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We report the discovery of two concentric Einstein rings around the gravitational lens SDSS J0946+ 1006. The main lens is at redshift z(l) = 0.222, while the inner ring ( 1) is at redshift z(s1) 0.609 (R-Ein1 = 1.43 '' +/- 0.01 ''). The wider image separation ( R-Ein2 = 2.07 '' +/- 0.02 '') of the outer ring (2) implies a higher redshift than that of ring 1; the detection of ring 2 in the F814WACS filter implies an upper limit of z(s2) less than or similar to 6.9. The main lens can be described by a power- law total mass density profile rho(tot) proportional to r(-gamma ') with gamma ' = 2.00 +/- 0.03 and velocity dispersion sigma(SIE) = 287 +/- 5 km s(-1) ( the stellar velocity dispersion is sigma(v,*) = 284 +/- 24 km s(-1)). The strong lensing configuration is inconsistent with light traces mass. Adopting a prior on the stellar mass- to- light ratio from previous SLACS work, we infer a 73% +/- 9% dark matter fraction within the cylinder of radius equal to the effective radius of the lens. We find that, for the case of SDSS J0946+ 1006, the geometry of the two rings does not place interesting constraints on cosmography because of the suboptimal redshifts of lens and sources. We then consider the perturbing effect of the mass associated with ring 1 building a compound lens model. This introduces minor changes to the mass of the main lens and provides an estimate of z(s2) = 3.1(-1.0)(+2.0) and of the mass of the source responsible for ring 1 (sigma(SIE,s1) = 94(-47)(+27) km s(-1)). We conclude by examining the prospects of doing cosmography with a sample of 50 double rings, expected from future space- based surveys. Accounting for uncertainties in the mass profile of the lens and the effects of the perturber, we find that such a sample would constrain Omega(m) and within 10%, assuming flatness.

We present H I observations of the edge-on galaxy NGC 891. These are among the deepest ever performed on an external galaxy. They reveal a huge gaseous halo, much more extended than seen previously and containing almost 30% of the H I. This H I halo shows structures on various scales. On one side, there is a filament extending ( in projection) up to 22 kpc vertically from the disk. Small (M(H I) greater than or similar to 10(6) M(circle dot)) halo clouds, some with forbidden (apparently counterrotating) velocities, are also detected. The overall kinematics of the halo gas is characterized by differential rotation lagging with respect to that of the disk. The lag, more pronounced at small radii, increases with height from the plane. There is evidence that a significant fraction of the halo is due to a galactic fountain. Accretion from inter-galactic space may also play a role in building up the halo and providing the low angular momentum material needed to account for the observed rotation lag. The long H I filament and the counterrotating clouds may be direct evidence of such accretion.

Context. The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in Ha emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. Aims. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in Ha emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. Methods. By comparing large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. Results. From the 24 mu m emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., similar to 30%. The additional asymmetry in Ha is modeled as additional symmetric dust attenuation which extends up to similar to 40 '' (1.9 kpc) above the plane of the galaxy with a mid-plane value of tau = 0.8 and a scale height of 0.5 kpc

We present Monte Carlo calculations of Ly alpha radiative transfer through optically thick, spherically symmetric, collapsing gas clouds. These represent simplified models of protogalaxies in the process of their assembly. Such galaxies produce Ly alpha flux over an extended solid angle, either from a spatially extended Ly alpha emissivity, or from scattering effects, or both. We present a detailed study of the effect of the gas distribution and kinematics and of the Ly alpha emissivity profile on the emergent spectrum and surface brightness distribution. The emergent Ly alpha spectrum is typically double peaked and asymmetric. In practice, however, we find energy transfer from the infalling gas to the Ly alpha photons to significantly enhance the blue peak and to render the red peak, in most cases, undetectable. The resulting effective blueshift, combined with scattering in the intergalactic medium, renders extended Ly alpha emission from collapsing protogalaxies difficult to detect beyond redshift z greater than or similar to 4. We find a strong wavelength dependence of the slope of the surface brightness distribution (with preferential flattening at the red side of the line) to be a robust indication that Ly alpha photons are being generated (rather than just scattered) in a spatially extended, collapsing region around the galaxy. For self-ionized clouds whose effective Ly alpha optical depth is less than or similar to 10(3), infall and outflow models can produce nearly identical spectra and surface brightness distributions and are practically indistinguishable. The presence of cosmic abundance of deuterium may produce a detectable dip in the spectra of systems with moderate hydrogen column densities, N(H) = 10(18)-10(20) cm(-2). Finally, we present a new analytic solution for the emerging Ly alpha spectrum in the limiting case of a static uniform sphere, extending previous solutions for slabs.

We model the spectra and surface brightness distributions for the Ly alpha radiation expected from protogalaxies that are caught in the early stages of their assembly. We use the results of a companion paper to characterize the radiation emerging from spherically collapsing gas clouds. We then modify these spectra to incorporate the effect of subsequent resonant scattering in the IGM. Using these models, we interpret a number of recent observations of extended Ly alpha blobs (LABs) at high redshift. We suggest, based on the angular size, energetics, relatively shallow surface brightness profiles, and double-peaked spectra, that several of these LABs may be associated with collapsing protogalaxies. We suggest two follow-up observations to diagnose the presence of gas infall. High-S/N spectra of LABs should reveal a preferential flattening of the surface brightness profile at the red side of the line. Complementary imaging of the blobs at redshifted H alpha wavelengths should reveal the intrinsic Ly alpha emissivity and allow its separation from radiative transfer effects. We show that Ly alpha scattering by infalling gas can reproduce the observed spectrum of the Steidel et al. LAB2 as accurately as a recently proposed outflow model. Finally, we find similar evidence for infall in the spectra of pointlike Ly alpha emitters. The presence of scattering by the infalling gas implies that the intrinsic Ly alpha luminosities and derived quantities, such as the star formation rate, in these objects may have been underestimated by about an order of magnitude.

