Greetings, I am Ignacio Ferreras, a reader at MSSL. This time our MSSL blog post is written from Tenerife, in the Canary Islands, a volcanic archipelago in the Atlantic Ocean, a special place for astronomical observations. These days I am visiting colleagues at the Instituto de Astrofísica de Canarias (IAC), a research centre built to support one of the largest observatories in the world. As mentioned in the title above, the reason for my stay here is a combination of volcanoes, colours and distant galaxies.
Volcanic islands, such as the Canary Islands or Hawaii are ideal places for astronomical observations. Their very steep slopes and special location, surrounded by a large mass of water, provide uniquely stable atmospheric conditions for the observation of faint sources. The Canary Islands have attracted astronomers from all over Europe, especially the UK, making possible one of the top ground-based observatories in the world. Although at MSSL we are mostly involved with space-based missions, we also work actively with ground-based data. I attach a photograph of the “Roque de los Muchachos” observatory, on the island of la Palma, at 2,396 metres above sea level. The view of a sea of clouds below the horizon – you may have seen such a view from an airliner – is a common sight. The steep cliffs you see in the picture are part of a collapsed volcano, forming a huge caldera. My visit to the IAC focuses on the analysis of the SHARDS survey, from data taken by the GTC, a 10.4 metre telescope located on that site. The GTC is at present the largest single optical collecting area in the world. The largest the collecting area of a telescope, the better its ability to observe faint sources. You may have heard in the news that several giant telescopes are planned in the future, including a European project, the E-ELT, with a diameter of 39 metres!
The vast majority of the information we receive from stars and galaxies is in the form of light. Photons (particles of light) carry information about the physical condition of the sources. An ideal way to collect this information is via spectroscopy, where fluxes of very narrow ranges of wavelengths (i.e. colours) are collected individually. You can see a past MSSL blog post with some information about spectroscopy and galaxies here. However, spectroscopy involves separating the photons into different wavelengths, making it very challenging for faint sources. An alternative strategy involves photometry, the use of filters that allow a wider range of wavelengths to get to the detector (simply put, you can have a blue filter, a red filter, a green filter: those will let photons with wavelengths over a 100-200 nanometre range  pass through; whereas the equivalent in spectroscopy would be many filters, each with a transmission window of a fraction of a nanometre). However, photometry lacks the detailed information one can gather from spectroscopy. For instance, in order to derive the chemical composition or the age of stars and galaxies, one needs the detailed information over a small range of wavelengths, whereas with photometric data, this sensitive information is washed out. A compromise between these two is the use of medium-band filters, whose very narrow range of wavelengths (around 10-20 nm) represents a midpoint between photometry and spectroscopy. The data I am working on uses this concept, and probes down to extremely faint galaxies. The next figure shows an example from the SHARDS survey I am working on. It shows a central massive galaxy and two satellite galaxies (the image is a high resolution picture from the Hubble Space Telescope). On the left, the photo-spectra taken by the GTC are equivalent to low-resolution spectroscopy of very faint sources. The numbers in the insets give our inferred estimates of galaxy mass and age for each object.
Why distant galaxies?
Cosmology studies the evolution of the Universe over large scales. There are two fundamental approaches to this field: One can either consider the evolution of the space-time on which our Universe is based, or focus instead on the evolution of galaxies, the building blocks of structure in the Universe. For instance, our Euclid team at MSSL focuses on the former. My research studies the latter, through the observations of galaxies across a huge range of cosmic time. During my visit at the IAC, my colleagues and I are exploring the build-up of structure over the past 8 billion years of cosmic history. The deep observations with medium-band filters allow us to compile a detailed sample of galaxies out to vast distances, probing down to galaxies with a very low mass, about 1/100th of the mass of our Milky Way. In order to achieve this limit, the survey is capable of detecting sources of magnitude 26.5. Although this sounds like a dry, technical number, the meaning of it is truly staggering. At magnitude 26.5, a galaxy appears about 160 million times fainter than the faintest star we could see with our naked eyes. The flux received from those galaxies is so low that only 2-3 photons are received by the huge telescope per second. Such a faint level can be detected not only because of the use of a large collecting area. Advances in detector technology and optics have made the cameras much more efficient than, for instance, the photographic plates used by Hubble (the astronomer!) at the 200 inch telescope on Mount Palomar. In the project that keeps me busy at the moment, we study how the clustering of galaxies evolves with time, allowing us to understand how Dark Matter structures (called halos) harbour a progressively larger number of galaxies. The fun never ends!