Faint Limits Research Articles

Satellite galaxies and fossil groups in the Millennium Simulation

We use a semianalytic galaxy catalogue constructed from the Millennium Simulation(MS) to study the satellites of isolated galaxies in theCDM cosmogony. The large volume surveyed by the MS (500 3 h 3 Mpc 3 ), together with its unprecedented numerical resolution, e nable the compilation of a large sample of � 80,000 bright (Mr < 20.5) primaries, surrounded by � 178,000 satellites down to the faint magnitude limit (Mr = 17) of our catalogue. This sample allows the characterization, with minimal statistical uncertainty, of the dynam- ical properties of satellite/primary galaxy systems in aCDM universe. The details of this characterization are sensitive to the details of the modeli ng, such as its assumptions on galaxy merging and dynamical friction timescales, but many of its general predictions should be ap- plicable to hierarchical formation models such asCDM. We find that, overall, the satellite population traces the dark matter rather well: its spatial d istribution and kinematics may be approximated by an NFW profile with a mildly anisotropic velo city distribution. Their spatial distribution is also mildly anisotropic, with a well-define d anti-Holmberg effect that reflects the misalignment between the major axis and angular momentum of the host halo. Our anal- ysis also highlights a number of difficulties afflicting stud ies that rely on satellite velocities to constrain the primary halo mass. These arise from variations in the star formation efficiency and assembly history of isolated galaxies, which result in a scatter of up to � 2 decades in halo mass at fixed primary luminosity. Our isolation criteri on (primaries may only have com- panions at least 2 mag fainter within 1 h 1 Mpc) contributes somewhat to the scatter, since it picks not only galaxies in sparse environments, but also a number of primaries at the centre of fossil groups. We find that the abundance and luminosity fu nction of these unusual systems are in reasonable agreement with the few available observational constraints. Much tighter halo mass-luminosity relations are found when splitting the sample by colour: red primaries inhabit halos more than twice as massive as those surrounding blue primaries, a difference that vanishes, however, when considering stellar mass instead of luminosity. The large scat- ter in the halo mass-luminosity relation hinders the interp retation of the velocity dispersion of satellites stacked according to the luminosity of the pri mary. We find L / σ 3 (the natu- ral scaling expected forCDM) for truly-isolated primaries, i.e., systems where the central galaxy contributes more than 85% of the total luminosity within its virial radius. Less stric t primary selection, however, leads to substantial modificat ion of the scaling relation: blindly stacking satellites of all primaries results in a much shall ower L-σ relation that is only poorly approximated by a power law.

The Identification of Blue Horizontal Branch Stars Using GALEX and Other Photometry

Halo samples that have no kinematic bias have been successfully isolated by photometric observations. We show that the NUV magnitude of the GALEX All-Sky Survey can be used, together with Johnson BV magnitudes in a (NUV - V)0 versus (B - V)0 plot, to distinguish blue horizontal branch (BHB) from other A stars of the same (B - V) color for 12 < V < 18. In addition, we use SDSS gr magnitudes in a (NUV - r)0 versus (g - r)0 plot for 14 < r < 18. The faint limit will be extended by ~3 mag for the GALEX Medium-Deep Survey. Attempts to use NUV in conjunction with 2MASS magnitudes and ROTSE mr magnitudes did not prove useful. The (NUV - V)0 versus (B - V)0 plot was used to examine BHB star candidates of varying quality near the south Galactic pole. We conclude that the addition of the GALEX NUV significantly adds to the reliability with which these stars can be identified.

Variable Stars in the Original CCD/Transit Instrument Survey

The existing CCD/Transit Instrument (CTI-I) survey database was searched for variable stars with 12 < V < 18.5. This magnitude range corresponds to the saturation limit and the practical faint limit of the original survey (1987-1992). The CTI-I imaged a strip of the sky with a narrow spread of declination over all right ascensions. The resulting survey area covers a large range of both Galactic latitude and longitude, and includes approximately 50 deg2. Numerous challenges created by the automated photometry pipeline and associated solutions to these problems implemented during the variable suspect selection process are described. The final list contains 1807 stars. Details and the photometry of each are provided to facilitate selection for follow-up observations and subsequent analysis. Known variable stars listed in the General Catalog of Variable Stars are identified, and follow-up observations at the US Air Force Academy Observatory of selected brighter suspects (V < 15) previously not identified as variable are also described.

