Kapteyn Institute Preprints (1999)

Kapteyn Institute Preprint Service.

Preprints of the Kapteyn Institute.
Paper copies can be ordered at:
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P.O. Box 800
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All the preprints are compressed using gzip.

1999 Preprints

* Preprint 290 Detection Efficiencies of Microlensing Datasets to Stellar and Planetary Companions B. Scott Gaudi and Penny D. Sackett
* Preprint 289 Astrometric Microlensing of Stars M. Dominik, K.C. Sahu
* Preprint 288 The discovery of a microarcsecond quasar: J1819+3845 J. Dennett-Thorpe, A.G. de Bruyn
* Preprint 287 Combined Analysis of the Binary-Lens Caustic-Crossing Event MACHO 98-SMC-1 : C. Afonso, C. Alard, J.N. Albert, J. Andersen, R. Ansari, E. Aubourg, P. Bareyre, F. Bauer, J.P. Beaulieu, A. Bouquet, S. Char, X. Charlot, F. Couchot, C. Coutures, F. Derue, R. Ferlet, J.F. Glicenstein, B. Goldman, A. Gould, D. Graff, M. Gros, J. Haissinski, J.C. Hamilton, D. Hardin, J. de Kat, A. Kim, T. Lasserre, E. Lesquoy, C. Loup, C. Magneville, J.B. Marquette, E. Maurice, A. Milsztajn, M. Moniez, N. Palanque-Delabrouille, O. Perdereau, L. Prevot, N. Regnault, J. Rich, M. Spiro, A. Vidal-Madjar, L. Vigroux, S. Zylberajch (The EROS Collaboration), C. Alcock, R.A. Allsman, D. Alves, T.S. Axelrod, A.C. Becker, K.H. Cook, A.J. Drake, K.C. Freeman, K. Griest, L.J. King, M.J. Lehner, S.L. Marshall, D. Minniti, B.A. Peterson, M.R. Pratt, P.J. Quinn, A.W. Rodgers, P.B. Stetson, C.W. Stubbs, W. Sutherland, A. Tomaney, T. Vandehei (The MACHO/GMAN Collaboration), S.H. Rhie, D.P. Bennett, P.C. Fragile, B.R. Johnson, J. Quinn (The MPS Collaboration), A. Udalski, M. Kubiak, M. Szymanski, G. Pietrzynski, P. Wozniak, K. Zebrun (The OGLE Collaboration), M.D. Albrow, J.A.R. Caldwell, D.L. DePoy, M. Dominik, B.S. Gaudi, J. Greenhill, K. Hill, S. Kane, R. Martin, J. Menzies, R.M. Naber, R.W. Pogge, K.R. Pollard, P.D. Sackett, K.C. Sahu, P. Vermaak, R. Watson, A. Williams (The PLANET Collaboration)
* Preprint 286 Radio imaging of core-dominated high redshift quasars Peter D. Barthel, Marianne Vestergaard, and Colin J. Lonsdale
* Preprint 285 A Complete Set of Solutions For Caustic-Crossing Binary Microlensing Events M. D. Albrow, J.-P. Beaulieu, J. A. R. Caldwell, D. L. DePoy, M. Dominik, B. S. Gaudi, A. Gould, J. Greenhill, K. Hill, S. Kane, R. Martin, J. Menzies, R. M. Naber, R. W. Pogge, K. R. Pollard, P. D. Sackett, K. C. Sahu, P. Vermaak, R. Watson, A. Williams (The PLANET Collaboration)
* Preprint 284 Detection of Warm and Cold Phases of the Neutral ISM in a Damped Ly-alpha Absorber W.M. Lane, F.H. Briggs, and A. Smette
* Preprint 283 ISO-SWS observations of interstellar solid 13CO2: heated ice and the Galactic 12C/13C abundance ratio A.C.A. Boogert, P. Ehrenfreund, P.A. Gerakines, A.G.G.M. Tielens, D.C.B. Whittet, W.A. Schutte, E.F. van Dishoeck, Th. de Graauw, L. Decin, T. Prusti
* Preprint 282 The Planetary Nebulae Spectrograph M. Arnaboldi, M. Capaccioli, N. Douglas, K. Kuijken, K. Freeman, T. Axelrod, K. Taylor, R. Gilmozzi and R. Kudritzki
* Preprint 281 Neutral hydrogen and dark matter in spiral galaxies Renzo Sancisi
* Preprint 280 On the axis ratio of the stellar velocity ellipsoid in disks of spiral galaxies P.C. van der Kruit and R. de Grijs
* Preprint 279 Properties of the Stellar Velocity Ellipsoid and Stability in Disks of Spiral Galaxies P.C. van der Kruit
* Preprint 278 The Milky Way compared to external galaxies P.C. van der Kruit
* Preprint 277 The binary gravitational lens and its extreme cases M. Dominik
* Preprint 276 Lensed galaxies in Abell 370. II. The ultraviolet properties of arclets and the star formation rate at high redshift J. Bezecourt, G. Soucail, R.S. Ellis, J.-P. Kneib
* Preprint 275 Lensed galaxies in Abell 370. I. Modeling the number counts and redshift distribution of background sources J. Bezecourt, J.P. Kneib, G. Soucail, T.M.D. Ebbels
* Preprint 274 Surface photometry of bulge dominated low surface brightness galaxies M. Beijersbergen, W.G.J. de Blok, and J.M. van der Hulst
* Preprint 273 Spectrophotometry of nearby field galaxies: the data, R.A. Jansen, D. Fabricant, M. Franx, N. Caldwell
* Preprint 272 Surface photometry of nearby field galaxies: the data, R.A. Jansen, M. Franx, D. Fabricant, N. Caldwell
* Preprint 271 Planetary Microlensing: Present Status and Long-term Goals, Penny D. Sackett
* Preprint 270 Galaxy kinematics from counter-dispersed imaging, N.G. Douglas, K. Taylor
* Preprint 269 The kinematics of the bulge and the disc of NGC 7331, Roelof Bottema
* Preprint 268 Limb-Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97-BLG-28, M.D. Albrow, J.-P. Beaulieu, J.A.R. Caldwell, M. Dominik, J. Greenhill, K. Hill, S. Kane, R. Martin, J. Menzies, R.M. Naber, J.-W. Pel, K. Pollard, P.D. Sackett, K.C. Sahu, P. Vermaak, R. Watson, A. Williams (The PLANET Collaboration), and M. S. Sahu
* Preprint 267 The Shape of Dark Matter Halos, Penny D. Sackett

