Kapteyn Institute Preprints (1998)

Kapteyn Institute Preprint Service.

Preprints of the Kapteyn Institute.
Paper copies can be ordered at:
Kapteyn Institute
P.O. Box 800
9700 AV Groningen
The Netherlands
 All the preprints are compressed using gzip.

1998 Preprints

* Preprint 266 Kinematically Lopsided Spiral Galaxies, R.A. Swaters, R.H.M. Schoenmakers, R. Sancisi and T.S. van Albada
* Preprint 265 Searching for Unseen Planets via Occultation and Microlensing, Penny D. Sackett
* Preprint 264 ORFEUS II echelle Spectra: Absorption by H2 in the LMC, K.S. de Boer, P. Richter, D.J. Bomans, A. Heithausen, and J. Koornneef
* Preprint 263 The Relative Lens-Source Proper Motion in MACHO 98-SMC-1, M. 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 K.R. Pollard, P.D. Sackett, K.C. Sahu, P. Vermaak R. Watson, A. Williams (The PLANET Collaboration) and R.W. Pogge
* Preprint 262 Luminosity Evolution of Early-Type Galaxies to Z = 0.83: Constraints on Formation Epoch and Omega, Pieter G. van Dokkum, Marijn Franx, Daniel D. Kelson and Garth D. Illingworth
* Preprint 261 The 1995 Pilot Campaign of PLANET: Searching for Microlensing Anomalies through Precise, Rapid, Round-the-Clock Monitoring, M. Albrow, J.-P. Beaulieu, P. Birch, J.A.R. Caldwell, S. Kane, R. Martin, J. Menzies, R.M. Naber, J.-W. Pel, K. Pollard, P.D. Sackett, K.C. Sahu, P. Vreeswijk, A. Williams, M.A. Zwaan (The PLANET Collaboration)
* Preprint 260 Redshifted 21cm Line Absorption by Intervening Galaxies, F.H. Briggs
* Preprint 259 Cosmologically Distant OH Megamasers: A Test of the Galaxy Merging Rate at Z Approximately 2 and a Contaminant of Blind HI Surveys in the 21cm Line, F.H. Briggs
* Preprint 258 The Color-Magnitude Relation in CL 1358+62 at Z = 0.33: Evidence for Significant Evolution in the S0 Population, Pieter G. van Dokkum, Marijn Franx, Daniel D. Kelson, Garth D. Illingworth, David Fisher and Daniel Fabricant
* Preprint 257 Star Formation and the Interstellar Medium in Low Surface Brightness Galaxies, II. Deep CO Observations of Low Surface Brightness Galaxies W.J.G. de Blok and J.M. van der Hulst
* Preprint 256 Star Formation and the Interstellar Medium in Low Surface Brightness Galaxies I. Oxygen Abundances and Abundance Gradients in Low Surface Brightness Disk Galaxies, W.J.G. de Blok and J.M. van der Hulst
* Preprint 255 The Global Structure of Galactic Discs, R. de Grijs
* Preprint 254 Weak Lensing Analysis of Cl 1358+62 using Hubble Space Telescope, H. Hoekstra, M. Franx, K. Kuijken, G. Squires
* Preprint 253 Red Clump Morphology as Evidence Against a New Intervening Stellar Population as the Primary Source of Microlensing toward the LMC, J.-P. Beaulieu and P.D. Sackett
* Preprint 252 The Gas/Solid Methane Abundance Ratio toward Deeply Embedded Protostars, A.C.A. Boogert, F.P. Helmich, E.F. van Dishoeck, W.A. Schutte, A.G.G.M. Tielens and D.C.B. Whittet
* Preprint 251 Rotation Curves of UMa Galaxies in the Context of Modified Newtonian Dynamics, N R.H. Sanders, M.A.W. Verheijen
* Preprint 250 On the Evolutionary Behaviour of BL Lac Objects, N. Bade, V. Beckmann, N.G. Douglas, P.D. Barthel, D. Engels, L. Cordis, P. Nass, W. Voges
* Preprint 249 Nature of 60 um Emission in 3C47, 3C207 and 3C334, I.M. van Bemmel, P.D. Barthel, M.S. Yun
* Preprint 248 Cosmology with modified Newtonian dynamics (MOND), R.H. Sanders
* Preprint 247 HST photometry of the stars near the center of PN NGC 650, J. Koornneef and S.R. Pottasch
* Preprint 246 Galaxy interactions: the HI signature, R. Sancisi
* Preprint 245 Testing the dark matter hypothesis with low surface brightness galaxies and other evidence, S.S. McGaugh and W.J.G. de Blok
* Preprint 244 Testing the hypothesis of modified dynamics with low surface brightness galaxies and other evidence, S.S. McGaugh and W.J.G. de Blok
* Preprint 243 Multifrequency radio continuum observations of head-tail galaxies in the Perseus cluster, D. Sijbring and A.G. de Bruyn

