Picture of the Week

Also see our pictures of the week on eso.org.
8 April 2013

Dusty detail in elliptical galaxy NGC 2768

The soft glow in the picture above is NGC 2768, an elliptical galaxy located in the northern constellation of Ursa Major (The Great Bear). It appears here as a bright oval on the sky, surrounded by a wide, fuzzy cloud of material. This image, taken by the NASA/ESA Hubble Space Telescope, shows the dusty structure encircling the centre of the galaxy, forming a knotted ring around the galaxy’s brightly glowing middle. Interestingly, this ring lies perpendicular to the plane of NGC 2768 itself, stretching up and out of the galaxy.

The dust in NGC 2768 forms an intricate network of knots and filaments. In the centre of the galaxy are two tiny, S-shaped symmetric jets. These two flows of material travel outwards from the galactic centre along curved paths, and are masked by the tangle of dark dust lanes that spans the body of the galaxy.

These jets are a sign of a very active centre. NGC 2768 is an example of a Seyfert galaxy, an object with a supermassive black hole at its centre. This speeds up and sucks in gas from the nearby space, creating a stream of material swirling inwards towards the black hole known as an accretion disc. This disk throws off material in very energetic outbursts, creating structures like the jets seen in the image above.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Judy Schmidt. One of her images, of newborn star XZ Tauri, was awarded third prize.


1 April 2013

Light and dust in a nearby starburst galaxy

Visible as a small, sparkling hook in the dark sky, this beautiful object is known as J082354.96+280621.6, or J082354.96 for short. It is a starburst galaxy, so named because of the incredibly (and unusually) high rate of star formation occurring within it.

One way in which astronomers probe the nature and structure of galaxies like this is by observing the behaviour of their dust and gas components; in particular, the Lyman-alpha emission. This occurs when electrons within a hydrogen atom fall from a higher energy level to a lower one, emitting light as they do so. This emission is interesting because this light leaves its host galaxy only after extensive scattering in the nearby gas — meaning that this light can be used as a pretty direct probe of what a galaxy is made up of.

The study of this Lyman-alpha emission is common in very distant galaxies, but now a study named LARS (Lyman Alpha Reference Sample) [1] is investigating the same effect in galaxies that are closer by. Astronomers chose fourteen galaxies, including this one, and used spectroscopy and imaging to see what was happening within them. They found that these Lyman-alpha photons can travel much further if a galaxy has less dust — meaning that we can use this emission to infer how dusty the source galaxy is.

The LARS study relies heavily on the high resolving power of Hubble. When Hubble is decommissioned, no telescope will be able to make observations like this in the far ultraviolet part of the spectrum — meaning that small, glittering galaxies imaged and probed by studies like LARS may give us some of the most detailed data we have to work with for some time to come.

Credit: ESA/Hubble & NASA, M. Hayes

Notes

[1] Hayes, Östlin et al., The Lyman Alpha Reference Sample: extended Lyman alpha halos produced at low dust content, The Astrophysical Journal, 2013.


25 March 2013

Masquerading as a double star

The object in this image is Jonckheere 900 or J 900, a planetary nebula — glowing shells of ionised gas pushed out by a dying star. Discovered in the early 1900s by astronomer Robert Jonckheere, the dusty nebula is small but fairly bright, with a relatively evenly spread central region surrounded by soft wispy edges.

Despite the clarity of this Hubble image, the two objects in the picture above can be confusing for observers. J 900’s nearby companion, a faint star in the constellation of Gemini, often causes problems for observers because it is so close to the nebula — when seeing conditions are bad, this star seems to merge into J 900, giving it an elongated appearance. Hubble’s position above the Earth’s atmosphere means that this is not an issue for the space telescope.

Astronomers have also mistakenly reported observations of a double star in place of these two objects, as the planetary nebula is quite small and compact.

J 900’s central star is only just visible in this image, and is very faint — fainter than the nebula’s neighbour. The nebula appears to display a bipolar structure, where there are two distinct lobes of material emanating from its centre, enclosed by a bright oval disc.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Josh Barrington.


