Faculty of Physics, Astronomy and Applied Computer Science, Jagiellonian University, Kraków, National Centre for Nuclear Research, Warsaw, Academia, Polish Academy of Sciences
Dr. Agnieszka Pollo is an observational cosmologist, studying the clustering
and evolution of galaxies..
Department of Particle and Astrophysical Science, Nagoya University, National Centre for Nuclear Research, Warsaw
Dr. Katarzyna Małek is an astrophysicist specializing in classifying galaxies, modeling their spectra and evolution.
Faculty of Mathematics and Natural Sciences, Jan Kochanowski University, Kielce
Dr. Janusz Krywult is an astrophysicist studying the shapes, evolution and properties of different types of galaxies.
Department of Particle and Astrophysical Science, Nagoya University
Dr. Aleksandra Solarz graduated from the Jagiellonian University, where she also defended her doctorate. She is currently working as a post-doc at Nagoya University. She specializes in galaxy classification and clustering..
During the first half of the last century, Edwin Hubble demonstrated that many objects clearly visible with simple telescopes and hitherto assumed to be nebulae in the Milky Way are, in fact, far larger and more distant “stellar islands,” in other words galaxies. Many galaxy classification systems have been developed since then, although the system proposed by Hubble himself remains the most popular, with just a few modifications. It situates galaxies on a tuning fork-shaped diagram, dividing them into three broad classes by appearance: elliptical, spiral, and lenticular. Although later classification systems include additional morphological characteristics, the two most important types of galaxies are elliptical (smooth, and egg-shaped or almost spherical) and disk-shaped (usually with a distinctive spiral structure).
Disks and ellipsoids
Spiral and elliptical galaxies differ in more than just shape. Elliptical galaxies are usually larger and brighter, they contain very little dust or gas, and since they are populated with older stars, they tend to be reddish. They have been termed “red and dead,” since no new stars are formed within them. In spiral galaxies, on the other hand, star formation occurs at a high rate, giving them a bluish hue (young stars are hotter than old stars, and therefore they emit light at shorter – bluer – wavelengths). Spiral galaxies are also usually smaller and less luminous.
It seems natural to interpret this in terms of progressive aging: elliptical galaxies are the mature, even old ones, while spiral galaxies are the youngsters, still growing. But is this really the case? The concept has always seemed tempting, but there is a crucial problem: for a spiral galaxy to have evolved into an elliptical galaxy passively – with no external input – a far longer time would need to have elapsed than the age of our Universe. Additionally, spiral galaxies do contain very old stars as well, which suggests that they must have been formed as long ago as elliptical galaxies. So what makes some galaxies preserve a semblance of youth, while others appear well-aged?
Nature or nurture?
The question of the origins of different classes of galaxies boils down to the kind of “nature vs. nurture” dichotomy that often arises in the natural sciences – for galaxies, “nature” would mean how a galaxy first came into being, “nurture” the later influence of its environment. There are strong arguments supporting both sides of this debate.
Arguments in favor of “nature” point to the conditions under which a galaxy was formed, in particular the mass of its dark matter halo. The gravitational potential of the most massive halos is powerful enough to rapidly attract matter from all around. Prior to such a galaxy’s formation matter would have been present in sufficiently high quantities to facilitate the formation of myriad young stars. The most massive stars soon end their lives as supernovae, with their explosions unleashing powerful stellar winds eliminating any remaining dust and gas from such a galaxy, which in turn makes it impossible for new generations of stars to be formed within it.
For less massive halos, the accretion of matter to form a young galaxy would have proceeded at a slower rate. On one hand, this causes the formation of a spinning galactic disk, and on the other it brings about a slower process of star formation, present until today.
What about “nurture”? As we all know, galaxies do not exist in isolation. Various types of interactions between them are hardly rare today, and observations of the distant Universe indicate that they were even more common in the past. An extreme example is a collision of two objects leading to the formation of a single, larger galaxy. However, it is sufficient for neighboring galaxies to pass near one another for the gravitational pull to affect their structure or even strip them of some of their matter. Elliptical galaxies occur almost exclusively in densely-packed clusters – so perhaps these frequent collisions and interactions between them accelerated their evolution and led them to take on their current form?
Both theories provide a good explanation for the fact that elliptical galaxies are generally larger and more massive than spiral galaxies, and that they are found where the density of dark matter is likely to have always been greater. So where does the truth lie? It’s likely somewhere in the middle; the current view is that the galaxies we see today were formed through a combination of both mechanisms. What we don’t know is their respective contribution at various stages of evolution of galaxies. We also do not fully understand the influence of other factors, such as active galactic nuclei.
