We report the discovery of a nearby, old, halo white dwarf candidate from the Sloan Digital Sky Survey. SDSS J110217.48+411315.4 has a proper motion of 1.75 arcsec/year and redder optical colors than all other known featureless (type DC) white dwarfs. We present SDSS imaging and spectroscopy of this object, along with near-infrared photometry obtained at the United Kingdom Infra-Red Telescope. Fitting its photometry with up-to-date model atmospheres, we find that its overall spectral energy distribution is fit reasonably well with a pure hydrogen composition and T_eff~3800 K (assuming log g=8). That temperature and gravity would place this white dwarf at 35 pc from the Sun with a tangential velocity of 290 km/s and space velocities consistent with halo membership; furthermore, its combined main sequence and white dwarf cooling age would be ~11 Gyr. However, if this object is a massive white dwarf, it could be a younger object with a thick disk origin. Whatever its origin, the optical colors of this object are redder than predicted by any current pure hydrogen, pure helium or mixed hydrogen-helium atmospheric model, indicating that there remain problems in our understanding of the complicated physics of the dense atmospheres of cool white dwarfs.
We have derived the oxygen abundance for a sample of nearby galaxies in the Data Release 5 of the Sloan Digital Sky Survey (SDSS) which possess at least two independent spectra of one or several HII regions with a detected [OIII]4363 auroral line. Since, for nearby galaxies, the [OII]3727 nebular line is out of the observed wavelength range, we propose a method to derive (O/H)_ff abundances using the classic Te method coupled with the ff relation. (O/H)_7325 abundances have also been determined, based on the [OII]7320,7330 line intensities, and using a small modification of the standard Te met…
We have used the publicly available data from the Sloan Digital Sky Survey and 2dF QSO Redshift Survey to test the hypothesis that QSOs are ejected from active galaxies with periodic non-cosmological redshifts. For two different intrinsic redshift models, namely the Karlsson $\log(1+z)$ model and Bell's decreasing intrinsic redshift (DIR) model, we do two tests respectively. First, using different criteria, we generate four sets of QSO-galaxy pairs and find there is no evidence for a periodicity at the predicted frequency in $\log(1+z)$, or at any other frequency. We then check the relationshi…
What can we learn from the somewhat arduous study of old stellar populations in nearby galaxies? Unless the nearby universe is subtly anomalous, it should contain a relatively normal selection of galaxies whose histories are representative of field galaxies in general throughout the Universe. We can therefore take advantage of our ability to resolve local galaxies into individual stars to directly, and accurately, measure star formation histories. The star formation histories are determined from numerical models, based on stellar evolution tracks, of colour-magnitude diagrams. The most accurat…
Most Tidal Dwarf Galaxies (TDGs) so-far discussed in the literature may be considered as young ones or even newborns, as they are still physically linked to their parent galaxies by an umbilical cord: the tidal tail at the tip of which they are usually observed. Old Tidal Dwarf Galaxies, completely detached from their progenitors, are still to be found. Using N--body numerical simulations, we have shown that tidal objects as massive as 10^9 solar masses may be formed in interacting systems and survive for more than one Gyr. Old TDGs should hence exist in the Universe. They may be identified lo…
Most Vega-like stars have far-infrared excess (60micron or longward in IRAS, ISO, or Spitzer MIPS bands) and contain cold dust (<~150K) analogous to the Sun's Kuiper-Belt region. However, dust in a region more akin to our asteroid belt and thus relevant to the terrestrial planet building process is warm and produces excess emission in mid-infrared wavelengths. By cross-correlating Hipparcos dwarfs with the MSX catalog, we found that EF Cha, a member of the recently identified, ~10 Myr old, ``Cha-Near'' Moving Group, possesses prominent mid-infrared excess. N-band spectroscopy reveals a strong …
We briefly discuss the evolutionary path and observational appearance of isolated neutron stars (INSs) focusing on radioquiet objects. There are many reasons to believe that these sources are extremely elusive once the star surface has cooled down: their high spatial velocities, the long propeller stage and/or the very low accretion efficiency. We describe recent population synthesis models of close-by young INSs, highlighting the major difficulties encountered in the past by these simulations in reproducing the observed properties of known sources. As we show, a likely possibility is that mos…
