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The XMM-Newton view of the X-ray halo and jet of NGC 6251
We present an XMM-Newton observation of the radio jet and diffuse haloof the nearby radio galaxy NGC 6251. The EPIC spectrum of the galaxy'shalo is best-fitted by a thermal model with temperature kT 1.6 keVand sub-solar abundances. Interestingly, an additional hard X-raycomponent is required to fit the EPIC spectra of the halo above 3 keV,and is independently confirmed by an archival Chandra observation.However, its physical origin is not clear. Contribution from apopulation of undetected Low Mass X-ray Binaries seems unlikely.Instead, the hard X-ray component could be due to inverse Comptonscattering of the CMB photons (IC/CMB) off relativistic electronsscattered throughout the halo of the galaxy, or non-thermalbremsstrahlung emission. The IC/CMB interpretation, together with limitson the diffuse radio emission, implies a very weak magnetic field, B≪ 1 μGauss, while a non-thermal bremsstrahlung origin implies thepresence of a large number of very energetic electrons. We also detectX-ray emission from the outer (3.5') jet, confirmingprevious ROSAT findings. Both the EPIC and ACIS spectra of the jet arebest-fitted by a power law with photon index Γ 1.2, fixedGalactic column density, and 1 keV flux F1 keV=2.1 nJy. Athermal model is formally ruled out by the data. Assuming an origin ofthe X-rays from the jet via IC/CMB, as suggested by energetic arguments,and assuming equipartition implies a large Doppler factor (δ 10). Alternatively, weaker beaming is possible for magnetic fieldsseveral orders of magnitude lower than the equipartition field.
| A spectroscopic study of NGC 6251 and its companion galaxies
Measurements of the velocities of galaxies thought to be associated withthe giant radio galaxy NGC 6251 confirm the presence of a poor clusterwith a systemic redshift of z=0.0244+/-0.0004 and a line-of-sightvelocity dispersion ofσz=283(+109,-52)kms-1. This suggests acluster atmosphere temperature of T=0.7(+0.6,-0.2)keV, which is notenough to confine the radio jet by gas pressure. The core of NGC 6251shows strong emission lines of [Oiii] and Hα+[Nii], but there isno evidence for line emission from the jet (detected in opticalcontinuum by Keel).
| The X-ray jet and halo of NGC 6251.
A new analysis of the ROSAT PSPC data of the giant radio galaxy NGC 6251is presented. In supplement to the already published results we used anew approach to disclose the X-ray morphology of NGC 6251. We havedetected an elliptically shaped X-ray halo out to a radial distance ofabout 100kpc from the host galaxy. Moreover, we have discovered a closepositional and intensity correlation of the radio continuum radiation ofthe jet with enhanced X-ray emission. Our analysis suggests that thebulk of the X-ray photons originates from a hot (T~10^7^K) thin plasma,while Inverse-Compton scattering is not important. The X-ray plasmaassociated with the radio continuum jet is most likely confined by themagnetic lines of forces within the jet.
| HI observations of galaxies in nearby Zwicky clusters
The results of a long term project of H I observations of galaxieswithin the boundaries of nearby Zwicky clusters are presented. Thedetection rate is rather low (233 out of 618, i.e., 38 percent) ascompared to other surveys carried out recently. Most of the radialvelocities of the detected galaxies are new determinations. The largespread in radial velocities for many of these clusters is a strongindication for the presence of several foreground and/or backgroundgalaxies.
| IUE UV spectra of extragalactic H II regions. I - The catalogue and the atlas
More than 150 spectrograms of giant H II complexes in spiral, bluecompact and irregular galaxies, excluding the Magellanic Clouds,obtained within the period March 1, 1978 and April 1, 1982 with theInternational Ultraviolet Explorer (IUE) in the wavelength range 1150 Ato 3200 A are cataloged. Many of the observations are presented ingraphic form. A comparison of repeated observations for two objects in M33 and M 101 is made to investigate the reliability of singleobservations. General considerations on the UV spectra of giantextragalactic H II regions are given and a comparison of the spectra ofH II regions at various distances from the centers of M 33 and M 101 ispresented.
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