Warm Gas Flows in the Coma Cluster
The flow of baryons into dark matter halos varies by halo mass, but is predicted to be filamentary in all cases. We propose to observe the flow of baryons relative to the most massive structures in the universe by creating the first map of the distribution of warm gas (Ly-alpha absorbers) in and around the Coma Cluster. The total amount of warm gas, covering fraction and structure will be compared to the less massive, irregular Virgo Cluster, the field, and simulation predictions. These absorbers are our only method of directly measuring the kinematics of Coma's diffuse gas. Coma is the benchmark cluster for models of cluster formation, and the data will be combined with the rich multi-wavelength data available and used to constrain these models.
This survey of the Coma cluster's warm ICM will use the Cosmic Origins Spectrograph in medium resolution mode with the G130M grating. We will observe 9 QSOs distributed over a range of distances, from the cluster center to ~2 times the virial radius. With the G130M grating, we will cover 1150 - 1450 AA, and our primary goal is to observe Ly-alpha absorbers.
The proposed observations will yield high S/N (S/N > 10), high resolution (~15 km/s) spectra. The combination of resolution and signal-to-noise is designed to allow us to detect weak Ly-alpha absorption features, W > 50 AA, at 5-sigma. This is consistent with our Virgo survey findings where we detected 25 absorbers toward 9 sightlines in the Virgo velocity range with W=50-500 AA. Our observations will be sensitive to Ly-alpha absorbers over the entire velocity range of Coma (4000-10000 km/s), as well as obtain information on the number of absorbers at a range of foreground and background velocities.
All sightlines were chosen by cross-matching the Veron-Cetty QSO catalog 13th ed. with the GALEX database. The GALEX data provide an estimate of the UV flux level, with the FUV imaging band covering the range 1344 - 1786 AA (effective wavelength 1516 AA). The SDSS images were checked to ensure there were no obvious stars or galaxies interfering with the GALEX flux. All our targets are selected to be brighter than 18.7 AB mag and the desired S/N level can be reached in 5 orbits or less.
The acquisition strategy (mirror A vs B) has been chosen using the COS Imaging Acquisition ETC and the measured GALEX fluxes to normalize a variety of input spectra (flat and redshifted QSO spectra). To be conservative, we chose the longest suggested exposure time among the various results, while ensuring count-rate limits are not exceeded. Science exposure times were estimated with the COS spectroscopy ETC and extended to fully pack the orbits. The BOT found all fields safe with two unknown objects that appear to be our actual targets.
QUASAR ABSORPTION LINES AND IGM
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SUPPORTED
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COS/FUV
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