Other
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21915435s&link_type=abstract
American Astronomical Society, AAS Meeting #219, #154.35
Other
Scientific paper
The high-throughput Cosmic Origins Spectrograph (COS) installed on the Hubble Space Telescope (HST) allows us to obtain high-quality UV spectra of active galactic nuclei (AGNs), many of which serve as background targets for studies of the low-redshift intergalactic medium (IGM). We present a composite spectrum produced by stacking spectra of 22 AGNs with redshifts 0.026 < z < 1.44, sufficient to explore the far ultraviolet (FUV) and extreme ultraviolet (EUV) ionizing continuum. The composite covers between 500 - 1800 Å, in the rest-frame, which includes the Lyman continuum of AGNs at z > 0.245. The high sensitivity and moderate resolution (20 km/s) of HST/COS in the G130M grating allows broad emission lines (Ne VIII, O V, O IV, O III, O II) to be resolved. These broad emission features and others contain much of the ionizing EUV flux. We fit the composite's underlying smooth continuum with a power-law, Fν = A να with index α, allowing the extrapolation of the continuum below 500 Å (E > 1.8 ryd). Describing the EUV continuum flux in such a way gives insight into the formation of emission lines, defines the "big blue bump" in the spectral energy distribution of AGNs, and constrains the ionization state of the IGM. We compare our COS-based power-law continuum fit to previous spectral fits from HST/FOS (Telfer et al. 2002) and Far Ultraviolet Spectroscopic Explorer (FUSE) (Scott et al. 2004). This work is supported by the COS-support grant from the STScI (NNX08-AC14G).
Danforth Charles W.
Shull Michael J.
Stevans Matthew L.
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