
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:
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.