Showing papers by "European Southern Observatory published in 1996"

SExtractor: Software for source extraction

Abstract: We present the automated techniques we have developed for new software that optimally detects, deblends, measures and classifies sources from astronomical images: SExtractor ( Source Extractor ). We show that a very reliable star/galaxy separation can be achieved on most images using a neural network trained with simulated images. Salient features of SExtractor include its ability to work on very large images, with minimal human intervention, and to deal with a wide variety of object shapes and magnitudes. It is therefore particularly suited to the analysis of large extragalactic surveys.

The Hubble Deep Field: Observations, data reduction, and galaxy photometry

Abstract: The Hubble Deep Field (HDF) is a Director’s Discretionary program on HST in Cycle 5 to image an undistinguished field at high Galactic latitude in four passbands as deeply as reasonably possible. These images provide the most detailed view to date of distant field galaxies and are likely to be important for a wide range of studies in galaxy evolution and cosmology. In order to optimize observing in the time available, a field in the northern continuous viewing zone was selected and images were taken for ten consecutive days, or approximately 150 orbits. Shorter 1-2 orbit images were obtained of the fields immediately adjacent to the primary HDF in order to facilitate spectroscopic follow-up by ground-based telescopes. The observations were made from 18 to 30 December 1995, and both raw and reduced data have been put in the public domain as a community service. We present a summary of the criteria for selecting the field, the rationale behind the filter selection and observing times in each band, and the strategies for planning the observations to maximize the exposure time while avoiding earth-scattered light. Data reduction procedures are outlined, and images of the combined frames in each band are presented. Objects detected in these images are listed in a catalog with their basic photometric parameters.

Internal Dynamics of Globular Clusters

Abstract: Galactic globular clusters, which are ancient building blocks of our Galaxy, represent a very interesting family of stellar systems in which some fundamental dynamical processes have taken place on time scales shorter than the age of the universe. In contrast with galaxies, these clusters represent unique laboratories for learning about two-body relaxation, mass segregation from equipartition of energy, stellar collisions, stellar mergers, and core collapse. In the present review, we summarize the tremendous developments, as much theoretical as observational, that have taken place during the last two decades, and which have led to a quantum jump in our understanding of these beautiful dynamical systems.

H emission in pre-main sequence stars. I. An atlas of line profiles

Abstract: We present an atlas of very high resolution ( R ~ 50000) H α line profiles of 63 pre-main sequence stars, divided among 43 T Tauri stars, 18 Herbig Ae/Be stars, and 2 FU Orionis objects. H α emission is the most common and prominent spectroscopic feature of pre-main sequence stars, and although it is optically very thick it is still the most frequently modelled emission line in young stars. In T Tauri stars the principal models involve magnetically driven winds, and more recently the role of infalling magnetospheric material has been explored. For Herbig Ae/Be stars a variety of models have been proposed, current emphasis is directed towards obscuration by clumpy circumstellar disk structures. In order to provide constraints on such models, we have made a statistical analysis of the 63 high resolution profiles. We here ignore the considerable variability of the H α emission, which is discussed in detail in a second paper. Most of our observed lines show complex profiles due to an interplay between emission and absorption features, and we suggest a two-dimensional classification scheme to describe these line profiles, based on the relative height of a secondary peak to the primary peak, as well as whether the absorption is blue- or red-shifted. Among T Tauri stars, 25% have symmetric profiles, 49% have blueshifted absorption dips, and 5% have P Cygni profiles; the remaining 21% show a variety of redshifted absorptions. For Herbig Ae/Be stars symmetric lines are quite rare (11%), indeed almost all of these stars have deep and prominent central absorptions. We have measured the extent of the line wings for all of our stars at the I max /40 level, and find that almost all have very extended wings, with typical extents of ± 350 km/s, but in high spectra the wings can be traced to lower intensities, and velocities as high as ± 900 km/s have been observed. Pronounced asymmetries of these extended wings are found for many stars, suggesting the possibility that the highest velocity material could be non-uniformly distributed. The equivalent widths of the H α emission in our sample of stars span two orders of magnitude, with a distribution that increases with decreasing equivalent width.