Kinematically Lopsided Spiral Galaxies

R.A. Swaters, R.H.M. Schoenmakers, R. Sancisi and T.S. van Albada

Preprint No. 266

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

Abstract

Asymmetries in the distribution of light and neutral hydrogen are often observed in spiral galaxies. Here, attention is drawn to the presence of large-scale asymmetries in their kinematics. Two examples of kinematically lopsided galaxies are presented and discussed. The shape of the rotation curve --rising more steeply on one side of the galaxy than on the other-- is the signature of the kinematic lopsidedness. It is shown that kinematic lopsidedness may be related to lopsidedness in the potential, and that even a mild perturbation in the latter can produce significant kinematical effects. Probably at least half of all spiral galaxies are lopsided.

Searching for Unseen Planets via Occultation and Microlensing

Penny D. Sackett

Preprint No. 265

Accepted for publication in: Planets outside the Solar System: Theory and Observations (eds. J.-M. Mariotti and D. Alloin), NATO-ASI Series, Kluwer, Dordrecht

Abstract

The fields of occultation and microlensing are linked historically. Early this century, occultation of the Sun by the Moon allowed the apparent positions of background stars projected near the limb of the Sun to be measured and compared with their positions six months later when the Sun no longer influenced the light path to Earth. The measured shift in the stellar positions was consistent with lensing by the gravitational field of the Sun during the occultation, as predicted by the theory of general relativity. This series of lectures explores the principles, possibilities and challenges associated with using occultation and microlensing to discover and characterize unseen planets orbiting distant stars. The two techniques are complementary in terms of the information that they provide about planetary systems and the range of system parameters to which they are most sensitive. Although the challenges are large, both microlensing and occultation may provide avenues for the discovery of extra-solar planets as small as Earth.

ORFEUS II echelle spectra: absorption by H2 in the LMC

K.S. de Boer, P. Richter, D.J. Bomans, A. Heithausen, and J. Koornneef

Preprint No. 264

Accepted for publication in: Astronomy and Astrophysics 338, L5

Abstract

We report the first detection of H2 absorption profiles of LMC gas on the line of sight to star 3120 in the association LH10 near the emission nebula N11B. Transitions found in the Lyman band are used to derive a total column density N(H2)= 6.6 × 1018 cm-2. Excitation temperatures of < 50 K for levels J < 1 and of ~ 470 K for levels 2 < J < 4 of H2 are derived. We conclude that moderate UV pumping influences the population even of the lowest rotational states in this LMC gas.

The Relative Lens-Source Proper Motion in MACHO 98-SMC-1

M. 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 K.R. Pollard, P.D. Sackett, K.C. Sahu, P. Vermaak R. Watson, A. Williams (The PLANET Collaboration) and R.W. Pogge

Preprint No. 263

Accepted for publication in: the Astrophysical Journal

Abstract

We present photometric and spectroscopic data for the second microlensing event seen toward the Small Magellanic Cloud (SMC), MACHO-98-SMC-1. The lens is a binary. We resolve the caustic crossing and find that the source took 2Delta t = 8.5 hours to transit the caustic. We measure the source temperature Teff = 8000 K both spectroscopically and from the color (V-I)0 ~ 0.22. We find two acceptable binary-lens models. In the first, the source crosses the caustic at phi=43°.2 and the unmagnified source magnitude is Is = 22.15. The angle implies that the lens crosses the source radius in time t*= Delta t sin phi = 2.92 hours. The magnitude (together with the temperature) implies that the angular radius of the source is theta* = 0.089 µas. Hence, the proper motion is µ = theta*/t* = 1.26 kms-1 kpc-1. For the second solution, the corresponding parameters are phi = 30°.6, Is = 21.81, t* = 2.15 hours, theta* = 0.104 µas, µ = theta*/t* = 2.00 kms-1 kpc-1. Both proper-motion estimates are slower than 99.5% of the proper motions expected for halo lenses. Both are consistent with an ordinary binary lens moving at ~75-120 kms within the SMC itself. We conclude that the lens is most likely in the SMC proper.

