Galactic Fossil Hunting

When astronomers peer into the heavens, they also look back in time. So, when scientists observe a galaxy that's a billion light-years from Earth, they are also seeing how the star system appeared a billion years in the past because light from the galaxy took a billion years to reach their instruments. This transforms telescopes into time machines that enable astronomers to explore ancient cosmic history. However, unlike earth-bound archeologists who study old rocks, astronomers examine fossils made of primordial light.

The origin and evolution of galaxies are two of the most actively researched areas in astrophysics. An overwhelming weight of evidence has convinced cosmologists that the Universe came into existence at a definite moment in time, some 13.6 billion years ago, in the form of a super hot, super dense fireball of energetic radiation known as the Big Bang event.

Today, the Lambda Cold Dark Matter (or LCDM) model is the latest incarnation of our understanding about the origin of the Cosmos. It represents an improvement of the big bang theory by positing most of the physical substance in the Universe consists of a material called dark matter.

Although it cannot be detected by current instrumentation, cosmologists believe dark matter is comprised of cold slow moving particles that do not emit electromagnetic radiation or scatter light, thus they also appear dark. However, the gravitational effect of dark matter can be observed on visible material, such as galaxies and observations of background radiation.

The Lambda in the theory's name accounts for the presence of dark energy, a hypothetical force that appears to be accelerating the expansion of the Universe. The theory was originally published in 1984 by United States physicists Joel R. Primack, George Blumenthal, and Sandra Moore Faber. Today, it's also referenced as the standard cosmological model.

According to the LCDM theory, the Universe was intensely hot, remarkably smooth and essentially homogeneous immediately following the Big Bang. However, small fluctuations in density, less than one part in a hundred thousand, began to appear and grow. As the universe cooled, clumps of dark matter began to condense and within them gas molecules formed. At this point the universe was almost exclusively composed of hydrogen, helium, and dark matter. This is the period when the cosmic microwave background radiation was emitted.

The gas and dark matter were gravitationally attracted to the areas of higher density and formed halos that represented the seeds for the first galaxies. As the halos became more massive, they began to collapse under their own weight and became proto-galaxies. Soon after, the hydrogen and helium gas within the halos began to make the first stars. Then, over time, the halos merged to form larger and larger galaxies.

Computer simulations have enabled theorists to follow the evolution of matter in the Universe from a short time after the Big Bang until recently. These simulations contend that galaxy formation in the presence of cold dark matter occurs hierarchically - the first galaxies to form are small dwarfs and these subsequently merge to form progressively larger star systems. Therefore, large galaxies like the Milky Way must have consumed a hundred or more small dwarf galaxies over time. However, many of them may be so thoroughly mixed with the stars of their parent galaxy they are no longer easily identifiable.


Mergers Made in Heaven

The most spectacular manifestation of this process may be the coalescence of comparably sized galaxies in a process known as a major merger. These events often result in the destruction of the spiral pattern in both merging galaxies. Major mergers can also trigger star bursts. Such events have been relatively rare within the past few billion years and only a small percentage of large galaxies are involved in an ongoing major merger at any point in time.

However, minor mergers that involve the disruption of a dwarf satellite galaxy by a much more massive companion are expected to be significantly more common. According to the LCDM model, minor mergers should still be occurring today. Since the stellar disk of the larger partner is not destroyed during a minor merger, signs of recent or on-going events should be apparent around many spirals, the most common type of large galaxy.


Cosmic Bread Crumbs

The ebb and flow of the tides are created when the Moon attracts our planet and its oceans differentially. Similarly, a large spiral deforms an orbiting satellite galaxy by exerting a stronger pull on one side than the other. In the process, some of the satellite's stars are removed like a trail of bread crumbs that leaves a fossil record in the form of a stellar tidal structure. During their merger, stars from a satellite galaxy can be pulled into long streams, deposited into vast shells of debris or swept into enormous umbrella shaped structures that surround the parent galaxy and remain detectable for several billion years like a gigantic cosmic relic.

Evidence of the first stellar stream was discovered in the Milky Way galaxy during the 90s. Known as the Sagittarius Dwarf Elliptical, this small satellite galaxy is orbiting on a perpendicular path to the Milky Way's broad stellar plane causing it to pass through our galaxy. With each pass through the disk, stars are being removed forming a thin stream.

Since the discovery of the Sagittarius dwarf galaxy, over 15 star streams have been identified in the halo our home galaxy and 4 have been discovered in the Andromeda galaxy, our nearest galactic neighbor.

However, evidence of tidal streams beyond the Local Group has been mostly anecdotal until recently.


Beyond the Local Group

For the past six years, Dr. David Martínez-Delgado of the Max Planck Institute for Astronomy has been leading an international team of professional and amateur astronomers searching for stellar streams around eight near-by spirals beyond the Local Group by analyzing ultra-deep images produced with modest-sized, commercially available instruments. Their efforts led to the discovery of six extensive stellar structures surrounding several of the galaxies surveyed. These previously undetected features were interpreted as debris from tidally disrupted satellites. In addition, their research confirmed and clarified several enormous stellar features that had been previously reported but never interpreted as fossil evidence of minor mergers.

The set of galaxies exhibited unexpected, highly diverse characteristics such as great circular features resembling the Milky Way's Sagittarius stream, remote shells and giant clouds of tidal debris as well as enormous jet-like features emerging from galactic disks. Together with the remains of already disrupted companions, the observations also captured surviving satellites caught in the act of tidal disruption.

Earlier in 2011, a new image of NGC 3521 was completed that improves on the data originally collected for the survey. Located 35 million light years from our planet towards the northern constellation of Leo, this star system has been classically categorized as flocculent galaxy due to the enormous amount of material partially obscuring its spiral structure.

However, the new ultra-deep image reveals evidence of one or more previous mergers with dwarf galaxies that left discernible substructures, such as an almost spherical cloud of debris visible on its eastern side and a large, elongated cloud to the west. Both represent debris shells belonging to an umbrella-like structure similar to the one seen in images of NGC 4651. But, their looser appearance suggests they were accreted much farther in the past. Additionally, the galaxy is enveloped a bubble of multiple debris shells that may represent further evidence of ancient mergers.

Other flocculent galaxies have also been shown to have stellar stream remnants, such as NGC 5055 (M63) leading some to speculate that this phenomenon may be linked to previous minor mergers.

A comparison with computer simulations confirmed the extraordinary variety of structures detected by the Delgado team. The existence of the tidal features around distant galaxies that appear to be normal, from all other aspects, and their match to the LCDM simulations constituted new evidence that the standard model also applies to a distant galaxies similar to the Milky Way.

Like cosmic archeologists, astronomers excavate ancient light to uncover the truth about the birth and development of galaxies. Stellar streams, relics of the hierarchical merger process predicted by the LCDM theory, were first identified in the Milky Way and other galaxies in the Local Group. Now, similar structures have been seen in much more distant galaxies. Combined, these observations support the current best theory explaining how the Universe, and everything within it, originated and evolved to the present day.

However, the book of scientific knowledge is written on loose leaf pages that are revised, re-sorted and sometimes removed over time when new information is dug up. Therefore, unlike earth-bound fossils, nothing cosmological should be considered etched in stone because there's always something new coming to light just over our horizon.


Also read:
- A Pilot Survey with Modest Aperture Telescopes
- The Model Universe