Kapteyn Institute Preprints (1997)

# 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.

## 1997 Preprints

Preprint 242 The Effects of a Disc Field on Bulge Surface Brightness, Y.C. Andredakis
Preprint 241 The Destruction of Interstellar Dust, A.G.G.M. Tielens
Preprint 240 Planet Detection via Microlensing, Penny D. Sackett
Preprint 239 The Circumnuclear Material in the Galactic Centre: A Clue to the Accretion Process, Bob Sanders
Preprint 238 A Pair of Lensed Galaxies at z=4.92 in the Field of CL1358+62, Marijn Franx, Garth D. Illingworth, Daniel D. Kelson, Pieter G. van Dokkum, Kim-Vy Tran
Preprint 237 The HI Halo of NGC 891, R.A. Swaters, R. Sancisi and J.M. van der Hulst
Preprint 236 Gas probed by MIR and FIR Spectroscopy, J.M. van der Hulst
Preprint 235 Stellar Distributions and NIR colours of Normal Galaxies, R.F. Peletier and R. de Grijs
Preprint 234 Measuring non-axisymmetry in Spiral Galaxies, R.H.M. Schoenmakers, M. Franx and P.T. Zeeuw
Preprint 233 The Z-structure of Disk Galaxies towards the Galaxy Planes, R. de Grijs, R.F. Peletier and P.C. van der Kruit
Preprint 232 The HI Mass Function of Galaxies from a Deep Survey in the 21cm Line, Martin Zwaan, Frank Briggs, David Sprayberry, and Ertu Sorar
Preprint 231 The maximum rotation of a galactic disc, Roelof Bottema
Preprint 230 The Maximum Optical Depth towards Bulge Stars from Axisymmetric Models of the Milky Way, Konrad Kuijken
Preprint 229 Cosmological Constraints from AGN Dust, P.D. Barthel
Preprint 228 The Dark and Visible Matter Content of low Surface Brightness Disk Galaxies, W.J.G. de Blok and S.S. McGaugh
Preprint 227 An Investigation of the Structure and Kinematics of the Spiral Galaxy NGC 6503, Roelof Bottema and Jeroen Gerritsen
Preprint 226 The Spectral Evolution of Post-AGB Stars, P.A.M. van Hoof, R.D. Oudmaijer and L.B.F.M. Waters
Preprint 225 The Ursa Major Cluster of Galaxies. II. Bimodality of the Distribution of Central Surface Brightnesses, R. Brent Tully and Marc A.W. Verheijen
Preprint 224 Bimodality of Freeman's Law, R. Brent Tully and Marc A.W. Verheijen
Preprint 223 Gravitational Lensing by Damped Ly-alpha Absorbers, Alain Smette, Jean-Francois Claeskens and Jean Surdej
Preprint 222 Does the Milky Way Have a Maximal Disk, Penny D. Sackett
Preprint 221 The Nature of Radio Emission in Radio-Quiet QSOs, Peter Barthel and Jeroen Gerritsen
Preprint 220 Low Surface Brightness Galaxies in de Local Universe. III. Implications for the Field Galaxy Luminosity Function, D. Sprayberry, C.D. Impey, M.J. Irwin and G.D. Bothun
Preprint 219 Dynamical Stability and Environmental Influences in Low Surface Brightness Disk Galaxies, J. Christopher Mihos, Stacy S. McGaugh, and W.J.G. de Blok
Preprint 218 Gass Mass Fractions and the Evolution of Spiral Galaxies, Stacy S. McGaugh and W.J.G. de Blok
Preprint 217 Heterodyned Holographic Spectroscopy, N.G. Douglas

## The Effects of a Disc Field on Bulge Surface Brightness

### Y.C. Andredakis

Preprint No. 242

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

### Abstract

Collisionless N-body simulations are used in an effort to reproduce the observed tendence of the surface brightness profile of bulges to change progressively from an R¼ law to an exponential, going from early to late type spirals. A possible cause for this is the formation of the disc, later in the history of the galaxy, and this is simulated by applying on the N-body bulge the force field of an exponential disc whose surface density increases with time. It is shown that n, the index of the Sersic law Sigman(tau) propto exp[-(tau/tau0)1/n] that best describes the surface brightness profile, does indeed decrease from 4 (de Vaucouleurs law) to smaller values; this decrease is larger for more massive and more compact discs. A large part of the observed trend of n with B/D ratio is explained, and many of the actual profiles can be matched exactly by the simulations. The correlation between the disc scalelength and bulge effective radius, used recently to support the "secular evolution" origin for bulges is also shown to arise naturally in a scenario like this. This mechanism, however, saturates at around n = 2 and exponential bulges cannot be produced; as n gets closer to 1, the profile becomes increasingly robust against a disc field. These results provide strong support to the old-bulge hypothesis for the early-type bulges. The exponential bulges, however, remain essentially unexplained; the results here suggest that they did not begin their lives as R¼ spheroids, and hence were probably formed, at least in part, by different processes than those of early type spirals.