Detection Efficiencies of Microlensing Datasets to Stellar and Planetary Companions

B. Scott Gaudi and Penny D. Sackett

Preprint no. 290

Accepted for publication in The Astrophysical Journal (Jan. 2000).

Abstract

Microlensing light curves are now being monitored with the temporal sampling and photometric precision required to detect small perturbations due to planetary companions of the primary lens. Microlensing is complementary to other planetary search techniques, both in the mass and orbital separation of the planets to which it is sensitive and its potential for measuring the abundance of planets beyond the solar neighborhood. We present an algorithm to analyze the efficiency with which the presence of lensing binaries of given mass ratio and angular separation can be detected in observed microlensing datasets; such an analysis is required in order to draw statistical inferences about lensing companions. Our method incorporates the actual sampling, photometric precision, and monitored duration of individual light curves. We apply the method to simulated (but realistic) data to explore the dependence of detection efficiencies on observational parameters, the impact parameter of the event, the finite size of the background source, the amount of unlensed (blended) light, and the criterion used to define a detection. We find that: (1) the detection efficiency depends strongly on the impact parameter of the monitored event, (2) the detection efficiency is robust to changes in detection criterion for strict criteria (Delta(chi^2) greater than ~100) and large mass ratios (q greater than ~0.01), (3) finite sources can dramatically alter detection efficiencies to companions with mass ratios q less than ~0.001, and (4) accurate determination of the blended light fraction is crucial for the accurate determination of the detection efficiency of individual events. Suggestions are given for addressing complications associated with computing accurate detection efficiences of observed datasets.

Astrometric Microlensing of Stars

M. Dominik, K.C. Sahu

Preprint no. 289

Accepted for publication in The Astrophysical Journal.

Abstract

Due to dramatic improvements in the precision of astrometric measurements, the observation of light centroid shifts in observed stars due to intervening massive compact objects ('astrometric microlensing') will become possible in the near future. Upcoming space missions, such as SIM and GAIA, will provide measurements with an accuracy of 4-60 micro-as depending on the magnitude of the observed stars, and an accuracy of ~1 micro-as is expected to be achieved in the more distant future. There are two different ways in which astrometric microlensing signals can be used to infer information: one possibility is to perform astrometric follow-up observations on photometrically detected microlensing events, and the other is to perform a survey based on astrometric observations alone. After the predictable effects of the Sun and the planets, stars in the Galactic disk play the dominant role in astrometric microlensing. The probability that the disk stars introduce a centroid shift larger than the threshold deltaT at a given time for a given source in the Galactic bulge towards Baade's window reaches 100% for a threshold of deltaT = 0.7 micro-as, while this probability is ~2% for deltaT = 5 micro-as. However, this centroid shift does not vary much during the time in which a typical photometric microlensing event differs from baseline. So astrometric follow-ups (e.g. with SIM) are not expected to be disturbed by the statistical astrometric microlensing due to disk stars, so that it is possible to infer additional information about the nature of the lens that caused the photometric event, as suggested. The probability to observe astrometric microlensing events within the Galaxy turns out to be large compared to photometric microlensing events. The probability to see a variation by more than 5 micro-as within one year and to reach the closest angular approach between lens and source is ~10-4 for a bulge star towards Baade's window, while this reduces to ~6x10-6 for a direction perpendicular to the Galactic plane. For the upcoming mission GAIA, we expect ~1000 of the observed stars to show a detectable astrometric microlensing signal within its 5 year lifetime. These events can be used to determine accurate masses of the lenses, and to derive the mass and the scale parameters (length and height) of the Galactic disk.

