Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/alfa/ealfa/meeting2/handouts/mediumdeep.pdf Дата изменения: Sat May 13 00:11:08 2006 Дата индексирования: Sun Dec 23 03:34:59 2007 Кодировка: Поисковые слова: storm

Medium Deep Survey Information The question of the legacy left by E-ALFA is one that we need to think very carefully about. A shallow survey that can be used as a reference point for the HI sky in many other studies is an important one. However, to keep HI astronomy vibrant into the next generation we also need to leave a legacy that encompasses new science, not just an improvement on what has come before. Therefore it will be important to consider whether we should use some of the available E-ALFA time for a medium deep survey to extend our knowledge of HI in the local universe. There has been some mention that a "medium deep" survey (we need to define what this means) that makes use of the sensitivity of Arecibo to probe the HI sky much more deeply than any of the other surveys have been able to. We present here some possible parameters for such a survey. Survey Science Objectives and Benefits: · HI mass function over a range of environments: A deep survey will be much more effective at probing the HI mass function over a range of environments (if the regions surveyed are well chosen) because it will probe more independent volumes than a shallower survey. A deep survey will also not suffer as greatly from the local distance uncertainties that can distort the low mass end of the HI mass function. · Serendipitous science: It is only in a survey that probes parameter space not covered by previous studies that we will be able to find new things. Using the sensitivity of Arecibo to push significantly deeper than previous large survey is one of the main paths to new science. · Deep surveying in Virgo and/or other nearby groups: If we pass the survey region through Virgo we will gain an opportunity to discover low mass sources in the cluster as well as being able to deeply probe the galaxies falling into Virgo on the backside, and the void behind the cluster. The region would only be surveyed over a small area, but it could still provide important insight into the gas rich galaxies in and around the cluster environment. Other possible considerations are surveying near some nearby galaxies for HVCs and possible overlap with SMUDGES and/or other optical galaxy surveys that will provide information at other wavelengths.


Survey Strategy: · Commensal observing: The pulsar group is proposing 2 surveys with observing times of 300 seconds/beam at |b|<5° and the other at |b|<10° so this might be a good survey to piggyback for deeper observations. The difficulty we will have to address in piggybacking with these surveys is the additional noise from continuum sources in the Galactic plane. We also have to consider how we can make the best use of the step and stare observing strategy likely to be implemented by the pulsar group. · Regions of interest for deep surveying: It is clear that commensal observing will give us a good opportunity to get deep observations of regions within the Galactic plane by piggybacking on other surveys. For the medium deep survey we need to determine whether there are independent regions that are important for our survey to select and observe to similar (or greater) depth. Virgo is clearly a region of great interest as other nearby groups may be as well. For deep strips overlapping with the SMUDGES 7° strip is also a possibility for synergies with optical data. We need precursor observing to see how we could mesh deeper observations with the shallower surveys already being proposed. · Possible survey schemes: If probing a range of environments is a high priority for the medium deep survey, covering long strips may be better than patches because you most rapidly increase the number of independent volumes that are being surveyed. However, in order to understand how the results fit in with the surrounding environment will require knowledge of the larger scale region that will have to be filled in with a larger area covered surveying scheme. Alternatively we get more information on a particular environment by looking at a patch, but we survey fewer independent volumes. The second important piece in putting together a survey strategy is determining whether step and stare or drift scan mode is the most appropriate. Step and stare will require some study of the optimal step size to tile the hexagonal shape of the footprint appropriately while dealing with the differing sensitivities of the beams. This method is the preferred one by the pulsar people so it will require some careful thought.


· Some numbers for comparison: Obs. Time Area Covered Distance @107 Distance @108 Distance @109 Indep 103 Vol @107 Indep 103 Vol @108 Num Gal @106-107 Num Gal @107-108 60 s 1200 2000 19.7 53.5 139.7 15 192 621 4789 300 s 1200 115.8 29.4 79.9 208.5 34 474 132 946 600 s 1200 57.9 35.0 95.1 248.2 49 362 110 780 ALFALFA VAVA 1990 1910 6600 13.0 35.3 92.2 3 7 674 4927 1600 19.7 53.5 139.7 5 11 487 3828