## The Destruction of Interstellar Dust,

### A.G.G.M. Tielens

Preprint No. 241

Accepted for publication in: "Formation and Evolution of Solids in Space" (eds. J.M. Greenberg and J. Kerridge), Erice, March 1997 (Kluwer, Dordrecht)

### Abstract

This paper reviews the detailed physics of the processes involved in the destruction of interstellar dust in interstellar shocks: sputtering by impacting gas atoms, and vaporization and shattering by grain-grain collisions. An analytical formalism is developed that describes these processes. The structure of interstellar shocks is discussed and the fate of grains in the cooling postshock gas is followed. Theoretical calculations show that high velocity shocks (v > 50 km/s) destroy interstellar dust efficiently. A simple stick model for evolution of interstellar dust in the interstellar medium (ISM), which includes dust destruction by shocks in the Warm ISM, accretion in the cloud phases, stardust injection, and mixing between the interstellar cloud and intercloud phases is described. It is concluded that fast shocks occur so frequently that the lifetime of refractory grains such as graphite and silicates is only 5 × 108 yr, much shorter than the stardust injection timescale. Consequently, growth of interstellar dust through accretion in the cloud phases of the ISM has to be rapid as well. This model is also used to derive rather directly dust lifetimes against destruction and accretion timescales from observations of depletions in the cloud and intercloud phases of the ISM. These are compared to the theoretically calculated lifetimes and the implications are discussed.

## Planet Detection via Microlensing

### Penny D. Sackett

Preprint No. 240
Figures are separated from the text: To appear in: Final Report of the ESO Working Group on the Detection of Extrasolar Planets (ESO Document: SPG-VLTI-97/002).

### Abstract

Microlensing is the most promising method to study the statistical frequency of extra-solar planets orbiting typical (random) stars in the Milky Way, even those several kiloparsecs from Earth. The lensing zone corresponds to orbital separations of a few times the Earth-Sun distance (AU) --- a good match to many planets in our own Solar System --- and the probability of detection is a rather weak function of planetary mass. Microlensing is thus a perfect complement to radial velocity and astrometric techniques that allow the detailed study of nearby planets with larger masses and smaller orbital separations. This report forms Appendix C of the Final Report of the European Southern Observatory (ESO) Working Group on the Detection of Extrasolar Planets, which contains recommendations to ESO for designing a competitive strategy in the field of extrasolar planets. The full report is available from ESO as document SPG-VLTI-97/002.

## The Circumnuclear Material in the Galactic Centre: A Clue to the Accretion Process

### Bob Sanders

Preprint No. 239
Figures are separated from the text: For Figure 1 and Figure 5 contact the author at sanders@astro.rug.nl

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

### Abstract

On the basis of "sticky particle" calculations, it is argued that the gas features observed within 10 pc of the Galactic Centre- the circumnuclear disk (CND) and the ionized gas filaments- as well as the newly formed stars in the inner one parsec can be understood in terms of tidal capture and disruption of gas clouds on low angular momentum orbits in a potential containing a point mass. The calculations demonstrate that a dissipative component forms a "dispersion ring", an asymmetric elliptical torus precessing counter to the direction of rotation, and that this shape can be maintained for many orbital periods. For a range of plausible initial conditions such a structure can explain the morphology and kinematics of the CND and of the most conspicuous ionized filament. While forming the dispersion ring, a small cloud with low specific angular momentum is drawn into a long filament which repeatedly collides with itself at high velocity. The compression in strong shocks is likely to lead to star formation even in the near tidal field of the point mass. This process may have general relevance to accretion onto massive black holes in normal and active galactic nuclei.

