Sky Survey Early Data Release Research Articles

The UKIRT Infrared Deep Sky Survey Early Data Release

This paper defines the UKIRT Infrared Deep Sky Survey (UKIDSS) Early Data Release (EDR). UKIDSS is a set of five large near-infrared surveys being undertaken with the United Kingdom Infrared Telescope Wide Field Camera (WFCAM). The programme began in 2005 May and has an expected duration of 7 yr. Each survey uses some or all of the broad-band filter complement ZY JHK. The EDR is the first public release of data to the European Southern Observatory (ESO) community. All worldwide releases occur after a delay of 18 months from the ESO release. The EDR provides a small sample data set, ∼50 deg2 (about 1 per cent of the whole of UKIDSS), that is a lower limit to the expected quality of future survey data releases. In addition, an EDR+ data set contains all EDR data plus extra data of similar quality, but for areas not observed in all of the required filters (amounting to ∼220 deg2). The first large data release, DR1, will occur in mid-2006. We provide details of the observational implementation, the data reduction, the astrometric and photometric calibration and the quality control procedures. We summarize the data coverage and quality (seeing, ellipticity, photometricity, depth) for each survey and give a brief guide to accessing the images and catalogues from the WFCAM Science Archive.

Galaxy clusters at 0.6 < z < 1.4 in the UKIDSS Ultra Deep Survey Early Data Release

Abstract We present the first cluster catalogue extracted from the UKIRT Infrared Deep Sky Survey (UKIDSS) Early Data Release. The catalogue is created using UKIDSS Ultra Deep Survey infrared J and K data combined with 3.6- and 4.5-μm Spitzer bands and optical BVRi′z′ imaging from the Subaru Telescope over 0.5 deg2 in the Subaru XMM–Newton Deep Field. We have created a new cluster detection algorithm, based on the friends-of-friends and Voronoi tessellation methods, which utilizes probability distribution functions derived from a photometric redshift analysis. We employ mock catalogues to understand the selection effects and contamination associated with the algorithm. The cluster catalogue contains 13 clusters at redshifts 0.61 ≤z≤ 1.39 with luminosities 10 L*≲Ltot≲ 50 L*, corresponding to masses 5 × 1013≲Mcluster≲ 3 × 1014 M⊙ for (M/M⊙)/(L/L⊙) = 75 h. The measured sky surface density of ∼10 clusters deg−2 for high-redshift (z= 0.5–1.5), massive (> 1014 M⊙) clusters is precisely in line with theoretical predictions presented by Kneissl et al.

New members in the Upper Scorpius association from the UKIRT Infrared Deep Sky Survey Early Data Release*†

We present the results of a 9.3-deg 2 infrared (ZYJHK) survey in the Upper Scorpius association extracted from the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Cluster Survey Early Data Release. We have selected a total of 112 candidates from the (Z - J, Z) colour-magnitude diagram over the Z = 12.5-20.5 magnitude range, corresponding to M = 0.25-0.01 M ⊙ at an age of 5 Myr and a distance of 145 pc. Additional photometry in J and K filters revealed most of them as reddened stars, leaving 32 possible members. Among them, 15 have proper motion consistent with higher-mass members from Hipparcos and optical spectra with strong Ha in emission and weak gravity features. We have also extracted two lower-mass candidate members for which no optical spectra are in hand. Three members exhibit strong Ha equivalent widths (>20 A), suggesting that they could still undergo accretion, whereas two other dwarfs show signs of chromospheric activity. The likelihood of the binarity of a couple of new stellar and substellar members is discussed as well.

SDSS J143030.22–001115.1: A Misclassified Narrow-line Seyfert 1 Galaxy with Flat X-ray Spectrum

We used multi-component profiles to model H$\beta$ and [O III]$\lambda \lambda $4959,5007 lines for SDSS J143030.22-001115.1, a narrow-line Seyfert 1 galaxy (NLS1) in a sample of 150 NLS1s candidates selected from the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR). After subtracting the H$\beta$ contribution from narrow line regions (NLRs), we found that its full width half maximum (FWHM) of broad H$\beta$ line is nearly 2900 \kms, significantly larger than the customarily adopted criterion of 2000 \kms. With its weak Fe II multiples, we think that SDSS J143030.22-001115.1 can't be classified as a genuine NLS1. When we calculate the virial black hole masses of NLS1s, we should use the H$\beta$ linewidth after subtracting the H$\beta$ contribution from NLRs.

