Stephen Gwyn

CFHTLenS: the Canada-France-Hawaii Telescope Lensing Survey - imaging data and catalogue products

We present data products from the Canada–France–Hawaii Telescope Lensing Survey (CFHTLenS). CFHTLenS is based on the Wide component of the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS). It encompasses 154 deg^2 of deep, optical, high-quality, sub-arcsecond imaging data in the five optical filters u*g′r′i′z′. The scientific aims of the CFHTLenS team are weak gravitational lensing studies supported by photometric redshift estimates for the galaxies. This paper presents our data processing of the complete CFHTLenS data set. We were able to obtain a data set with very good image quality and high-quality astrometric and photometric calibration. Our external astrometric accuracy is between 60 and 70 mas with respect to Sloan Digital Sky Survey (SDSS) data, and the internal alignment in all filters is around 30 mas. Our average photometric calibration shows a dispersion of the order of 0.01–0.03 mag for g′r′i′z′ and about 0.04 mag for u* with respect to SDSS sources down to i_(SDSS) ≤ 21. We demonstrate in accompanying papers that our data meet necessary requirements to fully exploit the survey for weak gravitational lensing analyses in connection with photometric redshift studies. In the spirit of the CFHTLS, all our data products are released to the astronomical community via the Canadian Astronomy Data Centre at http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/community/CFHTLens/query.html. We give a description and how-to manuals of the public products which include image pixel data, source catalogues with photometric redshift estimates and all relevant quantities to perform weak lensing studies.

The VIRMOS deep imaging survey - II: CFH12K BVRI optical data for the 0226-04 deep field

In this paper we describe in detail the reduction, preparation and reliability of the photometric catalogues which comprise the CFH12K-VIRMOS deep field. This region, consisting of four contiguous pointings of the CFH12K wide-field mosaic camera, covers a total area of 1: 2d eg 2 . The survey reaches a limiting magnitude of BAB 26:5, VAB 26:2, RAB 25:9 and IAB 25:0 (corresponding to the point at which our recovery rate for simulated point-like sources sources falls below 50%). In total the survey contains 90 729 extended sources in the magnitude range 18:0< IAB< 24:0. We demonstrate our catalogues are free from systematic biases and are complete and reliable down these limits. By comparing our galaxy number counts to previous wide-field CCD surveys, we estimate that the upper limit on bin-to-bin systematic photometric errors for the I lim- ited sample is10% in this magnitude range. We estimate that 68% of the catalogues sources have absolute per co-ordinate astrometric uncertainties less than j j0:38 00 andj j0:32 00 . Our internal (filter-to-filter) per co-ordinate astrometric uncertainties arej j0:08 00 andj j0:08 00 . We quantify the completeness of our survey in the joint space defined by object total magnitude and peak surface brightness. We also demonstrate that no significant positional incompleteness eects are present in our catalogues to IAB< 24:0. Finally, we present numerous comparisons between our catalogues and published literature data: galaxy and star counts, galaxy and stellar colours, and the clustering of both point-like and extended populations. In all cases our measurements are in excellent agreement with literature data to IAB< 24:0. This combination of depth and areal coverage makes this multi-colour catalogue a solid foundation to select galaxies for follow-up spectroscopy with VIMOS on the ESO-VLT and a unique database to study the formation and evolution of the faint galaxy population to z 1 and beyond.

MegaPipe: The MegaCam Image Stacking Pipeline at the Canadian Astronomical Data Centre

ABSTRACT. This paper describes the MegaPipe image processing pipeline at the Canadian Astronomical Data Centre. The pipeline combines multiple images from the MegaCam mosaic camera on Canada-France-Hawaii Telescope (CFHT) and combines them into a single output image. MegaPipe takes as input detrended MegaCam images and does a careful astrometric and photometric calibration on them. The calibrated images are then resampled and combined into image stacks. The astrometric calibration of the output images is accurate to within 0.15″ relative to external reference frames and 0.04″ internally. The photometric calibration is good to within 0.03 mag. The stacked images and catalogs derived from these images are available through the CADC Web site.