## A Pair of Lensed Galaxies at z=4.92 in the Field of CL1358+62

### Marijn Franx, Garth D. Illingworth, Daniel D. Kelson, Pieter G. van Dokkum, Kim-Vy Tran

Preprint No. 238

Accepted for publication in: Astrophysical Journal Letters, 486, L75 (Sept. 1997)

### Abstract

The cluster CL1358+62 displays a prominent red arc in WFPC2 images obtained with the Hubble Space Telescope. Keck spectra of the arc show Ly alpha emission at 7204 Angstrom, a continuum drop blueward of the line, and several absorption lines to the red. We identify the arc as a gravitationally lensed galaxy at a redshift of z=4.92. It is the highest redshift object currently known. A gravitational lens model was used to reconstruct images of the high-redshift galaxy. The reconstructed image is asymmetric, containing a bright knot and a patch of extended emission 0.4 arcsec from the knot. The effective radius of the bright knot is 0.022 arcsec or 130 h^-1 pc. The extended patch is partially resolved into compact regions of star formation. The reconstructed galaxy has I_AB= 24, giving a bolometric luminosity of about 3e11 Lsun. This can be produced by a star formation rate of 36 h^-2 Msun /yr (q0=0.5), or by an instantaneous star burst of 3e8 Msun. The spectral lines show velocity variations on the order of 300 km/s along the arc. The Si II line is blue shifted with respect to the Ly alpha emission, and the Ly alpha emission line is asymmetric with a red tail. These spectral features are naturally explained by an outflow model, in which the blue side of the Ly alpha line has been absorbed by outflowing neutral H I. Evidence from other sources indicates that outflows are common in starburst galaxies at high and low redshift. We have discovered a companion galaxy with a radial velocity only 450 km/s different than the arc's. The serendipitous discovery of these two galaxies suggests that systematic searches may uncover galaxies at even higher redshifts.

## The HI Halo of NGC 891

### R.A. Swaters, R. Sancisi and J.M. van der Hulst

Preprint No. 237

Accepted for publication in: the Astrophysical Journal

### Abstract

Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891 reveal the presence of an HI halo extending up to at least 5 kpc from the plane. This halo gas appears to rotate 25 to 100 km/s more slowly than the gas in the plane. If this velocity difference is due to the gradient in the gravitational potential, then it may serve to discriminate between disk and spheroidal mass models. The classic picture of a large outer flare in the HI layer of NGC 891 may no longer be valid.
A correlation is seen between the distributions of HI Halpha and radio continuum emission, which supports, in accordance with galactic fountain models, the picture of a substantial disk-halo circulation related to the star formation activity in the disk of NGC 891.
There is now also clear evidence for the presence of a rapidly rotating (v_rot ~ 230 km/s) disk or ring of HI in the central part of NGC 891.

## Gas probed by MIR and FIR Spectroscopy

### J.M. van der Hulst

Preprint No. 236

Accepted for publication in: "Extragalactic Astronomy in the Infrared" (eds. G.A. Mamon, Trinh Xuan Thuan and J. Tran Thanh Van), Proceedings of the 27th Symposium of the Rencontres de Moriond

### Abstract

In this paper I briefly discuss the various spectral lines in the mid- and far Infrared that can be used to probe the physical conditions of the interstellar medium (ISM) in a variety of circumstances. I will discuss the cold molecular phase of the ISM probed by the rotational lines of molecular Hydrogen, the cool diffuse ISM probed by the major cooling line of [CII] and the hot gas in HII regions which can be probed by fine structure lines of a number of elements and H-recombination lines.

## Stellar Distributions and NIR colours of Normal Galaxies

### R.F. Peletier and R. de Grijs

Preprint No. 235

Accepted for publication in: "Extragalactic Astronomy in the Infrared" (eds. G.A. Mamon, Trinh Xuan Thuan and J. Tran Thanh Van), Proceedings of the 27th Symposium of the Rencontres de Moriond

### Abstract

We discuss some results of a morphological study of edge-on galaxies, based on optical and especially near-infrared surface photometry. We find that the vertical surface brightness distributions of galaxies are fitted very well by exponential profiles, much better than by isothermal distributions. We find that in general the vertical scale height increases when going outward. This increase is strong for early-type spiral galaxies and very small for late types. We argue that it can be due to the presence of thick discs with scale lengths larger than the galaxy's main disc. Finally we discuss the colour-magnitude relation in I-K for spiral galaxies. We find that it is a tight relation, for which the scatter is similar to the observational uncertainties, with a steeper slope than for elliptical galaxies.