Two T dwarfs from the UKIDSS early data release

We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys. We aim to show that our methodologies for searching the ~27 sq degs of the LAS EDR are successful for finding both L and T dwarfs $via$ cross-correlation with the Sloan Digital Sky Survey (SDSS) DR4 release. While the area searched so far is small, the numbers of objects found shows great promise for near-future releases of the LAS and great potential for finding large numbers of such dwarfs. Ultracool dwarfs are selected by combinations of their YJH(K) UKIDSS colours and SDSS DR4 z-J and i-z colours, or, lower limits on these red optical/infrared colours in the case of DR4 dropouts. After passing visual inspection tests, candidates have been followed up by methane imaging and spectroscopy at 4m and 8m-class facilities. Our main result is the discovery following CH4 imaging and spectroscopy of a T4.5 dwarf, ULASJ 1452+0655, lying ~80pc distant. A further T dwarf candidate, ULASJ 1301+0023, has very similar CH4 colours but has not yet been confirmed spectroscopically. We also report on the identification of a brighter L0 dwarf, and on the selection of a list of LAS objects designed to probe for T-like dwarfs to the survey J-band limit. Our findings indicate that the combination of the UKIDSS LAS and SDSS surveys provide an excellent tool for identifying L and T dwarfs down to much fainter limits than previously possible. Our discovery of one confirmed and one probable T dwarf in the EDR is consistent with expectations from the previously measured T dwarf density on the sky.

A New Measurement of the Stellar Mass Density atz ≈ 5: Implications for the Sources of Cosmic Reionization

We present a new measurement of the integrated stellar mass per comoving volume at redshift 5 determined via spectral energy fitting drawn from a sample of 214 photometrically-selected galaxies with z'<26.5 in the southern GOODS field. Following procedures introduced by Eyles et al. (2005), we estimate stellar masses for various sub-samples for which reliable and unconfused Spitzer IRAC detections are available. A spectroscopic sample of 14 of the most luminous sources with <z>=4.92 provides a firm lower limit to the stellar mass density of 1e6 Msun/Mpc^3. Several galaxies in this sub-sample have masses of order 10^11 Msun implying significant earlier activity occurred in massive systems. We then consider a larger sample whose photometric redshifts in the publicly-available GOODS-MUSIC catalog lie in the range 4.4 <z 5.6. Before adopting the GOODS-MUSIC photometric redshifts, we check the accuracy of their photometry and explore the possibility of contamination by low-z galaxies and low-mass stars. After excising probable stellar contaminants and using the z'-J color to exclude any remaining foreground red galaxies, we conclude that 196 sources are likely to be at z~5. The implied mass density from the unconfused IRAC fraction of this sample, scaled to the total available, is 6e6 Msun/Mpc^3. We discuss the uncertainties as well as the likelihood that we have underestimated the true mass density. Including fainter and quiescent sources the total integrated density could be as high as 1e7 Msun/Mpc^3. Using the currently available (but highly uncertain) rate of decline in the star formationhistory over 5 <z< 10, a better fit is obtained for the assembled mass at z~5 if we admit significant dust extinction at early times or extend the luminosity function to very faint limits. [abridged]

Completeness – I. Revisited, reviewed and revived

We have extended and improved the statistical test recently developed by Rauzy for assessing the completeness in apparent magnitude of magnitude-redshift surveys. Our improved test statistic retains the robust properties - specifically independence of the spatial distribution of galaxies within a survey - of the T c statistic introduced in Rauzy's seminal paper, but now accounts for the presence of both a faint and bright apparent magnitude limit. We demonstrate that a failure to include a bright magnitude limit can significantly affect the performance of Rauzy's T c statistic. Moreover, we have also introduced a new test statistic, T v , defined in terms of the cumulative distance distribution of galaxies within a redshift survey. These test statistics represent powerful tools for identifying and characterizing systematic errors in magnitude-redshift data. We discuss the advantages of the T c and T v statistics over standard completeness tests, particularly the widely used ν/ν max test which assumes spatial homogeneity, and we demonstrate how our T v statistic can essentially be regarded as an improved, cumulative ν/ν max test which makes better use of the magnitude completeness information in a redshift survey. Finally, we apply our completeness test to three major redshift surveys: The Millennium Galaxy Catalogue (MGC), The Two Degree Field Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS). We confirm that MGC and SDSS are complete up to the published (faint) apparent magnitude limit of m bi = 20.00 mag and m r = 17.45 mag, respectively, indicating there are no residual systematic effects within the photometry. Furthermore, we show that, unless a bright limit is included for 2dFGRS, the data set displays significant incompleteness at magnitudes brighter than the published limit of m bj = 19.45 mag.