18 March 2013

Galactic glow worm

This charming and bright galaxy, known as IRAS 23436+5257, was captured by the the NASA/ESA Hubble Space Telescope. It is located in the northern constellation of Cassiopeia, which is named after an arrogant, vain, and yet beautiful mythical queen.

The twisted, wormlike structure of this galaxy is most likely the result of a collision and subsequent merger of two galaxies. Such interactions are quite common in the Universe, and they can range from minor interactions involving a satellite galaxy being caught by a spiral arm, to major galactic crashes. Friction between the gas and dust during a collision can have a major effect on the galaxies involved, morphing the shape of the original galaxies and creating interesting new structures.

When you look up at the calm and quiet night sky it is not always easy to picture it as a dynamic and vibrant environment with entire galaxies in motion, spinning like children’s toys and crashing into whatever crosses their path. The motions are, of course, extremely slow, and occur over millions or even billions of years.

The aftermath of these galactic collisions helps scientists to understand how these movements occur and what may be in store for our own Milky Way, which is on a collision course with a neighbouring galaxy, Messier 31.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Hidden Treasures was an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition has now closed and the results are published here.


11 March 2013

One ring to rule them all

Galaxies can take many forms — elliptical blobs, swirling spiral arms, bulges, and discs are all known components of the wide range of galaxies we have observed using telescopes like the NASA/ESA Hubble Space Telescope. However, some of the more intriguing objects in the sky around us include ring galaxies like the one pictured above — Zw II 28.

Ring galaxies are mysterious objects. They are thought to form when one galaxy slices through the disc of another, larger, one — as galaxies are mostly empty space, this collision is not as aggressive or as destructive as one might imagine. The likelihood of two stars physically colliding is minimal, and it is instead the gravitational effects of the two galaxies that causes the disruption.

This disruption upsets the material in both galaxies, causing it to redistribute to form a dense central core, encircled by bright stars. All this commotion causes clouds of gas and dust to collapse and triggers new periods of intense star formation in the outer ring, which is thus full of hot, young, blue stars and regions that are actively giving rise to new stars.

The sparkling pink and purple loop of Zw II 28 is not a typical ring galaxy due to its lack of a visible central companion. For many years it was thought to be a lone circle on the sky, but observations using Hubble have shown that there may be a possible companion lurking just inside the ring, where the loop appears to double back on itself. The galaxy has a knotty, swirling ring structure, with some areas appearing much brighter than others.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt.


4 March 2013

Blue bursts of hot young stars

This image, speckled with blue, white, and yellow light, shows part of the spiral galaxy IC 5052. Surrounded by distant stars and galaxies, it emits a bright blue-white glow which highlights its narrow, intricate structure. It is viewed side-on in the constellation of Pavo (The Peacock), in the southern sky.

When spiral galaxies are viewed from this angle, it is very difficult to fully understand their properties and how they are arranged. IC 5052 is actually a barred spiral galaxy – its pinwheeling arms do not begin from the centre point but are instead attached to either end of a straight "bar" of stars that cuts through the galaxy's middle. Approximately two thirds of all spirals are barred, including the Milky Way.

Bursts of pale blue light are visible across the galaxy's length, partially blocked out by weaving lanes of darker gas and dust. These are pockets of extremely hot newborn stars. The bars present in spirals like IC 5052 are thought to help these formation processes by effectively funnelling material from the swirling arms inwards towards these hot stellar nurseries. 

A version of this image was submitted to the Hubble's Hidden Treasures image processing competition by contestant Serge Meunier.


25 February 2013

Glowing, fiery shells of gas

It may look like something from The Lord of the Rings, but this fiery swirl is actually a planetary nebula known as ESO 456-67. Set against a backdrop of bright stars, the rust-coloured object lies in the constellation of Sagittarius (The Archer), in the southern sky.

Despite the name, these ethereal objects have nothing at all to do with planets; this misnomer came about over a century ago, when the first astronomers to observe them only had small, poor-quality telescopes. Through these, the nebulae looked small, compact, and planet-like — and so were labelled as such.

