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A Nature of Polar-Ring Candidate Galaxy ZGC 2315+03

G.M. Karataeva, V.A.Hagen-Thorn ,V.A.Yakovleva

Astronomical Institute of St.Petersburgh University

Cross-shaped galaxy ZGC 2315+03 (Fig.1) has been described by Zwicky (1971) as ``post-eruptive or the result of the crossing of a compact galaxy through an edge-on spiral'' .

This object was included in the Polar-Ring Catalog (C-71) by Whitmore et al. (1990) as a possible candidate for Polar-Ring Galaxies (PRG). According to this work there are two structures along the positional angles $\sim$
125 $^\circ$ and 64$^\circ$. One of them is galaxy 'disk' and the other is 'polar-ring'. The narrow dust lane extending along the major axis was found in the 'disk'

The CCD spectral and photometrical observations of this galaxy were carried out at the 6-meter telescope of Special Astrophysical Observatory (Russia) as a part of the PRGs research program of the Astronomical Institute of St.Petersburg University.

Assuming the Hubble constant to be H$_{0}$ = 65 km/s/Mpc we obtain for ZGC2315 a distance estimation of 295 Mpc (1 $^{\prime\prime}$= 1.43 kpc).

1 Data analysis

Figure 2 shows the radial velocity curves of the galaxy. The shape of these curves are very close to the case of standard undisturbed galaxies and are not at all resembling the situation of tidal interaction. At the same time they are not symmetrical relative the brightness center (radius = 0 $^{\prime\prime}$) and the dynamical center of the 'disk' does not coincide with that of the 'ring'. Taking into account the correction for Galaxy rotation (185 km/s) we receive the radial velocity of the 'disk'(19175 km/s $\pm$ 15 km/s) and the 'ring' (19215 km/s $\pm$ 15 km/s).

The outer part of B,V,R$_{\rm с}$-band isophotes do not show any distortion. The 2-D distribution of the observed colour index (B-V)in ZGC 2315+03 is shown on Fig. 3. The most light region are blue, the most dark are red.

The colour index difference between 'disk' and 'ring' is about 0^m.5. The smoothed rotation curves for the 'disk' and 'ring' are shown in Fig. 4. The increase of the rotation velocity of the 'ring' is linear to about 180 km/s at
11 $^{\prime\prime}$(15.7 kpc), then remains constant. The shape of the curve is very similar to the rotation curve of a disk with an exponential density distribution. It is necessary to stress the difference between the rotation curves of the 'disk' and 'ring' and in particular the greater value of the velocity gradient of the 'ring'. On the distance of 5 $^{\prime\prime}$.5 velocity in the 'ring' is greater on the 27 km/s.

2 Discussion

What is the nature of peculiarity of ZGC 2315+03? According to Zwicky this is a post-eruptive object or the pair of colliding galaxies. From the other hand, according to Whitmore et al. (1990) this is a PRG. We would like to offer a new assumption: the object ZGC 2315+03 is a (possibly occasionally) projection of one galaxy on the another.

The facts against the polar-ring nature of ZGC 2315+03 can be summarized as follows:

• The 'ring' isn't symmetrical relative the center of the 'disk galaxy'. Both the dynamical and the photometrical centers of the 'ring' (rather resembling a disk) are displaced to the SW direction relatively to the apparent brightness maximum.
• 'Ring' deviation of 30$^\circ$from the polar plane must cause the rapid destruction because of the differential precession.
• There is a displacement of the 'disk' and the 'ring' dynamical centers.

Thus it is possible to consider the structures elongated along positional angles of 130$^\circ$and 66$^\circ$as two spiral galaxies: the first one ('disk') has a dust lane and the second one ('ring') is a blue gas-rich spiral.

We prefer a projection model for this object instead of colliding galaxies scenario. The reasons for this are: the regular undistorted isophotes outside the intersection regions, lack of peculiarities of rotation curves shown on Fig. 4, difference in the velocity gradients and an undetectable IR-emission.

The analysis of the Fig. 3 representing the colour distribution in the galaxy, leads us to a conclusion that the blue galaxy is hided by dusty one due to coincidence of the maximum of the colour index with the position of the dust lane. Otherwise the contribution from the blue galaxy should be high and there should be the local colour index minimum in the lane region. Besides a radial velocity of the blue galaxy dynamical center is greater than the dusty one. This difference is not large (40 km/s) and could be explained by orbital movement in the pair. However if both galaxies are on the line of sight then their orbital velocity projection on the line of sight should be small. On the other side if we consider this difference as due to Hubble flow then difference of 40 km/s corresponds to a distance separation of 600 kpc. On such large distance a tidal interaction could not be high and the isophotes regularity is not a surprise.

Thus in accordance to these results we can conclude that object ZGC 2315+03 is the pair of galaxies projected one on the another . It is possible that they interact with each other but this interaction is small. Probably both galaxies are the giant spirals.

Bibliography

Whitmore B.C., Lucas R.A., McElroy D.B., Steiman-Cameron T.Y., Sackett P.D., Olling R.P. // Astron. J., 1990, V.100, P.1489.

Zwicky F. // CGPG. Publ. F. Zwicky, Guemligen (BE), Switzerland-388 (1971)