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The Realm of Ghosts
Will Gater dives into the region of Virgo, one of the most extraordinary swaths of the night sky, to reveal the science behind the faint galaxies that reside there.
Cast your eye across the band of sky between the star Spica and the handle of The Big Dipper on a clear spring night and what do you see? Save for few stars there doesn't appear to be much to write home about — at least not to the naked eye.
Yet this great swathe of the celestial sphere is arguably one of the most extraordinary patches of our night sky as it's peppered with hundreds of galaxies. Virgo is swarming with faintly glowing celestial forms; there are delicate spiral wisps, fuzzy ellipticals and even great clusters of galaxies gathering in their multitudes. And though they may be hidden to the naked eye, these spectral, stellar gatherings are of enormous interest to astrophysicists. So come with us as we explore the science of some of the more famous inhabitants of Virgo's ghostly galactic realm.
The Sombrero Galaxy
Virgo is a constellation famed for its huge population of distant celestial smudges, one of which is our first object, Messier 104. M104 actually sits close to the border between Virgo and the more southerly constellation Corvus, approximately 11° from the bright star Spica (Alpha Virginis). M104 is more commonly known as the Sombrero Galaxy and it's not difficult to see how it acquired this name when you look at it through a large telescope or see images of it taken by astrophotographers. Its scientific story is every bit as striking as its visual appearance too.
Perhaps its most obvious feature is the dark swathe across the bright mass of stars that make up its glowing oval shape. The swathe is a silhouetted portion of the galaxy's disc of dust and gas, which is viewed edge on from our line of sight. Hubble Space Telescope images have shown this disc in remarkable detail, revealing intricate structures in the dust lanes there. Infrared observations made with the Spitzer Space Telescope meanwhile have revealed that, unusually, M104's disc sits within another, larger elliptical galaxy, only part of which we see in visible light and which only becomes more fully apparent at longer infrared wavelengths.
Astronomy is full of mind-bending physics — and there's no shortage of weird and wonderful behavior in and around galaxies. Nowhere is this better demonstrated than when distant galaxies swarm together in vast clusters. Abell 1689 is one such galaxy cluster that astronomers have scrutinized intensely in recent decades. It lies at the heart of Virgo, around 7.5° east of the bright star Porrima (Gamma Virginis). At a distance of over two billion lightyears from us, and extremely faint, this cluster is not one you'll be tracking down through the eyepiece of a modest back-garden telescope. But thanks to the powerful orbiting eye of the Hubble Space Telescope, this faraway galactic gathering has been imaged in spectacular detail revealing a lot more than just the individual glowing members of the cluster itself.
Scan your eyes over Hubble's image of Abell 1689 (right) and you might see what makes the cluster so interesting. Scattered throughout it are thin, hair-like arcs of light. These aren't exotic celestial structures, but highly warped visions of other galaxies that sit far beyond the cluster. These arcs appear because the huge combined mass of the cluster galaxies distorts the space surrounding it, causing it to behave like a lens. Though the quality of the image provided by this gravitational lens might raise eyebrows in amateur telescope-making circles, the lens shares one key trait with the telescope lenses we use: it can reveal distant objects that we might otherwise be unable to see. Indeed in 2008 researchers announced that they'd used Hubble, in conjunction with the Abell 1689's gravitational lens, to observe a distant galaxy in the early Universe, some 700 million years after the Big Bang.
Look at any image of the rich fields of galaxies in and around the constellation of Virgo, and among the stars and galactic swirls that fill your view you'll see numerous bright ovals of light. These are elliptical galaxies and although they may not have the beauty or spectacular star-forming regions of their spiral cousins these often vast stellar swarms are some of the most enigmatic intergalactic inhabitants we know of. Foremost among the ellipticals in this part of the sky is the gargantuan M87. It's truly a giant — a recent study by astronomers at the European Southern Observatory was able to determine the size of the halo of stars around the galaxy: the ring of stars spans about 980,000 lightyears, dwarfing the Milky Way's stellar halo, which measures roughly 640,000 lightyears across.
However, M87's most famous feature is not its size but what lies at its heart: a supermassive black hole. Unlike the Milky Way's central black hole M87's is active. Images of the galaxy show an enormous jet emanating from the black hole; the jet is glowing due to light released by high-energy particles that are racing at tremendous speeds along magnetic field lines within it.
Aside from the jet and some globular clusters, though, the rest of M87 might seem rather bland in visible-light. At other wavelengths, however, a hidden maelstrom of activity in and around the enormous galaxy is revealed. Radio telescopes, for example, have observed huge glowing streams of material associated with the black-hole jet, while X-ray images from the orbiting Chandra observatory show immense swirling clouds of superheated gas within the galaxy. Something to consider the next time you set eyes on that seemingly placid, fuzzy patch in your telescope's eyepiece.