The Sloan Digital Sky Survey (SDSS) is making a multi-colour, three dimensional map of the nearby Universe. The survey is in two parts. The first part is imaging one quarter of the sky in five colours from the near ultraviolet to the near infrared. In this imaging survey we expect to detect around 50 million galaxies to a magnitude limit g ~ 23. The second part of the survey, taking place concurrently with the imaging, is obtaining spectra for up to 1 million galaxies and 100,000 quasars. From these spectra we obtain redshifts and hence distances, in order to map out the three-dimensional dist…
We present the results of 11 elliptical galaxies with strong nebular emission lines during our study of star formation history along the Hubble sequence. After removing the dilution from the underlying old stellar populations by use of stellar population synthesis model, we derive the accurate fluxes of all emission lines for these objects, which are later classified with emission line ratios into one Seyfert 2, six LINERs and four HII galaxies. We also identify one HII galaxy (A1216+04) as a hitherto unknown Wolf-Rayet galaxy from the presence of the Wolf-Rayet broad bump at 4650 Å. We propos…
The multi-wavelength study of old (>100 Myr) radio pulsars holds the key to understanding the long-term evolution of neutron stars, including the advanced stages of neutron star cooling and the evolution of the magnetosphere. Optical/UV observations are particularly useful for such studies because they allow one to explore both thermal and non-thermal emission processes. In particular, studying the optical/UV emission constrains temperature of the bulk of the neutron star surface, too cold to be measured in X-ray observations.Aim of this work is to identify the optical counterpart of the very …
We analyzed 1441 Chandra X-ray point sources in 32 nearby galaxies. The total point-source X-ray luminosity L_XP is well correlated with B, K, and FIR+UV luminosities of spiral host galaxies, and with the B and K luminosities for ellipticals. This suggests an intimate connection between L_XP and both the old and young stellar populations, for which K and FIR+UV luminosities are proxies for the galaxy mass M and star-formation rate SFR. We derive proportionality constants 1.3E29 erg/s/Msol and 0.7E39 erg/s/(Msol/yr), which can be used to estimate the old and young components from M and SFR, res…
We revisit the bimodal distribution of the galaxy population commonly seen in the local universe. Here we address the bimodality observed in galaxy properties in terms of spectral synthesis products, such as mean stellar ages and stellar masses, derived from the application of this powerful method to a volume-limited sample, with magnitude limit cutoff M_r = -20.5, containing about 50 thousand luminous galaxies from the SDSS Data Release 2. In addition, galaxies are classified according to their emission line properties in three distinct spectral classes: star-forming galaxies, with young stel…
Li abundances are derived for a sample of bulgelike stars with isochronal ages of 10-11 Gyr. These stars have orbits with pericentric distances, Rp, as small as 2-3 kpc and Zmax < 1 kpc. The sample comprises G and K dwarf stars in the metallicity range -0.80<[Fe/H]< +0.40. Few data of Li abundances in old turn-off stars (> 4.5 Gyr) within the present metallicity range are available. M67 (4.7 Gyr) and NGC 188 (6 Gyr) are the oldest studied metal-rich open clusters with late-type stars. Li abundances have also been studied for few samples of old metal-rich field stars. In the present work a high…
Using data derived from the deepest and finest angular resolution images of the universe yet acquired by astronomers at optical wavelengths using the Hubble Space Telescope (HST) in two postage-stamp sections of the sky (Williams et al. 1996a,b), plus simple geometrical and scaling arguments, we demonstrate that the faint blue population of point-source objects detected on those two fields (Méndez et al. 1996) could actually be ancient halo white dwarfs at distances closer than about 2 kpc from the Sun. This finding has profound implications, as the mass density of the detected objects would a…
We report the discovery of 39 Faint High Latitude Carbon Stars (FHLCs) from Sloan Digital Sky Survey commissioning data. The objects, each selected photometrically and verified spectroscopically, range over 16.6 < r* < 20.0, and show a diversity of temperatures as judged by both colors and NaD line strengths. At the completion of the Sloan Survey, there will be many hundred homogeneously selected and observed FHLCs in this sample. We present proper motion measures for each object, indicating that the sample is a mixture of extremely distant (>100 kpc) halo giant stars, useful for constraining …
We present a new analysis of the Sloan Digital Sky Survey data aimed at producing a detailed map of the nearby (z < 0.5) universe. Using neural networks trained on the available spectroscopic base of knowledge we derived distance estimates for about 30 million galaxies distributed over ca. 8,000 sq. deg. We also used unsupervised clustering tools developed in the framework of the VO-Tech project, to investigate the possibility to understand the nature of each object present in the field and, in particular, to produce a list of candidate AGNs and QSOs.