We investigate HST/ACS and WFPC2 images at the positions of five candidate microlensing events from a large survey of variability in M31 (MEGA). Three closely match unresolved sources, and two produce only flux upper limits. All are confined to regions of the color-magnitude diagram where stellar variability is unlikely to be easily confused with microlensing. Red variable stars cannot explain these events (although background supernovae are possible for two). If these lenses arise in M31's halo, they are due to masses 0.15

We investigate HST/ACS and WFPC2 images at the positions of five candidate microlensing events from a large survey of variability in M31 (MEGA). Three closely match unresolved sources, and two produce only flux upper limits. All are confined to regions of the color-magnitude diagram where stellar variability is unlikely to be easily confused with microlensing. Red variable stars cannot explain these events (although background supernovae are possible for two). If these lenses arise in M31's halo, they are due to masses 0.15

We report results from intensive monitoring of two fields on either side of M31,emphasizing microlensing involving stars and masses in M31. These results stem from the three-year VATT/Columbia survey of variability on 3d to 2m timescales. Observations were conducted intensively from 1997-1999, with baselines 1995-now,at the Vatican Advanced Technology Telescope, MDM 1.3-meter, and Isaac Newton telescopes, totaling about 200 epochs. The two fields cover 560 square arcmin total, along the minor axis on either side of M31. Candidate microlensing events are subject to tests to distinguish microlenses from variable stars. Fourprobable microlensing events total, compared to carefully computed event rate and efficiency models, indicate a marginally significant microlensing activity above expectations for the stellar lenses alone in M31 (and the Galaxy) acting as lenses. Maximum likelihood analyses of the event distribution in timescale and across the face of M31 indicates a microlensing dark matter halo fraction consistent with that seen in our Galaxy towards the LMC (Alcock et al. 2000). For a nearly singular isothermal sphere model, we find a microlensing halo mass fraction f=0.29 +0.30/-0.13 of total dark matter, and a poorly constrained lensing component mass 0.02 to 1.5 solar mass, 1 sigma limits). This study is the prototype for a larger one approaching completion; between them there is significant evidence for asymmetry in the distribution of microlensing events across the face of M31, and a large population of halo microlensing objects. (Abridged), Comment: 25 pages, LaTex; 11 figures. Revised, refereed manuscript resubmitted to The Astrophysical Journal. Corrected Figure 8 and other minor points

We have completed an intensive monitoring program of two fields on either side of the center of M31 and report here on the results concerning microlensing of stars in M31. These results stem from a 3 yr study ( the Vatican Advanced Technology Telescope [VATT]/Columbia survey) of microlensing and variability of M31 stars, emphasizing microlensing events of 3 day to 2 month timescales and likely due to masses in M31. These observations were conducted intensively from 1997 to 1999, with baselines 1995 to present, at the VATT and the 1.3 m telescope at MDM Observatory, with additional data from the Isaac Newton Telescope, including about 200 epochs total. The two fields monitored cover 560 arcmin(2) total, positioned along the minor axis on either side of M31. Candidate microlensing events are subject to a number of tests discussed here with the purpose of distinguishing microlensing from variable star activity. A total of four probable microlensing events, when compared with carefully computed event rate and efficiency models, indicate a marginally significant microlensing activity above that expected for the stars alone in M31 ( and the Galaxy) acting as lenses. A maximum likelihood analysis of the distribution of events in timescale and across the face of M31 indicates a microlensing dark matter halo fraction consistent with that seen by MACHO in our Galaxy toward the Large Magellanic Cloud. Specifically, for a nearly singular isothermal sphere model, we find a microlensing halo mass fraction fb = 0.29(-0.13)(+0.30) of the total dark matter and a poorly constrained lensing component mass (0.02 - 1.5 M-.; 1 sigma limits). This study serves as the prototype for a larger study approaching completion; between the two there is significant evidence for an asymmetry in the distribution of microlensing events across the face of M31 and therefore a large population of halo microlensing dark matter objects.

We present a study of the kinematics of the ionised gas in the nearby spiral galaxy NGC 2403 using deep long-slit spectra obtained with the 4.2-m William Herschel Telescope. The data show the presence of a halo component of ionised gas that is rotating more slowly than the gas in the disk. The kinematics of this ionised halo gas is similar to that of the neutral halo gas. On small scales, broad line profiles (up to 300 km/s wide) indicate regions of fast outflows of ionised gas. We discuss these new results in the context of galactic fountain models., 17 pages, 10 figures, accepted for publication in A&A

We report the discovery of significant stellar substructure in the halo and outer disk of our nearest large galactic neighbor, M31. Our deep panoramic survey with the Isaac Newton Telescope Wide Field Camera currently maps out an area of approximate to25 deg(2) around M31, extending along the semimajor axis to 55 kpc and is the first to allow an uninterrupted study of the density and color distribution of individual red giant branch stars across a large fraction of the halo of an external spiral galaxy. We find evidence for both spatial density and metallicity (as inferred from color information) variations, which are often, but not always, correlated. In addition to the previously reported giant stellar stream, the data reveal the presence of significant stellar over-densities at large radii close to the southwestern major axis, in the proximity of the very luminous globular cluster G1, and near the northeastern major axis, coinciding with and extending beyond the previously known northern spur. The most prominent metallicity variations are found in the southern half of the halo, where two large structures with above average metallicites are apparent; one of these coincides with the giant stellar stream while the other corresponds to a much lower level stellar enhancement. Our findings contrast with, but do not conflict with, past studies of the M31 halo and outer disk that have suggested a rather homogeneous stellar population at large radius: the bulk of our newly detected substructure lies in the previously uncharted far outer regions of the galaxy. We discuss the possible origin of the substructure observed and the implications it has for constraining the galaxy assembly process.

We report the discovery of significant stellar substructure in the halo and outer disk of our nearest large galactic neighbour, M31. Our deep panoramic survey with the Isaac Newton Telescope Wide Field Camera currently maps out an area of ~25 square degrees around M31, extending along the semi-major axis to 55 kpc, and is the first to allow an uninterrupted study of the density and color distribution of individual red giant branch stars across a large fraction of the halo of an external spiral galaxy. We find evidence for both spatial density and metallicity (as inferred from colour information) variations, which are often, but not always, correlated. In addition to the previously reported giant stellar stream (Ibata 2001b), the data reveal the presence of significant stellar overdensities at large radii close to the south-western major axis, in the proximity of the very luminous globular cluster G1, and near the north-eastern major axis, coinciding with and extending beyond the previously-known `northern spur'. The most prominent metallicity variations are found in the southern half of the halo, where two large structures with above average metallicites are apparent; one of these coincides with the giant stellar stream while the other corresponds to a much lower-level stellar enhancement. Our findings contrast with, but do not conflict with, past studies of the M31 halo and outer disk which have suggested a rather homogeneous stellar population at large radius: the bulk of our newly-detected substructure lies in the previously-uncharted far outer regions of the galaxy. We discuss the possible origin of the substructure observed and the implications it has for constraining the galaxy assembly process., Comment: 20 pages, 10 figures. Accepted for publication in AJ (Sep 2002). Version with high resolution embedded figures available at http://www.astro.rug.nl/~ferguson/m31.ps.gz

