1. Galaxy Evolution from z=2 to the present
Supervisors: Prof. R. Davies, Dr. R. Houghton, Dr. M. Cappellari
Star formation activity in the universe peaked between redshifts 1 < z < 2 (just before the universe reached half its current age). Observing how galaxies build up their mass during this period is central to understanding galaxy evolution. We are looking for students to work in a team devoted to understanding the evolution of galaxies from z=2 to the present day. Oxford is exploiting its guaranteed observing time on the near infrared multiple object integral field spectrograph KMOS on the VLT to study multiple high redshift clusters between 1 < z < 2. Unlike many high redshift studies, we aim to observe the rest frame V-band stellar absorption lines to investigate both the ages and composition of the stars, and the kinematics and dynamical masses of the galaxies.
Measuring galaxy evolution directly at z~2 with KMOS
To understand why galaxies in clusters have old passive populations now, we must investigate the formation of the stellar populations at high redshift (1 < z < 2) when the transition from star-forming to passive takes place. Comparing their masses, sizes, luminosities and stellar compositions to lower redshift observations allows us to measure and so understand the changes taking place. Furthermore, comparing the same absorption features at low and high redshifts dramatically reduces systematic uncertainties.
KMOS (K-band multi-object spectrograph) is a new multi-object infrared integral field spectrometer on the ESO VLT with 24 multiplexed mini-IFUs (3”x3”) operating from 0.8 um to 2.5 um. Oxford has guaranteed observing time time on KMOS because it designed and manufactured the spectrographs. KMOS is ideally suited to observe the rest-frame V-band spectra of galaxies in clusters at redshifts between 1 < z < 2: the increased observing efficiency of multiplexed observations allows not only study of (usually bright) nebula emission, but also the faint absorption lines produced by the stars. Oxford has already spent 9 nights observing >40 galaxies in two clusters at z=1.39 (right) and z=1.6 and will continue this observation programme over the next two years.
Top right: the 24 deployable arms in KMOS that ‘pick’ the light off the focal plane. The light is then split using image slicers and dispersed onto NIR detectors.
Bottom right: ESO VLT HAWK-I imaging of XMMUJ2235 at z-1.39 (false colour). This is one of the clusters Oxford observed with KMOS in November 2013, targeting >20 galaxies with masses as low as 7 x 1010 M.
Above: the velocity maps of early-type galaxies are bimodal: they can be classified as either slow rotators (left) or fast rotators (right). This is only possible with integral field spectroscopy, which provides the full 2D velocity map.
Slow rotators (SR)
Fast rotators (FR)
(Emsellem et al. 2007)
Vel Maps
The Morphology Density Relation
The morphology-density relation (MDR, lower left) shows us that early-type galaxies (ETGs: ellipticals, lenticulars) are more common in clusters while late-type galaxies (LTGs: spirals, irregulars) are more common in the field. This implies that the galaxy formation mechanisms at work in galaxy clusters produce more ETGs than LTGs, but we still don’t understand the details of how LTGs are transformed into ETGs in denser environments (if at all).
The ATLAS-3D survey has shown that the MDR is clearer when galaxies are classified kinematically into fast and slow rotators (upper right). This is the kinematic morphology density relation (kMDR, lower right); but this volume limited survey only extends to 42Mpc and therefore comprises mostly field galaxies, the densest region (and only cluster) being Virgo. This DPhil project will use data from the VLT FLAMES/GIRAFFE instrument, the AAO SAMI instrument and the MANGA survey to investigate the kMDR in a wider range of galaxy environments.
E, S0, S+Irr
Left: the original morphology density relation from Dressler 1980.
Above: the new kinematic morphology density relation from Cappellari et al