Inner Polar Rings and Disks: Observed Properties

arXiv:1204.4437v1 [astro-ph.CO] 19 Apr 2012 [email protected] o n ioasc hspeoeo smr orystud- poorly be- more scaled is usually phenomenon disks, This and kiloparsec. describes rings one literature low polar inner the of time, cases same the the is At rings polar known. external widely of phenomenon the galaxies, nearby review). (2006) Combes in and fil- references or- intergalactic massive (see two the most aments from of gas the merger of for rings—accretion the and extended galaxy, disks, donor capture oriented the the thogonally from are galaxy. matter the discussed of of usually axis mechanisms of rotation rotation basic the The of to moment interaction perpendicular the of is revolutions matter, which result surrounding the of the are lot PRGs with a the models make that Numerical can demonstrate undisturbed. pre- galaxy ring central differential the polar the the around halo, that to dark In so respect spheroidal solved. with cession, or stable considered triaxial is be a orientation now of the can presence in issues the formation key star the the of of on PRGs, history catalog data and detailed photographic of evolution a lack dynamics, the of Despite (1990) candidates. publication PRG al. the after et Whitmore began study late by the mass to their back date while positioned PRGs 1970s, of kiloparsecs, kinematics stud- internal of first the The of plane. tens ies galactic stellar-gaseous the several a to to perpendicular by roughly even up or sur- sized ring is disk, external cen- E/S0) an early-type, The by an (PRG). of rounded (typically, galaxies here ring galaxy galaxy polar tral a we so-called planes, in orthogonal the presence mutually mean the in rotating about subsystems talk of generally we When INTRODUCTION 1. edopitrqet to requests offprint Send rnltdfo srfiihsi ylee,21,vl6,No.2 vol.67, 2012, Byulleten, pp.123-135 Astrofizicheskij No.2, from 67, Translated vol. 2012, Bulletin, Astrophysical ept h eaieyrr curneaogthe among occurrence rare relatively the Despite h xenlevrnetadtepeec fa ne oa s polar inner an o of or presence axis interaction triaxial the major recent a and of the environment (or sign with external bar another the coincides a or usually structur one has rotation polar reveal galaxy galaxies of The a axis kpc. if its pola 1.5 time, of — exceed same parameters not the the do At of them galaxies. statistics of lis the majority The consider data. the We literature Irr. the to from compiled E is rings and disks Abstract. 2012 20, February 2012/Revised: 13, February Scie of Academy Russian Observatory Astrophysical Special ne oa ig n ik:Osre Properties Observed Disks: and Rings Polar Inner ito aaiswt ne ein eeln oa o str (or polar revealing regions inner with galaxies of list A ..Miev e-mail: Moiseev, A.V. : ..Moiseev A.V. i’hno&Aaaiv 00 i’hno 02,a elas methods well as 1997; 2002), the al., Sil’chenko, 2000; et using Afanasiev, (Sil’chenko & spectroscopy RAS) Sil’chenko (3D) (SAO panoramic Sciences of tele- Russian of BTA by of Observatory 6-m Academy made Astrophysical the Special researches at the the of colleagues scope her all, and of Sil’chenko first Olga early- de- Note, nearby the other galaxies. of within type kinematics authors internal of several studies 2217. decoupled by tailed NGC coin- kinematically found of practically bar similar were stellar decade, structures disk the following polar of per- Moreover, the axis this galaxy. plane, In major of the the the with rotation of in cides warped of disk occurs is axis stellar rotation gas the the that ionized to way of within pendicular a disk that such a shown in have kiloparsec, long- 2217 central the NGC of the of results the Later spectroscopy to convincing. according slit less (1990), be al. alternative et to disk Bettoni The plane—seems gaseous galaxy. galactic the the the in of of rotation cir- proposed—compression the of explanation of axis rotation the au- of with the axis to the ( axis that According cumnuclear