Large galaxies, like our own starlit spiral Milky Way, are usually accompanied by a sparkling entourage of smaller galactic satellites that travel on bound orbits around their massive, luminous host. This is similar to the way that planets within our own Solar System are gravitationally bound to our Sun. These smaller satellites lead tumultuous lives because of their messy interactions both with other satellites and their larger host galaxy, known as the primary galaxy. However, astronomers have learned to expect the unexpected when it comes to objects that dance their weird way throughout the observable Universe, and the distant, ancient galaxy dubbed Messier 94 (M94) has proven to be full of surprises. In January 2019 a team of astronomers announced their new findings that, even though M94 is about the same size as our Milky Way–that is richly endowed with a family of circling satellites–they have detected only two galaxies orbiting M94. Also, the distant dancing duo have very few stars each.
In astronomy long ago is the same as far away. The more distant an object is in Space, the more ancient it is in Time (Spacetime). The discovery of the relatively isolated M94 suggests that fewer galaxies were born in the early Universe than astronomers expected. This possibility could potentially create new questions for galaxy physics, according to the study conducted by University of Michigan (Ann Arbor) astronomers.
It has been known for a long time that our Milky Way is accompanied by about 10 smaller satellite galaxies that circle it, each hosting at least a million fiery stars. Indeed, our Milky Way’s largest satellite, the Large Magellanic Cloud (LMC) contains up to more than a billion stars.
Using the powerful Subaru Telescope, astronomers can now study galaxies five to 10 times the distance from our Milky Way, such as M94. They can use the physics explaining how satellite galaxies are born surrounding our Millky Way in order to predict the number of satellite galaxies a similar-sized galaxy may have.
So, for this reason, when the University of Michigan astronomers peered at M94, they expected to see a similar number of satellite galaxies in orbit around it. Alas, they only found the lonely, almost completely starless, dancing duo. Their results, led by Dr. Adam Smercina, are published in the journal Astrophysical Letters. Dr. Smercina is a National Science Foundation (NSF) fellow in the University of Michigan’s Department of Astronomy.
“More than just an observational oddity, we show that the current crop of galaxy formation models cannot produce such a satellite system. Our results indicate that Milky Way-like galaxies most likely host a much wider diversity of satellite populations than is predicted by any current model,” Dr. Smercina explained in a January 9, 2019 University of Michigan Press Release.
Galaxies are gravitationally bound systems of stars, dust, gas, stellar relics, and mysterious non-atomic dark matter. Galaxies come in different sizes, and can range from small dwarfs hosting only a few hundred million stars to giants that contain one hundred trillion stellar constituents, each in orbit around its galaxy’s center of mass.
In astronomical literature, the capitalized word “Galaxy” usually refers to our own Milky Way, thus distinguishing it from other galaxies. The English term Milky Way has been traced back to a story written by the English poet Geoffrey Chaucer (1340?-1400) in 1380:
“See yonder, Lo, the Galaxye
Which men clepeth the Milky Way,
For hit is whyt.”
Geoffrey Chaucer, The House of Fame
Galaxies are categorized according to their observed morphology, and are generally designated as spiral, elliptical and irregular–although the situation is somewhat more complex. Many, if not all, large galaxies are thought to host a supermassive black hole in their active hearts. Our own Milky Way’s supermassive heart of darkness is dubbed Sagittarius A* (pronounced Sagittarius-A-Star) –or Sgr A*, for short. It has a mass of about four million Suns, and it is dormant, only becoming active now and then when a buffet consisting of a star or cloud of gas floats too close to the gravitational snatching claws of this “frumious Bandersnatch” residing in the heart of our Galaxy.
Estimates of the number of galaxies that inhabit the observable Universe range from 200 billion to a trillion–or more. It has also been determined that there are more stars in the observable Universe than all of the grains of sand on Earth. Most galaxies are about 3000 to 300,000 light-years in diameter and are separated by distances on the order of megaparsecs (millions of parsecs). By comparison, our own Milky Way sports a diameter of at least 100,000 light-years and is separated from the Andromeda galaxy, its nearest large spiral neighbor, by 2.5 million light-years.
The space between galaxies is not empty–although it is close to it. Instead, intergalactic space is filled with a tenuous gas that is called the intergalactic medium, and it has an average density of less than one atom per cubic meter. Most galaxies are gravitationally organized into groups, clusters, and superclusters. Our own Milky Way is a member of the Local Group, which is dominated by it and the Andromeda galaxy, and is a part of the Virgo Supercluster. On the largest scale, these galactic associations are arranged into sheets and filaments surrounded by black, and almost empty, voids. The largest known structure of galaxies is a cluster of superclusters named Laniakea, which hosts the Virgo Supercluster. On this scale, the Universe resembles a honeycomb or a natural sponge, with filaments and voids intricately intertwined. Indeed, some astronomers think that the large-scale structure of the Universe is composed of only one filament wrapped around a single void.
