Gillian Wilson

THE FAINT END OF THE GALAXY LUMINOSITY FUNCTION IN MODERATE-REDSHIFT CLUSTERS

We present deep two-colour photometry of two rich clusters at z = 0.18, A665 and A1689. We use these data to construct number counts as a function of magnitude in the two fields. By combining these counts with similar observations from a large area field survey we subtract the field contamination statistically to produce luminosity functions for the two clusters. Great care has been taken to achieve agreement between the photometry of these two samples. The cluster data are complete to a limiting magnitude of I = 22.5 or an absolute magnitude in the cluster of I = 18.0 (M ⋆ + 5). The luminosity functions of both clusters are well described by a Gaussian function for the bright galaxies, combined with a Schechter function at the faint end, similar to that required to fit the luminosity function in local clusters. The slope at the faint end of the Schechter function in both clusters is extremely steep in V , � � 2. A shallower slope is seen to the limit of the I data, indicating that the cluster population is rapidly blueing as we reach fainter. The excellent agreement between the form of the luminosity function in our two distant clusters, as well as agreement with the luminosity function given by Driver et al. (1994) for a single z = 0.21 cluster, indicates that this faint blue population is a general constituent of distant clusters. We compare our results with those from studies of local clusters. Depending upon the degree of fading (or disruption) of these faint blue galaxies, we tentatively identify their remnants with the low surface brightness dwarf galaxies which are the dominant population in local clusters. We discuss the possible role of this population as the source of most of the X-ray gas in rich clusters.

AzTEC half square degree survey of the SHADES fields - II : Identifications, redshifts and evidence for large-scale structure

The Astronomical Thermal Emission Camera (AzTEC) 1.1 mm survey of the two SCUBA HAlf Degree Extragalactic Survey (SHADES) fields is the largest (0.7 deg2) blank-field millimetre-wavelength (mm-wavelength) survey undertaken to date at a resolution of ≃18 arcsec and a depth of ≃1 mJy. We have used the deep optical to radio multiwavelength data in the SHADES Lockman Hole East and SXDF/UDS fields to obtain galaxy identifications for ≃64 per cent (≃80 per cent including tentative identifications) of the 148 AzTEC-SHADES 1.1 mm sources reported by Austermann et al., exploiting deep radio and 24 μm data complemented by methods based on 8 μm flux density and red optical–infrared (i − K) colour. This unusually high identification rate can be attributed to the relatively bright mm-wavelength flux density threshold, combined with the relatively deep supporting multifrequency data now available in these two well-studied fields. We have further exploited the optical–mid-infrared–radio data to derive an ≃60 per cent (≃75 per cent including tentative identifications) complete redshift distribution for the AzTEC-SHADES sources, yielding a median redshift of z ≃ 2.2, with a high-redshift tail extending to at least z ≃ 4. Despite the larger area probed by the AzTEC survey relative to the original SCUBA-SHADES imaging, the redshift distribution of the AzTEC sources is consistent with that displayed by the SCUBA sources, and reinforces tentative evidence that the redshift distribution of mm/submm sources in the Lockman Hole field is significantly different from that found in the SXDF/UDS field. Comparison with simulated surveys of similar scale extracted from semi-analytic models based on the Millennium simulation indicates that this is as expected if the mm/submm sources are massive (M > 1011 M⊙) star-forming galaxies tracing large-scale structures over scales of 10–20 Mpc. This confirms the importance of surveys covering several deg2 (as now underway with SCUBA2) to obtain representative samples of bright (sub)mm-selected galaxies. This work provides a foundation for the further exploitation of the Spitzer and Herschel data in the SHADES fields in the study of the stellar masses and specific star formation rates of the most active star-forming galaxies in cosmic history.