Spectral classification of emission-line galaxies from the Sloan Digital Sky Survey. I. An improved classification for high redshift galaxies

We study the spectral classification of emission-line galaxies as star-forming galaxies or Active Galactic Nuclei (AGNs). From the Sloan Digital Sky Survey (SDSS) high quality data, we define an improved classification to be used for high redshift galaxies. We classify emission-line galaxies of the SDSS according to the latest standard recipe using [Oiii]5007, [Nii]6584, [Sii]6717+6731, H, and H emission lines. We obtain four classes: star-forming galaxies, Seyfert 2, LINERs, and composites. We then examine where these galaxies fall in the blue diagram used at high redshift (i.e. log([Oiii]5007/H) vs. log([Oii]3726+3729/H).We define new improved boundaries in the blue diagram for star-forming galaxies, Seyfert 2, LINERs, SF/Sy2, and SF-LIN/comp classes. We maximize the success rate to 99.7% for the detection of star-forming galaxies, to 86% for the Seyfert 2 (including the SF/Sy2 region), and to 91% for the LINERs. We also minimize the contamination to 16% in the region of star-forming galaxies. We cannot reliably separate composites from star-forming galaxies and LINERs, but we define a SF/LIN/comp region where most of them fall (64%).Comment: 6 pages, accepted for publication in A&

Spectral classification of emission-line galaxies from the Sloan Digital Sky Survey. II. A supplementary diagnostic for AGNs using the Dn(4000) index

In this paper we present a classification of emission-line galaxies at intermediate and high redshifts (0.52.5 for near-infrared spectra), using the Dn(4000) index as a supplementary diagnostic. Our goal is to complement the diagnostic based only on emission-line ratios from the blue part of the spectra, which suffer from some limitations for the classification of Seyfert 2 and composite galaxies. We used a sample of 89 379 galaxies with a good signal-to-noise ratio from the Sloan Digital Sky Survey (data release 7). Using the classification scheme presented in Paper I, we classified these galaxies with a diagnostic diagram involving the [Oiii]5007 /Hbeta and [Oii]3726+3729 /Hbeta emission-line ratios. Then we derived a supplementary diagnostic involving Dn(4000) to improve this classification, in the regions where objects of different types are mixed. To show the validity of our spectral classification we established success-rate and contamination charts, then we compared our results to those obtained with the reference classification that was scheme obtained also using Halpha, [Nii]6584, and [Sii]6717+6731 emission lines. We show that our supplementary classification based on the Dn(4000) index allows to separate unambiguously star-forming galaxies from Seyfert 2 in the region where they were mixed in Paper I. It also significantly reduces the region where star-forming galaxies are mixed with composites.Comment: accepted for publication in A\&A, 10 pages corrected bug in LateX file for equations 7 and

Emission-lines calibrations of the Star Formation Rate from the Sloan Digital Sky Survey

Our goal is to study the existing star formation rate calibrations based on emission-line luminosities and to provide new ones. We use the SDSS data release DR4, which gives star formation rates and emission-line luminosities of more than 100000 star-forming galaxies. We confirm that the best results are obtained with the Halpha calibration. This calibration has an uncertainty of 0.17 dex. We show that one has to check carefully the method used to derive the dust attenuation and to use the adequate calibration: in some cases, the standard scaling law has to be replaced by a more general power law. When data is corrected for dust attenuation but the Halpha emission line not observed, the use of the Hbeta emission line, has to be preferred to the [OII]3727 emission line. In the case of uncorrected data, the correction for dust attenuation can be assumed as a constant value but we show that such method leads to poor results, in terms of dispersion and residual slope. Self-consistent corrections, based e.g. on the absolute magnitude, give better results in terms of dispersion but still suffer from systematic shifts, and/or residual slopes. The best results with data not corrected for dust attenuation are obtained when using the observed [OII]3727 and Hbeta emission lines together. This calibration has an uncertainty of 0.23 dex