A new sample of broad absorption-line quasars exhibiting the ghost of Lyman α

We have searched the broad-absorption-line quasar (BAL QSO) sample presented by Reichard et al. for objects exhibiting the so-called `ghost of Lyman alpha'. This ghost manifests as a hump near -5900kms in the troughs of the broad absorption lines and provides strong evidence for the importance of line-driving in powering the outflows from BAL QSOs. Of the 224 sample BAL QSOs selected from the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR), 198 satisfy our redshift constraints and 58 show clear evidence of multiple-trough (MT) structure in the CIV line. A composite spectrum constructed from this MT sample already shows evidence for a ghost feature. Narrowing our classification scheme further, we define a set of 36 objects that individually show evidence of a ghost feature, and then apply further cuts to arrive at a final `best sample' that contains our seven strongest ghost candidates. A further five objects show evidence for a ghost feature that is almost strong enough to merit inclusion in our best sample. Despite its limited size, our best sample more than doubles the number of known BAL QSOs with clear ghost signatures and should make an excellent basis for detailed follow-up studies.

MgiiAbsorption Systems in Sloan Digital Sky Survey QSO Spectra

We present the results of a Mg II λλ2796, 2803 absorption-line survey using QSO spectra from the Sloan Digital Sky Survey (SDSS) Early Data Release. Over 1300 doublets with rest equivalent widths greater than 0.3 Å and redshifts 0.366 ≤ z ≤ 2.269 were identified and measured. We find that the λ2796 rest equivalent width (W) distribution is described very well by an exponential function ∂N/∂W = e, with N* = 1.187 ± 0.052 and W* = 0.702 ± 0.017 Å. Previously reported power-law fits drastically overpredict the number of strong lines. Extrapolating our exponential fit underpredicts the number of W ≤ 0.3 Å systems, indicating a transition in ∂N/∂W near W ≃ 0.3 Å. A combination of two exponentials reproduces the observed distribution well, suggesting that Mg II absorbers are the superposition of at least two physically distinct populations of absorbing clouds. We also derive a new redshift parameterization for the number density of W ≥ 0.3 Å lines: N* = 1.001 ± 0.132 (1 + z)0.226±0.170 and W* = 0.443 ± 0.032 (1 + z)0.634±0.097 Å. We find that the distribution steepens with decreasing redshift, with W* decreasing from 0.80 ± 0.04 Å at z = 1.6 to 0.59 ± 0.02 Å at z = 0.7. The incidence of moderately strong (0.4 Å ≲ W ≲ 2 Å) Mg II λ2796 lines does not show evidence for evolution with redshift. However, lines stronger than ≈2 Å show a decrease relative to the no-evolution prediction with decreasing redshift for z ≲ 1. The evolution is stronger for increasingly stronger lines. Since W0 in saturated absorption lines is an indicator of the velocity spread of the absorbing clouds, we interpret this as an evolution in the kinematic properties of galaxies from moderate to low redshift. Monte Carlo simulations do not reveal any strong systematic effects or biases in our results. While more recent SDSS QSO spectra offer the opportunity to increase the Mg II absorber sample by another order of magnitude, systematic errors in line identification and measurement will begin to dominate in the determination of absorber property statistics.

Partially Obscured Quasars in the Sloan Digital Sky Survey Early Data Release

We have compiled a sample of 21 low-redshift (z 0.3), luminous active galactic nuclei (AGNs) with large Balmer decrements (Hα/Hβ > 7) using the galaxy and QSO catalogs of the Sloan Digital Sky Survey Early Data Release. Using this sample we attempt to determine the fraction of quasars with large internal absorption. We find that these AGNs have strong [O III] λ5007 and broad Hα emission, and that starlight dominates the spectra in the blue band, suggesting that these objects are heavily reddened. Their narrow emission line ratios are similar to those of Seyfert 2 galaxies, yet the average [O III] λ5007 emission is ~10 times more luminous. Applying the empirical relation between the optical continuum and the Balmer line luminosity for blue quasars, we find that the intrinsic luminosities of these 21 objects are in the range for quasars. We propose that they are obscured, intermediate-type quasars analogous to type 1.8 and 1.9 Seyfert galaxies. The ratio of these optically selected, intermediate-type quasars to type 1 quasars is found to be around 1, similar to that for local Seyfert galaxies. Preliminary study indicates that most of these quasars are hosted in early-type galaxies.