An Observational Limit on the Dwarf Galaxy Population of the Local Group

We present the results of an all-sky, deep optical survey for faint Local Group dwarf galaxies. Candidate objects were selected from the second Palomar Observatory Sky Survey and ESO/Science Research Council survey plates, and follow-up observations were performed to determine whether they were indeed overlooked members of the Local Group. Only two galaxies (Antlia and Cetus) were discovered this way out of 206 candidates. Based on internal and external comparisons, we estimate that our visual survey is more than 77% complete for objects larger than 1' in size and with a surface brightness greater than an extremely faint limit over the 72% of the sky not obstructed by the Milky Way. Our limit of sensitivity cannot be calculated exactly, but it is certainly fainter than 25 mag arcsec-2 in R, probably 25.5 and possibly approaching 26. We conclude that there are at most one or two Local Group dwarf galaxies fitting our observational criteria still undiscovered in the clear part of the sky, and roughly a dozen hidden behind the Milky Way. Our work places the "missing satellite problem" on a firm quantitative observational basis. We present detailed data on all our candidates, including surface brightness measurements.

Panchromatic properties of 99 000 galaxies detected by SDSS, and (some by) ROSAT, GALEX, 2MASS, IRAS, GB6, FIRST, NVSS and WENSS surveys

We discuss the panchromatic properties of 99,088 galaxies selected from the Sloan Digital Sky Survey Data Release 1 “main” spectroscopic sample (a flux-limited sample for 1360 deg 2 ). These galaxies are positionally matched to sources detected by ROSAT, GALEX, 2MASS, IRAS, GB6, FIRST, NVSS and WENSS. The matching fraction varies from < 1% for ROSAT and GB6 to �40% for GALEX and 2MASS. In addition to its size, the advantages of this sample are well controlled selection effects, faint flux limits and the wealth of measured parameters, including accurate X-ray to radio photometry, angular sizes, and optical spectra. We find strong correlations between the detection fraction at other wavelengths and optical properties such as flux, colors, and emission-line strengths. For example, �2/3 of SDSS “main” galaxies classified as AGN using emission-line strengths are detected by 2MASS, while the corresponding fraction for star-forming galaxies is only �1/10. Similarly, over 90% of galaxies detected by IRAS display strong emission lines in their optical spectra, compared to �50% for the whole SDSS sample. Using GALEX, SDSS, and 2MASS data, we construct the UV-IR broad-band spectral energy distributions for various types of galaxies, and find that they form a nearly one-parameter family. For example, the SDSS u- and r- band data, supplemented with redshift, can be used to “predict” K-band magnitudes measured by 2MASS with an rms scatter of only 0.2 mag. When a dust content estimate determined from SDSS spectra with the aid of models is also utilized, this scatter decreases to 0.1 mag and can be fully accounted for by measurement uncertainties. We demonstrate that this interstellar dust content, inferred from optical SDSS spectra by Kauffmann et al. (2003a), is indeed higher for galaxies detected by IRAS and that it can be used to “predict” measured IRAS 60 µm flux density within a factor of two using only SDSS data. We also show that the position of a galaxy in the emission-line-based Baldwin-Phillips-Terlevich diagram is correlated with the optical light concentration index and u r color determined from the SDSS broad-band imaging data, and discuss changes in the morphology of this diagram induced by requiring detections at other wavelengths. Notably, we find that SDSS “main” galaxies detected by GALEX include a non-negligible fraction (10-30%) of AGNs, and hence do not represent a clean sample of starburst galaxies. We study the IR-radio correlation and find evidence that its slope

