2. General characteristics of polar-ring galaxies
As a definition of a polar-ring galaxy, we will use the definition of Category A objects in the PRC: spectroscopic evidence must exist for two nearly-perpendicular kinematical subsystems; centers of the two components must be aligned, and both subsystems must have similar systemic velocities; the ring must be comparable in size to the host galaxy, must be luminous and nearly planar. This definition allows to separate dust-lane ellipticals, galaxies with inclined HI rings etc. from PRGs. Using this rigorous definition, one can now consider only three additional galaxies to 6 the classic PRGs listed in the PRC: AM 2020-504 (Whitmore & Schweizer 1987, Arnaboldi et al. 1993), IC 1689 (Reshetnikov et al. 1995, Hagen-Thorn & Reshetnikov 1997) and NGC 5122 (Cox et al. 1995). (We do not consider ESO 603-G21 here due to the puzzling kinematics of the central galaxy (Arnaboldi et al. 1995).)
An examination of the optical images of PRGs (e.g. in the PRC) allows one to divide them into two groups (Whitmore 1991): galaxies with extended disk-like rings with the central region cut out and galaxies with relatively narrow rings, not extended in radius. This division is quite distinct since the first group of galaxies - A0136-0801 (A-1), UGC 7576 (A-4), NGC 4650A (A-5), UGC 9796 (A-6), and NGC 5122 (B-16) - possess optical rings extended out to 2-3 diameters of the central galaxies, while the second group - ESO 415-G26 (A-2), NGC 2685 (A-3), IC 1689 (B-3), and AM 2020-504 (B-19) - demonstrate optical rings with size not exceeding the diameter of the host galaxy.
In Table 1, we generalize the main observational characteristics of the two groups of PRGs. (Note that, due to the absence of optical data about NGC 5122, we did not consider this galaxy in the table.) In the case of incomplete data or large scatter of characteristics, we give in the table only limits or indicate the range of parameter changes. Absolute luminosities and colors in the table are corrected for Galactic absorption.
Table 1. General characteristics of PRGs
We discuss Table 1 in detail.
2.1. Central galaxies
The problem of the bulge-to-disk ratio determination for the PRGs is not quite simple. Reshetnikov et al. (1994) have noticed that bulge effective parameters ( and ) of PRGs with wide extended rings (our first group) lie in the plane of effective parameters below the mean relationship for normal galaxies (this means that at the same value of , bulges of PRGs have smaller radii in comparison with bulges of normal galaxies) - see Fig. 7 in the cited paper. But bulge characteristics of the second group of galaxies are located exactly along the standard relationship. Such a dichotomy suggests two explanations: the unusual compactness of the first group of PRG bulges or the underestimation of their sizes and luminosities due to the projection of gas and dust rich extended rings on central regions of the galaxies ( Reshetnikov et al. 1994).
A comparison of photometric cuts of the galaxies in different color bands allows one to solve this problem. In Fig. 1, we present surface brightness distributions for four PRGs with extended rings along the major axes of their central galaxies. We compare two profiles for each galaxy: one in the B passband and other in the red filter (R, i, or K). As one can see from figure, absorption in the rings barely changes the shape of the profiles. Therefore, PRGs with wide extended rings in fact possess unusually compact and faint bulges in comparison with normal early-type galaxies. As is evident from Fig. 1, the exponential disk dominates in the photometric structure of all these galaxies. The ratio of the total bulge luminosity to the luminosity of the exponential disk in the first group of PRGs is about 0.1 (we denoted this as in Table 1).
The photometric structure of PRGs with short rings (the second group of galaxies) looks usual for early-type galaxies. As was mentioned earlier, their characteristics in the plane of effective parameters follow the mean relation for normal galaxies. According to original papers, bulge-to-disk ratios in the B band for these galaxies are: ESO 415-G26 - 0.52 (Whitmore et al. 1987), NGC 2685 - 0.9 (Makarov et al. 1989), IC 1689 - 1.9 ( Reshetnikov et al. 1995). AM 2020-504 is an elliptical galaxy (Arnaboldi et al. 1993). Therefore, the second group PRGs are normal bulge-dominated galaxies by their photometric structure (conditionally, in the table).
2.2. Polar rings
Rings sizes in the two groups of galaxies are different, as well as their absolute luminosities ( in Table 1 denotes a diameter of the central galaxy measured at the surface brightness level 25). Both groups of rings show a large scatter of optical colors although, as it was shown by Reshetnikov et al. (1995), there is a general trend with a blue surface brightness; bluer rings have, on average, higher surface brightnesses. Extended rings also show large-scale color gradients, that is, they become bluer at larger radii (see, for instance, Figs. 4,6 in Reshetnikov et al. 1994 and Arnaboldi et al. 1995). Both group of rings contain a large amount of neutral hydrogen (we assume in Table 1 that all detected HI belongs to the rings), with larger scatter of HI mass in PRGs with narrow rings (Schechter et al. 1984, van Gorkom et al. 1987, Richter et al. 1994). The last line of Table 1 shows the range of ring inclinations (angular distance between the ring and the perpendicular to the central galaxy plane) according to Whitmore (1991). Both groups of PRGs demonstrate rings to be very close to perpendicular, with a somewhat larger deviation for extended rings. It should also be noted that global characteristics of extended rings resemble the disks of spiral galaxies (this was previously pointed out by Reshetnikov et al. 1994 and Reshetnikov & Combes 1994a, b). We will discuss this analogy in Sect. 4.
Summarizing the above analysis, one can conclude that there is a correlation between general properties of optical polar rings and characteristics of host galaxies. Extended, disk-like rings exist preferably around galaxies which have global photometric structure (and probably mass distribution) similar to late-type spiral galaxies. Host galaxies of PRGs with extended rings demonstrate a remarkable similarity - their characteristics fall in a relatively narrow range (see Table 1). From the other side, bulge-dominated galaxies have tighter and narrow rings. Host galaxies and rings of PRGs with non-extended rings demonstrate significantly larger scatter of intrinsic properties.
Although the statistics of PRGs are not great (8 - 9 objects only), the mentioned tendency is quite distinct - there are no bulge-dominated galaxies with extended luminous polar rings. It is significant since the known PRGs were selected on the base of optical morphology only, without any attention to the structure of the central galaxies. One can note also that the PRG dichotomy is quite analogous to a recently-found difference of ionized gas distribution in elliptical and S0 galaxies. According to Macchetto et al. (1996), more than half of elliptical galaxies with detected ionized gas have their gas concentrated in small ( 4 kpc) nuclear disks, while most S0 galaxies demonstrate more extended (up to 18 kpc) distribution of the gas.
A similar tendency was mentioned for the first time by Whitmore (1991), who found that only rapidly-rotating S0 disks have extended luminous polar rings, while dust-lane ellipticals rarely show an extended luminous component. He suggested that a more flattened potential of the S0 galaxy is able to stabilize the ring at greater radii than in an elliptical galaxy. In the next section, we will demonstrate that the observed dichotomy could be caused by another reason.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998