Anne’s Picture of the Day: Spiral Galaxy NGC 6118

May 11, 2013

NGC 6118, a grand design spiral galaxy in Serpens

The Blinking Galaxy

Image Credit: ESO

NGC 6118 (also known as the Blinking Galaxy) is a “grand design” spiral galaxy of roughly 110,000 light-years across (more or less the same size as our own Milky Way galaxy), located about 82.9 million light-years away from Earth in the southern constellation of Serpens (the Snake). It is receding from us at approximately 1573 kilometers per second.

A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms that extend clearly around the galaxy and can be observed over a large fraction of the galaxy’s radius. Approximately 10 percent of spiral galaxies are classified as grand design type spirals.

NGC 6118 is classified as a “SA(s)cd,” galaxy, meaning that it is a disk-shaped galaxy with several rather loosely wound spiral arms that spiral out directly from the nucleus. These arms show large numbers of bright bluish knots which are active star-forming regions where, in some of them, very luminous and young stars can be perceived. Although the “A’ means that the galaxy lacks a central bar, NGC 6118 does have a central bar, only a very small one.

On 1 August 2004, the Type Ib supernova SN 2004dk was discovered just north of the galaxy’s center.

A supernova is a phenomenon in which a star explodes in the final phase of its life. Type Ib supernovae are stellar explosions that are caused by the core collapse of massive stars (initially more than 8 times the mass of the Sun). These stars have somehow lost their entire hydrogen envelope, probably as a result of mass transfer in a binary system, before exploding. This type is often referred to as stripped core-collapse supernova.

Supernova explosions are enriching the intergalactic gas with heavy chemical elements like oxygen, iron and silicon, necessary to build new generations of stars and planets, and to create life.

NGC 6118 is a comparatively faint object of 13th magnitude with a rather low surface brightness, making it a pretty challenging object to see in small telescopes. It has gotten its nickname “The Blinking Galaxy” because the galaxy has a tendency to flick in and out of view with different eye positions.

This composite color image is taken at a distance of 80 million light-years on several nights around August 21, 2004 when the observing conditions were somewhat unstable. It is based on images obtained with the multi-mode VIMOS instrument on the ESO Very Large Telescope (VLT) in three different wavebands: R (red), V (green) and B (blue). North is up and East is to the left.

Anne’s Picture of the Day: Spiral Galaxy Messier 83

April 11, 2013

Messier 83, a barred spiral galaxy in Hydra

NGC 5236, the Southern Pinwheel

Image Credit: ESO

Messier 83 (also known as NGC 5236 and the Southern Pinwheel for its majestic spiral arms) is a barred spiral galaxy of some 40,000 light-years across, located about 14.7 million light-years away in the southern constellation of Hydra, the Water-Snake, while it is receding from us at roughly 513 kilometers per second. It is the central member of the M83 Group, a small nearby group of galaxies, including NGC 5253 and about 9 dwarf galaxies.

Despite its symmetric appearance, the central 1000 light-years of the galaxy shows an unusually high level of complexity, containing both a double nucleus and a double circumnuclear starburst ring. High resolution observations at radio wavelengths show the visible nucleus to be curiously offset from the true dynamical nucleus by some 200 light years. A gaseous accretion disk was found with a diameter of almost 400 light-years rotating at high velocity (180 kilometers per second) around the galaxy’s off centered nucleus.

The nature of the double nucleus is uncertain but some evidence suggests that the origin of the off centered nucleus may be a remnant core of a small galaxy that merged with Messier 83 in its distant past. Supporting this theory is other evidence (a 100,000 light year stellar arc) that Messier 83 recently cannibalized a small satellite galaxy.

Classified as a starburst galaxy, Messier 83 is undergoing more rapid star formation than our own Milky Way galaxy, especially in its nucleus. We can see hundreds of young star clusters, ancient swarms of globular star clusters, and hundreds of thousands of individual stars, mostly blue supergiants and red supergiants.

The image reveals in unprecedented detail the current rapid rate of star birth in this famous grand design spiral galaxy. The newest generations of stars are forming largely in clusters on the edges of the dark dust lanes, the backbone of the spiral arms. These fledgling stars, only a few million years old, are bursting out of their dusty cocoons and producing bubbles of reddish glowing hydrogen gas.