The discovery of a microarcsecond quasar: J1819+3845

J. Dennett-Thorpe, A.G. de Bruyn

Preprint no. 288

Accepted for publication in Astronomy and Astrophysics.

Abstract

We report on the discovery of a source which exhibits over 300% amplitude changes in radio flux density on the period of hours. This source, J 1819+3845, is the most extremely variable extragalactic source known in the radio sky. We believe these properties are due to interstellar scintillation, and show that the source must emit at least 55% of its flux density within a radius of <16 microarcseconds at 5 GHz. The apparent brightness temperature is >5.1012 K, and the source may be explained by a relativistically moving source with a Doppler factor ~15. The scattering occurs predominantly in material only a few tens of parsecs from the earth, which explains its unusually rapid variability. If the source PKS 0405-385 (Kedsiora-Chudszer et al 1997 is similarly affected by local scattering material, Doppler factors of ~ 10000 are not required to explain this source. The discovery of a second source whose properties are well modeled by interstellar scintillation strengthens the argument for this as the cause for much of the variations seen in intra-day variables (IDV).

Combined Analysis of the Binary-Lens Caustic-Crossing Event MACHO 98-SMC-1

C. Afonso, C. Alard, J.N. Albert, J. Andersen, R. Ansari, E. Aubourg, P. Bareyre, F. Bauer, J.P. Beaulieu, A. Bouquet, S. Char, X. Charlot, F. Couchot, C. Coutures, F. Derue, R. Ferlet, J.F. Glicenstein, B. Goldman, A. Gould, D. Graff, M. Gros, J. Haissinski, J.C. Hamilton, D. Hardin, J. de Kat, A. Kim, T. Lasserre, E. Lesquoy, C. Loup, C. Magneville, J.B. Marquette, E. Maurice, A. Milsztajn, M. Moniez, N. Palanque-Delabrouille, O. Perdereau, L. Prevot, N. Regnault, J. Rich, M. Spiro, A. Vidal-Madjar, L. Vigroux, S. Zylberajch (The EROS Collaboration), C. Alcock, R.A. Allsman, D. Alves, T.S. Axelrod, A.C. Becker, K.H. Cook, A.J. Drake, K.C. Freeman, K. Griest, L.J. King, M.J. Lehner, S.L. Marshall, D. Minniti, B.A. Peterson, M.R. Pratt, P.J. Quinn, A.W. Rodgers, P.B. Stetson, C.W. Stubbs, W. Sutherland, A. Tomaney, T. Vandehei (The MACHO/GMAN Collaboration), S.H. Rhie, D.P. Bennett, P.C. Fragile, B.R. Johnson, J. Quinn (The MPS Collaboration), A. Udalski, M. Kubiak, M. Szymanski, G. Pietrzynski, P. Wozniak, K. Zebrun (The OGLE Collaboration), M.D. Albrow, J.A.R. Caldwell, D.L. DePoy, M. Dominik, B.S. Gaudi, J. Greenhill, K. Hill, S. Kane, R. Martin, J. Menzies, R.M. Naber, R.W. Pogge, K.R. Pollard, P.D. Sackett, K.C. Sahu, P. Vermaak, R. Watson, A. Williams (The PLANET Collaboration)

Preprint no. 287

Accepted for publication in the Astrophysical Journal, Vol. 532 (March 2000).

Abstract

We fit the data for the binary-lens microlensing event MACHO 98-SMC-1 from 5 different microlensing collaborations and find two distinct solutions characterized by binary separation d and mass ratio q: (d,q)=(0.54,0.50) and (d,q)=(3.65,0.36), where d is in units of the Einstein radius. However, the relative proper motion of the lens is very similar in the two solutions, 1.30 km/(s kpc) and 1.48 km/(s kpc), thus confirming that the lens is in the Small Magellanic Cloud. The close binary can be either rotating or approximately static but the wide binary must be rotating at close its maximum allowed rate to be consistent with all the data. We measure limb-darkening coefficients for five bands ranging from I to V. As expected, these progressively decrease with rising wavelength. This is the first measurement of limb darkening for a metal-poor A star.

Radio imaging of core-dominated high redshift quasars

Peter D. Barthel, Marianne Vestergaard, and Colin J. Lonsdale

Preprint no. 286

Accepted for publication in the Astronomy and Astrophysics.

Abstract

VLA imaging at kiloparsec-scale resolution of sixteen core-dominated radio-loud QSOs is presented. Many objects appear to display variable radio emission and their radio morphologies are significantly smaller than those of steep-spectrum quasars, consistent with these objects being observed at sight lines close to their (relativistic, gamma~4-7) jet axes. The usefulness of the radio source orientation indicator R_V, being defined as ratio of radio core and rest frame optical V-band luminosity, is confirmed.

