ORCHIDSS

Principal Investigator

The Optical, Radio Continuum and HI Deep Spectroscopic Survey (ORCHIDSS) is an ESO Community Survey planned for the 4MOST Survey facility. The ORCHIDSS team consists of 33 co-investigators, with scientists spanning 4 continents and bringing together members of both the MIGHTEE and LADUMA collaborations.

Survey Goals

Galaxy evolution is regulated by the continuous cycle of gas accretion, consumption and feedback. Crucial in this cycle is the availability of neutral atomic (HI) and molecular hydrogen. Our current inventory of HI, however, is limited to the local Universe (\(z < 0.25\)), resulting in a very incomplete picture. ORCHIDSS will address this critical challenge with a spectroscopic survey of ~200,000 radio continuum detected objects selected from the MeerKAT deep extragalactic surveys (MIGHTEE, LADUMA and MeerKAT Fornax Surveys). Using the powerful combination of radio and 4MOST spectroscopy, ORCHIDSS will:

  1. Provide the most reliable measurement of the evolution of the cosmic HI density and HI mass function since \(z=1.4\),
  2. Measure the fundamental relations between HI and galaxy properties across a wide range of redshifts and environments for the first time, and
  3. Directly measure the effects of AGN and star-formation driven feedback on regulating the neutral gas reservoirs of galaxies across cosmic time.
The MeerKAT radio telescope, based in South Africa, is a pre-cursor for the Square Kilometre Array (SKA) undertaking a ground-breaking set of radio continuum and HI surveys. By complementing these observations with optical spectroscopy, ORCHIDSS will extend the scientific potential of MeerKAT HI studies to more galaxies and higher redshifts than otherwise possible, while also paving the way for future SKA surveys. Image Credit: xxx.

Survey Plan

Within the wider 4MOST Surveys, ORCHIDSS is integrated into the Wide-area Vista Extra-galactic Survey (WAVES) (PIs: J. Liske & S. Driver) along with the 4MOST-StEPS and 4C3R2 Community surveys. The current survey plan for ORCHIDSS is structured into two tiers, with the deep tier targeting all MIGHTEE detected radio continuum sources at \(z < 1.4\) within the WAVESdeepDDF footprint (in COSMOS, CDFS, ES1 and XMM-LSS), the wide tier then targets all continuum targets at z < 0.57 over the remaining ~20 sq.deg of deep MeerKAT observations (MIGHTEE XMM-LSS/CDFS flanking regions and MeerKAT Fornax Survey). With a target selection based on activity (both star-formation and accretion), the resulting spectra will also offer significant legacy value for tackling a broad range of scientific challenges and will be highly complementary to WAVES Deep.