Luminosity Evolution of Early-Type Galaxies to Z = 0.83: Constraints on Formation Epoch and Omega

Pieter G. van Dokkum, Marijn Franx, Daniel D. Kelson and Garth D. Illingworth

Preprint No. 262

Accepted for publication in: the Astrophysical Journal, 504, Letter 17

Abstract

We present deep spectroscopy with the Keck telescope of eight galaxies in the luminous X-ray cluster MS 1054-03 at z = 0.83. The data are combined with imaging observations from the Hubble Space Telescope (HST). The spectroscopic data are used to measure the internal kinematics of the galaxies, and the HST data to measure their structural parameters. Six galaxies have early-type spectra, and two have "E+A" spectra. The galaxies with early-type spectra define a tight Fundamental Plane (FP) relation. The evolution of the mass-to-light ratio is derived from the FP. The M/L ratio evolves as Delta log M/LB propto -0.40z (Omegam = 0.3, Omega_Lambda = 0). The observed evolution of the M/L ratio provides a combined constraint on the formation redshift of the stars, the IMF, and cosmological parameters. For a Salpeter IMF (x = 2.35) we find that zform > 2.8 and Omegam < 0.86 with 95% confidence. The constraint on the formation redshift is weaker if Omega_Lambda > 0: zform > 1.7 if Omegam = 0.3 and Omega_Lambda = 0.7. At present the limiting factor in constraining zform and Omega from the observed luminosity evolution of early-type galaxies is the poor understanding of the IMF. We find that if Omegam = 1 the IMF must be significantly steeper than the Salpeter IMF (x > 2.6).

The 1995 Pilot Campaign of PLANET: Searching for Microlensing Anomalies through Precise, Rapid, Round-the-Clock Monitoring

M. Albrow, J.-P. Beaulieu, P. Birch, J. A. R. Caldwell, S. Kane, R. Martin, J. Menzies, R. M. Naber, J.-W. Pel, K. Pollard, P. D. Sackett, K. C. Sahu, P. Vreeswijk, A. Williams, M. A. Zwaan (The PLANET Collaboration)

Preprint No. 261

Accepted for publication in: the 20 December 1998 issue of Astrophysical Journal

Abstract

PLANET (the Probing Lensing Anomalies NETwork) is a worldwide collaboration of astronomers whose primary goal is to monitor microlensing events densely and precisely in order to detect and study anomalies that contain information about Galactic lenses and sources that would otherwise be unobtainable. The results of PLANET's highly successful first year of operation are presented here. Details of the observational setup, observing procedures, and data reduction procedures used to track the progress in real time at the three participating observing sites in 1995 are discussed. The ability to follow several events simultaneously with a median sampling interval of 1.6 hours and a photometric precision of better than 0.10 mag even at I=19 has been clearly demonstrated. During PLANET's 1995 pilot campaign, 10 microlensing events were monitored; the binary nature of one of these, MACHO 95-BLG-12 was recognized by PLANET on the mountain. Another event, OGLE 95-BLG-04, displayed chromaticity that may betray the presence of blending with unresolved stars projected onto the same resolution element. Although lasting only about a month, the campaign may allow constraints to be placed on the number of planets with mass ratios to the parent star of 0.01 or greater.

Redshifted 21cm Line Absorption by Intervening Galaxies

F.H. Briggs

Preprint No. 260

to appear in "Highly redshifted radio lines" eds. Carilli, Radford, Menten, and Langston (ASP Conference Series).

Abstract

The present generation of radio telescopes, combined with powerful new spectrometers, is opening a new age of redshifted radio absorption-line studies. Outfitting of arrays of antennas, such as the European VLBI Network and the upgraded VLA, with flexibly tuned receivers, will measure sizes and kinematics of intervening galaxies as a function of cosmic time.

Cosmologically distant OH megamasers: A test of the galaxy merging rate at Z approximately 2 and a contaminant of blind HI surveys in the 21cm line

F.H. Briggs

Preprint No. 259

Accepted for publication in: Astronomy and Astrophysics.