The Canada-France-Hawaii Telescope Legacy Survey: Stacked Images and Catalogs

This paper describes the image stacks and catalogs of the CFHT Legacy Survey produced using the MegaPipe data pipeline at the Canadian Astronomy Data Centre. The Legacy Survey is divided into to two parts: The Deep Survey consists of 4 fields each of 1 square degree, with magnitude limits (50% completeness for point sources) of u = 27.5, g = 27.9, r = 27.7, i = 27.4, and z = 26.2, and contains 1.6× 10 sources. The Wide Survey consists of 150 square degrees split over 4 fields, with magnitude limits of u = 26.0, g = 26.5, r = 25.9, i = 25.7, and z = 24.6, and contains 3 × 10 sources. This paper describes the calibration, image stacking and catalog generation process. The images and catalogs are available on the web through several interfaces: normal image and text file catalog downloads, a “Google Sky” interface, an image cutout service, and a catalog database query service. Subject headings: methods: data analysis, astronomical data bases: miscellaneous, astrometry, techniques: photometric

The colour of galaxies in distant groups

We present new optical and near-infrared imaging for a sample of 98 spectroscopicallyselected galaxy groups at 0.25 < z < 0.55, most of which have velocity dispersions � < 500 km/s. We use psf-matched aperture photometry to measure accurate colours for group members and the surrounding field population. The samp le is statistically complete above a stellar mass limit of approximately M = 1 × 10 10 M⊙. The overall colour distribution is bimodal in both the field and group samples; but at fixed luminosity the fraction of group galaxies populating the red peak is larger, by � 20±7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in Hubble Space Telescope imaging, exhibit a tight red sequence, similar to that seen f or more massive clusters. Using optical and near-infrared colours, including data fr om the Spitzer Space Telescope, we show that approximately 20‐30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spi ral morphologies. This is true of both the field and group sample, and shows little dependence on nea r infrared luminosity. Thus, the fraction of bright ( 0.4 MK < 22) group members with no sign of star formation or AGN activity, as identified by their colours or [OII] emission, is 54 ± 6 per cent. Our field sample, which includes galaxies in all environments, contains 35±3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions, that dense enviro nments atz < 0.5 are associated with a premature cessation of star formation in some galaxies; in particular we find no evidence for significantly enhanced star formation in these environment s. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the timescale of this quenching equally for all group members distorts the colour distribution in a way that is inconsistent with observations.

Galaxies at the Extremes: Ultra-diffuse Galaxies in the Virgo Cluster

We report the discovery of three large (R29 >~ 1 arcminute) extremely low surface brightness (mu_(V,0) ~ 27.0) galaxies identified using our deep, wide-field imaging of the Virgo Cluster from the Burrell Schmidt telescope. Complementary data from the Next Generation Virgo Cluster Survey do not resolve red giant branch stars in these objects down to i=24, yielding a lower distance limit of 2.5 Mpc. At the Virgo distance, these objects have half-light radii 3-10 kpc and luminosities L_V=2-9x10^7 Lsun. These galaxies are comparable in size but lower in surface brightness than the large ultradiffuse LSB galaxies recently identified in the Coma cluster, and are located well within Virgos virial radius; two are projected directly on the cluster core. One object appears to be a nucleated LSB in the process of being tidally stripped to form a new Virgo ultracompact dwarf galaxy. The others show no sign of tidal disruption, despite the fact that such objects should be most vulnerable to tidal destruction in the cluster environment. The relative proximity of Virgo makes these objects amenable to detailed studies of their structural properties and stellar populations. They thus provide an important new window onto the connection between cluster environment and galaxy evolution at the extremes.

Stripped gas as fuel for newly formed H ii regions in the encounter between VCC 1249 and M 49: a unified picture from NGVS and GUViCS

Context. We study the peculiar interacting galaxy system of VCC 1249/M 49 located in the core of the Virgo B subcluster. Owing to a recent interaction between the dwarf galaxy VCC 1249 and the halo gas of the elliptical galaxy M 49, neutral hydrogen has been displaced from the interstellar medium of this dwarf into the Virgo intracluster medium. Observations also reveal multiple compact star-forming regions (aka H  II regions) that are embedded in this H  I cloud, with a projected separation up to 13 kpc from VCC 1249 in the northwest direction. Aims. Motivated by recent near-ultraviolet imaging from the GALEX Ultraviolet Virgo Cluster Survey (GUViCS) of the VCC 1249/M 49 system that shows significant ongoing/recent star formation in the compact regions, we aim to constrain the origin of these outlying H  II regions with a multi-wavelength approach. Methods. Using deep optical (u,g,i,z) imaging from the Next Generation Virgo Cluster Survey (NGVS) and new Hα imaging obtained at the San Pedro Martir observatory together with Keck long-slit spectroscopy, we characterize the star formation rates, ages, and metallicity of VCC 1249 and its outlying compact regions. Moreover, we analyze the color and luminosity profile of the galaxy to investigate its recent interaction with M 49. Results. Our new observations indicate that VCC 1249 underwent a recent interaction with M 49 in which both ram-pressure stripping and tidal interaction occured. The joint action of the two mechanisms led to the removal of the H  I gas from the interstellar medium of VCC 1249, while the gravitational tides triggered the stellar tail and counter-tail of VCC 1249. Our stellar population synthesis analysis reveals that the star formation in this galaxy was truncated around 200 Myr ago and that the outlying H  II regions were born in situ ≈10 Myr ago out of pre-enriched gas removed from the dwarf galaxy. These observations also reveal that interactions between central and satellite galaxies similar to those between VCC 1249/M 49 may be an effective way of dispersing metals into the halos of massive galaxies.

