Faint Limits Research Articles

A NEW RESULT ON THE ORIGIN OF THE EXTRAGALACTIC GAMMA-RAY BACKGROUND

In the paper, we continually use the method of image stacking to study the origin of the extragalactic gamma-ray background (EGB) at GeV bands, and find that the Faint Images of the Radio Sky at Twenty centimeters (FIRST) sources undetected by the Large Area Telescope on the Fermi Gamma-ray Space Telescope can contribute about (56{plus minus}6)% of the EGB. Because the FIRST is a flux limited sample of radio sources with incompleteness at the faint limit, we consider that the point-sources, including blazars, non-blazar AGNs,starburst galaxies, could produce a much larger fraction of the EGB.

DISCLOSING THE RADIO LOUDNESS DISTRIBUTION DICHOTOMY IN QUASARS: AN UNBIASED MONTE CARLO APPROACH APPLIED TO THE SDSS-FIRST QUASAR SAMPLE

We investigate the dichotomy in the radio loudness distribution of quasars by modelling their radio emission and various selection effects using a Monte Carlo approach. The existence of two physically distinct quasar populations, the radio-loud and radio-quiet quasars, is controversial and over the last decade a bimodal distribution of radio loudness of quasars has been both affirmed and disputed. We model the quasar radio luminosity distribution with simple unimodal and bimodal distribution functions. The resulting simulated samples are compared to a fiducial sample of 8,300 quasars drawn from the SDSS DR7 Quasar Catalog and combined with radio observations from the FIRST survey. Our results indicate that the SDSS-FIRST sample is best described by a radio loudness distribution which consists of two components, with 12+/-1 % of sources in the radio-loud component. On the other hand, the evidence for a local minimum in the loudness distribution (bimodality) is not strong and we find that previous claims for its existence were probably affected by the incompleteness of the FIRST survey close to its faint limit. We also investigate the redshift and luminosity dependence of the radio loudness distribution and find tentative evidence that at high redshift radio-loud quasars were rarer, on average "louder", and exhibited a smaller range in radio loudness. In agreement with other recent work, we conclude that the SDSS-FIRST sample strongly suggests that the radio loudness distribution of quasars is not a universal function, and that more complex models than presented here are needed to fully explain available observations.

THE GALACTIC PLANE INFRARED POLARIZATION SURVEY (GPIPS)

The scientific motivation, data collection strategy, data reduction, and analysis methods are presented for the Galactic Plane Infrared Polarization Survey (GPIPS). The chief goal for the Survey was to reveal the nature of the magnetic field threading the Galactic disk, in particular through regions of low to moderate extinction (1-20 mag of A{sub V} ) and star formation in the cool interstellar medium. The Survey region spans 76 deg{sup 2} of the northern Milky Way disk, from l = 18 Degree-Sign to 56 Degree-Sign and b =-1 Degree-Sign to +1 Degree-Sign . Linear polarimetric imaging observations began in 2006 in the near-infrared H band (1.6 {mu}m) using the Mimir instrument on the 1.8 m Perkins telescope, located outside Flagstaff, AZ. Mimir used a cold, fixed wire grid and a rotateable cold, compound half-wave plate to obtain 'step-and-integrate' polarimetry over its full 10 Multiplication-Sign 10 arcmin field of view. The GPIPS bright and faint polarimetric limits are approximately 7th and 15th mag, respectively, set by saturation and photon noise. Polarimetric uncertainties track with stellar magnitude, from about 0.1% to 25%, on average, from the brightest to faintest stars. Across the 3237 field GPIPS region, approximately 0.5 million stars are estimated tomore » show detectable linear polarization (P/{sigma}{sub P} > 3); most of these have m{sub H} < 12. This represents many orders of magnitude improvement in the number of polarization measurements across this region. GPIPS observations are more than 90% complete and should finish in 2012.« less

LOWER-LUMINOSITY GALAXIES COULD REIONIZE THE UNIVERSE: VERY STEEP FAINT-END SLOPES TO THE UV LUMINOSITY FUNCTIONS AT z ⩾ 5-8 FROM THE HUDF09 WFC3/IR OBSERVATIONS

