2008 (IPP)
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Item Properties of warm absorbers in active galaxies: a systematic stability curve analysis(2008-10-14) Chakravorty, Susmita; Kembhavi, A.K.Signatures of warm absorbers are seen in soft X-ray spectra of about half of all Seyfert1 galaxies observed and in some quasars and blazars. We use the thermal equilibrium curve to study the influence of the shape of the ionizing continuum, density and the chemical composition of the absorbing gas on the existence and nature of the warm absorbers. We describe circumstances in which a stable warm absorber can exist as a multiphase medium or one with continuous variation in pressure. In particular we find the following results: i) the warm absorber exists only if the spectral index of the X-ray power-law ionizing continuum α > 0.2 and has a multiphase nature if α ∼ 0.8, which interestingly is the spectral index formost of the observed Seyfert 1 galaxies; ii) thermal and ionization states of highly dense warm absorbers are sensitive to their density if the ionizing continuum is sufficiently soft, i.e. dominated by the ultraviolet iii) absorbing gas with super Solar metallicity is more likely to have a multiphase nature; iv) the nature of the warm absorber is significantly influenced by the absence of iron and associated elements which are produced in the later stages of star formation history in supernovae of type Ia.Item Molecular hydrogen in high-redshift damped lyman-alpha systems: the VLT/UVES database(2008-02) Noterdaeme, P.; Srianand, R.Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 < zabs ≤ 4.2) Damped Lyman-α systems (DLAs) capitalising on observations performed with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). Methods. We identify 77 DLAs/strong sub-DLAs, with log N(Hi) ≥ 20 and zabs > 1.8, which have data that include redshifted H2 Lyman and/or Werner-band absorption lines. This sample of Hi, H2 and metal line measurements, performed in an homogeneous manner, is more than twice as large as our previous sample (Ledoux et al. 2003) considering every system in which searches for H2 could be completed so far, including all non-detections. Results. H2 is detected in thirteen of the systems, which have molecular fractions of values between f ≃ 5 × 10−7 and f ≃ 0.1, where f = N(H2)/(2N(H2) + N(Hi)). Upper limits are measured for the remaining 64 systems with detection limits of typically log N(H2) ∼ 14.3, corresponding to log f < −5. We find that about 35% of the DLAs with metallicities relative to solar [X/H] ≥ −1.3 (i.e., 1/20th solar), with X = Zn, S or Si, have molecular fractions log f > −4.5, while H2 is detected – regardless of the molecular fraction – in ∼ 50% of them. In contrast, only about 4% of the [X/H] < −1.3 DLAs have log f > −4.5. We show that the presence of H2 does not strongly depend on the total neutral hydrogen column density, although the probability of finding log f > −4.5 is higher for log N(Hi) ≥ 20.8 than below this limit (19% and 7% respectively). The overall H2 detection rate in log N(Hi) ≥ 20 DLAs is found to be about 16% (10% considering only log f > −4.5 detections) after correction for a slight bias towards large N(Hi). There is a strong preference for H2-bearing DLAs to have significant depletion factors, [X/Fe] > 0.4. In addition, all H2-bearing DLAs have column densities of iron into dust grains larger than log N(Fe)dust ∼ 14.7, and about 40% of the DLAs above this limit have detected H2 lines with log f > −4.5. This demonstrates the importance of dust in governing the detectability of H2 in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H2-bearing DLAs nor that of the molecular fraction in systems with H2 detections over the redshift range 1.8 < zabs ≤ 3.Item Giant Radio Jet Ejected by An Ultramassive Black Hole in a Single-Lobed Radio Galaxy(2008-01) Bagchi, JoydeepWe report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ∼ 440 kpc away. This jet emanates from an extremely massive black-hole (> 109 M⊙) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counter-jet, even though it is similar to the main jet in brightness up to a scale of tens of kilo parsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic field, this Fanaroff-Riley type II mega-jet is an exceptionally useful laboratory for testing the role of magnetic field in jet stabilization and radio lobe formation.