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L. Wisotzki

Publications and source records attributed to L. Wisotzki.

At least 145 records · Page 8Linked to original sources
arXiv2003

Integral-field spectroscopy of the quadruple QSO HE 0435-1223: Evidence for microlensing

We present the first spatially resolved spectroscopic observations of the recently discovered quadruple QSO and gravitational lens HE0435-1223. Using the Potsdam Multi-Aperture Spectrophotometer (PMAS), we show that all four QSO components have very similar but not identical spectra. In particular, the spectral slopes of components A, B, and D are indistinguishable, implying that extinction due to dust plays no major role in the lensing galaxy. While also the emission line profiles are identical within the error bars, as expected from lensing, the equivalent widths show significant differences between components. Most likely, microlensing is responsible for this phenomenon. This is also consistent with the fact that component D, which shows the highest relative continuum level, has brightened by 0.07 mag since Dec 2001. We find that the emission line flux ratios between the components are in better agreement with simple lens models than broad band or continuum measurements, but that the discrepancies between model and data are still unacceptably large. Finally, we present a detection of the lensing galaxy, although this is close to the limits of the data. Comparing with a model galaxy spectrum, we obtain a redshift estimate of z_lens=0.44+-0.02.

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arXiv2003

The evolution of faint AGN between z~1 and z~5 from the COMBO-17 survey

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 350nm < lambda_obs < 930nm, we could simultaneously determine photometric redshifts with an accuracy of sigma_z<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 M_B = -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.

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arXiv2003

Disentangling microlensing and differential extinction in the double QSO HE0512-3329

We present the first separate spectra of both components of the small-separation double QSO HE0512-3329 obtained with HST/STIS in the optical and near UV. The similarities especially of the emission line profiles and redshifts strongly suggest that this system really consists of two lensed images of one and the same source. The emission line flux ratios are assumed to be unaffected by microlensing and are used to study the differential extinction effects caused by the lensing galaxy. Fits of empirical laws show that the extinction properties seem to be different on both lines of sight. With our new results, HE0512-3329 becomes one of the few extragalactic systems which show the 2175 AA absorption feature, although the detection is only marginal. We then correct the continuum flux ratio for extinction to obtain the differential microlensing signal. Since this may still be significantly affected by variability and time-delay effects, no detailled analysis of the microlensing is possible at the moment. This is the first time that differential extinction and microlensing could be separated unambiguously. We show that, at least in HE0512-3329, both effects contribute significantly to the spectral differences and one cannot be analysed without taking into account the other. For lens modelling purposes, the flux ratios can only be used after correcting for both effects.

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arXiv2003

Integral Field Spectroscopy of SN 2002er with PMAS

We present observations of the Type Ia supernova SN 2002er during the brightening phase. The observations were performed with the Potsdam Multi Aperture Spectrophotometer (PMAS) integral field instrument. Due to the 8"*8", field of view of the spectrograph an accurate background subtraction was possible. Results from analyses of the evolution of absorption features in comparisons with other SNe show that SN 2002er is a fairly bright Type Ia supernova with a peak brightness of M_B=-19.6+-0.1.

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arXiv2002

On-axis spatially resolved spectroscopy of low redshift quasar host galaxies: HE 1503+0228 at z=0.135

We present the first result of a comprehensive spectroscopic study of quasar host galaxies. On-axis, spatially resolved spectra of low redshift quasars have been obtained with FORS1, mounted on the 8.2m ESO Very Large Telescope, Antu. The spectra are spatially deconvolved using a spectroscopic version of the ``MCS deconvolution algorithm''. The algorithm decomposes two dimensional spectra into the individual spectra of the central point-like nucleus and of its host galaxy. Applied to HE1503+0228, at z=0.135 (M_B=-23.0), it provides us with the spectrum of the host galaxy between 3600A, and 8500A, (rest-frame), at a mean resolving power of 700. The data allow us to measure several of the important Lick indices. The stellar populations and gas ionization state of the host galaxy of HE1503+0228, are very similar to the ones measured for normal non-AGN galaxies. Dynamical information is also available for the gas and stellar components of the galaxy. Using deconvolution and a deprojection algorithm, velocity curves are derived for emission lines, from the center up to 4arcsec, away from the nucleus of the galaxy. Fitting a simple three-components mass model (point mass, spherical halo of dark matter, disk) to the position-velocity diagram, we infer a mass of M(r<1kpc) = (2.0 +- 0.3) 10^{10} solar mass within the central kiloparsec of the galaxy, and a mass integrated over 10 kpc of M(r<10kpc) = (1.9 +- 0.3) 10^{11} solar mass, with an additional 10% error due to the uncertainty on the inclination of the galaxy. This, in combination with the analysis of the stellar populations indicates that the host galaxy of HE1503+0228, is a normal spiral galaxy.

