DECPHOT: An Optimal Deconvolution-based Photometric Reduction Method

in Transiting Extrapolar Planets Workshop (2007, July 01)

A high accuracy photometric reduction method is needed to take full advantage of the potential of the transit method for the detection and characterization of exoplanets, especially in deep crowded fields ... [more ▼]

A high accuracy photometric reduction method is needed to take full advantage of the potential of the transit method for the detection and characterization of exoplanets, especially in deep crowded fields. In this context, we present DECPHOT, a new deconvolution-based photometry algorithm able to deal with a very high level of crowding and large variations of seeing. It also increases the resolution of astronomical images, an important advantage for the discrimination of false positives in transit photometry. [less ▲]

On-axis spectroscopy of the host galaxies of 20 optically luminous quasars at z similar to 0.3

in Monthly Notices of the Royal Astronomical Society (2007), 378(1), 83-108

We present the analysis of a sample of 20 bright low-redshift quasars (M-B < -23 and z < 0.35) observed spectroscopically with the Very Large Telescope (VLT). The Focal Reducer/low dispersion Spectrograph ... [more ▼]

We present the analysis of a sample of 20 bright low-redshift quasars (M-B < -23 and z < 0.35) observed spectroscopically with the Very Large Telescope (VLT). The Focal Reducer/low dispersion Spectrograph (FORS1) spectra, obtained in Multi-Object Spectroscopy (MOS) mode, allow to observe simultaneously the quasars and several reference stars used to spatially deconvolve the data. Applying the Magain, Courbin, & Sohy (MCS) deconvolution method, we are able to separate the individual spectra of the quasar and of the underlying host galaxy. Contrary to some previous claims, we find that luminous quasars are not exclusively hosted by massive ellipticals. Most quasar host galaxies harbour large amounts of gas, irrespective of their morphological type. Moreover, the stellar content of half of the hosts is a young Sc-like population, associated with a rather low-metallicity interstellar medium. A significant fraction of the galaxies contain gas ionized at large distances by the quasar radiation. This large distance ionization is always associated with signs of gravitational interactions (as detected from images or disturbed rotation curves). The spectra of the quasars themselves provide evidence that gravitational interactions bring dust and gas in the immediate surrounding of the super massive black hole, allowing to feed it. The quasar activity might thus be triggered (1) in young gas-rich spiral galaxies by local events and (2) in more evolved galaxies by gravitational interactions or collisions. The latter mechanism gives rises to the most powerful quasars. Finally, we derive mass models for the isolated spiral host galaxies and we show that the most reliable estimators of the systemic redshift in the quasar spectrum are the tips of the Hα and Hβ lines. [less ▲]

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses. V. The time delay in SDSS J1650+4251

in Astronomy and Astrophysics (2007), 464

Aims.Our aim is to measure the time delay between the two gravitationally lensed images of the z_qso = 1.547 quasar SDSS J1650+4251, in order to estimate the Hubble constant H_0. Methods: Our measurement ... [more ▼]

Aims.Our aim is to measure the time delay between the two gravitationally lensed images of the z_qso = 1.547 quasar SDSS J1650+4251, in order to estimate the Hubble constant H_0. Methods: Our measurement is based on R-band light curves with 57 epochs obtained at Maidanak Observatory, in Uzbekistan, from May 2004 to September 2005. The photometry is performed using simultaneous deconvolution of the data, which provides the individual light curves of the otherwise blended quasar images. The time delay is determined from the light curves using two very different numerical techniques, i.e., polynomial fitting and direct cross-correlation. The time delay is converted into H[SUB]0[/SUB] following analytical modeling of the potential well. Results: Our best estimate of the time delay is Delta t = 49.5 ± 1.9 days, i.e., we reach a 3.8% accuracy. The R-band flux ratio between the quasar images, corrected for the time delay and for slow microlensing, is F_A/F[SUB]B[/SUB] = 6.2 ± 5%. Conclusions: .The accuracy reached on the time delay allows us to discriminate well between families of lens models. As for most other multiply imaged quasars, only models of the lensing galaxy that have a de Vaucouleurs mass profile plus external shear give a Hubble constant compatible with the current most popular value (H[SUB]0[/SUB] = 72 ± 8 km s[SUP]-1[/SUP] Mpc[SUP]-1[/SUP]). A more realistic singular isothermal sphere model plus external shear gives H[SUB]0[/SUB] = 51.7[SUP]+4.0[/SUP][SUB]-3.0[/SUB] km s[SUP]-1[/SUP] Mpc[SUP]-1[/SUP]. Table [see full text] is only available in electronic form at http://www.aanda.org [less ▲]

