M {sub BH} or small M {sub star} compared to the local relation. While this could indicate that SMBHs grow earlier than do their host galaxies, such an argument is not conclusive, as the effect may be dominated by observational biases.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Metallicity inhomogeneities in local star-forming galaxies as a sign of recent metal-poor gas accretion

We measure the oxygen metallicity of the ionized gas along the major axis of seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14, show ≅0.5 dex metallicity decrements in inner regions with enhanced star formation activity. This behavior is similar to the metallicity drop observed in a number of local tadpole galaxies by Sánchez Almeida et al., and was interpreted as showing early stages of assembling in disk galaxies, with the star formation sustained by external metal-poor gas accretion. The agreement with tadpoles has several implications. (1) It proves that galaxies other than the local tadpoles present the same unusual metallicity pattern. (2) Our metallicity inhomogeneities were inferred using the direct method, thus discarding systematic errors usually attributed to other methods. (3) Taken together with the tadpole data, our findings suggest a threshold around one-tenth the solar value for the metallicity drops to show up. Although galaxies with clear metallicity drops are rare, the physical mechanism responsible for them may sustain a significant part of the star formation activity in the local universe. We argue that the star formation dependence of the mass-metallicity relationship, as well as other general properties followed by most local disk galaxies, is naturally interpreted as side effects of pristine gas infall. Alternatives to the metal-poor gas accretion are examined as well.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The magnetic field of active region 11158 during the 2011 February 12-17 flares: Differences between photospheric extrapolation and coronal forward-fitting methods

We developed a coronal nonlinear force-free field (COR-NLFFF) forward-fitting code that fits an approximate nonlinear force-free field (NLFFF) solution to the observed geometry of automatically traced coronal loops. In contrast to photospheric NLFFF codes, which calculate a magnetic field solution from the constraints of the transverse photospheric field, this new code uses coronal constraints instead, and this way provides important information on systematic errors of each magnetic field calculation method, as well as on the non-force-freeness in the lower chromosphere. In this study we applied the COR-NLFFF code to NOAA Active Region 11158, during the time interval of 2011 February 12-17, which includes an X2.2 GOES-class flare plus 35 M- and C-class flares. We calculated the free magnetic energy with a 6 minute cadence over 5 days. We find good agreement between the two types of codes for the total nonpotential E{sub N} and potential energy E{sub P} but find up to a factor of 4 discrepancy in the free energy E {sub free} = E{sub N} – E{sub P} and up to a factor of 10 discrepancy in the decrease of the free energy ΔE {sub free} during flares. The coronal NLFFF code exhibits a larger time variability and yields a decrease of free energy during the flare that is sufficient to satisfy the flare energy budget, while the photospheric NLFFF code shows much less time variability and an order of magnitude less free-energy decrease during flares. The discrepancy may partly be due to the preprocessing of photospheric vector data but more likely is due to the non-force-freeness in the lower chromosphere. We conclude that the coronal field cannot be correctly calculated on the basis of photospheric data alone and requires additional information on coronal loop geometries.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Verification of the helioseismology travel-time measurement technique and the inversion procedure for sound speed using artificial data

We performed three-dimensional numerical simulations of the solar surface acoustic wave field for the quiet Sun and for three models with different localized sound-speed perturbations in the interior with deep, shallow, and two-layer structures. We used the simulated data generated by two solar acoustics codes that employ the same standard solar model as a background model, but utilize different integration techniques and different models of stochastic wave excitation. Acoustic travel times were measured using a time-distance helioseismology technique, and compared with predictions from ray theory frequently used for helioseismic travel-time inversions. It is found that the measured travel-time shifts agree well with the helioseismic theory for sound-speed perturbations, and for the measurement procedure with and without phase-speed filtering of the oscillation signals. This testing verifies the whole measuring-filtering-inversion procedure for static sound-speed anomalies with small amplitude inside the Sun outside regions of strong magnetic field. It is shown that the phase-speed filtering, frequently used to extract specific wave packets and improve the signal-to-noise ratio, does not introduce significant systematic errors. Results of the sound-speed inversion procedure show good agreement with the perturbation models in all cases. Due to its smoothing nature, the inversion procedure may overestimate sound-speed variations in regions with sharp gradients of the sound-speed profile.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Parallax beyond a kiloparsec from spatially scanning the wide field Camera 3 on the Hubble space telescope

We use a newly developed observing mode on the Hubble Space Telescope (HST) and Wide Field Camera 3, spatial scanning, to increase source sampling a thousand-fold and measure changes in source positions to a precision of 20-40 μas, more than an order of magnitude better than attainable in pointed observations. This observing mode can usefully measure the parallaxes of bright stars at distances of up to 5 kpc, a factor of 10 farther than achieved thus far with HST. The technique should also provide a unique crosscheck of future parallax measurements from Gaia. Long-period classical Cepheid variable stars in the Milky Way, nearly all of which reside beyond 1 kpc, are especially compelling targets for parallax measurements from scanning, as they may be used to anchor a determination of the Hubble constant to ∼1%. We illustrate the method by measuring to high precision the parallax of a classical Cepheid, SY Aurigae, at a distance of more than 2 kpc, using five epochs of spatial-scan data obtained at intervals of six months. Rapid spatial scans also enable photometric measurements of bright Milky Way Cepheids—which would otherwise saturate even in the shortest possible pointed observations—on the same flux scale as extragalactic Cepheids, which is a necessity for reducing a leading source of systematic error in the Hubble constant. We demonstrate this capability with photometric measurements of SY Aur on the same system used for Cepheids in Type Ia supernova host galaxies. While the technique and results presented here are preliminary, an ongoing program with HST is collecting such parallax measurements for another 18 Cepheids to produce a better anchor for the distance scale.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The diffuse source at the center of LMC SNR 0509–67.5 is a background galaxy at z = 0.031

