Origins of the Highly Ionized Gas toward the X-ray Bright BL Lac Object Mrk 421

Details Origins of the Highly Ionized Gas toward the X-ray Bright BL Lac Object Mrk 421 Origins of the Highly Ionized Gas toward the X-ray Bright BL Lac Object Mrk 421

Jul 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10266s&link_type=abstract

HST Proposal ID #10266

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Hst Proposal Id #10266 Ism

Scientific paper

The X-ray bright BL Lac object Mrk 421 is one of the small number of extragalactic objects for which it is possible to study the highly ionized gas in the ISM and Local Group using the full diagnostic power of UV, far-UV and X-ray observatories. High quality far-UV observations of OVI and other ions have been obtained by FUSE, whereas Chandra and XMM-Newton have provided X-ray measurements of extremely strong OVII and OVIII absorption at redshift zero. It has been proposed that the OVI, OVII, and OVIII originate together in the warm-hot intergalactic medium, of the kind predicted by numerical simulations to contain a substantial fraction of the baryons at z=0. However, the properties of the high-ion absorption are also consistent with an origin in an extended Galactic Corona. Data for other species such as SiIII, SiIV, and CIV will be essential to determine whether or not a Local Group WHIM filament has been detected. These species are not expected to be seen in a hot {T > 10^6 K} medium and their detection at the same velocity as OVI would suggest that the OVI samples a different phase of the gas than the OVII and OVIII. We propose to obtain a high quality 7 km/s resolution STIS UV spectrum of Mrk 421 from 1150 to 1700 A. The STIS measurements of CII, CIV, SiIII, SiIV, and other species will allow us to evaluate the ionization conditions in the absorbing gas in the Galactic thick disk and in an anomalous high positive velocity absorption wing currently only traced by OVI and CIII. The ionic ratios among these species will provide critical tests of the ionization mechanisms affecting the OVI absorption.

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