## Measuring non-axisymmetry in Spiral Galaxies

### R.H.M. Schoenmakers, M. Franx and P.T. de Zeeuw

Preprint No. 234

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

### Abstract

We present a method for measuring small deviations from axisymmetry of the potential of a filled gas disk. The method is based on a higher order harmonic expansion of the full velocity field of the disk. This expansion is made by first fitting a tilted-ring model to the velocity field of the gas disk and subsequently expanding the velocity field along each ring into its harmonic terms. We use epicycle theory to derive equations for the harmonic terms in a distorted potential. The phase of each component of the distortion can vary with radius. We show that if the potential has a distortion of harmonic number m, the velocity field as seen on the sky exhibits an m - 1 and m + 1 distortion. As is to be expected, the effects of a global elongation of the halo are similar to an m=2 spiral arm. The main difference is that the phase of the spiral arm can vary with radius. Our method allows a measurement of epsilon_pot sin 2 varphi_2, where epsilon_pot is the elongation of the potential and varphi_2 is one of the viewing angles. The advantage of this method over previous approaches to measure the elongations of disk galaxies is that, by using HI data, one can probe the potential at radii beyond the stellar disk, into the regime where dark matter is thought to be the dominant dynamical component. The method is applied to the spiral galaxies NGC 2403 and NGC 3198 and the harmonic terms are measured up to ninth order.
The residual velocity field of NGC2403 shows some spiral-like structures. The harmonic analysis indicates that the m = 3 term is dominant, with an average value of ~0.02 v_c. This is consistent with an average ellipticity of the potential of epsilon_pot sin 2 varphi_2 = 0.064plm0.003, but spiral arms may couple significantly to this result.
In the harmonic analysis of the kinematics of NGC3198 the m = 2 and m = 3 terms are strongest (~0.01 v_c). The inferred average elongation of the potential is 0.019plm0.003. Since the amplitude of the elongation is coupled to the viewing angles and may be influenced by spiral arms, more galaxies should be examined to separate these effects from true elongation in a statistical way.

## The Z-structure of Disk Galaxies towards the Galaxy Planes

### R. de Grijs, R.F. Peletier and P.C. van der Kruit

Preprint No. 233

Accepted for publication in: Astronomy and Astrophysics

### Abstract

We present a detailed study of a statistically complete sample of highly inclined disk galaxies in the near-infrared K' band. Since the K'-band light is relatively insensitive to contamination by galactic dust, we have been able to follow the vertical light distributions all the way down to the galaxy planes. The mean levels for the sharpness of the K'-band luminosity peaks indicate that the vertical luminosity distributions are more peaked than expected for the intermediate sech(z) distribution, but rounder than exponential. After fitting a generalized family of fitting functions characterised by an exponent 2/n (n = \infty for exponential, n = 2 for sech and n = 1 for sech^2; van der Kruit, 1988) we find that the mean value for 2/n in the K' band equals <2/n>_{K'} = 0.538, sigma_{K'} = 0.198. To explain this distribution, we have run simulations and show that projection of not completely edge-on galaxies onto the plane of the sky flattens vertical surface brightness profiles near the plane of the galaxy. The observed distribution of 2/n is consistent with the fact that all galaxies intrinsically have purely exponential vertical profiles. We find that the profile shape is independent of galaxy type, and varies little with position along the major axis. The fact that we observe this in all our sample galaxies indicates that the formation process of the galaxy disks perpendicular to the galaxy planes is a process intrinsic to the disks themselves.

## The HI Mass Function of Galaxies from a Deep Survey in the 21cm Line

### Martin Zwaan, Frank Briggs, David Sprayberry, and Ertu Sorar

Preprint No. 232

Accepted for publication in: the Astrophysical Journal

### Abstract

The HI mass function (HIMF) for galaxies in the local universe is constructed from the results of the Arecibo HI Strip Survey, a blind extragalactic survey in the 21cm line. The survey consists of two strips covering in total ~65 square degrees of sky, with a depth of cz=7400 km s^{-1}, and was optimized to detect column densities of neutral gas N_HI > 10^{18} cm^{-2} (5 sigma). The survey yielded 66 significant extragalactic signals of which approximately 50% are cataloged galaxies. No free floating HI clouds without stars are found. VLA follow-up observations of all signals have been used to obtain better measurements of the positions and fluxes and allow an alternate determination of the achieved survey sensitivity. The resulting HIMF has a shallow faint end slope (alpha approx 1.2), and is consistent with earlier estimates computed for the population of optically selected gas rich galaxies. This implies that there is not a large population of gas rich low luminosity or low surface brightness galaxies that has gone unnoticed by optical surveys. The influence of large scale structure on the determination of the HIMF from the Arecibo HI Strip Survey is tested by numerical experiments and was not found to affect the resulting HIMF significantly. The cosmological mass density of HI at the present time determined from the survey, Omega_HI (z=0) = (2.0 plm 0.5) x 10^{-4} h^{-1}, is in good agreement with earlier estimates. We determine lower limits to the average column densities of the galaxies detected in the survey and find that none of the galaxies have < 10^{19.7} cm^{-2}, although there are no observational selection criteria against finding lower density systems. Eight percent of the signals detected in the original survey originated in groups of galaxies, whose signals chanced to coincide in frequency.