A Complete Set of Solutions For Caustic-Crossing Binary Microlensing Events

M. D. Albrow, J.-P. Beaulieu, J. A. R. Caldwell, D. L. DePoy, M. Dominik, B. S. Gaudi, A. Gould, J. Greenhill, K. Hill, S. Kane, R. Martin, J. Menzies, R. M. Naber, R. W. Pogge, K. R. Pollard, P. D. Sackett, K. C. Sahu, P. Vermaak, R. Watson, A. Williams (The PLANET Collaboration)

Preprint no. 285

Accepted for publication in the Astrophysical Journal, 522, 1022.

Abstract

We present a method to analyze binary-lens microlensing light curves with one well-sampled fold caustic crossing. In general, the surface of chi2 shows extremely complicated behavior over the 9-parameter space that characterizes binary lenses. This makes it difficult to systematically search the space and verify that a given local minimum is a global minimum. We show that for events with well-monitored caustics, the caustic-crossing region can be isolated from the rest of the light curve and easily fit to a 5-parameter function. Four of these caustic-crossing parameters can then be used to constrain the search in the larger 9-parameter space. This allows a systematic search for all solutions and thus identification of all local minima. We illustrate this technique using the PLANET data for MACHO 98-SMC-1, an excellent and publicly available caustic-crossing data set. We show that a very broad range of parameter combinations are compatible with the PLANET data set, demonstrating that observations of binary-lens lightcurves with sampling of only one caustic crossing do not yield unique solutions. The corollary to this is that the time of the second caustic crossing cannot be reliably predicted on the basis of early data including the first caustic crossing alone. We investigate the requirements for determination of a unique solution and find that occasional observations of the first caustic crossing may be sufficient to derive a complete solution.

Detection of Warm and Cold Phases of the Neutral ISM in a Damped Ly-alpha Absorber

W.M. Lane, F.H. Briggs, and A. Smette

Preprint no. 284

Accepted for publication in the Astrophysical Journal, 2000

Abstract

We present a detailed study of the HI 21cm absorption system at z=0.0912 towards the radio quasar B0738+313. The uncommonly narrow main absorption line and weak secondary line are resolved for the first time. In addition we find it necessary to add a third, broader shallow component to obtain a good fit to the spectrum. Although the harmonic mean spin temperature calculated by comparison of the 21cm lines to the damped Ly-alpha line is Ts = 775 K, the thermal kinetic temperatures of the two narrow components, calculated from their widths, are much lower: Tk < 297 and < 103 K respectively. This is the first case of a redshifted absorption system for which Tk is measured to be less than Ts. We discuss this result in the context of a two phase gas model, in which the damped Ly-alpha gas is sensitive to a significant neutral column density of warm phase gas as well as the cold phase gas of the narrow 21cm lines. The third component is interpreted as representing the warm phase gas with with Tk <5050 K. The combined column density of the three 21cm components is approximately equal to that derived from fits to the damped Ly-alpha line.

ISO-SWS observations of interstellar solid 13CO2: heated ice and the Galactic 12C/13C abundance ratio

A.C.A. Boogert, P. Ehrenfreund, P.A. Gerakines, A.G.G.M. Tielens, D.C.B. Whittet, W.A. Schutte, E.F. van Dishoeck, Th. de Graauw, L. Decin, T. Prusti

Preprint no. 283

Accepted for publication in Astronomy and Astrophysics

Abstract

We present observations of the stretching mode of 13CO2 ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory (ISO-SWS). Remarkable variations are seen in the absorption band profile in the different lines of sight. The main feature is attributed to 13CO2 mixed with polar molecules such as H2O, and CH3OH. The high-mass protostars GL 2136, GL 2591, S 140 : IRS1, and W 3 : IRS5 show an additional narrow substructure at 2282 cm-1 (4.382 um), which we attribute to a polar, CH3OH-containing CO2 ice, that experienced heating. This heating effect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar CO2 ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing.

Furthermore, we determine for the first time the Galactic 12C/13C ratio from the solid state as a function of Galacto-centric radius. The 12CO2/13CO2 ratio for the local ISM (69+/-15), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase (C18O, H2CO) studies. For the few individual objects for which gas phase values are available, the 12C/13C ratios derived from CO2 tend to be higher compared to CO studies (albeit with 2.5 sigma significance only). We discuss the implications of this possible difference for the chemical origin of interstellar CO2.

The Planetary Nebulae Spectrograph

M. Arnaboldi, M. Capaccioli, N. Douglas, K. Kuijken, K. Freeman, T. Axelrod, K. Taylor, R. Gilmozzi and R. Kudritzki

Preprint no. 282

To be published in the Proceedings of the SAIT* symposium, Naples 1999
*Societa Astronomica Italiana

Abstract

We present the Planetary Nebulae Spectrograph project. This is an international collaboration aimed at building a dedicated slitless spectrograph for the detection of extragalactic planetary nebulae to be mounted at the 4 metre WHT and TNG. The scientific projects and the scope of such an instrument are outlined. The feasibility studies and the final optical design are presented.

Neutral hydrogen and dark matter in spiral galaxies

Renzo Sancisi

Preprint no. 281

Figures 1 and 2 are separated from the text

figure 1, figure 2.

