Introduction

It has been know for some 25 years now that star formation (SF) in dwarf irregulars (dIrrs) occurs in bursts - the simple argument being  that dwarf galaxies do not have enough fuel in order to form stars at the observed high SF rate for, say, a Hubble time.  The picture to emerge is that this bursting nature converts a dIrr galaxy into an HII-galaxy, a Blue Compact Dwarf (BCD), or Wolf Rayet (WR) galaxy, depending on which characteristics are chosen to classify an actively star forming dIrr. II Zw 33, shown here, is a prominent example of a WR galaxy (see, e.g., Vacca & Conti 1992).
The main question about starbursting galaxies is what causes the burst. One suggestion is that interactions are responsible (see, e.g., Taylor 1997 and references therein). Brinks (1990) indeed discovered a companion HI cloud near II Zw 33 at nearly the same redshift. This makes it tempting to ascribe the current starburt in II Zw 33 to an interaction with this companion.
This formed one of the motivations for us to obtain better HI data using the Very Large Array and additional B-band photometry of both objects. Recently, Mendez et al. (1998) presented high resolution H-alpha imaging of II Zw 33, reporting that star formation seems to be propagating from the center outwards and along the prominent bar-like feature (see below). In what follows we will try to demonstrate how their result nicely fits into our picture that the bursting phase was indeed triggered by an interaction with the low surface brightness companion.

Observations

HI observations

II Zw 33 and its companion were observed with the NRAO Very Large Array (VLA) in B- and C-array in the 21cm line of neutral hydrogen (HI). A full account of the data reduction is given elsewhere (Walter et al. 1997). A map of the distribution of the HI in both galaxies is shown in Fig. 1 (left). Adopting a distance of 38 Mpc, we derive the following HI masses: IIZw33 (north): 1.0^.10⁹ M_sun; companion (south): 0.6^.10⁹ M_sun. Note that the HI masses of both galaxies are of the same order of magnitude. Also their HI sizes are fairly similar. A blowup of both galaxies is shown in Fig. 2. The velocity field of both galaxies is superimposed on the optical map in Fig. 3.

Figure 1: Left: HI surface brightness map of II Zw 33 (north) and its companion (south). Right: B-band image of the same area of the sky. Note that although the HI masses of both galaxies are rather similar, their optical properties are quite different.
 

II Zw 33 and its companion were observed with the 4-m telescope of the Kitt Peak National Observatory (KPNO). The B-band map is shown in Fig. 1 (both galaxies) and Fig. 3 (blowups). The contours in Fig. 3 represent the velocity field.  Note the almost linear north-south chain of massive, young stars in II Zw 33.  The companion is hardly visible in the optical - in fact the companion would not have been detected at all in optical surveys if one would not have been specifically searching for it! Note that both II Zw 33 and its companion are more extended in the HI then in the optical. Using our optical observations, we derive an HI-mass to blue luminosity, in solar units, of 0.31 (II Zw 33) and 1.83 (the companion).

Figure 2: HI surface brightness map of II Zw 33 (left) and its companion (right).

Dynamical Analysis

Walter et al. (1997) performed a detailed dynamical analysis of both II Zw 33 and its companion. Only the results are summarized here and the interested reader is referred to their paper. Somewhat unexpected,  neither IIZw33 nor the companion need dark matter to explain the observed rotation curve out to the last measured point. This is a major difference with other dwarf galaxies studied thus far, most of which seem to be dark matter dominated throughout. It is likely, though, from an analysis of their relative orbit, that a considerable amount of dark matter is hidden between the objects.
 

 

Discussion

In the following we assume that II Zw 33 and its companion are indeed a bound system and that we are not observing a chance superposition or a chance encounter. The rather small velocity difference between them of about 50 km/s suggests that their relative orbit lies more or less in the plane of the sky. The projected separation of the galaxies (about 50 kpc) is therefore probably close to the true separation. Taylor et al. (1996) show that in their sample of BCD galaxies, many of which have companions, the difference in systemic velocity between dwarf galaxy and companion is always less then 150 km/s. Accepting this latter value as an upper limit to the true space velocity and 50 kpc as a lower limit for the separation, we derive an elapsed time since perigalacticon of 3^.10⁸ years.

Note that the HII regions in II Zw 33 cannot be older than 10⁷ years. Therefore the time since a possible flyby seems at first sight too high for this to be the trigger of the observed burst. However there are a few arguments which favor the scenario that this encounter indeed triggered the observed burst in in II Zw 33. First of all, Mendez et al. (1998) argue that strong SF has proceeded in the galaxy over a longer period than that corresponding to a single burst. In fact they find clear indications that star formation is propagating from the center outwards along the apparent bar. Another argument is provided by the model developed by Noguchi & Ishibashi (1986). They show that the maximum activity of a burst produced by an interaction takes place after about 3.10⁸ years of closest approach, exactly what we find for the time since perigalacticon. In summary, there is considerable circumstancial evidence that the interaction with the companion transformed the 'quiescent' dwarf Irregular II Zw 33 to the Wolf Rayet galaxy we witness today.
 

Summary

II Zw 33 is a gas rich Wolf-Rayet galaxy. A high resolution H-alpha study by Mendez et al. (1998) shows that the current starburst is propagating from the center outwards, along the apparent bar of II Zw 33.
A companion galaxy is detected at almost the same redshift as II Zw 33.
The companion has virtually the same HI mass as the parent galaxy although their optical properties are quite different. The companion can be classified as a Low Surface Brightness galaxy.  A dynamical analysis shows that surprisingly neither II Zw 33 nor its companion require any dark matter component.
Using best estimates for the orbital parameters, we derive a time elapsed since perigalacticon of 3^.10⁸ years. This number is in excellent agreement with the model by Noguchi & Ishibashi who derive that one indeed expects the maximum star formation activity to occur on this timescale.
In summary, our HI and optical observations as well as Mendez's H-alpha study lends support to the scenario that the companion has triggered the current burst of star formation in II Zw 33 hence transforming a quiet dwarf irregular into a Wolf-Rayet galaxy.
 

References

Brinks, E., 1990, in 'Dynamics and Interactions of Galaxies', ed. R. Wielen,  Springer, p. 146
Mendez, D.I., Cairos, L.M., Esteban, C., Vilchez, J.M., 1998, 'Proceedings: The Magellanic Clouds and other
       Dwarf Galaxies', eds. T.  Richtler, J.M. Braun, Shaker Verlag, click here for online paper
Noguchi, M., Ishibashi, S., 1986, MNRAS, 219, 305
Taylor, C., Thomas, D., Brinks, E., Skillman, E.D., 1996, ApJS, 107, 143
Taylor, C., 1997, ApJ, 480, 524
Vacca, W.D., Conti, P.S., 1992, ApJ, 401, 543
Walter, F., Brinks, E., Duric, N., Klein, U., AJ, 113, 2031