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Register a new userThe VLT-VIMOS Mask Preparation Software
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VIMOS (VIsible Multi-Object Spectrograph) is a multi-object imaging spectrograph installed at the VLT (Very large Telescope) at the ESO (European Southern Observatory) Paranal Observatory, especially suited for survey work. VIMOS is characterized by its very high multiplexing factor: it is possible to take up to 800 spectra with 10 arcsec long slits in a single exposure. To fully exploit its multiplexing potential, we designed and implemented a dedicated software tool: the VIMOS Mask Preparation Software (VMMPS), which allows the astronomer to select the objects to be spectroscopically observed, and provides for automatic slit positioning and slit number maximization within the instrumental constraints. The output of VMMPS is used to manufacture the slit masks to be mounted in the instrument for spectroscopic observations.
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The VIRMOS Consortium has the task to design and manufacture two spectrographs for ESO VLT, VIMOS (Visible Multi-Object Spectrograph) and NIRMOS (Near Infrared Multi-Object Spectrograph). This paper describes how the Mask Manufacturing Unit (MMU), which cuts the slit masks to be used with both instruments, meets the scientific requirements and manages the storage and the insertion of the masks into the instrument. The components and the software of the two main parts of the MMU, the Mask Manufacturing Machine and the Mask Handling System, are illustrated together with the mask material and with the slit properties. Slit positioning is accurate within 15 micron, equivalent to 0.03 arcsec on the sky, while the slit edge roughness has an rms on the order of 0.03 pixels on scales of a slit 5 arcsec long and of 0.01 pixels on the pixel scale (0.205 arcsec). The MMU has been successfully installed during July/August 2000 at the Paranal Observatory and is now operational for spectroscopic mask cutting, compliant with the requested specifications.
OSIRIS is a Day One instrument that will be available at the 10m GTC telescope which is being built at La Palma observatory in the Canary Islands. This optical instrument is designed to obtain wide-field narrow-band images using tunable filters and to do low-resolution spectroscopy in both long-slit and multislit modes. For the multislit spectroscopy mode, we have developed a software to assist the observers to design focal plane masks. In this paper we describe the characteristics of this Mask Designer tool. We discuss the main design concepts, the functionality and particular features of the software.
[Abridged] We present a homogeneous and complete catalogue of optical groups identified in the purely flux limited (17.5<=I<=24.0) VIMOS-VLT Deep Survey (VVDS). We use mock catalogues extracted from the MILLENNIUM simulation, to correct for potential systematics that might affect the overall distribution as well as the individual properties of the identified systems. Simulated samples allow us to forecast the number and properties of groups that can be potentially found in a survey with VVDS-like selection functions. We use them to correct for the expected incompleteness and also to asses how well galaxy redshifts trace the line-of-sight velocity dispersion of the underlying mass overdensity. In particular, we train on these mock catalogues the adopted group-finding technique (the Voronoi-Delaunay Method, VDM). The goal is to fine-tune its free parameters, recover in a robust and unbiased way the redshift and velocity dispersion distributions of groups and maximize the level of completeness (C) and purity (P) of the group catalogue. We identify 318 VVDS groups with at least 2 members within 0.2<=z<=1.0, among which 144 (/30) with at least 3 (/5) members. The sample has globally C=60% and P=50%. Nearly 45% of the groups with at least 3 members are still recovered if we run the algorithm with a parameter set which maximizes P (75%). We exploit the group sample to study the redshift evolution of the fraction f_b of blue galaxies (U-B<=1) within 0.2<=z<=1. We find that f_b is significantly lower in groups than in the whole ensemble of galaxies irrespectively of their environment. These quantities increase with redshift, with f_b in groups showing a marginally significant steeper increase. We also confirm that, at any explored redshift, f_b decreases for increasing group richness, and we extend towards fainter luminosities the magnitude range over which this result holds.
We present the type-1 active galactic nuclei (AGN) sample extracted from the VIMOS VLT Deep Survey first observations of 21000 spectra in 1.75 square degree. This sample, which is purely magnitude limited, free of morphological or color selection biases, contains 130 broad line AGN (BLAGN) spectra with redshift up to 5. Our data are divided into a wide (Iab < 22.5) and a deep (Iab < 24) subsample containing 56 and 74 objects respectively. Because of its depth and selection criteria, this sample is uniquely suited to study the population of faint type-1 AGN. Our measured surface density (~ 472 +- 48 BLAGN per square degree with Iab < 24) is significantly higher than that of any other optically selected sample of BLAGN with spectroscopic confirmation. By applying a morphological and color analysis to our AGN sample we find that: (1)~23% of the AGN brighter than Iab=22.5 are classified as extended; this percentage increases to ~42% for those with z < 1.6; (2) a non-negligible fraction of our BLAGN are lying close to the color space area occupied by stars in u*-g versus g-r color-color diagram. This leads us to the conclusion that classical optical ultraviolet preselection technique, if employed at such deep magnitudes (Iab=22.5) in conjuction with a preselection of point-like sources, can miss miss up to ~35% of the AGN population. Finally, we present a composite spectrum of our sample of objects. While the continuum shape is very similar to that of the SDSS composite at short wavelengths, it is much redder than it at lambda > 3000 A. We interpret this as due to significant contamination from emission of the host galaxies, as expected from the faint absolute magnitudes sampled by our survey.
We have investigated the dependence of galaxy clustering on their stellar mass at z~1, using the data from the VIMOS-VLT Deep Survey (VVDS). We have measured the projected two-point correlation function of galaxies, wp(rp) for a set of stellar mass selected samples at an effective redshift <z>=0.85. We have control and quantify all effects on galaxy clustering due to the incompleteness of our low mass samples. We find that more massive galaxies are more clustered. When compared to similar results at z~0.1 in the SDSS, we observed no evolution of the projected correlation function for massive galaxies. These objects present a stronger linear bias at z~1 with respect to low mass galaxies. As expected, massive objects at high redshift are found in the highest pics of the dark matter density field.
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