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Register a new userEvolution of optically faint AGN from COMBO-17 and GEMS
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We have mapped the AGN luminosity function and its evolution between z=1 and z=5 down to apparent magnitudes of $R<24$. Within the GEMS project we have analysed HST-ACS images of many AGN in the Extended Chandra Deep Field South, enabling us to assess the evolution of AGN host galaxy properties with cosmic time.
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We present the first application of the 3D cosmic shear method developed in Heavens et al. (2006) and the geometric shear-ratio analysis developed in Taylor et al. (2006), to the COMBO-17 data set. 3D cosmic shear has been used to analyse galaxies with redshift estimates from two random COMBO-17 fields covering 0.52 square degrees in total, providing a conditional constraint in the (sigma_8, Omega_m) plane as well as a conditional constraint on the equation of state of dark energy, parameterised by a constant w= p/rho c^2. The (sigma_8, Omega_m) plane analysis constrained the relation between sigma_8 and Omega_m to be sigma_8(Omega_m/0.3)^{0.57 +- 0.19}=1.06 +0.17 -0.16, in agreement with a 2D cosmic shear analysis of COMBO-17. The 3D cosmic shear conditional constraint on w using the two random fields is w=-1.27 +0.64 -0.70. The geometric shear-ratio analysis has been applied to the A901/2 field, which contains three small galaxy clusters. Combining the analysis from the A901/2 field, using the geometric shear-ratio analysis, and the two random fields, using 3D cosmic shear, w is conditionally constrained to w=-1.08 +0.63 -0.58. The errors presented in this paper are shown to agree with Fisher matrix predictions made in Heavens et al. (2006) and Taylor et al. (2006). When these methods are applied to large datasets, as expected soon from surveys such as Pan-STARRS and VST-KIDS, the dark energy equation of state could be constrained to an unprecedented degree of accuracy.
We present a detailed empirical assessment of how the galaxy luminosity function and stellar luminosity density evolves over the last half of the universes age (0.2<z<1.2) for galaxies of different spectral energy distributions (SED). The results are based on ~25,000 galaxies (R<24) with redshift measurements (sigma_z~0.03) and SEDs across 350..930 nm, derived from medium-band photometry in 17 filters, observed as part of the COMBO-17 survey (``Classifying Objects by Medium-Band Observations in 17 Filters) over three disjoint fields with a total area of 0.78 square degrees. Luminosity functions (LF), binned in redshift and SED-type, are presented in the restframe passbands of the SDSS r-band, the Johnson B-band and a synthetic UV continuum band at 280 nm. We find that the luminosity function depends strongly on SED-type at all redshifts covered. The shape of the LF, i.e. the faint-end power-law slope, does depend on SED type, but not on redshift. However, the redshift evolution of the characteristic luminosity M* and density phi* depends strongly on SED-type: (1) Early-type galaxies, defined as redder than a present-day reference Sa spectrum, become drastically more abundant towards low redshift, by a factor of 10 in the number density phi* from z=1.1 to now, and by a factor of 4 in their contribution to the co-moving r-band luminosity density, j_r. (2) Galaxies resembling present-day Sa- to Sbc-colours show a co-moving number density and contribution to j_r that does not vary much with redshift. (3) Galaxies with blue spectra reflecting strong star formation decrease towards low redshift both in luminosity and density, and by a factor of 4 in their j_r contribution. (abridged)
We present an update to the photometric calibration of the COMBO-17 catalogue on the Extended Chandra Deep Field South, which is now consistent with the GaBoDS and MUSYC catalogues. As a result, photometric redshifts become slightly more accurate, with <0.01 rms and little bias in the delta_z/(1+z) of galaxies with R<21 and of QSOs with R<24. With increasing photon noise the rms of galaxies reaches 0.02 for R<23 and 0.035 at R~23.5. Consequences for the rest-frame colours of galaxies at z<1 are discussed.
We present a morphological study of galaxies in the A901/902 supercluster from the COMBO-17 survey. A total of 570 galaxies with photometric redshifts in the range 0.155 < z_phot < 0.185 are visually classified by three independent classifiers to M_V=-18. These morphological classifications are compared to local galaxy density, distance from the nearest cluster centre, local surface mass density from weak lensing, and photometric classification. At high local galaxy densities, log(Sigma_10 /Mpc^2) > 1.5, a classical morphology-density relation is found. A correlation is also found between morphology and local projected surface mass density, but no trend is observed with distance to the nearest cluster. This supports the finding that local environment is more important to galaxy morphology than global cluster properties. The breakdown of the morphological catalogue by colour shows a dominance of blue galaxies in the galaxies displaying late-type morphologies and a corresponding dominance of red galaxies in the early-type population. Using the 17-band photometry from COMBO-17, we further split the supercluster red sequence into old passive galaxies and galaxies with young stars and dust according to the prescription of Wolf et al. (2005). We find that the dusty star-forming population describes an intermediate morphological group between late-type and early-type galaxies, supporting the hypothesis that field and group spiral galaxies are transformed into S0s and, perhaps, ellipticals during cluster infall.
We discuss a sample of 29 AGN (16 narrow-lined and 13 broad-lined) discovered in a spectroscopic survey of ~1000 star-forming Lyman-break galaxies (LBGs) at z~3. Reaching apparent magnitudes of R_{AB}=25.5, the sample includes broad-lined AGN approximately 100 times less UV-luminous than most surveys to date covering similar redshifts, and the first statistical sample of UV/optically-selected narrow-lined AGN at high redshift. The fraction of objects in our survey with clear evidence for AGN activity is ~3%. A substantial fraction, perhaps even most, of these objects would not have been detected in even the deepest existing X-ray surveys. We argue that these AGN are plausibly hosted by the equivalent of LBGs. The UV luminosities of the broad-lined AGN in the sample are compatible with Eddington-limited accretion onto black holes that satisfy the locally determined M_{BH} versus M_{bulge} relation given estimates of the stellar masses of LBGs. The clustering properties of the AGN are compatible with their being hosted by objects similar to LBGs. The implied lifetime of the active AGN phase in LBGs, if it occurs some time during the active star-formation phase, is ~10^7 years.
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