On the Significance of Absorption Features in HST/COS Data

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 <= 5% when the wavelength and Doppler parameter are in the ranges of lambda = 1150-1750 angstrom and b > 10 km s(-1). We have tested a number of co-addition algorithms and find the noise properties of individual exposures to be closer to the Poissonian ideal than are co-added data in all cases. For unresolved absorption lines, limiting equivalent widths for co-added data are 6% larger than limiting equivalent widths derived from individual exposures with the same signal-to-noise ratio. This ratio scales with b-value for resolved absorption lines, with co-added data having a limiting equivalent width that is 25% larger than individual exposures when b approximate to 150 km s(-1).