minor indicates 1977). its this al., et along thors, (Rubin velocities detected gra- line-of-sight was significant the a 3672 of NGC spectral galaxy dient the Sc during the of instance, rec- observations For been has confirmed. PRG and of ognized phenomenon the before even known galaxies. of regions in- stellar central and or the gaseous ob- the in polar to of components motions internal especially the and on the data bulge, identify detailed bright tain to the against effort disk requires of clined It lot PRGs. “classical” images a the optical quite to the contrast in in “clarity” galaxies invisible lower of usually the by structures, explained such be of well as may which ied, egn.Ti atidctsadrc eainbetween relation direct a indicates fact This merging. from types, morphological all of galaxies 47 contains t cs ihi rhz 617 Russia 369167, Arkhyz, Nizhnij nces, neetnl,teinrplrsrcue IS were (IPS) structures polar inner the Interestingly, tutrskonfo bevtos h ai of radii The observations. from known structures r tructure. h a.Mr hntotid falconsidered all of thirds two than More bar. the f ug) hslast h oa ikstabilization disk polar the to leads this bulge), saeeulycmo nbre n unbarred and barred in common equally are es nl nlndt h anglci plane) galactic main the to inclined ongly < r 5 c a a osdrbeangle considerable a has gas pc) 350 oebr1,2018 10, November 2 Moiseev: Inner Polar Rings and Disks the publications related with the group of the University to Shalyapina et al. (2004); of Padua (Bertola & Corsini, 2000; Pizzella et al., 2001). (4) the distance (D) in Mpc in accordance with the The list of galaxies with confirmed IPSs, published in HyperLeda; 2003, already contained 17 objects (Corsini et al., 2003). (5), (6) the external radius of the polar structure in the In the subsequent decade, various groups have presented angular and linear scales (r). In many papers the au- a fairly extensive observational material, dedicated to the thors themselves gave this value. In other cases, we made detection and investigation of such structures. This was the estimates based on the data presented in the origi- much facilitated by the survey of kinematics and stel- nal papers: the diagrams of radial variations of the kine- lar population of nearby early-type galaxies, performed matic axis position angle (P Akin ) or the published veloc- at the 4.2-m WHT telescope, and the study of chemi- ity fields. Sometimes the problem was simplified by the cally decoupled galactic nuclei at the SAO RAS 6-m BTA fact that all the ionized gas, observed at the center of telescope, performed using the SAURON and MPFS the galaxy belongs to the polar structure. In this case, in- integral-field spectrographs, respectively. The study by stead of looking for a region of the line-of-sight velocity Sil’chenko & Afanasiev (2004) is also representative. Here, gradient variation, it was sufficient to estimate the size of the authors selected for the MPFS observations eight the region, occupied by the gas emission lines (for exam- galaxies, the optical images of the central regions of which ple, NGC 4552, and NGC 5129 according to the SAURON are clearly revealing the dust lanes, projected onto the nu- data). For some galaxies, only to the lower limit of this cleus, which argues in favor of the presence of gas-dust parameter is known, limited by the spectrograph field-of- disks, strongly inclined to the line of sight. The derived view; velocity fields of ionized gas and stars have confirmed the (7), (8) the parameters of orientation of the main galactic existence of internal disks or polar rings in all the sample disk: the position angle (P A0) and the inclination to the objects. Coccato et al. (2004) have demonstrated that in line of sight (i0) in degrees are in most cases taken from 50–60% of bright unbarred galaxies the remarkable gra- the original papers, and in the remaining cases—according dient of the line-of-sight velocity is observed along the to the HyperLeda database; minor axis, which