Decrease in the space density of quasars at high redshift

Abstract: QUASARS have been found up to redshifts of almost five, corresponding to an epoch when the Universe was less than ten per cent of its present age. This leaves a rather short time for the formation of the galaxies in which quasars are believed to be embedded. Indeed, some optical studies1–3 indicate that the space density of quasars does decline rapidly for redshifts z > 3, as expected if this is the epoch of galaxy formation. The interpretation of this decline is equivocal, however, as it could result simply from the obscuration of distant quasars by dust in intervening galaxies4. Radio emission from quasars, on the other hand, is unaffected by dust, and we show here that the space density of radio-loud quasars also decreases strongly forz > 3, demonstrating that the decline is real, at least for these objects. We argue that this conclusion probably applies to all quasars. If quasars are associated with galaxy formation and/or interactions between galaxies, the decline in their space density at high redshift provides a measure of the timescale for the onset of these processes.

The properties of the peculiar type Ia supernova 1991bg — I. Analysis and discussion of two years of observations

Abstract: Observations of the peculiar type Ia SN 1991bg in NGC 4374 collected at ESO–La Silla and Asiago are presented and discussed. The photometric coverageextends for 530 days and the spectroscopy for the first seven months after the explosion. The broad– band light curves in the early months have a narrower peak and a luminosity decline faster than other SNIa (14.6 and 11.7 mag×(100d) 1 in B and V respectively). The R and I light curves do not show the secondary peak typical of normal SNIa. The SN is intrinsically very red ((B V )max = 0.74) and faint (Bmax = 16.54). The light curves flatten with age but remain significantly steeper (2.0 and 2.7 mag×(100d) 1 in B and V between 70 and 200 day) than the average. Consequently the uvoir bolometric light curve of SN 1991bg is fainter with a steeper decline than that of the normal SNIa (e.g. 1992A). This object enhances the correlation which exists between the peak luminosity of SNIa, the decline rate and the kinetic energy. Peculiarities are evident in the spectra at various phases. The continuum at maximum is very red and the photospheric expansion velocity extremely low. There are a number of unusual spectral features, in particular a broad absorption between 4200 and 4500 u which is attributed to TiII and the appearance, as early as one month after maximum, of nebular emission of possibly [CoIII] �5890 5908. Nevertheless, contrary to previous claims (Ruiz-Lapuente et al 1993), the spectral evolution retains a general resemblance to that of other SNIa until the latest available observation (day 203). At this epoch one sees the typical emission features of SNIa at late times although they are significantly narrower (FWHM� 2300 km s 1 ). This facilitates the identification of most lines with forbidden emission of [FeII], [FeIII] and [CoIII]. The emission feature centered at about �6590 is difficult to reconcile with the previous identification as H�, unless asymmetries in the ejecta or ad hoc binary configurations are invoked. This work suggests that the explosion energy was probably a factor 3 to 5 lower than in normal SNIa. Whether this resulted from an explosion of a sub–Chandrasekhar mass WD is not unambiguously established.

The tilt of the fundamental plane of elliptical galaxies — I. Exploring dynamical and structural effects

Abstract: In this paper we explore several structural and dynamical effects on the projected velocity dispersion as possible causes of the fundamental plane (FP) tilt of elliptical galaxies. Specifically, we determine the size of the systematic trend along the FP in the orbital radial anisotropy, in the dark matter (DM) content and distribution relative to the bright matter, and in the shape of the light profile that would be needed to produce the tilt, under the assumption of a constant stellar mass to light ratio. Spherical, non rotating, two--components models are constructed, where the light profiles resemble the $R^{1/4}$ law. For the investigated models anisotropy cannot play a major role in causing the tilt, while a systematic increase in the DM content and/or concentration may formally produce it. Also a suitable variation of the shape of the light profile can produce the desired effect, and there may be some observational hints supporting this possibility. However, fine tuning is always required in order to produce the tilt, while preserving the {\it tightness} of the galaxies distribution about the FP.