The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5 mag. Luminosity functions (LF) from these new HUDF09 data for 132 z\sim7 and z\sim8 galaxies are combined with new LFs for z\sim5-6 galaxies and the earlier z\sim4 LF to reach to very faint limits (<0.05 L*(z=3)). The faint-end slopes alpha are steep: -1.79+/-0.12 (z\sim5), -1.73+/-0.20 (z\sim6), -2.01+/-0.21 (z\sim7), and -1.91+/-0.32 (z\sim8). Slopes alpha\lesssim-2 lead to formally divergent UV fluxes, though galaxies are not expected to form below \sim-10 AB mag. These results have important implications for reionization. The weighted mean slope at z\sim6-8 is -1.87+/-0.13. For such steep slopes, and a faint-end limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux. While current results show that galaxies can reionize the universe by z\sim6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z>8, taking alpha to be -1.87+/-0.13 (the mean value at z\sim6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061_{-0.006}^{+0.009}. However, this result will change if the faint-end slope alpha is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (dalpha/dz\sim-0.05+/-0.04), that would increase the Thomson optical depths to 0.079_{-0.017}^{+0.063}, consistent with recent WMAP estimates (tau=0.088+/-0.015). It may thus not be necessary to resort to extreme assumptions about the escape fraction or clumping factor. Nevertheless, the uncertainties remain large. Deeper WFC3/IR+ACS observations can further constrain the ionizing flux from galaxies.

Galaxy populations in the Antlia cluster - III. Properties of faint early-type galaxies★

(Abridge) We present a new analysis of the early-type galaxy population in the central region of the Antlia cluster, focusing on the faint systems like dwarf ellipticals (dE) and dwarf spheroidals (dSph). We confirm 22 early-type galaxies as Antlia members, using GEMINI-GMOS and MAGELLAN-MIKE spectra. Among them, 2 belong to the rare type of compact ellipticals (cE), and 5 are new faint dwarfs that had never been catalogued before. In addition, we present 16 newly identified low surface brightness galaxy candidates, almost half of them displaying morphologies consistent with being Antlia's counterparts of Local Group dSphs, that extend the faint luminosity limit of our study down to MB = -10.1 (BT = 22.6) mag. We built an improved CMR in the Washington photometric system, i.e. integrated T1 magnitudes versus (C - T1) colours, which extends \sim 4 mag faintwards the limit of spectroscopically confirmed Antlia members. When only confirmed early-type members are considered, this relation extends over 10 mag in luminosity with no apparent change in slope or increase in colour dispersion towards its faint end. The intrinsic colour scatter of the relation is compared with those reported for other clusters of galaxies; we argue that it is likely that the large scatter of the CMR, usually reported at faint magnitudes, is mostly due to photometric errors along with an improper membership/morphological classification. The distinct behaviour of the luminosity versus mean effective surface brightness relation at the bright and faint ends is analyzed, while it is confirmed that dE galaxies on the same relation present a very similar effective radius, regardless of their colour. The projected spatial distribution of the member sample confirms the existence of two groups in Antlia, each one dominated by a giant elliptical galaxy and with one cE located close to each giant.

