SHARP: Search for HI absorptionwith APERTIF

Members of the Team: Raffaella Morganti, Filippo Maccagni, Tom Oosterloo, Bjorn Adebahr, Robert Schultz, Brad Frank, Nicolas Vilchez, Elaine Sadler (PI FLASH), James Allison, Vanessa Moss, Elizabeth Mahony, Neeraj Gupta (PI MALS), Rajeshwari Dutta

The SHARP HI absorption surveys aims at using the broad band of APERTIF (300 MHz) over the frequency range 1050 - 1400 MHz (redshift 0 - 0.3) combined with the large field of view to perform a blind search for HI absorption against radio sources.

The survey has two main science goals:

• Intervening HI absorption ➔ study evolution of cold neutral medium in galaxies using unbiased samples. The number of these absorption expected is relatively low : for 100 mJy sources over 4000 sq degrees, we expect about 50 intervening absorbers because of the low redshift range covered by APERTIF.
  Nevertheless this survey is critical because: 1) expansion compared to available samples (detections at z<0.3 is about a dozen) AND unbiased surveys. Benchmark for high-redshift studies of HI absorbers - allowing us to measure HI in the same way across a wide range in redshift; 2) critical to understanding how best to convert the observed 21cm HI optical depths into a robust estimate of the HI column density distribution => only at z ~0 that we can DIRECTLY compare the estimated HI column densities inferred from absorption systems with the actual distribution measured from HI emission surveys.

• Associated HI absorption ➔ study the conditions and distribution of the gas in the central regions of radio AGN in 4000 sqdeg, ~200-300 associated detections down to 50-100 mJy, and around 500 at the level of 30 mJy. The detailed physical conditions of the gas will be further studied by follow up VLBI observations. Staking for galaxies with known spectroscopic redshift will also be performed.

Requirements of the SHARP survey: we plan to piggyback from all surveys, including observations with poor imaging performances (i.e. during "transients" configuration or with no full-12h tracks); "blind" unbiased search ➔ large area needed: ~ 4000 sqdeg x 12h; medium-deep survey will be used to explore a new parameter space: low HI optical depth and/or HI in low power sources

Other important requirements:
(i) overlap as much as possible with SDSS footprint (this also guarantees that FIRST is available!), and/or other spectroscopic surveys (for redshifts) ➔ for fast ID and redshifts for stacking - some of the detections can also be followed up with targeted optical observations;
(ii) continuum images needed to extract HI spectra;
(iii) access to HI emission or the N(HI) limit to determine the origin of absorbing gas.

There is a strong complementarity between SHARP and the other two HI absorption surveys planned with SKA pathfinders, FLASH and MALS. Unlike the other surveys, APERTIF covers only low redshifts and, because of the less favourable site, is more affected by RFI. SHARP has a major advantage (when it comes to quickly deriving science using optical data or go for a follow-up) due to the easier possibility for available optical ID for SDSS fields and a good VLBI network (including eVLBI) for follow up (in addition to GMRT+VLA) in the Northern hemisphere.