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arXiv2001

The Double Quasar HE1104-1805: a case study for time delay determination with poorly sampled lightcurves

We present a new determination of the time delay of the gravitational lens system HE1104-1805 ('Double Hamburger') based on a previously unpublished dataset. We argue that the previously published value of dt_(A-B)=0.73 years was affected by a bias of the employed method. We determine a new value of dt_(A-B)=0.85+/-0.05 years (2 sigma confidence level), using six different techniques based on non interpolation methods in the time domain. The result demonstrates that even in the case of poorly sampled lightcurves, useful information can be obtained with regard to the time delay. The error estimates were calculated through Monte Carlo simulations. With two already existing models for the lens and using its recently determined redshift, we infer a range of values of the Hubble parameter: Ho=48+/-4 km/s Mpc^-1 (2 sigma) for a singular isothermal ellipsoid (SIE) and Ho=62+/-4 km/s Mpc^-1 (2 sigma) for a constant mass-to-light ratio plus shear model (M/L+gamma). The possibly much larger errors due to systematic uncertainties in modeling the lens potential are not included in this error estimate.

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arXiv2001

Deep BVR photometry of the Chandra Deep Field South from the COMBO-17 survey

We report on deep multi-color imaging (R_5-sigma = 26) of the Chandra Deep Field South, obtained with the Wide Field Imager (WFI) at the MPG/ESO 2.2 m telescope on La Silla as part of the COMBO-17 survey. As a result we present a catalog of 63501 objects in a field measuring 31.5' x 30' with astrometry and BVR photometry. A sample of 37 variable objects is selected from two-epoch photometry. We try to give interpretations based on color and variation amplitude.

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arXiv2001

The stellar content of the Hamburg/ESO survey. II. A large, homogeneously-selected sample of high latitude carbon stars

We present a sample of 403 faint high latitude carbon (FHLC) stars selected from the digitized objective prism plates of the Hamburg/ESO Survey (HES). Because of the ~15 Angstroem spectral resolution and high signal-to-noise ratio of the HES prism spectra, our automated procedure based on the detection of C_2 and CN molecular bands permits high-confidence identification of carbon stars without the need for follow-up spectroscopy. From a set of 329 plates (87% of the survey), covering 6400 square degrees to a magnitude limit of V~16.5, we analyze the selection efficiency and effective surface area of the HES FHLC survey to date. The surface density of FHLC stars that we detect (0.072 +/- 0.005 square degrees) is 2-4 times higher than that of previous objective prism and CCD surveys at high galactic latitude, even though those surveys claimed a limiting magnitude up to 1.5 magnitudes fainter. This attests to the highest selection sensitivity yet achieved for these types of stars.

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arXiv2001

The luminosity function of QSO host galaxies

We report on results from H band imaging observations of a complete sample of high-luminosity low-redshift QSOs. The luminosity function of QSO hosts is similar in shape to that of normal galaxies, although offset in normalisation by a factor of 10^(-4). This supports the hypothesis that the parent population of quasars is identical to the general population of early-type field galaxies.

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arXiv2001

QSO host galaxy star formation history from multicolour data

We investigate multicolour imaging data of a complete sample of low redshift (z<0.2) QSO host galaxies. The sample was imaged in four optical (BVRi) and three near-infrared bands (JHKs), and in addition spectroscopic data is available for a majority of the objects. We extract host luminosities for all bands by means of two-dimensional modeling of galaxy and nucleus. Optical and optical-to-NIR colours agree well with the average colours of inactive early type galaxies. The six independent colours are used to fit population synthesis models. We assess the presence of young populations in the hosts for which evidence shows to be very weak.

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arXiv2000

The stellar content of the Hamburg/ESO survey. I. Automated selection of DA white dwarfs

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 narrow band (Strömgren c_1) colours can be derived directly from HES spectra, with precisions of sigma(U-B)=0.092mag; sigma(B-V)=0.095mag; sigma(c_1)=0.15mag. We describe simulation techniques that allow 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_λ(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.