The White Dwarf Distance to the Globular Cluster NGC 6752 (and Its Age) with the HUBBLE SPACE TELESCOPE

Abstract: Deep Hubble Space Telescope (HST) observations with WFPC2 of the nearby globular cluster NGC 6752 have allowed us to obtain accurate photometry for the cluster white dwarfs (WDs). A sample of local WDs of known trigonometric parallax and mass close to that of the cluster WDs have also been observed with WFPC2. Matching the cluster and the local WD sequences provides a direct measure of the distance to the cluster: (m - M)0 = 13.05, with an uncertainty less than ±0.1 mag, which allows a substantial reduction in the uncertainty in the age of the cluster. Indeed, coupling this value of the cluster distance to the cluster metallicity, helium abundance, and α-element enhancement [α/Fe] = 0.5 yields an age of 15.5 and 14.5 Gyr using evolutionary models that do not include or do include helium diffusion, respectively. The uncertainty affecting these age determinations is ~10%. The majority of the cluster WDs appear to be of the DA variety, while the color-magnitude location of two WDs is consistent with the DB type. This suggests a cluster DB/DA ratio similar to that of WDs in the solar neighborhood.

The Kinematics of the Planetary Nebulae in the Outer Regions of NGC 4406

Abstract: We have measured the velocities of 19 planetary nebulae (PNs) in the outer regions of the giant early-type galaxy NGC 4406 in the Virgo cluster. The kinematics of the outermost regions, at a mean radius of 142" or 11 kpc, show rapid rotation and a low observed velocity dispersion (~ 166 and 96 km s−1, respectively) , compared with the central velocity dispersion of about 210 km s−1. This supports the classification of this galaxy as an S0. Although this galaxy has a systemic velocity of -227 km s−1, we found three PNs with velocities close to the mean velocity of the Virgo cluster (about 1400 km s−1). We argue that these three PNs may be members of an intracluster stellar population in the Virgo cluster.

Biasing in the Galaxy Distribution

Abstract: We investigate the variation of galaxy clustering with luminosity using the recently completed SSRS2 sample. Clustering measurements based on the two-point correlation function and the variance of counts in cells reveal a strong dependence of clustering on luminosity for galaxies brighter than L*, while no significant variation is detected for fainter galaxies. We derive a relative bias versus magnitude relation that can be compared with theoretical predictions. Existing models of galaxy formation cannot reproduce adequately the simultaneous steep rise of biasing at high luminosities and the plateau at the low-luminosity end. Improved modeling of halo-galaxy relation and larger samples including low-luminosity galaxies are required to draw more definitive conclusions.

A Burst of Herbig-Haro Flows in NGC 1333

Abstract: We report the discovery of over 20 groups of new Herbig-Haro (HH) objects in the NGC 1333 region of the Perseus molecular cloud, including some highly collimated jets. Our images contain over 30 groups of HH objects driven by over a dozen active outflow sources. Several of the new jets appear to be driven by optically visible stars, including HH 333, HH 334, and possibly HH 335 and HH 336. A spectacular jet, HH 333, lies nearly in the plane of the sky and has a length-to-width ratio exceeding 100 and may exhibit S-shaped point symmetry about a faint Hα emission-line star. HH 336 is located toward the cloud edge and is also centered on a visible star. A large number of new HH objects lie to the south of the concentration of known young stellar objects near SVS 13. We use published millimeter-wavelength CO and near-infrared H2 maps and images to associate HH objects, H2 emitting shocks, and CO outflows with more than a dozen potential driving sources. The high density of objects results in source confusion that limits the extent to which this can be done. Some HH objects are seen toward low-extinction regions far from the opaque cloud cores and may trace parts of parsec scale outflows from embedded sources. The large number of collisionally excited nebulae in this young stellar cluster requires a nearly coeval microburst of star formation.