When a star like the Sun approaches the end of its life, it flings material out into space. Planetary nebulae are the intricate, glowing shells of dust and gas pushed outwards from such a star. At their centres lie the remnants of the original stars themselves — small, dense white dwarf stars.

In this image of ESO 456-67, it is possible to see the various layers of material expelled by the central star. Each appears in a different hue — red, orange, yellow, and green-tinted bands of gas are visible, with clear patches of space at the heart of the nebula. It is not fully understood how planetary nebulae form such a wide variety of shapes and structures; some appear to be spherical, some elliptical, others shoot material in waves from their polar regions, some look like hourglasses or figures of eight, and others resemble large, messy stellar explosions — to name but a few.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Jean-Christophe Lambry


18 February 2013

A glowing jet from a young star

This image shows an object known as HH 151, a bright jet of glowing material trailed by an intricate, orange-hued plume of gas and dust. It is located some 460 light-years away in the constellation of Taurus (The Bull), near to the young, tumultuous star HL Tau.

In the first few hundred thousand years of life, new stars like HL Tau pull in material that falls towards them from the surrounding space. This material forms a hot disc that swirls around the coalescing body, launching narrow streams of material from its poles. These jets are shot out at speeds of several hundred kilometres per second and collide violently with nearby clumps of dust and gas, creating wispy, billowing structures known as Herbig-Haro objects — like HH 151 seen in the image above.

Such objects are very common in star-forming regions. They are short-lived, and their motion and evolution can actually be seen over very short timescales, on the order of years. They quickly race away from the newly-forming star that emitted them, colliding with new clumps of material and glowing brightly before fading away.

A version of this image was entered into the Hidden Treasures image processing competition by Gilles Chapdelaine.


11 February 2013

Cosmic “flying V” of merging galaxies

This large “flying V” is actually two distinct objects — a pair of interacting galaxies known as IC 2184. Both the galaxies are seen almost edge-on in the large, faint northern constellation of Camelopardalis (The Giraffe), and can be seen as bright streaks of light surrounded by the ghostly shapes of their tidal tails.

These tidal tails are thin, elongated streams of gas, dust and stars that extend away from a galaxy into space. They occur when galaxies gravitationally interact with one another, and material is sheared from the outer edges of each body and flung out into space in opposite directions, forming two tails. They almost always appear curved, so when they are seen to be relatively straight, as in this image, it is clear that we are viewing the galaxies side-on.

Also visible in this image are bursts of bright blue, pinpointing hot regions where the stars from both galaxies have begun to crash together during the merger.

The image consists of visible and infrared observations from Hubble’s Wide Field and Planetary Camera 2.

A version of this picture was entered into the Hubble’s Hidden Treasures image-processing competition by contestant Serge Meunier.


4 February 2013

A side-on spiral streak

This thin, glittering streak of stars is the spiral galaxy ESO 121-6, which lies in the southern constellation of Pictor (The Painter's Easel). Viewed almost exactly side-on, the intricate structure of the swirling arms is hidden, but the full length of the galaxy can be seen — including the intense glow from the central bulge, a dense region of tightly packed young stars sitting at the centre of the spiral arms.

Tendrils of dark dust can be seen across the frame, partially obscuring the bright centre of the galaxy and continuing out towards the smattering of stars at its edges, where the dust lanes and shapes melt into the inky background. Numerous nearby stars and galaxies are visible as small smudges in the surrounding sky, and the brightest stars are dazzlingly prominent towards the bottom left of the image.

ESO 121-6 is a galaxy with patchy, loosely wound arms and a relatively faint central bulge. It actually belongs to a group of galaxies, a clump of no more than 50 similar structures all loosely bound to one another by gravity. The Milky Way is also a member of a galactic group, known as the Local Group. 


