The early universe can be studied by looking at light coming from distant galaxies. The further away the galaxy is, the longer it takes light to reach us, allowing us to see extremely distant galaxies as they were billions of years ago. However, the further back in time we look through this method, the more difficult it becomes to clearly resolve all the features of the galaxies we are looking at.
An easier task, and one that researchers have been doing for decades, is to study the gas around early galaxies. When light passes through this gas, certain wavelengths of light are absorbed and others are not. As a result, there are certain gaps or lines in this light. But until now, researchers mostly couldn’t go: they couldn’t learn much about the galaxies themselves.
In a new study, researchers directly imaged two galaxy-like galaxies through observations from ALMA (the Atacama Large Millimeter/submillimeter Array). The galaxies appear to have the properties of massive, star-forming galaxies.
The birth of galaxies
The first galaxies are believed to have formed from a largely uniform distribution of gas and dark matter. This material was not precisely however uniform: there were regions that were denser than average. These dense regions began to collapse due to the gravitational pull, eventually forming the galaxies we see today.
Stars are also formed when gas contracts due to gravity, so the more gas a galaxy has, the more stars it can form at any given time. But the original galaxies are not expected to contain enough gas to account for the rate of star formation we see in them, so the galaxies must get more gas from their surroundings. It is possible that the earliest galaxies were surrounded by giant halos of gas, stretching many tens of thousands of light-years beyond the galaxies’ disks.
For that reason, researchers are very interested in studying the gas clouds around early galaxies. This could increase our understanding of the formation and evolution of galaxies. Neutral hydrogen is the most common gas, and while its emission is difficult to detect, it is relatively easy to spot hydrogen absorption lines when the gas is between us and a light source. Researchers can use the light from an object, such as a quasar, that lies behind the gas. The light passes through the gas to reach us, allowing us to identify the elements in the gas.
But if the gas is associated with a galaxy, it’s difficult to study the galaxy. That’s because any bright light sources behind it interfere with imaging, and because the gas also absorbs light from the galaxy. As a result, we have largely studied the nearby gases.
This problem can be circumvented by looking into other wavelengths, such as the submillimetre wavelengths in which ALMA specializes. The researchers used ALMA to search for the galaxies associated with two hydrogen-filled gas halos. The special objects have been selected for their above-average metal content. (In astronomy, “metals” are simply all elements other than hydrogen and helium on the periodic table). And thanks to ALMA’s capabilities, they were able to find both galaxies.
The researchers were able to resolve the galaxies well enough to detect their rotation. The portion of a galaxy’s disk moving away from us has a spectrum shifted to the longer wavelength, the red end of the spectrum, or red-shifted; the part coming towards us is shifted blue. Comparing these parts of the galaxy gives an idea of how fast it rotates.
This allowed the researchers to estimate the mass of one of the galaxies. One of the two has at least 60 billion times the mass of the sun. (The other had more complicated dynamics, making it difficult to get a clear mass estimate.) They were also able to get an estimate of the star formation rates of both galaxies, finding that both speeds are “moderately high”. That result is comparable to other bright galaxies previously observed.
Their results indicate that these galaxies, with their hydrogen-rich gas clouds, have characteristics similar to a type of previously identified star-forming galaxies identified by their lack of ultraviolet brightness. The envelope of neutral hydrogen gas surrounding the galaxies does not appear to be moving away from the host galaxies, implying that the galaxies will likely attract the gas and eventually convert it into stars. This could help researchers build better models of the early formation and evolution of galaxies.
Science2017. DOI: 10.1126/science.aal1737 (About DOIs)