may partly be explained by the pres- (9) the position angle of the major axis of the bar (P Abar), ence of IPSs. In Moiseev et al. (2010) we briefly presented specified in the original papers, in degrees. In some cases, a new list of 37 galaxies with IPSs. Despite the fact that (NGC2768, NGC2841, NGC6340, NGC7217) we deal the number of such objects surpasses the number of kine- with the “triaxial bulge”, rather than the contrasting bar. matically confirmed external polar rings, their nature re- For NGC 4548 we list the orientation of the internal tri- mains vague. Up to date, there is no clear self-consistent axial structure instead of the external bar (in agreement scenario of their formation, the issues of stability of such with Sil’chenko (2002)). In some cases, the authors of the structures are not resolved either. The views on the re- original papers suspected “hidden triaxiality” of the in- lationship of IPSs with bars of galaxies and their exter- ner regions based on the indirect evidence, but could not nal environment (the presence of companions, traces of specify the exact position angle (e.g., see NGC 3607 in interaction, etc.) are contradictory (Corsini et al., 2003; Afanasiev & Sil’chenko (2007)); Moiseev et al., 2010). (10) the major axis of the circumnuclear structure (P A1) This paper presents an updated list of galaxies with in- in degrees. It was given by the authors or estimated by ner polar rings and disks, compiled based on the data pub- us from their diagrams of radial variations P Akin (r). The lished in the literature, including our own observational value is missing for some galaxies, the data on the in- data obtained with the 6-m telescope. A large enough ternal kinematics of which are based only on the long- number of objects allowed us to consider some statistical slit spectroscopy (NGC4424, NGC4698, NGC4941) or relations in the properties of IPSs. for which the authors have constructed the spatial model of the internal structure (Arp 220, NGC 1068, NGC 2855, NGC3227, NGC7049). For the NGC3368 and NGC4111 2. COMPILATION OF THE LIST galaxies, the adopted parameter P A1 significantly differs from the estimates based on P Akin and was determined 2.1. Selection Criteria from the orientation of the internal dust ring. We believe Table 1 presents the main parameters of internal polar that here, in the velocity field along the line of sight, we structures, described in the literature. The columns with observe a superposition of two gaseous subsystems, or the respective numbers contain the following data: inclined structure is nonstationary. For NGC 5014 the pa- (1) the name of the galaxy; rameters of orientation were determined from the SDSS (2), (3) its morphological type according to the NED/RC3 image, which reveals a narrow blue ring (Moiseev et al., and its digital code adopted from the HyperLeda 2011); database1, (T = −2 corresponds to S0, T = 0—to S0a, (11) the inclination of the internal structure to the line etc.). For NGC 7468, which is clearly not elliptical (as of sight (i1), in degrees. Evaluated the same way as specified in the LEDA), T = 9 was adopted according P A1, with the same remarks on individual galaxies. Unfortunately, in many cases it is impossible to estimate 1 http://leda.univ-lyon1.fr this parameter, we hence either list the assumed range of Moiseev: Inner Polar Rings and Disks 3 values, or the lower limit when it is clear that the inner line-of-sight velocity gradient along the minor axis is not disk is significantly inclined (NGC 3414, NGC 7742, etc.). a sufficient criterion of the presence of an IPS. Hence, our For NGC2787, and NGC2911 our own estimate of orien- list lacks plenty of candidates from Coccato et al. (2004). tation of the dust structure is given; On the other hand, our sample includes most of the early- (12) the angle of inclination ∆i of the internal structure type galaxies from the SAURON survey, for which the ∗ to the plane of the disk. The sign marks the estimates difference between the P