DNA Fingerprinting of Galaxies

These days we are inundated with “hi-tech” TV shows where science plays the leading role to solve a crime scene. One of the main revolutions in the field of criminology (… or even biology!) has been DNA fingerprinting, where any trace of human tissue, blood or hair can lead to the identification of a person via its genetic code. In extragalactic astrophysics, research focuses on understanding the formation process of galaxies by observing their properties. We follow an analogous method to DNA-fingerprint our “suspects”, the stellar populations.

The stellar component of a galaxy can be modelled as an ensemble created during the different stages in its formation. If we are capable of translating the information we get at the telescopes into these ensembles, we recover the formation history of a galaxy. In this quest, it is of course very important to use a method that enables us to identify a specific ensemble. How is it possible to determine the age of a galaxy?

I am Dr Ignacio Ferreras, a reader in the astrophysics group at MSSL. My work focuses on understanding the formation of galaxies by studying in minute detail the light they emit. The DNA of a stellar population is in the form of a spectrum.

The rainbow can be considered the “DNA-fingerprint” of the Sun. If we observe it in detail (left), we would find a large number of dark patches which reveal the presence of different chemical elements in the solar atmosphere.
The rainbow can be considered the “DNA-fingerprint” of the Sun. If we observe it in detail (right), we find a large number of dark patches revealing the presence of different chemical elements in the external layers of the Sun.

The most familiar spectrum to all of us is the rainbow. Droplets of water in the Earth’s atmosphere refract the light from the Sun, separating the different wavelengths into a colourful display. We can think of the rainbow as the spectrum (i.e. the DNA) of the Sun. If we could look at this rainbow in detail, we would find that it has variations in intensity, with maximum emission around yellow light (the Sun is yellow!) and with an abundant set of “dark patches” (see the first figure). These patches only extend over a small range of colour, and are caused by the absorption of light of a specific wavelength by chemical elements in the external layers of the Sun. The good news is that the position of these dark patches can be known to a high level of precision, as the transitions involved are governed by the laws of atomic physics. For instance, the orange colour of a sodium floodlight is neatly explained by a specific transition between energy levels of the sodium atom (the same transition is seen in the solar spectrum!). Our task is then to identify these atomic and molecular lines in the spectrum of far away galaxies to understand their “chemistry”, which is intimately related to their formation.

An example of galactic DNA-fingerprinting: by taking the spectrum of a galaxy (left), we can explore in detail the different absorption regions and thus measure the amount of elements such as Magnesium or Iron. A strong trend is found between the relative amount of these two elements and the total mass of the galaxy (right). This trend is related to the amount of time it takes to form the galaxy. Massive galaxies seem to form very quickly.
An example of galactic DNA-fingerprinting: by taking the spectrum of a galaxy (left), we can explore in detail the different absorption regions and thus measure the amount of elements such as Magnesium or Iron. A strong trend is found between the relative amount of these two elements and the total mass of the galaxy (right). This trend is related to the amount of time it takes to form the galaxy. Massive galaxies seem to form very quickly.

Let me illustrate this with an example related to work we do in the astrophysics group at MSSL.

Chemistry as a stellar clock: The lifetime of any star is strongly dependent on its mass: The most massive stars (several tens of solar masses) live much shorter lifetimes than our Sun. During their short lives, they build up a good amount of chemical elements from Helium to Iron via thermonuclear reactions inside their very hot cores. When they reach their final stages, massive stars release these elements in a gigantic explosion (termed type II supernova). However, the heaviest material (Iron) gets trapped inside the core, and only a small amount of iron is ejected, whereas other elements – like oxygen or magnesium – are released in vast amounts. All this material is mixed up with the gas in the interstellar medium of the galaxy, and gets incorporated into the next generations of stars. If only these supernovae were present, the amount of iron in stars would be very low indeed. However, there is a second type of explosion (type Ia supernova) involving a binary system, where at least one of the stars is a compact object called a white dwarf (our Sun will evolve into such an object at the end of its life). In this case, gas from the companion star in the binary system falls on the white dwarf, eventually triggering a thermonuclear explosion. This explosion blows the whole system up, so the iron generated in the process is not trapped. This type of scenario takes long to develop, which means the iron-rich type Ia supernovae occur much later than the iron-poor type II. So, if all the stars in a galaxy were formed over a very short period of time (before type Ia supernovae went off), then little iron would be seen in their “DNA-spectrum”, whereas a more extended period of formation would allow for the recycling of type Ia ejecta, giving a higher iron content in their stars.

