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Going Deep with ALFA
E-ALFA Ultra-deep field group February 17, 2004

Abstract The improved sensitivity, RFI excisions and baselines expected with ALFA will open a new area of high sensitivity observations for the Arecibo Telescope. There is a significant scientific interest in single sight-line observations to unprecedented depth with integration times up to about 100 hours per beam. We propose a set of precursor observation to test the feasibility of very deep ALFA observations and find the limitations of such observations. The proposed strategy is to obtain multiple drift scans over a field with a known late-type disk galaxy close to the redshift limit and close to the optimal Arecibo declination at night. The total proposed integration time is 35 hours per beam. By defining the ultimate sensitivity limit of ALFA, these observations will serve as a reference for other surveys and general observations.

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Scientific case for Deep Surveys

The Arecibo dish is the largest and most sensitive instrument of its kind. This makes it possible to conduct unique extragalactic HI surveys which are more sensitive than any other surveys. Before the commissioning of ALFA, it was not possible to exploit this possibility because ultimate sensitivity requires large bandpass and multiple independent beams for baseline subtraction. ALFA makes it feasible, in principle, to observe down to frequencies of 1225 MHz, corresponding to a redshift of 0.16, which corresponds to a look-back time of 15% of the age of the Universe. With sufficient integration time, ALFA should be able to reach a 1 column density sensitivity of 1016 cm-2 per 5 km s-1 for gas filling the beam. The Doppler parameters of clouds which correspond to the Lyman- forest are typically 30 km s-1 (Shull et al. 2000, ApJ, 538, 13), which means that they would be resolved with the proposed 5 km s-1 resolution. This depth is sufficient to distinguish between evolutionary scenarios. While such column densities are similar to that reached by Parkes multibeam HIDEEP survey (Minchin et al. 2003), an ALFA survey would have three significant advantages: 1. Higher resolution. 2. Higher sensitivity to unresolved sources. 3. Higher velocity resolution. To exploit this unique capabilities, the ALFA Extragalactic Consortium intends to propose a large scale ultra deep survey which will reach sensitivities never explored with Arecibo before.

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The Ultimate Sensitivity of ALFA

While sensitivity calculations show that ALFA should be able to reach the sensitivity outlined above, it is not clear whether in practice it can be reached. There are several limitation which might prevent observations to reach such depth: 1. RFI 2. receiver limitations 3. limits on the possible accuracy of baseline subtraction and/or standing waves The impact of these problems on any observations are partially known and will be documented by other commissioning observations. Here we propose to characterize these effects to an depth which has not previously been explored. These data will characterize the ALFA system and will therefore be useful for calibration purposes and as a reference of low-level effects and intermittent effects.

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Prop osed Observing Strategy

To reach the deepest possible observations with ALFA, we propose to observe in "drift and chase" mode. The basic strategy is to point about 10 arcmin ahead of the target field with the central beam. We will then let the sky drift past for 1.3 min of time (or 20 arcmin). Subsequently, we will drive to the starting point and repeat the procedure. We intend to test several variations of this basic strategy. The total integration time to reach the cosmological interesting sensitivity outlined above is 70 hour per beam. We propose precursor observations to reach half this integration time per beam using both channels of the WAPPS with 100 MHz bandwidth. We estimate that with our "drift and chase" mode, this can be achieved in a total of 70 hours of scheduled observing time. By collecting repeated observations of the same source over an extended period of time, we will be able to investigate stability and robustness of calibration of the whole system. The ultimate goal of the proposed observations is to test the feasibility and efficiency of our our proposed observing mode and evaluate whether noise as a function of integration times keeps decreasing at the expected rate for such long exposure times. If it does not, we will attempt to identify and quantify any additional sources of noise. Results from this experiment will be: 1. RMS noise at as a function of integration time for frequencies throughout the ALFA bandwidth 2. Characterization of low-level RFI and identification of RFI-free regions of the spectrum, investigation of methods to mitigate low-level RFI


3. quantification of gain stability over the period of observation 4. quantification of sensitivity limits due to baseline artifacts and standing waves

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Target Selection

We have used the SDSS Query Server to identify fields at low zenith angle for nighttime observations which contain strong H emitting galaxies at a redshift z 0.15. The galaxies we selected are clearly disk galaxies and have similar properties as the typical HI rich counterparts. The exact selection criteria were identical to the ones successfully used by Catinella, Gardner, Connolly, Haynes and Giovanelli (2004, private communication) to identify HI rich galaxies detectable with Arecibo. Our prime target is at a redshift of 14.2. In case this part of the spectrum is unusable due to RFI, we selected another field with a similar galaxy at z=0.10.