## The maximum rotation of a galactic disc

### Roelof Bottema

Preprint No. 231

Accepted for publication in: Astronomy and Astrophysics

### Abstract

The observed stellar velocity dispersions of galactic discs show that the maximum rotation of a disc is on average 63% of the observed maximum rotation. This criterion can, however, not be applied to small or low surface brightness (LSB) galaxies because such systems show, in general, a continuously rising rotation curve until the outermost measured radial position. That is why a general relation has been derived, giving the maximum rotation for a disc depending on the luminosity, surface brightness, and colour of the disc. As a physical basis of this relation serves an adopted fixed mass-to-light ratio as a function of colour. That functionality is consistent with results from population synthesis models and its absolute value is determined from the observed stellar velocity dispersions. The derived maximum disc rotation is compared with a number of observed maximum rotations, clearly demonstrating the need for appreciable amounts of dark matter in the disc region and even more so for LSB galaxies. Matters have been illustrated for two examples; the galaxy NGC 6503 and LSB galaxy NGC 1560.

## The Maximum Optical Depth towards Bulge Stars from Axisymmetric Models of the Milky Way

Preprint No. 230

Accepted for publication in: Astrophysical Journal Letters

### Abstract

It has been known that recent microlensing results towards the bulge imply mass densities that are surprisingly high given dynamical constraints on the Milky Way mass distribution. We derive the maximum optical depth towards the bulge that may be generated by axisymmetric structures in the Milky Way, and show that observations are close to surpassing these limits. This result argues in favor of a bar as a source of significantly enhanced microlensing. Several of the bar models in the literature are discussed.

## Cosmological Constraints from AGN Dust

### P.D. Barthel

Preprint No. 229

Accepted for publication in: "The far infrared and submillimetre Universe" (ed. A. Wilson, ESTEC), Proceedings of the ESA Symposium, Grenoble, 15-17 April, 1997

### Abstract

The far-infrared emission of radio-loud active galaxies and quasars is a composite of various types of radiation with their own specific signatures. These different components can be isolated by combining radiometric and spectrographic measurements. The warm dust component re-radiating the AGN luminosity in radio-loud AGN is likely to display little or no cosmic evolution. Its distribution as a function of redshift may be used to address cosmological models.

## The Dark and Visible Matter Content of low Surface Brightness Disk Galaxies

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

Preprint No. 228

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

### Abstract

We present mass models of a sample of 19 low surface brightness (LSB) galaxies and compare the properties of their constituent mass components with those of a sample of high surface brightness (HSB) galaxies. We find that LSB galaxies are dark matter dominated. Their halo parameters are only slightly affected by assumptions on stellar mass-to-light ratios. Comparing LSB and HSB galaxies we find that mass models derived using the maximum disk hypothesis result in the disks of LSB galaxies having systematically higher stellar mass-to-light ratios than HSB galaxies of similar rotation velocity. This is inconsistent with all other available evidence on the evolution of LSB galaxies. We argue therefore that the maximum disk hypothesis does not provide a representative description of the LSB galaxies and their evolution. Mass models with stellar mass-to-light ratios determined by the colors and stellar velocity dispersions of galactic disks imply that LSB galaxies have dark matter halos that are more extended and less dense than those of HSB galaxies. Surface brightness is thus related to the halo properties. LSB galaxies are slowly evolving, low density and dark matter dominated galaxies.

## An Investigation of the Structure and Kinematics of the Spiral Galaxy NGC 6503

### Roelof Bottema and Jeroen Gerritsen

Preprint No. 227

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

### Abstract

The spiral galaxy NGC 6503 exhibits a regular kinematical structure except for a remarkable drop of the stellar velocity dispersion values in the central region. To investigate the dynamics of the disc in general, and that of the central region in particular, a theoretical framework has been described. This includes a mass decomposition of the galaxy into a family of disc/halo realizations compatible with the observed photometry and rotation curve. For this family stellar velocity dispersion values and stability parameters were calculated, showing that the more massive discs, although having larger dispersions, are less stable. However, a reliable theoretical description of the inner regions where the drop occurs cannot be given.
That is why we have resorted to numerical calculations. Not only to study the central region, but also to investigate the appearance of the disc in a general sense. Pure stellar 3d simulations have been performed for the family of decompositions. A clear result is that disc/dark halo mass ratios approaching those of the maximum disc limit generate a large bar structure. This is incompatible with the observed morphology of NGC 6503. For radii larger than approximately 0.2 scalelengths, the stellar kinematics resulting from the simulations essentially agrees with that predicted by the theory. But, unfortunately, the central velocity dispersion drop could not be reproduced.
A close inspection reveals that the central nuclear region is very small and bright. Therefore, tentatively, this nucleus was considered as an isothermal sphere and a core fitting procedure was applied. For an adopted equal mass-to-light ratio of disc and nucleus, a velocity dispersion of 21.5 km/s is predicted, in excellent agreement with the observed central value. An analysis, in retrospect, of the local densities involved proves that the nucleus is local and gravitationally dominant such that its approximation as an isothermal sphere, is justified. The observed dispersion drop can thus be explained by a separate kinematically distinct galactic component.

