Received 03rd March 1998
Abstract.
We present the main properties of tidal dwarf galaxies, a class of galaxies
formed out of tidal material expelled in the intergalactic medium during
collisions of parent galaxies.
We point out that tidal dwarfs offer instances of young galaxies in formation
in the nearby Universe.
1. Introduction
The life and evolution of galaxies are dramatically affected by environmental
effects. Interactions with the intergalactic medium and collisions with
companions cause major perturbations in the morphology and contents of
galaxies: in particular stars and gas clouds may be gravitationally pulled
out from their parent galaxies during tidal encounters,
forming rings, tails and bridges.
This debris of collisions are at the origin of a new generation of small
galaxies, the so-called ``tidal dwarf galaxies'' (hereafter TDGs).
Recent studies, based on optical and H I observations,
have shown that TDGs actually form a class of ``recycled'' objects with
some properties similar to the more classical dwarf irregulars (dIrrs)
and blue compact dwarf galaxies (BCDGs).
Figure 1 presents different examples of
interacting systems.
The tidal tails emanating from the parent galaxies contain blue compact
clumps hosting star-forming regions.
Their spectra show emission lines typical of H II regions,
ionized by massive OB stars younger than 10 Myr.
Given the time scale for the formation of clumps in tidal tails, greater than
500 Myr, the stars at the tip of the arms must have been born in situ.
Tidal tails host large reservoirs of hydrogen gas
(Hibbard & van Gorkom 1996; Duc & Mirabel 1997, and references therein)
that fuel the star forming episodes.
The H I distributions of two interacting systems are
shown in Fig. 2.
It is striking that the central regions of the parent galaxies contain little
atomic gas, whereas the optical tails, and especially the tidal dwarfs at their
tip, are associated with H I clouds as massive as
5·109 Msun.
2. Properties of tidal dwarf galaxies
2.1. Metallicity
Figure 3a shows the oxygen abundance vs.
absolute magnitude of a sample of TDGs and nearby dIrrs.
Clearly TDGs are more metal rich than classical dwarfs of the same luminosity:
Zsun/3 on average, a value typical of the outer regions
of spirals.
They actually do not follow the correlation found for field dwarf and giant
galaxies between luminosity (hence mass) and metallicity.
Being ``recycled'' objects, formed from pre-enriched material,
tidal dwarfs inherited from their parents their relatively enriched
interstellar medium.
2.2. Stellar populations
Tidal dwarf galaxies are made of two stellar components: young stars,
formed from the recent collapse of expelled H I clouds,
and an older star population, also tidally pulled out from the disks of their
interacting parent galaxies.
U. Fritze-v. Alvensleben discusses in
this volume the relative contribution
of the young and old components.
[Click here to see Fig. 1!]
[Click here to see Fig. 2!]
[Click here to see Fig. 3!]
2.3. Dynamics
Little is still known about the internal dynamics of tidal dwarfs.
First indications are that the most massive TDGs may be gravitationally bound
(Malphrus et al. 1997; Hibbard et al. 1998).
Hints for rotation were found in the H I cloud associated
with a TDG in Arp 105 (Duc et al. 1997).
Strong velocity gradients of the ionized gas were also measured
(Fig. 3b).
This suggests that some objects in tidal tails may already be dynamically
independent.
Strikingly enough, we found that some TDGs are counter-rotating with
respect to their parent galaxies.
Further kinematical studies would be necessary to probe their internal
dynamics and estimate their dark matter content, predicted to be low in
numerical simulations (Barnes & Hernquist 1992).
4. The fate of tidal dwarf galaxies
Do tidal dwarf galaxies contribute significantly to the overall
population of dwarf galaxies?
The answer to this fundamental question depends on the frequency of tidal
interactions between galaxies and on the survival time of tidal dwarfs.
The latter is limited by the hostile environment of TDGs, in the vicinity
of giant parent galaxies.
