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NGC 4449: Star Stream for a Dwarf Galaxy January 26, 2012

Posted by jtintle in Deep Space, Space Fotos.
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See Explanation. Moving the cursor over the image will bring up an annotated version. Clicking on the image will bring up the highest resolution version available.

Image Credit & Copyright:

R Jay Gabany (Blackbird Obs.),


Subaru/Suprime-Cam (NAOJ),


David Martinez-Delgado (MPIA, IAC), et al.


A mere 12.5 million light-years from Earth, irregular dwarf galaxy NGC 4449 lies within the confines of Canes Venatici, the constellation of the Hunting Dogs. About the size of our Milky Way’s satellite galaxy the Large Magellanic Cloud, NGC 4449 is undergoing an intense episode of star formation, evidenced by its wealth of young blue star clusters, pinkish star forming regions, and obscuring dust clouds in this deep color portrait. It also holds the distinction of being the first dwarf galaxy with an identified tidal star stream, faintly seen at the lower right. Placing your cursor over the image reveals an inset of the stream resolved into red giant stars. The star stream represents the remains of a still smaller infalling satellite galaxy, disrupted by gravitational forces and destined to merge with NGC 4449. With relatively few stars, small galaxies are thought to possess extensive dark matter halos. But since dark matter interacts gravitationally, these observations offer a chance to examine the significant role of dark matter in galactic merger events. The interaction is likely responsible for NGC 4449’s burst of star formation and offers a tantalizing insight into how even small galaxies are assembled over time.


Fasten Your Seatbelts September 12, 2008

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Hubble and Chandra Image of colliding cluster

X-ray(NASA/CXC/Stanford/S.Allen); Optical/Lensing(NASA/STScI/UC Santa Barbara/M.Bradac)


Simple experiments are the best, especially when performed on a cosmic scale. A few years ago, astronomers posited a seemingly impossible question: Does dark matter, that mysterious stuff that makes up about one fourth of the present-day Universe, interact any other way than by gravity? For example, if you threw a bucket of “normal” matter and the dark stuff, which would go farther? Fortunately they had access to the tools and the testbed they needed to help answer this question. The tools were the Hubble Space Telescope and the Chandra X-ray Observatory, and the testbed was the Bullet Cluster, a cosmic train wreck consisting of one cluster of galaxies speeding through another. Astronomers used Chandra to measure the distribution of the “normal” matter, which is slowed by gravity and pressure forces and which can be seen by the tell-tale X-rays it produces. Using Hubble, astronomers were able to measure the distribution of gravitating matter from the amount of light bending (Einstein’s “gravitational lensing“), and they found that, indeed, the distribution of normal and dark matter was different: the dark matter was not slowed down as much as the normal stuff. The image above shows another example of this experiment, using the colliding cluster called MACS J0025.4-1222. In this image the pink shows the X-ray emission produced by the normal matter, and the blue shows the distribution of dark matter determined from observations of the gravitational lensing measured by Hubble. Once again, the dark matter distribution lies outside the location of the normal matter, giving further evidence that dark matter only interacts via gravity.

A Clash of Clusters Provides Another Clue to Dark Matter September 5, 2008

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MACS J0025.4-1222

X-ray(NASA/CXC/Stanford/S.Allen); Optical/Lensing(NASA/STScI/UC Santa Barbara/M.Bradac)

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Another powerful collision of galaxy clusters has been captured with NASA’s Chandra X-ray Observatory and Hubble Space Telescope. Like its famous cousin, the so-called Bullet Cluster, this clash of clusters provides striking evidence for dark matter and insight into its properties.

Like the Bullet Cluster, this newly studied cluster, officially known as MACS J0025.4-1222, shows a clear separation between dark and ordinary matter. This helps answer a crucial question about whether dark matter interacts with itself in ways other than via gravitational forces.

This finding is important because it independently verifies the results found for the Bullet Cluster in 2006. The new results show the Bullet Cluster is not an exception and that the earlier results were not the product of some unknown error.

Just like the original Bullet Cluster, MACS J0025 formed after an incredibly energetic collision between two large clusters in almost the plane of the sky. In some ways, MACS J0025 can be thought of as a prequel to the Bullet Cluster. At its much larger distance of 5.7 billion light years, astronomers are witnessing a collision that occurred long before the Bullet Cluster’s.

Using optical images from Hubble, the team was able to infer the distribution of the total mass (colored in blue) — dark and ordinary matter — using a technique known as gravitational lensing. The Chandra data enabled the astronomers to accurately map the position of the ordinary matter, mostly in the form of hot gas, which glows brightly in X-rays (pink.)

An important difference between the Bullet Cluster and the new system is that MACS J0025 does not actually contain a “bullet”. This feature is a dense, X-ray bright core of gas that can be seen moving through the Bullet Cluster. Nonetheless, the amount of energy involved in this mammoth collision is nearly as extreme as that found in the Bullet Cluster.

As the two clusters that formed MACS J0025 (each almost a whopping million billion times the mass of the Sun) merged at speeds of millions of miles per hour, the hot gas in each cluster collided with the hot gas in the other and slowed down, but the dark matter did not. The separation between the material shown in pink and blue therefore provides direct evidence for dark matter and supports the view that dark matter particles interact with each other only very weakly or not at all, apart from the pull of gravity.

One of the great accomplishments of modern astronomy has been to establish a complete inventory of the matter and energy content of the Universe. The so-called dark matter makes up approximately 23% of this content, five times more than the ordinary matter that can be detected by telescopes. The latest results with MACS J0025 once again confirms these findings.

The international team of astronomers in this study was led by Marusa Bradac of the University of California Santa Barbara (UCSB), and Steve Allen of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC. Their results will appear in an upcoming issue of The Astrophysical Journal. Other collaborators included Tommaso Treu (UCSB), Harald Ebeling (University of Hawaii), Richard Massey (Royal Observatory Edinburgh), and R. Glenn Morris, Anja von der Linden, and Douglas Applegate (Stanford).

3D distribution of dark matter in the Universe January 8, 2007

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3D distribution of dark matter

Larger Version


NASA, ESA and R. Massey (California Institute of Technology)


This three-dimensional map, obtained thanks to HST and XMM-Newton data, offers a first look at the web-like large-scale distribution of dark matter, an invisible form of matter that accounts for most of the Universe’s mass.

The map reveals a loose network of dark matter filaments, gradually collapsing under the relentless pull of gravity, and growing clumpier over time.

The three axes of the box correspond to sky position (in right ascension and declination), and distance from the Earth increasing from left to right (as measured by cosmological redshift). Note how the clumping of the dark matter becomes more pronounced, moving right to left across the volume map, from the early Universe to the more recent Universe.

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