On the Significance of Absorption Features in HST/COS Data

Abstract

We present empirical scaling relations for the significance of absorption features detected in medium resolution, far-UV spectra obtained with the Cosmic Origins Spectrograph (COS). These relations properly account for both the extended wings of the COS line spread function and the non-Poissonian noise properties of the data, which we characterize for the first time, and predict limiting equivalent widths that deviate from the empirical behavior by \leq 5% when the wavelength and Doppler parameter are in the ranges λ= 1150-1750 A and b > 10 km/s. We have tested a number of coaddition algorithms and find the noise properties of individual exposures to be closer to the Poissonian ideal than coadded data in all cases. For unresolved absorption lines, limiting equivalent widths for coadded data are 6% larger than limiting equivalent widths derived from individual exposures with the same signal-to-noise. This ratio scales with b-value for resolved absorption lines, with coadded data having a limiting equivalent width that is 25% larger than individual exposures when b \approx 150 km/s.

arXiv comment25 pages, 3 tables, 7 figures, accepted for publication in PASP

References in corpus (8)

• The Cosmic Origins Spectrograph
• HST/COS Observations of the Ly alpha Forest toward the BL Lac Object 1ES1553+113
• The Cosmic Origins Spectrograph: On-Orbit Instrument Performance
• Broad HI Absorbers as Metallicity-Independent Tracers of the Warm-Hot Intergalactic Medium
• A Multiphase Absorber Containing O VI and Broad H I Directly Tracing 10^6 K Plasma at Low-Redshift Toward HE 0153-4520
• Cosmic Origins Spectrograph Detection of Ne VIII Tracing Warm - Hot Gas Towards PKS 0405-123
• Cosmic Origins Spectrograph and FUSE Observations of T ~ 10^5 K Gas In A Nearby Galaxy Filament
• HST/COS Observations of Thirteen New He II Quasars

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