Two studies have recently reported the discovery of pronounced Halo substructure in the Sloan Digital Sky Survey (SDSS) commissioning data. Here we show that this Halo substructure is almost in its entirety due to the expected tidal stream torn off the Sagittarius dwarf galaxy during the course of its many close encounters with the Milky Way. This interpretation makes strong predictions on the kinematics and distances of these stream stars. Comparison of the structure in old horizontal branch stars, detected by the SDSS team, with the carbon star structure discovered in our own survey, indicat…
Recent results on the detection of substructure in dwarf spheroidal galaxies are discussed. In most cases they show that, when a galaxy experiences multiple SF episodes, the intermediate age population is more centrally concentrated than the old population, and that the recent SF is even more concentrated towards the central regions. Moreover, it appears that the spatial distribution of stars becomes more and more irregular as younger and younger subpopulations are considered. We illustrate how wide-area observations, allowing a substantial coverage of dwarf spheroidals (in particular the sate…
We report on the detection in Sloan Digital Sky Survey data of a 63 degree-long tidal stream of stars, extending from Ursa Major to Cancer. The stream has no obvious association with the orbit of any known cluster or galaxy. The contrast of the detected stream is greatest when using a star count filter that is matched to the color-magnitude distribution of stars in M 13, which suggests that the stars making up the stream are old and metal poor. The visible portion of the stream is very narrow and about 8.5 kpc above the Galactic disk, suggesting that the progenitor is or was a globular cluster…
If Ultra-High Energy Cosmic Rays (UHECRs) are originated from nearby galaxies, modeling of the distribution of nearby galaxies is important to an accurate estimate the source number density of UHECRs. We investigate uncertainty of the selection function of the Optical Redshift Survey (ORS), which we used to construct a source model of UHECRs. The investigation is based on a comparison of numbe counts of ORS galaxies with those of the spectroscopic sample of the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR). We carefully count galaxies in the same absolute magnitude bin from the two …
A search of the Sloan Digital Sky Survey spectroscopic database has turned up 23 new ultra-cool subdwarfs, low-mass metal-poor stars of spectral subtype M 7.0 or later. Spectra from these red objects all show very strong molecular bands of CaH but relatively weak bands of TiO, indicative of a cool, metal-poor atmosphere. Five of the stars are formally classified as M subdwarfs (sdM7.0-sdM8.5), 13 as more metal-poor extreme subdwarfs (esdM7.0-esdM8.0), and five as extremely metal-poor ultra subdwarfs (usdM7.0-usdM7.5). In the [H_r,r-z] reduced proper motion diagram, these subdwarfs clearly popu…
We report the discovery of two binary M dwarf systems in the immediate solar neighborhood using 2MASS. The first is an M6.5 companion to the nearby G star HD 86728 (Gl 376). The known properties of HD 86728 indicate that the M dwarf (Gl 376B) is old, metal-rich and only 14.9 parsecs away. The M dwarf is highly active with both H alpha and X-ray emission. Thus, Gl 376B offers the opportunity to study an old, bright, active M dwarf with known metallicity, age, and luminosity. We show that it is probable that Gl 376B is itself an unresolved pair. The other system consists of an M6.5 and an M8 dwa…
We report the discoveries of two, two-image gravitationally lensed quasars selected from the Sloan Digital Sky Survey: SDSS J0806+2006 at z_s=1.540 and SDSS J1353+1138 at z_s=1.629 with image separations of 1.40" and 1.41" respectively. Spectroscopic and optical/near-infrared imaging follow-up observations show that the quasar images have identical redshifts and possess extended objects between the images that are likely to be lens galaxies at z_l~0.6 in SDSS J0806+2006 and z_l~0.3 in SDSS J1353+1138. The field of SDSS J0806+2006 contains several nearby galaxies that may significantly perturb …