They may fall back on their progenitors on time scales of 1 Gyr,
as pointed out by Hibbard & Mihos (1995), or be tidally disrupted.
It is therefore expected that only the most massive TDGs and those that are
far away enough from their progenitors will survive.
This limits the number of galaxies produced to one or two per colliding system.
From an observational point of view, the census of TDGs is not an easy task.
TDGs should obviously be searched for in the environment of interacting
galaxies.
Hunsberger et al. (1996) claim from the analysis of photometric data that half
of the dwarf galaxies in the Hickson compact groups could be of tidal origin.
However, one should note that once the stellar/gaseous bridge between the
parent and child galaxies has dissipated, it is difficult to re-establish
a link between the two.
Our study has shown that a good genetic fingerprint of TDGs recently formed
is their higher metallicity.
Several studies have actually put forward trends for dwarf galaxies in groups
or clusters to be more metal rich than field dwarfs (Bothun et al. 1985;
Vilchez 1995).
Since the collision rate is enhanced in denser environments, it is tempting to
argue that a significant fraction of dwarfs in clusters could be recycled
objects.
A bimodal star formation history is also a strong signature for tidal dwarfs.
In this respect, it would be useful to have reference color-magnitude diagrams
of tidal dwarf galaxies.
Evolutionary Synthesis Models simulating a burst of star formation on top of
the underlying component of old galaxies will give constraints for their
future evolution (Fritze - v. Alvensleben,
this volume).
What is the ultimate fate of a tidal dwarf that has survived the tidal
shearing from their parent?
How does a faded TDG compare with dwarf spheroidals?
On the other hand, could some dwarf spheroidals have a tidal origin,
as suggested for the local group dSph lying along the Magellanic Stream?
5. Tidal dwarf galaxies: young galaxies in
the nearby Universe
The interest for a more detailed study of these objects lies in their
young age and actually in the knowledge of an upper limit for it:
the epoch of the tidal interaction that can be estimated thanks to
numerical simulations of the collision.
The objects embedded in evolving tidal tails are observed
in a particularly interesting period of their history:
at the moment of their formation.
Tidal dwarfs offer therefore instances of young galaxies in the nearby
Universe.
They also provide a good laboratory to study star formation outside the
galactic disk.
How is it triggered?
What is the role on SF parameters (IMF, SFR) of a particular interstellar
medium characterized by a high turbulence due to the collision and
a relative high metallicity inherited from their parent galaxies?
Of a particular interest for such a study are TDGs born in pure
H I tidal tails (e.g. NGC 5291, Duc & Mirabel 1998)
not contaminated by an old stellar population from the parent galaxies.
References
- Barnes J.E., Hernquist L., 1992, Nature 360, 715
- Bothun G.D., Mould J.R., Wirth A., Caldwell N., 1985, AJ 90, 697
- Duc P.-A., Mirabel I.F., 1997, The Messenger 89, 14
- Duc P.-A., Mirabel I.F., 1998, A&A 333, 813
- Duc P.-A., Brinks E., Wink J.E., Mirabel I.F., 1997, A&A 326, 537
- Hibbard J., van der Hulst J., Barnes J., 1998, in preparation
- Hibbard J.E., Guhathakurta P., van Gorkom J.H., Schweizer F., 1994,
AJ 107, 67
- Hibbard J.E., Mihos J.C., 1995, AJ 110, 140
- Hibbard J.E., van Gorkom J.H., 1996, AJ 111, 655
- Hunsberger S.D., Charlton J.C., Zaritsky D., 1996, ApJ 462, 50
- Malphrus B., Simpson C., Gottesman S. and Hawarden, T.G., 1997,
AJ 114, 1427
- Richer M.G., McCall M.L., 1995, ApJ 445, 642
- Vilchez J.M., 1995, AJ 110, 1090
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| First version: | 17th | March, | 1998
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| Last update: | 08th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)