Subaru Telescope Captures Images of the "Stealth Merger" of Dwarf Galaxies

Figure 1: Suprime-Cam captured this image of the nearby dwarf galaxy NGC 4449 (lower left) and its companion (upper right), a dwarf galaxy that has been gravitationally pulled apart into a stellar stream. It shows individual stars composing the galaxies. R. Jay GaBany (Blackbird Observatory) produced the composite: blue hues in the center of the larger galaxy are brought about by a burst of recent star formation, while the red in its periphery and in its satellite indicates the presence of older, red-giant stars. (Hi-Res image)

An international team of scientists led by David Martinez-Delgado (Max Planck Institute for Astronomy, Germany) has conducted research that reveals a "stealth merger" of dwarf galaxies, where an in-falling satellite galaxy is nearly undetectable by conventional means yet has a substantial influence on its host galaxy. Aaron Romanowsky (University of California Observatories in Santa Cruz) along with graduate student Jacob Arnold (UCSC) used the Subaru Telescope to obtain high-resolution images of individual stars in a dense stream of stars in the outer regions of a nearby dwarf galaxy (NGC 4449); these outlying stars are the remains of an even smaller companion galaxy in the process of merging with its host (Figure 1). NGC 4449, the host galaxy, is the smallest primary galaxy in which a stellar stream from an ongoing merger has been identified and studied in detail. Romanowsky commented, "I don't think I'd ever seen a picture of a galaxy merger where you can see the individual stars. It's really an impressive image."

Martinez-Delgado organized a campaign to follow up on an initial report of the stellar stream in NGC 4449, first detected by Russian astronomers as a mysterious, faint smudge in digitized photographic plates from the Digitized Sky Survey project. Had the objects been slightly fainter, more diffuse, or farther from the host galaxy, the stellar stream could easily have been missed. NGC 4449 is located 12.5 million light years from Earth and is a member of a group of galaxies in the constellation Canes Venatici. Martinez-Delgado recruited astrophotographer R. Jay GaBany (recipient of the 2010 AAS Chambliss Amateur Achievement Award) to obtain deep, wide-field images with the half-meter telescope at Black Bird Observatory; these images confirmed the presence of a faint substructure in the galaxy's halo. Romanowsky and Arnold then took advantage of the wide field of view and light collecting power of Subaru Telescope's 8.2 meter mirror and its prime focus camera (Suprime-Cam) to capture high resolution images of the faint objects in the halo. These final observations at Subaru in 2011 clearly showed the stealth merger of two dwarf galaxies.

Modern cosmological theory posits that large galaxies were built up from smaller ones through an orderly succession of mergers. Although astronomers have observed many mergers involving massive galaxies, it has been difficult to find mergers of two dwarf galaxies. Theory suggests that similar processes of merging should occur on a smaller scale, with small galaxies eating even smaller ones; this is how galaxies grow. Romanowsky commented on the significance of the Subaru image: "Now we have this beautiful image of a dwarf galaxy consuming a smaller dwarf. You can see a smaller galaxy coming in and getting shredded, eventually leaving its stars scattered through the halo of the host galaxy. "

The new observations support the idea that the stellar halos around many dwarf galaxies are the remnants of smaller satellites that were shredded in past merger events. The ongoing merger in NGC 4449 may also be responsible for the intense burst of star formation seen in the galaxy. "This galaxy is famous for its starburst activity, and it seems we've found the reason for that. The gravitational interaction with the incoming galaxy has probably disturbed the gas in the main galaxy and caused it to start forming stars," Romanowsky said.


Notes:

This research was supported by the National Science Foundation (NSF, USA), National Aeronautics and Space Administration (NASA, USA) and the University of California at Santa Cruz-University Affiliated Research Center (UCSC-UARC) Aligned Research Program.


The paper describing the research, “Dwarfs gobbling dwarfs: A stellar tidal stream around NGC 4449 and hierarchical galaxy formation on small scales”, will be published in Astrophysical Journal Letters. Coauthors of this international study include:

D. Martinez-Delgado (Max Planck Institute for Astronomy, Germany)
A. Romanowsky (University of California Observatories at Santa Cruz, USA)
J. Arnold (University of California at Santa Cruz, USA)
R. J. GaBany (astrophotographer, amateur astronomer, USA)
J. Brodie (UC Santa Cruz, USA)
F. Annibali (Astronomical Observatory of Bologna, Italy)
J. Fliri (Observatory of Paris, France)
S. Zibetti (University of Copenhagen, Denmark)
R. van der Marel (Space Telescope Science Institute, USA)
A. Aloisi (Space Telescope Science Institute, USA)
H.-W. Rix (Max Planck Institute for Astronomy, Germany)
A. Macciò (Max Planck Institute for Astronomy, Germany)
T. Chonis (University of Texas at Austin, USA)
J. Carballo-Bello (Canary Astrophysics Institute, Spain)
J. Gallego-Laborda (Fosca Nit Observatory Spain)
M. Merrifield (University of Nottingham, UK)