Despite the common picture of an early-type dwarf (dE) as a quiescent galaxy with no star formation and little gas, we identify 23 dEs that have blue central colors caused by recent or ongoing star formation in our sample of 476 Virgo cluster dEs. In addition, 14 objects that were mostly classified as (candidate) BCDs have similar properties. Among the certain cluster members, the dEs with blue centers reach a fraction of more than 15% of the dE population at brighter (B<=16) magnitudes. A spectral analysis of the centers of 16 galaxies reveals in all cases an underlying old population that do…
We report the detection of a faint old stellar system at $(α,δ)=(194.29^\circ,~34.32^\circ)$ (SDSS J1257+3419), based on the spatial distribution of bright red-giant branch stars in the Sloan Digital Sky Survey Data Release 4. SDSS J1257+3419 has a half-light radius of $38\pm 12$ pc and an absolute integrated $V$-magnitude of $M_V=-4.8^{+1.4}_{-1.0}$ mag at a heliocentric distance of $150\pm 15$ kpc. A comparison between SDSS J1257+3419 and known Galactic halo objects suggests that SDSS J1257+3419 is either (a) a faint and small dwarf galaxy or (b) a faint and widely extended globular cluster.…
The Sloan Digital Sky Survey (SDSS) is collecting photometry and intermediate resolution spectra for ~ 10**5 stars in the thick-disk and stellar halo of the Milky Way. This massive dataset can be used to infer the properties of the stars that make up these structures, and considerably deepen our vision of the old components of the Galaxy. We devise tools for automatic analysis of the SDSS photometric and spectroscopic data based on plane-parallel line-blanketed LTE model atmospheres and fast optimization algorithms. A preliminary study of about 5000 stars in the Early Data Release gives a hint…
We detected a nearby (d=360 pc), old (5 Myr) pulsar B1133+16 with Chandra. The observed pulsar's flux is $(0.8\pm 0.2)\times 10^{-14}$ ergs cm$^{-2}$ s$^{-1}$ in the 0.5-8 keV band. Because of the small number of counts detected, the spectrum can be described by various models. A power-law fit of the spectrum gives a photon index $Γ= 2.5$ and an isotropic luminosity of $1.4\times 10^{29}$ ergs s$^{-1}$ in the 0.5-8 keV band, which is about $1.6\times 10^{-3}$ of the spin-down power $\dot{E}$. The spectrum can also be fitted by a blackbody model with a temperature of $\approx 2.8$ MK and a proj…
The nearby, 30 to 40 Myr old, starburst region of the Gould Belt has formed numerous massive stars. Within its 300 pc radius, it produces core-collapse supernovae at an enhanced rate which is shown to be 75 to 95 Myr{-1} kpc{-2}, i.e. 3 to 5 times higher than the local Galactic rate, over the past and future few million years. A population of persistent, but unidentified, gamma-ray sources has been recently singled out at medium latitudes above 100 MeV. Their distribution across the sky is shown to be quite significantly and better correlated with the tilted Gould Belt than with other Galactic…
In this paper we present a new statistic for quantifying galaxy morphology based on measurements of the Gini coefficient of galaxy light distributions. This statistic is easy to measure and is commonly used in econometrics to measure how wealth is distributed in human populations. When applied to galaxy images, the Gini coefficient provides a quantitative measure of the inequality with which a galaxy's light is distributed amongst its constituent pixels. We measure the Gini coefficient of local galaxies in the Early Data Release of the Sloan Digital Sky Survey and demonstrate that this quantit…
We present the first detailed X-ray observations, using Chandra, of NGC 6482 - the nearest known `fossil group'. The group is dominated by an optically luminous giant elliptical galaxy and all other known group members are at least two magnitudes fainter. The global X-ray properties (luminosity, temperature, extent) of NGC 6482 fall within the range of other groups, but the detailed properties show interesting differences. We derive the gas temperature and total mass profiles for the central 30 kpc using ACIS spatially resolved spectroscopy. The temperature profile shows a continuous decrease …