ChandraObservations of X‐Ray–weak Narrow‐Line Seyfert 1 Galaxies

We present Chandra observations of 17 optically-selected, X-ray weak narrow-line Seyfert 1 (NLS1) galaxies. These objects were optically identified by Williams et al. (2002) in the Sloan Digital Sky Survey Early Data Release, but were not found in the ROSAT All-Sky Survey (RASS) despite having optical properties similar to RASS-detected NLS1s. All objects in this sample were detected by Chandra and exhibit a range of 0.5-2 keV photon indices Gamma=1.1-3.4. One object was not detected in the soft band, but has a best-fit Gamma=0.25 over the full 0.5-8 keV range. These photon indices extend to values far below what are normally observed in NLS1s. A composite X-ray spectrum of the hardest objects in this sample does not show any signs of absorption, although the data do not prohibit one or two of the objects from being highly absorbed. We also find a strong correlation between Gamma and L_1keV; this may be due to differences in L_bol/L_Edd among the NLS1s in this sample. Such variations are seemingly in conflict with the current paradigm that NLS1s accrete near the Eddington limit. Most importantly, this sample shows that strong, ultrasoft X-ray emission is not a universal characteristic of NLS1s; in fact, a substantial number may exhibit weak and/or low-Gamma X-ray emission.

A Catalog of Compact Groups of Galaxies in the SDSS Commissioning Data

Compact groups (CGs) of galaxies--relatively poor groups of galaxies in which the typical separations between members is of the order of a galaxy diameter--offer an exceptional laboratory for the study of dense galaxian environments with short (<1Gyr) dynamical time-scales. In this paper, we present an objectively defined catalog of CGs in 153 sq deg of the Sloan Digital Sky Survey Early Data Release (SDSS EDR). To identify CGs, we applied a modified version of Hickson's (1982) criteria aimed at finding the highest density CGs and thus reducing the number of chance alignments. Our catalog contains 175 CGs down to a limiting galaxy magnitude of r* = 21. The resulting catalog has a median depth of approximately z = 0.13, substantially deeper than previous CG catalogs. Since the SDSS will eventually image up to one quarter of the celestial sphere, we expect our final catalog, based upon the completed SDSS, will contain on the order of 5,000-10,000 CGs. This catalog will be useful for conducting studies of the general characteristics of CGs, their environments, and their component galaxies.

Merging Galaxies in the Sloan Digital Sky Survey Early Data Release

We present a new catalog of merging galaxies obtained through an automated systematic search routine. The 1479 new pairs of merging galaxies were found in ≈462 square degrees of the Sloan Digital Sky Survey Early Data Release (SDSS EDR) photometric data, and the pair catalog is complete for galaxies in the magnitude range 16.0 ≤ g* ≤ 20. The selection algorithm, implementing a variation on the original Karachentsev criteria, proved to be very efficient and fast. Merging galaxies were selected such that the intergalaxy separations were less than the sum of the component galaxies' radii. We discuss the characteristics of the sample in terms of completeness, pair separation, and the Holmberg effect. We also present an on-line atlas of images for the SDSS EDR pairs obtained using the corrected frames from the SDSS EDR database. The atlas images also include the relevant data for each pair member. This catalog will be useful for conducting studies of the general characteristics of merging galaxies, their environments, and their component galaxies. The redshifts for a subset of the interacting and merging galaxies and the distribution of angular sizes for these systems indicate the SDSS provides a much deeper sample than almost any other wide-area catalog to date.

