The Strange Case Of The Sombrero Galaxy

Myriad sparkling stars light up the billions of galaxies that dwell in the observable Universe. The observable, or visible, Universe is that relatively small domain of the unimaginably vast Cosmos that we are able to observe.

The Sombrero Galaxy

The Sombrero Galaxy - Source: By Pablo Carlos Budassi - Own work, CC BY-SA 4.0,

The light traveling to us from more distant regions has not had enough time to reach us since the Big Bang. This is because of the expansion of Space, and the universal speed limit set by light. No known signal can travel faster than light in a vacuum, although Space itself can, and so the very secret of our existence may reside in regions of Spacetime that are far beyond the horizon of our visibility. The galaxies of the Cosmos are far away and mysterious, and the Sombrero Galaxy (Messier 104) stands out in the crowd as one of the most bewitching and bewildering of its starlit kind. In February 2020, a team of astronomers announced that evidence derived from the Hubble Space Telescope (HST) indicates that the Sombrero's many weird and unexplained attributes are the result of major galaxy mergers--even though its smooth disk displays no signs of a recent catastrophic disruption. However, the Sombrero's disk may hide the secret of a turbulent past.

The Sombrero has long been a seductively tantalizing object because it seems to travel to the beat of a different drum than other known galaxies. It displays a mystifying mix of shapes found in disk-shaped spiral galaxies (like our own Milky Way), as well as football-shaped elliptical galaxies. The tantalizing mystery of how it acquired its unusual structure becomes ever more bewitching and bewildering with the new evidence from the HST.

The galaxy's faint halo provides some tattle-tale clues. It is splattered with innumerable stars that are well-endowed with heavier atomic elements--called metals by astronomers. This is because they are later-generation stars. In the terminology astronomers use, a metal is any atomic element heavier than hydrogen and helium, and so the same term has a different meaning for astronomers than it does for chemists. The Big Bang produced only hydrogen, helium, and traces of lithium--but the stars created all the rest. The first generation of stars to dance in the Cosmos were the first to cook up the heavier atomic elements in their nuclear-fusing hearts--and then they sent them screaming into Space when they went supernova. The newly-forged metals were eventually incorporated into later generations of stars. The first stars (Population III) were born depleted of heavy metals, because no stars existed before them to cook them up. The second generation of stars (Population II) were almost, but not quite, depleted of metals, because they were "polluted" with the batch produced in the searing-hot hearts of the first stars. The youngest generation of stars (Population I)--of which our Sun is a member--contain the largest quantity of metals, having received these elements from previous generations of stars.

For this reason, stars with an abundance of heavy metals are usually seen only in a galaxy's disk. The Sombrero's metal-rich stars must have been hurled into its halo, as the result of ancient mergers with mature galaxies, that were heavily endowed with metals. The Sombrero galaxy, in its current "adulthood", is more settled than it was in its "youth". It is also isolated. This means that there is nothing else dwelling nearby for it to "eat". This discovery provides a new twist on the way galaxies form in our Cosmos.

"The Sombrero has always been a bit of a weird galaxy, which is what makes it so interesting. Hubble's metallicity measurements (i.e.: the abundance of heavy elements in the stars) are another indication that the Sombrero has a lot to teach us about galaxy assembly and evolution,"

commented Dr. Paul Goudfrooij in a February 20, 2020 Hubblesite Press Release. Dr. Goudfrooij is of the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

"Hubble's observations of the Sombrero's halo are turning our accepted understanding of galaxy makeup and metallicity on its head,"

added co-investigator Dr. Roger Cohen, who is also of the STScI.

HST's sensitivity was able to resolve tens of thousands of individual stars inhabiting the Sombrero's vast, extended halo. The halo is the region situated beyond a galaxy's central portion, that is typically composed of older stars. These recent observations of the Sombrero are intriguing because they show only a small percentage of older, metal-poor stars in the halo, plus the abundance of metal-rich stars in its disk and central bulge. Therefore, ancient, turbulent galaxy collisions and major mergers provide a possible explanation.

Strange And Beautiful

The Sombrero has captivated skywatchers for years because of its strange structure and great beauty. However, thanks to HSTs recent observations, astronomers are now seeing the Sombrero in a new light. The galaxy displays an extended halo brimming with metal-rich stars with barely any evidence of the predicted metal-poor stars observed in the halos of other galaxies. Astronomers, pouring over the data from the HST, have turned to sophisticated computer simulations to find a solution to this perplexing puzzle that poses a challenge to conventional galaxy-formation theory.

Those results indicate the surprising possibility that major mergers occurred in this weird galaxy's past, even though the Sombrero's elegant and lovely structure shows no evidence of recent disruption. The results of these new findings are published in the Astrophysical Journal.