DIFFUSE Lyα EMITTING HALOS: A GENERIC PROPERTY OF HIGH-REDSHIFT STAR-FORMING GALAXIES

Using a sample of 92 UV continuum-selected, spectroscopically identified galaxies with <z> = 2.65, all of which have been imaged in the Ly-a line with extremely deep narrow-band imaging, we examine galaxy Ly-a emission profiles to very faint surface brightness limits. The galaxies are representative of spectroscopic samples of LBGs at similar redshifts in terms of apparent magnitude, UV luminosity, inferred extinction, and star formation rate, and were selected without regard to Ly-a emission properties. We use extremely deep stacks of UV continuum and Ly-a emission line images to show that all sub-samples exhibit diffuse Ly-a emission to radii of at least 10" (80 physical kpc), including galaxies whose spectra exhibit Ly-a in net absorption. The intensity scaling, but not the surface brightness distribution, is strongly correlated with the emission observed in the central ~1". The characteristic scale length for Ly-a line emission exceeds that of the UV continuum light for the same galaxies by factors of at least 5-10, regardless of the spectral morphology of Ly-a. Including the extended Ly-a halos increases the total Ly-a flux [and rest equivalent width W_0(Lya)] by an average factor of 5. We argue that most, if not all, of the observed Ly-a emission in the diffuse halos originates in the galaxy H II regions and is scattered in our direction by H I gas in the galaxy's circum-galactic medium (CGM). We show that whether or not a galaxy is classified as a giant "Lyman-a Blob" (LAB) depends sensitively on the Ly-a surface brightness threshold reached by an observation. Accounting for diffuse Ly-a halos, all LBGs would be LABs if surveys were routinely sensitive to 10 times lower surface brightness thresholds; also, essentially all LBGs would qualify as LAEs (W_0(Lya) > 20 A).

An E-ELT case study: colour–magnitude diagrams of an old galaxy in the Virgo cluster

One of the key science goals for a diffraction limited imager on an Extremely Large Telescope (ELT) is the resolution of individual stars down to faint limits in distant galaxies. The aim of this study is to test the proposed capabilities of a multi-conjugate adaptive optics (MCAO) assisted imager working at the diffraction limit, in IJHK$_s$ filters, on a 42m diameter ELT to carry out accurate stellar photometry in crowded images in an Elliptical-like galaxy at the distance of the Virgo cluster. As the basis for realistic simulations we have used the phase A studies of the European-ELT project, including the MICADO imager (Davies & Genzel 2010) and the MAORY MCAO module (Diolaiti 2010). We convolved a complex resolved stellar population with the telescope and instrument performance expectations to create realistic images. We then tested the ability of the currently available photometric packages STARFINDER and DAOPHOT to handle the simulated images. Our results show that deep Colour-Magnitude Diagrams (photometric error, $\pm$0.25 at I$\ge$27.2; H$\ge$25. and K$_s\ge$24.6) of old stellar populations in galaxies, at the distance of Virgo, are feasible at a maximum surface brightness, $\mu_V \sim$ 17 mag/arcsec$^2$ (down to M$_I > -4$ and M$_H \sim$ M$_K > -6$), and significantly deeper (photometric error, $\pm$0.25 at I$\ge$29.3; H$\ge$26.6 and K$_s\ge$26.2) for $\mu_V \sim$ 21 mag/arcsec$^2$ (down to M$_I \ge -2$ and M$_H \sim$ M$_K \ge -4.5$). The photometric errors, and thus also the depth of the photometry should be improved with photometry packages specifically designed to adapt to an ELT MCAO Point Spread Function. We also make a simple comparison between these simulations and what can be expected from a Single Conjugate Adaptive Optics feed to MICADO and also the James Webb Space Telescope.

THINK OUTSIDE THE COLOR BOX: PROBABILISTIC TARGET SELECTION AND THESDSS-XDQSOQUASAR TARGETING CATALOG

We present the SDSS-XDQSO quasar targeting catalog for efficient flux-based quasar target selection down to the faint limit of the Sloan Digital Sky Survey (SDSS) catalog, even at medium redshifts (2.5 <~ z <~ 3) where the stellar contamination is significant. We build models of the distributions of stars and quasars in flux space down to the flux limit by applying the extreme-deconvolution method to estimate the underlying density. We convolve this density with the flux uncertainties when evaluating the probability that an object is a quasar. This approach results in a targeting algorithm that is more principled, more efficient, and faster than other similar methods. We apply the algorithm to derive low-redshift (z < 2.2), medium-redshift (2.2 <= z <= 3.5), and high-redshift (z > 3.5) quasar probabilities for all 160,904,060 point sources with dereddened i-band magnitude between 17.75 and 22.45 mag in the 14,555 deg^2 of imaging from SDSS Data Release 8. The catalog can be used to define a uniformly selected and efficient low- or medium-redshift quasar survey, such as that needed for the SDSS-III's Baryon Oscillation Spectroscopic Survey project. We show that the XDQSO technique performs as well as the current best photometric quasar-selection technique at low redshift, and outperforms all other flux-based methods for selecting the medium-redshift quasars of our primary interest. We make code to reproduce the XDQSO quasar target selection publicly available.

