Jean-Luc Gach

Bio: Jean-Luc Gach is an academic researcher from Aix-Marseille University. The author has contributed to research in topics: Wavefront & Photon counting. The author has an hindex of 23, co-authored 82 publications receiving 2207 citations. Previous affiliations of Jean-Luc Gach include Centre national de la recherche scientifique & University of Provence.

SPHERE: A ‘Planet Finder’ Instrument for the VLT

Abstract: Direct detection and spectral characterization of extra-solar planets is one of the most exciting but also one of the most challenging areas in modern astronomy. The challenge consists in the very large contrast between the host star and the planet, larger than 12.5 magnitudes at very small angular separations, typically inside the seeing halo. The whole design of a "Planet Finder" instrument is therefore optimized towards reaching the highest contrast in a limited field of view and at short distances from the central star. Both evolved and young planetary systems can be detected, respectively through their reflected light and through the intrinsic planet emission. We present the science objectives, conceptual design and expected performance of the SPHERE instrument.

GHASP: an Hα kinematic survey of spiral and irregular galaxies - VI. New Hα data cubes for 108 galaxies

Abstract: We present the Fabry-Perot observations obtained for a new set of 108 galaxies in the frame of the Gassendi Ha survey of SPirals (GHASP). The GHASP survey consists of 3D Ha data cubes for 203 spiral and irregular galaxies, covering a large range in morphological types and absolute magnitudes, for kinematics analysis. The new set of data presented here completes the survey. The GHASP sample is by now the largest sample of Fabry-Perot data ever published. The analysis of the whole GHASP sample will be done in forthcoming papers. Using adaptive binning techniques based on Voronoi tessellations, we have derived Ha data cubes from which are computed Ha maps, radial velocity fields as well as residual velocity fields, position-velocity diagrams, rotation curves and the kinematical parameters for almost all galaxies. Original improvements in the determination of the kinematical parameters, rotation curves and their uncertainties have been implemented in the reduction procedure. This new method is based on the whole 2D velocity field and on the power spectrum of the residual velocity field rather than the classical method using successive crowns in the velocity field. Among the results, we point out that morphological position angles have systematically higher uncertainties than kinematical ones, especially for galaxies with low inclination. The morphological inclination of galaxies having no robust determination of their morphological position angle cannot be constrained correctly. Galaxies with high inclination show a better agreement between their kinematical inclination and their morphological inclination computed assuming a thin disc. The consistency of the velocity amplitude of our rotation curves has been checked using the Tully-Fisher relationship. Our data are in good agreement with previous determinations found in the literature. Nevertheless, galaxies with low inclination have statistically higher velocities than expected and fast rotators are less luminous than expected.

GHASP : An Halpha kinematic survey of spiral and irregular galaxies - VI. New Halpha data cubes for 108 galaxies

Abstract: We present the Fabry-Perot observations obtained for a new set of 108 galaxies that completes the GHASP survey (Gassendi HAlpha survey of SPirals). The GHASP survey consists of 3D Ha data cubes for 203 spiral and irregular galaxies, covering a large range in morphological types and absolute magnitudes, for kinematics analysis. The GHASP sample is by now the largest sample of Fabry-Perot data ever published. We have derived Ha data cubes from which are computed Ha maps, radial velocity fields as well as residual velocity fields, position-velocity diagrams, rotation curves and the kinematical parameters for almost all galaxies. Original improvements in the determination of the kinematical parameters, rotation curves and their uncertainties have been implemented in the reduction procedure. This new method is based on the whole 2D velocity field and on the power spectrum of the residual velocity fieldrather than the classical method using successive crowns in the velocity field. Among the results, we point out that morphological position angles have systematically higher uncertainties than kinematical ones, especially for galaxies with low inclination. Morphological inclination of galaxies having no robust determination of their morphological position angle cannot be constrained correctly. Galaxies with high inclination show a better agreement between their kinematical inclination and their morphological inclination computed assuming a thin disk. The consistency of the velocity amplitude of our rotation curves have been checked using the Tully-Fisher relationship. Our data are in good agreement with previous determinations found in the literature. Nevertheless, galaxies with low inclination have statistically higher velocities than expected and fast rotators are less luminous than expected.