"The absence of metal-poor stars was a big surprise, and the abundance of metal-rich stars only added to the mystery,"

Dr. Goudfrooij noted in the February 20, 2020 Hubblesite Press Release.

The Sombrero got its name because it resembles the broad rim and high-topped Mexican hat with this name--and, observed from Earth, it is seen almost edge-on. This galaxy is also very bright, and it is easily observed with small telescopes. Because of its brightness, it is just a little beyond the limit of the unaided human eye. The strange and beautiful galaxy resides at the southern edge of the heavily populated Virgo Cluster, and it is one of the most massive objects in that group of galaxies. Indeed, its mass is equivalent to 800 billion Suns. The Sombrero is 50,000 light-years across and is situated 28 million light-years from Earth.

The Sombrero also hosts a heavy population of globular clusters, which are tightly bound spherical collections of stars. It is estimated that this galaxy is orbited by nearly 2,000 globulars--10 times as many as orbit our own barred-spiral Milky Way Galaxy. However, the ages of the clusters are similar to the ages of those circling our Galaxy, ranging from 10 to 13 billion years old.

A smaller disk is embedded within the Sombrero's bright core, and it is tilted relative to the large disk. X-ray emission indicates that material is swirling down into the galaxy's compact core, where a supermassive black hole resides in sinister secret, waiting for its dinner to come tumbling down into its voracious maw. This supermassive heart of darkness weighs in at 1 billion times our Sun's mass. In contrast, our Milky Way's resident supermassive black hole weighs "only" 4 million times solar mass.

In the 19th century, some astronomers suggested that the Sombrero was just an edge-on disk of brightly shining luminous gas encircling a youthful star--which is the way our own Solar System evolved. However, in 1912, the American astronomer V.M. Slipher (1875-1969) found that the strange object appeared to be flying away from Earth at 700 miles per second. That great velocity provided some of the earliest clues that the Sombrero was really another galaxy, and that the Universe was expanding in all directions.

How Bizarre!

Astronomers expect to find earlier generations of stars, with very small quantities of metals, in a galaxy's halo--as compared to the more densely populated regions within a galaxy's main disk. Heavier atomic elements are forged by way of the process of stellar nucleosynthesis, whereby increasingly heavier and heavier atomic elements are created out of lighter ones in a star's searing-hot core. The longer a galaxy has hosted stars that create heavy metals, the more metal-rich its gas becomes--and the higher the metallictity of the stars that are born "polluted" from that gas. These youthful, high-metallicity stars are usually seen in the main disk of a galaxy, because this is where there is a heavier stellar population.

However, in the mysterious case of the Sombrero, things get complicated because of the presence of a large number of globular clusters containing elderly, metal-poor stars. Such metal-poor, elderly stars are normally expected to travel out of their host clusters and become part of the stellar halo. However, that process failed to work in the Sombrero galaxy. The team of HST astronomers compared their results with recent computer simulations in order to see what could be the origin of such a numerous population of metal-rich stars in this bizarre galaxy's halo.

Their results were surprising, because they showed that the currently well-ordered, peaceful Sombrero had suffered violent accretion, or major merger events, billions of years ago. In contrast, our Milky Way is thought to have eaten many small satellite galaxies in "minor" accretions over the passage of billions of years. A major accretion event, however, is very different because it involves the merger of two or more similarly massive galaxies that are both well-endowed with higher-metallicity, later-generation stars.

The satellite galaxies only contained low-metallicity stars that were composed primarily of hydrogen and helium--the two lightest atomic elements--born in the Big Bang birth of the Universe almost 14 billion years ago (Big Bang nucleosynthesis). The heavier elements had to be manufactured in the searing-hot interiors of the stars through stellar nucleosynthesis and ultimately incorporated into later stellar generations. This process was somewhat inefficient in the case of small dwarf galaxies such as those circling our own Milky Way--while being considerably more effective in larger, more well-endowed galaxies, such as our own.

The new results for the Sombrero galaxy are surprising because its smooth, unperturbed disk displays no signs of disruption. By comparison, many interacting galaxies, such as the iconic Antennae galaxies, get their name from the distorted appearance of their spiral arms that result from the tidal forces of past violent interactions. Mergers of similarly massive galaxies usually coalesce into single large, smooth elliptical galaxies with extended halos--a process that takes billions of years. However, the Sombrero has always traveled to the beat of a different drum, and it never quite fit into the traditional definition of either a spiral or an elliptical galaxy. Like human nonconformists, it does what it will. It is a galactic mixture of the two.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.

November 30, 2021