The Large-scale Structure in the Chandra Deep Field South

AbstractWe have studied the large scale distribution of matter in the Chandra Deep Field South on the basis of photometric redshifts and we have identified several over-densities between redshift 0.6 and 2.3. We analyse two of these structures using the deepest X-ray observations ever obtained: 4 Ms with the Chandra satellite and 2.5 Ms with XMM-Newton. We set a very faint upper limit on the X-ray luminosity of a structure at redshift 1.6, and we find an extended X-ray emission from a structure at redshift 0.96 of which we can estimate the gas temperature and make a comparison with the scaling relations between the X-ray luminosity and mass or temperature of high redshift galaxy clusters.

FOCUSING COSMIC TELESCOPES: EXPLORING REDSHIFTz∼ 5-6 GALAXIES WITH THE BULLET CLUSTER 1E0657 – 56

The gravitational potential of clusters of galaxies acts as a cosmic telescope allowing us to find and study galaxies at fainter limits than otherwise possible and thus probe closer to the epoch of formation of the first galaxies. We use the Bullet cluster 1E0657 − 56 (z = 0.296) as a case study, because its high mass and merging configuration makes it one of the most efficient cosmic telescopes we know. We develop a new algorithm to reconstruct the gravitational potential of the Bullet cluster based on a non-uniform adaptive grid, combining strong and weak gravitational lensing data derived from deep Hubble Space Telescope/Advanced Camera for Surveys F606W−F775W−F850LP and ground-based imaging. We exploit this improved mass map to study z ∼ 5–6 Lyman break galaxies (LBGs), which we detect as dropouts. One of the LBGs is multiply imaged, providing a geometric confirmation of its high redshift, and is used to further improve our mass model. We quantify the uncertainties in the magnification map reconstruction in the intrinsic source luminosity, and in the volume surveyed, and show that they are negligible compared to sample variance when determining the luminosity function of high-redshift galaxies. With shallower and comparable magnitude limits to Hubble Ultra Deep Field (HUDF) and Great Observatories Origins Deep Survey (GOODS), the Bullet cluster observations, after correcting for magnification, probe deeper into the luminosity function of the high-redshift galaxies than GOODS and only slightly shallower than HUDF. We conclude that accurately focused cosmic telescopes are the most efficient way to sample the bright end of the luminosity function of high-redshift galaxies and—in case they are multiply imaged—confirm their redshifts.

Preface - SpS8 - The Galactic Plane

The first digital astronomical surveys emphasised exploration of the sky away from the crowded Galactic Plane. But now, increased computing power has made it possible to take on comprehensive surveying of the Galactic Plane even at high spatial resolution and down to faint magnitude limits. A number of ambitious wide-area surveys sampling high energies, optical wavelengths, the infrared, sub-millimetre and radio ranges are complete, in process, or about to begin. The goals of these surveys are as broad as Galactic science itself, but are mainly focused either on solving key problems in star formation and stellar evolution, or on mapping the complex substructures of the Galactic bulge and disk in order to see more clearly how the whole is constructed. This meeting brought together researchers directly involved in the many surveys, along with specialists in the observations and modelling of the ISM, stellar evolution, and the structure of the Galactic Disk and Bulge.

