We present radio observations of the radio galaxy PKS 2152-699 obtained with the Australia Telescope Compact Array. The much higher resolution and signal-to-noise ratio of the new radio maps reveal ...the presence of a bright radio component about 10 arcsec north-east of the nucleus. This lies close to the highly ionized cloud previously studied in the optical and here shown in a broad-band red snapshot image with the HST PC 2. It suggests that PKS 2152-699 may be a jet/cloud interaction similar to 3C 277.3. This could cause the change in the position angle (of ∼ 20°) of the radio emission from the inner to the outer regions. On the large scale, the source has Fanaroff & Riley type II morphology although the presence of the two hotspots in the centres of the lobes is unusual. The northern lobe shows a particularly relaxed structure while the southern one has an edge-brightened, arc-like structure.
Context. The Great Observatories Origins Deep Survey (GOODS) has provided us with one of the deepest multi-wavelength views of the distant universe. The combination of multi-band photometry and ...optical spectroscopy has resulted in the identification of sources whose redshifts extend to values in excess of six. Amongst these distant sources are Lyα emitters whose nature must be deduced by clearly identifying the different components that contribute to the measured SED. Aims. From a sample of Lyα emitters in the GOODS-S field with uncontaminated photometry and optical (red) spectroscopy, we select a spatially compact object at a redshift of 5.563 (Lyα) that shows a second emission line, identified as $\ion{N}{iv}$ 1486 Å. The SED is modelled in a way that accounts for both the $\ion{N}{iv}$ line emission and the photometry in a self-consistent way. Methods. The photoionization code CLOUDY is used to calculate a range of nebular models as a function of stellar ionizing source temperature, ionization parameter, density and nebular metallicity. We compare the theoretical and observed magnitudes and search for the model parameters that also reproduce the observed $\ion{N}{iv}$ luminosity and equivalent width. Results. A nebular model with a hot blackbody ionizing source of around 100 kK and a nebular metallicity of ~5% of solar is able to fit the observed SED and, in particular, explain the large apparent Balmer break which is inferred from the pure stellar population model fitting conventionally applied to multi-band photometric observations. In our model, an apparent spectral break is produced by strong $\ion{O}{iii}$ 4959, 5007 Å emission falling in one of the IR bands (IRAC1 in this case). A lower limit on the total baryonic mass of a model of this type is 3.2 $\times$ $10^8~M_{\odot}$. Conclusions. It is argued that objects with Lyα emission at high redshift that show an apparent Balmer break may have their SED dominated by nebular emission and so could possibly be identified with very young starbursting galaxies rather than massive evolved stellar populations. Detailed studies of these emission nebulæ with large telescopes will provide a unique insight into very early chemical evolution.
New optical, radio and X-ray observations are used to investigate the morphology, ionization and kinematics of extended structures in the host galaxy of the radio source PKS 2250 -41 (z= 0.308). The ...data provide clear evidence for a powerful interaction between the radio jets and the ambient interstellar medium. Not only do Oiii emission-line images show spectacular extended arc structures associated with the radio lobes on both sides of the galaxy nucleus on a scale of 40–65 kpc,1 but there is also large depolarization in the western radio lobe at the position of the western emission-line arc. Optical long-slit spectra of the extended emission-line regions provide the first convincing evidence that the radio jets can have a major ionizing effect on the warm gas in powerful radio galaxies: while the low-ionization Ni and Sii lines are broad (FWHM∼300–500 km s−1) across the entire emission-line nebulosity, the higher ionization O iii lines are significantly narrower in the western emission-line arc (FWHM = 240 ± 10 km sα1). This anticorrelation between linewidth and ionization is difficult to explain in terms of central source photoionization, but is entirely consistent with the compression or direct ionization effects of fast shocks driven by the radio jets. Further evidence for shocks is provided by the minimum in the ionization state at the position of the radio lobes, and by O iii (5007 + 4959)/4300 and He ii/Hβ diagnostic ratios which are more consistent with shocks than with photoionization. The spectacular nature of the jet/cloud interaction in this object is best explained in terms of a direct collision between the radio jet and a companion galaxy in the surrounding group. This model is supported by the detection of a blue continuum source in the western arc with a colour and luminosity characteristic of late-type spiral galaxies.
We present spatially resolved spectropolarimetric measurements of the 100 kpc scale gaseous environment of the z = 2.34 radio galaxy TXS 0211-122. The polarization level of the narrow Ly{alpha} ...emission is low centrally (P < 5%), but rises to P = 16.4% {+-} 4.6% in the eastern part of the nebula, indicating that the nebula is at least partly powered by the scattering of Ly{alpha} photons by H I. Not only is this the first detection of polarized Ly{alpha} around a radio-loud active galaxy, it is also the second detection to date for any kind of Ly{alpha} nebula. We also detect a pair of diametrically opposed UV continuum sources along the slit, at the outer edges of the Ly{alpha} nebula, which we suggest may be the limb of a dusty shell, related to the large-scale H I absorbers often associated with high-z radio galaxies.
We present new observations, including both narrow-band imaging and long-slit spectroscopy, of the optical filaments in Centaurus A. The line-emitting filaments which compose part of the well-known ...‘optical jet’ are complex and highly structured at the limit of ground-based resolution. Variations in the surface brightness, line intensity ratios and the velocity field occur on spatial scales ~ 100 pc. We find no evidence of O III λ5007 emission associated with either the ‘inner jet’ or a possible ‘counter jet’. The emission line spectra of the filaments are very similar in character to those of the spatially extended nebulosities in more distant radio galaxies and radio quasars. We conclude from an analysis of the line intensity ratios that the filaments are predominantly photoionized by the radiation field of a nuclear continuum source which is hidden from our direct view either by obscuration, intrinsic anisotropy, or both. Spatial variations in the line intensity ratios can be explained by local differences in the distribution of densities among an ensemble of clouds. The radiation field which is required to account for the ionization state of the emitting gas is such that the nuclear continuum source seen by them has an intensity in the 2–6 keV X-ray band which is 200 times greater than that which is observed directly from Earth. This ‘X-ray deficit’ can be explained if the nuclear continuum is relativistically beamed in the direction of the optical jet. We show that, if this is the case, then Centaurus A has a beam power similar to that of BL Lac.
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