The Virgo Galaxy Cluster
Ask a seasoned stargazer to name one of their favorite spring galaxies to observe and chances are it'll be located in Virgo. The constellation boasts an extraordinary array of galactic treasures, including some of the most famous in the sky. It's perhaps no surprise then that many of the galaxies crowding into this part of the heavens are associated, that is they're all part of an enormous — and in cosmic terms relatively nearby — grouping known as the Virgo Galaxy Cluster. Recent surveys suggest that there are some 1,900 galaxies in this cluster, which sits roughly 56 million lightyears from the Milky Way.
The cluster counts within its number many relatively bright galaxies that are familiar to amateur astronomers — for example M87 as well as M86, M84 and the others that make up the sweeping curve of galaxies known as Markarian's Chain. The heart of the cluster itself lies in the region around 6° west of the star Vindemiatrix (Epsilon Virginis). However, modern studies have shown that there are members of the cluster spread all over this patch of sky, with some in neighboring constellations of Coma Berenices and Leo too.
Looking out into the cosmos, the distances to even the nearest galaxies can seem immense, and yet spiral galaxies collide frequently. As two galaxies approach, their gravitational interactions cause them to distort each other.
You can get a sense of what happens when spiral galaxies engage this way if you look into the constellation of Virgo — specifically within Markarian's Chain. In the chain are two galaxies — known as The Eyes — that lie roughly 50 million lightyears from us. The pair are catalogued as NGC 4438 and NGC 4435, and deep images of NGC 4438 show a contorted jumble of scattered dust lanes and ribbon-like streams of stars around a brighter, central region. Astronomers think that what we're seeing in NGC 4438 is actually a spiral galaxy that's been disrupted by a violent encounter with the elliptical galaxy M86, which now sits less than 0.5° away on the sky.
The Whirlpool Galaxy
This journey across Virgo's ethereal realm of galaxies has taken us across some 60° of the sky and we end our exploration of this extraordinary region with one of the most beautiful galaxies anywhere on the celestial sphere. M51, otherwise known as the Whirlpool Galaxy, has captivated astronomers for centuries and continues to intrigue both amateurs and professionals today. M51 was being scrutinized by astronomers long before its true nature — as a galaxy in its own right and not just another glowing nebula within the Milky Way — was really known.
William Parsons, the third Earl of Rosse, famously sketched M51 in 1845 using the enormous Leviathan of Parsonstown, a 72-inch reflecting telescope housed at Birr Castle in Ireland. His exquisite drawing clearly depicts the sweeping form of the Whirlpool — and its neighbor, the galaxy NGC 519 that's instantly recognizable in the astro images taken with today's photographic equipment.
Our perspective of M51, looking down on the galaxy's disc, affords us a superb view of the physics unfolding there. Within the disc, density waves have formed spiral arms, which are home to vast numbers of hot, relatively young, blue stars. Photographs of the galaxy reveal another striking feature of these arms: numerous crimson patches of light scattered throughout M51's disc. This feature is one that, just like the hot young stars, is testament to the star formation occurring there. These crimson patches are regions where the radiation of infant and newborn stars is exciting their surrounding maternal nebulae, causing the gas clouds to shine with the characteristic ruby hue of glowing hydrogen.
These dramatic flourishes of star formation aren't the only dynamism on display with the Whirlpool Galaxy either. NGC 5195 is interacting with M51 and long-exposure images of the pair show extensive swathes of stars — known as tidal streams — near the galaxies that have been drawn out during this gravitational dance.
See the Galaxies
Although none of the galaxies we've covered here are visible to the naked eye, several, such as M104 (the Sombrero Galaxy) and M51 (the Whirlpool Galaxy), are fine sights through amateur telescopes. If you've never observed a distant galaxy through a telescope before, you'll soon realize why many astronomers affectionately refer to deep-sky objects as faint fuzzies. It's a description that sums up rather well the view of many galaxies through the eyepiece of a modest amateur telescope: a faint, fuzzy blob. That's not to say there aren't brighter examples that show more structure or interesting features, such as M104's dark bar, though. As with many celestial objects the key to seeing more detail is to get away from light pollution and use a larger aperture telescope. If you don't have one then pay a visit to your local astronomical society observing evening or star party during the galaxy seasons of spring and autumn. These events often provide access to large-aperture instruments.
ABOUT THE WRITER
Will Gater is an astronomy journalist, author and presenter. Follow him on Twitter at @willgater or visit willgater.com
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