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:
Figure 1,
Figure 2a ,
Figure 2b ,
Figure 3a ,
Figure 3b ,
Figure 4a
and
Figure 4b
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:
Figure 2,
Figure 3a ,
Figure 3b ,
Figure 3c ,
Figure 4 ,
Figure 6 ,
Figure 7a ,
Figure 7b ,
Figure 7c ,
Figure 8 and
Figure 9
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
Konrad Kuijken
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.
The Nature of Radio Emission in Radio-Quiet QSOs
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.