Wednesday.
1309.0047
The primordial helium abundance from updated emissivities
Aver, Olive, Porter, Skillman
Observations of the metal-poor extragalactic H II regions allow the determination of the primordial He abundance, Y_p. The He I emissivities are the foundation of the model of the H II region's emission. Others have recently published updated He I emissivities based on improved photoionization cross-sections. Incorporate these new atomic data and update the recent Markov Chain Monte Carlo analysis of the dataset published by some others. As before, cuts are made to promote quality and reliability, and only solutions which fit the data within 95% CL are used to determine the primordial He abundance, The previously qualifying dataset is almost entirely retained and with strong concordance between the physical parameters. Overall, an upward bias from the new emissivities leads to a decrease in Y_p. In addition, find a general trend to larger uncertainties in individual objects (due to changes in the emissivities) and an increased variance (due to additional objects included). From a regresion to zero metallicity, determine Y_p = 0.2465 pm 0.0097, in good agreement with the Planck result of Y_p = 0.2485 pm 0.0002. In the future, a better understanding of why a large fraction of spectra are not well fit by the model will be crucial to achieving an increase in the precision of the primordial He abundance determination.
1309.0504
A new method to measure galaxy bias
Pollack, Smith, Porciani
A new approach for modeling halo bias that utilizes the full non-linear information contained in the moments of the matter density field. The basis of the technique is to derive this information directly from a set of numerical simulations. Although this method is general, perform a case study based on the local Eulerian bias scheme truncated to second-order. Using 200 N-body sims that cover a total comoving volume of 675 (Gpc/h)^3, measure several 2- and 3-point statistics of the halo distribution to unprecedented accuracy; then use the bias model to fit the halo-halo PS, the halo-matter cross spectrum and the corresponding 3 bispectra for wavenumbers in the range 0.04<k<0.12 h/Mpc. While all fits accurately describe the data, find that the constraints on the bias parameters obtained using the full NL information differ significantly from those derived using leading-order standard perturbation theory. Hence, neglecting the full NL information leads to biased results for this particular scale range. Also test the validity of the second-order Eulerian local biasing scheme by comparing the parameter constraints derived from different statistics. Find the halo-matter cross spectrum gives incompatible results compared to all other statistics that give mutually consistent constraints. The main difference is that the halo-matter cross spectrum favours a value for the NL bias parameter b_2 that is very close to zero, whereas the other spectra prefer a negative b_2. Analysis of the halo-matter cross-correlation coefficients defined for the 2- and 3-pt statistics reveals further inconsistencies contained in the second-order Eulerian bias scheme, suggesting it is too simple a model to describe halo bias with high accuracy.
1309.0638
Probing the jet base of the blazar PKS1930-211 from the chromatic variability of its lensed images. Serendipitous ALMA observations of a strong gamma-ray flare
Marti-Vidal, et al
The launching mechanism of the jets of AGN is observationally poorly constrained, due to the large distances to these objects and the very small scales (sub-parsec) involved. In order to better constrain theoretical models, it is especially important to get information from the region close to the physical base of the jet, where the plasma acceleration takes place. In this paper, report multi-epoch and multi-frequency continuum observations of the z=2.5 blazar PKS1830-211 with ALMA, serendipitously coincident with a strong gamma-ray flare reported by Fermi-LAT. The blazar is lensed by a FG z=0.89 galaxy, with two bright images of the compact core separated by 1". The ALMA observations individually resolve these two images (although not any of their substructures); study the change of their relative flux ratio with time (four epochs spread over nearly 3 times the time delay between the 2 lensed images) and frequency ( between 350 and 1050 GHz, rest-frame of the blazar), during the gamma-ray flare. In particular, detect a remarkable frequency-dependent behavior of the flux ratio, which implies the presence of a chromatic structure in the blazar (i.e., a core-shift effect). Rule out the possibility of micro- and milli-lensing effects and propose instead a simple model of plasmon ejection in the blazar's jet to explain the time and frequency variability of the flux ratio. Suggest that PKS1830-211 is likely on of the best sources to probe the activity at the base of a blazar's jet at submillimeter wavelengths, thanks to the peculiar geometry of the system. The implications of the core-shift in absorption studies of the FG z=0.89 galaxy (e.g., constraints on the cosmological variations of fundamental constants) are discussed.
