Galaxy populations are observed to be bimodal in several physical properties across a wide redshift range. The first class of galaxies are characterised by 'disky' morphologies (indicating rotation), blue colours and appreciable star-formation rates, represented on the image to the right by the late-type galaxies along the forks of Hubble's famous diagram. The second class are characterised by spherical morphologies (indicating the prevalence of random motions), red colours and a dearth of star-formation, represented by the early-type galaxies along the handle of Hubble's tuning fork. These two classes are thought to form an evolutionary sequence which moves from right to left along the diagram, in that galaxies are born blue, star-forming and with their baryons confined to a rotating disk. Some combination of physical processes must conspire to shut-down star-formation and cause both morphological and dynamical transformation in late-type galaxies. The timescales invovled in this transition are dependent on the physical processes invovled, and currently there is no unified theory for what these should be.
There is a connection between the star-formation rate and stellar mass of 'normal' late-type galaxies, shown in the graph to the left. This relationship appears to hold out to high redshift, and is often used to select samples of typical star-forming galaxies at the appropriate redshift. Not only do individual galaxies evolve along and eventually off this 'main-sequence' when star-formation ceases, but the sequence itself is evolving with redshift. This is a crucial point; since we can't observe evolution in individual galaxies because of the timescales involved, we must study the evolution of the mean properties of 'similar' galaxies at different redshifts.
My (and others) Work
The photons emitted by young stars in late-type galaxies ionise the gas, which then emits strongly at wavelengths which correspond to the energy gap of atomic transitions. By studying these emission lines with integral field spectroscopy we can learn how the physical properties of late-type galaxies change across their spatial extent. This has now been done at many redshift slices in order to track galaxy evolution as explained above.
An issue affecting these types of studies, which focus on resolving small patches within each galaxy, is the degrading effects of beam-smearing on the angular resolution of a set of observations. This is particularly problematic when observing from the ground, and thus staring through a turbulent atmosphere. In short, photons emitted from a spatial region of the galaxy equivalent to the point-spread function of the observations cannot be distinguished from one another, resulting in a loss of information. Since the angular size of galaxies decreases with distance from the observer, this effect becomes more severe with increasing redshift. We must therefore disentangle galaxy evolution from the effects of decreasing resolution. Fortunately adaptive-optics technology is becoming more widespread, as well as the sophistication of computational methods to remove the effects of beam-smearing after the data have been collected and analysed.
The KMOS Deep Survey (KDS) is a project aiming to study the spatially resolved properties of 77 late-type galaxies at the highest redshift yet, z = 3.5. This uses data from the KMOS instrument (rightmost of the interactive images), mounted on ESO's Very Large Telescope in Chile.
KDS I: Galaxy Dynamics
We have carried out an extensive analysis of the dynamical properties of 32 isolated and spatially resolved galaxies from the KDS parent sample. This uses Hubble Space Telescope imaging and derived properties form the KMOS datacubes in a combined morpho-kinematic analysis. The kinematic properties are determined using three-dimensional beam-smeared models which are fit to the data, primarily to determine the rotation velocities and velocity dispersions of the galaxies via the intrinsic positions and widths of the [OIII]λ5007 emission line. An example of the results of this process is shown below for one of the KDS galaxies.
Previous studies have shown that late-type galaxies appear to be becoming more turbulent with increasing redshift, as traced by the mean velocity dispersion of samples of typical star-forming galaxies across cosmic time. The KDS results are in agreement with this, showing a sample mean velocity dispersion value which is a factor 3-4 higher than in the local universe. We explore a model first discussed in Wisnioski et al. (2015), using this to explain the redshift dependence of velocity dispersions as a consequence of the increasing gas fractions, increasing specific star-formation rates and decreasing gas depletion timescales observed with redshift in star-forming galaxies. As a result of the increase in velocity dispersions, only 34% of the sample are classed as 'rotation-dominated', which is significantly less than in star-forming samples at low and intermediate redshift. The left plot below shows the increase in velocity dispersion with redshift, and the right plot shows the decrease in the rotation dominated fraction, with the KDS datapoint shown by the red circle at z = 3.5 in both cases.
Dispersion-dominated galaxies are therefore more representative of the star-forming population at z = 3.5. These turbulent systems presumably stabilise over time as gas is consumed and a more massive stellar population is accumulated. Full details of this work can be found here arxiv.org/abs/1704.06263v1
KDS II: Angular Momentum and the Tully-Fisher relationship
New results to appear soon!
KDS III: Spatially resolved metallicities
New results to appear soon!