## The Spectral Evolution of Post-AGB Stars

### P.A.M. van Hoof, R.D. Oudmaijer and L.B.F.M. Waters

Preprint No. 226

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

### Abstract

A parameter study of the spectral evolution of a typical post-AGB star, with particular emphasis on the evolution of the infrared colours, is presented. The models are based on the latest evolutionary tracks for hydrogen burning post-AGB stars. For such tracks the evolutionary rate is very dependent on the assumed mass loss rate as a function of time. We investigate this effect by modifying the mass loss prescription. The newly calculated evolutionary rates and density distributions are used to model the spectral evolution of a post-AGB star with the photo-ionization code Cloudy, including dust in the radiative transfer. Different assumptions for the dust properties and dust formation are considered. It is shown that by varying these parameters in a reasonable way, entirely different paths are followed in the IRAS colour-colour diagram. First of all, the effects of the evolution of the central star on the expanding dust shell can not be neglected. Also the dust properties and the definition of the end of the AGB phase have an important effect. The model tracks show that objects occupying the same location in the IRAS colour-colour diagram can have a different evolutionary past, and therefore the position in the IRAS colour-colour diagram alone can not a priori give a unique determination of the evolutionary status of an object. An alternative colour-colour diagram, the K-[12] vs. [12]-[25] diagram, is presented. The tracks in this diagram seem less affected by particulars of the grain emission. This diagram may be a valuable additional tool for studying post-AGB evolution.

## The Ursa Major Cluster of Galaxies. II. Bimodality of the Distribution of Central Surface Brightnesses

### R. Brent Tully and Marc A.W. Verheijen

Preprint No. 225

Accepted for publication in: The Astrophysical Journal

### Abstract

The Ursa Major Cluster appears to be unevolved and made up of HI-rich spiral galaxies like one finds in the field. B,R,I,K' photometry has been obtained for 79 galaxies, including 62 in a complete sample with M(B)<-16.5 mag (with a distance to the cluster of 15.5 Mpc). The K' information is particularly important for the present discussion because it is not seriously affected by obscuration. There is reasonably convincing evidence that the distribution of exponential disk central surface brightnesses is bimodal. There is roughly an order of magnitude difference in the mean luminosity densities of high and low surface brightness disks. Disks avoid the domain between the high and low surface brightness zones. The few intermediate surface brightness examples in the sample all have significant neighbors within a projected distance of 80 kpc. The high surface brightness galaxies exhibit a range -21

## Bimodality of Freeman's Law

### R. Brent Tully and Marc A.W. Verheijen

Preprint No. 224

To be published in: "Galaxy Scaling Relations: Origins, Evolution and Applications", Proceedings of 3rd ESO-VLT Workshop, Garching, 18-20 November 1996

### Abstract

A cluster sample of 62 galaxies complete to M()=-16.5 mag has been observed at B,R,I,K' bands with imaging detectors. The distribution of exponential disk central surface brightnesses is found to be bimodal. The bimodality is particularly significant at K' because obscuration is not a problem and because the high surface brightness galaxies are redder than the low surface brightness galaxies so the bifurcation is greater. The bimodality signal is especially clear when galaxies with close companions are excluded from consideration. High and low surface brightness pairs with essentially identical luminosities and maximum rotation characteristics are compared. It is suggested that the high surface brightness galaxies have self-gravitating disks while the low surface brightness galaxies are halo dominated at all radii. Evidently the intermediate surface brightness regime is unstable. If a disk has sufficiently low angular momentum and it shrinks enough that the disk potential begins to dominate the halo potential locally, then the disk must secularly evolve to the high surface brightness state characterized by a flat rotation curve.