Gradually, the young stars’ fierce winds (streams of charged particles) blow away the gas, revealing bright blue star clusters. These stars are about 1 million to 10 million years old. The older populations of stars are not as blue.

A bar of stars, gas, and dust slicing across the core of the galaxy may be instigating most of the star birth in the galaxy’s core. The bar funnels material to the galaxy’s center, where the most active star formation is taking place. The brightest star clusters reside along an arc near the core.

The remains of about 60 supernova explosions, the deaths of massive stars, can be seen in the image. Among them are six supernovae (SN 1923A, SN 1945B, SN 1950B, SN 1957D, SN 1968L and SN 1983N) which have been observed in the past century. By studying these remnants, astronomers can better understand the nature of the progenitor stars, which are responsible for the creation and dispersal of most of the galaxy’s heavy elements.

This image is taken by the European Southern Observatory’s Wide Field Imager on the ESO/MPG 2.2-meter telescope at La Silla, Chile.

Anne’s Picture of the Day: The Pinwheel Galaxy

January 11, 2012

The Pinwheel Galaxy, a Grand Design spiral in Ursa Major

Messier 101 or NGC 5457

Image Credit & Copyright: Robert Gendler (

The Pinwheel Galaxy (also known as Messier 101 or NGC 5457) is a giant asymmetrical spiral galaxy of about 170,000 light-years across (what makes it one of the largest disk galaxies known), located 21 – 27 million light-years away in the constellation of Ursa Major, while moving away from us at approximately 267 kilometers per second.

It is the brightest member of the M101 Group (a group of at least 9 galaxies, with as brightest companions NGC 5474 and NGC 5585), with a luminosity of about 30 billion times that of our Sun, and is estimated to contain at least one trillion stars of which some 100 billion of these stars could be like our Sun in terms of temperature and lifetime.

Although its size exceeds that of our Milky Way its overall mass is similar to our galaxy, with a disk mass on the order of 100 billion solar masses, along with a small bulge of about 3 billion solar masses.

The Pinwheel Galaxy is also one of the most prominent Grand Design spirals in the sky (a type of spiral galaxy with prominent and well-defined spiral arms, that extend clearly around the galaxy through many radians and can be observed over a large fraction of the galaxy’s radius).

The galaxy’s tightly-wound spiral arms, laced with dark dust lanes, are sprinkled with brilliant young clusters of hot, blue, newborn stars, and large and extremely bright star-forming regions, of which a total of about 3,000 can be counted, ionized by large numbers of very bright and hot young stars.

Because galaxy’s core is considerably displaced from the center of its thin disk, it is thought that in the recent past (speaking in galactic terms) the Pinwheel underwent a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this encounter also amplified the density waves in the spiral arms of the galaxy. The amplification of these waves leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.

The gravitational interaction between the Pinwheel and its companion galaxies may also have triggered the formation of its grand design pattern, and probably distorted its companion galaxy NGC 5474.

Four supernovae have been recorded in this galaxy. The first one was SN 1909A in January 1909, the second, a Type II was SN 1951H in September 1951, the third one, also a Type II was SN 1970G in January 1970, and the last one, a Type Ia supernova, was discovered on August 24, 2011.

A supernova is a phenomenon in which a star explodes in the final phase of its life. There are two types: Type I and Type II. The Type I does not show hydrogen in the spectra. Among these, Type Ia are explosions of white dwarf stars. The other group, Type II, are explosions of massive stars (initially more than 8 times the mass of the Sun). The Type II supernovae do show hydrogen in their spectra.

Anne’s Picture of the Day: NGC 2997

November 5, 2012

NGC 2997, a grand design spiral galaxy in Antlia

NGC 2997

Image Credit & Copyright: Robert Gendler (

NGC 2997 is a bright, unbarred grand-design spiral galaxy, located about 40 million light-years away in the southern constellation Antlia (the Air pump). It is the brightest member of a group of galaxies of the same name in the Local Supercluster of galaxies along with the Local Group.

NGC 2997 contains hundreds of billions of stars and is thought to have a mass of about 100 billion times that of our Sun, but is probably less massive than our own Milky Way Galaxy. The galaxy is speeding away from us at about 1085 kilometers per second.