High-sensitivity H I observations of the nearby spiral galaxy NGC 2403 obtained with the VLA are presented and discussed. The properties of the extended, differentially rotating H I layer with its H I holes, spiral structure, and outer warp are described. In addition, these new data reveal the presence of a faint, extended, and kinematically anomalous component. This shows up in the H I line profiles as extended wings of emission toward the systemic velocity. In the central regions these wings are very broad (up to 150 km s(-1)) and indicate large deviations from circular motion. We have separated the anomalous gas component from the cold disk and have obtained for it a separate velocity field and a separate rotation curve. The mass of the anomalous component is 1/10 of the total H I mass. The rotation velocity of the anomalous gas is 25-50 km s(-1) lower than that of the disk. Its velocity field has nonorthogonal major and minor axes that we interpret as due to an overall inflow motion of 10-20 km s(-1) toward the center of the galaxy. The picture emerging from these observations is that of a cold H I disk surrounded by a thick and clumpy H I layer characterized by slower rotation and inflow motion toward the center. The origin of this anomalous gas layer is unclear. It is likely, however, that it is related to the high rate of star formation in the disk of NGC 2403 and that its kinematics is the result of a galactic fountain type of mechanism. We suggest that these anomalous H I complexes may be analogous to a part of the high-velocity clouds of our Galaxy.

We discuss new, high-sensitivity H I observations of the spiral galaxy NGC 2403, which show extended emission at anomalous velocities with respect to the "cold" disk. This "anomalous" gas component (similar to1/10 of the total H I mass) is probably located in the region of the halo and rotates more slowly (similar to 20-50 km s(-1)) than the gas in the disk. Moreover, it shows a distortion in the velocity field that we interpret as a large-scale radial motion (10-20 km s(-1) inflow) toward the center of the galaxy. The most likely explanation for its origin and kinematics seems to be that of a galactic fountain. There is, however, a significant part of the anomalous gas that seems to be moving contrary to rotation and is difficult to understand in such a picture. These anomalous gas complexes discovered in NGC 2403 may be analogous to the high-velocity clouds of our Galaxy. They may be rather common in spiral galaxies and not have been detected yet for lack of sensitivity.

We discuss new, high-sensitivity H I observations of the spiral galaxy NGC 2403, which show extended emission at anomalous velocities with respect to the "cold" disk. This "anomalous" gas component (similar to1/10 of the total H I mass) is probably located in the region of the halo and rotates more slowly (similar to 20-50 km s(-1)) than the gas in the disk. Moreover, it shows a distortion in the velocity field that we interpret as a large-scale radial motion (10-20 km s(-1) inflow) toward the center of the galaxy. The most likely explanation for its origin and kinematics seems to be that of a galactic fountain. There is, however, a significant part of the anomalous gas that seems to be moving contrary to rotation and is difficult to understand in such a picture. These anomalous gas complexes discovered in NGC 2403 may be analogous to the high-velocity clouds of our Galaxy. They may be rather common in spiral galaxies and not have been detected yet for lack of sensitivity.

We examine the detectability and interpretation of debris trails caused by satellite disruption in external galaxies using semianalytic approximations for the dependence of streamer length, width, and surface brightness on satellite and primary galaxy characteristics. The semianalytic method is tested successfully against N-body simulations and then applied to three representative astronomical applications. First, we show how streamer properties can be used to estimate mass-to-light ratios Y and streamer ages of totally disrupted satellites, and we apply the method to the stellar arc in NGC 5907. Second, we discuss how the lack of observed tidal debris around a satellite can provide an upper limit on its mass-loss rate and, as an example, derive the implied limits on mass-loss rates for M32 and NGC 205 around Andromeda. Finally, we point out that a statistical analysis of streamer properties might be applied to test and refine cosmological models of hierarchical galaxy formation, and we use the predicted debris from a standard A cold dark matter realization to test the feasibility of such a study. Using the Local Group satellites and the few known examples of debris trails in the Galaxy and in external systems, we estimate that the best current techniques could characterize the brightest (R

We examine the dynamics of the stars and globular clusters in the nearby giant elliptical galaxy M87 and constrain the mass distribution, using all the available data over a large range of radii, including higher order moments of the stellar line-of-sight velocity distributions and the discrete velocities of over 200 globular clusters. We introduce an extension of spherical orbit modeling methods that makes full use of all the information in the data and provides very robust constraints on the mass models. We conclusively rule out a constant mass-to-light ratio model and infer that the radial density profile of the galaxy's dark halo falls off more slowly than r(-2), suggesting that the potential of the Virgo Cluster is already dominant at r similar to 300" similar to 20 kpc.

We present the first unambiguous case of external variability of a radio gravitational lens, CLASS B1600+434. The Very Lai-ge Array (VLA) 8.5-GHz difference light curve of the lensed images, taking the proper time-delay into account, shows the presence of external variability with 14.6-sigma confidence. We investigate two plausible causes of this external variability: scattering by the ionized component of the Galactic interstellar medium and microlensing by massive compact objects in the bulge/disk and halo of the lens galaxy. Based on the tight relation between the modulation-index (fractional rms variability) and variability time scale and the quantitative difference between the light curves of both lensed images, we conclude that the observed short-term variability characteristics of the lensed images are incompatible with scintillation in our Galaxy. This conclusion is strongly supported by multi-frequency Westerbork Synthesis Radio Telescope (WSRT) observations at 1.4 and 5 Gi;z, which are in disagreement with predictions based on the scintillation hypothesis. Several arguments against scintillation might need to be reevaluated if evidence is found for significant scatter-broadening of lensed image B seen through the lens galaxy. However, the frequency-dependence and time scale of variability from image A are not affected by this and remain strong arguments against scintillation. On the other hand, a single superluminal jet-component in the source, having an apparent velocity 9 less than or similar to(nu(app)/c)less than or similar to 26, a radius of 2-5 mu as and containing 5-11 % of the observed 8.5-GHz source flux density, can reproduce the observed modulation-indices and variability time scale at 8.5 GHz, when it is microlensed by compact objects in the lens galaxy. It also reproduces the frequency-dependence of the modulation-indices, determined from the independent WSRT 1.4 and 5-GHz observations. The difference between the modulation-indices of the lensed images (i.e. 2.8% and 1.6% at 8.5 GHz in 1998 for images A and B, respectively), if not affected by scatter-broadening of image B by the ionized ISM of the lens galaxy, can be explained through a different mass-function for the compact objects in the bulge/disk and halo of the lens galaxy. Comparing the observations with microlensing simulations, we place a tentative lower limit of greater than or similar to 0.5 M. on the average mass of compact objects in the halo line-of-sight. The above-mentioned set of mass-function and source parameters is consistent, although not unique, and should only be regarded as indicative. The only conclusion fully consistent with the data gathered thus far is that we have indeed detected radio microlensing. The far reaching consequence of this statement is that a significant fraction of the mass in the dark-matter halo at similar to 6kpc (h=0.65) above the lens-galaxy disk in B1600+434 consists of massive compact objects.