To be published in: "Toward a New Millennium in Galaxy Morphology" (eds. D.L. Block, I. Puerari, A. Stockton and D. Ferreira), Conf. Proc., Kluwer, Dordrecht.

Abstract

The first part presents a brief review of the main HI properties of isolated, normal spiral galaxies and of the phenomena which seem to characterize and dominate their internal metabolism. In the second part attention is drawn to all those processes, such as tidal interactions, accretion and mergers, that depend on the galaxy environment and may play a significant role in galaxy formation and evolution. In the third part the observational evidence for the dark matter component of spiral galaxies is discussed.

On the axis ratio of the stellar velocity ellipsoid in disks of spiral galaxies

P.C. van der Kruit and R. de Grijs

Preprint no. 280

Accepted for publication in: Astronomy and Astrophysics.

Abstract

The spatial distribution of stars in a disk of a galaxy can be described by a radial scalelength and a vertical scaleheight. The ratio of these two scale parameters contains information on the axis ratio of the velocity ellipsoid, i.e. the ratio of the vertical to radial stellar velocity dispersions of the stars, at least at some fiducial distance from the center. The radial velocity dispersion correlates well with the amplitude of the rotation curve and with the disk integrated magnitude, as was found by Bottema (1993). These relations can be understood as the result of the stellar disk being (marginally) stable against local instabilities at all length scales. This is expressed by Toomre's well-known criterion, which relates the sheer in the rotation to a minimum value that the radial stellar velocity dispersion should have for stability for a given surface density. Via the Tully-Fisher (1977) relation, the velocity dispersion then becomes related to the integrated magnitude and hence to the scalelength. The vertical velocity dispersion relates directly to the scaleheight through hydrostatic equilibrium. It can be shown that the ratio of the two length scales relates to the axis ratio of the velocity ellipsoid only through the Toomre parameter Q and in particular does not require a choice of the mass-to-light ratio or a distance scale.
We have applied this to the statistically complete sample of edge-on galaxies, for which de Grijs (1997) has performed surface photometry and has determined the length scales in the stellar light distribution.

Properties of the Stellar Velocity Ellipsoid and Stability in Disks of Spiral Galaxies

P.C. van der Kruit

Preprint no. 279

To be published in: "Toward a New Millenium in Galaxy Morphology", Conf. Proc. (Johannesburg, South Africa, September 1999).

Abstract

Disks of spiral galaxies are characterized by effectively exponential brightness and presumably density distributions in both the radial and vertical directions. It is to be expected that the ratio between the scalelength and -height bears a relation to the axis ratio of the stellar velocity ellipsiod. Hydrostatic equilibrium connects the vertical velocity dispersion to the scaleheight. In the radial direction the velocity dispersion relates to the scalelength through conditions of local stability. Preliminary applications are presented.

The Milky Way compared to external galaxies

P.C. van der Kruit

Preprint No. 278

To be published in: "The Legacy of J.C. Kapteyn", Conf. Proc. (Groningen, the Netherlands, June 1999).

Abstract

Kapteyn's work to map the Sidereal System was based on star counts. Now that we know that the stellar system is comparable to many external spiral galaxies, this information can be used to further constrain models for the structure of our Galaxy. It is now possible to also use surface brightness distributions, both in the optical from the Pioneer spacecrafts and in the near-infrared with the COBE/DIRBE experiment. Star counts and surface photometry are discussed in a comparative sense. It turns out, that at least away from directions close to the Galactic center, surface brightness is dominated by the contribution from the disk, while faint star counts are dominated by the halo population. If low Galactic latitudes are ignored, surface brightness data can be used to constrain the parameters of the old stellar disk.
I repeat an earlier discussion in which older star counts are compared with current models for the distribution of stars in the Galactic disk. Herschel, in his famous "Star Gauges" of 1785, counted stars rather consistently to a visual magnitude of 14.5 to 15. This is consistent with Herschel's own values for the "gage or space penetrating powers" of his telescopes and his "extent of telescopic vision". Counts early this century by Kapteyn and van Rhijn seem to have significantly incorrect magnitude scales compared to the prediction of models, in the sense of being too bright by half a magnitude or more at photograpic magnitude 18.5. Yet, the counts are consistent across the sky and with limits on current parameters for the Galactic disk.
A discussion is given of the value of the exponential scalelength of the Galactic disk. A value of 4.5 to 5.0 kpc is still preferred in spite of smaller values from surveys in the near-infrared. Finally, I show that as far as structural parameters are concerned, NGC 891, 5033 and 5375 closely resemble the Milky Way Galaxy.

The binary gravitational lens and its extreme cases

M. Dominik

Preprint No. 277

Accepted for publication in: Astronomy and Astrophysics, 349, 108, 1999.

