When Did Galaxies Get Their Spirals?
What makes a spiral galaxy, well, spiral? And how long does it take them to get in a spin? Continue reading →
Look in any given point in the sky and you will see galaxies. Billions and billions and billions of galaxies. Look closer and you'll find they can be categorized into three main types of galaxy, based on their apparent shape: elliptical, spiral, and irregular. But what makes a spiral galaxy, well, spiral? And how long does it take them to get in a spin?
In a fascinating study to be published in the Astrophysical Journal, married astronomer team Debra Elmegreen (of Vassar College in Poughkeepsie, New York) and Bruce Elmegreen (at IBM's T.J. Watson Research Center in Yorktown Heights, New York) looked to the famous Hubble Ultra-Deep Field (UDF) observation of a tiny, ‘empty' patch of sky in the constellation Fornax. The observation gathered data from September 2003 to January 2004, capturing light that was generated right at the dawn of the Universe.
The ground-shaking revelation to come from the UDF is that even a tiny region of the sky that appears to be empty is actually stuffed full of faint, distant galaxies and in this particular observation, around 10,000 galaxies can be seen.
After some intense scrutiny, the researchers were able to pick out 269 spiral galaxies in the UDF, but whittled that number down to 41 - the others were discarded due to the lack of red-shift data (a metric that would reveal the galaxy's distance and therefore its age) or the inability to clearly see a spiral pattern.
But of those 41 galaxies, the Elmegreens were able to sub-divide them into five morphological classifications - from the clumpy-armed spirals that had a "wooly" appearance and two symmetrical spiral arm galaxies (designated "Grand Design" galaxies) to more mature, multi-armed spiral structures, not too dissimilar to our galaxy. The different classifications painted a picture of spiral galaxy evolution and has now given astronomers a very privileged look into when the spirals of a galaxy formed in the early Universe.
"The onset of spiral structure in galaxies appears to occur between redshifts 1.4 and 1.8 when disks have developed a cool stellar component, rotation dominates over turbulent motions in the gas, and massive clumps become less frequent," write the astronomers.
The redshift of a galaxy directly relates to that galaxy's age. As the Universe expands, ancient light traveling through the universe will get stretched. This ‘light-stretching' is known as redshift. The higher the redshift, the further the light has traveled, so the older it is.
Therefore, from the redshift measurements of this small collection of galaxies in the UDF, the researchers have found that a definite spiral galaxy structure begins to form for galaxies at redshift 1.8, which equates to approximately 3.7 billion years after the Big Bang. However, these are only the embryos of spiral galaxies, the "woolly"-type galaxies with very basic structures smeared with nebulous clouds of star formation. It's not until approximately 8 billion years after the Big Bang (redshift 0.6) that more complex, multi-arm spiral structures form.
"The observations of different spiral types are consistent with the interpretation that clumpy disks form first and then transition to spirals as the accretion rate and gas velocity dispersion decrease, and the growing population of old fast-moving stars begins to dominate the disk mass," they write.
In a nutshell, early galaxies are a turbulent mess of gas, dust and voracious star formation. These tumultuous times are not conducive to the galaxy settling into a more refined spiral structure. But given enough time, older stars begin to dominate the galactic landscape as the once-giant star formation regions shrink. These factors limit the instabilities throughout the galaxy, heralding a long, quiescent spiral galaxy structure not too dissimilar to the Milky Way's shape some 13.75 billion years after the Big Bang.
As pointed out by The Physics arXiv Blog, although this research goes a long way to describe the evolution of galaxies in the earlier phases of the Universe, it would be interesting to see how dark matter factors in. Dark matter is known to pervade the known Universe and has been linked with galaxy growth. Also, how do the supermassive black holes, known to lurk in the centers of the majority of galaxies, factor into the evolution of spiral galaxies?
This study highlights the incredible power of the Hubble Space Telescope and proves that the data it provides continues to transform how we view the Cosmos.
Pre-print of publication: The Onset of Spiral Structure in the Universe, Elmegreen and Elmegreen, 2013. arXiv:1312.2215 [astro-ph.CO]
The Hubble Ultra Deep Field, is an image of a small region of space in the constellation Fornax, composited from Hubble Space Telescope data accumulated over a period from Sept. 3, 2003 through Jan. 16, 2004. The patch of sky in which the galaxies reside was chosen because it had a low density of bright stars in the near-field.
To celebrate its 23rd year in space, the Hubble Space Telescope snapped this view of the famous Horsehead nebula in infrared light. Usually obscured by the thick clouds of dust and gas, baby stars can be seen cocooned inside this stellar nursery. For the last 23 years, Hubble has been looking deep into the Cosmos returning over a million observations of nebular such as this, but also planets, exoplanets, galaxies and clusters of galaxies. The mission is a testament to the the human spirit to want to explore and discover. Here are some of our favorite recent observations to come from the veteran mission.
Light from an ancient galaxy 10 billion light-years away has been bent and magnified by the galaxy cluster RCS2 032727-132623. Without the help of this lensing effect, the distant galaxy would be extremely faint.
This is 30 Doradus, deep inside the Tarantula Nebula, located over 170,000 light-years away in the Large Magellanic Cloud, a small satellite galaxy of the Milky Way. 30 Doradus is an intense star-forming region where millions of baby stars are birthed inside the thick clouds of dust and gas.
NGC 3314 is actually two galaxies overlapping. They’re not colliding – as they are separated by tens of millions of light-years – but from our perspective, the pair appears to be in a weird cosmic dance.
Arp 116 consists of a very odd galactic couple. M60 is the huge elliptical galaxy to the left and NGC 4647 is the small spiral galaxy to the right. M60 is famous for containing a gargantuan supermassive black hole in its core weighing in at 4.5 billion solar masses.
With help from the Karl G. Jansky Very Large Array (VLA) radio telescope in New Mexico, Hubble has observed the awesome power of the supermassive black hole in the core of elliptical galaxy Hercules A. Long jets of gas are being blasted deep into space as the active black hole churns away inside the galaxy’s nucleus.
The striking Sharpless 2-106 star-forming region is approximately 2,000 light-years from Earth and has a rather beautiful appearance. The dust and gas of the stellar nursery has created a nebula that looks like a ‘snow angel.’
NGC 922 is a spiral galaxy with a difference. Over 300 million years ago, a smaller galaxy (called 2MASXI J0224301-244443) careened through the center of its disk causing a galactic-scale smash-up, blasting out the other side. This massive disruption generated waves of gravitational energy, triggering pockets of new star formation – highlighted by the pink nebulae encircling the galaxy.
Four hundred years ago a star exploded as a type 1a supernova in the Large Magellanic Cloud (LMC) some 170,000 light-years from Earth. This is what was left behind. The beautiful ring-like structure of supernova remnant (SNR) 0509-67.5 is highlighted by Hubble and NASA’s Chandra X-ray space observatory observations. The X-ray data (blue/green hues) are caused by the shockwave of the supernova heating ambient gases.
The intricate wisps of thin gas (billions of times less dense than smoke in our atmosphere) from Herbig-Haro 110 are captured in this stunning Hubble observation. Herbig-Haro objects are young stars in the throes of adolescence, blasting jets of gas from their poles.
Contained within an area a fraction of the diameter of the moon, astronomers counted thousands of galaxies in the deepest observation ever made by Hubble. Combining 10 years of Hubble observations, the Hubble eXtreme Deep Field (XDF) has picked out galaxies that were forming when the Universe was a fraction of the age it is now.