We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens images over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to 11% peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999).From these VLA light curves we determine a median time delay between the lens images of 47(-6)(+5), d (68%) or 47(-9)(+12) d (95%). We use two different methods to derive the time delay; both give the same result within the errors. We estimate an additional systematic error between -8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter H(0)=57(-11)(+14) (95% statistical) (+26)(-15) (systematic) km s(-1) Mpc(-1) (Omega(m)=1 and Omega(Lambda)=0). Similarly, the Modified-Hubble-Profile mass model would give H(0)=74(-15)(+18) (95% statistical) (+22)(-22) (systematic) km s(-1) Mpc(-1) For Omega(m)=0.3 and Omega(Lambda)=0.7, these values increase by 5.4%. We emphasize that the slope of the radial mass profile of the lens-galaxy dark-matter halo in B1600+434 is extremely ill-constrained. Hence, an accurate determination of Ho from this system is very difficult, if no additional constraints on the mass model are obtained. These values of Ho should therefore be regarded as indicative.Once H(0) (from independent methods) and the time delay have been determined with sufficient accuracy, it will prove more worthwhile to constrain the radial mass profile of the dark-matter halo around the edge-on spiral lens galaxy at z=0.4.

We present H-band images obtained with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) of a field 75 " (5 kpc) above the plane of the disk of the edge-on spiral galaxy NGC 5907. Ground-based observations have shown that NGC 5907 has a luminous halo with a shallow radial profile between 4 and 8 kpc that roughly traces the dark matter distribution of the galaxy deduced from its rotation curve. Our NICMOS observations were designed to resolve bright giants in the halo of NGC 5907 to constrain its stellar composition with the goal of understanding its nature and origin. More than 100 stars are expected in the NICMOS images if the dwarf-to-giant ratio in the halo of NGC 5907 is consistent with that expected from standard stellar initial mass functions and if ground-based estimates of the distance to NGC 5907 and the integrated colors of its halo are correct. Instead we observe only one candidate giant star.-This apparent discrepancy can be resolved by assuming either a significantly larger distance than that suggested by several studies or a halo metallicity much lower than that suggested by ground-based colors and as low as that in metal-poor Galactic globular clusters. If previous distance and halo color estimates for NGC 5907 are correct, our NICMOS results suggest that its extended light is composed of stars that formed with an initial mass function different than that observed locally, leading to a much higher ratio of dwarfs to giants. We describe how these three possible explanations for the absence of bright giants in our NICMOS images of the halo of NGC 5907 might be constrained by future observations.

High-resolution H alpha rotation curves are presented for five low surface brightness galaxies. These H alpha rotation curves have shapes different from those previously derived from H I observations, probably because of the higher spatial resolution of the H alpha observations. The H alpha rotation curves rise more steeply in the inner parts than the H I rotation curves, and they reach a hat part beyond about two disk scale lengths. With radii expressed in optical disk scale lengths, the rotation curves of the low surface brightness galaxies presented here and those of the high surface brightness galaxies have almost identical shapes. Mass modeling shows that the contribution of the stellar component to the rotation curves may be scaled to explain most of the inner parts of the rotation curves, albeit with high stellar mass-to-light ratios. On the other hand, well-fitting mass models can also be obtained with lower contributions of the stellar disk. These observations suggest that the luminous mass density and the total mass density are coupled in the inner parts of these galaxies.

High-resolution H alpha rotation curves are presented for five low surface brightness galaxies. These H alpha rotation curves have shapes different from those previously derived from H I observations, probably because of the higher spatial resolution of the H alpha observations. The H alpha rotation curves rise more steeply in the inner parts than the H I rotation curves, and they reach a hat part beyond about two disk scale lengths. With radii expressed in optical disk scale lengths, the rotation curves of the low surface brightness galaxies presented here and those of the high surface brightness galaxies have almost identical shapes. Mass modeling shows that the contribution of the stellar component to the rotation curves may be scaled to explain most of the inner parts of the rotation curves, albeit with high stellar mass-to-light ratios. On the other hand, well-fitting mass models can also be obtained with lower contributions of the stellar disk. These observations suggest that the luminous mass density and the total mass density are coupled in the inner parts of these galaxies.

We present high-sensitivity, high-velocity resolution VLA H I observations of the giant low surface brightness (LSB) galaxy, UGC 2936. Like the giant LSBs presented in Pickering et al., UGC 2936 is a large and massive galaxy. Its H I mass is nearly 10(10) M-circle dot h(75)(-2), it has detectable H I extending beyond 30 kpc h(75)(-1), and it is a fast rotator (V-max similar or equal to 250 km s(-1)) with a slowly rising rotation curve. This galaxy also exhibits warping in the outermost isophotes of the optical images that appears to be visible in the H I distribution and kinematics as well. This galaxy's high inclination and relatively large amount of Her emission provides a unique opportunity to compare high-quality H I and optical rotation curves in the same LSB galaxy. The optical and H I data show good agreement as long as the effects of beam smearing on the H I rotation curve are taken into account. A large part of the disk of UGC 2936 lies above the critical density for star formation as described by Kennicutt. This is consistent with the relatively large amount of star formation occurring within the disk of this galaxy and perhaps brings into question whether this galaxy should be considered a true LSB galaxy.

