Nicholas Amos
Integral Field Units (IFUs) are a class of instrument that offer unique opportunities to spatially map the chemical signatures of galaxies. Specifically, I am using these instruments to observe galaxies within distant cluster environments to determine how their star formation is being quenched (slowed down). We know that the peak of star formation in galaxies occurred during a period 7-10 billion years ago and this peak has been well studied for isolated galaxies in field environments (e.g. the KMOS redshift one spectroscopic survey, KROSS) but there has been relatively little investigation of galaxies in dense environments (e.g. galaxy clusters). I will use IFU data from the KMOS Cluster Survey (KCS) to study how the distant cluster environment affects the star-formation in galaxies and determine which quenching mechanisms are responsible. In order to do this, I will map the kinematics, star-formation and chemical abundances of these galaxies combining data from Very Large Telescope (VLT) KMOS and the Hubble Space Telescope (HST), to draw robust conclusions. Comparing these data with other external data sets will allow us to draw direct comparisons between galaxy clusters and the field to isolate the effect of environment.
An Integral Field Unit (IFU) is a mixture of imaging and spectroscopy and provides us with a spectrum for each pixel of our ‘image’. This tells us about the chemical signatures of our target per pixel. This of this as an image slice across a range of wavelengths within a cube.
Slide 2 – Galaxies in Clusters
To investigate galaxies in clusters we should understand and compare to their counterparts in the field. Field galaxies are those that are isolated and gravitationally independent whereas cluster galaxies reside in denser environments that can gravitationally interact. In field galaxies we see a clear reduction in star formation rate at approximately 10 billion years ago. The reduction in star formation rate is also known as ‘quenching’. For field galaxies this is thought to be due to internal processes within these galaxies with no external interference due to their isolation, these processes are called secular processes. Such a process is star formation feedback which is caused by outflows from forming stars preventing gas from condensing to form stars. A large volume of data exists for field galaxies however this is not the case for cluster galaxies at greater distances and larger ‘lookback’ times. For cluster galaxies we cannot just consider secular processes as these galaxies are interacting with each other and their environment in a variety of different ways.
Slide 3 – How are we Investigating?
In order to form these comparisons, we need more information on cluster galaxies at this point of slowing in star formation, i.e approximately 10 billion years ago. This data is taken from the KMOS Cluster Survey (KCS) which used an IFU on the Very Large Telescope called KMOS. This data provides us with 2D information on the chemical signatures in these galaxies, primarily H⍺ emission as this tells us how many stars are being formed. In turn this will allow us to study the internal motions and external interactions via velocity mapping. In addition to this we have images of these galaxies from the Hubble Space Telescope (HST). This imaging data will allow us to assess the shape, size, and mass of the galaxies which is essential to disentangle the effects we discover in the 2D spectroscopy data.
Slide 4 – What Next?
In order to make conclusions and answer these questions about why these cluster galaxies are reducing the rate at which they form stars we must be able to form quantitative comparisons between field and cluster galaxies and their respective quenching mechanisms. The best way to do this is to form models from the data we have in both imaging and spectroscopy. Beyond this we can use the velocity information to learn more about the dark-matter content of the galaxies and the motions of the individual galaxies within the cluster to understand their relation to their environment. We can also form emission line ratio maps to assess the metal and active galactic nucleus content, and the impact of these on star formation rate. All of this will allow us to see how these galaxies evolve over time and is being investigated in Amos et al. (in Prep) and future papers by Nicholas Amos and John Stott.