Abstract

Bright OH megamaser galaxies, radiating 1667/1665 MHz lines, could be detected at redshifts from z approximately 1 to 3 in moderate integration times with existing radio telescopes. The superluminous FIR galaxies that host the megamasers are relatively rare at z equal 0, but they may have been more common at high redshift, if the galaxy merger rate increases steeply with redshift. Therefore, blind radio spectroscopic surveys at frequencies of 400 to 1000 Mhz can form an independent test of the galaxy merger rate as a function of time over the redshift interval z = 4 to 0.7.
The redshift range z = 0.17 to 0.4 will be difficult to survey for OH masers, since spectroscopic survey signals will be confused with HI emission from normal galaxies at redshifts less than 0.3. In fact, the signals from OH masers are likely to dominate over 21cm line emission from normal galaxies at frequencies below 1200 MHz (i.e. large redshifts zHI > 0.18 and zOH > 0.4). Surveyors of nearby galaxies in the 21cm line may find that OH masers form a contaminant to deep, blind HI surveys for redshift velocities less than a few hundred kilometers per second. At frequencies just above 1420 MHz, sensitive sky surveys might detect OH masers, which could be mistaken for a population of "infalling, compact High Velocity Clouds" but would ultimately be traced to luminous FIR background galaxies at z approximately 0.17 once optical and IR follow-up has been performed.

The color-magnitude relation in CL 1358+62 at Z = 0:33: Evidence for significant evolution in the S0 population

Pieter G. van Dokkum, Marijn Franx, Daniel D. Kelson, Garth D. Illingworth, David Fisher and Daniel Fabricant

Preprint No. 258

Accepted for publication in the Astrophysical Journal.

Abstract

We use a large, multi-color mosaic of HST WFPC2 images to measure the colors and morphologies of 194spectroscopically confirmed members of the rich galaxy cluster CL 1358+62 at z = 0.33. We study the color-magnitude (CM) relation as a function of radius in the cluster, to a limit of 4.6 arcmin from the center, equivalent to 1.6 h-150 Mpc.
The intrinsic scatter in the restframe B - V CM relation of the elliptical galaxies is very small: ~0.022 magnitudes. The CM relation of the ellipticals does not depend significantly on the distance from the cluster center. In contrast, the CM relation for the S0 galaxies does depend on radius: the S0s in the core follow a CM relation similar to the ellipticals, but at large radii (R > 0.7 h-150 Mpc) the S0s are systematically bluer and the scatter in the CM relation approximately doubles to ~0.043 magnitudes. The blueing of the S0s at large radii is significant at the 95% confidence level.
These results imply that the S0 galaxies in the outer parts of the cluster have formed stars more recently than the S0s in the inner parts. A likely explanation is that clusters at z = 0.33 continue to accrete galaxies and groups from the field and that infall extinguishes star formation. The apparent homogeneity of the elliptical galaxy population implies that star formation in recently accreted ellipticals was terminated well before accretion occurred.
We have constructed models to explore the constraints that these observations place on the star formation history of cluster galaxies. The best constrained parameter is the scatter in the luminosity-weighted age Delta tauL<tauL>, which is less than 18% for the ellipticals and the S0s in the cluster core, and less than 35% for the S0s in the outer parts of the cluster. The constraints on the most recent period of star formation are model dependent, but we show that star formation in ellipticals likely ceased at z = 0.6 or higher. If we assume that the galaxies have a constant star formation rate up to a randomly distributed truncation time, we find that the S0s in the outer parts of the cluster have experienced star formation until the epoch of observation at z = 0.33.
We conclude that the population of S0s in clusters is likely to evolve as star forming galaxies are converted into passively evolving galaxies. Assuming a constant accretion rate after z = 0.33, we estimate ~15% of the presentday early-type galaxy population in rich clusters was accreted between z = 0.33 and z = 0. The ellipticals (and the brightest S0s) are probably a more stable population, at least since z = 0.6.

Star Formation and the Interstellar Medium in Low Surface Brightness Galaxies II. Deep CO Observations of Low Surface Brightness Galaxies

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

Preprint No. 257

Accepted for publication in: Astronomy and Astrophysics.