We present high-sensitivity, high-velocity resolution VLA H I observations of the giant low surface brightness (LSB) galaxy, UGC 2936. Like the giant LSBs presented in Pickering et al., UGC 2936 is a large and massive galaxy. Its H I mass is nearly 10(10) M-circle dot h(75)(-2), it has detectable H I extending beyond 30 kpc h(75)(-1), and it is a fast rotator (V-max similar or equal to 250 km s(-1)) with a slowly rising rotation curve. This galaxy also exhibits warping in the outermost isophotes of the optical images that appears to be visible in the H I distribution and kinematics as well. This galaxy's high inclination and relatively large amount of Her emission provides a unique opportunity to compare high-quality H I and optical rotation curves in the same LSB galaxy. The optical and H I data show good agreement as long as the effects of beam smearing on the H I rotation curve are taken into account. A large part of the disk of UGC 2936 lies above the critical density for star formation as described by Kennicutt. This is consistent with the relatively large amount of star formation occurring within the disk of this galaxy and perhaps brings into question whether this galaxy should be considered a true LSB galaxy.

We present modified Newtonian dynamics (MOND) fits to 15 rotation curves of low surface brightness (LSB) galaxies. Good fits are readily found, although for a few galaxies minor adjustments to the inclination are needed. Reasonable values for the stellar mass-to-light ratios are found, as well as an approximately constant value for the total (gas and stars) mass-to-light ratio. We show that the LSB galaxies investigated here lie on the one, unique Tully-Fisher relation, as predicted by MOND. The scatter on the Tully-Fisher relation can be completely explained by the observed scatter in the total mass-to-light ratio. We address the question of whether MOND can fit any arbitrary rotation curve by constructing a plausible fake model galaxy. While MOND is unable to fit this hypothetical galaxy, a normal dark-halo fit is readily found, showing that dark matter fits are much less selective in producing fits. The good fits to rotation curves of LSB galaxies support MOND, especially because these are galaxies with large mass discrepancies deep in the MOND regime.

We present the analysis of new, deep Hi observations of the spiral galaxy NGC 3198 as part of the Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey, with the main aim of investigating the presence, amount, morphology, and kinematics of extraplanar gas. We present models of the Hi observations of NGC 3198. The model that matches best the observed data cube features a thick disk with a scale height of ~3 kpc and an Hi mass of about 15% of the total Hi mass; this thick disk also has a decrease in rotation velocity as a function of height (lag) of 7-15 km s-1 kpc-1 (though with large uncertainties). This extraplanar gas is detected for the first time in NGC 3198. Radially, this gas appears to extend slightly beyond the actively star-forming body of the galaxy (as traced by the H? emission), but it is not more radially extended than the outer, fainter parts of the stellar disk. Compared to previous studies, thanks to the improved sensitivity we trace the rotation curve out to larger radii. We model the rotation curve in the framework of modified Newtonian dynamics (MOND) and confirm that, with the allowed distance range we assumed, fit quality is modest in this galaxy, but the new outer parts are explained in a satisfactory way.

In recent years it has become clear that the vertical structure of disk galaxies is a key ingredient for understanding galaxy evolution. In particular, the presence and struc- ture of extra-planar gas has been a focus of research. The Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey aims to provide a census on the rate of cold neutral gas accretion in nearby galaxies as well as a statistically significant set of galaxies that can be investigated for their extra-planar gas properties. In order to better understand the the vertical structure of the neutral hydrogen in the two edge-on HALOGAS galaxies NGC 5023 and UGC 2082 we construct de- tailed tilted ring models. The addition of distortions resembling arcs or spiral arms significantly improves the fit of the models to these galaxies. In the case of UGC 2082 no vertical gradient in rotational velocity is required in either symmetric models nor non-symmetric models to match the observations. The best fitting model features two arcs of large vertical extent that may be due to accretion. In the case of NGC 5023 a vertical gradient is required in symmetric models (dV/dz =?14.9±3.8 km s?1 kpc?1) and its magnitude is significantly lowered when non-symmetric models are considered (dV/dz =?9.4 ± 3.8 km s?1 kpc?1). Additionally it is shown that the underlying disk of NGC 5023 can be made symmetric, in all parameters except the warp, in non- symmetric models. In comparison to the "classical" modeling these models fit the data significantly better with a limited addition of free parameters.

We present 21 cm observations and models of the neutral hydrogen in NGC 4565, a nearby, edge-on spiral galaxy, as part of the Westerbork Hydrogen Accretion in LOcal GAlaxieS survey. These models provide insight concerning both the morphology and kinematics of H I above, as well as within, the disk. NGC 4565 exhibits a distinctly warped and asymmetric disk with a flaring layer. Our modeling provides no evidence for a massive, extended H I halo. We see evidence for a bar and associated radial motions. Additionally, there are indications of radial motions within the disk, possibly associated with a ring of higher density. We see a substantial decrease in rotational velocity with height above the plane of the disk (a lag) of -40+5 - 20 km s-1 kpc-1 and -30+5 - 30 km s-1 kpc-1 in the approaching and receding halves, respectively. This lag is only seen within the inner ~4farcm75 (14.9 kpc) on the approaching half and ~4farcm25 (13.4 kpc) on the receding half, making this a radially shallowing lag, which is now seenin the H I layers of several galaxies. When comparing results for NGC 4565 and those for other galaxies, there are tentative indications of high star formation rate per unit area being associated with the presence of a halo. Finally, H I is found in two companion galaxies, one of which is clearly interacting with NGC 4565.