Atlas of high-resolution emission and shell lines in Be stars. Line profiles and short-term variability

Abstract: We present an atlas of high-S=N , high-resolution (v 6k m s 1 ) data of Be star emission and shell proles. We have collected proles of H and of Fe ii ,m ostly of the5317 transition. These lines have been selected to provide measures for the overall emission strength and for the velocity eld in these disks. We have collected data for 77 southern and equatorial programme stars, covering the period 1982-1993. This is the most comprehensive overview of prole shapes in Be disks. We propose a three-dimensional scheme in which most observed proles can be classied. The parameters are i) inclination, ii) optical depth, and iii) the pattern of the velocity eld. A search for short-term variability (timescales between ve days and a few minutes) in six stars ended with negative result. Shortest observed timescale for variability is a few days for well-developed disks in binary systems (HR 1910, HR 2142).

Discovery of 800 HZ Quasi-Periodic Oscillations in 4U 1608-52

Abstract: We present results of Rossi X-Ray Timing Explorer observations of the low-mass X-ray binary and atoll source 4U 1608-52 made over 9 days during the decline of an X-ray intensity outburst in 1996 March. A fast-timing analysis shows a strong and narrow quasi-periodic oscillation (QPO) peak at frequencies between 850 and 890 Hz on March 3 and 6, as well as a broad peak around 690 Hz on March 9. Observations on March 12 show no significant signal. On March 3, the X-ray spectrum of the QPO is quite hard; its strength increases steadily from 5% at ~2 keV to ~20% at ~12 keV. The QPO frequency varies between 850 and 890 Hz on that day, and the peak widens and its rms decreases with centroid frequency in a way very similar to the well-known horizontal branch oscillations (HBO) in Z sources. We apply the HBO beat frequency model to atoll sources and suggest that, whereas the model could produce QPOs at the observed frequencies, the lack of correlation we observe between QPO properties and X-ray count rate is hard to reconcile with this model.

Infrared aperture photometry at ESO (1983-1994) and its future use.

Abstract: We describe the infrared (IR) photometric system for the single channel photometers at ESO, which have been used from 1983 until 1994. In addition to the broadband near infrared (NIR, 1 5 m) photometric system presented in 1991 by Bouchet et al. and Bersanelli et al., we describe a narrow-band NIR photometric system and a mid infrared (MIR, 7 20 m) photometric system. We also extend the set of NIR standard stars by Bouchet et al. towards fainter objects (K'9). The photometric data of the standard stars in these systems were extracted from the complete IR photometric data archive of ESO, covering 10 years. The zeropoints of the NIR photometry are set by assuming that HR 3314 has a V -magnitude of 3.89, and that V K= 0.05, J K= 0.01, H K= 0.01, K L 0 =0.00, K M=0.00. The zeropoints of the MIR photometry are set by assuming that the colours of Hyi (HR 0098) and CenA (HR 5459) are equal to the colours of the Sun. We adopt the absolute calibration of M egessier (1995) for the NIR and we argue that this calibration can be extrapolated to 20 m, using the MIR calibrations by Rieke et al. (1985) and Cohen et al. (1992). The denition of the zeropoints is consistent with the absolute calibration. We obtained accurate (' 0.02 mag.) NIR photometry of about 240 standard stars and MIR photometry of about 40 standard stars (' 0.04 mag). Comparison of our NIR photometric system with other well established systems shows that there are some small colour dependencies and zeropoint osets which are always smaller than about 0.02 mag. except for the L 0 band.

Sakurai's Object—A Possible Final Helium Flash in a Planetary Nebula Nucleus

Abstract: UBVRIJK photometry and optical spectroscopy of Sakurai's object and direct imaging of the surrounding nebulosity are presented. The progenitor star is identified as a faint blue object of mj = 21. The circular planetary nebula has a diameter of 32'', its distance, derived from the Hβ flux, is 5.5 kpc. At present, the object is a luminous supergiant of approximate spectral type F2, having MV = -4.1, and has clear indications of hydrogen underabundance, with carbon and oxygen overabundance. Its photospheric radial velocity is the same as that of the surrounding planetary nebula, indicating that they are associated, and that there is currently no mass loss by a thick wind. A marginal IR excess suggests the presence of some hot dust.