A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph

Abstract We present a catalogue of positions, magnitudes and velocities for 3300 emission-line objects found by the Planetary Nebula Spectrograph in a survey of the Andromeda galaxy, M31. Of these objects, 2615 are found likely to be planetary nebulae (PNe) associated with M31. The survey area covers the whole of M31's disc out to a radius of . Beyond this radius, observations have been made along the major and minor axes, and the Northern Spur and Southern Stream regions. The calibrated data have been checked for internal consistency and compared with other catalogues. With the exception of the very central, high surface brightness region of M31, this survey is complete to a magnitude limit of m5007∼ 23.75, 3.5 mag into the PN luminosity function. We have identified emission-line objects associated with M31's satellites and other background galaxies. We have examined the data from the region tentatively identified as a new satellite galaxy, Andromeda VIII, comparing it to data in the other quadrants of the galaxy. We find that the PNe in this region have velocities that appear to be consistent with membership of M31 itself. The luminosity function of the surveyed PNe is well matched to the usual smooth monotonic function. The only significant spatial variation in the luminosity function occurs in the vicinity of M31's molecular ring, where the luminosities of PNe on the near side of the galaxy are systematically ∼0.2 mag fainter than those on the far side. This difference can be explained naturally by a modest amount of obscuration by the ring. The absence of any difference in luminosity function between bulge and disc suggests that the sample of PNe is not strongly populated by objects whose progenitors are more massive stars. This conclusion is reinforced by the excellent agreement between the number counts of PNe and the R-band light. The number counts of kinematically selected PNe also allow us to probe the stellar distribution in M31 down to very faint limits. There is no indication of a cut-off in M31's disc out to beyond four scalelengths, and no signs of a spheroidal halo population in excess of the bulge out to 10 effective bulge radii. We have also carried out a preliminary analysis of the kinematics of the surveyed PNe. The mean streaming velocity of the M31 disc PNe is found to show a significant asymmetric drift out to large radii. Their velocity dispersion, although initially declining with radius, flattens out to a constant value in the outer parts of the galaxy. There are no indications that the disc velocity dispersion varies with PN luminosity, once again implying that the progenitors of PNe of all magnitudes form a relatively homogeneous old population. The dispersion profile and asymmetric drift results are shown to be mutually consistent, but require that the disc flares with radius if the shape of its velocity ellipsoid remains invariant.

Quasars Associated with NGC 613, NGC 936 and NGC 941

The association of low redshift, active galaxies with high redshift quasars is here tested in two regions uniformly surveyed for quasars to faint limits. Three active galaxies in these regions all show significant association with nearby quasars. Radio, optical and X-ray data available for these galaxies give new information on the processes which accompany the proposed ejection of the quasars. It is concluded that the quasars are originally relatively compact compared to the galactic medium through which they exit but can be slowed. It is argued that effects of ejection can be seen on the galaxies and may be connected with spiral structure. Evidence for periodicity of the redshifts is discussed.

The Arecibo Legacy Fast ALFA Survey. I. Science Goals, Survey Design, and Strategy

The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map ~7000 deg2 of the high Galactic latitude sky visible from Arecibo, providing a H I line spectral database covering the redshift range between -1600 and 18,000 km (s-1) with ~5 km s(-1) resolution. Exploiting Arecibo's large collecting area and small beam size, ALFALFA is specifically designed to probe the faint end of the H I mass function in the local universe and will provide a census of H I in the surveyed sky area to faint flux limits, making it especially useful in synergy with wide-area surveys conducted at other wavelengths. ALFALFA will also provide the basis for studies of the dynamics of galaxies within the Local Supercluster and nearby superclusters, allow measurement of the H I diameter function, and enable a first wide-area blind search for local H I tidal features, H I absorbers at z < 0.06, and OH megamasers in the redshift range 0.16 < z < 0.25. Although completion of the survey will require some 5 years, public access to the ALFALFA data and data products will be provided in a timely manner, thus allowing its application for studies beyond those targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum intrusion, drift scan observing technique that samples the same region of sky at two separate epochs to aid in the discrimination of cosmic signals from noise and terrestrial interference. Survey simulations, which take into account large-scale structure in the mass distribution and incorporate experience with the ALFA system gained from tests conducted during its commissioning phase, suggest that ALFALFA will detect on the order of 20,000 extragalactic H I line sources out to z ~ 0.06, including several hundred with H I masses M(HI) < 10(7.5) M ?.

