4 Discussion and conclusions

The global kinematical structure of ESO 603-G21 - stellar rotation along two orthogonal position angles (Arnaboldi et al. 1994) - suggests that the object is a polar-ring galaxy. The host galaxy is probably an early-type galaxy with an exponential-like surface brightness distribution. The central galaxy is surrounded by a warped star-forming ring or disk. In general, ESO 603-G21 looks similar to other classic PRG (e.g. NGC 4650A).

There are, nonetheless, several facts complicating such an interpretation. First, the central round component shows very low surface brightness which may indicate that the central object is not an early-type galaxy like in most classic PRG (PRC). Second, in the near-infrared region (K passband) most of the stellar light comes from a bright nearly-exponential disk. Third, the central round object, clearly visible in the optical images (Fig. 2), is quite faint in the K passband (Arnaboldi et al. 1995). The totality of the observed characteristics (optical and NIR colors, color gradients, HI and H₂ content, FIR luminosity and star-formation rate, rotation-curve shape - Fig. 6 -, the agreement with the Tully-Fisher relation, etc.) suggests that ESO 603-G21 could be an unusual late-type spiral galaxy with a kinematically-decoupled extended "bulge''. Therefore, it may be similar in some respects to NGC 4672 and NGC 4698, which are early-type disk galaxies with geometric and kinematical orthogonal decoupling between the bulge and disk (Bertola et al. 1999; Sarzi et al. 2000), or to NGC 2748, which is a late-type spiral galaxy with possible ongoing accretion of a dwarf companion onto the central region of the galaxy (Hagen-Thorn et al. 1996). The bulge-like feature of ESO 603-G21 can be a "true" polar ring that is formed during almost perpendicular accretion of an early-type companion onto central region of a pre-existing disk galaxy.

Another interesting interpretation of the observed ESO 603-G21 peculiarities is that the galaxy may be the result of a dissipative merger event (this scenario was proposed recently by Iodice et al. 2001 to explain the NGC 4650A puzzles). According to Bekki (1997, 1998), dissipative galaxy merging with a near polar orbit orientation can transform two late-type spirals into one PRG. In this scenario, a spiral galaxy falling from the polar axis of the target galaxy triggers the outwardly propogating density wave in the gaseous disk of the victim galaxy. Then, gaseous dissipation and star formation transform the victim disk into polar ring or disk. The central object is the intruding galaxy that has been turned into an early-type-like galaxy during the merging.

Figure 7 depicts the various internal substructures of ESO 603-G21 as revealed in this work (see item 3.2). Such complex, non-settled, fine structure of the galaxy supports our supposition about relatively late formation of the "bulge'' due to external accretion or a merger.

Figure 7: Sketch of ESO 603-G21 as seen in careful visual inspection of the images in the various filters. North is up and East is on the left.

Interestingly enough, the companion of ESO 603-G21 is ESO 603-G20, an edge-on disk-galaxy without any "explicit" evidence of interaction. The relative velocity between both objects is 65 kms^-1 (see NED and references therein); this suggests that both objects may form a bound system! There is, nonetheless, a third faint object between them, which seems to bear a very faint and narrow bridge to ESO 603-G21. The coordinates of the centroid (J2000) of this object are $\alpha = 22^{\rm h}$51$^{\rm m}$10.4$^{\rm s}$ and $\delta = -20^{\rm o}$14'59.5 $\hbox {$^{\prime \prime }$ }$ within 1 $\hbox {$^{\prime \prime }$ }$ of error, as calculated from the Digitized Sky Survey (DSS) image (see Fig. 8). Therefore, we have denoted this object Anon J225110.4-201459.5. This is probably a low-surface brightness galaxy. On the basis of the DSS image we have found that the B-band total magnitude of the galaxy is $B_{\rm T}=18.0\pm0.5$.

Figure 8: In spite of the DSS resolution, a high-pass filtering has slightly enhanced the probable bridge between ESO 603-G21 (on the left) and Anon J225110.4-201459.5 at the center (see text). ESO 603-G20 is on the right. North is to the top, east on the left.

It is essential to note that the disks of ESO 603-G21 (Fig. 1) and ESO 603-G20 (Fig. 8) are strongly warped. This feature, as well as the probable bridge, may be an indication of ongoing interaction in the ESO 603-G21-ESO 603-G20-Anon J225110.4-201459.5 triple system (e.g. Reshetnikov & Combes 1999). So ESO 603-G21 is not an isolated object, but a member of a group of galaxies (like, for instance, NGC 4650A). Such dense spatial environment supports the idea that ESO 603-G21 may represent the result of a merging event. To test this scenario, detailed numerical simulations are needed.

Acknowledgements

We would like to thank the referee, Dr. G. Galletta, for useful comments and suggestions. V.R. acknowledges partial support from the "Integration" programme (A0145) and the DAAD (Germany). M.F.-A. and M. de O.-A. thank the partial support of the Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG, grant CEX 1864/95) and the Ministério da Ciência e Tecnologia (MCT, Brazil). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration. The Digitized Sky Survey was produced at the Space Telescope Science Institute (ST ScI) under US Government grant NAG W-2166. Also, this publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.