28 January 2013

The moment the lights went out

The further away you look, the further back in time you see. Astronomers use this fact to study the evolution of the Universe by looking at nearby and more distant galaxies and comparing their features. Hubble is particularly well suited for this type of work because of its extremely high resolution and its position above the atmosphere. This has allowed it to detect many of the most distant galaxies known, as well as making detailed images of faraway objects.

Comparing galaxies in the distant past with those around us today, astronomers have noticed that the nearby galaxies are far quieter and calmer than their distant brethren, seen earlier in their lives. Nearby galaxies (although not the Milky Way) are often large, elliptical galaxies with little or no ongoing star formation, and their stars tend to be elderly and red in colour. These galaxies, in astronomers' language, are "red and dead".

This is not so for galaxies further away, which typically show more vigorous star birth.

The reason for this appears to be that as the Universe has aged, galaxies have often collided and merged together, and these events disrupt gas clouds within them. A merger will usually be a trigger for such intense star formation that the supply of gas is used up, and no more star formation occurs afterwards. The merged elliptical galaxy then creeps into old age, getting redder as its stars get older. This is expected to happen to the Milky Way when it merges with the nearby Andromeda Galaxy, some four billion years from now.

The galaxy in this image, catalogued as 2MASX J09442693+0429569, marks a transitional phase in this process as young, star-forming galaxies settle to become massive, red and dead galaxies.

The galaxy has tail-like features extending from it, typical of a galaxy that has recently undergone a merger. Studying the properties of the light from this galaxy, astronomers see no sign of ongoing star formation; in other words, the merger triggered an event which has used up all the gas. However, the observations suggest that star formation was strong until the very recent past, and has ceased only within the last billion years. This image therefore shows a snapshot of the moment star formation stopped forever in a galaxy. 

A version of this image was entered into the Hidden Treasures image processing competition by contestant Nick Rose.


21 January 2013

Appearances can be deceptive

Globular clusters are roughly spherical collections of extremely old stars, and around 150 of them are scattered around our galaxy. Hubble is one of the best telescopes for studying these, as its extremely high resolution lets astronomers see individual stars, even in the crowded core. The clusters all look very similar, and in Hubble’s images it can be quite hard to tell them apart – and they all look much like NGC 411, pictured here.

And yet appearances can be deceptive: NGC 411 is in fact not a globular cluster, and its stars are not old. It isn’t even in the Milky Way.

NGC 411 is classified as an open cluster. Less tightly bound than a globular cluster, the stars in open clusters tend to drift apart over time as they age, whereas globulars have survived for well over 10 billion years of galactic history. NGC 411 is a relative youngster — not much more than a tenth of this age. Far from being a relic of the early years of the Universe, the stars in NGC 411 are in fact a fraction of the age of the Sun.

The stars in NGC 411 are all roughly the same age, having formed in one go from one cloud of gas. But they are not all the same size. Hubble’s image shows a wide range of colours and brightnesses in the cluster’s stars. These tell astronomers many facts about the stars, including their mass, temperature and evolutionary phase. Blue stars, for instance, have higher surface temperatures than red ones.

The image is a composite produced from ultraviolet, visible and infrared observations made by Hubble’s Wide Field Camera 3. This filter set lets the telescope “see” colours slightly further beyond red and the violet ends of the spectrum.


14 January 2013

A busy patch of the Great Attractor

A busy patch of space has been captured in this image from the NASA/ESA Hubble Space Telescope. Scattered with many nearby stars, the field also has numerous galaxies in the background.

Located on the border of Triangulum Australe (The Southern Triangle) and Norma (The Carpenter’s Square), this field covers part of the Norma Cluster (Abell 3627) as well as a dense area of our own galaxy, the Milky Way.

The Norma Cluster is the closest massive galaxy cluster to the Milky Way, and lies about 220 million light-years away. The enormous mass concentrated here, and the consequent gravitational attraction, mean that this region of space is known to astronomers as the Great Attractor, and it dominates our region of the Universe.

The largest galaxy visible in this image is ESO 137-002, a spiral galaxy seen edge on. In this image from Hubble, we see large regions of dust across the galaxy’s bulge. What we do not see here is the tail of glowing X-rays that has been observed extending out of the galaxy — but which is invisible to an optical telescope like Hubble.

