Unexpected discovery of faraway neutral hydrogen
While developing the data analysis pipeline for MeerKAT imaging projects, the IDIA team stumbled upon strong detections of hydrogen in galaxies far away, which was not expected from early low-resolution MeerKAT data for the MIGHTEE experiment. These detections have been confirmed and are proving an exciting science result complementing other MeerKAT experiments.
Astronomers use hydrogen to trace the evolution of galaxies in the universe, specifically neutral hydrogen. Hydrogen is the most abundant element in the universe and is therefore one of the best probes into the universe and into galaxies. But hydrogen cannot be seen by optical telescopes. Radio telescopes like the MeerKAT are needed to look for hydrogen both nearby and far away. Understanding the formation and evolution of galaxies using hydrogen is in fact one of the main science goals of the SKA telescope.
Several of the MeerKAT projects are also focusing on mapping hydrogen. MIGHTEE looks at hydrogen relatively far away and LADUMA will be the deepest (i.e. furthest) survey of hydrogen in the universe so far. The Fornax survey will map hydrogen in the nearby Fornax galaxy cluster, a group of galaxies held together by gravity.
The IDIA pipeline team, led by Dr. Bradley Frank, was developing the imaging tools and testing those on some initial data from MeerKAT for the MIGHTEE experiment. This data was 1/8th of the resolution of a full MeerKAT dataset. Such data sets are often used in the development and testing of tools for data analysis. It wasn’t expected that data at this resolution would be enough to detect any neutral hydrogen from faraway galaxies. Yet, that is exactly what happened.
As the data was analysed, what appeared was unambiguous detections of neutral hydrogen. The team was compelled to look for counterparts to those detections in other data sets and found galaxies at the exact locations of the detections, in the existing Sloan Digital Sky Survey (SDSS) – an optical survey of the sky.
This was a lucky find, as most astronomers are waiting for the full resolution data from MeerKAT. For comparison, while one of those full resolution data sets can take up to 17 hours to be turned into a data cube that astronomers can analyse. 20 data cubes of the lower resolution data can be processed in the same amount of time and computing.
In March and April 2019, the team started to clean up some of the lower resolution data cubes to look for further detection of hydrogen in galaxies. More than 30 detections were found in one small slice of one cube, all with optical counterparts in SDSS too. Some previously known hydrogen–heavy galaxies were found, but many fainter ones also showed up in the data. Structures such as a group of galaxies was detected, and one galaxy was found to have a companion.
The rest of the volume of data remains to be looked at, but this is already beyond anything astronomers could have expected. This is particularly interesting because while LADUMA will measure hydrogen in galaxies over a much wider range of masses, MIGHTEE is finding galaxies in a diversity of cosmic environments, such as large scale cosmic structures; walls, voids, clusters, etc.