The BL-Lac gamma-ray blazar PKS 0447-439 as a probable member of a group of galaxies at z=0.343

The BL-Lac blazar PKS 0447-439 is one of the brightest HE gamma-ray sources that were first detected by Fermi-LAT. It was also detected by H.E.S.S. at VHE gamma-rays, which allowed constraining the redshift of PKS 0447-439 by considering the attenuation caused by gamma-ray interactions with ambient photons in the extragalactic background light (EBL). This constraint agreed with color-magnitude and spectroscopic redshift constraints (0.179 < z < 0.56), Recently, however, a much higher redshift was proposed for this blazar (z > 1.2). This value was debated because if true, it would imply either that the relevant absorption processes of gamma-rays are not well understood or that the EBL is dramatically different from what is believed today. This high redshift was not confirmed by three independent new spectroscopic observations at high signal-to-noise ratios. Given that BL-Lac are typically hosted by elliptical galaxies, which in turn are associated with groups, we aim to find the host group of galaxies of PKS 0447-439. The ultimate goal is to estimate a redshift for this blazar. Spectra of twenty-one objects in the field of view of PKS 0447-439 were obtained with the Gemini Multi-Object Spectrograph. Based on the redshifts and coordinates of these galaxies, we searched for groups of galaxies. Using a deep catalog of groups, we studied the probability of finding by chance a group of galaxies in the line of sight of PKS 0447-439. We identified a group of galaxies that was not previously cataloged at z = 0.343 with seven members, a virial radius of 0.42 Mpc, and a velocity dispersion of 622 km s^-1. We found that the probability of the host galaxy of PKS 0447-439 to be a member of the new group is >= 97%. Therefore, we propose to adopt z = 0.343 +- 0.002 as the most likely redshift for PKS 0447-439.Comment: Accepted for publication in A&

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In this study we modelled and measured the U-turn trajectories of individual magnetotactic bacteria under the application of rotating magnetic fields, ranging in ampitude from 1 to 12 mT. The model is based on the balance between rotational drag and magnetic torque. For accurate verification of this model, bacteria were observed inside 5 m tall microfluidic channels, so that they remained in focus during the entire trajectory. From the analysis of hundreds of trajectories and accurate measurements of bacteria and magnetosome chain dimensions, we confirmed that the model is correct within measurement error. The resulting average rate of rotation of Magnetospirillum Gryphiswaldense is 0.74 +- 0.03 rad/mTs.Comment: 17 pages, 12 figure

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On the redshift of the gamma-ray blazar PKS 0447-439: Optical spectroscopy using Gemini observations with high S/N ratio

Abstract: The BL-Lac blazar PKS 0447-439 was detected at very high energy gamma-rays by HESS following the discovery by Fermi-LAT. The lack of both emission and absorption lines in BL-Lacs make the estimation of their redshifts very difficult. Modeling the drop in gamma-ray spectra it was possible to have an estimation of redshift for PKS 0447-439 of z approx 0.2, which is compatible with the value z=0.205 reported by the identification of Ca II absorption lines in optical spectra. By the identification of a weak line of Mg II using spectra with average signal-to-noise S/N approx 80, it has been recently reported a lower limit for the redshift of this blazar of z&lt;=1.246. Triggered by this controversy, we have proposed new optical observations with the Gemini South telescope to perform further spectroscopic analysis with very high S/N ratio (approx 200-500). In this work we present a new optical spectrum of PKS 0447-439, and report on the analysis and results of such observations. Even with this significantly high quality signal we were not able to identify any spectral features to allow an estimation of the redshift. In agreement with other recent studies, we identify the Mg II line reported previously as originated in the Earth´s atmosphere, and conclude the lower limit of the redshift is incorrect. More interestingly, we could not identify the Ca II absorption lines used to report a redshift of 0.205.Fil: Donzelli, Carlos. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina.Fil: Donzelli, Carlos. Conicet. Instituto de Astronomía Teórica y Experimental; Argentina.Fil: Muriel, Hernán. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina.Fil: Muriel, Hernán. Conicet. Instituto de Astronomía Teórica y Experimental; Argentina.http://adsabs.harvard.edu/abs/2013arXiv1307.6907RAstronomía (incluye Astrofísica y Ciencias del Espacio