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Little Galaxies Are Big on Dark Matter

The stellar stream in the halo of the nearby dwarf starburst galaxy NGC 4449 is resolved into its individual starry constituents in this exquisite image taken with the 8.2-meter Subaru Telescope and Suprime-Cam. Image credit: R. Jay GaBany and Aaron J. Romanowsky (UCSC) in collaboration with David Martinez-Delgado (MPIA) and NAOJ. Image processed by R. Jay GaBany

Dark matter… It came into existence at the moment of the Big Bang. Within its confines, galaxies formed and evolved. If you add up all the parts contained within any given galaxy you derive its mass, yet its gravitational effects can only be explained by the presence of this mysterious subatomic particle. It would be easy to believe that the larger the galaxy, the larger the amount of dark matter should be present, but new research shows that isn’t so. Dwarf galaxies have even higher proportions of dark matter than their larger counterparts. Although the dwarfs are the most common of all, we know very little about them – even when they consume each other. Enter the star stream…

“Several of my previous images feature the fossil remnants of these ancient mergers as faint stellar rivers called tidal streams. These stellar streams are the table crumbs from small dwarf galaxies that were gravitationally dismembered as they were devoured by the larger galaxy they orbited.” says astrophotographer, R. Jay Gabany. “The theory implies dwarf galaxies also merged and are still merging with each other. But, there has never been clear photographic evidence or a close investigation of dwarf galactic mergers until now.”

The target is NGC 4449, a small, irregular dwarf galaxy much like the the Milky Way’s Large Magellanic Cloud. What makes it interesting to astronomers is the presence of thousands of hot blue stars and massive red regions interspaced with thick dust clouds. It isn’t just forming new stars… it’s experiencing an explosion of star birth! According to current theory, dwarf galaxies such as this one could be undergoing a merger event, but there hasn’t been photographic proof until now.

“The picture I am sharing is of a small, dwarf galaxy known as NGC 4449 that’s located about 12.5 million light years from Earth towards the northern constellation of Canes Venatici, the Hunting Dogs. This galaxy is about the size of our Milky Way’s largest satellite galaxy, the Magellanic Cloud. But, NGC 4449 is much farther away and it is experiencing a major star burst event- an episode characterized by the production of new stars at a furious rate.” says Gabany. “This image is unique because is it captures the first dwarf galaxy known to have its own tidal stream of stars. Therefore, it represents the first closely studied example of a dwarf galaxy merging with an even smaller dwarf star system! The professional astronomers with whom I work also suspect the merger may have contributed to the ferocious production rate of new stars inside NGC 4449.”

The research done by the team led by Dr. David Martinez-Delgado has some very interesting ramifications and their paper has been accepted for publication in the Astrophysical Journal Letters.. As so well put in Jay’s photographic explanation in his webpage; “Although the cold dark matter theory predicts mergers and interactions between dwarf galaxies, there is scant observational evidence that these types of mergers are still happening in the nearby local Universe. Interactions between dwarf galaxies invoke the possibility of exploring a very different merger regime. For example, research has shown that multiple dwarf galaxies with different stellar masses may exist in similar sized dark matter halos, hence what appears as a minor merger of stars could be a major dark matter merger. Studying interactions on a small scale, such as NGC 4449, provides unique insights on the role of stars versus dark matter in galactic merger events.”

Where once amateur astrophotographers painted beautiful portraits of what lay just beyond human perception in deep space, they are now crafting images capable of true science. The eyes of their telescopes are being combined with professional instruments and producing amazing results.

“We live in an age where science has become unfettered from examining the Universe with only our physical six senses.” concludes Gabany. “This has unlocked a profound new level of understanding, resolved ancient mysteries and unlatched a Pandora’s chest filled with new questions begging for answers. We still have much to learn.”

For Further Reading: Dwarfs Gobbling Dwarfs: A Stellar Tidal Stream Around NGC 4449 and Hierarchical Galaxy Formation On Small Scales and The Big Deal About Dwarf Galaxies.

by Tammy Plotner

Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

Source: Universe Today

More about → Little Galaxies Are Big on Dark Matter