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We have performed Hubble Space Telescope imaging of a sample of 23 high-redshift (1.8<z<2.75) active galactic nuclei (AGNs), drawn from the COMBO-17 survey. The sample contains moderately luminous quasars (M(B)similar to-23). The data are part of the Galaxy Evolution from Morphologies and SEDs imaging survey that provides high-resolution optical images obtained with the Advanced Camera for Surveys in two bands (F606W and F850LP), sampling the rest-frame UV flux of the targets. To deblend the AGN images into nuclear and resolved (host galaxy) components, we use a point-spread function subtraction technique that is strictly conservative with respect to the flux of the host galaxy. We resolve the host galaxies in both filter bands in nine of the 23 AGNs, whereas the remaining 14 objects are considered nondetections, with upper limits of less than 5% of the nuclear flux. However, when we co-add the unresolved AGN images into a single high signal-to-noise ratio composite image, we find again an unambiguously resolved host galaxy. The recovered host galaxies have apparent magnitudes of 23.0<F606W<26.0 and 22.5<F850LP<24.5, with rest-frame UV colors in the range -0.2<(F606W-F850LP)(obs)<2.3. The rest-frame absolute magnitudes at 200 nm are -20.0<M-200 nm<-22.2. The photometric properties of the composite host are consistent with the individual resolved host galaxies. We find that the UV colors of all host galaxies are substantially bluer than expected from an old population of stars with formation redshift z<=5, independent of the assumed metallicities. These UV colors and luminosities range up to the values found for Lyman break galaxies (LBGs) at z=3. Our results suggest either a recent starburst of, e. g., a few percent of the total stellar mass at 100 Myr before observation, with mass fraction and age strongly degenerate, or the possibility that the detected UV emission may be due to young stars forming continuously. For the latter case we estimate star formation rates of typically &SIM;6 M&ODOT; yr(-1) (uncorrected for internal dust attenuation), which again lies in the range of rates implied from the UV flux of LBGs. Our results agree with the recent discovery of enhanced blue stellar light in AGN hosts at lower redshifts
We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely "best and brightest stars first". Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., R similar to 2000) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., R > 20 000) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z(cr) = 10(-3.4) Z(circle dot) reproduces the sharp drop at [Fe/H] similar to -3.6 present in the HES MDF. Although currently about ten stars at [Fe/H] < -3.6 are known, the evidence for the existence of a tail of the halo MDF extending to [Fe/H] similar to -5.5 is weak from the sample considered in this paper, because it only includes two stars [Fe/H] < -3.6. Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant.
We describe automatic procedures for the selection of DA white dwarfs in the Hamburg/ESO objective-prism survey (HES). For this purpose, and the selection of other stellar objects (e.g., metal-poor stars and carbon stars), a flexible, robust algorithm for detection of stellar absorption and emission lines in the digital spectra of the HES was developed. Broad band (U-B, B-V) and intermediate band (Strömgren c_1) colours can be derived directly from HES spectra, with precisions of sigma U-B=0.092 mag; sigma B-V=0.095 mag; sigma c_1=0.15 mag. We describe simulation techniques that allow one to convert model or slit spectra to HES spectra. These simulated objective-prism spectra are used to determine quantitative selection criteria, and for the study of selection functions. We present an atlas of simulated HES spectra of DA and DB white dwarfs. Our current selection algorithm is tuned to yield maximum efficiency of the candidate sample (minimum contamination with non-DAs). DA candidates are selected in the B-V versus U-B and c_1 versus W_lambda (Hbeta +Hgamma +Hdelta ) parameter spaces. The contamination of the resulting sample with hot subdwarfs is expected to be as low as ~ 8%, while there is essentially no contamination with main sequence or horizontal branch stars. We estimate that with the present set of criteria, ~ 80% of DAs present in the HES database are recovered. A yet higher degree of internal completeness could be reached at the expense of higher contamination. However, the external completeness is limited by additional losses caused by proper motion effects and the epoch differences between direct and spectral plates used in the HES. Based on observations collected at the European Southern Observatory, La Silla and Paranal, Chile.
We present a determination of the optical/UV AGN luminosity function and its evolution, based on a large sample of faint (R < 24) QSOs identified in the COMBO-17 survey. Using multi-band photometry in 17 filters within 350 nm <~ lambdaobs <~ 930 nm, we could simultaneously determine photometric redshifts with an accuracy of sigmaz <0.03 and obtain spectral energy distributions. The redshift range covered by the sample is 1.2 < z < 4.8, which implies that even at z =~ 3, the sample reaches below luminosities corresponding to MB = -23, conventionally employed to distinguish between Seyfert galaxies and quasars. We clearly detect a broad plateau-like maximum of quasar activity around z =~ 2 and map out the smooth turnover between z =~ 1 and z =~ 4. The shape of the LF is characterised by some mild curvature, but no sharp ``break'' is present within the range of luminosities covered. Using only the COMBO-17 data, the evolving LF can be adequately described by either a pure density evolution (PDE) or a pure luminosity evolution (PLE) model. However, the absence of a strong L*-like feature in the shape of the LF inhibits a robust distinction between these modes. We present a robust estimate for the integrated UV luminosity generation by AGN as a function of redshift. We find that the LF continues to rise even at the lowest luminosities probed by our survey, but that the slope is sufficiently shallow that the contribution of low-luminosity AGN to the UV luminosity density is negligible. Although our sample reaches much fainter flux levels than previous data sets, our results on space densities and LF slopes are completely consistent with extrapolations from recent major surveys such as SDSS and 2QZ.