In order to understand the formation and subsequent evolution of galaxies one must first distinguish between the two main morphological classes of massive systems: spirals and early-type systems. This paper introduces a project, Galaxy Zoo, which provides visual morphological classifications for nearly one million galaxies, extracted from the Sloan Digital Sky Survey (SDSS). This achievement was made possible by inviting the general public to visually inspect and classify these galaxies via the internet. The project has obtained more than 40,000,000 individual classifications made by ~100,000 …
We report the discovery of two doubly-imaged quasars, SDSS J100128.61+502756.9 and SDSS J120629.65+433217.6, at redshifts of 1.838 and 1.789 and with image separations of 2.86'' and 2.90'', respectively. The objects were selected as lens candidates from the Sloan Digital Sky Survey (SDSS). Based on the identical nature of the spectra of the two quasars in each pair and the identification of the lens galaxies, we conclude that the objects are gravitational lenses. The lenses are complicated; in both systems there are several galaxies in the fields very close to the quasars, in addition to the l…
We present the two--point correlation function (2PCF) of narrow-line active galactic nuclei (AGN) selected within the First Data Release of the Sloan Digital Sky Survey. Using a sample of 13605 AGN in the redshift range 0.055 < z < 0.2, we find that the AGN auto--correlation function is consistent with the observed galaxy auto--correlation function on scales 0.2h^{-1}Mpc to >100h^{-1}Mpc. The AGN hosts trace an intermediate population of galaxies and are not detected in either the bluest (youngest) disk--dominated galaxies or many of the reddest (oldest) galaxies. We show that the AGN 2PCF is …
We report discovery of actively star forming elliptical galaxies in a morphologically classified sample of bright galaxies at a low redshift obtained from the Sloan Digital Sky Survey. The emission lines of these galaxies do not show the characteristics of active galactic nuclei, and thus their strong H$α$ emission is ascribed to star formation with a rate nearly as high as that is seen in typical late spiral galaxies. This is taken as evidence against the traditional view that all elliptical galaxies formed early and now evolve only passively. The frequency of such star forming elliptical gal…
A spiral galaxy partially ovelapping a more distant elliptical offers an unique opportunity to measure the dust extinction in the foreground spiral. From the Sloan Digital Sky Survey DR4 spectroscopic sample, we selected 83 occulting galaxy pairs and measured disk opacity over the redshift range z = 0.0-0.2 with the goal to determine the recent evolution of disk dust opacity. The enrichment of the ISM changes over the lifetime of a disk and it is reasonable to expect the dust extinction properties of spiral disks as a whole to change over their lifetime. When they do, the change will affect ou…
This conference has brought together many of the world's `cosmographers', particularly those focussed on the mapping nearby large-scale structures. Not surprisingly, numerous portrayals of the Local Cosmic Web and its characteristics have been presented, with some reconciliation to the structures found by the much deeper 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey. In recent years, Near-infrared and H I surveys have greatly narrowed the `Zone of Avoidance' caused by the foreground obscuration of the Milky Way. There has been emphasis towards southern large-scale structures, wit…
(abridged) The old Star Cluster (SC) systems surrounding any sofar investigated galaxy represent a powerful tool for the understanding of the cosmological evolution of their host galaxies. Phases of enhanced cluster formation can be identified by reliably age dating the clusters. These are symptoms of violent star formation, results of intense starbursts as commonly seen in merging nearby galaxies and high-z forming galaxies. However, the young SC systems of nearby merging galaxies and the old SC systems of nearby passive galaxies (most likely resulting from high-z starburst events of these ga…