Type Ia supernovae (SNe Ia) are well-known for their use in the measurement of cosmological distances, but our continuing lack of concrete knowledge about their progenitor stars is both a matter of debate and a source of systematic error. In our attempts to answer this question, we presented unambiguous evidence that LMC SNR 0509–67.5, the remnant of an SN Ia that exploded in the Large Magellanic Cloud 400 ± 50 yr ago, did not have any point sources (stars) near the site of the original supernova explosion, from which we concluded that this particular supernova must have had a progenitor system consisting of two white dwarfs. There is, however, evidence of nebulosity near the center of the remnant, which could have been left over detritus from the less massive WD, or could have been a background galaxy unrelated to the supernova explosion. We obtained long-slit spectra of the central nebulous region using GMOS on Gemini South to determine which of these two possibilities is correct. The spectra show Hα emission at a redshift of z = 0.031, which implies that the nebulosity in the center of LMC SNR 0509–67.5 is a background galaxy, unrelated to the supernova.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Consistent use of type Ia supernovae highly magnified by galaxy clusters to constrain the cosmological parameters

We discuss how Type Ia supernovae (SNe) strongly magnified by foreground galaxy clusters should be self-consistently treated when used in samples fitted for the cosmological parameters. While the cluster lens magnification of a SN can be well constrained from sets of multiple images of various background galaxies with measured redshifts, its value is typically dependent on the fiducial set of cosmological parameters used to construct the mass model. In such cases, one should not naively demagnify the observed SN luminosity by the model magnification into the expected Hubble diagram, which would create a bias, but instead take into account the cosmological parameters a priori chosen to construct the mass model. We quantify the effect and find that a systematic error of typically a few percent, up to a few dozen percent per magnified SN may be propagated onto a cosmological parameter fit unless the cosmology assumed for the mass model is taken into account (the bias can be even larger if the SN is lying very near the critical curves). We also simulate how such a bias propagates onto the cosmological parameter fit using the Union2.1 sample supplemented with strongly magnified SNe. The resulting bias on the deduced cosmological parameters is generally at the few percent level, if only few biased SNe are included, and increases with the number of lensed SNe and their redshift. Samples containing magnified Type Ia SNe, e.g., from ongoing cluster surveys, should readily account for this possible bias.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Redshift evolution of the dynamical properties of massive galaxies from SDSS-III/BOSS

We study the redshift evolution of the dynamical properties of ∼180, 000 massive galaxies from SDSS-III/BOSS combined with a local early-type galaxy sample from SDSS-II in the redshift range 0.1 ≤ z ≤ 0.6. The typical stellar mass of this sample is M{sub *} ∼2 × 10{sup 11} M{sub ☉}. We analyze the evolution of the galaxy parameters effective radius, stellar velocity dispersion, and the dynamical to stellar mass ratio with redshift. As the effective radii of BOSS galaxies at these redshifts are not well resolved in the Sloan Digital Sky Survey (SDSS) imaging we calibrate the SDSS size measurements with Hubble Space Telescope/COSMOS photometry for a sub-sample of galaxies. We further apply a correction for progenitor bias to build a sample which consists of a coeval, passively evolving population. Systematic errors due to size correction and the calculation of dynamical mass are assessed through Monte Carlo simulations. At fixed stellar or dynamical mass, we find moderate evolution in galaxy size and stellar velocity dispersion, in agreement with previous studies. We show that this results in a decrease of the dynamical to stellar mass ratio with redshift at >2σ significance. By combining our sample with high-redshift literature data, we find that this evolution of the dynamical to stellar mass ratio continues beyond z ∼ 0.7 up to z > 2 as M{sub dyn}/M{sub *} ∼(1 + z){sup –0.30±0.12}, further strengthening the evidence for an increase of M{sub dyn}/M{sub *} with cosmic time. This result is in line with recent predictions from galaxy formation simulations based on minor merger driven mass growth, in which the dark matter fraction within the half-light radius increases with cosmic time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

An empirical method for improving the quality of RXTE PCA spectra

We fitted all of the several hundred RXTE PCA spectra of the Crab individually to a simple power-law model; the total number of counts in the composite spectrum is >10{sup 9}. We then used the spectrum of residuals to derive a calibration tool, called pcacorr, that we apply to large samples of spectra for GX 339-4, H1743-322, and XTE J1550-564. Application of the tool improved the quality of all the fits, and the improvement is dramatic for spectra with ≳ 10{sup 7} counts. The Crab residual spectrum is somewhat different for each of the five Proportional Counter Array (PCA) detectors, but it was relatively stable over the course of the mission. We recommend that pcacorr be routinely applied to spectra with ≳ 10{sup 6} counts and that one include a systematic error of 0.1%, rather than the 0.5-1% value that has customarily been used. We expect that application of the tool will result in an increase in sensitivity of the PCA to faint spectral features by up to an order of magnitude.

79 ASTRONOMY AND ASTROPHYSICS↗