Kinematics of tidal tails in interacting galaxies: Tidal Dwarf Galaxies and projection effects

Abstract: The kinematics of tidal tails in colliding galaxies has been studied via Fabry-Perot observations of the Halpha emission. With their large field of view and high spatial resolution, the Fabry-Perot data allow to probe simultaneously, in 2-D, two kinematical features of the tidal ionized gas: large-scale velocity gradients due to streaming motions along the tails, and small-scale motions related to the internal dynamics of giant HII regions within the tails. In several interacting systems, massive (10^9 Msun) condensations of HI, CO and stars are observed in the outer regions of tails. Whether they are genuine accumulations of matter or not is still debated. Indeed a part of the tidal tail may be aligned with the line-of-sight, and the associated projection effect may result in apparent accumulations of matter that does not exist in the 3-D space. Using numerical simulations, we show that studying the large-scale kinematics of tails, it is possible to know whether these accumulations of matter are the result of projection effects or not. We conclude that several ones are genuine accumulations of matter. We also study the small-scale motions inside these regions: several small-scale velocity gradients are identified with projected values as large as 50-100 km/s accross the observed HII regions. In one system, the spatial resolution of our observations is sufficient to detail the velocity field; we show that it is rotating and self-gravitating, and discuss its dark matter content. The Fabry-Perot observations have thus enabled us to prove that some 10^9 Msun condensations of matter are real structures, and are kinematically decoupled from the rest of the tail. Such massive and self-gravitating objects are the progenitors of the so-called \'\'Tidal Dwarf Galaxies\'\'.

Fabry‐Pérot Observations Using a New GaAs Photon‐counting System

Abstract: A third-generation image photon-counting system (IPCS) camera is presented, based on a GaAs photocathode that can achieve a quantum efficiency of up to 23%, which is comparable to a thick CCD but without readout noise. This system is 10 times more sensitive at Hα than previous photon-counting cameras. In terms of signal-to-noise ratio, the system outperforms CCDs for extremely faint fluxes, including antireflection-coated, low-noise, thin CCDs. This system, with up to 1K × 1K pixels, is one of the largest monolithic IPCSs. A unique cooling system, based on a Ranque-Hilsh vortex tube, is used for this camera. Real-time centering is done by a scalable digital signal processor board. Astrophysical projects and preliminary results obtained with this new camera coupled with a scanning Fabry-Perot interferometer at the Cassegrain focus of the 3.6 m ESO telescope, the 1.93 m Observatoire de Haute Provence telescope, and the 1.6 m Observatoire du Mont Megantic telescope are presented.

The Occurrence and Architecture of Exoplanetary Systems

Abstract: The basic geometry of the Solar System—the shapes, spacings, and orientations of the planetary orbits—has long been a subject of fascination as well as inspiration for planet-formation theories. For exoplanetary systems, those same properties have only recently come into focus. Here we review our current knowledge of the occurrence of planets around other stars, their orbital distances and eccentricities, the orbital spacings and mutual inclinations in multiplanet systems, the orientation of the host star's rotation axis, and the properties of planets in binary-star systems.

Detection and characterization of exoplanets and disks using projections on karhunen-loève eigenimages

Abstract: We describe a new method to achieve point-spread function (PSF) subtractions for high-contrast imaging using principal component analysis that is applicable to both point sources or extended objects (disks). Assuming a library of reference PSFs, a Karhunen–Lo` eve transform of these references is used to create an orthogonal basis of eigenimages on which the science target is projected. For detection this approach provides comparable suppression to the Locally Optimized Combination of Images (LOCI) algorithm, albeit with increased robustness to the algorithm parameters and speed enhancement. For characterization of detected sources, the method enables forward modeling of astrophysical sources. This alleviates the biases in the astrometry and photometry of discovered faint sources, which are usually associated with LOCI-based PSF subtractions schemes. We illustrate the algorithm performance using archival Hubble Space Telescope images, but the approach may also be considered for ground-based data acquired with angular differential imaging or integral-field spectrographs.

