This post is by Martha Haynes
As we wind up 2011 with about 95% of the ALFALFA survey observations completed, we are already starting actively to think about what science we want to do when we have finished the ALFALFA survey. That of course won’t be for some time; we won’t get the final observations until Fall 2012, and then we will still have to process and analyze the data to produce the final ALFALFA legacy catalog. But, already, we can imagine starting a new survey in a few years, and as was the case with ALFALFA itself, advanced planning (and a bit of dreaming) is critical to pushing the frontiers of science forward. What science do we want to do and what observational and computational capabilities will be required in order to achieve that science? What new instrumentation might be available in a few years? In the last few months, we’ve been thinking seriously about what will come after ALFALFA.
One of the science frontier questions related to “Understanding the Cosmic Order” identified in the Astro 2010 decadal survey asks how baryons cycle in and out of galaxies. A number of papers have shown that dark matter halos of mass below log Mtot/Msun < 10 are baryon-poor. Studying the process(es) which impose this depletion requires advances in (g)astrophysics (reionization, feedback, etc) and better observational constraints on the baryon content of low mass dwarfs. ALFALFA has discovered a set of ultracompact HI clouds which might be associated with low mass dark matter halos in the Local Group, but determining their distances and thus proving their extragalactic nature by detecting their associated stellar counterparts is very difficult. We realize that the clinching evidence that the lowest HI mass clouds are in fact the cores of isolated low mass dark matter halos will require a deep census of several very nearby groups of galaxies. At distances of 5-10 Mpc, low HI mass dwarfs will separate in velocity from the range of galactic and perigalactic phenomena, thereby allowing a direct estimate of distance without requiring the detection of stellar counterparts. Besides exploring the baryon content of low mass halos, an accurate counting of such dwarfs in nearby groups may potentially constrain dark matter decay models (decay injects kinetic energy that can unbind dwarfs).
A survey with 10 times ALFALFA’s sensitivity covering several hundred square degrees in 2-3 nearby groups would deliver the required sampling. To undertake such a survey with ALFA and its 7 feed horns would, however, take an enormous amount of telescope time. We need more pixels! The exciting news is that a 40-beam focal plane phased array feed (PAF) is now in development. A PAF on a single dish can be cooled, and Arecibo’s unmatched instantaneous sensitivity is optimal for local universe detection experiments. So, besides working on ALFALFA in the next years, we will also start the advanced planning for a new survey with this new device dubbed “AO40”, perhaps to start in 2014. You can read more about our thinking on this survey here.
We started the advanced planning for ALFALFA in 2002, and the survey observations themselves began in 2005. So it seems about the right time to start laying the foundations for the next generation survey. Dreaming about the future is always fun, especially when you realize that the future isn’t so far away.