Observing the Great Attractor is difficult at optical wavelengths. The plane of the Milky Way — responsible for the numerous bright stars in this image — both outshines (with stars) and obscures (with dust) many of the objects behind it. There are some tricks for seeing through this — infrared or radio observations, for instance — but the region behind the centre of the Milky Way, where the dust is thickest, remains an almost complete mystery to astronomers.

This image consists of exposures in blue and infrared light taken by Hubble’s Advanced Camera for Surveys.


7 January 2013

An archetypal dwarf galaxy

The constellation of Ursa Major (The Great Bear) is home to Messier 101, the Pinwheel Galaxy. One of the biggest and brightest spiral galaxies in the night sky, Messier 101 is also the subject of one of Hubble's most famous images (heic0602). Like the Milky Way, Messier 101 is not alone, with smaller dwarf galaxies in its neighbourhood.

NGC 5477, one of these dwarf galaxies in the Messier 101 group, is the subject of this image from the NASA/ESA Hubble Space Telescope. Without obvious structure, but with visible signs of ongoing starbirth, NGC 5477 looks much like an archetypal dwarf irregular galaxy. The bright nebulae that extend across much of the galaxy are clouds of glowing hydrogen gas in which new stars are forming. These glow pinkish red in real life, although the selection of green and infrared filters through which this image was taken makes them appear almost white.

The observations were taken as part of a project to measure accurate distances to a range of galaxies within about 30 million light-years from Earth, by studying the brightness of red giant stars.

In addition to NGC 5477, the image includes numerous galaxies in the background, including some that are visible right through NGC 5477. This serves as a reminder that galaxies, far from being solid, opaque objects, are actually largely made up of the empty space between their stars.

This image is a combination of exposures taken through green and infrared filters using Hubble's Advanced Camera for Surveys. The field of view is approximately 3.3 by 3.3 arcminutes. 


31 December 2012

Don’t trust your eyes

The Universe loves to fool our eyes, giving the impression that celestial objects are located at the same distance from Earth. A good example can be seen in this spectacular image produced by the NASA/ESA Hubble Space Telescope. The galaxies NGC 5011B and NGC 5011C are imaged against a starry background.

Located in the constellation of Centaurus, the nature of these galaxies has puzzled astronomers. NGC 5011B (on the right) is a spiral galaxy belonging to the Centaurus Cluster of galaxies lying 156 million light-years away from the Earth. Long considered part of the faraway cluster of galaxies as well, NGC 5011C (the bluish galaxy at the centre of the image) is a peculiar object, with the faintness typical of a nearby dwarf galaxy, alongside the size of an early-type spiral.

Astronomers were curious about the appearance of NGC 5011C. If the two galaxies were at roughly the same distance from Earth, they would expect the pair to show signs of interactions between them. However, there was no visual sign of interaction between the two. How could this be possible?

To solve this problem, astronomers studied the velocity at which these galaxies are receding from the Milky Way and found that NGC 5011C is moving away far more slowly than its apparent neighbour, and its motion is more consistent with that of the nearby Centaurus A group at a distance of 13 million light-years. Thus, NGC 5011C, with only about ten million times the mass of the Sun in its stars, must indeed be a nearby dwarf galaxy rather than member of the distant Centaurus Cluster as was believed for many years.

Problem solved.

This image was taken with Hubble’s Advanced Camera for Surveys using visual and infrared filters.


24 December 2012

A wanderer dancing the dance of stars and space

The NASA/ESA Hubble Space Telescope provides us this week with a spectacular image of the bright star-forming ring that surrounds the heart of the barred spiral galaxy NGC 1097. In this image, the larger-scale structure of the galaxy is barely visible: its comparatively dim spiral arms, which surround its heart in a loose embrace, reach out beyond the edges of this frame.