The 2MASS Redshift Survey: description and data release

Abstract: We present the results of the 2MASS Redshift Survey (2MRS), a ten-year project to map the full three-dimensional distribution of galaxies in the nearby universe. The Two Micron All Sky Survey (2MASS) was completed in 2003 and its final data products, including an extended source catalog (XSC), are available online. The 2MASS XSC contains nearly a million galaxies with K_s ≤ 13.5 mag and is essentially complete and mostly unaffected by interstellar extinction and stellar confusion down to a galactic latitude of |b| = 5° for bright galaxies. Near-infrared wavelengths are sensitive to the old stellar populations that dominate galaxy masses, making 2MASS an excellent starting point to study the distribution of matter in the nearby universe. We selected a sample of 44,599 2MASS galaxies with K_s ≤ 11.75 mag and |b| ≥ 5° (≥8° toward the Galactic bulge) as the input catalog for our survey. We obtained spectroscopic observations for 11,000 galaxies and used previously obtained velocities for the remainder of the sample to generate a redshift catalog that is 97.6% complete to well-defined limits and covers 91% of the sky. This provides an unprecedented census of galaxy (baryonic mass) concentrations within 300 Mpc. Earlier versions of our survey have been used in a number of publications that have studied the bulk motion of the Local Group, mapped the density and peculiar velocity fields out to 50 h^(–1) Mpc, detected galaxy groups, and estimated the values of several cosmological parameters. Additionally, we present morphological types for a nearly complete sub-sample of 20,860 galaxies with K_s ≤ 11.25 mag and |b| ≥ 10°.

Review of snapshot spectral imaging technologies

Abstract: Within the field of spectral imaging, the vast majority of instruments used are scanning devices. Recently, several snapshot spectral imaging systems have become commercially available, providing new functionality for users and opening up the field to a wide array of new applications. A comprehensive survey of the available snapshot technologies is provided, and an attempt has been made to show how the new capabilities of snapshot approaches can be fully utilized.

Gas-to-dust mass ratios in local galaxies over a 2 dex metallicity range

Abstract: Aims. The goal of this paper is to analyse the behaviour of the gas-to-dust mass ratio (G/D) of local Universe galaxies over a wide metallicity range. We especially focus on the low-metallicity part of the G/D vs metallicity relation and investigate several explanations for the observed relation and scatter.Methods. We assembled a total of 126 galaxies, covering a 2 dex metallicity range and with 30% of the sample with 12 + log(O/H)≤ 8.0. We homogeneously determined the dust masses with a semi-empirical dust model including submm constraints. The atomic and molecular gas masses have been compiled from the literature. We used two XCO scenarios to estimate the molecular gas mass: the Galactic conversion factor, XCO,MW, and a XCO that depends on the metallicity XCO,Z (∝Z-2). We modelled the observed trend of the G/D with metallicity using two simple power laws (slope of –1 and free) and a broken power law. Correlations with morphological type, stellar masses, star formation rates, and specific star formation rates are also discussed. We then compared the observed evolution of the G/D with predictions from several chemical evolution models and explored different physical explanations for the observed scatter in the G/D values.Results. We find that out of the five tested galactic parameters, metallicity is the main physical property of the galaxy driving the observed G/D. The G/D versus metallicity relation cannot be represented by a single power law with a slope of –1 over the whole metallicity range. The observed trend is steeper for metallicities lower than ~8.0. A large scatter is observed in the G/D values for a given metallicity: in metallicity bins of ~0.1 dex, the dispersion around the mean value is ~0.37 dex. On average, the broken power law reproduces the observed G/D best compared to the two power laws (slope of –1 or free) and provides estimates of the G/D that are accurate to a factor of 1.6. The good agreement of observed values of the G/D and its scatter with respect to metallicity with the predicted values of the three tested chemical evolution models allows us to infer that the scatter in the relation is intrinsic to galactic properties, reflecting the different star formation histories, dust destruction efficiencies, dust grain size distributions, and chemical compositions across the sample. Conclusions. Our results show that the chemical evolution of low-metallicity galaxies, traced by their G/D, strongly depends on their local internal conditions and individual histories. The large scatter in the observed G/D at a given metallicity reflects the impact of various processes occurring during the evolution of a galaxy. Despite the numerous degeneracies affecting them, disentangling these various processes is now the next step.