Time Dilation in the Light Curve of the Distant Type Ia Supernova SN 1995K

Abstract: The light curve of a distant Type Ia supernova acts like a clock that can be used to test the expansion of the universe. SN 1995K, at a spectroscopic redshift of z = 0.479, provides one of the first meaningful data sets for this test. We find that all aspects of SN 1995K resemble local Type Ia supernova events when the light curve is dilated by (1 + z), as prescribed by cosmological expansion. In a static, nonexpanding universe, SN 1995K would represent a unique object with a spectrum identifying it as a regular Type Ia supernova but with a light-curve shape and luminosity that do not follow the well-established relations for local events. We conclude that SN 1995K provides strong evidence for an interpretation of cosmological redshifts as being due to universal expansion. Theories in which photons dissipate their energy during travel are excluded as are age-redshift dependencies.

Time Dilation in the Light Curve of the Distant Type Ia Supernovae SN 1995K

Abstract: The light curve of a distant Type Ia supernova acts like a clock that can be used to test the expansion of the Universe. SN~1995K, at a spectroscopic redshift of $z = 0.479$, provides one of the first meaningful data sets for this test. We find that all aspects of SN~1995K resemble local supernova Ia events when the light curve is dilated by $(1+z)$, as prescribed by cosmological expansion. In a static, non-expanding universe SN~1995K would represent a unique object with a spectrum identifying it as a regular Type Ia supernova but a light curve shape and luminosity which do not follow the well-established relations for local events. We conclude that SN~1995K provides strong evidence for an interpretation of cosmological redshifts as due to universal expansion. Theories in which photons dissipate their energy during travel are excluded as are age-redshift dependencies.

Dwarf Galaxies Also Have Stellar Halos

Abstract: We present evidence for the existence of an old stellar halo in the dwarf irregular galaxy WLM, an isolated member of the Local Group. The halo consists of Population II stars, with low metallicities and age ≥1010 yr. The finding of a halo in a dwarf irregular galaxy argues for a generic mode of galaxy formation that requires a halo in the presence of a disk, regardless of galaxy size. This implies that formation mechanisms are similar along the spiral Hubble sequence, and favors the scenarios where the formation takes place during the original collapse and accretion of the protogalactic gas clouds. Halo formation also appears not necessarily to be related to the presence of a bulge, or a nucleus, since WLM lacks both of these components.

Biasing in the Galaxy Distribution

Abstract: We investigate the variation of galaxy clustering with luminosity using the recently completed SSRS2 sample. Clustering measurements based on the two-point correlation function and the variance of counts in cells reveal the existence of a strong dependence of the clustering amplitude on luminosity for galaxies brighter than L*, while no significant variation is detected for fainter galaxies. We derive a relative bias versus magnitude relation which can be compared with theoretical predictions. Existing models of galaxy formation cannot adequately reproduce the simultaneous steep rise of biasing at high luminosities and the plateau at the low-luminosity end. Improved modeling of the halo-galaxy relation and larger samples including low luminosity galaxies are required to draw more definitive conclusions.