Akin , determined by the velocity from the literature, in other cases it was evaluated by us fields of gas and stars, is in excess of 30◦. The only ex- (see Section 3.4 below). For NGC1068 and NGC3227 the ception were the objects where the disk of ionized gas ex- value ∆i> 90 is listed, meaning that the innermost parts tends beyond the edge of the spectrograph’s field of view, of the circumnuclear gaseous disks are warped so much obviously, being an internal part of the polar structures, that the orbits are re-approaching the main plane of the observed in neutral hydrogen far beyond the stellar disk galaxy; of the galaxy. This applies to NGC 2685—the prototype of (13) the comment, pointing to the observed composition of the classical PRG, and a galaxy with an external UV-ring, the structure (H II—ionized, H I—neutral, CO—molecular NGC 4262 (Bettoni et al. (2010), also see Oosterloo et al. gas, s—stars) and its structure: w—a strong warp, r—a (2010) for the HI map). ring, i.e. there is a hole in the center; In addition to the perturbing effect produced by the (14) references to the literature used. triaxial potential, a sharp change in the direction of the In total, Table 1 lists 47 galaxies, for which there ex- line-of-sight velocity gradient can be also due to the radial 2 ist strong arguments that in their inner regions a sub- gas flows, triggered by the central burst of star formation stantial part of the emitting matter steadily rotates in or by a jet from the active nucleus. Thus, Coccato et al. the plane, strongly inclined to the plane the main disk. (2004), based on the long-slit spectroscopy data suspected As a rule, such a dynamic configuration is directly stated the existence of an IPS in NGC 6810. However, the sub- by the authors of papers, referred to in the last column sequent studies have shown that the central kinemat- of Table 1. In the case of other objects, we believe that ics of gas there is determined by the starburst super- the presence of polar (or highly inclined) orbits is the wind (Sharp & Bland-Hawthorn, 2010). For a similar rea- most reasonable explanation of the observed circumnu- son we have excluded from consideration a number of clear kinematics. To make this conclusion we must have known galaxies with ionization cones, in which the mo- a velocity field, obtained with a sufficiently high spatial tions of ionized gas in the central kiloparsec region are resolution. Historically, the first technique was to make likely to be caused by the activity of the nucleus, de- several spectral sections with a long slit, while more re- spite the fact that a number of authors have found in- liable results can be obtained with the panoramic (3D, clined disks here (Mrk 3, Afanasiev & Sil’chenko (1991), integral-field) spectroscopy in the optical range, or using NGC5252, Morse et al. (1998), etc.). radio interferometry in the molecular gas lines. An inter- Note that the existence of a twist of P Akin is esting example is the NGC 253 galaxy, in which an IPS not strictly necessary. For example, in NGC3607 and was revealed from the observations in the radio recombi- NGC 7742, visible almost face-on, the kinematic position nation H92α line (Anantharamaiah & Goss, 1996). angles for the external and internal regions are almost In Krajnovićet al. (2008) the terms “kinematic twist” identical (P A1 ≈ P A0). However, the amplitude of the and “kinematically decoupled component” identify the ◦ ◦ line-of-sight velocity of gas, observed in the center is so cases of significant (exceeding 10 –20 ) variations of high that the most reasonable explanation for this is ro- P Akin with increasing distance from the center in the ob- tation of the disk, strongly inclined to the line of sight. served line-of-sight velocity field. Note that such a mis- The case of an elliptical galaxy NGC 5198 is challeng- match should not always be associated with rotation in ing. This object, according to the velocity fields, presented orbits that lie outside the galactic disk