We report on deep VLT observations of the fields of two nearby, isolated millisecond pulsars PSR J1744-1134 and PSR J1024-0719. Both objects are old neutron stars with characteristic age $τ\ge 10^9$ yr and have relatively high spin-down flux. They have been detected earlier as X-ray sources by ROSAT HRI observations and were considered good candidates for non-thermal emission in the optical bands. Our observations set an upper limit of $B=26.9$, $V=26.3$ and $R=26.0$ for PSR J1744-1134. In the case of PSR J1024-0719, we find two faint objects near the radio position of the pulsar. Using multi-…
The oldest and most metal-poor Milky Way stars form a kinematically hot halo, which motivates the two major formation scenarios for our galaxy: extended hierarchical accretion and rapid collapse. RR Lyrae stars are excellent tracers of old and metal-poor populations. We measure the kinematics of 43 RR Lyrae stars in the inner regions of the nearby Large Magellanic Cloud (LMC) galaxy. The velocity dispersion, σ_{true}=53\pm10 km/s, indicates that a kinematically hot metal-poor old halo also exists in the LMC. This suggests that our galaxy and smaller late-type galaxies like the LMC have similar…
We present a weak gravitational lensing analysis of 22 early-type strong lens galaxies, based on deep HST images obtained as part of the Sloan Lens ACS Survey. Using the most advanced techniques to control systematic uncertainties related to the variable PSF and charge transfer efficiency of the ACS, we detect weak lensing signal out to 300 kpc/h. We analyze blank control fields from the COSMOS survey in the same manner, inferring that the residual systematic uncertainty in the tangential shear is <0.3%. A joint strong and weak lensing analysis shows that the average total mass density profile…
We report the results of spectroscopic observations, obtained with the GEMINI Multi-Object Spectrograph, of 8 planetary nebulae (PNe) in the dwarf spheroidal (dSph) galaxy NGC147, a companion of M31. The physico-chemical properties of the six brightest PNe (Corradi et al. 2005) were derived using both the empirical ICF method and photoionization modelling with CLOUDY. Different aspects of the evolution of low and intermediate mass stars in a low-metallicity environment are analysed using relationships between chemical abundances. In addition, certain features of the chemical evolution of NGC14…
(Abridged) We describe a sample of low-mass Seyfert 2 galaxies selected from the Sloan Digital Sky Survey, having a median absolute magnitude of M_g = -19.0 mag. These galaxies are Type 2 counterparts to the Seyfert 1 galaxies with intermediate-mass black holes identified by Greene & Ho (2004). Spectra obtained with the Echellette Spectrograph and Imager at the Keck Observatory are used to determine the central stellar velocity dispersions and to examine the emission-line properties. Overall, the stellar velocity dispersions are low (40-90 km/s), and we find 12 objects having sigma < 60 km/s, …
The Pisces-Cetus supercluster (redshift z~0.06) is one of the richest nearby (z<0.1) superclusters of galaxies, and emerges as a remarkable filament of galaxies at the edges of the two-degree-field galaxy redshift survey and the ongoing Sloan Digital Sky Survey. We explore the extent of the supercluster on the sky and in redshift space, and map the distribution of its constituent clusters and groups. We find evidence of enhanced star formation in galaxies belonging to groups in the supercluster compared to those in the field. This effect appears to be higher among the poorer groups than in the…
Properties of nearby dwarf galaxies are briefly discussed. Dwarf galaxies vary widely in their star formation histories, the ages of their subpopulations, and in their enrichment history. Furthermore, many dwarf galaxies show evidence for spatial variations in their star formation history; often in the form of very extended old populations and radial gradients in age and metallicity. Determining factors in dwarf galaxy evolution appear to be both galaxy mass and environment. We may be observing continuous evolution from low-mass dwarf irregulars via transition types to dwarf spheroidals, where…