The Millennium Galaxy Catalogue: the photometric accuracy, completeness and contamination of the 2dFGRS and SDSS-EDR/DR1 data sets

The Millennium Galaxy Catalogue (MGC) is a deep (μ B,lim = 26 mag arcsec -2 ), wide-field, charge-coupled device imaging survey, covering 37.5 deg 2 . The MGC survey region is completely contained within the Two-Degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey Early Data Release (SDSS-EDR). We compare the photometry and completeness of the 2dFGRS and the SDSS-EDR with the MGC over the range 16 < B < 20 mag. We have also undertaken a photometric comparison to SuperCosmos (SCOS) and the Sloan Digital Sky Survey First Data Release (SDSS-DR1). We find that B MGC - B 2dF = (0.035 ± 0.005) mag with an uncertainty of 0.142 mag per galaxy; B MGC - B SCOS = (0.032 ± 0.005) mag with an uncertainty of 0.108 mag; B MGC -B SDSS-EDR = (0.032 ± 0.005) mag with an uncertainty of 0.094 mag; and B MGC - B SDSS-DR1 = (0.039 ± 0.005) mag with an uncertainty of 0.086 mag. We find that high surface brightness 2dFGRS galaxies are systematically too faint, which leads to a significant scale error in magnitude. This effect is significantly reduced with the SCOS photometry. In the SDSS there is a weak non-linear scale error, which is negligible for faint galaxies. Low surface brightness galaxies in the SDSS are systematically fainter, consistent with the relative shallowness of this survey. We find that the 2dFGRS catalogue has (5.2 ± 0.3) per cent stellar contamination, (7.0 ± 0.4) per cent of objects resolved into two or more by the MGC, and is (8.7 ± 0.6) per cent incomplete compared to the MGC. From our all-object spectroscopic survey we find that the MGC is itself misclassifying (5.6 ± 1.3) per cent of galaxies as stars, and hence the 2dFGRS misses (14.3 ± 1.4) per cent of the galaxy population. The SDSS-EDR galaxy catalogue has (1.3 ± 0.1) per cent stellar contamination and (5.3 ± 1.0) per cent of galaxies misclassified as stars, with (0.18 ± 0.04) per cent of objects resolved into two or more by the MGC, and is (1.8 ± 0.1) per cent incomplete compared to the MGC. The total fraction of galaxies missing from the SDSS-EDR galaxy catalogue to B MGC = 20 mag, from incompleteness and misclassification, is (7.1 ± 1.0) per cent.

A Search for Nitrogen-Enriched Quasars in the Sloan Digital Sky Survey Early Data Release

A search for nitrogen-rich quasars in the Sloan Digital Sky Survey Early Data Release (SDSS EDR) catalog has yielded 16 candidates, including five with very prominent emission, but no cases with nitrogen emission as strong as in Q0353-383. The quasar Q0353-383 has long been known to have extremely strong nitrogen intercombination lines at lambda 1486 and lambda 1750 Angstroms, implying an anomalously high nitrogen abundance of about 15 times solar. It is still the only one of its kind known. A preliminary search through the EDR using the observed property of the weak C IV emission seen in Q0353-383 resulted in a sample of 23 objects with unusual emission or absorption-line properties, including one very luminous redshift 2.5 star-forming galaxy. We present descriptions, preliminary emission-line measurements, and spectra for all the objects discussed here.

The Millennium Galaxy Catalogue: star counts and the structure of the Galactic stellar halo

We derive a star catalogue generated from the images taken as part of the 37.5 sq. deg Millennium Galaxy Catalogue. These data, alone and together with colours gained from the Sloan Digital Sky Survey Early Data Release, allow the analysis of faint star counts (B(MGC) < 20) at high Galactic latitude (41 < b < 63), as a function of Galactic longitude (239 < l < 353). We focus here on the inner stellar halo, providing robust limits on the amplitude of substructure and on the large-scale flattening. In line with previous results, the thick disk, an old, intermediate-metallicity population, is clearly seen in the colour-magnitude diagram. We find that the Galactic stellar halo within ~10 kpc (the bulk of the stellar mass) is significantly flattened, with an axial ratio of (c/a) =0.56 +/- 0.01, again consistent with previous results. Our analysis using counts-in-cells, angular correlation functions and the Lee 2D statistic, confirms tidal debris from the Sagittarius dwarf but finds little evidence for other substructure in the inner halo, at heliocentric distances of < 5 kpc. This new quantification of the smoothness in coordinate space limits the contribution of recent accretion/disruption to the build-up of the bulk of the stellar halo.

