New Discoveries - Lecture 23 - MeerKAT Detects the Faint Hydrogen Glow of the Distant Universe

Greetings and welcome to the introduction to astronomy. In this episode of new discoveries in astronomy, we will talk about how astronomers used the Mircat radio telescope to detect the faint collective signal of neutral hydrogen from the distant universe. So, let's get started. How can astronomers map galaxies that are too distant and faint to detect individually? Normally, we might imagine finding one galaxy, measuring it, and then moving on to the next. But when astronomers want to study enormous volumes of the universe, there is another approach. Instead of identifying every galaxy separately, they can search for the combined radio emission from neutral hydrogen distributed among many galaxies. And a new mircat observation shows that this extremely faint signal can now be detected directly. Neutral hydrogen produces a characteristic radio signal known as the 21 cm line. As this radiation travels across the expanding universe, its wavelength is stretched or redshifted. That means astronomers can use its observed wavelength not only to detect hydrogen but also to determine approximately where in cosmic history that emission originated. There is a problem, however. At cosmological distances, the signal is extraordinarily faint. Astronomers must separate it from the much brighter foreground radio emission, human-made radio interference, and subtle instrumental effects. So, the challenge is not simply receiving radio waves. The challenge is extracting a very weak cosmological signal from [snorts] everything else the telescope detects. The researchers tackled that problem using about 96 hours of observations from Mircat in South Africa. And rather than attempting to detect neutral hydrogen in each distant galaxy separately, they used a technique called hydrogen intensity mapping. With intensity mapping, astronomers measure the collective radio brightness produced by hydrogen across a large region of space. Think of it as measuring the glow of a distant city without needing to identify every individual light bulb. The individual galaxies do not have to be resolved as separate hydrogen detections. Instead, small variations in the combined 21 cm emission reveal how neutral hydrogen is distributed across very large cosmic volumes. And in this study, the researchers detected that intensity mapping signal directly from the Mircat radio observation alone. That distinction is important. Previous reliable detections at these distances have typically depended on combining radio measurements with information from optical galaxy surveys. Here Mircat's radio data provided the direct detection. The researchers detected hydrogen emission from two periods in cosmic history at red shifts of approximately 32 and 0.44. The radio waves they measured had been traveling toward us for roughly 4 to 5 billion years. And the measurements trace variations in hydrogen across scales of several million light years. In other words, astronomers are no longer limited to asking where individual hydrogen-rich galaxies are located. They can begin measuring the collective distribution of hydrogen atoms across enormous regions of the universe. This is where intensity mapping becomes especially powerful. Position on the sky gives astronomers two dimensions. The red shift of the 21 cm signal provides information about depth. Together, those measurements can be used to construct a three-dimensional picture of how hydrogen is distributed throughout space and cosmic time. And because neutral hydrogen is closely connected with galaxies and their evolution, those maps can help astronomers investigate how largecale cosmic structure developed over billions of years. Future larger surveys could therefore provide a new way to study both galaxy evolution and the distribution of matter across the universe. There is another interesting aspect of this result. The observations analyzed in this study were collected in 2018 when Mircat had only recently begun science operations. They were not originally designed for hydrogen intensity mapping. The signal was already present in the data. What changed was the ability of astronomers to isolate and interpret it. That is part particularly encouraging for future surveys with Mircat and eventually the square kilometer array where intensity mapping is expected to become an important cosmological technique. [snorts] This discovery in illustrates an important change in how we map the distant universe. We do not need to see every galaxy individually. Sometimes the collective signal contains the information we need. By learning how to recover the faint radio glow of hydrogen from beneath much stronger signals, astronomers can begin mapping structures that would otherwise remain hidden. Sometimes the universe becomes visible when we stop looking at one galaxy at a time. So that concludes our discussion on Mircat detects the faint hydrogen glow of the distant universe. We'll be back again next week for another new discovery in astronomy. So until then, have a great day everyone and I will see you in class.