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G.M. Karataeva, V.A.Hagen-Thorn ,V.A.YakovlevaA Nature of Polar-Ring Candidate Galaxy ZGC 2315+03
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'' .
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 = 65 km/s/Mpc
we obtain for ZGC2315
a distance estimation of 295 Mpc (1
= 1.43 kpc).
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
) 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
15 km/s) and
the 'ring' (19215 km/s
15 km/s).
The outer part of B,V,R-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
(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
.5 velocity
in the 'ring' is greater on the 27 km/s.
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:
Thus it is possible to consider the structures elongated along
positional angles of 130and 66
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.
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)