The Next Generation Virgo Cluster Survey-Infrared (NGVS-IR). I. A New Near-Ultraviolet, Optical, and Near-Infrared Globular Cluster Selection Tool

The NGVS-IR project (Next Generation Virgo Cluster Survey-Infrared) is a contiguous, near-infrared imaging survey of the Virgo cluster of galaxies. It complements the optical wide-field survey of Virgo (NGVS). In its current state, NGVS-IR consists of K-s-band imaging of 4 deg(2) centered on M87 and J- and K-s-band imaging of similar to 16 deg(2) covering the region between M49 and M87. We present observations of the central 4 deg(2) centered on Virgos core region. The data were acquired with WIRCam on the Canada-France-Hawaii Telescope, and the total integration time was 41 hr distributed over 34 contiguous tiles. A survey-specific strategy was designed to account for extended galaxies while still measuring accurate sky brightness within the survey area. The average 5s limiting magnitude is K-s = 24.4 AB mag, and the 50% completeness limit is K-s = 23.75 AB mag for point-source detections, when using only images with better than 0.” 7 seeing (median seeing 0.” 54). Star clusters are marginally resolved in these image stacks, and Virgo galaxies with mu(Ks) similar or equal to 24.4 AB mag arcsec(-2) are detected. Combining the K-s data with optical and ultraviolet data, we build the uiK(s) color-color diagram, which allows a very clean color-based selection of globular clusters in Virgo. This diagnostic plot will provide reliable globular cluster candidates for spectroscopic follow-up campaigns, needed to continue the exploration of Virgos photometric and kinematic substructures, and will help the design of future searches for globular clusters in extragalactic systems. We show that the new uiK(s) diagram displays significantly clearer substructure in the distribution of stars, globular clusters, and galaxies than the gzK(s) diagram-the NGVS + NGVS-IR equivalent of the BzK diagram that is widely used in cosmological surveys. Equipped with this powerful new tool, future NGVS-IR investigations based on the uiK(s) diagram will address the mapping and analysis of extended structures and compact stellar systems in and around Virgo galaxies.

Spectacular tails of ionized gas in the Virgo cluster galaxy NGC 4569

Using MegaCam at the CFHT, we obtained a deep narrow band H alpha+[NII] wide-field image of NGC 4569 (M90), the brightest late-type galaxy in the Virgo cluster. The image reveals the presence of long tails of diffuse ionized gas, without any associated stellar component extending from the disc of the galaxy up to similar or equal to 80 kpc (projected distance) and with a typical surface brightness of a few 10(-18) erg s(-1) cm(-2) arcsec(-2). These features provide direct evidence that NGC 4569 is undergoing a ram-presure stripping event. The image also shows a prominent 8 kpc spur of ionized gas that is associated with the nucleus that spectroscopic data identify as an outflow. With some assumptions on the 3D distribution of the gas, we use the Ha surface brightness of these extended low-surface brightness features to derive the density and the mass of the gas that has been stripped during the interaction of the galaxy with the intracluster medium. The comparison with ad hoc chemo-spectrophotometric models of galaxy evolution indicates that the mass of the Ha emitting gas in the tail is a large fraction of that of the cold phase that has been stripped from the disc, suggesting that the gas is ionized within the tail during the stripping process. The lack of star-forming regions suggests that mechanisms other than photoionization are responsible for the excitation of the gas (shocks, heat conduction, magneto hydrodynamic waves). This analysis indicates that ram pressure stripping is efficient in massive (M-star similar or equal to 10(10.5) M-circle dot) galaxies located in intermediate-mass (similar or equal to 10(14) M-circle dot) clusters under formation. It also shows that the mass of gas expelled by the nuclear outflow is only similar to 1% than that removed during the ram pressure stripping event. Together these results indicate that ram pressure stripping, rather than starvation through nuclear feedback, can be the dominant mechanism that is responsible for the quenching of the star formation activity of galaxies in high density environments.

THE NEXT GENERATION VIRGO CLUSTER SURVEY (NGVS). XIII. THE LUMINOSITY AND MASS FUNCTION OF GALAXIES IN THE CORE OF THE VIRGO CLUSTER AND THE CONTRIBUTION FROM DISRUPTED SATELLITES

We present measurements of the galaxy luminosity and stellar mass function in a 3.71 deg