Amateur astronomer investigates a galaxy’s tidal stream

Posted by David Eicher
on Wednesday, December 14, 2011

Frequent Astronomy magazine contributor R. Jay GaBany, who operates the Blackbird II Observatory near Alder Springs, California, recently participated in a study of the dwarf starburst galaxy NGC 4449 and its attendant tidal stream. He is working with several professional astronomers on the project.

R. Jay GaBany
Here is the story of this project from GaBany’s website:

“We have mapped and analyzed a stellar stream in the halo of the nearby dwarf starburst galaxy NGC 4449, detecting it in deep integrated-light images using the Black Bird II Observatory’s 0.5-meter telescope, and resolving it into a river of individual red giant branch stars using the 8.2-meter Subaru/Suprime-CAM (NAOJ). The properties of the stream imply a massive dwarf spheroidal progenitor, which will continue to disrupt and deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The ratio between luminosity or stellar-mass between the two galaxies represents a stealth major merger.

This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.

Within the hierarchical framework for galaxy formation, the stellar bodies of massive galaxies are expected to form and evolve not only through the inflow of cold gas, but also the in fall and successive mergers of low-mass, initially bound systems — commonly referred to as dwarf satellites, they consist of dark matter, gas, and, in most cases, stars — that span a wide range of mass. While the present-day interaction rate is expected to decrease, numerous cosmological models built within the Cold Dark Matter (CDM) paradigm predict that such satellite disruptions still occur around all massive galaxies. As a consequence, the stellar halos of these galaxies should contain a wide variety of diffuse structural features such as stellar streams or shells that result from interactions and mergers with dwarf satellites.

The most spectacular examples of tidal debris feature long, dynamically cold stellar streams formed from a disrupted dwarf satellite that wrap around the host galaxy’s disk and roughly trace the orbit of the progenitor satellite. Although these fossil records disperse into amorphous clouds of debris in a few billion-years, CDM-based simulations predict that stellar streams may be detected nowadays, with sufficiently deep observations, in the outskirts of almost all nearby galaxies.

However, the detection of these faint tidal remnants is a ubiquitous aspect of galaxy formation that has not yet been fully exploited, mainly because they are challenging to observe. Although the most luminous examples of diffuse stellar streams and shells around massive elliptical galaxies have been known for many decades, recent studies have showed that fainter analogues of these structures are common around spiral galaxies in the local universe, including the Milky Way and Andromeda.

Although the CDM model also predicts mergers and interactions among low-mass galaxies, there is scant observational evidence that these minuscule mergers are still happening in the local universe. Interactions between low-mass galaxies allow the possibility of exploring a very different merger regime. Studies based on halo occupation distribution and abundance matching techniques have shown that the stellar mass is a very rapidly decreasing function of (decreasing) dark matter halo mass.

This implies that low-mass dwarf galaxies with different stellar masses may live in similar dark matter halos, hence what seems a minor merger in stars could be a major merger from a dark halo perspective. Studying interactions on a small scale could provide unique insights on the role of stars versus dark matter in galactic mergers.

While understanding the mutual influences of these two galaxies on each other will require further modeling, we suspect it’s not a coincidence that such a novel stream was first found around one of the most intense star-forming galaxies located nearby. The accretion event may well be acting as the trigger for the starburst.”

For more on this story and to see more images by GaBany, see: http://www.cosmotography.com/img_masters/img_resources.html.

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