Abstract

The transition of the binary gravitational lens from the equal mass case to small (planetary) mass ratios q is studied. It is shown how the limit of a (pure shear) Chang-Refsdal lens is approached, under what conditions the Chang-Refsdal approximation is valid, and how the 3 different topologies of the critical curves and caustics for a binary lens are mapped onto the 2 different topologies for a Chang-Refsdal lens with pure shear. It is shown that for wide binaries, the lensing in the vicinity of both lens objects can be described by a Taylor-expansion of the deflection term due to the other object, where the Chang-Refsdal approximation corresponds to a truncation of this series. For close binaries, only the vicinity of the secondary, less massive, object can be described in this way. However, for image distances much larger than the separation of the lens objects, any binary lens can be approximated by means of multipole expansion, where the first non-trivial term is the quadrupole term. It is shown that an ambiguity exists between wide and close binary lenses, where the shear at one of the objects due to the other object for the wide binary is equal to the absolute value of the eigenvalues of the quadrupole moment for the close binary. This analysis provides the basis for a classification of binary lens microlensing events, especially of planetary events, and an understanding of present ambiguities.

Lensed galaxies in Abell 370. II. The ultraviolet properties of arclets and the star formation rate at high redshift

J. Bezecourt, G. Soucail, R.S. Ellis, J.-P. Kneib

Preprint No. 276

Accepted for publication in: Astronomy and Astrophysics.

Abstract

We discuss the statistical properties of faint background galaxies detected in a deep Hubble Space Telescope ultraviolet (F336W) Wide Field Planetary Camera 2 image of the lensing cluster Abell 370 (z=0.37). By combining modelled field galaxy counts at this wavelength with a detailed mass model for the cluster, we develop techniques for predicting the expected redshift distribution of lensed sources and find the bulk of the sources should be moderately magnified sources whose redshifts lie between 0.5 and 2. We compare these predictions with redshift estimates derived from the lens inversion method which utilises the shapes of individual arclets in the context of a known field ellipticity distribution. This allows us to infer the comoving star formation rate for the range 0.5<z<2 where there is currently little spectroscopic data. We discuss our results in the context of contemporary pictures for the history of star formation.

Lensed galaxies in Abell 370. I. Modeling the number counts and redshift distribution of background sources

J. Bezecourt, J.P. Kneib, G. Soucail, T.M.D. Ebbels

Preprint No. 275

Accepted for publication in: Astronomy and Astrophysics.

Abstract

We present new observations of the cluster-lens Abell 370: a deep HST/WFPC2 F675W image and ESO 3.6m spectroscopy of faint galaxies. These observations shed new light on the statistical properties of faint lensed galaxies. In particular, we spectroscopically confirm HST/WFPC2 F675W image and ESO 3.6m spectroscopy of faint galaxies. These observations shed new light on the statistical properties of faint lensed galaxies. In particular, we spectroscopically confirm the multiple image nature of the B2-B3 gravitational pair (Kneib et al. 1993), and determine a redshift of z=0.806±0.002 which is in very good agreement with earlier predictions. A refined mass model of the cluster core (that includes cluster galaxy halos) is presented, based on a number of newly identified multiple images. Following Bezecourt et al. (1998a), we combine the new cluster mass model with a spectrophotometric prescription for galaxy evolution to predict the arclets number counts and redshift distribution in the HST image. In particular, the ellipticity distribution of background sources is taken into account, in order to properly estimate the statistical number and redshift distribution of arclets. We show that the redshift distribution of arclets, and particularly its high redshift tail can be used as a strong constraint to disentangle different galaxy evolution scenarios. A hierarchical model which includes number density evolution is favored by our analysis. Finally, we compute the depletion curves for the faint galaxy number counts and discuss their wavelength dependence.

Surface photometry of bulge dominated low surface brightness galaxies

M. Beijersbergen, W.G.J. de Blok, and J.M. van der Hulst

Preprint No. 274

Accepted for publication in: Astronomy and Astrophysics.

Abstract

We present results of broad band BVRI observations of a sample of galaxies with a low surface brightness (LSB) disk and a bulge. These galaxies are well described as exponential disks and exponential bulges with no preferred value for either scale length or central surface brightness. The median B band disk scale length is 12.6 kpc (H0 = 75 km/s/Mpc) which is much larger than scale lengths of typical (disk dominated) LSB or high surface brightness (HSB) galaxies. Furthermore, the disk and bulge scale lengths are correlated, suggesting a coupling in the formation. Bulge dominated LSB galaxies are observed to be redder than disk dominated LSB galaxies and their bulge-to-disk ratios are increasing towards redder wavelengths. We find colors that are comparable to or bluer than HSB galaxies of the same morphological types. Bulge dominated LSB galaxies are therefore not faded HSB galaxies with no current star formation. We find that bulge dominated LSB galaxies fit in with the general trends defined by the HSB galaxies. The properties of these bulge dominated LSB galaxies show that LSB galaxies do not just come in two varieties. They cover the entire range in optical and morphological properties between late-type disk dominated LSBs and giant Malin-1-like LSBs. LSB galaxies thus also form a LSB Hubble sequence, parallel to the classical HSB one.

