Galaxy Formation
(Stevens)
Work on galaxy formation at Hertfordshire is a recent addition but activity is accelerating. Our work focuses on the early stages of galaxy evolution when the bulk of the future stellar mass is still in the form of molecular gas, and when the supermassive black holes found dormant in the centres of local massive galaxy bulges were growing by accretion. Our aim is to trace to sequence of events by which such a protogalaxy evolves over time to become a fully formed local galaxy spheroid.
The currently favoured cosmological model predicts that today's most massive galaxies - cluster ellipticals - formed at early epochs and at highly biased peaks of the dark matter distribution. Fig. 1 shows a simulation by Joerg Colberg and Antonaldo Diaferio which has dimensions 21x21x8 Mpc. The white dots are the dark matter distribution while the coloured disks show the galaxies; these are colour-coded by star-formation rate where blue represents a galaxy with high star-formation activity and red a galaxy with an old stellar population. At the highest redshifts, notice that there are relatively few galaxies and that almost all of them are `blue'. As the structure evolves with time, more galaxies form while others drop in activity. At the current epoch a cluster of `red' objects, which can be identified as elliptical galaxies, lie at the density peak of the dark matter distribution. .
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Fig 1. Semi-analytic simulations as a function of redshift (credit: Joerg Colberg and Antonaldo Diaferio). See text for details. |
Recent Progress and open questions
We follow a multiwavelength approach but it is driven by observations at submillimetre and far-infrared wavelengths. These wavelengths are sensitive to dust heated by hot young stars and hence the star-formation rate. Fig 2 shows SCUBA images of two fields centred on X-ray selected quasi-stellar objects (QSOs). We targetted these fields because the high-redshift QSO they contain signposts an over-dense region of the early universe. The field should therefore also display an over density of star-forming galaxies which the SCUBA images reveal to be the case.
Fig 2. SCUBA submillimetre imaging of the fields of 2 X-ray selected QSOs. The colour scale and black contours are 450 micron data while the white contours are lower resolution 850 micron data. The images reveal significant over-densities of star-forming galaxies. |
Questions we are now trying to answer include:
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Are the dusty companion galaxies at the same redshift as the QSO? The most direct way to investigate this is to obtain spectroscopic redshifts. We have used multi-object spectrographs on Gemini North and Keck to this end. Given the faintness of the optical counterparts, a second method is to measure photometric redshifts which we are pursuing by fitting our stellar population models to mid-infrared (Spitzer IRAC), near-infrared and optical imaging data. Fig. 3 shows examples of how the companion galaxies appear on mid-infrared through optical images.
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Do they contain buried AGN, the signature of growing black holes? We are addressing this question with our mid-infrared (Spitzer IRAC/MIPS) and X-ray (XMM-Newton) data.
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What is the evolutionary status of the stellar population? In other words, how many stars have formed compared to how much fuel is available to form new stars? Stellar masses can be calculated from our population synthesis models while molecular gas masses can be deduced from interferometric observations of CO - a field that will be opened up by ALMA but which is also accessible to PdB and the SMA.
Fig 3. The left-hand panel shows 850 micron contours overlaid on a UKIRT K-band image while the right-hand panel show the same contours on the Spitzer 8.0 micron image. The QSO is one of the cluster of galaxies at the centre of these panels. Note that the dust peaks on the 850 micron image are often detected at 8.0 microns even if they are not detected at K. These galaxies are very red or contain buried AGN or both. |