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

MURIEL, HERNÁN; DONZELLI, C. J; ROVERO, A.C.; PICHEL, A.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2015 vol. 574 p. 1 - 6

0004-6361

Context. 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 theattenuation caused by gamma-ray interactions with ambient photons in the extragalactic background light (EBL). This constraintagreed with color-magnitude and spectroscopic redshift constraints (0.179 < z < 0.56). Recently, however, a much higher redshiftwas proposed for this blazar (z > 1.2). This value was debated because if true, it would imply either that the relevant absorptionprocesses of gamma-rays are not well understood or that the EBL is dramatically different from what is believed today. This highredshift was not confirmed by three independent new spectroscopic observations at high signal-to-noise ratios. The scenario is clearevidence of the difficulties in estimating the redshift of BL-Lac objects, whose optical spectra are typically featureless. Neither of theestimated redshifts for PKS 0447-439 are confirmed as yet.Aims. Given that BL-Lac are typically hosted by elliptical galaxies, which in turn are associated with groups, we aim to find the hostgroup of galaxies of PKS 0447-439. The ultimate goal is to estimate a redshift for this blazar.Methods. Spectra of twenty-one objects in the field of view of PKS 0447-439 were obtained with the Gemini Multi-ObjectSpectrograph. Based on the redshifts and coordinates of these galaxies, we searched for groups of galaxies. Using a deep catalogof groups, we studied the probability of finding by chance a group of galaxies in the line of sight of PKS 0447-439.Results. We identified a group of galaxies that was not previously cataloged at z = 0.343 with seven members, a virial radius of0.42 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 memberof the new group is >∼97%. Therefore, we propose to adoptz = 0.343±0.002 as the most likely redshift for PKS 0447-439.