By observing specific lines attributed to chemical elements produced during either type II or type Ia supernovae, we can determine whether the stars in a galaxy were formed very quickly, or at a slow pace. The second figure of this post shows that the relative contribution of iron is found to decrease strongly with the mass of a galaxy. The stars in the most massive galaxies, therefore, are expected to form over a very short period of cosmic history. This is quite a challenging result for our understanding of galaxy formation, and  the search to find the progenitors of these very massive galaxies is on. Stay tuned.

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Observing the heavens from the summit of Mauna Kea

Observing the heavens from the summit of Mauna Kea

Hello, I’m Dr. Missagh Mehdipour, a research scientist at the Mullard Space Science Lab. In this post I will tell you a little bit about my first observing trip to a telescope.

As an X-ray astromoner, I am interested in studying the hottest objects in the universe which radiate in the X-ray energy band. However, as a consequence I don’t often travel to ground-based telescopes to observe them; X-ray astronomy can only be done from space since the Earth’s atmosphere absorbs the X-rays (on the upside those harmful radiations cannot reach us!). So when in the final year of my PhD the opportunity of an observing trip to a ground-based telescope came along, I grabbed it with both hands. My colleague Prof. Mat Page, and I travelled to the James Clerk Maxwell Telescope (JCMT) on the summit of Mauna Kea in the Island of Hawaii.

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Here is a fine view of Mauna Kea from close to our hotel in Hilo.

With a diameter of 15 m, JCMT is the largest single-dish telescope in the world operating in the sub-millimeter region of the spectrum. With its sensitive instruments it detects light from the coldest materials in the universe, with temperatures of only a few degrees above absolute zero. The observations that we carried out were part of a major survey to study our galaxy and the universe in sub-milimeter wavelengths. Since water vapour in the Earth’s atmosphere attenuates radiation in this part of the spectrum, the high altitude and dry conditions of Mauna Kea (an extinct volcano standing at 4200 m above sea level), makes it the ideal place for submilimeter astronomy. In fact since the creation of an access road in 1964, more than a dozen world-class telescopes have been constructed at the summet, making Mauna Kea one of the most important sites for ground-based astronomy.

 James Clerk Maxwell Telescope (image credit: Joint Astronomy Centre http://www.jach.hawaii.edu)
James Clerk Maxwell Telescope (image credit: Joint Astronomy Centre http://www.jach.hawaii.edu)

Atmospheric pressure at the summit is about 40% less than at sea level; this poses a health risk which can have a range of effects on humans from minor discomfort to life threatening conditions. Therefore we were strongly adviced about sympotoms of altitude sickness and how to prevent it and respond to it. Before travelling to the summit, we stayed at our base camp (known as Hale Pohaku, Hawaiian for “stone building”) on the slope of Mauna Kea for two night to allow us to acclimate to the high altitude. Hale Pohaku at an elevation of 2800 m is a cluster of buildings which includes few dormetories and a main building containing a caferteria, offices, recreation facilities. The many cinder cones and lava rocks in the surrounding areas are testement to the former volcanic activity of the mountain.

Here is a view from top of a cinder cone looking down at the base camp. The zigazagging dirt road to the mountain top can be seen in the distance.
Here is a view from top of a cinder cone looking down at the base camp. The zigazagging dirt road to the mountain top can be seen in the distance.

Our observing program ran for 7 consecutive nights; we would leave our base at 6 pm each night and return by 8 am the following day. After the first two nights, staying up all night became easy! For safety reasons, no one is allowed to spend more than 14 hours above the base camp in a 24 hour peroid. The drive from Hale Pohaku on the winding dirt road to the summit takes about 20 minutes. The view from the top is spectacular; we often saw tourists driving up the mountain to catch a glimpse of the beautiful sunset.

Some of the telescopes at the top of Mauna Kea, including JCMT shown on the far left hand side.
Some of the telescopes at the top of Mauna Kea, including JCMT shown on the far left hand side.

In the couple of days we had before and after our observing run, we visited some amazing places in the Big Island. One interesting location is the Volcanoes National Park. The park is full of mind-bogglingly huge crators and fascinating lava fields. The cracks on the ground from which steam and sulphur gases are expelled are a reminder the region is an active geothermal area. The diverse and beautiful landscape of Hawaii Island makes it a unique place definately worth visiting!