XBootes: An X‐Ray Survey of the NDWFS Bootes Field. I. Overview and Initial Results

We obtained a 5 ks deep Chandra X-ray Observatory ACIS-I map of the 9.3 deg2 Bootes field of the NOAO Deep Wide-Field Survey. Here we describe the data acquisition and analysis strategies leading to a catalog of 4642 (3293) point sources with 2 or more (4 or more) counts, corresponding to a limiting flux of roughly 4(8) ? 10-15 ergs cm-2 s-1 in the 0.5-7 keV band. These Chandra XBootes data are unique in that they constitute the widest contiguous X-ray field yet observed to such a faint flux limit. Because of the extraordinarily low background of the ACIS, we expect only 14% (0.7%) of the sources to be spurious. We also detected 43 extended sources in this survey. The distribution of the point sources among the 126 pointings (ACIS-I has a 16' ? 16' field of view) is consistent with Poisson fluctuations about the mean of 36.8 sources per pointing. While a smoothed image of the point source distribution is clumpy, there is no statistically significant evidence of large-scale filamentary structure. We do find however, that for ? > 1', the angular correlation function of these sources is consistent with previous measurements, following a power law in angle with slope ~-0.7. In a 1.4 deg2 sample of the survey, approximately 87% of the sources with 4 or more counts have an optical counterpart to R ~ 26 mag. As part of a larger program of optical spectroscopy of the NDWFS Bootes area, spectra have been obtained for ~900 of the X-ray sources, most of which are quasars or active galactic nuclei.

The Extragalactic Radio Sky at Faint Flux Densities

In this paper we present simulations of the extragalactic radio sky at 151, 325, and 1400 MHz based on number density predictions from models of the evolution of the radio luminosity function. From these simulations we predict typical source distributions, allowing us to estimate the natural confusion limit to faint flux density limits. The results are relevant to the science and design goals of future radio facilities, namely the Square Kilometre Array, the low-frequency array, and their demonstrator projects.

Discovery of optically faint obscured quasars with Virtual Observatory tools

We use Virtual Observatory (VO) tools to identify optically faint, obscured (i.e., type 2) active galactic nuclei (AGN) in the two Great Observatories Origins Deep Survey (GOODS) fields. By employing publicly available X-ray and optical data and catalogues we discover 68 type 2 AGN candidates. The X-ray powers of these sources are estimated by using a previously known correlation between X-ray luminosity and X-ray-to-optical flux ratio. Thirty-one of our candidates have high estimated powers (Lx > 10 44 erg/s) and therefore qualify as optically obscured quasars, the so-called 2. Based on the derived X-ray powers, our candidates are likely to be at relatively high redshifts, z ∼ 3, with the QSO 2 at z ∼ 4. By going ∼ 3m ag fainter than previously known type 2 AGN in the two GOODS fields we are sampling a region of redshift - power space which was previously unreachable with methods. Our method brings to 40 the number of QSO 2 in the GOODS fields, an improvement of a factor ∼ 4 when compared to the only 9 such sources previously known. We derive a QSO 2 surface density down to 10 −15 erg cm −2 s −1 in the 0.5−8 keV band of >330 deg −2 , ∼30% of which is made up of previously known sources. This is larger than current estimates and some predictions and suggests that the surface density of QSO 2 at faint flux limits has been underestimated. This work demonstrates that VO tools are mature enough to produce cutting-edge science results by exploiting astronomical data beyond classical identification limits (R < 25) with interoperable tools for statistical

The dwarf galaxy population in Abell 2218

We present results from a deep photometric study of the rich galaxy cluster Abell 2218 (z = 0.18) based on archival Hubble Space Telescope WFPC2 F606W images. These have been used to derive the luminosity function to extremely faint limits (M F606W -13.2 mag, μ 0 24.7 mag arcsec -2 ) over a wide field of view (1.3 h -2 Mpc 2 ). We find the faint-end slope of the luminosity function to vary with environment within the cluster, going from a = -1.23 ± 0.13 within the projected central core of the cluster (100 < r < 300 h -1 kpc) to a = -1.49 ± 0.06 outside this radius (300 < r < 750 h -1 kpc). We infer that the core is 'dwarf depleted', and further quantify this by studying the ratio of 'dwarf' to 'giant' galaxies and its dependency as a function of clustercentric radius and local galaxy density. We find that this ratio varies strongly with both quantities, and that the dwarf galaxy population in A2218 has a more extended distribution than the giant galaxy population.

The Liverpool-Edinburgh high proper motion catalogue

We present a machine selected catalogue of 11 289 objects with proper motions exceeding 0.18 �� yr −1 and an R-band faint magnitude limit of 19.5 mag. The catalogue was produced using SuperCOSMOS digitized R-Band ESO and UK Schmidt Plates in 287 Schmidt fields covering almost 7000 square degrees (∼17% of the whole sky) at the South Galactic Cap. The catalogue includes UK Schmidt BJ and I magnitudes for all of the stars as well as 2MASS magnitudes for 10 447 of the catalogue stars. We also show that the NLTT is ∼95% complete for Dec > −32.5 ◦ .