Spectrophotometry of nearby field galaxies: the data

R.A. Jansen, D. Fabricant, M. Franx, N. Caldwell

Preprint No. 273

Accepted for publication in: Astrophysical Journal Suppl. (Vol. 127, March 2000)

Abstract

We have obtained integrated and nuclear spectra, as well as U, B, R surface photometry, for a representative sample of 196 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (M_B=-14 to -22). Here we present the spectrophotometry for these galaxies. The selection of the sample and the U, B, R surface photometry is described in a companion paper (Paper I). Our goals for the project include measuring the current star formation rates and metallicities of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt (1992a) to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high redshift.
We describe the observing, data reduction and calibration techniques, and demonstrate that our spectrophotometry agrees well with that of Kennicutt. The spectra span the range 3550--7250 A at a resolution (FWHM) of ~6 A, and have an overall relative spectrophotometric accuracy of +/- 6 per cent. We present a spectrophotometric atlas of integrated and nuclear rest-frame spectra, as well as tables of equivalent widths and synthetic colors.
We study the correlations of galaxy properties determined from the spectra and images. Our findings include: (1) galaxies of a given morphological class display a wide range of continuum shapes and emission line strengths if a broad range of luminosities are considered, (2) emission line strengths tend to in- crease and continua tend to get bluer as the luminosity decreases, and (3) the scatter on the general correlation between nuclear and integrated H_alpha emission line strengths is large.

Surface photometry of nearby field galaxies: the data

R.A. Jansen, M. Franx, D. Fabricant, N. Caldwell

Preprint No. 272

Accepted for publication in: Astrophysical Journal Suppl. (Vol. 127, March 2000)

Abstract

We have obtained integrated spectra and multi-filter photometry for a repre- sentative sample of ~200 nearby galaxies. These galaxies span the entire Hubble sequence in morphological type, as well as a wide range of luminosities (M_B = -14 to -22) and colors (B-R=0.4 to 1.8). Here we describe the sample selection criteria and the U, B, R surface photometry for these galaxies. The spectro- photometric results will be presented in a companion paper. Our goals for the project include measuring the current star formation rates and metallicity of these galaxies, and elucidating their star formation histories, as a function of luminosity and morphology. We thereby extend the work of Kennicutt (1992) to lower luminosity systems. We anticipate that our study will be useful as a benchmark for studies of galaxies at high redshift.
We discuss the observing, data reduction and calibration techniques, and show that our photometry agrees well with previous work in those cases where earlier data are available. We present an atlas of images, radial surface brightness profiles and color profiles, as well as tables of derived parameters. The atlas and tables of measurements will be made available electronically.
We study the correlations of galaxy properties determined from the galaxy images. Our findings include: (1) colors determined within the effective radius correlate better with morphological type than with M_B and (2) 50 per cent of the low luminosity galaxies are bluest in their centers.

Planetary Microlensing: Present Status and Long-term Goals

Penny D. Sackett

Preprint No. 271

To appear in the Springer-Verlag series "ESO Astrophysics Symposia".
Invited Review at VLT Opening Symposium, Antofagasta, Chile, March 1999.

Abstract

Massive gravitational microlensing programs were begun about a decade ago as a means to search for compact baryonic dark matter in the Galaxy, but before the first events were detected the technique was also proposed as a means of detecting extra-solar planets in our Galaxy. Current microlensing planet searches, which have been underway for four years, are sensitive to jovian-mass planets orbiting a few to several AU from their parent Galactic stars. Within two years, sufficient data should be in hand to characterize or meaningfully constrain the frequency of massive planets in this range of parameter space, nicely complementing information about planets at smaller orbital radii now being provided by radial velocity searches. In principle, the technique could be pushed to smaller planetary masses, but only if a larger number of faint microlensed sources can be monitored with higher precision and temporal sampling. The VST on Paranal, with spectroscopic follow-up with the VLT, may be the ideal instrument for such an ambitious program.

Galaxy kinematics from counter-dispersed imaging

N.G. Douglas, K. Taylor

Preprint No. 270

Accepted for publication in: Monthly Notices of the Royal Astronomical Society.

Abstract

Determining the internal kinematics of a galaxy from planetary nebulae (PN) is usually a two-step process in which the candidate PN are first identified in a target galaxy and then, in a follow up run, spectra are obtained. We have implemented a new technique in which two dispersed images at the wavelength of the [OIII] emission line at 5007 Å are combined to yield positions, magnitudes and velocities of the PN population in a single step. A reduction in observing time of about a factor 2 is attainable. We present here the proof-of-principle results.

