• Yifan Ding
  • Ayushi Parmar

Extra-Galactic Astronomy

Extra-Galactic astronomy is an incredibly broad topic which covers all observations of objects outside of our own Milky Way galaxy.  All such observations must be interpreted in the context of an expanding Universe, which induces a cosmological redshift, and with look-back times that can be an appreciable fraction of the (current) age of the Universe.  Particularly at the high redshifts and large look-back times that are the focus of the Group’s research efforts, the focus is on observing the evolution of the cosmological populations over cosmic time, with a particular focus on dusty star-forming galaxies and high-redshift quasars.

Dusty Star-Forming Galaxies

The 1996 discovery of the cosmic infrared background demonstrated that roughly half of the energy generation by stars and supermassive lack holes in the history of the universe is hidden from our view by dust. The largest optical telescopes, in space or on the ground, are thus incapable of determining the full history of star and galaxy formation, and have to conduct observations in the far-infrared (far IR) and sub-millimetre to unveil this hidden history of the universe. Since the atmosphere of the Earth is opaque at far-IR wavelengths, and only partially transparent in the sub-mm, we must use space-based observatories like Herschel, and ground-based telescopes on high mountains, like JCMT in Hawaii, and ALMA in Chile, to pursue this work.

Dusty star-forming galaxies (DSFGs) are particularly important to the hidden history of the universe. These objects are forming a prodigious amount of new stars - 100s to 1000s of solar masses of per year. While rare in the local universe, DSFGs are many times more common at higher redshifts, as we approach the peak of the cosmic star formation rate density at redshifts of about 2.5, roughly 2.5 billion years after the Big Bang. It now seems likely that DSFGs played a major role in this peak of the star formation rate, possibly becoming the dominant sites of star formation. Less clear, though, is what happened at higher redshifts, at earlier stages of the universe, where DSFGs become much more difficult to find, identify and study. Also unclear is why the number of DSFGs declined to lower redshifts, whether they preferentially lie in the dense environments of galaxy clusters/protoclusters, and what they evolve into in the local universe. These are key aspects of the work of the IR Astronomy Group at Imperial College, where we look for high redshift DSFGs, determine what role they play in galaxy cluster evolution, and study the detailed properties of their local-universe equivalents.

Imperial was heavily involved with planning, constructing and using past generations of Far-IR space missions, from IRAS to ISO, Spitzer, AKARI and, most recently, Herschel and Planck. We are now contributing to the planning and preparation of the next generation of space missions to study this enigmatic but very important part of the electromagnetic spectrum.

High-Redshift Quasars

A typical galaxy has super-massive black hole at its centre, which grows sporadically by accrediting material that can be so hot that it outshines all the stars in the galaxy; this is a quasar.  Quasars are the most luminous non-transient sources in the Universe, and can hence be seen at high redshifts (z > 6), corresponding to the first billion years after the Big Bang.

Such high-redshift quasars are incredibly valuable as they reveal the first super-massive black holes and can be used to probe cosmological reionization; unfortunately, they are also incredibly rare, and hence difficult to find.  Members of the Imperial Astrophysics Group have been pioneering the development of techniques to efficiently and reliably find these remarkable but rare objects.  A particular highlight of this work was the discovery using the UKIRT Infrared Deep Sky Survey (UKIDSS) of ULAS J1120+0641, the first quasar known with z > 7.  The spectrum of this object revealed a Lyman alpha damping wing, indicating the inter-galactic medium was significantly neutral at that time.  It also showed that the central black hole had a mass two billion times that of the Sun, an object that standard theories of black hole formation and growth cannot explain.

The main focus of the Imperial group’s research is now Euclid, an ESA space telescope scheduled for launch in 2023.  The Euclid Wide survey will make near-infrared images of 1/3 of the sky and will allow the identification of quasars at redshifts of z > 8, pushing the observational frontier further back towards  the Big Bang.