A. A. Kaas

Bio: A. A. Kaas is an academic researcher. The author has contributed to research in topics: Nordic Optical Telescope & Gravitational lens. The author has an hindex of 8, co-authored 10 publications receiving 523 citations.

Very High Column Density and Small Reddening toward GRB 020124 at z = 3.20*

Abstract: We present optical and near-infrared observations of the dim afterglow of GRB 020124, obtained between 2 and 68 hr after the gamma-ray burst. The burst occurred in a very faint (R 29.5) damped Lyα absorber (DLA) at a redshift of z = 3.198 ± 0.004. The derived column density of neutral hydrogen is log(N) = 21.7 ± 0.2, and the rest-frame reddening is constrained to be E(B-V) < 0.065, i.e., AV < 0.20 for standard extinction laws with RV ≈ 3. The resulting dust-to-gas ratio is less than 11% of that found in the Milky Way but consistent with the SMC and high-redshift QSO DLAs, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. A gray extinction law (large RV), produced through preferential destruction of small dust grains by the gamma-ray burst, could increase the derived AV and dust-to-gas ratio. The dimness of the afterglow is, however, fully accounted for by the high redshift: if GRB 020124 had been at z = 1, it would have been approximately 1.8 mag brighter—in the range of typical bright afterglows.

Very high column density and small reddening towards GRB 020124 at z = 3.20

Abstract: We present optical and near-infrared observations of the dim afterglow of GRB 020124, obtained between 2 and 68 hours after the gamma-ray burst. The burst occurred in a very faint (R > 29.5) Damped Ly-alpha Absorber (DLA) at a redshift of z = 3.198 +- 0.004. The derived column density of neutral hydrogen is log(N_H) = 21.7 +- 0.2 and the rest-frame reddening is constrained to be E(B-V) < 0.065, i.e., A_V < 0.20 for standard extinction laws with R_V ~ 3. The resulting dust-to-gas ratio is less than 11 % of that found in the Milky Way, but consistent with the SMC and high-redshift QSO DLAs, indicating a low metallicity and/or a low dust-to-metals ratio in the burst environment. A grey extinction law (large R_V), produced through preferential destruction of small dust grains by the GRB, could increase the derived A_V and dust-to-gas ratio. The dimness of the afterglow is however fully accounted for by the high redshift: If GRB 020124 had been at z = 1 it would have been approximately 1.8 mag brighter--in the range of typical bright afterglows.

Time delay and lens redshift for the doubly imaged BAL quasar SBS 1520+530 ?;??

Abstract: We present optical R-band light curves of the gravitationally lensed quasar SBS 1520+530 derived from data obtained at the Nordic Optical Telescope. A time delay of 130 ± 3 days (1σ) is determined from the light curves. In addition, spectra of SBS 1520+530 obtained at the Keck Observatory are spatially deconvolved in order to extract the spectrum of the faint lensing galaxy, free of any contamination by the light from the bright quasar images. This spectrum indicates a lens redshift z = 0.717, in agreement with one of the absorption systems found in the quasar spectra. The best mass model of the system includes a second nearby galaxy and a cluster of galaxies in addition to the main lensing galaxy. Adopting this model and an Ω = 0.3, Λ = 0.7 cosmology, our time-delay measurement yields a Hubble constant of H_0 = 51 ± 9 km s^(-1) Mpc^(-1) (1σ error).

Time delay and lens redshift for the doubly imaged BAL quasar SBS1520+530

Abstract: We present optical R-band light curves of the gravitationally lensed quasar SBS1520+530 derived from data obtained at the Nordic Optical Telescope. A time delay of 130+/-3 days (1 sigma) is determined from the light curves. In addition, spectra of SBS1520+530 obtained at the Keck Observatory are spatially deconvolved in order to extract the spectrum of the faint lensing galaxy, free of any contamination by the light from the bright quasar images. This spectrum indicates a lens redshift z=0.717, in agreement with one of the absorption systems found in the quasar spectra. The best mass model of the system includes a second nearby galaxy and a cluster of galaxies in addition to the main lensing galaxy. Adopting this model and an Omega=0.3, Lambda=0.7 cosmology, our time-delay measurement yields a Hubble constant of H_0=51+/- 9, km/s/Mpc (1 sigma error).

The Time Delay of the Quadruple Quasar RX J0911.4+0551*

Abstract: We present optical light curves of the gravitationally lensed components A (≡A1+A2+A3) and B of the quadruple quasar RX J0911.4+0551 (z = 2.80). The observations were primarily obtained at the Nordic Optical Telescope between 1997 March and 2001 April and consist of 74 I-band data points for each component. The data allow the measurement of a time delay of 146 ± 8 days (2 σ) between A and B, with B as the leading component. This value is significantly shorter than that predicted from simple models and indicates a very large external shear. Mass models including the main lens galaxy and the surrounding massive cluster of galaxies at z = 0.77, responsible for the external shear, yield H0 = 71 ± 4 (random, 2 σ) ± 8 (systematic) km s-1 Mpc-1. The systematic model uncertainty is governed by the surface-mass density (convergence) at the location of the multiple images.

