The stellar stream in the halo of nearby dwarf starburst galaxy NGC 4449 was detected in deep integrated-light images from the Black Bird Remote Observatory. Mouse-over the image to view an inset that resolves the stream into its constituent stars.Click the image for a larger view.

This image was produced with a half meter RCOS telescope, Apogee Alta U16M camera and Astrodon E-Series filters. Exposure times were 1,080 minutes Luminance, 240 minutes Red, 240 minutes Green, 240 minutes Blue (All 1X1). Black Bird observations were conducted over a series of nights spanning April 13 through June 10, 2010 and again between January 13 and January 28, 2011



Image copyright ©2009- 2012 R Jay GaBany

Dwarfs gobbling dwarfs: a tidal star stream around NGC 4449

Hierarchical Galaxy Formation on Small Scales


An investigation by:
David Martínez-Delgado: (Max-Planck-Institut fur Astronomy); Aaron Romanowsky: (University of California); R. Jay GaBany: (Black Bird Observatories); Francesca Annibali: (Osservatorio Astronomico di Bologna); Jacob A. Arnold: (University of California); Juergen Fliri: (Paris Observatory- GEPI); Stefano Zibetti: (Dark Cosmology Centre Niels Bohr Institute); Roeland P. van der Marel: (Space Telescope Science Institute); Hans-Walter Rix: (Max-Planck-Institut fur Astronomy); Taylor S. Chonis: (University of Texas); Julio A. Carballo-Bello: (Instituto de Astrofisica de Canarias); Andrea V. Maccio: (Max-Planck-Institut fur Astronomy); J.Gallego-Laborda: (Fosca Nit Observatory); Alessandra Aloisi: (Osservatorio Astronomico di Bologna); Jean P. Brodie: (University of California); Michael R. Merrifield: (University of Nottingham)


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 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.



  • Download the full resolution version of the scientific paper (PDF)
  • Click here for a larger view of the image
  • Click here for an interactive view from the Subaru telescope
  • Also see: NASA APOD January 26, 2012
  • Also read: The Big Deal About Dwarf Galaxies



  • 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 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. (See NGC 5907 and NGC 4013) 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 an 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 shown 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 very different stellar masses may live in very similar dark matter halos, hence what seems a minor merger in stars could be a major merger from an dark halo perspective. Studying interactions on a small scale could provide unique insights on the role of stars versus dark matter in galactic mergers.

    We suspect exact analogues to this stream are not very common or they would have been noticed already in DSS or SDSS images. However, if the stream had been only a bit fainter, more diffuse, or at a larger radius, it could have been missed, and thus there may be many more dwarf-hosted stellar streams awaiting detection.

    If streams such as NGC 4449 are common among dwarf galaxies, they re-ignite classic ideas about galaxy interactions triggering starbursts. Given the high rates of star formation in dwarf galaxies, it is natural to ask if satellites are responsible.

    Also read: The Formation and Evolution of Galaxies







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