plane. The twist in Sarzi et al. (2006); Krajnovićet al. (2008) has two po- of the P Akin can also be related with the non-circular lar structures—an internal stellar (r < 2′′) and a more motions in the plane of the galaxy caused by the non- extended gaseous (r< 5′′) structure, not coinciding with spherical potential of the central bar or triaxial bulge. each other. At that, the position angle of the gaseous disk Fortunately, comparing the velocity field with the results almost coincides with the P Akin of the external regions of of isophote analysis of the galaxy images, we can under- the stellar velocity field, however it looks that the counter- stand what type of motion takes place (see the discussion rotation occurs. But as the observed amplitude of the ion- and references in Moiseev et al. (2004)). In addition, the ized gas rotation curve is very large, it is clearly outside triaxial potential and the inclined disk should manifest the main plane of the stellar spheroid, in which the out- themselves in different ways in the distribution of the line- ermost gas is rotating. of-sight velocities along the major and minor axes of the galaxy (Corsini et al., 2003). However, the absence of the 2.2. Galaxies, not Included in the List 2 By inner regions we mean the scales smaller than or com- parable with the characteristic scales of a bulge, or an external Compiling the list we have tried to review the largest disk. possible number of observational papers on this issue. 4 Moiseev: Inner Polar Rings and Disks

Table 1. List of galaxies with inner polar structures Ref (14) Eckart & Downes (2001) Sil’chenko & Afanasiev (2008) Hagen-Thorn & Reshetnikov (1995) Melchior & Combes (2011) Moiseev (2011) Moiseev (2011) Anantharamaiah & Goss (1996) Sarzi et al. (2006), Krajnovi´cet al. (2008) Schinnerer et al. (2000b) Bettoni et al. (1990) Sil’chenko & Afanasiev (2004) Dumas et al. (2007) Sparke et al. (2008) Moiseev et al. (2004) Sil’chenko & Afanasiev (2004) Fried & Illingworth (1994) Sil’chenko & Afanasiev (2004) Crocker et al. (2008) Sil’chenko & Afanasiev (2004) Sil’chenko et al. (1997) Afanasiev & Sil’chenko (1999) Coccato et al. (2007) Sil’chenko & Afanasiev (2004) Schinnerer et al. (2000a) Sil’chenko et al. (2003) Moiseev et al. (2004) Sil’chenko et al. (2003) Sarzi et al. (2006) Krajnovi´cet al. (2008) Sil’chenko et al. (2003) Sil’chenko & Afanasiev (2004) Sil’chenko et al. (2010) Afanasiev & Sil’chenko (2007) Afanasiev & Sil’chenko (2007) Sil’chenko et al. (2010) Fridman et al. (2005) Sil’chenko & Afanasiev (2004) Sil’chenko & Afanasiev (2004) Coccato et al. (2005) Sil’chenko (2002) s HII HII HII HII HII HII HII HII HII HII HII HII HII HII (13) HII, r HII+s HII+s HII+s HII+s CO, w HII, w CO, w HII, w HII, w CO, w HII, r? Comm. HII+s, r HII+CO HII, w, r HI+HII, r CO+HII, r ◦ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ ∗ – – – – – – – – – – – – 90 88 90 83 81 90 i, 73 60 90 90 90 90 70 87 55 (12) – 80 , , − − − − ∆ 71,88 32,76 − − > > 39, 86 56, 86 58 25 78 77 74 84 40 55 ◦ – – – – – – – – – – – – – – – – – – – – , 30 40 47 75 60 90 70 60 60 60 50 1 i (11) − − > > 30 30 ◦ – – – – – , 20 90 68 35 74 20 72 63 68 60 76 1 290 296 195 236 349 325 163 260 324 256 351 347 226 320 190 358 (10) − PA 335 ◦ – – – – – – – – – – – – – – – – – – – – – , 48 90 25 (9) 154 138 125 200 110 111 149 132 bar PA ◦ , 59 90 40 54 25 90 65 42 56 56 48 40 52 28 34 47 84 87 63 37 40 77 59 45 79 26 21 90 62 57 32 73 (8) 0 i ◦ 6 , 78 85 67 47 95 40 35 95 55 (7) 0 278 150 117 158 135 253 255 145 164 116 346 230 330 148 109 140 179 300 341 346 150 176 PA , r 65 76 59 (6) . . . 0.62 0.16 0.78 0.31 0.51 0.08 0.20 0.21 0.69 0.44 0.21 0.23 0.12 3.28 0.70 1.41 0.61 0.30 0.32 1.83 0.22 0.91 0.33 1.10 0.22 0.39 1.14 1.19 1 1 0 0.083 > > > 2 5 5 4 3 3 4 3 3 7 5 3 5 4 5 9 2 4 7 8 6 ′′ 15 10 12 11 16 12 10 r, 1.5 0.9 (5) 0.3 180 > > > > , D (4) 3.4 7.5 85.1 67.6 16.1 24.2 32.4 23.6 12.6 26.5 18.3 13.7 14.3 20.3 22.9 16.8 15.6 81.3 0.78 24.2 12.0 32.7 20.7 13.2 47.0 13.7 25.2 22.8 20.0 19.7 14.9 35.1 -4.0 -2.0 5.1 3.0 0.1 -2.0 -4.5 3.0 -0.2 1.5 2.2 -4.8 -2.0 -4.8 1.0 3.1 T (3) 9.3 3.0 9.9 0.0 -2.0 -0.6 0.4 -1.0 -2.0 -2.7 -2.0 -3.2 -0.9 4.1 -1.3 -2.0 (rs) (rs) + + (r)? (s)? (r) (s) (s)? 