## Gravitational Lensing by Damped Ly-alpha Absorbers

### Alain Smette, Jean-Francois Claeskens and Jean Surdej

Preprint No. 223

Accepted for publication in: New Astronomy

### Abstract

Assuming that (i) damped Ly-alpha absorbers (DLAs) arise in present-day-like spiral galaxies which are immersed in isothermal dark matter halos, (ii) that these galaxies obey the Tully-Fisher sigma/sigma_* = (L/L_*)^{1/alpha_TF} and the Holmberg R_L/R_* = (L/L_*)^{alpha_H} relations, and (iii) that they follow the Schechter luminosity distribution, we describe how their observed number density (dN/dz), distribution of column density (f(N)) as well as inferred cosmological density of HI (Omega_HI) derived from DLA surveys are affected by gravitational lensing (GL). The by-pass' effect causes the lines-of-sight (LOSs) towards background QSOs to avoid the central parts of galaxies and reduces their effective cross-section for absorption; the amplification bias' leads observers to select QSOs whose LOSs preferentially cross galaxies close to their Einstein radius. As a consequence, the determination of the quantities dN/dz, f(N) and Omega_HI from DLA surveys does not only depend on the redshift z and luminosity L of galaxies responsible for the absorbers but also on the column density profile of HI within the galaxies and on the redshift z_q and magnitude b_q of the background QSOs.
For most of the existing surveys using b_q <~ 19 QSOs, the amplification bias dominates the combined effect resulting in a slight overestimate of dN/dz, f(N) and Omega_HI. We mainly find that observational strategies presently used to produce high-z DLA surveys result in avoiding the signature of significant GL effects: following our model, we determine that an overestimate of Omega_HI by more than 10% is unlikely for the z > 1.7 existing surveys, but may reach ~-35% for the low redshift ones.
However, we show that, in the absence of extinction by dust and micro-lensing effects, surveys ideally designed to enhance GL effects, i.e. to search for DLAs at z ~ 0.5 in front of very bright (b_q ~- 16), high-z (z_q > 1) QSOs, may lead 1) to over-estimate by up to ~-90% the number of DLAs per unit redshift; 2) to bias the survey towards high HI column density systems so that it could contain up to 4 times as many such systems, thus 3) to overestimate by up to ~-170% the cosmological density of gas associated with those DLAs. Identification of the galaxies responsible for the DLAs may be severely biased towards luminous galaxies if 2/alpha_TF - alpha_H > 0; this latter effect is greatly increased for log N_HI > 21 DLAs.
Hence, GL effects on the quantities derived from surveys for z ~ 0.5 DLAs are of the same order, but of opposite direction, as the effects of extinction by dust (cf. Fall & Pei 1993). However, the GL and dust extinction effects do not compensate each other: combining them in a consistent way is necessary to interpret existing DLA surveys. Furthermore, the effects due to micro-lensing should be simultaneously taken into account. We intend to report the results on the complex interplay between macro-lensing, micro-lensing and dust in a subsequent paper.
We briefly present statistical tests specifically designed to check whether GL affects existing DLA surveys, and assuming that extinction by dust is negligible. We only find indications of GL effects for the z < 1 ones which, if confirmed, might even be stronger than predicted by our model.
We show that an independent work on the same subject by Bartelmann & Loeb (1996) incorrectly treats the inclination effects for the intervening galaxies, thus undermining some of their main results and conclusions.

## Does the Milky Way Have a Maximal Disk

### Penny D. Sackett

Preprint No. 222

Accepted for publication in: The Astrophysical Journal

### Abstract

The Milky Way is often considered to be the best example of a spiral for which the dark matter not only dominates the outer kinematics, but also plays a major dynamical role in the inner galaxy: the Galactic disk is therefore said to be "sub-maximal". This conclusion is important to the understanding of the evolution of galaxies and the viability of particular dark matter models. The Galactic evidence rests on a number of structural and kinematic measurements, many of which have recently been revised. The new constraints indicate not only that the Galaxy is a more typical member of its class (Sb-Sc spirals) than previously thought, but also require a re-examination of the question of whether or not the Milky Way disk is maximal. By applying to the Milky Way the same definition of "maximal disk" that is applied to external galaxies, it is shown that the new observational constraints are consistent with a Galactic maximal disk of reasonable M/L. In particular, the local disk column can be substantially less than the oft-quoted required Sigma_sun approx 100 M_sun pc^{-2} -- as low as 40 M_sun pc^{-2} in the extreme case -- and still be maximal, in the sense that the dark halo provides negligible rotation support in the inner Galaxy. This result has possible implications for any conclusion that rests on assumptions about the potentials of the Galactic disk or dark halo, and in particular for the interpretation of microlensing results along both LMC and bulge lines of sight.

