Received 05th March 1998
Abstract.
We report on first results of a comprehensive study of interacting and merging
processes between satellites and spiral galaxies to investigate the effects of
such events on the disk component of spirals.
Analysis of our newly obtained photometric data of about 150 edge-on
galaxies (i.e. interacting, non interacting and superthin galaxies)
in optical and in NIR shows that there are considerable differences
between interacting and non-interacting galaxies concerning their absolute disk
scale parameters as well as their ratios h/z0.
It is obvious that, in comparison with normal spirals, the kinematically heated
disks of mergers possess a 1.5-4-times larger scale height and thus velocity
dispersion <vz2>1/2,
while the average heating factor perpendicular to the disk plane is about 1.5.
Most striking feature of the distributions of h/z0 for both
normal galaxies and mergers, is the total lack of typical flat disk axis ratios
(i.e. that of late type ones) of h/z0 > 6 for mergers.
1. Introduction
Observations of interacting and merging processes between satellites and disk
components of spiral galaxies in the range
Msat/Mdisk≅0.1 provide us with
significant information on the structural and kinematic changes of the
affected galaxy disks.
The changes of characteristic disk parameters in the course of such merging
events, especially the efficiency of the disk heating mechanism itself, are
not yet well studied.
Moreover it is still unclear in how far
different disk components of present-day galaxies are the result of
merging events in the past. Earlier studies (Quinn et al. 1993;
Toomre & Toomre 1972; Walker et al. 1996) have shown that galactic
disks can be very sensitive to tidal perturbations,
depending on the disk stability characterized by the so called
Toomre parameter Q.
Also recently conducted numerical simulations of realistic merging events
(Quinn et al. 1993; Walker et al. 1996) support that already a single merger
is sufficient to destroy a thin galactic disk,
even if the mass of the satellite is only a few percent of the disk.
On the other hand unknown parameters such as gas content, star formation rates
or unsufficient number of test particles could significantly modify these
results.
From this point of view it is therefore desirable to derive significant
parameters of such galaxy disks from observational data obtained
from an extensive statistical sample.
For that reason we investigated a comprehensive sample of about 150
interacting and non-interacting galaxies and report on our observations,
data reduction methods, first results, and
some implications for the raised questions.
2. Observations
In order to have a representative sample and the possibility of
comparing the results we selected edge-on interacting galaxies in different
stages of the merging process as well as non interacting galaxies,
including also extreme flat ''superthin'' spirals of different morphological
types ranging from T = 0...9.
Due to the size of the sample and the required observing time observations
were carried out at different telescopes during the last 2 years
(2.2 m ESO/MPI, 1.54 m Danish and 24 inch Bochum telescope
at La Silla Obs., 2.2 m, and
1.23 m telescopes at Calar Alto
Obs. as well as 42 inch telescope at Lowell, and
1 m telescope at Hoher List Observatory).
For most of the galaxies which were observed in R-band we also obtained
NIR photometry (bands H and K) to avoid extreme dust extinction
near the galactic plane and to get information about colors and different
aging disk populations.
3. Description of disk model and fitting procedure
A detailed description of applied disk models and the disk fitting procedure
can be found in Schwarzkopf (1996), Schwarzkopf & Dettmar (1997) and in
the world wide web
(http://www.astro.ruhr-uni-bochum.de/schwarz/index.html).
4. Results
4.1. Disk parameters of non interacting galaxies
Investigating the ratio of horizontal and vertical disk scale parameters
of non interacting galaxies we find that h/z0 correlates
well with the morphological type of galaxies, meaning that the ratio increases
from an average h/z0≅1.0 for early types like S0 to
h/z0≅ 7 for late type spirals Sc/Sd.
Although the mean error is about ±2.0 the correlation is quite obvious
and it seems that there is a smooth transition between these two extremes
(Fig. 1).
It should be stressed that the upper limit is characterized by disks of
superthin late type spirals with T = 8...9, which can possess extreme
ratios up to h/z0≅10 or 11.
Such values are in the range of theoretically derived so called ``maximum
disk fits'' (see Bottema 1993).
4.2. Distribution of h/z0 and absolute scale
heights z0 for mergers and other galaxies
Our study reveals that distribution of ratio h/z0, a clear
indicator for vertical disk thickening, is completely different for normal and
for merging galaxies (Figs. 3 and 4).
The center of both distributions is at (h/z0)n =
3.9 and (h/z0)m = 2.7, respectively, leading to
a vertical disk heating factor of about 1.5 on average.
But this factor considerably underestimates this difference, because in
comparison to non interacting galaxies, mergers show a clear drop-off at
h/z0≅4 and a total lack for extreme ratios
h/z0>5.
The above mentioned correlation between h/z0 and
morphological type is, as expected, destroyed when all mergers are taken into
account, i.e. mergers possess disturbed disks and do not follow the general
h/z0-trend (Fig. 2).
Distribution of absolute disk scale heights z0 confirm these
results of disk thickening by merging events, i.e. while disks of non
interacting galaxies possess a scale height in the range z0
= (0.3...2.8) kpc having a maximum around
z0≅1.0 kpc mergers are spread in a wide range
from z0 = (1.0...4.2) kpc with a wide maximum around
z0≅2.0 kpc.
To summarize these first results one can say that even small infalling
satellites can significantly thicken galactic disks by a factor of at least
1.5.
The majority of kinetic energy leads to an increasing vertical velocity
dispersion <vz2>1/2 of disk
stars.
No thin, cool disk of any kind (like types T = 8...9) is detectable
after such a merging event.
[Click here to see Figs. 1-4!]
References
- Bottema R., 1993, A&A 275, 16
- Quinn P.J., Hernquist L., Fullager D.P., 1993, ApJ 403, 74
- Reshetnikow V., Combes F., 1997, A&A 324, 80
- Schwarzkopf U., 1996, Diploma Thesis, Ruhr-Universität Bochum
- Schwarzkopf U., Dettmar R.-J., 1997, AG Abstr. Ser. 13, 238
- Toomre A., Toomre J., 1972, ApJ 405, 142
- Walker I.R., Mihos J.C., Hernquist L., 1996, ApJ 460, 121
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| First version: | 17th | July, | 1998
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| Last update: | 30th | August, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)