The primordial helium abundance from updated emissivities
Aver, Olive, Porter, Skillman
Observations of the metal-poor extragalactic H II regions allow the determination of the primordial He abundance, Y_p. The He I emissivities are the foundation of the model of the H II region's emission. Others have recently published updated He I emissivities based on improved photoionization cross-sections. Incorporate these new atomic data and update the recent Markov Chain Monte Carlo analysis of the dataset published by some others. As before, cuts are made to promote quality and reliability, and only solutions which fit the data within 95% CL are used to determine the primordial He abundance, The previously qualifying dataset is almost entirely retained and with strong concordance between the physical parameters. Overall, an upward bias from the new emissivities leads to a decrease in Y_p. In addition, find a general trend to larger uncertainties in individual objects (due to changes in the emissivities) and an increased variance (due to additional objects included). From a regresion to zero metallicity, determine Y_p = 0.2465 pm 0.0097, in good agreement with the Planck result of Y_p = 0.2485 pm 0.0002. In the future, a better understanding of why a large fraction of spectra are not well fit by the model will be crucial to achieving an increase in the precision of the primordial He abundance determination.
1309.0504
A new method to measure galaxy bias
Pollack, Smith, Porciani
A new approach for modeling halo bias that utilizes the full non-linear information contained in the moments of the matter density field. The basis of the technique is to derive this information directly from a set of numerical simulations. Although this method is general, perform a case study based on the local Eulerian bias scheme truncated to second-order. Using 200 N-body sims that cover a total comoving volume of 675 (Gpc/h)^3, measure several 2- and 3-point statistics of the halo distribution to unprecedented accuracy; then use the bias model to fit the halo-halo PS, the halo-matter cross spectrum and the corresponding 3 bispectra for wavenumbers in the range 0.04<k<0.12 h/Mpc. While all fits accurately describe the data, find that the constraints on the bias parameters obtained using the full NL information differ significantly from those derived using leading-order standard perturbation theory. Hence, neglecting the full NL information leads to biased results for this particular scale range. Also test the validity of the second-order Eulerian local biasing scheme by comparing the parameter constraints derived from different statistics. Find the halo-matter cross spectrum gives incompatible results compared to all other statistics that give mutually consistent constraints. The main difference is that the halo-matter cross spectrum favours a value for the NL bias parameter b_2 that is very close to zero, whereas the other spectra prefer a negative b_2. Analysis of the halo-matter cross-correlation coefficients defined for the 2- and 3-pt statistics reveals further inconsistencies contained in the second-order Eulerian bias scheme, suggesting it is too simple a model to describe halo bias with high accuracy.
1309.0638
Probing the jet base of the blazar PKS1930-211 from the chromatic variability of its lensed images. Serendipitous ALMA observations of a strong gamma-ray flare
Marti-Vidal, et al
The launching mechanism of the jets of AGN is observationally poorly constrained, due to the large distances to these objects and the very small scales (sub-parsec) involved. In order to better constrain theoretical models, it is especially important to get information from the region close to the physical base of the jet, where the plasma acceleration takes place. In this paper, report multi-epoch and multi-frequency continuum observations of the z=2.5 blazar PKS1830-211 with ALMA, serendipitously coincident with a strong gamma-ray flare reported by Fermi-LAT. The blazar is lensed by a FG z=0.89 galaxy, with two bright images of the compact core separated by 1". The ALMA observations individually resolve these two images (although not any of their substructures); study the change of their relative flux ratio with time (four epochs spread over nearly 3 times the time delay between the 2 lensed images) and frequency ( between 350 and 1050 GHz, rest-frame of the blazar), during the gamma-ray flare. In particular, detect a remarkable frequency-dependent behavior of the flux ratio, which implies the presence of a chromatic structure in the blazar (i.e., a core-shift effect). Rule out the possibility of micro- and milli-lensing effects and propose instead a simple model of plasmon ejection in the blazar's jet to explain the time and frequency variability of the flux ratio. Suggest that PKS1830-211 is likely on of the best sources to probe the activity at the base of a blazar's jet at submillimeter wavelengths, thanks to the peculiar geometry of the system. The implications of the core-shift in absorption studies of the FG z=0.89 galaxy (e.g., constraints on the cosmological variations of fundamental constants) are discussed.
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