We investigate the structure of cold dark matter halos using advanced models of spherical collapse and accretion in an expanding Universe. These base on solving time-dependent equations for the moments of the phase-space distribution function in the fluid approximation; our approach includes non-radial random motions, and most importantly, an advanced treatment of both dynamical relaxation effects that takes place in the infalling matter: phase-mixing associated to shell crossing, and collective collisions related to physical clumpiness. We find self-similar solutions for the spherically-averaged profiles of mass density rho(r), pseudo phase-space density Q(r) and anisotropy parameter beta(r). These profiles agree with the outcomes of state-of-the-art N-body simulations in the radial range currently probed by the latter; at smaller radii, we provide specific predictions. In the perspective provided by our self-similar solutions we link the halo structure to its two-stage growth history, and propose the following picture. During the early fast collapse of the inner region dominated by a few merging clumps, efficient dynamical relaxation plays a key role in producing a closely universal mass density and pseudo phase-space density profiles; in particular, these are found to depend only weakly on the detailed shape of the initial perturbation and the related collapse times. The subsequent inside-out growth of the outer regions feeds on the slow accretion of many small clumps and diffuse matter; thus the outskirts are only mildly affected by dynamical relaxation but are more sensitive to asymmetries and cosmological variance., 31 pages, 16 figures. Typos corrected. Accepted by ApJ

We report the discovery of two concentric Einstein rings around the gravitational lens SDSS J0946+ 1006. The main lens is at redshift z(l) = 0.222, while the inner ring ( 1) is at redshift z(s1) 0.609 (R-Ein1 = 1.43 '' +/- 0.01 ''). The wider image separation ( R-Ein2 = 2.07 '' +/- 0.02 '') of the outer ring (2) implies a higher redshift than that of ring 1; the detection of ring 2 in the F814WACS filter implies an upper limit of z(s2) less than or similar to 6.9. The main lens can be described by a power- law total mass density profile rho(tot) proportional to r(-gamma ') with gamma ' = 2.00 +/- 0.03 and velocity dispersion sigma(SIE) = 287 +/- 5 km s(-1) ( the stellar velocity dispersion is sigma(v,*) = 284 +/- 24 km s(-1)). The strong lensing configuration is inconsistent with light traces mass. Adopting a prior on the stellar mass- to- light ratio from previous SLACS work, we infer a 73% +/- 9% dark matter fraction within the cylinder of radius equal to the effective radius of the lens. We find that, for the case of SDSS J0946+ 1006, the geometry of the two rings does not place interesting constraints on cosmography because of the suboptimal redshifts of lens and sources. We then consider the perturbing effect of the mass associated with ring 1 building a compound lens model. This introduces minor changes to the mass of the main lens and provides an estimate of z(s2) = 3.1(-1.0)(+2.0) and of the mass of the source responsible for ring 1 (sigma(SIE,s1) = 94(-47)(+27) km s(-1)). We conclude by examining the prospects of doing cosmography with a sample of 50 double rings, expected from future space- based surveys. Accounting for uncertainties in the mass profile of the lens and the effects of the perturber, we find that such a sample would constrain Omega(m) and within 10%, assuming flatness.

Context. The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in Ha emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. Aims. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in Ha emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. Methods. By comparing large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. Results. From the 24 mu m emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., similar to 30%. The additional asymmetry in Ha is modeled as additional symmetric dust attenuation which extends up to similar to 40 '' (1.9 kpc) above the plane of the galaxy with a mid-plane value of tau = 0.8 and a scale height of 0.5 kpc

The properties of the gas in halos of galaxies constrain global models of the interstellar medium. Kinematical information is of particular interest since it is a clue to the origin of the gas. Here we report observations of the kinematics of the thick layer of the diffuse ionized gas in NGC 891 in order to determine the rotation curve of the halo gas. We have obtained a Fabry-P\'erot data cube in Halpha to measure the kinematics of the halo gas with angular resolution much higher than obtained from HI 21 cm observations. The data cube was obtained with the TAURUS II spectrograph at the WHT on La Palma. The velocity information of the diffuse ionized gas extracted from the data cube is compared to model distributions to constrain the distribution of the gas and in particular the halo rotation curve. The best fit model has a central attenuation tau_H-alpha=6, a dust scale length of 8.1 kpc, an ionized gas scale length of 5.0 kpc. Above the plane the rotation curve lags with a vertical gradient of -18.8 km/s/kpc. We find that the scale length of the H-alpha must be between 2.5 and 6.5 kpc. Furthermore we find evidence that the rotation curve above the plane rises less steeply than in the plane. This is all in agreement with the velocities measured in the HI., Comment: A&A, in press. 13 pages, 19 figures

We present Monte Carlo calculations of Ly alpha radiative transfer through optically thick, spherically symmetric, collapsing gas clouds. These represent simplified models of protogalaxies in the process of their assembly. Such galaxies produce Ly alpha flux over an extended solid angle, either from a spatially extended Ly alpha emissivity, or from scattering effects, or both. We present a detailed study of the effect of the gas distribution and kinematics and of the Ly alpha emissivity profile on the emergent spectrum and surface brightness distribution. The emergent Ly alpha spectrum is typically double peaked and asymmetric. In practice, however, we find energy transfer from the infalling gas to the Ly alpha photons to significantly enhance the blue peak and to render the red peak, in most cases, undetectable. The resulting effective blueshift, combined with scattering in the intergalactic medium, renders extended Ly alpha emission from collapsing protogalaxies difficult to detect beyond redshift z greater than or similar to 4. We find a strong wavelength dependence of the slope of the surface brightness distribution (with preferential flattening at the red side of the line) to be a robust indication that Ly alpha photons are being generated (rather than just scattered) in a spatially extended, collapsing region around the galaxy. For self-ionized clouds whose effective Ly alpha optical depth is less than or similar to 10(3), infall and outflow models can produce nearly identical spectra and surface brightness distributions and are practically indistinguishable. The presence of cosmic abundance of deuterium may produce a detectable dip in the spectra of systems with moderate hydrogen column densities, N(H) = 10(18)-10(20) cm(-2). Finally, we present a new analytic solution for the emerging Ly alpha spectrum in the limiting case of a static uniform sphere, extending previous solutions for slabs.