With a disk that is inclined 45 degrees to our line of sight, NGC 2997 has an oval appearance. The inner disk is covered with dust lanes, silhouetted against the central part of the galaxy, which shows a high surface brightness.

Like all grand design spirals, NGC 2997 has prominent and well-defined spiral arms, which appear to originate in the yellow nucleus. These sprawling arms are peppered with bright red blobs of ionized hydrogen which are regions of star formation, where the bright blue stars are born that generate most of the light in the arms of the galaxy.

Its small nucleus, that most likely hosts a supermassive black hole, also shows an interesting structure and concentrates an older population of yellowish stars. The nucleus is surrounded by a chain of hot giant clouds of ionized hydrogen.

Astounding Spiral Galaxy form the Early Universe Discovered


A team led by an astronomer at the Dunlap Institute for Astronomy & Astrophysics, University of Toronto, has discovered a spiral galaxy that appears to have formed a billion years before other spirals. The galaxy is 10.5 billion light-years from Earth, putting it at a time when the Universe was only three billion years old and spirals were extremely rare.

An artist’s rendering of galaxy BX442 and its companion dwarf galaxy (above the center on the right). Credit: Dunlap Institute for Astronomy & Astrophysics/Joe Bergeron

According to Dunlap Institute postdoctoral fellow and Principal Investigator David Law, “Seeing this galaxy amongst the irregular, young galaxies of that epoch is like seeing a fully-formed adult in a room of grade-school children.”

Law says, “The fact that this galaxy exists is astounding. Current wisdom holds that such grand-design spiral galaxies simply didn’t exist at such an early time in the history of the Universe.” Most galaxies in the three billion year old Universe are clumpy and irregularly-shaped; they haven’t formed the well-defined spiral arms we see in galaxies like the Whirlpool Galaxy (Messier 51).

The rest of Law’s team comprises researchers from UCLA, Caltech, UC Riverside, Steward Observatory, and UW Milwaukee. The Space Telescope Science Institute provided principal funding for the work, the results of which will be published in today’s issue of the science journal Nature.

HST/Keck false color composite image of galaxy BX442. Credit: David Law/Dunlap Institute for Astronomy & Astrophysics

The researchers noticed the galaxy, identified as BX442, in images they obtained using the Hubble Space Telescope (HST). Law’s co-investigator Alice Shapley, from UCLA, remembers coming across the oddity. “Among the irregular and clumpy galaxies of the early Universe, this well-ordered spiral stuck out like a sore thumb—a beautiful and amazing sore thumb.”

But, while the Hubble image revealed the galaxy’s spiral structure, it didn’t prove conclusively that the galaxy rotated. In order to settle this question, Law and Shapley used the Keck II telescope in Hawaii to study the object’s internal motions. The twin Keck telescopes, each with 10-metre diameter primary mirrors, are the largest optical/infrared telescopes in the world. The Keck II is equipped with a laser-guide-star adaptive-optics system which corrects for the distortion of in-coming light caused by the Earth’s turbulent atmosphere, resulting in images as sharp as those taken with the HST.

U of T’s David Law pictured in front of an artist’s rendering of the spiral galaxy BX442. Credit: Dunlap Institute for Astronomy & Astrophysics

Law and Shapley used an integral-field spectrograph called OSIRIS (OH-Suppressing Infrared Imaging Spectrograph) on the Keck II telescope to sample light from different parts of the galaxy. These samples showed that those parts were moving at different speeds relative to us—revealing that it is indeed a spiral disk, rotating roughly as fast as our own Milky Way Galaxy, but much thicker and forming stars more rapidly.

While the spiral structure and rotation have been confirmed, the reason for the spiral structure remains a mystery; it’s unclear why this galaxy has been able to form such sweeping spiral structures so much earlier than other galaxies. According to Shapley, “Immediately, we started wondering how such a spiral galaxy might form in the early Universe.” One possibility, Law suggests, is the presence of a dwarf companion galaxy that they observe in the process of merging with the main galaxy.

Just as Messier 51 is subject to tidal forces from a dwarf companion of its own, gravitational interaction with the newly-discovered galaxy’s dwarf companion might help excite transient spiral structure within the main galaxy. Understanding this mechanism in greater detail could help explain the formation and evolution of modern spirals like our own Milky Way Galaxy.

Source: The University of Toronto