Abstract Details
| Name: Tanvir Hussain Affiliation: Tezpur University Conference ID: ASI2015_367 Title : HST/COS detection of a Ne VIII absorber towards PG 1407+265: An unambiguous tracer of collisionally ionized hot gas? Authors and Co-Authors : Sowgat Muzahid, PSU (USA) Anand Narayanan, IIST (India) Raghunathan Srianand, IUCAA (India) Bart P. Wakker, Wisconsin (USA) Jane C. Charlton, PSU (USA) Amit Pathak, Tezpur University (India) Abstract Type : Poster Abstract Category : Extragalactic astronomy Abstract : We report the detection of Ne VIII in a z_abs = 0.59961 intervening absorber in the high signal-to-noise HST /COS spectrum of the QSO PG1407+265 (z_em= 0.94). Besides Ne VIII, absorption from H I Lyman series lines (H I λ1025 – λ915) and several other low (C II, N II, O II and S II), intermediate (C III, N III, N IV, O III, S IV and S V) and high (S VI, O VI and Ne VIII) ionization metal lines is detected with > 3σ significance. Disparity in the absorption line kinematics between low and high ions implies that the absorber has a multiphase structure. Photoionization models suggest that the low and intermediate ions (except S V) trace a compact (∼ 410 pc), metal-rich (Z ∼ Z_⊙) and over-dense (log n_H ∼ −2.6) region. The estimated relative abundances of C, N, O, S, and Si are consistent with the solar values within 0.2 dex indicating that the photoionized gas was part of a region that sustained star-formation for a prolonged period. The observed column densities of Ne VIII and O VI in different components and the absence of Lyβ absorption in one of the components can be explained as arising in a low density (−5.3 < log n_H < −5.0), metal-rich (Z ~ Z_⊙) and diffuse (∼ 180 kpc) photoionized gas. The S V, S VI and C IV (detected in an earlier FOS spectrum) column densities are consistent with an intermediate ionization phase with −4.2 < log n_H < −3.5. Alternatively, a pure collisional ionization model with 5.65 < log T < 5.72 can reproduce the observed column densities of S VI, O VI, and Ne VIII simultaneously in a single phase. However, even such models require an intermediate phase to reproduce any observable S V and/or C IV. Therefore, we conclude that a photoionization model is equally plausible as a collisionally ionized model for the origin of Ne VIII absorption in this system. This demonstrates that Ne VIII is not necessarily an unambiguous tracer of collisionally ionized hot gas as was suggested by all previous analyses of intervening Ne VIII systems. Searching for faint dwarf galaxies at low impact parameters in deeper images will provide further insights on the absorber that shows signature of being associated to star forming region. |