This face-on galaxy, lying 45 million light-years away from Earth in the southern constellation of Fornax (The Furnace), is particularly attractive for astronomers. NGC 1097 is a Seyfert galaxy. Lurking at the very centre of the galaxy, a supermassive black hole 100 million times the mass of our Sun is gradually sucking in the matter around it. The area immediately around the black hole shines powerfully with radiation coming from the material falling in.

The distinctive ring around the black hole is bursting with new star formation due to an inflow of material toward the central bar of the galaxy. These star-forming regions are glowing brightly thanks to emission from clouds of ionised hydrogen. The ring is around 5000 light-years across, although the spiral arms of the galaxy extend tens of thousands of light-years beyond it.

NGC 1097 is also pretty exciting for supernova hunters. The galaxy experienced three supernovae (the violent deaths of high-mass stars) in the 11-year span between 1992 and 2003. This is definitely a galaxy worth checking on a regular basis.

However, what it is really exciting about NGC 1097 is that it is not wandering alone through space. It has two small galaxy companions, which dance “the dance of stars and the dance of space” like the gracious dancer of the famous poem The Dancer by Khalil Gibran.

The satellite galaxies are NGC 1097A, an elliptical galaxy orbiting 42 000 light-years from the centre of NGC 1097 and a small dwarf galaxy named NGC 1097B. Both galaxies are located out beyond the frames of this image and they cannot be seen. Astronomers have indications that NGC 1097 and NGC 1097A have interacted in the past.

This picture was taken with Hubble’s Advanced Camera for Surveys using visual and infrared filters.

A version of this image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Eedresha Sturdivant.


17 December 2012

The Needle Galaxy

Like finding a silver needle in the haystack of space, the NASA/ESA Hubble Space Telescope has produced this beautiful image of the spiral galaxy IC 2233, one of the flattest galaxies known.

Typical spiral galaxies like the Milky Way are usually made up of three principal visible components: the disc where the spiral arms and most of the gas and dust is concentrated; the halo, a rough and sparse sphere around the disc that contains little gas, dust or star formation; and the central bulge at the heart of the disc, which is formed by a large concentration of ancient stars surrounding the Galactic Centre.

However, IC 2233 is far from being typical. This object is a prime example of a super-thin galaxy, where the galaxy’s diameter is at least ten times larger than the thickness. These galaxies consist of a simple disc of stars when seen edge on. This orientation makes them fascinating to study, giving another perspective on spiral galaxies. An important characteristic of this type of objects is that they have a low brightness and almost all of them have no bulge at all.

The bluish colour that can be seen along the disc gives evidence of the spiral nature of the galaxy, indicating the presence of hot, luminous, young stars, born out of clouds of interstellar gas. In addition, unlike typical spirals, IC 2233 shows no well-defined dust lane. Only a few small patchy regions can be identified in the inner regions both above and below the galaxy’s mid-plane.

Lying in the constellation of Lynx, IC 2233 is located about 40 million light-years away from Earth. This galaxy was discovered by British astronomer Isaac Roberts in 1894.

This image was taken with the Hubble’s Advanced Camera for Surveys, combining visible and infrared exposures. The field of view in this image is approximately 3.4 by 3.4 arcminutes.

A version of this image was entered into the Hubble's Hidden Treasures image processing competition by contestant Luca Limatola.


10 December 2012

Smoky Shells

Located in a relatively vacant region of space about 4200 light-years away and difficult to see using an amateur telescope, the lonesome planetary nebula NGC 7354 is often overlooked. However, thanks to this image captured by the NASA/ESA Hubble Space Telescope we are able to see this brilliant ball of smoky light in spectacular detail.

Just as shooting stars are not actually stars and lava lamps do not actually contain lava, planetary nebulae have nothing to do with planets. The name was coined by Sir William Herschel because when he first viewed a planetary nebula through a telescope, he could only identify a hazy smoky sphere, similar to gaseous planets such as Uranus. The name has stuck even though modern telescopes make it obvious that these objects are not planets at all, but the glowing gassy outer layers thrown off by a hot dying star.