We model the spectra and surface brightness distributions for the Lyman alpha (Lya) radiation expected from protogalaxies that are caught in the early stages of their assembly. We use the results of a companion paper to characterize the radiation emerging from spherically collapsing gas clouds. We then modify the intrinsic spectra to incorporate the effect of subsequent resonant scattering in the intergalactic medium (IGM). Using these models, we interpret a number of recent observations of extended Lya blobs (LABs) at high redshift. We suggest, based on the angular size, energetics, as well as the relatively shallow surface brightness profiles, and double-peaked spectra, that several of these LABs may be associated with collapsing protogalaxies. We suggest two follow-up observations to diagnose the presence of gas infall. High S/N spectra of LABs should reveal a preferential flattening of the surface brightness profile at the red side of the line. Complementary imaging of the blobs at redshifted Balmer alpha wavelengths should reveal the intrinsic Lya emissivity and allow its separation from radiative transfer effects. We show that Lya scattering by infalling gas can reproduce the observed spectrum of Steidel et al's LAB2 as accurately as a recently proposed outflow model. Finally, we find similar evidence for infall in the spectra of point-like Lyman alpha emitters. The presence of scattering by the infalling gas implies that the intrinsic Lya luminosities, and derived quantities, such as the star-formation rate, in these objects may have been underestimated by about an order of magnitude., Comment: Accepted for Publication in ApJ, 11 emulateapj pages with 6 figures, together with a companion paper

The confined nature of the debris from the Sagittarius dwarf to a narrow trail on the sky has recently prompted the suggestion that the dark matter halo of our Galaxy should be nearly spherical (Ibata et al. 2001; Majewski et al. 2003). This would seem to be in strong contrast with predictions from cold dark matter (CDM) simulations, where dark halos are found to have typical density axis ratios of 0.6 to 0.8. Here I present numerical simulations of the evolution of a system like the Sagittarius dSph in a set of Galactic potentials with varying degrees of flattening. These simulations show that the Sagittarius streams discovered so far are too young dynamically to be sensitive to the shape of the dark halo of the Milky Way. The data presently available are entirely consistent with a Galactic dark matter halo that could either be oblate or prolate, with density axis ratios c/a that range from 0.6 to 1.6 within the region of the halo probed by the orbit of the Sagittarius dwarf.

We discuss new, high sensitivity HI observations of the spiral galaxy NGC2403 which show extended emission at anomalous velocities with respect to the `cold' disk. This `anomalous' gas component (~1/10 of the total HI mass) is probably located in the region of the halo and rotates more slowly (~20-50 km/s) than the gas in the disk. Moreover, it shows a distortion in the velocity field that we interpret as a large-scale radial motion (10-20 km/s inflow) towards the centre of the galaxy. The most likely explanation for its origin and kinematics seems to be that of a galactic fountain. There is, however, a significant part of the anomalous gas which seems to be moving contrary to rotation and is difficult to understand in such a picture. These anomalous gas complexes discovered in NGC 2403 may be analogous to the High Velocity Clouds of our Galaxy. They may be rather common in spiral galaxies and not have been detected yet for lack of sensitivity., 11 pages, 4 figures, accepted for publication in ApJ Letters

We examine the detectability and interpretation of debris trails caused by satellite disruption in external galaxies using semi-analytic approximations for the dependence of streamer length, width and surface brightness on satellite and primary galaxy characteristics. The semi-analytic method is tested successfully against N-body simulations and then applied to three representative astronomical applications. First, we show how streamer properties can be used to estimate mass-to-light ratios $\Upsilon$ and streamer ages of totally disrupted satellites, and apply the method to the stellar arc in NGC 5907. Second, we discuss how the lack of observed tidal debris around a satellite can provide an upper limit on its mass-loss rate, and, as an example, derive the implied limits on mass-loss rates for M32 and NGC 205 around Andromeda. Finally, we point out that a statistical analysis of streamer properties might be applied to test and refine cosmological models of hierarchical galaxy formation, and use the predicted debris from a standard $\Lambda$CDM realization to test the feasibility of such a study. Using the Local Group satellites and the few known examples of debris trails in the Galaxy and in external systems, we estimate that the best current techniques could characterize the brightest ($R < 29$ mag/ arcsec$^{2}$) portions of the youngest (3 dynamical periods) debris streamers. If systematics can be controlled, planned large-aperture telescopes such as CELT and OWL may allow fainter trails to be detected routinely and thus used for statistical studies such as those required for tests of galaxy formation., Comment: 33 pages, 8 figures. Revised version, accepted for publication in ApJ

We examine the dynamics of the stars and globular clusters in the nearby giant galaxy M87 and constrain the mass distribution, using all the available data over a large range of radii, including higher-order moments of the stellar line-of-sight velocity distributions and the discrete velocities of over two hundred globular clusters. We introduce an extension of spherical orbit modeling methods that makes full use of all the information in the data, and provides very robust constraints on the mass models. We conclusively rule out a constant mass-to-light ratio model, and infer that the radial density profile of the galaxy's dark halo falls off more slowly than 1/r^2, suggesting that the potential of the Virgo Cluster is already dominant at r ~ 300" ~ 20 kpc., Comment: Revised version accepted by ApJ: expanded Results with new mass models; 26 pages, with 12 inline Postscript figures, LaTeX, aaspp4.sty

We present the first unambiguous case of external variability of a radio gravitational lens, CLASS B1600+434. The Very Lai-ge Array (VLA) 8.5-GHz difference light curve of the lensed images, taking the proper time-delay into account, shows the presence of external variability with 14.6-sigma confidence. We investigate two plausible causes of this external variability: scattering by the ionized component of the Galactic interstellar medium and microlensing by massive compact objects in the bulge/disk and halo of the lens galaxy. Based on the tight relation between the modulation-index (fractional rms variability) and variability time scale and the quantitative difference between the light curves of both lensed images, we conclude that the observed short-term variability characteristics of the lensed images are incompatible with scintillation in our Galaxy. This conclusion is strongly supported by multi-frequency Westerbork Synthesis Radio Telescope (WSRT) observations at 1.4 and 5 Gi;z, which are in disagreement with predictions based on the scintillation hypothesis. Several arguments against scintillation might need to be reevaluated if evidence is found for significant scatter-broadening of lensed image B seen through the lens galaxy. However, the frequency-dependence and time scale of variability from image A are not affected by this and remain strong arguments against scintillation. On the other hand, a single superluminal jet-component in the source, having an apparent velocity 9 less than or similar to(nu(app)/c)less than or similar to 26, a radius of 2-5 mu as and containing 5-11 % of the observed 8.5-GHz source flux density, can reproduce the observed modulation-indices and variability time scale at 8.5 GHz, when it is microlensed by compact objects in the lens galaxy. It also reproduces the frequency-dependence of the modulation-indices, determined from the independent WSRT 1.4 and 5-GHz observations. The difference between the modulation-indices of the lensed images (i.e. 2.8% and 1.6% at 8.5 GHz in 1998 for images A and B, respectively), if not affected by scatter-broadening of image B by the ionized ISM of the lens galaxy, can be explained through a different mass-function for the compact objects in the bulge/disk and halo of the lens galaxy. Comparing the observations with microlensing simulations, we place a tentative lower limit of greater than or similar to 0.5 M. on the average mass of compact objects in the halo line-of-sight. The above-mentioned set of mass-function and source parameters is consistent, although not unique, and should only be regarded as indicative. The only conclusion fully consistent with the data gathered thus far is that we have indeed detected radio microlensing. The far reaching consequence of this statement is that a significant fraction of the mass in the dark-matter halo at similar to 6kpc (h=0.65) above the lens-galaxy disk in B1600+434 consists of massive compact objects.