The Morphological Decomposition of Abell 868

We report on the morphological luminosity functions (LFs) and radial profiles derived for the galaxy population within the rich cluster Abell 868 (z = 0.153) based purely on Hubble Space Telescope imaging in F606W. We recover Schechter functions (-24.0 < MF606W - 5 log h0.65 < -16.0) within a 0.65h0.65 Mpc radius for early (E/S0) , mid (Sabc), and late (Sd/Irr) type galaxies of The early, mid, and late types are all consistent with the recent field morphological LFs based on recent analysis of the Sloan Digital Sky Survey Early Data Release. From a detailed error analysis, including clustering of the background population, we note that improved statistics can only come from combining data from many clusters. We also examine the luminosity-density and number-density profiles as a function of morphology and draw the following conclusions: (1) the galaxies responsible for the steep faint-end slope are predominantly of late-type morphology; (2) the cluster core is dominated by elliptical galaxies; (3) the core is devoid of late-type systems; (4) the luminosity density as a function of morphological type is skewed toward early types when compared with the field; (5) up to half of the elliptical galaxies may have formed from the spiral population through core disk-destruction process(es). We believe the most plausible explanation is the conventional one that late types are destroyed during transit through the cluster core and that mid types are converted into early types through a similar process, which destroys the outer disk and results in a more tightly bound population of core elliptical galaxies.

Stellar‐Mass Black Holes in the Solar Neighborhood

We search for nearby, isolated, accreting, "stellar-mass" (3-100 M☉) black holes. Models suggest a synchrotron spectrum in visible wavelengths and some emission in X-ray wavelengths. Of 3.7 million objects in the Sloan Digital Sky Survey Early Data Release, about 150,000 objects have colors and properties consistent with such a spectrum, and 87 of these objects are X-ray sources from the ROSAT All-Sky Survey. Thirty-two of these have been confirmed not to be black holes using optical spectra. We give the positions and colors of these 55 black hole candidates and quantitatively rank them on their likelihood to be black holes. We discuss uncertainties in the expected number of sources and the contribution of black holes to local dark matter.

The subdwarf luminosity function

Using data from the Sloan Digital Sky Survey Early Data Release and SuperCOSMOS Sky Survey scans of POSS-I plates we identify a sample of �2600 subdwarfs using reduced proper motion methods and strict selection criteria. This forms one of the largest and most reliable samples of candidate subdwarfs known, and enables us to determine accurate luminosity functions along many different lines of sight. We derive the subdwarf luminosity function with unprecedented accuracy to MV . 13, finding good agreement with recent local estimates but discrepancy with results for the more distant spheroid. This provides further evidence that the inner and outer parts of the stellar halo cannot be described by a single density distribution. We also find that the form of the inner spheroid density profile within distances of 2.5 kpc is closely matched by a power law with an index of α = 3.15± 0.3.

Long-Term Variability of Sloan Digital Sky Survey Quasars

We use a sample of 3791 quasars from the Sloan Digital Sky Survey (SDSS) Early Data Release and compare their photometry with historic plate material for the same set of quasars in order to study their variability properties. The time baseline we attain this way ranges from a few months to up to 50 yr. In contrast to monitoring programs, where relatively few quasars are photometrically measured over shorter time periods, we use existing databases to extend this baseline as much as possible, at the cost of sampling per quasar. Our method, however, can easily be extended to much larger samples. We construct variability structure functions and compare these with the literature and model functions. From our modeling, we conclude that (1) quasars are more variable toward shorter wavelengths, (2) their variability is consistent with an exponentially decaying light curve with a typical timescale of ~2 yr, and (3) these outbursts occur on typical timescales of ~200 yr. With the upcoming first data release of the SDSS, a much larger quasar sample can be used to put these conclusions on a more secure footing.