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arXiv2000

Cool helium-rich white dwarfs from the Hamburg/ESO Survey

We present an analysis of 40 cool helium-rich white dwarfs found in the Hamburg/ESO survey. They were selected for follow-up spectroscopy because of their U-B colour below -0.18, the absence of strong absorption lines, and a continuum shape similar to that of a quasar. Effective temperatures for individual stars were determined by fitting model atmospheres of nearly pure helium with a small admixture of hydrogen. As a consequence of the selection criteria all but one sample stars have Teff below 20000 K. Four stars clearly show helium and hydrogen lines in their spectra. In the spectra of another three, helium, hydrogen, and metal lines can be detected. For these stars hydrogen and metal abundances were also determined by fitting appropriate model atmospheres containing these elements. Seven sample stars most likely have helium-rich atmospheres but do not show any helium lines. They either have featureless spectra or show calcium lines.

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arXiv2000

The Hamburg/ESO survey for bright QSOs. III. A large flux-limited sample of QSOs

We present a new sample of 415 bright QSOs and Seyfert~1 nuclei drawn from the Hamburg/ESO survey (HES). The sample is spectroscopically 99 % complete and well-defined in terms of flux and redshift limits. Optical magnitudes are in the interval 13 < B_J < 17.5, redshifts range within 0< z < 3.2. More than 50 % of the objects in the sample are new discoveries. We describe the selection techniques and discuss sample completeness and potential selection effects. There is no evidence for redshift-dependent variations of completeness; in particular, low-redshift QSOs - notoriously missed by other optical surveys - are abundant in this sample, since no discrimination against extended sources is imposed. For the same reason, the HES is not biased against multiply imaged QSOs due to gravitational lensing. The sample forms the largest homogeneous set of bright QSOs currently in existence, useful for a variety of statistical studies. We have redetermined the bright part of the optical quasar number-magnitude relation. We confirm that the Palomar-Green survey is significantly incomplete, but that its degree of incompleteness has recently been overestimated.

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arXiv1999

FAUST Observations in the Fourth Galactic Quadrant

We analyze UV observations with FAUST of four sky fields in the general direction of the fourth Galactic quadrant, in which we detect 777 UV sources. This is ~50% more than detected originally by Bowyer et al (1995). We discuss the source detection process and the identification of UV sources with optical counterparts. For the first time in this project we use ground-based objective-prism information for two of the fields, to select the best-matching optical objects with which to identify the UV sources. Using this, and correlations with existing catalogs, we present reliable identifications for \~75% of the sources. Most of the remaining sources have assigned optical counterparts, but lacking additional information we offer only plausible identification. We discuss the types of objects found, and compare the observed population with predictions of our UV Galaxy model.

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arXiv1999

Finding the First Stars: The Hamburg/ESO Objective Prism Survey

We report on a search for extremely metal-poor ([Fe/H]<-3.0) turnoff stars in the Hamburg/ESO objective prism survey (HES). Metal-poor stars are selected by automatic spectral classification. Extensive simulations show that the selection efficiency for turnoff stars of [Fe/H]<-3.0 is >25% at B<16.5. Since the HES is more than 1 mag deeper than the HK survey of Beers et al. (1992), the HES offers the possibility to efficiently increase the total number of metal-poor stars by at least a factor of 4.

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arXiv1999

The new complex gravitational lens system HE 0230-2130

We report the discovery of the new gravitational lens system HE~0230$-$2130, a QSO at redshift $z=2.162$ consisting of at least five distinct components. Three of these are clearly lensed images of the QSO, one is most likely the lensing galaxy, while for the fifth component the identity is unclear: It could be a fourth QSO image (if so, then highly reddened), or another intervening galaxy, or a superposition of the two. Differential reddening seems to be important also for the first three QSO images. The surface density of faint galaxies near the QSO appears to be enhanced by a factor of $\ga 2$, indicating the presence of a distant cluster close to the line of sight.

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arXiv1998

Four magnetic DB white dwarfs discovered by the Hamburg/ESO survey

We report on seven peculiar faint blue stars found in the course of the Hamburg/ESO survey (HES) which appear to be magnetic white dwarfs (WDs) with non-hydrogen spectra. We show in particular that four of them (HE 0338-3853, HE 0107-0158, HE 0026-2150, and HE 0003-5701) have He I lines split by magnetic fields of roughly 20MG, since the πcomponents of He I 5876 Angstroem and He I 4929 Angstroem can be identified unambiguously in their spectra, and the $σ^+$, $σ^-$ components can be identified in the spectra of two of these stars (HE 0338-3853 and HE 0003-5701). Besides GD 229, these are the first magnetic DB white dwarfs discovered so far. In addition, three further WDs with broad, unidentifiable features have been found: HE 1043-0502, HE 0236-2656, and HE 0330-0002. We argue that in all three of these stars H I can not be responsible for the broad features, and He I most probably not for the features in HE 0236-2656 and HE 0330-0002, while it still remains possible that the broad features of HE 1043-0502 are due to He I.

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