Biased Galaxy Formation at high redshifts
(Stevens, Priddey)
According to the currently favoured Lambda-CDM model of structure formation, the most massive structures in the Universe (super-clusters) are still in the process of assembly while progressively less massive systems were in place at increasingly earlier times. This is the well-known hierarchical, bottom-up scenario of structure formation. On the other hand, there is increasing evidence that the formation of galaxies within this underlying structure of cold dark matter proceeds in an "anti-hierarchical" manner, i.e. big galaxies form first. Combining these two pieces of information leads to the prediction that the most massive galaxies (ellipticals) should form at high redshifts, and in the seed high-density peaks that are destined to evolve into today's rich clusters.
To investigate this formation epoch we require two tools. The first is a means of observing massive bursts of star-formation which are likely to be highly obscured at optical and even near-infrared wavelengths. The best technique available from the ground, where higher spatial resolution is available than from space-based facilities, is to observe the reprocessed submillimetre emission from warm dust. The second requirement is a method of locating over-dense regions of the early universe. Here the problem we face is that high-density peaks are inherently rare and since blank-field surveys at submillimetre wavelengths currently cover very small areas (<1 degree) they have a vanishingly small chance of including one. We have thus pioneered a technique of targeting the fields of known high-redshift active galactic nuclei (AGN) at submillimetre wavelengths. Such AGN must be very luminous in order to be detected in flux-limited optical and X-ray surveys meaning that they must host very massive black holes. Such AGN are therefore amongst the the most massive objects in existence at their epoch and should signpost rare high-density peaks of the dark matter distribution.
We have pursued a number of programmes using the SCUBA camera on the JCMT. Our target fields were centred on high-redshift radio galaxies (HzRGs) at 2<z<4 and quasi-stellar objects (QSOs) at 2<z<6. In these fields we find a statistical over-density (by a factor of at least a few on average) of ultraluminous star-forming galaxies compared to the number counts measured from blank-field surveys. This is exactly what we would expect to observe if these surveys are locating the progenitors of present day cluster elliptical galaxies, i.e. a number of dust-rich (and probably gas-rich) galaxies simultaneously forming large numbers of stars (SFR>1000 Solar masses per year) in a small region of space (the SCUBA images have a co-moving diameter of about 1 Mpc at 2<z<6).
We are currently investigating the properties of the galaxies discovered in these over-dense regions (AGN content, gas masses, stellar masses, morphologies, redshifts) in order to determine their evolutionary status.
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Submillimetre (SCUBA) and mid-infrared (Spitzer) imaging of the field around a z~1.8 QSO (Stevens et al. 2004, ApJ, 604, L17). The left-hand panel shows 450um emission (white contours) overlaid on the 850um emission (colour scale and black contours). We detect several ultraluminous star-forming galaxies which are all detected by Spitzer at 4.5um (middle panel) and 24um (right-hand panel). The submillimetre emission traces star-formation while the Spitzer data traces predominantly star-light and PAH emission. |