The kinematics of the bulge and the disc of NGC 7331

Roelof Bottema

Preprint No. 269

Accepted for publication in: Astronomy & Astrophysics, Main Journal

Abstract

Results are presented of spectroscopic emission and absorption line observations along the major axis of the Sb galaxy NGC 7331. The kinematics of the ionized gas and stellar component are derived, being regular and symmetric with respect to the centre of the galaxy. Contrary to what may be expected, for radii less than 40 arcsec the emission line gas appears to rotate slower than the stars. A likely explanation for this phenomenon is an inclined and warped gas layer in those inner regions. In the bulge dominated region the absorption line profiles are asymmetric in the sense that they have a shallow extension towards the systemic velocity. No counterrotating stellar component is observed, which is contrary to previous claims. As demonstrated, these claims might be based on a wrong interpretation of the employed analysis method. Outside the bulge dominated region the stellar radial velocities are in agreement with the neutral hydrogen dynamics and the stellar velocity dispersion decreases towards larger radii.
A detailed bulge/disc light decomposition has been made. This has been used to construct a kinematical model of NGC 7331 from which model absorption line profiles were calculated. These profiles have been compared with the observations and model parameters have been adjusted to obtain a good match. It appeared necessary to combine a rapidly rotating disc having a radially decreasing velocity dispersion with a slowly rotating constant dispersion bulge. Then, simultaneously, the observed stellar radial velocities, the velocity dispersions and the observed asymmetry of the line profile could be explained satisfactorily. An even better fit to the data can be achieved when the disc is relatively thinner and colder inside the bulge region.
For the disc the stellar velocity dispersions and photometry result in a mass-to-light ratio of 1.6±0.7 in the I-band. This value agrees with previous determinations for other discs using observed velocity dispersions. A rotation curve analysis allows the calculation of the mass-to-light ratio of the bulge which amounts to 6.8 in the I-band; considerably larger than the disc value. It appears that the mass distribution of NGC 7331 is completely dominated by the combination of bulge and dark halo at all radii. Comparing well determined mass-to-light ratios of a number of bulges with disc values the mass-to-light ratio of the bulge is, on average, three times as large as the mass-to-light ratio of the disc in the I-band. For the B-band this ratio goes up to 7.2, a fact which should have cosmological consequences.

Limb-Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97-BLG-28

M.D. Albrow, J.-P. Beaulieu, J.A.R. Caldwell, M. Dominik, J. Greenhill, K. Hill, S. Kane, R. Martin, J. Menzies, R.M. Naber, J.-W. Pel, K. Pollard, P.D. Sackett, K.C. Sahu, P. Vermaak, R. Watson, A. Williams (The PLANET Collaboration), and M. S. Sahu

Preprint No. 268

Accepted for publication in: volume 522 of the 1999 Astrophysical Journal

Abstract

We present the PLANET photometric dataset for the binary-lens microlensing event MACHO 97-BLG-28 consisting of 696 I and V-band measurements, and analyze it to determine the radial surface brightness profile of the Galactic bulge source star. The microlensed source, demonstrated to be a K giant by our independent spectroscopy, crossed the central isolated cusp of the lensing binary, generating a sharp peak in the light curve that was well-resolved by dense (3 - 30 minute) and continuous monitoring from PLANET sites in Chile, South Africa, and Australia. Our modeling of these data has produced stellar profiles for the source star in the I and V bands that are in excellent agreement with those predicted by stellar atmospheric models for K giants. The limb-darkening coefficients presented here are the first derived from microlensing, among the first for normal giants by any technique, and the first for any star as distant as the Galactic bulge. Modeling indicates that the lensing binary has a mass ratio q = 0.23 and an (instantaneous) separation in units of the angular Einstein ring radius of d = 0.69. For a lens in the Galactic bulge, this corresponds to a typical stellar binary with a projected separation between 1 and 2 AU. If the lens lies closer, the separation is smaller, and one or both of the lens objects is in the brown dwarf regime. Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source proper motion is mu = 19.4 +/- 2.6 km/s kpc, consistent with a disk or bulge lens. If the non-lensed blended light is due to a single star, it is likely to be a young white dwarf in the bulge, consistent with the blended light coming from the lens itself.

The Shape of Dark Matter Halo

Penny D. Sackett

Preprint No. 267

Invited Review to appear in "Galaxy Dynamics" (eds. D. Merritt, J.A. Sellwood and M. Valluri), proceedings of the Rutgers Conferece, A.S.P. Conf. Series, 1999

Abstract

Techniques for inferring the radial and geometric form of dark matter halos and the results they have produced to date are reviewed. Dark halos appear to extend to at least ~50 kpc with total enclosed masses that rise linearly with radius R. Whether this behavior can be extrapolated to distances as large as 200 kpc and beyond is controversial; results at this radius are model-dependent. Observationally, the geometrical form of the dark halo can be characterized by the equatorial axis ratio b/a (ovalness) and vertical-to-equatorial axis ratio c/a (flattening) of the total density. Different techniques consistently yield b/a 0.7 (and thus b/a 0.9 for the potential) at R~20 kpc, with more axisymmetric values, b/a >~ 0.8, being more likely. Results are less consistent for the vertical flattening, perhaps due to the difference in the spatial regions probed by different techniques or inappropriate assumptions. Techniques that probe furthest from the stellar plane z~15 kpc consistently implicate substantially flattened c/a = 0.5 +/- 0.2 dark halos. These axis ratios are in acceptable agreement with expectations from N-body simulations of cold dark matter mixed with ~10% dissipational gas.


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