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How do we measure the distance to a galaxy and why is it so important?

On March 3, 1912, Henrietta Swan Leavitt made a short contribution to the Harvard College Observatory Circular. With it she laid the foundations of modern Astronomy. Locked in solitude due to her deafness, Leavitt was the first person to discover how to measure distance to galaxies, thus expanding our understanding of the Universe in one giant leap.

Leavitt discovered that the period of oscillation of Cepheid stars, a type of variable stars, is related to their intrinsic brightness. Brighter Cepheids take longer to complete their oscillation cycle. Simply finding Cepheids on other galaxies and measuring their oscillation period would allow one to measure the distance to such objects based on their apparent brightness. Leavitt’s result is so important that it would have earned her a Nobel Prize if it had not been for her premature death.

Leavitt’s discovery was the first step of what we know today as the cosmic distance ladder, a set of techniques that allows us to measure the distance to objects located millions of light years away. Although the use of Cepheid stars is one of our best tools for measuring distances to nearby galaxies, we cannot always find these variable stars in other galaxies. For this reason, astronomers have been developing multiple (but less secure) alternatives to measure distance to galaxies. One possibility, for example, is to measure the brightness of red giant stars when they reach the brightest phase during their stellar evolution. This happens at a well-defined brightness that is known as the Tip of the Red Giant Branch. This technique gives very precise distances when galaxies are relatively close. However, there are times when the detection of individual stars is at the limit of the capacity of the telescopes. On these occasions, astronomers can resort to other techniques that are based on the variation of brightness from one area to another of the galaxies. As galaxies are made up of individual stars, if we observe relatively small regions of a galaxy we will appreciate that they do not have exactly the same number of stars and therefore their brightness can change from one area to another. The closer the galaxy is, the greater will be the variation from one region to another, since the number of stars observed in the same angular region will be lower. This way of measuring distances is known as surface brightness fluctuation (SBF) technique and it is considered a secondary distance indicator as it is not as reliable as those tools based on individual stars.

Henrietta Swan Leavitt by unknown. Public domain via Wikimedia Commons.

While astronomers continued to refine their methods of measuring distances to galaxies, a revolution in astronomical imaging techniques was taking place. In recent years, new observation strategies have allowed us to discover galaxies that are thousands of times fainter than the brightness of the night sky in terrestrial observatories. Suddenly, thousands of galaxies with very low surface brightness appeared in images obtained by telescopes. These galaxies have come popular under the name of Ultra Diffuse Galaxies and their ultimate nature is the subject of a very lively debate within astronomers. As with any astronomical object, the first problem posed to astronomers was to measure the distance at which these galaxies are located. The measurement of distance is key if we want to unravel the properties of these newly discovered galaxies.

Of all the galaxies discovered to date, one has generated massive attention: KKS2000 04 (also known as NGC1052-DF2). This galaxy, located in the constellation of Cetus, has aroused much interest because of its apparent absence of dark matter. This is something that is difficult (if not impossible) to understand within the current framework of galaxy formation. The amount of dark matter that we infer in a galaxy depends strongly on the distance at which the object is located. As a general rule, if we incorrectly place the galaxy very far away, scientists measure less dark matter. Using the SBF technique, to measure the distance to KKS2000 04 resulted in a distance of 64 million light years. A galaxy at that distance would represent the first example of a galaxy without dark matter. However, if the galaxy were closer, its content of dark matter would be greater and the object would not be anomalous.

Puzzled by the fact that all the properties of KKS2000 04 that depend on the distance were anomalous (their dark matter content and the brightness of their global clusters), it was important to explore other different distance indicators in addition to the SBF technique. Surprisingly, all the other methods (including the more secure Tip of the Red Giant Branch estimator) agreed on a much closer distance to the galaxy (of only 42 million light years away). With this revised distance, all the properties of the galaxy are normal, and fit well within all the observed trends traced by similar galaxies of low surface brightness. So, what is wrong with the SBF technique? Why does it produce larger distances for ultra-diffuse galaxies like KKS2000 04?

The SBF technique has been so far only explored in bright galaxies, where the density of stars (and therefore their brightness) is very high. These galaxies are very different compared to objects with a very low density of stars such as KKS2000 04. Would that mean that the SBF technique should be revisited when dealing with ultra-diffuse galaxies? New works indicate that this must be the case. In fact, without such a revision for diffuse galaxies, the SBF tool provides greater distances than those at which the object is actually located. This does not mean that the SBF technique cannot be used to measure distances for diffuse galaxies, but rather that it’s time to recalibrate the technique. With the new calibration, adapted to this type of galaxy, the distance to KKS2000 04 coincides among different methods and the problem of the absence of dark matter in this galaxy is solved.

More than 100 years later, heirs to a tradition that began with Henrietta Leavitt, the results of this work show once again the fundamental relevance of having accurate distances to extragalactic objects. For a long time, this has been (and continues to be) one of the most difficult tasks in Astronomy: how to measure the distance to objects we cannot touch.

Featured image credit: Colour composite image of [KKS2000]04 combining F606W and F814W filters with black and white background using g-band very deep imaging from Gemini. The ultra-deep g-band Gemini data reveals a significant brightening of the galaxy in the northern region. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society

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