The 2dF QSO Redshift Survey - XII. The spectroscopic catalogue and luminosity function

We present the final catalogue of the 2dF QSO Redshift Survey (2QZ), based on Anglo-Australian Telescope 2dF spectroscopic observations of 44576 colour-selected (u b_J r) objects with 18.25<b_J<20.85 selected from APM scans of UK Schmidt Telescope (UKST) photographic plates. The 2QZ comprises 23338 QSOs, 12292 galactic stars (including 2071 white dwarfs) and 4558 compact narrow-emission-line galaxies. We obtained a reliable spectroscopic identification for 86 per cent of objects observed with 2dF. We also report on the 6dF QSO Redshift Survey (6QZ), based on UKST 6dF observations of 1564 brighter 16<b_J<18.25 sources selected from the same photographic input catalogue. In total, we identified 322 QSOs spectroscopically in the 6QZ. The completed 2QZ is, by more than a factor 50, the largest homogeneous QSO catalogue ever constructed at these faint limits (b_J<20.85) and high QSO surface densities (35 QSOs deg^-2). As such it represents an important resource in the study of the Universe at moderate-to-high redshifts. As an example of the results possible with the 2QZ, we also present our most recent analysis of the optical QSO luminosity function and its cosmological evolution with redshift. For a flat, Omega_m=0.3 and Omega_lam=0.7, Universe, we find that a double power law with luminosity evolution that is exponential in look-back time, t, of the form L*(z) exp(6.15t), equivalent to an e-folding time of 2Gyr, provides an acceptable fit to the redshift dependence of the QSO luminosity function over the range 0.4 < z < 2.1 and M_bJ<-22.5. Evolution described by a quadratic in redshift is also an acceptable fit, with L*(z)~10^(1.39z-0.29z^2).

Dominant galaxies in 2dF groups

We investigate whether the spectral-type of a locally dominant (most luminous) galaxy can be used to select sets of galaxies that are physically associated (groups). We assume that passive dominants trace a group-like potential, and SF-dominants a field-like environment. The group sample includes 988 groups selected in the 2dFGRS applying a maximum magnitude difference criterion. We find that the average number of passive galaxies associated to a dom- inant is larger when the dominant is passive, a result supporting our assumption that galaxy associations around a passive dominant are reliable groups. Finally we show that, to reduce the contamination by unbound galaxy associations (SF-dominant), a >3 passive-members criterion is more efficient than a>6 all-members criterion. 1. Dominant Group Galaxy selection criteria Each 2dF galaxy with >4 neighbours (projected separation less than 1h 1 Mpc and line of sight velocity difference less than 1000kms 1 ), that is more luminous (by >0.2 mag) than all of its neighbours, is a dominant group galaxy and identifies a group. Given the bright and faint apparent magnitude limits of the 2dF, a maximum magnitude difference criterion has been applied providing, for each dominant, complete identification of all neighbours fromequally-luminous to �2÷2.5 magnitudes fainter. Dominants are selected in the range bj2(17 ÷ 17.5), neighbours in range bj2(17 ÷ 19.5). The criteria for selection are 0.03 6 zdom 6 0.12 and 22 6 (M 5logh)dom 6 19. The sample includes 988 groups (7281 galaxies): 639 with a passive (Type 1 in Madgwick et al. 2002) dominant galaxy (P-dG), and 349 with a SF (Type>1) dominant (SF-dG). P-dGs are typically richer than SF-dGs. 2. Passive and Star-Forming dominants If we assume that P-dGs trace a group-like potential, and SF-dGs a field-like envi- ronment (galaxy associations not embedded within a common massive halo), differences in the spectral mix of P-dGs and SF-dGs members should reflect the role of a group environment on the spectral evolution of galaxies. Figure 1 shows the abundance of P- dGs, and SF-dGs (top panels) as a function of absolute magnitude of the dominant. Non-luminous dominants are equally likely to be passive or SF, but the former are much more common than the latter among luminous dominants. That SF galaxies represent only a small fraction among luminous dominant galaxies is consistent with X-ray studies that show diffuse hot X-ray halo typically occur in groups hosting a dominant luminous elliptical. It remains unclear whether groups with a faint passive dominant exist at all, or whether these are typically embedded within larger systems (Kelm & Focardi 2004). The bottom panels show the distributions of non-dominants that are passive and star- forming, as a function of the luminosity of the dominant, in P-dGs (left) and SF-dGs (right). P-dGs with a luminous dominant exhibit more passive than SF non-dominant