A SAMPLE OF CANDIDATE RADIO STARS IN FIRST AND SDSS

We conduct a search for radio stars by combining radio and optical data from the FIRST survey and the Sloan Digital Sky Survey (SDSS). The faint limit of SDSS makes possible a homogeneous search for radio emission from stars of low optical luminosity. We select a sample of 112 candidate radio stars in the magnitude range 15 <i 19.1 and with radio flux S20 1.25 mJy, from about 7000 deg 2 of sky. The selection criteria are positional coincidence within 1 �� , radio and optical point source morphology, and an SDSS spectrum classified as stellar. The sample contamination is estimated by random matching to be 108 ± 13, suggesting that at most a small fraction of the selected candidates are genuine radio stars. Therefore, we rule out a very rare population of extremely radio-loud stars: no more than 1.2 of every million stars in the magnitude range 15 <i <19.1 stars has radio flux S20 1.25 mJy. We investigate the optical and radio colors of the sample to find candidates that show the largest likelihood of being real radio stars. The significant outliers from the stellar locus, as well as the magnetically active stars, are the best candidates for follow-up radio observations. We conclude that, while the present wide-area radio surveys are not sensitive enough to provide homogeneous samples of the extremely rare radio stars, upcoming surveys that exploit the great sensitivity of current and planned telescopes do have sufficient sensitivity and will allow the properties of this class of object to be investigated in detail.

DEEP 2MASS PHOTOMETRY OF M67 AND CALIBRATION OF THE MAIN-SEQUENCEJ–KSCOLOR DIFFERENCE AS AN AGE INDICATOR

We present an analysis of Two Micron All Sky Survey (2MASS) calibration photometry of the old open cluster M67 (NGC 2682). The proper motion-cleaned color-magnitude diagram (CMD) resulting from these data extends ~3 magnitudes deeper than one based on data from the point source catalog. The CMD extends from above the helium-burning red clump to a faint limit that is more than 7 magnitudes below the main sequence turnoff in the Ks band. After adopting a reddening of E(B-V) = 0.041 +/- 0.004 and a metal abundance of [Fe/H] = -0.009 +/- 0.009 based on a survey of published values, we fit the unevolved main sequence of M67 to field main sequence stars with 2MASS photometry and Hipparcos parallaxes. This analysis yields distance moduli of (m-M)Ks = 9.72 +/- 0.05 and (m-M)o = 9.70 +/- 0.05, which are consistent with published values. We compare the theoretical isochrones of Girardi et al. and Dotter et al. to the CMD of M67 and comment on the relative merits of each set of models. These comparisons suggest an age between 3.5 and 4.0 Gyr for M67. The depth of the M67 data make them ideal for the calibration of a new age indicator that has recently been devised by Calamida et al.- the difference in (J-Ks) color between the main sequence turnoff (TO) and the point on the lower main sequence where it turns down (TD) and becomes nearly vertical [D(J-Ks)]. Coupled with deep 2MASS photometry for three other open clusters, NGC 2516, M44, and NGC 6791, we calibrate D(J-Ks) in terms of age and find D(J-Ks) = (3.017 +/- 0.347) - (0.259 +/- 0.037)*Log Age (yrs).

MEASURING THE UNDETECTABLE: PROPER MOTIONS AND PARALLAXES OF VERY FAINT SOURCES

The near future of astrophysics involves many large solid-angle, multi-epoch, multi-band imaging surveys. These surveys will, at their faint limits, have data on large numbers of sources that are too faint to be detected at any individual epoch. Here we show that it is possible to measure in multi-epoch data not only the fluxes and positions, but also the parallaxes and proper motions of sources that are too faint to be detected at any individual epoch. The method involves fitting a model of a moving point source simultaneously to all imaging, taking account of the noise and point-spread function in each image. By this method it is possible to measure the proper motion of a point source with an uncertainty close to the minimum possible uncertainty given the information in the data, which is limited by the point-spread function, the distribution of observation times (epochs), and the total signal-to-noise in the combined data. We demonstrate our technique on multi-epoch Sloan Digital Sky Survey imaging of the SDSS Southern Stripe. We show that we can distinguish very red brown dwarfs by their proper motions from very high-redshift quasars more than $1.6\mag$ fainter than with traditional technique on these SDSS data, and with better better fidelity than by multi-band imaging alone. We re-discover all 10 known brown dwarfs in our sample and present 9 new candidate brown dwarfs, identified on the basis of high proper motion.