### Peter Barthel and Jeroen Gerritsen

Preprint No. 221

To appear in: "Quasar Hosts" (eds. D. Clements and I. Perez-Fournon), Proceedings of ESO/IAC Conference, Tenerife, September 1996.

### Abstract

Investigations of the origin of the weak radio emission in radio-quiet QSOs are reviewed. A picture emerges where - in varying degrees - optically thick radio emission from active nuclei combines with optically thin emission from galactic disks with large scale and/or circumnuclear star formation and optically thin emission from weak AGN-fed components. High resolution radio observations in combination with far-infrared (FIR) data are suggestive of an evolutionary link with ultraluminous far-infrared galaxies.

## Low Surface Brightness Galaxies in de Local Universe. III. Implications for the Field Galaxy Luminosity Function

### D. Sprayberry, C.D. Impey, M.J. Irwin and G.D. Bothun

Preprint No. 220

Accepted for publication in: The Astrophysical Journal

### Abstract

We present a luminosity function for low surface brightness (LSB) galaxies identified in the APM survey of Impey et al. (1996). These galaxies have central surface brightnesses (mu(0)) in B in the range 22.0 < mu(0) < 25.0. Using standard maximum-likelihood estimators, we determine that the best-fit Schechter function parameters for this luminosity function (LF) are alpha = -1.42, M* = -18.34, and phi* = 0.0036, assuming H_0 = 100 h_100 km s-1 Mpc-1. We compare the luminosity and number densities derived from this luminosity function to those obtained from other recent field galaxy studies and find that surveys which do not take account of the observation selection bias imposed by surface brightness are missing a substantial fraction of the galaxies in the local universe. Under our most conservative estimates, our derivation of the LF for LSB galaxies suggests that the CfA redshift survey has missed one third of the local galaxy population. This overlooked fraction is not enough by itself to explain the large number of faint blue galaxies observed at moderate redshift under no-evolution models, but it does help close the gap between local and moderate-redshift galaxy counts.

## Dynamical Stability and Environmental Influences in Low Surface Brightness Disk Galaxies

### J. Christopher Mihos, Stacy S. McGaugh, and W.J.G. de Blok

Preprint No. 219

Accepted for publication in: The Astrophysical Journal Letters

### Abstract

Using analytic stability criteria, we demonstrate that, due to their low surface mass density and large dark matter content, LSB disks are quite stable against the growth of global nonaxisymmetric modes such as bars. However, depending on their (poorly constrained) stellar velocity dispersions, they may be only marginally stable against local instabilities. We simulate a collision between an LSB and HSB galaxy and find that, while the HSB galaxy forms a strong bar, the response of the LSB disk is milder, manifesting weaker rings and spiral features. The lack of sufficient disk self-gravity to amplify dynamical instabilities naturally explains the rarity of bars in LSB disks. The stability of LSB disks may also inhibit interaction-driven gas inflow and starburst activity in these galaxies.

## Gass Mass Fractions and the Evolution of Spiral Galaxies

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

Preprint No. 218
Figures are separated from the text:
Figures
Accepted for publication in: The Astrophysical Journal

### Abstract

We show that the gas mass fraction of spiral galaxies is strongly correlated with luminosity and surface brightness. It is not correlated with linear size. Gas fraction varies with luminosity and surface brightness at the same rate, indicating evolution at fixed size.
Dim galaxies are clearly less evolved than bright ones, having consumed only ~1/2 of their gas. This resolves the gas consumption paradox, since there exist many galaxies with large gas reservoirs. These gas rich galaxies must have formed the bulk of their stellar populations in the last half of a Hubble time. The existence of such immature galaxies at z = 0 indicates that either galaxy formation is a lengthy or even ongoing process, or the onset of significant star formation can be delayed for arbitrary periods in tenuous gas disks.

## Heterodyned Holographic Spectroscopy

### N.G. Douglas

Preprint No. 217

Accepted for publication in: Publications of the Astronomical Society of the Pacific, 1997

### Abstract

In Holographic Spectroscopy an image of an interference pattern is projected onto a detector and transformed back to the input spectrum. The general characteristics are similar to those of Fourier Transform Spectroscopy, but the spectrum is obtained without scanning. In the Heterodyned arrangement one or more diffraction gratings are used so that high resolution spectra can be recorded without a corresponding increase in the recording density. The technique offers interesting possibilities, but is not without difficulties. The purpose of this paper is to review recent developments, especially in astronomical applications, to describe the theoretical background to typical problems such as coherence losses, and to discuss the extent to which these can be solved in practical instrumentation.

 Maintained by:  Gineke Alberts