The QUEST RR Lyrae Survey. I. The First Catalog

With the 1 m Schmidt telescope of the Llano del Hato Observatory and the QUEST CCD camera, 380 deg2 of the sky have been surveyed for RR Lyrae variables in a band 23 wide in declination (centered at δ = -1°) and covering right ascensions from 41 to 61 and from 80 to 170. The bright limit (due to CCD saturation) and the faint limit are V ~ 13.5 and ~19.7, respectively, which correspond to ~4 and ~60 kpc from the Sun. We present a catalog of the positions, amplitudes, mean magnitudes, periods, and light curves of the 498 RR Lyrae variables that have been identified in this region of the sky. The majority of these stars (86%) are new discoveries. The completeness of the survey has been estimated from simulations that model the periods and light curves of real RR Lyrae variables and take into account the pattern of epochs of observation. While the completeness of the survey varies with apparent magnitude and with position, almost everywhere in the surveyed region it is quite high (>80%) for the type ab RR Lyrae variables and between 30% and 90% for the low-amplitude type c variables.

Star Formation atz ∼ 6:i‐Dropouts in the Advanced Camera for Surveys Guaranteed Time Observation Fields

Using an i - z dropout criterion, we determine the space density of z ~ 6 galaxies from two deep ACS GTO fields with deep optical-IR imaging. A total of 23 objects are found over 46 arcmin2, or ~0.5 ± 0.1 objects arcmin-2 down to zAB ~ 27.3 (6 σ), or a completeness-corrected ~0.5 ± 0.2 objects arcmin-2 down to zAB ~ 26.5 (including one probable z ~ 6 active galactic nucleus). Combining deep ISAAC data for our RDCS 1252-2927 field (JAB ~ 25.7 and Ks,AB ~ 25.0; 5 σ) and NICMOS data for the Hubble Deep Field-North (J110,AB and H160,AB ~ 27.3, 5 σ), we verify that these dropouts have relatively flat spectral slopes, as one would expect for star-forming objects at z ~ 6. Compared with the average-color (β = -1.3) U-dropout in the Steidel et al. z ~ 3 sample, i-dropouts in our sample range in luminosity from ~1.5L* (zAB ~ 25.6) to ~0.3L* (zAB ~ 27.3) with the exception of one very bright candidate at z850,AB ~ 24.2. The half-light radii vary from 009 to 021, or 0.5 kpc to 1.3 kpc. We derive the z ~ 6 rest-frame UV luminosity density (or star formation rate density) by using three different procedures. All three procedures use simulations based on a slightly lower redshift (z ~ 5) V606-dropout sample from Chandra Deep Field-South ACS images. First, we make a direct comparison of our findings with a no-evolution projection of this V-dropout sample, allowing us to automatically correct for the light lost at faint magnitudes or lower surface brightnesses. We find 23% ± 25% more i-dropouts than we predict, consistent with no strong evolution over this redshift range. Adopting previous results to z ~ 5, this works out to a mere 20% ± 29% drop in the luminosity density from z ~ 3 to z ~ 6. Second, we use the same V-dropout simulations to derive a detailed selection function for our i-dropout sample and compute the UV-luminosity density [(7.2 ± 2.5) × 1025 ergs s-1 Hz-1 Mpc-3 down to zAB ~ 27]. We find a 39% ± 21% drop over the same redshift range (z ~ 3-6), consistent with the first estimate. This is our preferred value and suggests a star formation rate of 0.0090 ± 0.0031 M☉ yr-1 Mpc-3 to zAB ~ 27, or ~0.036 ± 0.012 M☉ yr-1 Mpc-3 by extrapolating the luminosity function to the faint limit, assuming α = -1.6. Third, we follow a very similar procedure, except that we assume no incompleteness, and find a rest-frame continuum luminosity that is ~2-3 times lower than our other two determinations. This final estimate is to be taken as a lower limit and is important if there are modest changes in the colors or surface brightnesses from z ~ 5 to z ~ 6 (the other estimates assume no large changes in the intrinsic selectability of objects). We note that all three estimates are well within the canonical range of luminosity densities necessary for reionization of the universe at this epoch by star-forming galaxies.