Sambatriniaina Rajohnson
Neutral hydrogen atoms illuminate structures not traced by stars. They are detectable by the so-called 21cm hyperfine transition – The HI line. The study of the HI component of galaxies gives us an insight into their evolution over cosmic time, their overall gas content, star formation rate, as well as the way they are connected with their environments. However, due to the flux of HI photons being extremely weak, not much is known about the HI content of galaxies located in the more distant Universe (distance D > 1.45 Gly). Thanks to the capabilities of the new, powerful telescope MeerKAT, extending the current HI view of the Universe up to intermediate redshifts (D ~ 7.2 Gly) will be feasible with the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey.
Before conducting the full survey with the new telescope, a series of tests have to be made to ensure the reliability of the resulting flux values. To do so, a 2º × 2º early science observation over one of the MIGHTEE fields has searched for galaxies within a volume of D < 0.86 Gly. This has led to a detection of 50 sources ranging from dwarf galaxies to large spirals, galaxy groups and satellite companions. We have derived their HI properties such as flux density and HI masses for science validation purposes. The catalogue of sources, along with their moment maps, were exported into cubelets and form part of the early science data products of the MIGHTEE-HI survey.
HI imaging has been used to understand the gas content in galaxies, their masses, their star formation rate over time, and is an excellent environmental tracer. However, we are facing some challenges since HI emission is very faint. Our knowledge to date is based on HI detection of galaxies in the very local Universe only (D < 1.45 Gly). As a result, little is known for galaxies located at higher distances due to technical limitations: not enough frequency coverage, lack of sensitivity, and resolution.
Extending the current HI view of the Universe up to intermediate redshifts (D ~ 7.2 Gly) will be feasible thanks to the capabilities of the new, powerful telescope MeerKAT (Fig. 2), an SKA precursor, and currently the most sensitive telescope for HI studies to date (Jonas et al. 2018). The MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) is a survey designed to collect radio continuum, spectral line and polarization information over 20 square degrees of the sky (Jarvis et al. 2016). It therefore has the combination of an excellent resolution, a good sensitivity and a large frequency coverage.
How can we trust the flux values that we get with a new telescope?
In order to answer these key questions, a 2º × 2º pointing of one of the MIGHTEE fields has been investigated in order to detect galaxies with distances D < 0.86 Gly. Our primary goals are to assess the imaging quality of the early survey and verify whether the derived HI masses are consistent with existing previous measurements.
Once corrections are applied to the raw data, a 3D data cube (Right Ascension R.A, Declination DEC and Velocity) showing the emission of gas in the field has been obtained, then a catalogue list has been created after visual inspection of the cube.
Fifty detections were found visually in the cube, ranging from small dwarfs to large spiral galaxies. Their HI properties such as flux densities, HI masses, linewidths and rms noises have been derived. Most of them were found at D ~ 0.6 Gly and have a mean log HI mass of 9.4 (Fig. 4). From their distributions in the cube, we have noticed that the pointing center of the telescope beam is more sensitive to low mass galaxies (< 10^9 solar mass), while the edge is almost only sensitive to massive nearby galaxies (Fig. 3). For each detection a small cubelet was extracted for which we generated a spectrum (flux vs. velocity) indicating the presence of HI emission, their moment 0 maps featuring their overall gas distribution, and their moment 1 maps showing the gas motion (Fig. 6).
A few detections were also showing the presence of a dwarf satellite companion next to the main large spiral galaxy. One of them was previously found by Meyer et al. (2004), obtained with the single dish Parkes telescope, which could not resolve the object into its two components. With MeerKAT’s excellent spatial resolution and sensitivity, various sources are now resolved and found to be composed of a main galaxy and a satellite companion (Fig. 5).