It is believed that winds from the central star play an important role in determining the shape and morphology of planetary nebulae. The structure of NGC 7354 is relatively easy to distinguish. It consists of a circular outer shell, an elliptical inner shell, a collection of bright knots roughly concentrated in the middle and two symmetrical jets shooting out from either side. Research suggests that these features could be due to a companion central star, however the presence of a second star in NGC 7354 is yet to be confirmed.

NGC 7354 resides in Cepheus, a constellation named after the mythical King Cepheus of Aethiopia and is about half a light-year in diameter.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Bruno Conti.


3 December 2012

Glitter galaxy — An edge-on view of the ESO 318-13 galaxy

The brilliant cascade of stars through the middle of this image is the galaxy ESO 318-13 as seen by the NASA/ESA Hubble Space Telescope. Despite being located millions of light-years from Earth, the stars captured in this image are so bright and clear you could almost attempt to count them.

Although ESO 318-13 is the main event in this image, it is sandwiched between a vast collection of bright celestial objects. Several stars near and far dazzle in comparison to the neat dusting contained within the galaxy. One that particularly stands out is located near the centre of the image, and looks like an extremely bright star located within the galaxy. This is, however, a trick of perspective. The star is located in the Milky Way, our own galaxy, and it shines so brightly because it is so much closer to us than ESO 318-13.

There are also a number of tiny glowing discs scattered throughout the frame that are more distant galaxies. In the top right corner, an elliptical galaxy can be clearly seen, a galaxy which is much larger but more distant than ESO 318-13. More interestingly, peeking through the ESO 318-13, near the right-hand edge of the image, is a distant spiral galaxy.

Galaxies are largely made up of empty space; the stars within them only take up a small volume, and providing a galaxy is not too dusty, it can be largely transparent to light coming from the background. This makes overlapping galaxies like these quite common. One particularly dramatic example of this phenomenon is the galaxy pair NGC 3314 (heic1208). 


26 November 2012

A Peculiar Compact Blue Dwarf Galaxy

The NASA/ESA Hubble Space Telescope provides us this week with an impressive image of the irregular galaxy NGC 5253.

NGC 5253 is one of the nearest of the known Blue Compact Dwarf (BCD) galaxies, and is located at a distance of about 12 million light-years from Earth in the southern constellation of Centaurus. The most characteristic signature of these galaxies is that they harbour very active star-formation regions. This is in spite of their low dust content and comparative lack of elements heavier than hydrogen and helium, which are usually the basic ingredients for star formation.

These galaxies contain molecular clouds that are quite similar to the pristine clouds that formed the first stars in the early Universe, which were devoid of dust and heavier elements. Hence, astronomers consider the BCD galaxies to be an ideal testbed for better understanding the primordial star-forming process.

NGC 5253 does contain some dust and heavier elements, but significantly less than the Milky Way galaxy. Its central regions are dominated by an intense star forming region that is embedded in an elliptical main body, which appears red in Hubble’s image. The central starburst zone consists of a rich environment of hot, young stars concentrated in star clusters, which glow in blue in the image. Traces of the starburst itself can be seen as a faint and diffuse glow produced by the ionised oxygen gas.

The true nature of BCD galaxies has puzzled astronomers for a long time. Numerical simulations following the current leading cosmological theory of galaxy formation, known as the Lambda Cold Dark Matter model, predict that there should be far more satellite dwarf galaxies orbiting big galaxies like the Milky Way. Astronomers refer to this discrepancy as the Dwarf Galaxy Problem.

This galaxy is considered part of the Centaurus A/Messier 83 group of galaxies, which includes the famous radio galaxy Centaurus A and the spiral galaxy Messier 83. Astronomers have pointed out the possibility that the peculiar nature of NGC 5253 could result from a close encounter with Messier 83, its closer neighbour.

This image was taken with the Hubble’s Advanced Camera for Surveys, combining visible and infrared exposures. The field of view in this image is approximately 3.4 by 3.4 arcminutes.

A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Nikolaus Sulzenauer.


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