Received 27th March 1998
Abstract.
A statistical relationship between interactions and elevated star formation
rates was found by Taylor (1997) for gas rich dwarf galaxies.
That study was not able to conclude what physical mechanism was responsible
for this relationship.
To study the physics involved, we have obtained
VLA
C-array observations in the H
I line.
These data show that H
II galaxies tend to have dense
concentrations of H
I overlapping spatially with their
stellar components, often have large H
I holes, and have
velocity fields indicating solid-body rotation.
We plan to combine the new observations of 6 H
II galaxies
with previous observations of 5 others.
By comparing the kinematics and morphology of the isolated systems with
those of the interacting systems, we hope to determine how the interactions
affect the physical properties of the H
II galaxies.
1. Introduction
H II galaxies, also known as Blue Compact Dwarfs, are
a class of dwarf galaxy with some interesting properties.
They tend to be H I rich, with compact optical morphologies
and optical spectra containing features typical of H II
regions.
The recent star formation rates inferred from spectra are quite high for
dwarf galaxies (0.2-1 Msun yr-1,
Sage et al. 1992), rapid enough to exhaust the available gas in well under
a Hubble time.
Because at the current epoch they still contain substantial amounts of gas
(Thuan & Martin 1981), H II galaxies are believed to
experience a bursting mode of star formation, in which the bursts last
∼107 yr and quiescent periods last ∼108 to
109 yr.
Galaxy-galaxy interactions are known to be associated with increased star
formation activity (e.g. Larson & Tinsley 1978; Bushouse 1987; Kennicutt
et al. 1987).
Work on this connection has focussed on "normal" galaxies - spirals
and ellipticals.
Brinks (1990) suggested that interactions might trigger the star formation
in H II galaxies as well, based upon observations of
II Zw 33, in which a
VLA map
of the H I distribution revealed a previously
unknown companion galaxy.
To test the idea that H II galaxies might be triggered by
interactions, even in cases with no obvious companion, we conducted a VLA
H I survey of H II galaxies,
searching for gas rich companions (Taylor et al. 1993 - TBS; Taylor et al.
1995 - TBGS).
We found that 53% (10 of 19) H II galaxies had
H I rich companions, with a lower limit on the true
companion rate for the sample of 32%.
For comparison, a sample of low surface brightness galaxies - which have
low star formation rates - was also surveyed with the VLA (Taylor et al.
1996).
These galaxies were chosen to span the same range in recession velocity,
H I mass, and H I line width as
the H II galaxies, to prevent any relative selection bias
between the two samples (Taylor 1997).
Only 24% (4 of 17) of the low surface brightness dwarfs were found
to have H I rich companions.
Taylor (1997) ruled out a spurious origin for this difference between the two
samples, concluding that the small scale environments (distances of
≤250 kpc) are different, with the H II galaxies
having significantly more nearby neighbors.
This work has established a statistical relationship between the presence of
a companion near a dwarf galaxy and the existence of a burst of star formation
in that galaxy.
However, it was not able to say anything about physics involved in this
process.
In order to address the underlying physical mechanism responsible for the
bursts of star formation, we decided to apply for high resolution
H I data.
2. Observations
New 21 cm observations of 7 H II galaxies were
made with the VLA in
the C-array.
The correlator was configured to give 5.2 km s-1 velocity
resolution.
Standard VLA calibration procedures were followed, and the C-array uv
data were combined with D-array data from TBGS and TBS prior to mapping with
the AIPS task IMAGR.
Data cubes were prepared from the CLEANed maps using the conditional blanking
process as described in TBS as well as from applying a 4σ cut off
directly to the original maps.
3. Preliminary Results
We observed 3 H II galaxies without H I
companions (UM 323, UM 372 and Haro 21) and 3 interacting
systems (UM 422, UM 456, UM 500/501) in order to compare their
characteristics.
All the galaxies have a core-halo morphology in common, with a dense
concentration of H I at the center, overlapping with
the optical component of the galaxy, and a more diffuse extended distribution.
An exception to this is UM 422 (UGC 6345B) which shows a highly
irregular and clumpy H I distribution, with a large
depression in the H I column density at the center
(Figs. 1 and
2).
This depression may correspond to a supernovae driven hole, or Mosaic of
several such holes overlapping.
Such features are discussed in detail by Brinks
(1998) in this volume.
Four of the seven systems have such H I holes, although
UM 422 is the most extreme case.
Another feature shared by these galaxies is solid body rotation, which is
generally common in dwarfs except for those of the lowest masses (Young &
Lo 1997).
These results are consistent with the findings of Taylor et al. (1994) who
made similar high resolution observations of H II galaxies.
The core-halo morphology, the violent ISM with H I holes,
and solid-body rotation are thus properties which in general are shared by
H II galaxies.
[Click here to see Fig. 1 and 2!]
4. Future Goals
The primary objective of this program is to gain an understanding of the
physical mechanisms behind the observed relationship between interactions and
elevated star formation rates in dwarf galaxies.
To that end, we have observed 3 H II galaxies, and
3 interacting systems to compare and contrast their properties.
One property that ought to be affected by an interaction is the velocity
dispersion of the gas, which should be increased in interacting galaxies as
the gravitational influence of the companion modifies the orbits of gas clouds.
However, with the data currently available, we are unable to measure the
velocity dispersion.
Even with our high resolution data, the beam size is large relative to the
velocity gradient caused by rotation implying that the measured dispersion is
dominated by bulk motion over scales of ∼1 kpc rather than a true
measure of the random motions along the line of sight.
Other methods to measure the effect of an interaction remain.
In particular, we plan to fit rotation curves to both the isolated and
interacting H II galaxies, and use these to create model
velocity fields.
These models can then be subtracted from the data, yielding a residual map
which will show departures from symmetry in the velocity fields.
We predict these residuals will be larger for the interacting galaxies
than in the isolated ones.
In addition, we will compare orbital parameters of the interacting galaxies
(e.g. projected separations, relative radial velocities and orbit orientations)
to measures of the star formation rate (e.g. Halpha luminosity
or line width).
To increase the number of galaxies in the study we will add the data from the
5 H II galaxies of Taylor et al. (1994), for a total
of 11 galaxies.
With this larger sample, we hope to determine the physical process by which
interactions influence the star formation rates in dwarf galaxies.
References
- Brinks E., 1990, in 'Dynamics and Interactions of Galaxies', Wielen R.
(ed.), Springer, Berlin, p. 146
- Bushouse H.A., 1987, ApJ 320, 49
- Kennicutt R.C., Keel W.C., van der Hulst J.M., Hummel E., Roettiger K.A.,
1987, AJ 93, 1011
- Larson R.B., Tinsley B.M., 1978, ApJ 219, 46
- Sage L.J., Salzer J.J., Loose H.-H., Henkel C., 1992, A&A 265, 19
- Taylor C.L., 1997, ApJ 480, 524
- Taylor C.L., Brinks E., Grashuis R.M., Skillman E.D., 1995, ApJS 99, 427,
1996, ApJS 102, 189 (TBGS)
- Taylor C.L., Brinks E., Pogge R.W., Skillman E.D., 1994, AJ 107, 971
- Taylor C.L., Brinks E., Skillman E.D., 1993, AJ 105, 128 (TBS)
- Taylor C.L., Thomas D.L., Brinks E., Skillman E.D., 1996, ApJS 107, 143
- Thuan T.X., Martin G.E., 1981, ApJ 247, 823
- Young L.M., Lo K.Y., 1997, ApJ 490, 710
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| First version: | 12th | August, | 1998
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| Last update: | 28th | September, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)