GREEN BANK TELESCOPE OBSERVATIONS OF LOW COLUMN DENSITY H I AROUND NGC?2997 AND NGC?6946

摘要

Observations of ongoing H I accretion in nearby galaxies have only identified about 10% of the fuel necessary to sustain star formation in these galaxies. Most of these observations have been conducted using interferometers and may have missed lower column density, diffuse, H I gas that may trace the missing 90% of gas. Such gas may represent the so-called cold flows predicted by current theories of galaxy formation to have never been heated above the virial temperature of the dark matter halo. As a first attempt to identify such cold flows around nearby galaxies and complete the census of H I down to N H I ?~?1018?cm–2, I used the Robert C. Byrd Green Bank Telescope (GBT) to map the circumgalactic (r ?100-200?kpc) H I environment around NGC?2997 and NGC?6946. The resulting GBT observations cover a 4?deg2 area around each galaxy with a 5σ detection limit of N H I ~ 1018?cm–2 over a 20?km?s–1 line width. This project complements absorption line studies, which are well-suited to the regime of lower N H I . Around NGC?2997, the GBT H I data reveal an extended H I disk and all of its surrounding gas-rich satellite galaxies, but no filamentary features. Furthermore, the H I mass as measured with the GBT is only 7% higher than past interferometric measurements. After correcting for resolution differences, the H I extent of the galaxy is 23% larger at the N H I = 1.2 × 1018?cm–2 level as measured by the GBT. On the other hand, the H I observations of NGC?6946 reveal a filamentary feature apparently connecting NGC?6946 with its nearest companions. This H I filament has N H I ~?5 × 1018?cm–2 and an FWHM of 55 ±?5?km?s–1 and was invisible in past interferometer observations. The properties of this filament are broadly consistent with being a cold flow or debris from a past tidal interaction between NGC?6946 and its satellites.