A huge crater in the Volcanoes National Park
A huge crater in the Volcanoes National Park

Teenage Galaxies

Hello, I am Dr Myrto Symeonidis a postdoctoral researcher at UCL’s Mullard Space Science Laboratory (MSSL), and this week I will be telling you about some of my research here at MSSL on galaxies “in their teenage years”: some of the youngest, dustiest and most energetic, galaxies in the Universe.

Infrared Radiation

For most people, the word “astronomy” brings to mind beautiful, colourful images of galaxies taken by the Hubble Space Telescope. Those images are usually showing you what galaxies look like in the optical part of the  electromagnetic spectrum. However, few people realise that what we can “see” with our own eyes is only a very small part of the picture.

EM Spectrum from http://en.wikipedia.org/wiki/Electromagnetic_spectrum

In fact, more than half of the total light produced by all galaxies in the Universe since the beginning of time emerges as infrared radiation, invisible to the human eye. The reason is dust.

Cosmic dust is somewhat of a misnomer; unlike the dust that you find in your home which has many constituents, cosmic dust is a conglomeration of tiny grains of rock-like material typically of carbon and silicon. It is prevalent in all galaxies and readily absorbs ultraviolet and visible light, subsequently re-radiating it in the infrared. Discovering the dust-obscured side of the Universe was one of the major scientific breakthroughs of the last century, enabling us to make immense progress in our understanding of the cosmos.

Star Formation or Supermassive Black holes?

Since the start of my career in astronomy I have been very keen to make sense of the role of dust in galaxy formation and evolution. One reason is that the dust content of a galaxy is intricately linked to its ability to make new stars, what astronomers refer to as the ‘star-formation rate’. The most dusty galaxies appear to make the most stars, which means that, on a cosmic scale, dust-rich galaxies are the main contributors to the total  cosmic energy budget. The other main way in which galaxies release energy into the cosmos are through Active Galactic Nuclei (AGN). This is a mundane name for an awesome phenomenon: AGN are supermassive black holes (many billions of times the mass of the Sun) that reside in the centres of galaxies and devour all infalling matter. In the process of consuming matter, AGN release copious amounts of energy. To understand the energy content of the Universe we therefore need to disentangle the contributions from star formation and AGN.

Measuring star-formation rates in galaxies that host AGN is a challenging feat because in many cases the AGN is luminous enough to completely drown the stellar emission from its host. To make things even more challenging, dusty galaxies were far more common in the early Universe, and thus identifying and studying them requires cutting-edge observational facilities to look back in time.

Herschel and the Rosetta Nebula from http://spaceinimages.esa.int/Images/2010/04/Herschel_and_Rosette_Nebula

The Herschel Space Observatory 

We are now fortunate to be living in the most data-rich era that infrared astronomy has ever seen, thanks to the Herschel Space Observatory, one of ESA’s most ambitious missions. Herschel is the largest space telescopes ever launched and is the only facility to date and for the foreseeable future that can span the part of the electromagnetic spectrum in which most of the Universe’s radiation from galaxies emerges.

MSSL has had a major role in the Herschel mission since its conception, with particular involvement in designing and building Herschel’s Spectral and Photometric Imaging receiver (SPIRE). Seeing the planing, building and eventual scientific results from space missions is one of things that makes MSSL such an exciting place to work.

The mechanical and thermal engineering group at MSSL assembled and tested the whole SPIRE structure, additionally providing key components such as thermal straps, detector boxes, mirror mounts and filter holders. Moreover, since Herschel’s successful launch in 2009, the MSSL astronomy group has been heavily involved in the exploitation of Herschel data with a strong focus on measuring the physical attributes of the primary energy production mechanisms in dust-rich galaxies, determining how these mechanisms interact and charting the changes in their characteristics back to when the Universe was less than a quarter of its current age.

With the advent of Herschel, we have been able to exploit the far-infrared part of the electromagnetic spectrum, where the energetic balance between AGN and star-formation tilts towards the latter. Taking advantage of Herschel data in a recent project that was led by the MSSL Herschel team, we were able to measure the star-formation rates of the galaxies which host the most powerful AGN at the time when the Universe was less than half its current age.

We discovered that although the host galaxies of AGN experience intense star-formation, if the AGN becomes powerful enough it seems that it is able to slow down and eventually terminate the star-formation, placing the whole galaxy in a rapid course towards old age.

An artistic impression of an AGN from here http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=5033

Although we still have a long way to go with respect to solving the mysteries of the cosmos, thanks to Herschel we are now step closer to understanding how galaxies evolve, from young ,dusty and active into old age.

 

 

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