LAMA Memo 805 ALMA Calibration Source Counts at 250 GHz

Quasar source counts at millimeter and sub-millimeter wavelengths will be important for phase and delay calibration as well as focus and pointing observations. Some of these observations can be done at low frequencies, but some observations will need to be done at the target frequency. Having a good knowledge of the high frequency quasar source counts will help us with the ALMA calibration plan. We compare source counts at 250 GHz derived from wide field imaging with mJy sensitivity with predictions based on models constrained by lower frequency observations of known flat spectrum radio source samples. While the statistics remain few, current data are consistent with flat spectrum AGN having an areal density N(> 10mJy) ∼ 5±3.5 deg−2 at 250 GHz. The observed counts exceed by a factor of 6 what is expected from source counts extrapolated upwards from 90 GHz. This level of source counts may be adequate for phase calibration via fast switching without requiring frequency boot-strapping. These sources show a high degree of variability, with the maximum measured flux being ∼3 times higher than the minimum measured flux. We also show that thermal sources show a cut-off in the counts at a few mJy, and that these thermal sources must be larger than 0.1′′, and hence are not useful calibrators on baselines longer than 1 km.