We present CCD surface photometry of 16 nearby dwarf galaxies, many of which were only recently discovered. Our sample comprises both isolated galaxies and galaxies that are members of nearby galaxy groups. The observations were obtained in the Johnson B and V bands (and in some cases in Kron-Cousins I). We derive surface brightness profiles, total magnitudes, and integrated colors. For the 11 galaxies in our sample with distance estimates the absolute B magnitudes lie in the range of -10>Mb>-13. The central surface brightness ranges from 22.5 to 27.0 mag/sq.arcsec. Most of the dwarf galaxies …
The so-called ``G-dwarf problem'' emerged in the 1960's when it was noticed that the observed metal distribution of nearby stars differs from what is expected in simple models of the evolution of the Galaxy. We have determined metallicities for a sample of over 100 K dwarfs drawn from the Hipparcos catalog, using R-I colours to estimate effective temperatures and Geneva photometry to estimate [Fe/H]. We show there is a K-dwarf problem. K-dwarfs have life-times older than the Galactic disc, so these are an ideal star with which to investigate the disc's chemical evolution. Further photometric o…
Using a new grid of models of cooling white dwarfs, we calculate isochrones and luminosity functions in the Johnson-Kron/Cousins and HST filter sets for systems containing old white dwarfs. These new models incorporate a non-grey atmosphere which is necessary to properly describe the effects of molecular opacity at the cool temperatures of old white dwarfs. The various functions calculated and extensively tabulated and plotted are meant to be as utilitarian as possible for observers so all results are listed in quantities that observers will obtain. The tables and plots developed should eventu…
Using a nearest neighbor analysis, we construct a sample of void galaxies from the Sloan Digital Sky Survey (SDSS) and compare the photometric properties of these galaxies to the population of non-void (wall) galaxies. We trace the density field of galaxies using a volume-limited sample with z_{max}=0.089. Galaxies from the flux-limited SDSS with z\leq z_{max} and fewer than three volume-limited neighbors within 7h^{-1}Mpc are classified as void galaxies. This criterion implies a density contrast δρ/ ρ< -0.6 around void galaxies. From 155,000 galaxies, we obtain a sub-sample of 13,742 galaxies…
Ibata et al (1999) have recently discovered faint, moving objects in the Hubble Deep Field. The quantity, magnitudes and proper motions of these objects are consistent with old white dwarfs making up the Galactic dark halo. We review a number of ground-based proper motion surveys in which nearby dark halo white dwarfs might be present, if they have the colours and absolute magnitudes proposed. No such objects have been found, whereas we argue here that several times more would be expected than in the Hubble Deep Field. We conclude it is unlikely that hydrogen atmosphere white dwarfs make up a …
Young, massive star clusters are the most notable and significant end products of violent star-forming episodes triggered by galaxy collisions, mergers, and close encounters. The question remains, however, whether or not at least a fraction of the compact YMCs seen in abundance in extragalactic starbursts, are potentially the progenitors of globular cluster (GC)-type objects. However, because of the lack of a statistically significant sample of similar nearby objects we need to resort to either statistical arguments or to the painstaking approach of case by case studies of individual objects i…