0 0 + + − )SAB0/a(rs) )SA(rs)bc ′ ′ 0 Type (2) S? S0 S0? SA(s)b Im pec? pec? SAB(s)c SA0 (R)SA(rs)b (R)SB0 SAB0/a(s) (R S0 E6? SB0 SA(r)b? (R)SA0/a(rs) SA0(s)? pec SAB(s)a pec SAB(rs)ab E1 SB0 S0 pec SA0? SA0 E2 (R)SA0 (R SA0 S0 SB(s)a? SB(rs)b Name Arp 220 (1) IC 1548 IC 1689 M 31 Mrk 33 Mrk 370 NGC 253 NGC 474 NGC 1068 NGC 2217 NGC 2655 NGC 2681 NGC 2732 NGC 2768 NGC 2787 NGC 2841 NGC 2855 NGC 2911 NGC 3227 NGC 3368 NGC 3379 NGC 3384 NGC 3414 NGC 3599 NGC 3607 NGC 3608 NGC 3626 NGC 4100 NGC 4111 NGC 4233 NGC 4424 NGC 4548 Moiseev: Inner Polar Rings and Disks 5

Table 1. (continue) Ref Sarzi et al. (2006), Krajnovi´cet al. (2008) Dumas et al. (2007) Bertola & Corsini (2000) Pizzella et al. (2001) Bertola & Corsini (2000) Coccato et al. (2005) Noordermeer et al. (2005) Moiseev et al. (2011) Sarzi et al. (2006) Krajnovi´cet al. (2008) Moiseev et al. (2004) Sil’chenko (2000) Chilingarian et al. (2009) Coccato et al. (2007) Sil’chenko & Afanasiev (2000) Sil’chenko & Moiseev (2006) Sil’chenko et al. (2011) Afanasiev & Sil’chenko (2000) Sil’chenko (2005) Shalyapina et al. (2004) Sil’chenko & Moiseev (2006) Shalyapina et al. (2002, 2007) (14) s HII HII HII HII HII HII HII HII HI,r (13) HII+s HII+s HII+s HII, w Comm. HII, w, r ◦ ∗ ∗ ∗ – – – – – – – 90 56 26 i, 65 80 89 (12) , , − − > ∆ 49,87 68-80 40 65 88 52 ◦ – – – – – – – – – , 60 80 60 60 80 35 1 i (11) − > > 40 ◦ – 9 – – , 32 47 32 35 1 154 134 330 329 120 130 260 (10) PA ◦ – – – 0 – – – – – – , 58 85 60 60 (9) 115 bar PA ◦ 9 , 14 39 90 74 37 81 49 37 20 30 52 45 50 60 (8) 0 i ◦ , 96 46 15 14 70 58 (7) 0 110 170 100 335 268 258 180 128 182 PA , r (6) 0.38 2.06 1.32 0.39 0.21 2.08 0.76 1.13 1.30 0.24 0.92 0.36 2.17 0.73 0.27 5 6 5 2 4 6 3 6 5 2 ′′ 18 22 12 10 r, (5) , D (4) 15.6 23.6 45.7 16.1 21.2 19.5 39.3 38.7 22.4 16.7 31.8 24.7 44.7 29.9 28.2 -4.6 2.9 1.1 1.7 2.1 1.4 -4.8 3.1 0.4 2.5 9.0 2.8 -1.8 T (3) -1.9 -1.3 (r) + (s) 0 Type (2) E0-1 SAB(rs)b SA(s)a pec SA(s)ab (R)SAB(r)ab? Sa? E1-2? SB(r)b SA0/a(s) SA0 (R)SA(r)ab SAB0 E3? pec SA(r)b S0? (1) NGC 4579 NGC 4672 NGC 4698 NGC 4941 NGC 5014 NGC 5198 NGC 5850 NGC 6340 NGC 7049 NGC 7217 NGC 7280 NGC 7468 NGC 7742 UGC 5600 Name NGC 4552 6 Moiseev: Inner Polar Rings and Disks

Unfortunately, the abstracts and conclusions do not al- Local Volume (D< 10 Mpc). When we mean the PRGs, it ways contain information on the presence of polar struc- is reasonable to talk about the rings, even if they are rather tures. Therefore, we apologize in advance to the authors wide, on the other hand, considering the IPSs, we are as a of papers that we may have missed. At the same time, our rule (in 39 out of 47, i.e. 83% of cases) talking about the sample does not include some known candidates for the disk geometry in which the inner diameter is negligible presence of IPSs. Besides the already mentioned galax- in comparison with the outer one. Unfortunately, we do ies possessing extended polar disks and active nuclei with not possess enough observational data of high spatial res- ionization cones, these are objects on which, in our opin- olution to refine their detailed inner structure. As follows ion, the available data is insufficient or too controver- from Table 1, only the gaseous disks and rings are mainly sial. These include: NGC 3672, mentioned in the intro- found, the list contains only one galaxy (NGC 3384) with duction (the galaxy where the existence of an IPS was a purely stellar polar disk, and six more structures are first reported), as well as most of the other galaxies marked as stellar-gaseous. The selection effect is present with the remarkable line-of-sight velocity gradient along here, since methodologically it is much easier to identify the minor axis from the list of Coccato et al. (2004); the emission lines of ionized or molecular gas than to sep- the kinematically