Spirals eat dwarfs:
Galactic tendrils shed light on evolution of spiral galaxies
Spiral galaxies grow by swallowing smaller dwarf galaxies. As they are digested, these dwarf galaxies are severely distorted, forming structures such as surreal tendrils and stellar streams that surround their captors. Now, for the first time, a new survey has detected such tell-tale structures in galaxies more distant than our immediate galactic neighbourhood. This opens up the possibility of testing our current views of galaxy evolution in a new way.
Figure 1: Stellar streams around the galaxy M 63: remnants of a satellite galaxy that M 63 has swallowed. The central part is an ordinary positive image; in the outer regions, the negative of the image is shown. In this way, the faint structures that are the target of this survey are more readily discerned. This galaxy's distance from Earth is around 30 million light-years. The new survey has, for the first time, shown the presence of such tell-tale traces of spiral galaxies swallowing smaller satellites for galaxies more distant than our own “Local Group” of 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 with its three spiral galaxies and numerous dwarfs is much too small a sample to see whether theoretical predictions of the frequency of such digestive processes match observations. Now, for the first time, 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.
When a spiral galaxy is approached by a much smaller companion, such as a dwarf galaxy, the larger galaxy's uneven gravitational pull severely distorts the smaller star system. Over the course of a few billions of years, tendril-like structures develop that can be detected by sensitive observation. In one typical outcome, the smaller galaxy is transformed into an elongated “tidal stream” consisting of stars that, over the course of additional billions of years, will 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 galaxy's total mass are quite common in spiral galaxies.
Detailed simulations depicting the evolution of galaxies predict both tidal streams and a number of other distinct features that indicate mergers, 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 – impressive evidence that current models of galaxy evolution are indeed on the right track.
The ultra-deep images obtained by Delgado and his colleagues open the door to a new round of systematic galactic interaction studies. Next, with a more complete survey that is currently in progress, the researchers intend to subject the current models to more quantitative tests, checking whether current simulations make the correct predictions for the relative frequency of the different morphological features.
Remarkably, these cutting-edge results were obtained with the telescopes of ambitious amateur astronomers: For their observations, the researchers used telescopes 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 the 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.
Further information can be found on the project's webseite.
The research described here will be published as a letter in the October issue of the Astronomical Journal as D. Martínez-Delgado et al., “Stellar Tidal Streams in Spiral Galaxies of the Local Volume: A Pilot Survey with Modest Aperture Telescopes”. An electronic preprint is avalailable under http://arxiv.org/abs/1003.4860
The observations were carried out with 50 cm telescopes at Black Bird Observatory (New Mexico, USA) and Ranco del Sol (California, USA), the 37 cm telescope at Moorook (South Australia) and the 16 cm telescope at New Mexico Skies (New Mexico, USA).
The research group consists of David Martínez-Delgado (Max Planck Institute for Astronomy, Heidelberg, Germany, and Instituto de Astrofísica de Canarias, Spain), R. Jay Gabany (Black Bird Observatory), Ken Crawford (Rancho del Sol Observatory), Stefano Zibetti and Hans-Walter Rix (Max Planck Institute for Astronomy), Steven R. Majewski and David A. McDavid (University of Virginia), Jürgen Fliri (Instituto de Astrofísica de Canarias and Observatoire de Paris, Meudon), Julio A. Carballo-Bello and Ignacio Trujillo (Instituto de Astrofísica de Canarias), Daniella C. Bardalez-Gagliuffi (MIT and Instituto de Astrofísica de Canarias), Jorge Penarrubia (Cambridge University), Taylor S. Chonis (University of Texas), Barry Madore (Carnegie Institution of Washington) and Mischa Schirmer (Argelander Institute for Astronomy, Bonn University).
Figure 2: Examples for traces of spirals swallowing dwarf galaxies, as found with the new survey. For all images, the central part is an ordinary positive image. In the outer regions, the negative of the image is shown. In this way, the faint structures that are the target of this survey are more readily discerned. Wisps, plumes, stellar streams, partially disrupted satellites or stellar cloud indicate that we are witnessing merger processes.
Left column from top to bottom:
M 63, NGC 4651, NGC 7531, NGC 5866. Right column from top to bottom:
NGC 1084, NGC 4651, NGC 3521, NGC 1055.
Figure 3: Smaller satellite galaxies caught by a spiral galaxy are distorted into elongated structures consisting of stars, which are known as tidal streams, as shown in this artist's impression. The new survey has, for the first time, shown the presence of such tell-tale traces of spiral galaxies swallowing smaller satellites for galaxies more distant than our own “Local Group” of galaxies.
Figure 4: Closer inspection of this spiral galaxy, NGC 1055 in the constellation Cetus, reveals a weak box-shaped halo of stars: a trace of NGC 1055 having swallowed and assimilated a smaller satellite galaxy. This galaxy's distance from Earth is 60 million light-years. The new survey has, for the first time, shown the presence of such tell-tale traces of spiral galaxies swallowing smaller satellites for galaxies more distant than our own “Local Group” of galaxies.
Figure 5: One of the galaxies in the survey, NGC 4651, sports a remarkable umbrella-like structure. It is composed of tidal star streams, the remnants of a smaller satellite galaxy which NGC 4651 has attracted and torn apart. This galaxy's distance from Earth is 35 million light-years. The new survey has, for the first time, shown the presence of such tell-tale traces of spiral galaxies swallowing smaller satellites for galaxies more distant than our own “Local Group” of galaxies.