Abstract

We present deep, pointed 12CO(J=2-1) observations of three late-type LSB galaxies. The beam-size was small enough that we could probe different environments (HI maximum, HI mininum, star forming region) in these galaxies. No CO was found at any of the positions observed. We argue that the implied lack of molecular gas is real and not caused by conversion factor effects. The virtual absence of a molecular phase may explain the very low star formation rates in these galaxies

Star Formation and the Interstellar Medium in Low Surface Brightness Galaxies I. Oxygen Abundances and Abundance Gradients in Low Surface Brightness Disk Galaxies

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

Preprint No. 256

Accepted for publication in: Astronomy and Astrophysics.

Abstract

We present measurements of the oxygen abundances in 64 HII regions in 12 LSB galaxies. We find that oxygen abundances are low. No regions with solar abundance have been found, and most have oxygen abundances ~0.5 to 0.1 solar. The oxygen abundance appears to be constant as a function of radius, supporting the picture of quiescently and sporadically evolving LSB galaxies.

The Global Structure of Galactic Discs

R. de Grijs

Preprint No. 255

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

Abstract

A statistical study of global galaxy parameters can help to improve our understanding of galaxy formation processes. In this paper we present the analysis of global galaxy parameters based on optical and near-infrared observations of a large sample of edge-on disc galaxies.
We found a correlation between the ratio of the radial to vertical scale parameter and galaxy type: galaxies become systematically thinner when going from S0's to Sc's, whereas the distribution seems to level off for later types.
The observed scale length ratios (and thus the radial colour gradients) largely represent the galaxies' dust content. On average the colour gradients indicated by the scale length ratios increase from type Sa to at least type Sc. For galaxy types later than Sc, the average colour gradient seems to decrease again.
The distribution of K-band (edge-on) disc central surface brightnesses is rather flat, although with a large scatter. However, the latest-type sample galaxies (T > 6) show an indication that their average disc central surface brightnesses may be fainter than those of the earlier types. This effect is probably not the result of dust extinction.

Weak lensing analysis of Cl 1358+62 using Hubble Space Telescope observations

H. Hoekstra, M. Franx, K. Kuijken, G. Squires

Preprint No. 254

Accepted for publication in: Astrophysical Journal.

Abstract

We report on the detection of weak gravitational lensing of faint, distant background objects by
Cl 1358+62, a rich cluster of galaxies at a redshift of z = 0.33. The observations consist of a large, multi-color mosaic of HST WFPC2 images.
The number density of approximately 50 background objects arcmin-2 allows us to do a detailed weak lensing analysis of this cluster. We detect a weak lensing signal out to ~1.5 Mpc from the cluster centre. The observed distortion is consistent with a singular isothermal sphere model with a velocity dispersion of 780±50 km/s. The total projected mass within a radius of 1 Mpc, corresponding to this model is (4.4±0.6) × 1014 M_odot. The errors given here, represent the random error due to the ellipticities of the background galaxies. The uncertainty in the redshift distribution introduces an additional, systematic error of ~10% in the weak lensing mass. The weak lensing mass is slightly lower than dynamical estimates and agrees well with X-ray mass estimates. The mass distribution is elongated similar to the light. The axis ratio of 0.30±0.15 and position angle of -21°±7° were measured directly from the observations and agree very well with a previous strong lensing determination. A two-dimensional reconstruction of the cluster mass surface density shows that the peak of the mass distribution coincides with the peak of the light distribution. We find a value of (90±13)h50 M_odot/L_{Vodot} for the mass-to-light ratio, consistent with being constant with radius. The point spread function of HST is highly anisotropic at the edges of the individual chips. This systematically perturbs the shapes of objects and we present a method for applying the appropriate correction.

Red clump morphology as evidence against a new intervening stellar population as the primary source of microlensing toward the LMC

J.-P. Beaulieu and P.D. Sackett

Preprint No. 253
Figures are separated from the text:

figure 1, figure 2, figure 3, figure 4, figure 5, figure 6, figure 7, figure 8, figure 9

Accepted for publication in: Astronomical Journal.

