THE SEARCH FOR Hi EMISSION AT z ≈ 0.4 IN GRAVITATIONALLY LENSED GALAXIES WITH THE GREEN BANK TELESCOPE

Abstract

Neutral hydrogen (HI) provides a very important fuel for star formation, but is difficult to detect at high redshift due to weak emission, limited sensitivity of modern instruments, and terrestrial radio frequency interference (RFI) at low frequencies. We report the first attempt to use gravitational lensing to detect HI line emission from three gravitationally lensed galaxies behind the cluster Abell 773, two at redshifts of 0.398 and one at z = 0.487, using the Green Bank Telescope. We find that a 3σ upper limit for a galaxy with a rotation velocity of 200 km s −1 is M H I = 6.58 × 10 9 and 1.5 × 10 10 M e at z = 0.398 and z = 0.487. The estimated HI masses of the sources at z = 0.398 and z = 0.487 are factors of 3.7 and ∼30 times lower than our detection limits at the respective redshifts. To facilitate these observations we have used sigma-clipping to remove both narrow-and wideband RFI but retain the signal from the source. We are able to reduce the noise of the spectrum by ∼25% using our routine instead of discarding observations with too much RFI. The routine is most effective when ∼10% of the integrations or fewer contain RFI. These techniques can be used to study HI in highly magnified distant galaxies that are otherwise too faint to detect.

Towards the first detection of strongly lensed H i emission

“…The only previously published upper-limit on integrated lensed H i flux (Hunt et al 2016) placed competitive 3σ MHI upper limits of 6.58 × 10 9 M at z = 0.398 and 1.5 × 10 10 M at z = 0.487, however the assumption was made that the H i magnification is equal to the optical magnification which is inconsistent with our simulations (see next section). Their method constrains the H i mass by taking a factor of the spectral RMS as an upper limit on the lensed H i signal.…”

Toward Cosmicflows-4: The HI data catalog

“…We chose to ignore integrations of a single scan contaminated by this RFI when deriving the final HI spectra. The FFT methodology (sigma clipping in Fourier space) suggested by Hunt et al (2016) has been tested on contaminated spectra with no success. Please refer to this publication for more details on the method.…”

LADUMA: Looking at the Distant Universe with the MeerKAT Array

“…LADUMA will also pursue a range of "beyond-the-headlines" science that includes searching for gravitationally lensed H I emission, which promises to extend the study of Ω H I (z) described above (e.g., [20,32]), and probing the demographics of low column density H I absorbers. The survey will also encompass non-H I science, specifically observations of OH megamasers, which arise in galaxy major mergers associated with extreme star formation, and thus probe mergers across cosmic time as well as the most extreme starbursts [18,66].…”