A month in the life of NGC 4151: velocity-delay maps of the broad-line region

Abstract: We analyse continuum and emission-line variability in NGC 4151 based on IUE spectra taken with nearly complete I-d sampling over a 35-d interval during 1991 November­ December. The 1320-A continuum undertook a bumpy exponential decline by a factor -3 over 25 d, and then made a dramatic recovery in -2 d near the end of the campaign. The same pattern of variations occurred in all continuum bands with an amplitude that decreases with wavelength out to 3000 A. Cross-correlations reveal emission-line variations lagging behind the continuum variations. The red wing, core and blue wing of the C IV emission line lag by 1.8, 4.4 and 3.2d respectively. Maximum entropy fits to continuum light curves and line profile variations yield more detailed velocity-delay maps of the CIV and Herr responses at delays from 0 to 20 d. The line maps generally decline from a peak at 0 delay, implying bigh­ velocity gas inside 1 light-day. The velocity-delay structure of CIV is asymmetric, with a strong red wing at T :5 2 d, and a faint blue wing at T :5 10 d. This suggests inflow in the bigh­ velocity inner regions, but might also be interpreted as outflow combined with an inward anisotropy. The C IV velocity range decreases with delay in a way that suggests vitial motions around a mass of _107 M0 .

Herbig-Haro Jets, CO Flows, and CO Bullets: The Case of HH 111

Abstract: We have carried out high spatial resolution, 12'', CO J = 2-1 observations at the IRAM 30 m telescope of the molecular outflow in the HH 111 jet complex. The Herbig-Haro jet is found to coincide with a highly collimated CO flow, with two distinct velocities, possibly providing kinematic evidence that the CO flow surrounds the HH jet. A second well-defined bipolar molecular flow, at large angles to the principal flow axis, coincides with the HH 121 infrared flow that emanates from the (presumably binary) VLA driving source; the region thus harbors one of the rare quadrupolar molecular flows. Extremely high velocity CO is found toward the principal HH working surface at the same velocity as the optically emitting gas, whereas this emission is weak toward the Herbig-Haro jet. Since the inclination of the HH jet is known from optical observations to be 10° to the plane of the sky, we conclude that there is CO in the flow with space velocities of up to 500 km s-1! Further out, and precisely along the flow axis, we have discovered three equidistant CO bullets with space velocities of about 240 km s-1, which are not detected in the optical. We interpret these bullets as the result of earlier eruptions from the energy source that are now moving through an ambient medium so tenuous that no observable shock interaction takes place. Finally, we discuss the physical relation between the Herbig-Haro jet, the CO bullets, and the low-velocity molecular outflow. We favor the view that HH jets and CO bullets, which represent different manifestations of the same physical phenomena, are driving the low-velocity molecular outflow.

Obscured asymptotic giant branch stars in the Magellanic Clouds — II. Near-infrared and mid-infrared counterparts

Abstract: We have carried out an infrared search for obscured asymptotic giant branch (AGB) stars in the Magellanic Clouds. Fields were observed in the vicinity of IRAS sources with colours and flux densities consistent with such a classification. The survey uncovered a number of obscured AGE stars as well as some supergiants with infrared excess. We present photometry of the sources and discuss the colour diagrams and bolometric luminosities. One of the supergiants is close to the maximum luminosity allowed for red supergiants, implying a progenitor mass around 50 M.. Its late spectral type (M7.5) is surprising for such a massive star. Most of the AGE stars are luminous, often dose to the classical limit of M(bol)=-7.1. To determine whether the stars are oxygen-rich or carbon-rich, we have acquired narrow-band mid-infrared photometry with the ESO TIMMI camera for several sources. All but one are found to show the silicate feature and therefore to have oxygen-rich dust; the colours of the remaining source are consistent with either an oxygen-rich or a carbon-rich nature. A method to distinguish carbon and oxygen stars based on H-K versus K-[12] colours is presented. We discuss several methods of calculating the mass-loss rate: for the AGE stars the mass-loss rates vary between approximately 5 x 10(-4) and 5 x 10(-6) M. yr(-1), depending on the assumed dust-to-gas mass ratio. We present a new way to calculate mass-loss rates from the OH maser emission, We find no evidence for a correlation of the mass-loss rates with luminosity in these obscured stars. Nor do the mass-loss rates for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) stars differ in any clear systematic way from each other. Expansion velocities appear to be slightly lower in the LMC than in the Galaxy, Period determinations are discussed for two sources: the periods are comparable to those of the longer-period Galactic OH/IR stars. All of the luminous stars for which periods are available have significantly higher luminosities than predicted from the period-luminosity relations.