The VIMOS-VLT Deep Survey: Dependence of galaxy clustering on stellar mass

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 =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.Comment: 4 pages, 2 figures, 43rd Rencontres de Moriond - March 15-22, 2008 - La Thuile (Val d'Aosta, Italy

Astrometry with "Carte du Ciel" plates, San Fernando zone. I. Digitization and measurement using a flatbed scanner

We present an original method of digitizing and astrometrically reducing "Carte du Ciel" plate material using an inexpensive flatbed scanner, to demonstrate that for this material there is an alternative to more specialized measuring machines that are very few in number and thus not readily available. The sample of plates chosen to develop this method are original "Carte du Ciel" plates of the San Fernando zone, photographic material with a mean epoch 1903.6, and a limiting photographic magnitude ~14.5, covering the declination range of -10 < dec < -2. Digitization has been made using a commercial flatbed scanner, demonstrating the internal precision that can be attained with such a device. A variety of post-scan corrections are shown to be necessary. In particular, the large distortion introduced by the non-uniform action of the scanner is modelled using multiple scans of each plate. We also tackle the specific problems associated with the triple-exposure images on some plates and the grid lines present on all. The final measures are reduced to celestial coordinates using the Tycho-2 Catalogue. The internal precision obtained over a single plate, 3microns ~ 0.18" in each axis, is comparable to what is realized with similar plate material using slower, less affordable, and less widely available conventional measuring machines, such as a PDS microdensitometer. The accuracy attained over large multi-plate areas, employing an overlapping plate technique, is estimated at 0.2".Comment: 16 pages, 19 figures and 3 tables. Accepted for publication in A&

2D Kinematics and Physical Properties of z~3 Star-Forming Galaxies

We present results from a study of the kinematic structure of star-forming galaxies at redshift z~3 selected in the VVDS, using integral-field spectroscopy of rest-frame optical nebular emission lines, in combination with rest-frame UV spectroscopy, ground-based optical/near-IR and Spitzer photometry. We also constrain the underlying stellar populations to address the evolutionary status of these galaxies. We infer the kinematic properties of four galaxies: VVDS-20298666, VVDS-020297772, VVDS-20463884 and VVDS-20335183 with redshifts z = 3.2917, 3.2878, 3.2776, and 3.7062, respectively. While VVDS-20463884 presents an irregular velocity field with a peak in the local velocity dispersion of the galaxy shifted from the centre of the galaxy, VVDS-20298666 has a well-resolved gradient in velocity over a distance of ~4.5 kpc with a peak-to-peak amplitude of v = 91 km/s . We discovered that the nearby galaxy, VVDS-020297772 (which shows traces of AGN activity), is in fact a companion at a similar redshift with a projected separated of 12 kpc. In contrast, the velocity field of VVDS-020335183 seems more consistent with a merger on a rotating disk. However, all of the objects have a high local velocity dispersion (sigma ~ 60-70 km/s), which gives v/sigma < 1. It is unlikely that these galaxies are dynamically cold rotating disk of ionized gas.Comment: 14 pages and 16 figure

The metallicity properties of zCOSMOS galaxies at 0.2<z<0.8

We study the metallicity properties of galaxies in the zCOSMOS sample between 0.2<z<0.8. At z<0.46, where Ha and [NII] are detected, we find the same dependence of metallicity on stellar mass and Star Formation Rate (SFR), the Fundamental Metallicity Relation, found by Mannucci et al. (2010) in SDSS galaxies on a similar redshift range. We extend this relation to higher redshift, 0.49<z<0.8 where the R23 metallicity index can be measured in our data, finding no evidence for evolution, and a metallicity scatter around the relation of about 0.16 dex. This result confirms, with a much higher level of significance with respect to previous works, the absence of evolution of the FMR during the last half of cosmic history.Comment: 9 pages, 7 figures, accepted for publication in MNRA