Deconvolution of HST images of the Cloverleaf gravitational lens - Detection of the lensing galaxy and a partial Einstein ring

in Astronomy and Astrophysics (2007), 470(2), 467-473

Archival HST/NICMOS-2 images of the Cloverleaf gravitational lens ( H1413+ 117), a quadruply-imaged quasar, were analysed with a new method derived from the MCS deconvolution algorithm (Magain et al. 1998 ... [more ▼]

Archival HST/NICMOS-2 images of the Cloverleaf gravitational lens ( H1413+ 117), a quadruply-imaged quasar, were analysed with a new method derived from the MCS deconvolution algorithm (Magain et al. 1998). This method is based on an iterative process which simultaneously allows us to determine the Point Spread Function ( PSF) and to perform a deconvolution of images containing several point sources plus extended structures. As such, it is well-adapted to the processing of gravitational lens images, especially in the case of multiply-imaged quasars. Two sets of data were analysed: the first one, which was obtained through the F160W filter in 1997, basically corresponds to a continuum image, while the second one, obtained through the narrower F180M filter in 2003, is centered around the forbidden [OIII] emission lines at the source redshift, thus probing the narrow-line region of the quasar. The deconvolution gives astrometric and photometric measurements in both filters and reveals the primary lensing galaxy as well as a partial Einstein ring. The high accuracy of the results is particularly important in order to model the lensing system and to reconstruct the source undergoing the strong lensing. The reliability of the method is checked on a synthetic image similar to H1413+117. [less ▲]

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses. II. SDSS J0924+0219: the redshift of the lensing galaxy, the quasar spectral variability and the Einstein rings

in Astronomy and Astrophysics (2006), 451

Aims.To provide the observational constraints required to use the gravitationally lensed quasar SDSS J0924+0219 for the determination of H[SUB]0[/SUB] from the time delay method. We measure here the ... [more ▼]

Aims.To provide the observational constraints required to use the gravitationally lensed quasar SDSS J0924+0219 for the determination of H[SUB]0[/SUB] from the time delay method. We measure here the redshift of the lensing galaxy, we show the spectral variability of the source, and we resolve the lensed host galaxy of the source. <BR />Methods.We present our VLT/FORS1 deep spectroscopic observations of the lensed quasar SDSS J0924+0219, as well as archival HST/NICMOS and ACS images of the same object. The two-epoch spectra, obtained in the Multi Object Spectroscopy (MOS) mode, allow for very accurate flux calibration and spatial deconvolution. This strategy provides spectra for the lensing galaxy and for the quasar images A and B, free of any mutual light contamination. We deconvolve the HST images as well, which reveal a double Einstein ring. The mass distributions in the lens, reconstructed in several ways, are compared. <BR />Results.We determine the redshift of the lensing galaxy in SDSS J0924+0219: z_lens = 0.394±0.001. Only slight spectral variability is seen in the continuum of quasar images A and B, while the C III] , Mg II and Fe II emission lines display obvious changes. The flux ratio between the quasar images A and B is the same in the emission lines and in the continuum. One of the Einstein rings found using deconvolution corresponds to the lensed quasar host galaxy at z=1.524 and a second bluer one, is the image either of a star-forming region in the host galaxy, or of another unrelated lower redshift object. A broad range of lens models give a satisfactory fit to the data. However, they predict very different time delays, making SDSS J0924+0219 an object of particular interest for photometric monitoring. In addition, the lens models reconstructed using exclusively the constraints from the Einstein rings, or using exclusively the astrometry of the quasar images, are not compatible. This suggests that multipole-like structures play an important role in SDSS J0924+0219. <BR /> [less ▲]

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses - III. Redshift of the lensing galaxy in eight gravitationally lensed quasars

in Astronomy and Astrophysics (2006), 451(3), 759-766

Aims. We measure the redshift of the lensing galaxy in eight gravitationally lensed quasars in view of determining the Hubble parameter H-0 from the time delay method. Methods. Deep VLT/FORS1 spectra of ... [more ▼]

Aims. We measure the redshift of the lensing galaxy in eight gravitationally lensed quasars in view of determining the Hubble parameter H-0 from the time delay method. Methods. Deep VLT/FORS1 spectra of lensed quasars are spatially deconvolved in order to separate the spectrum of the lensing galaxies from the glare of the much brighter quasar images. A new observing strategy is devised. It involves observations in Multi-Object-Spectroscopy (MOS) which allows the simultaneous observation of the target and of several PSF and flux calibration stars. The advantage of this method over traditional long-slit observations is a much more reliable extraction and flux calibration of the spectra. Results. For the first time we measure the redshift of the lensing galaxy in three multiply-imaged quasars: SDSS J1138+0314 (zlens = 0.445), SDSS J1226-0006 (z(lens) = 0.517), SDSS J1335+0118 (z(lens) = 0.440), and we give a tentative estimate of the redshift of the lensing galaxy in Q 1355- 2257 (z(lens) = 0.701). We confirm four previously measured redshifts: HE 0047-1756 (z(lens) = 0.407), HE 0230-2130 (z(lens) = 0.523), HE 0435-1223 (z(lens) = 0.454) and WFI J2033-4723 (z(lens) = 0.661). In addition, we determine the redshift of the second lensing galaxy in HE 0230-2130 (z(lens) = 0.526). The spectra of all lens galaxies are typical for early-type galaxies, except for the second lensing galaxy in HE 0230-2130 which displays prominent [OII] emission. [less ▲]