Abstract

We examine the morphology of the color-magnitude diagram (CMD) for core helium-burning (red clump) stars to test the recent suggestion by Zaritsky & Lin (1997) that an extension of the red clump in the Large Magellanic Cloud (LMC) toward brighter magnitudes is due to an intervening population of stars that is responsible for a significant fraction of the observed microlensing toward the LMC. Using our own CCD photometry of several fields across the LMC, we confirm the presence of this additional red clump feature, but conclude that it is caused by stellar evolution rather than a foreground population. We do this by demonstrating that the feature
(1) is present in all our LMC fields,
(2) is in precise agreement with the location of the blue loops in the isochrones of intermediate age red clump stars with the metallicity and age of the LMC,
(3) has a relative density consistent with stellar evolution and LMC star formation history, and
(4) is present in the Hipparcos CMD for the solar neighborhood where an intervening population cannot be invoked.
Assuming there is no systematic shift in the model isochrones, which fit the Hipparcos data in detail, a distance modulus of µLMC = 18.3 provides the best fit to our dereddened CMD.

The gas/methane abundance ratio toward deeply embedded protostars

A.C.A. Boogert, F.P. Helmich, E.F. van Dishoeck, W.A. Schutte, A.G.G.M. Tielens and D.C.B. Whittet

Preprint No. 252

Accepted for publication in: Astronomy and Astrophysics.

Abstract

We present the detection of ro-vibrational absorption lines of the deformation mode of gaseous CH4 toward the massive protostars W 33A, and NGC 7538 : IRS9, using the SWS spectrometer on board of the Infrared Space Observatory. The observed lines indicate that the CH4 gas is warm (T ~ 90 K), and has a low abundance (~4 10-7). The CH4 ice in these lines of sight is embedded in a polar matrix (Boogert et al. 1996), and the gas/solid state abundance ratio is low (~0.5). We discuss models for the formation of interstellar CH4. The observations impose strong limitations on time dependent gas phase models, e.g. a low initial CO/C ratio would be required, the CH4 must have been formed and subsequently condensed on the grains within a narrow time window, and an additional mechanism would be necessary to form polar ice mantles. More likely, interstellar CH4 is formed through grain surface reactions at a high CO/C ratio, which explains the low observed CH4 abundance (~10-6), the presence of CH4 in a polar ice, the low gas/solid ratio, and the absence of a strong cold CH4 gas component. The observed warm CH4 gas probably has sublimated from the grains in the `hot core' region surrounding the protostar.

Rotation curves of UMa galaxies in the context of modified Newtonian Dynamics

R.H. Sanders, M.A.W. Verheijen

Preprint No. 251
Figures are separated from the text:

figure 1a, figure 1b, figure 1c, figure 1d, figure 1e, figure 2, figure 3, figure 4, figure 5

Accepted for publication in: Astrophysical Journal.

Abstract

This is the third in a series of papers in which spiral galaxy rotation curves are considered in the context of Milgrom's modified dynamics (MOND). The present sample of 30 objects is drawn from a complete sample of galaxies in the Ursa Major cluster with photometric data by Tully et al. (1996) and 21 cm line data by Verheijen (1997). The galaxies are roughly all at the same distance (15 to 16 Mpc). The radio observations are made with the Westerbork Synthesis Array which means that the linear resolution of all rotation curves is comparable. The greatest advantage of this sample is the existance of K'-band surface photometry for all galaxies; the near-infrared emission, being relatively free of the effects of dust absorption and less sensitive to recent star formation, is a more precise tracer of the mean radial distribution of the dominant stellar population. The predicted rotation curves are calculated from the K'-band surface photometry and the observed distribution of neutral hydrogen using the simple MOND prescription where the one adjustable parameter is the mass of the stellar disk or the implied mass-to-light ratio. The predicted rotation curves generally agree with the observed curves and the mean M/L in the near-infrared is about 0.9 with a small dispersion. The fitted M/L in the B-band is correlated with B-V color in the sense predicted by population synthesis models. Including earlier work, about 80 galaxy rotation curves are now well-reproduced from the observed distribution of detectable matter using the MOND formula to calculate the gravitational acceleration; this lends considerable observational support to Milgrom's unconventional hypothesis.

On the Evolutionary Behaviour of BL Lac Objects

N. Bade, V. Beckmann, N.G. Douglas, P.D. Barthel, D. Engels, L. Cordis, P. Nass and W. Voges

Preprint No. 250

Accepted for publication in: Astronomy and Astrophysics.

