One of the most impressive accomplishments of the Chandra mission has been the improved understanding of the distant X-ray Universe. Chandra has accomplished this through deep X-ray surveys that generally involve pointing Chandra at a particular region of the sky that is not known to have any bright nearby objects and letting the camera collect X-ray light for an extended period of time.
The deepest of these surveys are the Chandra Deep Fields, which consist of two small patches of the sky in a northern and southern region that are collectively twice the size of the full moon. Over Chandra's nine years of operation these fields have been spied on by Chandra for a total of nearly two months, which have returned some impressive images and informative data of sources in the distant Universe. In addition to the Chandra observations, world-class observatories that observe at other wavelengths of light (the Hubble Space Telescope, Spitzer Space Telescope, and many other telescopes) have invested large amounts of time observing the Chandra Deep Fields to give multi-wavelength information about the distant objects.
In my research, which is done in collaboration with an international team, we use the Chandra Deep Fields to acquire an understanding of what typical galaxies in the distant universe are like from an X-ray perspective; this is one of many different scientific areas being studied using the Chandra Deep Fields. As we look to more and more distant galaxy populations, we are seeing them as they were earlier and earlier in the history of the Universe. In the Chandra Deep Fields, we can see galaxies in X-rays at a variety of distances ranging from nearby galaxies to galaxies that are so distant that the Universe was only 1-2 billion years old (about 10% of the age of our roughly 13.5 billion year old Universe) when the X-ray light that we observe was emitted. By observing galaxies over this broad range of cosmic history, we are able to see the progression of how these galaxy populations have changed and evolved over the last 10 billion years of the Universe's history.
For typical galaxies, X-rays come from a combination of sources including young binary stars that contain an ordinary star that dumps material (through a powerful stellar wind or normal changes in the size of the star through evolution that allows the stellar envelope to overflow) onto a compact neutron star or black hole; this material is then heated to produce X-rays as it falls in. Additional X-ray emission in galaxies is often supplied by a hot interstellar medium of gas, supernovae and supernova remnants that interact with the interstellar medium, and atmospheric winds from young stars. The total X-ray emission coming from a galaxy is related to the amount of star formation going on in that galaxy. So the larger the intensity of galaxy growth (through star formation) the brighter the galaxy will appear in X-rays. As anyone who has ever had an X-ray taken by a doctor knows, X-rays are very penetrating and can therefore allow astronomers to see sites of galaxy growth that are often blocked by intervening gas and dust at other wavelengths of light.
In our work with the Chandra Deep Fields, we have found that as we look to more and more distant galaxy populations, we find the X-ray emission to be more and more powerful on average, a direct signature that provides clues for how galaxies assembled and grew into galaxies that we observe in the nearby Universe. Our work has revealed that over the last 10 billion years, giant massive galaxies have relatively weak X-ray emission in comparison to their mass, suggesting that most of their growth must have occurred very early in the history of the Universe. By contrast, over the same time period, lower mass galaxies have more intense X-ray emission in relation to their mass, signifying that they are growing more intensely than their high mass cousins.
This research pushes the limits of Chandra's capabilities (or those of any other X-ray observatory yet) for studying typical galaxies in the distant Universe and can only be achieved through large investments of telescope time. However, this work has opened up new and exciting avenues of research for future generations of X-ray telescopes that will allow astronomers to study in greater detail the evolution of typical galaxies by using their X-ray properties.
Bret Lehmer is an astrophysicist who is currently studying the distant Universe from his vantage point at Durham University in the United Kingdom.
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