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 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.