Have Astronomers Found a Galaxy Without Dark Matter? Not so Far
Dark matter is fundamental to understanding how galaxies form, so it came as a surprise in 2018 when astronomers reported finding a galaxy with no dark matter within its structure. New research shows that idea is likely in error, and NGC 1052-DF2 is much closer to the Earth than found in the original study, affecting the calculation of how much dark matter the galaxy contains.
Astronomers announced the discovery of a “galaxy without dark matter,” and the story was covered by numerous science news outlets. However, nearly all the data regarding the distance to this galaxy deviated from expected values, leading researchers to take a closer look at this object. They found the galaxy is significantly closer to Earth than previously calculated. Analysis of data, with the revised distance from Earth, reveals NGC 1052-DF2 does possess a normal amount of dark matter for a typical ultra-diffuse galaxy.
“The original article published in Nature stated that the galaxy is at a distance of some 64 million light years from the Earth. However, this new research has revealed that the real distance is much less, around 42 million light years. Thanks to these new results, the parameters of the galaxy inferred from its distance have become ‘normal’ and fit the observed trends traced by galaxies with similar characteristics,” the Astrofísica de Canarias (IAC) reports.
This new data suggests this galaxy has a mass just half that previously calculated. However, the total quantity of all visible stars within the system is only one-quarter of the mass in earlier measurements, suggesting dark matter makes up roughly half of what lies within the galaxy.
Before there were Computers, there were the Computers
Measuring the distance to far-flung galaxies is a challenging task, but there are numerous methods by which distances can be discovered.
In 1912, astronomer Henrietta Leavitt realized that Cepheid variable stars shine at specific brightnesses, dependent entirely on their cycle. Brighter Cepheids take longer than dimmer ones to complete each cycle. By knowing where such a star is in its cycle allows astronomers to determine its intrinsic brightness. Comparing this to its apparent brightness as seen from Earth permits a calculation of the distance the Cepheid sits from our home world. This discovery gave astronomers the first tool ever devised to accurately measure distances to far-flung galaxies.
Leavitt was one of the original human “computers” working at Harvard University. Her deafness drove her toward solitary activities, including mathematics and astronomy. At the time, women were considered to be better-suited for tedious rote work. The fact that women could be paid significantly less than men made it so that most of the computers employed in the US at the time were female.
Meanwhile, Harvard forbade women from using their telescopes, leaving Leavitt with her calculations on Cepheid variables. She was the first to find that while Cepheids varied in intensity over periods ranging from days to weeks, the brightest members had the longest cycles.
And if Plan A Doesn’t Work, Bring in the Big Stars
However, it is not always possible to measure the brightness of these variable stars. In these cases, other methods must be employed. One means utilized to accomplish this task involves measuring the brightness of red giant stars as they reach the brightest point of their evolution. This method is only effective for galaxies which are close enough to distinguish individual stars. A third method of determining the distance to galaxies takes advantage of the fact that stars are unevenly distributed around galaxies. Therefore, astronomers can measure the brightness of one region of a galaxy, and compare it to another region. Observers see the greatest variation in galaxies closer to the Earth.
“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,” writes Ignacio Trujillo of the IAC.
This SBF method was used to calculate the distance to NGC 1052-DF2, producing a result of 64 million light years from Earth. Using this figure to determine the properties of the galaxy suggested that this object did not contain dark matter, a characteristic which has never before been observed in galaxies.
However, all other distance indicators, including an examination of red giants, suggested a much lower distance to the galaxy — just 42 million light years. If the galaxy were located at that distance, all the properties of the object would be normal, including the contribution of dark matter to the mass of the structure.
Maybe You CAN be Too Thin…
Ultra Diffuse Galaxies like NGC 1052-DF2, as their name suggests, are thinly populated by stars. These are among the most-recently discovered class of galaxies. Given their lack of obvious targets, it is challenging to measure the distance to these collections of stars. Researchers suggest the SBF technique is especially susceptible to errors when studying these tenuous objects.
Dark matter can not be seen (hence the name), as it does not emit or radiation as normal matter does. However, the gravitational influence of dark matter holds star clusters together and affects the rotational rates of stars around galaxies.
“Any man who can hitch the length and breadth of the galaxy, rough it, slum it, struggle against terrible odds, win through, and still knows where his towel is is clearly a man to be reckoned with.”
— Douglas Adams
No one is certain what dark matter is, but it makes up a large majority of all the mass in the Universe — around five times all matter we can see around us. More than 95 percent of everything in the Cosmos is unseen, coming in the form of either dark matter or dark energy. Every star, planet, moon, black hole, and particle of dust we see, put together, make up just one small part of the Cosmos.
Current theories state that dark matter drives the collapse of gas into stars, making the presence of this enigmatic “something” essential to the formation of galaxies. It seems this diffuse galaxy is no exception to the rule that galaxies everywhere are intricately linked with dark matter.