The Collimation-corrected Gamma-Ray Burst Energies Correlate with the Peak Energy of Their νFν Spectrum

Abstract: We consider all bursts with known redshift and νFν peak energy, E. For a good fraction of them an estimate of the jet opening angle is available from the achromatic break of their afterglow light curve. This allows the derivation of the collimation-corrected energy of the bursts, Eγ. The distribution of the values of Eγ is more spread out than in previous findings, covering about 2 orders of magnitude. We find a surprisingly tight correlation between Eγ and the source frame Epeak: E(1 + z) ∝ E. This correlation can shed light on the still uncertain radiation processes for the prompt GRB emission. More importantly, if the small scatter of this newly found correlation could be confirmed by forthcoming data, it would be possible to use it for cosmological purposes.

Constraining galaxy formation and cosmology with the conditional luminosity function of galaxies

Abstract: We use the conditional luminosity function �(L|M)dL, which gives the number of galaxies with luminosities in the range L±dL/2 that reside in a halo of mass M, to link the distribution of galaxies to that of dark matter haloes. Starting from the number density of dark matter haloes predicted by current models of structure formation, we seek the form of �(L|M) that reproduces the galaxy luminosity function and the luminosity dependence of the galaxy clustering strength. We test the models of �(L|M) by comparing the resulting mass-to-light ratios with constraints from the Tully-Fisher (TF) relation and from galaxy clusters. A comparison between model predictions and current observations yields a number of stringent constraints on both galaxy formation and cosmology. In particular, this method can break the degeneracy between 0 and the power-spectrum normalization �8, inherent in current weak-lensing and cluster

The Hubble Diagram to Redshift >6 from 69 Gamma-Ray Bursts

Abstract: One of the few ways to measure the properties of dark energy is to extend the Hubble diagram (HD) to higher redshifts with gamma-ray bursts (GRBs). GRBs have at least five properties (their spectral lag, variability, spectral peak photon energy, time of the jet break, and the minimum rise time) that have correlations to the luminosity of varying quality. In this paper I construct a GRB HD with 69 GRBs over a redshift range from 0.17 to >6, with half the bursts having a redshift larger than 1.7. This paper uses over 3.6 times as many GRBs and 12.7 times as many luminosity indicators as any previous GRB HD work. For the gravitational lensing and Malmquist biases, I find that the biases are small, with an average of 0.03 mag and an rms scatter of 0.14 mag in the distance modulus. The GRB HD is well behaved and nicely delineates the shape of the HD. The reduced χ2 for the fit to the concordance model is 1.05, and the rms scatter about the concordance model is 0.65 mag. This accuracy is just a factor of 2.0 times that gotten for the same measure from all the big supernova surveys. I fit the GRB HD to a variety of models, including where the dark energy has its equation of state parameter varying as w(z) = w0 + waz/(1 + z). I find that the concordance model is consistent with the data, that is, the dark energy can be described well as a cosmological constant that does not change with time.

Constraining Galaxy Formation and Cosmology with the Conditional Luminosity Function of Galaxies

Abstract: We use the conditional luminosity function (CLF), which gives the number of galaxies with luminosities in the range [L, L+dL] that reside in a halo of mass M, to link the distribution of galaxies to that of dark matter haloes. We seek the CLF that reproduces the galaxy luminosity function and the luminosity dependence of the galaxy clustering strength and test the models by comparing the resulting mass-to-light ratios (M/L) with constraints from the Tully-Fisher (TF) relation. We obtain a number of stringent constraints on both galaxy formation and cosmology. In particular, this method can break the degeneracy between Omega_0 and the power-spectrum normalization sigma_8, inherent in current weak-lensing and cluster-abundance studies. For flat LCDM cosmogonies with sigma_8 normalized by recent weak lensing observations, the best results are obtained for Omega_0~0.3; Omega_0 0.4 gives too high M/L. The best-fit model for the LCDM concordance cosmology (Omega_0=0.3) predicts M/L that are slightly too high. We discuss a number of possible effects that might remedy this problem, including small modification of cosmological parameters, warm in stead of cold dark matter, systematic errors in current observational data, and the existence of dark galaxies. Finally we use the CLF to predict several statistics about the distribution of galaxy light in the local Universe.

Implications for Cosmic Reionization from the Optical Afterglow Spectrum of the Gamma-Ray Burst 050904 at z = 6.3

Abstract: The gamma-ray burst (GRB) 050904 at z =6 .3 provides the first opportunity to probe the intergalactic medium (IGM) by GRBs at the epoch of reionization. Here, we present a spectral modeling analysis of the optical afterglow spectrum taken by the Subaru Telescope, aiming to constrain the reionization history. The spectrum shows a clear damping wing at wavelengths redward of the Lyman break, and the wing shape can be fitted either by a damped Lyα system with a column density of log[NHI(cm −2 )] ∼ 21.6 at a redshift close to the detected metal absorption lines (zmetal =6 .295), or by almost neutral IGM extending to a slightly higher redshift of zIGM,u ∼ 6.36. In the latter case, the difference between the two redshifts may be explained by the acceleration of metal absorbing shells in the activities of the GRB or its progenitor. However, we exclude this possibility by using the light transmission feature around the Lyβ resonance, leading to a firm upper limit of zIGM,u ≤ 6.314. We then show evidence that the IGM was already largely ionized at z =6 .3 with the best-fit neutral fraction of IGM, xHI (≡ nHI/nH )=0 .00, and upper limits of xHI 6. Various systematic uncertainties are examined, but none of them appears large enough to change our conclusion. To get further information on the reionization, it is important to increase the sample size of z 6 GRBs, in order to find GRBs with low column densities (log NHI 20) within their host galaxies and to make statistical studies of Lyα line emission from host galaxies.