We present the discovery of two new late-T dwarfs identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) Data Release 2 (DR2). These T dwarfs are nearby old T dwarfs along the line of sight to star-forming regions and open clusters targeted by the UKIDSS GCS. They are found towards the Alpha Per cluster and Orion complex, respectively, from a search in 54 square degrees surveyed in five filters. Photometric candidates were picked up in two-colour diagrams, in a very similar manner to candidates extracted from the UKIDSS Large Area Survey (LAS) but taking advan…
We revisit the shapes of isophotes for elliptical (E) and lenticular (S0) galaxies by studying 847 nearby early-type galaxies selected from the Sloan Digital Sky Survey Data Release 4 with velocity dispersions above 200 km/s. The IRAF task {\tt ellipse} was used to derive the deviations of the isophotes from pure ellipses (Fourier coefficients a_3/a and a_4/a), position angles and ellipticities as a function of radius. We show the statistical distributions of the a_3/a and a_4/a parameters as a function of velocity dispersion, magnitude, and colour. The a_4/a parameter is correlated with the e…
Taking advantage of an improved genetic optimization algorithm for fitting unconstrained Kepler orbits to the Hipparcos intermediate astrometric data, we obtain additional orbital solutions for 81 Hipparcos stars with previous stochastic (failed) solutions. The sample includes astrophysically interesting objects, including the old disk wide binary HIP 754, the nearby AGB star HIP 34922 (L2 Pup), and the nearby M2 dwarf HIP 5496 (GJ 54, at 8 pc from the Sun) which has a resolved M dwarf companion.
We present the discovery and spectroscopic identification of two very high proper motion ancient white dwarf stars, found in a systematic proper motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool Halo white dwarf WD0346+246. Canonical stellar halo models predict a white dwarf volume density of two orders of magnitude less than the approx 7*10^{-4} Solar masses per pc^{-3} inferred from this survey. With the caveat that the sample size is very small, it appears that …
This review aims to give a summary of our understanding of galaxy evolution as infered from studies of nearby galaxies; how observations made with the Hubble Space Telescope have contributed significantly to our detailed understanding of the older stellar populations in Local Group dwarf galaxies. Recent results from VLT are also promising interesting future prospects for the study of resolved stellar populations in nearby dwarf galaxies.
Using spectra of normal emission line galaxies from the First Data Release of the Sloan Digital Sky Survey (SDSS) we have investigated the relations between the extinction $C({\rm H}β)$ as derived from the \Ha/\Hb emission line ratio and various global parameters of the galaxies. Our main findings are that: 1) $C({\rm H}β)$ is linked with the galaxy spectral type and colour, decreasing from early- to late-type spirals. 2) $C({\rm H}β)$ increases with increasing metallicity. 3) $C({\rm H}β)$ is larger in galaxies with an older stellar population. 4) $C({\rm H}β)$ is larger for more luminous gal…
We present an analysis of the optical colors of 413 Virgo cluster early-type dwarf galaxies (dEs), based on Sloan Digital Sky Survey imaging data. Our study comprises (1) a comparison of the color-magnitude relation (CMR) of the different dE subclasses that we identified in Paper III of this series, (2) a comparison of the shape of the CMR in low and high-density regions, (3) an analysis of the scatter of the CMR, and (4) an interpretation of the observed colors with ages and metallicities from population synthesis models. We find that the CMRs of nucleated (dE(N)) and non-nucleated dEs (dE(nN…
Up to now, most planet search projects have concentrated on F to K stars. In order to considerably widen the view, we have stated a survey for planets of old, nearby brown dwarfs and very low mass stars. Using UVES, we have observed 26 brown dwarfs and very low mass stars. These objects are quite inactive and are thus highly suitable for such a project. Two objects were found to be spectroscopic binaries. Another object shows significant radial velocity variations. From our measurements, we conclude that this object either has a planetary-mass companion, or the variations are caused by surface…
