Galactic tendrils shed light on the evolution of spiral galaxies

Spiral Galaxies devour their satellites

  • Smaller satellite galaxies caught by a spiral galaxy are distorted into elongated star filled structures called tidal streams, as shown in this artist's impression. A new pilot survey has, for the first time, shown the tell-tale traces of spiral galaxies swallowing smaller satellites around star systems more distant than our own Local Group of galaxies.

    Illustration credit: Jon Lomberg
  • By: Markus Pössel (MPIA and HdA)

Cannibal Galaxies

It's a "galaxy-eats-galaxy" Universe out there! According to current models, galaxies grow by ingesting other star systems. While gigantic elliptical galaxies are believed to result when two or more massive precursor galaxies merge, spiral galaxies, such as our own Milky Way, grow by swallowing smaller dwarf galaxies.

Around the Milky Way galaxy and in the vicinity of our immediate cosmic neighborhood, known as the "Local Group" of galaxies, traces of spiral galaxies swallowing dwarf galaxies have been known since 1997. But the Local group's three principal spiral galaxies and retinue of dwarfs is not a large enough sample to test theoretical predictions about the frequency of such digestive processes. Now, a new survey has managed to detect the tell-tale tendrils of galactic digestion beyond the Local Group. An international group of researchers led by David Martínez-Delgado (Max-Planck Institute for Astronomy and Instituto de Astrofísica de Canarias) has completed a pilot survey of spiral galaxies at distances of up to 50 million light-years from Earth, discovering the tell-tale signs of spirals eating dwarfs.
  • Messier 63 represents one of several recently discovered examples of an isolated spiral galaxy with a warped disk showing strong evidence of an ongoing satellite merger.
  • Image credit: R. Jay GaBany

When a spiral galaxy is approached by a much smaller companion, such as a dwarf galaxy, the larger galaxy's uneven gravitational pull will start to severely distort the smaller star system. Over the course of a few billion years, tendril-like structures develop. In one typical outcome, the smaller galaxy is transformed into an elongated tidal stream consisting of stars that, over the course of billions more years, eventually join the galaxy's regular stellar inventory through a process of complete assimilation. The study shows that major tidal streams with masses between 1 and 5 percent of the parent's total mass are quite common in spiral galaxies.
  • Rings and arcs aren't the only evidence of an ancient satellite mergers. This animation simulates the merger of numerous companion galaxies and demonstrates that plumes, spears, spikes and shells that surround the primary galaxy are also possible.
  • Model animation credit: Jorge Peñarrubia (University Victoria)

Detailed simulations depicting the evolution of galaxies predict both tidal streams and a number of other distinct features as merger indicators such as giant debris clouds or jet-like features emerging from galactic discs. Interestingly, all these various features are indeed seen in the new observations offering impressive evidence that current galaxy evolution models are on the right track.

The ultra-deep images obtained by Dr. Delgado and his international team open the door to a new round of systematic galactic studies. Next, with a more complete survey that is currently in progress, the researchers intend to subject the current models to more quantitative tests to verify that current simulations make correct morphology predictions.

Remarkably, these cutting-edge results were obtained with the telescopes of ambitious amateur astronomers: For their observations, the researchers used instruments with apertures between 10 and 50 cm, equipped with commercially available CCD cameras. The telescopes are robotic (that is, they can be controlled remotely), and are located at two private observatories in the US and one in Australia.

The results attest to the power of systematic work that is possible even with smaller instruments: while larger telescopes have an undeniable edge in detecting very distant but comparatively bright star systems such as active galaxies, this survey provides some of the deepest insight yet when it comes to detecting ordinary galaxies that are similar to our own cosmic home, the Milky Way.

* Source: Max Planck Institute for Astronomy