A search for massive ultra-compact dwarf galaxies in the Centaurus galaxy cluster

<i>Context. <i/>In a previous paper, we initiated a search for ultra-compact dwarf galaxies (UCDs) in the Centaurus galaxy cluster, resulting in the discovery of 27 compact objects with mag. Our overall survey completeness was 15–20% within 120 kpc projected clustercentric distance.<i>Aims. <i/>To constrain the luminosity distribution of the brightest UCDs in Centaurus in a more accurate way, we continue our search by improving substantially our survey completeness specifically for brightnesses of mag ( mag).<i>Methods. <i/>Using VIMOS at the VLT, we obtained low resolution spectra of 400 compact objects with mag ( mag at the Centaurus distance) in the central of the Centaurus cluster, which corresponds to a projected radius of ~150 kpc. Our survey had complete area coverage within a radius of kpc.<i>Results. <i/>For 94% of the sources included in the masks, we measure successfully a redshift. Because of incompleteness in the slit assignment, our final completeness in the area surveyed is 52%. Among our targets, we find three new UCDs in the magnitude range mag, hence at the faint limit of our survey. For one of these targets HST WFPC2 imaging is available, yielding a size estimate of pc. At 95% confidence, we can reject the hypothesis that in the area surveyed there are more than 2 massive UCDs with mag and pc. Hence our survey confirms the extreme rarity of massive UCDs. We find that the radial distributions of Centaurus and Fornax UCDs with respect to their host clusters' centres agree within the 2<i>σ<i/> level.

AHubbleandSpitzerSpace Telescope Survey for Gravitationally Lensed Galaxies: Further Evidence for a Significant Population of Low‐Luminosity Galaxies beyond $z=7$

We present the results of a systematic search for gravitationally lensed continuum Lyman break "dropouts" beyond a redshift 7 conducted via very deep imaging through six foreground clusters undertaken with the Hubble and Spitzer space telescopes. The survey has yielded 10 z-band and two J-band dropout candidates to photometric limits of J110≃ 26.2 AB (5 σ). Taking into account the magnifications afforded by our clusters (1-4 mag), we probe the presence of z > 7 sources to unlensed limits of J110≃ 30 AB, fainter than those charted in the Hubble Ultradeep Field. To verify the fidelity of our candidates we conduct a number of tests for instrumental effects which would lead to spurious detections, and carefully evaluate the likelihood of foreground contamination by considering photometric uncertainties in the dropout signature, the upper limits from stacked IRAC data and the statistics of multiply imaged sources. Overall, we conclude that we can expect about half of our sample of z-band dropouts to be at high redshift. An ambitious infrared spectroscopic campaign undertaken with the NIRSPEC spectrograph at the W. M. Keck Observatory for seven of the most promising candidates failed to detect any Lyα emission highlighting the challenge of making further progress in this field. While the volume density of high-redshift sources will likely remain uncertain until more powerful facilities are available, our data provides the first potentially interesting constraints on the UV luminosity function at z≃ 7.5 at intrinsically faint limits. We discuss the implications of our results in the context of the hypothesis that the bulk of the reionizing photons in the era 7 < z < 12 arise in low-luminosity galaxies undetected by conventional surveys.

A pilot survey for KX QSOs in the UKIDSS Ultra Deep Survey Field

We have undertaken a pilot survey for faint quasi-stellar objects (QSOs) in the UKIRT Infrared Deep Survey (UKIDSS) Ultra Deep Survey (UDS) Field using the KX selection technique. These observations exploit the very deep near-infrared and optical imaging of this field from United Kingdom Infrared Telescope (UKIRT) and Subaru to select candidate QSOs based on their VJK colours and morphologies. We determined redshifts for 426 candidates using the AAOmega spectrograph on the Anglo-Australian Telescope in service time. We identify 17 QSOs (M B ≤ -23) in this pilot survey at z = 1.57-3.29. We combine our sample with an X-ray-selected sample of QSOs in the same field (a large fraction of which also comply with our KX selection) to constrain the surface density of QSOs with K ≤ 20, deriving limits on the likely surface density of 85-150 deg -2 . We use the good image quality available from our near-infrared imaging to detect a spatially extended component of the QSO light which probably represents the host galaxies. We also use our sample to investigate routes to improve the selection of KX QSOs at faint limits in the face of the significant contamination by compact, foreground galaxies. The brightest examples from our combined QSO sample will be used in conjunction with a large Very Large Telescope VIMOS spectroscopic survey of high-redshift galaxies in this region to study the structures inhabited by gas, galaxies and growing supermassive black holes at high redshifts in the UKIDSS UDS.

