Received 13th May 1998
Abstract.
We present observations in the 21 cm line of five dwarf galaxies in
the Centaurus A group of galaxies (NGC 5237, NGC 5264,
NGC 5408, ESO 324-G24, ESO 383-G87) obtained with the
Australia Telescope Compact
Array.
The extended region (
d ≅ 400 kpc) of X-ray emission around
Cen A indicates the existence of a diffuse hot intra-group medium (IGM)
in this nearby (3.5 Mpc) and loose group.
Combined with optical data and the sample of dwarf irregular galaxies in this
group observed by Côté (1995), the data is used to investigate
the effects of tidal perturbations and possible interaction with the IGM of
the weakly bound H
I halos of low mass dwarf galaxies.
We find that one of the galaxies shows signs of ram pressure stripping.
Most of the galaxies have extended H
I halos and are
dominated by rotation.
The data will be analysed with respect to the dark matter content of the
individual galaxies and the overall properties of the group.
1. Introduction
The Centaurus A (NGC 5128) group is one of nearest groups of galaxies
beyond the Local Group (D ≅ 3.5 Mpc).
Côté et al. (1997) find a rich population of dwarf irregulars in
this group.
The 20 known dwarf galaxies are concentrated in several subcondensations
around the 7 major members, forming a loose chain of galaxies which covers
more than 25° on the sky.
All main members are active galaxies and the group shows a unusual diversity
of (disturbed) morphologies.
The long crossing time of 4.5·109 years
(Côté et al. 1997) indicates that this group is probably
not virialised.
Tidal interaction in close galaxy-galaxy encounters might not be the only
perturbating effect in this poor group of galaxies.
Arp (1994) finds an extended (≅400 kpc), diffuse X-ray emission
near Cen A, not linked to the radio lobes.
Due to the low galactic latitude of Cen A, leading to the absorption of
the soft X-ray band, the nature of the emission remains unclear.
But it is a strong indication for the presence of a hot intra group medium
(IGM), found in 1/3 of similar poor groups (Mulchaey et al. 1996).
2. Observations
We observed five dwarf galaxies in the 21 cm line with two compact
configurations (750D & 375 array) of the
Australia Telescope Compact
Array (ATCA, ATNF) in 1997.
The observing frequency was 1418 MHz
(≅528 km s-1), the bandwidth 8 MHz with
512 channels, resulting in a velocity resolution of
3.3 km s-1.
The observing time was 5 × 10 h (750D) and
5 × 12 h (375).
The data was calibrated and processed with the standard methods, using AIPS.
For the five pointing centers the mean synthesised beam (uniform weighting)
is 40" (<1 kpc at 3.5 Mpc), the rms noise is
2.9 mJy/beam.
Our data is combined with optical data and ATCA H I
observations of three other dwarf galaxies in the Centaurus group from
Côté (1995) to cover the entire dwarf population in the inner
part of the group.
3. Properties of the Dwarfs
- NGC 5264: This BCDG is close to M83 in the northern subcondensation
of the group.
It has a compact appearance in the optical as well as in
H I.
- ESO 383-G87: The recession velocity of this Sm is more than
200 km s-1 lower than the group average and it
might therefore not be a member of the group.
The optical and the H I show a clear spiral structure.
- NGC 5408: This gas rich star forming BCDG has a disturbed
H I distribution with the optical component between
two H I concentrations and extended tidal tails
possibly indicating a merger scenario of two H I
clouds.
- NGC 5237: Classified as a dI0, this gas poor dwarf surprisingly
shows an extended, undisturbed H I disk
although relatively close to Cen A and in the region of X ray
emission.
- ESO 324-G24: With a projected distance of only 100 kpc, this
LSBG is the galaxy closest to Cen A and the X ray emitting
region.
The H I distribution and the velocity field show
some features which can be best explained by ram pressure interaction.
4. Intra Group Medium
If ESO 324-G24 was affected by ram pressure stripping, one can derive
some properties of the IGM from the effects on the ISM of the galaxy.
Using the basic ram pressure stripping condition by Gunn & Gott (1972)
that the force resulting from ram pressure exceeds (or is comparable to)
the gravitational restoring force
ρ v2≥2π G σtot σgas,
it is possible to give a lower limit on the density of the IGM ρ with
the knowledge of the velocity of the motion through the IGM v and the
measured surface densities σ.
Modifying this condition according to Davis et al. (1995), deducing from
the morphological characteristics and assuming a velocity derived from
the group velocity dispersion, we obtain an estimate for a lower limit of
ρ≥10-5 cm-3.
This is a reasonable value for the density of the IGM close to the center of
a poor group.
5. Summary
- Most of the Centaurus dwarf galaxies have extended H I
halos.
- Their kinematics are dominated by rotation.
- The optical and the H I morphologies are often
disturbed.
- There is no obvious dependence of their properties on the projected
distance to Cen A.
This rules out that a dense IGM fills the entire inner part of the group.
- There is some evidence for a ram pressure interaction of ESO 324-G24
with an IGM in the Centaurus group.
References
- Arp H., 1994, A&A 288, 738
- Côté S., 1995, PhD thesis, Australian National University
- Côté S., Freeman K.C., Carignan C., Quinn P.J., 1997,
AJ 114, 1313
- Davis D.S., Mushotzky R.F., Mulchaey J.S., Worrall D.M., Birkinshaw M.,
1995, ApJ 444, 582
- Gunn J.E., Gott J.R., 1972, ApJ 176, 1
- Mulchaey J.S., Davis D.S., Mushotzky R.F., Burstein D., 1996, ApJ 456, 80
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| First version: | 04th | August, | 1998
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| Last update: | 29th | September, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)