The Mass distribution in the nearby universe

Abstract: We present a new reconstruction of the mass density and the peculiar velocity fields in the nearby universe using recent measurements of Tully-Fisher distances for a sample of late spirals. We find significant differences between our reconstructed fields and those obtained in earlier work: overdensities tend to be more compact while underdense regions, consisting of individual voids, are more abundant. Our results suggest that voids observed in redshift surveys of galaxies represent real voids in the underlying matter distribution. While we detect a bulk velocity of ~300 km s-1, within a top-hat window 6000 km s-1 in radius, the flow is less coherent than previously claimed, exhibiting a bifurcation toward the Perseus-Pisces and the Great Attractor complexes. This is the first time that this feature is seen from peculiar velocity measurements. The observed velocity field resembles, more closely than any previous reconstruction, the velocity field predicted from self-consistent reconstructions based on all-sky redshift surveys. This better match is likely to affect estimates of the parameter β = Ω0.6/b and its uncertainty based on velocity-velocity comparisons.

A binary lensing event toward the LMC: Observations and dark matter implications

Abstract: The MACHO collaboration has recently analyzed 2.1 years of photometric data for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis has revealed 8 candidate microlensing events and a total microlensing optical depth of τ meas = 2.9 −0.9 +1.4 × 10 −7 . This significantly exceeds the number of events (1.1) and the microlensing optical depth predicted from known stellar populations: τ back = 5.4 × 10 −8 , but it is consistent with models in which about half of the standard dark halo mass is composed of Machos of mass ~ 0.5M ⊙ . One of these 8 events appears to be a binary lensing event with a caustic crossing that is partially resolved, and the measured caustic crossing time allows us to estimate the distance to the lenses. Under the assumption that the source star is a single star and not a short period binary, we show that the lensing objects are very likely to reside in the LMC. However, if we assume that the optical depth for LMC-LMC lensing is large enough to account for our entire lensing signal, then the binary event does not appear to be consistent with lensing of a single LMC source star by a binary residing in the LMC. Thus, while the binary lens may indeed reside in the LMC, there is no indication that most of the lenses reside in the LMC.

The Mass Distribution in the Nearby Universe

Abstract: We present a new reconstruction of the mass density and the peculiar velocity fields in the nearby universe using recent measurements of Tully-Fisher distances for a sample of late spirals. We find significant differences between our reconstructed fields and those obtained in earlier work: overdensities tend to be more compact while underdense regions, consisting of individual voids, are more abundant. Our results suggest that voids observed in redshift surveys of galaxies represent real voids in the underlying matter distribution. While we detect a bulk velocity of $\sim$ 300 \kms, within a top-hat window 6000 \kms in radius, the flow is less coherent than previously claimed, exhibiting a bifurcation towards the Perseus-Pisces and the Great Attractor complexes. This is the first time that this feature is seen from peculiar velocity measurements. The observed velocity field resembles, more closely than any previous reconstruction, the velocity field predicted from self-consistent reconstructions based on all-sky redshift surveys. This better match is likely to affect estimates of the parameter $\beta = \Omega^{0.6}/b$ and its uncertainty based on velocity-velocity comparisons.

The Host Galaxies of IRAS-selected Quasi-stellar Objects

Abstract: We present Hubble Space Telescope (HST) images of three QSOs selected on the basis of their IRAS properties. The data were taken with the Planetary Camera primarily in order to examine the host galaxies. All three QSOs appear embedded in spectacular interactions between two or more luminous galaxies, probably spirals. We discuss the evolutionary connection, if any, between these three objects and the far more numerous ultraluminous infrared galaxies. We argue that these three objects are probably young and therefore do not fit a scenario in which QSOs emerge only in the later stages of an interaction when most of the dust has been blown away. It may be that we are simply viewing them from a fortuitous angle that allows a clear view into the cores.