Continuum and Emission-Line Properties of Broad Absorption Line Quasars

We investigate the continuum and emission-line properties of 224 broad absorption line quasars (BALQSOs) with 0.9<z<4.4 drawn from the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR), which contains 3814 bona fide quasars. We find that low-ionization BALQSOs (LoBALs) are significantly reddened as compared to normal quasars, in agreement with previous work. High-ionization BALQSOs (HiBALs) are also more reddened than the average nonBALQSO. Assuming SMC-like dust reddening at the quasar redshift, the amount of reddening needed to explain HiBALs is E(B-V)~0.023 and LoBALs is E(B-V)~0.077 (compared to the ensemble average of the entire quasar sample). We find that there are differences in the emission-line properties between the average HiBAL, LoBAL, and nonBAL quasar. These differences, along with differences in the absorption line troughs, may be related to intrinsic quasar properties such as the slope of the intrinsic (unreddened) continuum; more extreme absorption properties are correlated with bluer intrinsic continua. Despite the differences among BALQSO sub-types and nonBALQSOs, BALQSOs appear to be drawn from the same parent population as nonBALQSOs when both are selected by their UV/optical properties. We find that the overall fraction of traditionally defined BALQSOs, after correcting for color-dependent selection effects due to different SEDs of BALQSO and nonBALQSOs, is 13.4+/-1.2% and shows no significant redshift dependence for 1.7<z<3.45. After a rough completeness correction for the effects of dust extinction, we find that approximately one in every six quasars is a BALQSO.

Damped Ly Gas Metallicities at Redshifts z =0.9-2.0 from Sloan Digital Sky Survey Spectra

Using Sloan Digital Sky Survey early data release spectra, we have identified 370 Mg II absorption systems with Mg II λ2796 rest equivalent widths ≥1 Å and redshifts z = 0.9-2.2. From our previous and ongoing Hubble Space Telescope UV spectroscopic studies, we estimate that the mean neutral hydrogen column density of a system selected in this manner is N = × 1020 atoms cm-2, which corresponds to the damped Lyα (DLA) regime. We have formed high signal-to-noise ratio composite spectra using 223 of these systems with z = 0.9-2.0 in order to study the strength of the Zn II and Cr II absorption lines corresponding to this mean neutral hydrogen column density. After making a correction for missed DLAs, overall we find that [Zn/H] = -1.13 ± 0.19. We find [Cr/Zn] = -0.45 ± 0.13, which indicates that ≈65% of the Cr is depleted onto grains, but this does not correct for the missed DLAs. We have also derived Zn and Cr abundances in two kinematic regimes, and within each regime we consider two redshift intervals. We find trends that indicate that metallicities are higher in the composites in which the absorption has larger velocity spreads as measured by Mg II λ2796 rest equivalent width. Larger velocity spreads may correspond to deeper gravitational potential wells that represent more massive and chemically evolved structures and/or regions associated with winds from starbursting galaxies, also leading to kinematically broad structures of chemically enriched gas. Within the large velocity spread regime, we find that at lower redshifts the Zn metallicity is larger and more Cr is depleted onto grains.

Star formation and metallicity history of the SDSS galaxy survey: unlocking the fossil record

Using MOPED we determine non-parametrically the star-formation and metallicity history of over 37,000 high-quality galaxy spectra from the Sloan Digital Sky Survey (SDSS) early data release. We use the entire spectral range, rather than conce ntrating on specific features, and we estimate the complete star formation history without prior assumptions about its form (by constructing so-called ‘population boxes’). The main resu lts of this initial study are that the star formation rate in SDSS galaxies has been in decline for � 6 Gyr; a metallicity distribution for star-forming gas which is peaked � 3 Gyr ago at about solar metallicity, inconsistent with closed-box models, but consistent with infall models. We also determine the infall rate of gas in SDSS and show that it has been significant for the last 3 G yr. We investigate errors using a Monte-Carlo Markov Chain algorithm. Further, we demonstrate that recovering star formation and metallicity histories for such a large sample becomes intractable without data compression methods, particularly the exploration of the likelihood surface. By exploring the whole likelihood surface we show that age-metallicity degeneracies are not as severe as by using only a few spectral features. We find that 65% of galaxie s contain a significant old population (with an age of at least 8 Gyr), including recent starburst galaxies, and that over 97% have some stars older than 2 Gyr. It is the first time that a c omplete star formation and metallicity history, without restrictive assumptions abo ut its form have been derived for such a large dataset of integrated stellar populations, and the fi rst time that the past star formation history has been determined from the fossil record of the present-day spectra of galaxies.