Void statistics and void galaxies in the 2dF Galaxy Redshift Survey

In the 2dF Galaxy Redshift Survey, we study the properties of voids and of fainter galaxies within voids that are defined by brighter galaxies. Our results are compared with simulated galaxy catalogues from the Millennium simulation coupled with a semi-analytical galaxy formation recipe. We derive the void size distribution and discuss its dependence on the faint magnitude limit of the galaxies defining the voids. While voids among faint galaxies are typically smaller than those among bright galaxies, the ratio of the void sizes to the mean galaxy separation reaches larger values. This is well reproduced in the mock galaxy samples studied. We provide analytic fitting functions for the void size distribution. Furthermore, we study the galaxy population inside voids defined by galaxies with BJ− 5 log h < −20 and diameter larger than 10 h−1Mpc. We find a clear bimodality of galaxies inside voids and in the average field but with different characteristics. The abundance of blue cloud galaxies inside voids is enhanced. There is an indication of a slight blueshift of the blue cloud. Furthermore, galaxies in void centres have slightly higher specific star formation rates as measured by the η parameter. We determine the radial distribution of the ratio of early- and late-type galaxies through the voids. We find and discuss some differences between observations and the Millennium catalogues.

Orbital periods of cataclysmic variables identified by the SDSS - II. Measurements for six objects, including two eclipsing systems

Continuing our work from Paper I (Southworth et al., 2006) we present medium-resolution spectroscopy and broad-band photometry of seven cataclysmic variables (CVs) discovered by the SDSS. For six of these objects we derive accurate orbital periods, all which are measured for the first time. For SDSS J013132.39+090122.2, which contains a non-radially pulsating white dwarf, we find an orbital period of 81.54 +/- 0.13 min and a low radial velocity variation amplitude indicative of an extreme mass ratio. For SDSS J205914.87+061220.4, we find a period of 107.52 +/- 0.14 min. This object is a dwarf nova and was fading from its first recorded outburst throughout our observations. INT photometry of SDSS J155531.99-001055.0 shows that this system undergoes total eclipses which are 1.5 mag deep and occur on a period of 113.54 +/- 0.03 min. A NOT light curve of SDSS J075443.01+500729.2 shows that this system is also eclipsing, on a period of 205.965 +/- 0.014 min, but here the eclipses are V-shaped and only 0.5 mag deep. Its low emission-line strengths, orbital period and V-shaped eclipse unambiguously mark it as a novalike object. WHT photometry of SDSS J005050.88+000912.6 and SDSS J210449.94+010545.8 yields periods of 80.3 +/- 2.2 and 103.62 +/- 0.12 min, respectively. Photometry of the seventh and final system, SDSS J165658.12+212139.3, shows only flickering. Our results strengthen the conclusion that the faint magnitude limit of the SDSS spectroscopic database implies that the sample of CVs contained in it has quite different characteristics to previously studied samples of these objects. Five of the six orbital periods measured here are shorter than the observed 2-3 hr CV period gap. Two systems have periods very close to the minimum orbital period for hydrogen-rich CVs.

The Gaia mission: science, organization and present status

AbstractThe ESA space astrometry mission Gaia will measure the positions, parallaxes and proper motions of the 1 billion brightest stars on the sky. Expected accuracies are in the 7–25 μas range down to 15 mag and sub-mas accuracies at the faint limit (20 mag). The astrometric data are complemented by low-resolution spectrophotometric data in the 330–1000 nm wavelength range and, for the brighter stars, radial velocity measurements. The scientific case covers an extremely wide range of topics in galactic and stellar astrophysics, solar system and exoplanet science, as well as the establishment of a very accurate, dense and faint optical reference frame. With a planned launch around 2012 and an (extended) operational lifetime of 6 years, final results are expected around 2021. We give a brief overview of the science goals of Gaia, the overall project organisation, expected performance, and some key technical features and challenges.