Abstract

We present a new well defined sample of BL Lac objects selected from the ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming a flux limited sample down to fX(0.5 - 2.0 keV) = 8 · 10-13 ergs cm-2 s-1, redshifts are known for 33 objects (and 31 of the complete sample). X-ray spectral properties were determined for each object individually with the RASS data. The luminosity function of RASS selected BL Lac objects is compatible with results provided by objects selected with the Einstein observatory, but the RASS selected sample contains objects with luminosities at least tenfold higher. Our analysis confirms the negative evolution for X-ray selected BL Lac objects found in a sample by the Einstein observatory, the parameterization provides similar results. A subdivision of the sample into halves according to the X-ray to optical flux ratio yielded unexpected results. The extremely X-ray dominated objects have higher redshifts and X-ray luminosities and only this subgroup shows clear signs of strong negative evolution. The evolutionary behaviour of objects with an intermediate spectral energy distribution between X-ray and radio dominated is compatible with no evolution at all. Consequences for unified schemes of X-ray and radio selected BL Lac objects are discussed. We suggest that the intermediate BL Lac objects are the basic BL Lac population. The distinction between the two subgroups can be explained if extreme X-ray dominated BL Lac objects are observed in a state of enhanced X-ray activity.

Nature of 60 µm Emission in 3C47, 3C207 and 3C334

I.M. van Bemmel, P.D. Barthel, M.S. Yun

Preprint No. 249

Accepted for publication in: Astronomy and Astrophysics.

Abstract

We try to explain the unusually high far-infrared emission seen by IRAS in the double-lobed radio-loud quasars 3C47, 3C207 and 3C334. High resolution cm-mm observations were carried out to determine their radio core spectra, which are subsequently extrapolated to the far-infrared in order to determine the strength of the synchrotron far-infrared emission. The extrapolated flux densities being considerably lower than the observed values, a significant nonthermal far-infrared component is unlikely in the case of 3C47 and 3C334. However, this component could be responsible for the far-infrared brightness of 3C207. Our analysis demonstrates that nonthermal emission cannot readily account for the difference between quasars and radio galaxies in the amount of their far-infrared luminosity. On the other hand, a significant role for this mechanism is likely; full sampling of the mm-submm spectral energy distributions is needed to address the issue quantitatively.

Cosmology with Modified Newtonian Dynamics (MOND)

R.H. Sanders

Preprint no. 248
Figures are separated from the text:

figure 1, figure 2, figure 3, figure 4

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

Abstract

It is well-known that the application of Newtonian dynamics to an expanding spherical region leads to the correct relativistic expression (the Friedmann equation) for the evolution of the cosmic scale factor. Here, the cosmological implications of Milgrom's modified Newtonian dynamics (MOND) are considered by means of a similar procedure. Earlier work by Felten demonstrated that in a region dominated by modified dynamics, the expansion cannot be uniform (separations cannot be expressed in terms of a scale factor) and that any such region will eventually re-collapse regardless of the initial expansion velocity and mean density. Here I show that, because of the acceleration threshold for the MOND phenomenology, a region dominated by MOND will have a finite size which, in the earlier universe (z > 3), is smaller than the horizon scale. Therefore, uniform expansion and homogeneity on horizon scale are consistent with MOND-dominated non-uniform expansion and the development of inhomogeneities on smaller scale. In the radiation-dominated era, the amplitude of MOND-induced inhomogeneities is much smaller than that implied by observations of the cosmic background radiation, and the thermal and dynamical history of the universe is identical to that of the standard big bang. In particular, the standard results for primordial nucleosynthesis are retained. When matter first dominates the energy density of the universe, the cosmology diverges from that of the standard model. Objects of galaxy mass are the first virialized objects to form (by z=10) and larger structure develops rapidly. At present, the universe would be inhomogeneous out to a substantial fraction of the hubble radius.

HST photometry of the stars near the center of PN NGC 650

J. Koornneef and S.R. Pottasch

Preprint No. 247

Accepted for publication in: Astronomy and Astrophysics.

Abstract

Colours and magnitudes of the central star of NGC 650 and its two companions have been obtained from HST images. The spectral types of the companions, which have been deduced from the colours, suggest a probable distance to the companions of between 5 and 6 kpc. Using the spectral and photometric characteristics of the central star, which are very similar to the very well studied PN PG 1159, we show that its distance is probably close to 1.2 kpc. Thus it appears unlikely that the central star is physically related to the two visible companions.

