Integrated HI emission in galaxy groups and clusters
  The integrated HI emission from hierarchical structures such as groups and
clusters of galaxies can be detected by FAST at intermediate redshifts. Here we
propose to use FAST to study the evolution of the global HI content of clusters
and groups over cosmic time by measuring their integrated HI emissions. We use
the Virgo cluster as an example to estimate the detection limit of FAST, and
have estimated the integration time to detect a Virgo type cluster at different
redshifts (from z=0.1 to z=1.5). We have also employed a semi-analytic model
(SAM) to simulate the evolution of HI contents in galaxy clusters. Our
simulations suggest that the HI mass of a Virgo-like cluster could be 2-3 times
higher and the physical size could be more than 50\% smaller when redshift
increases from z=0.3 to z=1. Thus the integration time could be reduced
significantly and gas rich clusters at intermediate redshifts can be detected
by FAST in less than 2 hour of integration time. For the local universe, we
have also used SAM simulations to create mock catalogs of clusters to predict
the outcomes from FAST all sky surveys. Comparing with the optically selected
catalogs derived by cross matching the galaxy catalogs from the SDSS survey and
the ALFALFA survey, we find that the HI mass distribution of the mock catalog
with 20 second of integration time agrees well with that of observations.
However, the mock catalog with 120 second integration time predicts much more
groups and clusters that contains a population of low mass HI galaxies not
detected by the ALFALFA survey. Future deep HI blind sky survey with FAST would
be able to test such prediction and set constraints to the numerical simulation
models. Observational strategy and sample selections for the future FAST
observations of galaxy clusters at high redshifts are also discussed.