Before the 20th century, astronomers generally thought that our Milky Way was the only galaxy in the Cosmos, and the existence of other galaxies was not well established. Indeed, the idea that other galaxies danced throughout the Universe was so controversial at that earlier era that it led to what is now called the Shapley-Curtis Great Debate, named after the two American astronomers Harlow Shapley (1885-1972) and Heber Doust Curtis (1872-1942). During this debate, the two scientists articulated their opposing views on the identity of “nebulae” and the size of our Galaxy. The debate was held at the National Academy of Sciences in Washington D.C. on April 26, 1920. Shapley contended that the Milky Way was the entire Universe, and that all of the observed nebulae (clouds)–which are now recognized as being galaxies in their own right–resided within the Milky Way. In dramatic contrast, Curtis argued correctly that the Milky Way was smaller than the entire Universe, and that the observed nebulae were really other galaxies similar to our own.
At last, in late 1923, the astronomer Edwin Hubble–referred to as the “father of modern observational astronomy”–measured the distance to the Andromeda galaxy (M31) using a type of variable star called Cepheid Variables to make his measurements. By measuring the period of these variable stars, Hubble was able to calculate their intrinsic luminosity and upon combining this with their measured apparent magnitude he arrived at a distance of 300 kiloparsecs, which is an order of magnitude greater than the estimated size of the Universe made by Shapley. This measurement verified that not only was the Universe much, much bigger than previously proposed, it also revealed that the observed nebulae were really distant galaxies with a wide variety of morphologies.
Despite Hubble’s discovery that the Universe played host to myriad galaxies, most satellite galaxies of our Milky Way and the entire Local Group remained undiscovered, and undiscoverable, until the advent of modern astronomical surveys–although the duo of small satellite galaxies, the Large and Small Magellanic Clouds, have been observable in the Southern Hemisphere with the unaided eye since ancient times.
A Lonely Galaxy Far, Far Away
Dr. Smercina noted in the January 9, 2019 University of Michigan Press Release that the new observations of the lonely galaxy M94 may have important implications for the current understanding of how galaxies are born and evolve–which takes place within “cradles” of the mysterious form of material called dark matter. The dark matter is much more abundant than the ordinary atomic matter that composes the world that human beings find most familiar, and it is thought to be made up of exotic non-atomic particles that refuse to dance with light or any form of electromagnetic radiation. This means that the ghostly dark matter is invisible. However, most astronomers think that this transparent material really exists because of its gravitational effects on objects that can be observed, such as stars and starlit galaxies.
The halos composed of the dark matter that surrounds galaxies possess a powerful gravitational force. For this reason, these dark matter halos can pull in gas from their nearby surroundings. Large galaxies like our own and Andromeda usually form halos of approximately the same mass. However, the smaller satellite galaxies, which are surrounded by smaller “subhalos”, seem to travel to the beat of a different drum.
The birth-rate of high-mass stars in these small satellite galaxies actually modulates their growth. This means that a nascent satellite galaxy gives birth to too many high-mass stars at any one time, and their eventual supernova blasts might evict all of their gas screaming into space–thus causing all further growth to come to a screeching halt. The more massive the star, the shorter its hydrogen burning “life” on the Hertzsprung-Russell Diagram of Stellar Evolution. Massive stars live fast and die young, and a fatal supernova blast usually heralds their demise. However, astronomers have not yet determined at what size halo this “scatter” in galactic formation becomes important.
Dr. Smercina continued to explain that M94 suggests that galaxy formation in intermediate-sized dark matter halos may be much more mysterious than previously believed.
“We think that that scatter–the range of galaxies we expect to see–may be a lot higher than what people currently think for dark matter halos of a certain mass. Nobody’s under any illusions as to there being this huge scatter at the very lowest halo masses, but it’s at these intermediate dark matter halos that this discussion is happening,” Dr. Smercina added.
In order to observe the number of dwarf satellite galaxies surrounding M94, the astronomers took a composite image of the large galaxy. The image obtained covered approximately 12 square degrees of the night sky. By comparison, Earth’s Moon appears as about one square degree. This kind of image includes layers upon layers of “noise”. This “noise” includes scattered light and cosmic rays, which make dim dwarf galaxies hard to detect.
In order to make sure that they weren’t missing satellite galaxies surrounding M94, the scientists engineered artificial galaxies into the image and recovered them using the same methods that would be used for real satellite galaxies. By using this technique, the Dr. Smercina and his team were able to confirm that there were no more that two galaxies dancing around M94.
Dr. Smercina went on to explain that “The real kicker is whether or not the community expected this could be possible. That is the real curiosity of this finding–the result is something the simulations don’t predict. When you can discover something we didn’t really think we could find, you can make a contribution to our understanding of how our Universe works, that’s really rewarding.”