Ever deeper, ever further…
To understand the history of galaxies, we need to peer very far back – ideally back to the time when the first clumps of luminescent matter were being formed. We are now reasonably familiar with the Universe closest to us – the “modern Universe” – at distances of around 2-3 billion light years from Earth. Does that sound like a lot? Only at first glance, since the Universe is around 13 billion years old. In order to solve the puzzle of the origin of galaxies, we must first look deeper into space and further back into the past.
And that’s not easy. Distant objects may be numerous, but they are very faint, so their observation requires extremely powerful telescopes and long observation periods. Checking which objects actually are distant galaxies requires expensive and time-consuming spectroscopic measurements. Images produced by coupled-charge devices show distant galaxies as blurry spots, so studying their physical properties is more difficult than for light sources found closer to Earth.
Deep sky catalogues, listing distant galaxies in their thousands – which allow astronomers to conduct statistical analyses – have only arisen during the last decade.
Halfway from the Big Bang
The most extensive catalogue of the distant Universe is the VIMOS Public Extragalactic Redshift Survey (VIPERS), aiming to create the world’s largest 3D map of the Universe as it existed 7-8 billion years ago. The target is to provide precise measurements of the locations and properties of 100,000 galaxies. In 2013, VIPERS published a 3D map of the Universe at half its present age, based on data from measurements of 55,000 galaxies. It is the largest and most detailed map of such a distant Universe available at present. Soon after that, the VIPERS team released redshift measurements to the international astronomical community, while the following year, after an arduous data reduction process, it also released their spectra.
VIPERS is a large program carried out by the European Southern Observatory’s Very Large Telescope (ESO VLT). The project requires a total of 440 hours of observation time from Melipal, one of ESO’s 8.2 meter VLT units. The project is being coordinated by an international team, which includes an active Polish “node” of astronomers from the National Centre for Nuclear Research, the Jagiellonian University and the Jan Kochanowski University in Kielce.
The map of the Universe at half its present age, created using data provided by VIPERS, is surprisingly similar to the map of the present-day Universe. In particular, it clearly shows that galaxies were already exhibiting properties allowing us to classify them into the types we see today. In large clusters – nodes of the cosmic network – groups of red galaxies, similar to today’s elliptical galaxies, are visible. Blue galaxies – centers of star formation and the counterparts of present-day spiral galaxies – are distributed in less densely-populated regions. Observations of their shapes show that red galaxies were already spheroidal, and blue galaxies were disk-shaped. This means that the basic types of galaxies must have been shaped far earlier.
All available information indicates that the differing types of galaxies we see today are actually of similar age. They were all formed during relatively early stages of the Universe’s evolution; the young appearance of some of them is due to a combination of external and internal factors, most likely to do with the mass of the local dark mater halo.
So are there really no genuinely young galaxies in our Universe? Perhaps there are. Astronomers believe that some such objects might be among the inconspicuous dwarf galaxies. The majority have a long history, but some may be located in halos with such low masses that they have only recently accumulated sufficient baryonic matter to be able to form their first stars. Astronomers are very keen to find such galaxies in our vicinity, since they may act as laboratories – testing grounds allowing them to follow the processes that were taking place within young galaxies in the early Universe. But the similarities are limited. Dwarfs which have only recently started along the way of star formation do not have a bright future ahead. Given their relatively minuscule mass, it would take just a few supernovae for the galactic wind to completely remove any remaining dust and gas to beyond their gravitational fields. For such galaxies, the first episodes of star formation may also prove to be the last – unless a close encounter with another galaxy stirs them into life again.
The research is supported by the Polish National Science Centre grants UMO-2012/07/B/ST9/ and UMO-2013/09/D/ST9/04030.
Guzzo L. et al. (2014). The VIMOS Public Extragalactic Redshift Survey (VIPERS). An unprecedented view of galaxies and large-scale structure at 0.5 < z < 1.2. Astronomy & Astrophysics, 566. A108, 1-21.
Garilli B. et al. (2013). The VIMOS Public Extragalactic Survey (VIPERS). First Data Release of 57 204 spectroscopic measurements. Astronomy & Astrophysics, 562, A23, 1-18.
Malek K. et al. (2013). The VIMOS Public Extragalactic Redshift Survey (VIPERS). A support vector machine classification of galaxies, stars, and AGNs. Astronomy & Astrophysics, 557, A16, 1-16.
© Academia 3 (43) 2014