High precision photometry in crowded stellar fields

in SPIE Newsroom (2006)

A deconvolution-based method which allows to derive high-precision photometry of stars in crowded fields, proves very useful for a variety of astronomical projects, including transit searches for ... [more ▼]

A deconvolution-based method which allows to derive high-precision photometry of stars in crowded fields, proves very useful for a variety of astronomical projects, including transit searches for extrasolar planets. [less ▲]

Photometric monitoring of the doubly imaged quasar UM 673: possible evidence for chromatic microlensing

in Astronomy and Astrophysics (2005), 441

We present the results of two-band CCD photometric monitoring of the gravitationally lensed quasar Q 0142-100 (UM 673). The data, obtained at ESO-La Silla with the 1.54 m Danish telescope in the Gunn i ... [more ▼]

We present the results of two-band CCD photometric monitoring of the gravitationally lensed quasar Q 0142-100 (UM 673). The data, obtained at ESO-La Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998-September 1999) and in the Johnson V-band (October 1998 to December 2001), were analyzed using three different photometric methods. The light-curves obtained with all methods show variations, with a peak-to-peak amplitude of 0.14 mag in V. Although it was not possible to measure the time delay between the two lensed QSO images, the brighter component displays possible evidence for microlensing: it becomes bluer as it gets brighter, as expected under the assumption of differential magnification of a quasar accretion disk. [less ▲]

Chemical abundances in 43 metal-poor stars

in Astronomy and Astrophysics (2005), 440

We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Ni, and Ba for 43 metal-poor field stars in the solar neighbourhood, most of them subgiants or turn-off-point stars with iron ... [more ▼]

We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Ni, and Ba for 43 metal-poor field stars in the solar neighbourhood, most of them subgiants or turn-off-point stars with iron abundances [Fe/H] ranging from -0.4 to -3.0. About half of this sample has not been spectroscopically analysed in detail before. Effective temperatures were estimated from uvby photometry, and surface gravities primarily from Hipparcos parallaxes. The analysis is differential relative to the Sun, and was carried out with plane-parallel MARCS models. Various sources of error are discussed and found to contribute a total error of about 0.1-0.2 dex for most elements, while relative abundances, such as [Ca/Fe], are most probably more accurate. For the oxygen abundances, determined in an NLTE analysis of the 7774 Å triplet lines, the errors may be somewhat larger. We made a detailed comparison with similar studies and traced the reasons for the, in most cases, relatively small differences. Among the results we find that [O/Fe] possibly increases beyond [Fe/H] = -1.0, though considerably less so than in results obtained by others from abundances based on OH lines. We did not trace any tendency toward strong overionization of iron, and find the excesses, relative to Fe and the Sun, of the alpha elements Mg, Si, and Ca to be smaller than those of O. We discuss some indications that also the abundances of different alpha elements relative to Fe vary and the possibility that some of the scatter around the trends in abundances relative to iron may be real. This may support the idea that the formation of Halo stars occurred in smaller systems with different star formation rates. We verify the finding by Gratton et al. (2003b, A&A, 406, 131) that stars that do not participate in the rotation of the galactic disk show a lower mean and larger spread in [ alpha/Fe] than stars participating in the general rotation. The latter stars also seem to show some correlation between [ alpha/Fe] and rotation speed. We trace some stars with peculiar abundances, among these two Ba stars, <ASTROBJ>HD 17072</ASTROBJ> and <ASTROBJ>HD 196944</ASTROBJ>, the second already known to be rich in s elements. Finally we advocate that a spectroscopic study of a larger sample of halo stars with well-defined selection criteria is very important, in order to add to the very considerable efforts that various groups have already made. [less ▲]

Time Delay in Gravitational Lenses: an Alternative Route to the Hubble Constant

in Sterken, C. (Ed.) The Light-Time Effect in Astrophysics: Causes and cures of the O-C diagram (2005, July 01)

The gravitational lens phenomenon is briefly introduced. When several images of the same variable quasar are observed, a determination of the time delay, i.e. the difference in light travel time between ... [more ▼]

The gravitational lens phenomenon is briefly introduced. When several images of the same variable quasar are observed, a determination of the time delay, i.e. the difference in light travel time between these different images can lead to a determination of distances between the source, the lens and the observer and, thus, of the Hubble constant. These determinations are compared to the more classical ones based on standard candles (Cepheids and Type Ia Supernovae). [less ▲]