Galaxy interactions: the HI signature

R. Sancisi

Preprint No. 246

To appear in: IAU Symposium 186 "Galaxy Interactions at Low and High Redshift" (eds. D. Sanders and J. Barnes), Kluwer Academic Publ., Dordrecht (review paper)

Abstract

HI observations are an excellent tool for investigating tidal interactions. Ongoing major and minor interactions which can lead to traumatic mergers or to accretion and the triggering of star formation, show distinct HI signatures. Interactions and mergers in a recent past can also be recognized in the HI structure and kinematics. Recent 21cm line surveys of large samples of galaxies indicate that at least one out of every four galaxies shows signs of a present interaction or merger/accretion events in the recent past.

Testing the dark matter hypothesis with low surface brightness galaxies and other evidence

S.S. McGaugh and W.J.G. de Blok

Preprint No. 245

Accepted for publication in: the Astrophysical Journal.

Abstract

The severity of the mass discrepancy in spiral galaxies is strongly correlated with the central surface brightness of the disk. Progressively lower surface brightness galaxies have ever larger mass discrepancies. No other parameter (luminosy, size, velocity, morphology) is so well correlated with the magnitude of the mass deficit.
The rotation curves of low surface brightness disks thus provide a unique data set with which to probe the dark matter distribution in galaxies. The mass discrepancy is apparent from R = 0 giving a nearly direct map of the halo mass distribution. The luminous mass is insignificant.
Interpreting the data in terms of dark matter leads to troublesome fine-tuning problems. Different observations require contradictory amouns of dark matter. Structure formation theories are as yet far from able to explain the observations.

Testing the hypothesis of modified dynamics with low surface brightness galaxies and other evidence

S.S. McGaugh and W.J.G. de Blok

Preprint No. 244

Accepted for publication in: the Astrophysical Journal.

Abstract

The rotation curves of low surface brightness galaxies provide a unique data set with which to test alternative theories of gravitation over a large dynamic range in size, mass, surface density, and acceleration. Many clearly fail, including any in which the mass discrepancy appears at a particular length-scale. One hypothesis MOND (Milgrom 1983, ApJ, 270, 371) is consistent with the data. Indeed, it accurately predicts the observed behavior. We find no evidence on any scale which clearly contradicts MOND, and a good deal which supports it.

Multifrequency radio continuum observations of head-tail galaxies in the Perseus cluster

D. Sijbring and A.G. de Bruyn

Preprint No. 243

Accepted for publication in: Astronomy and Astrophysics.

Abstract

New observations of head-tail radio galaxies in the Perseus cluster at 21, 49 and 92 cm using the WSRT are presented. Two new head-tail galaxies were discovered bringing the total for the cluster to five. We show total intensity maps for all of them and spectral index distribution and polarization maps for NGC 1265 and IC 310.
The observations with high dynamic range, obtained with the WSRT and the redundancy and self-calibration techniques, reveal a large extension of the tail of NGC 1265 both at 49 cm and at 92 cm. The projected tail bends over an angle of almost 360o. There are several clear differences between the bright and the faint part of the radio tail: within a distance of one or two resolution beams the intensity decreases by more than an order of magnitude and the spectral index steepens from -1 to -2. The simplest model for the tail is that it delineates the orbit of the galaxy through the cluster. Because the tail is seen in projection, the faint part of the tail is visible only when it is not superimposed onto the bright part of the tail. It is likely, however, that large scale motions of the hot intracluster medium also help shape the tail. In this respect it is interesting to note that the tails of both NGC 1265 and IC 310, as well as the low brightness emission of NGC 1275 appear to have their faint extensions shifted to the east.
The very long tail of NGC 1265 makes it possible to test at low frequencies models of the ageing process of the radiating electrons. In the faint part of the tail the brightness temperature is very low, which either indicates a very weak magnetic field or a large deviation from equipartition between particle and magnetic energy. The most remarkable properties of the faint part of the tail of NGC 1265, namely its constant surface brightness and spectral index, are compared with three synchrotron ageing models, making some standard assumptions. All these models disagree with either the data or one of the assumptions. The Kardashev-Pacholczyk model and the Komissarov-Gubanov model require magnetic fields stronger than the equipartition value; the Jaffe-Perola model and the Komissarov-Gubanov model require an additional process, like in situ reacceleration or bulk streaming motions of electrons in the tail, far in excess of a plausible galaxy speed, to explain the observed spectra.

Maintained by:  Gineke Alberts