We present H-band images obtained with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) of a field 75 " (5 kpc) above the plane of the disk of the edge-on spiral galaxy NGC 5907. Ground-based observations have shown that NGC 5907 has a luminous halo with a shallow radial profile between 4 and 8 kpc that roughly traces the dark matter distribution of the galaxy deduced from its rotation curve. Our NICMOS observations were designed to resolve bright giants in the halo of NGC 5907 to constrain its stellar composition with the goal of understanding its nature and origin. More than 100 stars are expected in the NICMOS images if the dwarf-to-giant ratio in the halo of NGC 5907 is consistent with that expected from standard stellar initial mass functions and if ground-based estimates of the distance to NGC 5907 and the integrated colors of its halo are correct. Instead we observe only one candidate giant star.-This apparent discrepancy can be resolved by assuming either a significantly larger distance than that suggested by several studies or a halo metallicity much lower than that suggested by ground-based colors and as low as that in metal-poor Galactic globular clusters. If previous distance and halo color estimates for NGC 5907 are correct, our NICMOS results suggest that its extended light is composed of stars that formed with an initial mass function different than that observed locally, leading to a much higher ratio of dwarfs to giants. We describe how these three possible explanations for the absence of bright giants in our NICMOS images of the halo of NGC 5907 might be constrained by future observations.

We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens images over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to 11% peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999). From these VLA light curves we determine a median time delay between the lens images of 47^{+5}_{-6} d (68%) or 47^{+12}_{-9} d (95%). We use two different methods to derive the time delay; both give the same result within the errors. We estimate an additional systematic error between -8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter, H_0=57^{+14}_{-11} (95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED), 14 pages, 6 figures, accepted for publication in Astronomy & Astrophysics (Figs 1 and 3 with degraded resolution)

High resolution Halpha rotation curves are presented for five low surface brightness galaxies. These Halpha rotation curves have shapes different from those previously derived from HI observations, probably because of the higher spatial resolution of the Halpha observations. The Halpha rotation curves rise more steeply in the inner parts than the HI rotation curves and reach a flat part beyond about two disk scale lengths. With radii expressed in optical disk scale lengths, the rotation curves of the low surface brightness galaxies presented here and those of HSB galaxies have almost identical shapes. Mass modeling shows that the contribution of the stellar component to the rotation curves may be scaled to explain most of the inner parts of the rotation curves, albeit with high stellar mass-to-light ratios. On the other hand, well fitting mass models can also be obtained with lower contributions of the stellar disk. These observations suggest that the luminous mass density and the total mass density are coupled in the inner parts of these galaxies., Comment: Accepted for publication in ApJ Letters

We present modified Newtonian dynamics (MOND) fits to 15 rotation curves of low surface brightness (LSB) galaxies. Good fits are readily found, although for a few galaxies minor adjustments to the inclination are needed. Reasonable values for the stellar mass-to-light ratios are found, as well as an approximately constant value for the total (gas and stars) mass-to-light ratio. We show that the LSB galaxies investigated here lie on the one, unique Tully-Fisher relation, as predicted by MOND. The scatter on the Tully-Fisher relation can be completely explained by the observed scatter in the total mass-to-light ratio. We address the question of whether MOND can fit any arbitrary rotation curve by constructing a plausible fake model galaxy. While MOND is unable to fit this hypothetical galaxy, a normal dark-halo fit is readily found, showing that dark matter fits are much less selective in producing fits. The good fits to rotation curves of LSB galaxies support MOND, especially because these are galaxies with large mass discrepancies deep in the MOND regime.

We present 21-cm line observations of the spiral galaxy NGC 4559, made with the Westerbork Synthesis Radio Telescope. We have used them to study the HI distribution and kinematics, the relative amount and distribution of luminous and dark matter in this galaxy and, in particular, the presence of extra-planar gas. Our data do reveal the presence of such a component, in the form of a thick disk, with a mass of 5.9 x 10^8 Mo (one tenth of the total HI mass) and a mean rotation velocity 25-50 km/s lower than that of the thin disk. The extra-planar gas may be the result of galactic fountains but accretion from the IGM cannot be ruled out. With this study we confirm that lagging, thick HI layers are likely to be common in spiral galaxies., Comment: 17 pages, 10 figures. Accepted for publication in A&A

We present H I observations of the edge-on galaxy NGC 891. These are among the deepest ever performed on an external galaxy. They reveal a huge gaseous halo, much more extended than seen previously and containing almost 30% of the H I. This H I halo shows structures on various scales. On one side, there is a filament extending ( in projection) up to 22 kpc vertically from the disk. Small (M(H I) greater than or similar to 10(6) M(circle dot)) halo clouds, some with forbidden (apparently counterrotating) velocities, are also detected. The overall kinematics of the halo gas is characterized by differential rotation lagging with respect to that of the disk. The lag, more pronounced at small radii, increases with height from the plane. There is evidence that a significant fraction of the halo is due to a galactic fountain. Accretion from inter-galactic space may also play a role in building up the halo and providing the low angular momentum material needed to account for the observed rotation lag. The long H I filament and the counterrotating clouds may be direct evidence of such accretion.

We investigate $HST$/ACS and WFPC2 images at the positions of five candidate microlensing events from a large survey of variability in M31 (MEGA). Three closely match unresolved sources, and two produce only flux upper limits. All are confined to regions of the color-magnitude diagram where stellar variability is unlikely to be easily confused with microlensing. Red variable stars cannot explain these events (although background supernova are possible for two). If these lenses arise in M31's halo, they are due to masses $0.15 < m / M_\odot < 0.49$ (95% certainty, for a $��$-function mass distribution), brown dwarfs for disk lenses, and stellar masses for bulge lenses., Accepted for publication in ApJL. Higher resolution version available at http://www.astro.columbia.edu/~patrick/hst/hst_ml.pdf