decoupled nucleus in NGC 4150, known arate the absorption spectrum, observed along the line of from the SAURON/OASIS data McDermid et al. (2006); sight, into stars, belonging to the bulge, the polar disk and NGC 7332, although having some indications of the pres- the main disk of the galaxy. It is obvious that quite a few ence of an inclined disk Sil’chenko (2005), its velocity field described IPSs are indeed stellar-gaseous, just that it is has a very irregular shape; NGC524, where a polar disk problematic to detect the kinematic manifestation of stars. was suspected (Sil’chenko, 2000), but the new data re- For example, the stellar polar disk in NGC 7217, identified veal a more complex structure consisting of two counter- by its kinematic properties is remarkably more compact rotating disks Katkov et al. (2011a); NGC 3367, in which than the gaseous one Sil’chenko & Moiseev (2006). The according to Gabbasov et al. (2009) there is a reversal ionization of gas in the IPSs can (at least partially) be of internal isovels in the line-of-sight velocity field, but explained by the ongoing star formation. the interpretation is difficult; M83, which was suspected in the presence of an IPS from the morphology of dust lanes Sofue & Wakamatsu (1994), but the follow-up studies of perinuclear kinematics did not confirm this hypoth- 3.2. Radial scales esis. A recent paper Davis et al. (2011) provides the data on the kinematic parameters of early-type galaxies within the Figure 1 shows the histogram of distribution of the sizes of internal polar structures. It it clear that with respect to ATLAS3D surveys. Their lists reveal a dozen more galax- them it would be fair to use the term the “central kilopar- ies in which the difference of position angles, measured ◦ ◦ sec”: the average median radius is about 600 pc, 85% of by the velocity fields of gas and stars exceeds 40 –50 . all IPSs are sized below 1.5 kpc. This compactness is likely However, we can not be sure that we are dealing with the due to the fact that for a stable existence of polar orbits inner polar disks, since the authors do not give neither any a stabilizing factor is required, i.e. a spheroidal or triaxial P Akin (r) dependencies, nor the velocity fields of ionized potential. For the classical PRGs it is the gravitational po- gas. We have also eliminated from our consideration the tential of dark halos, and for the internal structures—the objects in which the P Akin coincides with the photometric potential of a bulge or a bar (see Section 3.5 below). This major axis in the inner regions, and with the minor axis may practically explain the lack of known polar struc- in the external regions (NGC4365, NGC4406, etc.), what tures of intermediate size, with r = 2–10 kpc. Evidently, is most likely related with the triaxiality of these elliptical at these scales, the differential precession that occurs un- galaxies (see the survey deZeeuw & Franx (1991)) der the influence of gravitational potential of the stellar disk leads to a catastrophically fast decrease of orbit incli- 3. STATISTICAL PROPERTIES nations and their “fallout onto the disk.” Note that even in the IPS in IC 1689—the largest in our list—the radius is 3.1. General Remarks smaller than the effective radius of the bulge (re = 4 kpc The compiled list confirms the idea that the inner polar according to Reshetnikov et al. (1995)). It is possible that structures are a very common phenomenon Coccato et al. it is exactly the picture, observed in NGC 7743—a lenticu- lar galaxy where the entire ionized gas at r =1.5–5.4 kpc (2004). Indeed, the number of known galaxies, contain- ◦ ◦ ing IPSs is one and a half times greater than the total is located in the plane, inclined by 34 or 77 with respect number of galaxies with kinematically confirmed external to the stellar disk Katkov et al. (2011b). polar rings (about 30 objects, see Moiseev et al. (2011)). Unlike the case of “classical” PRGs, among the IPSs only The dip in the distribution for r < 200 pc is appar- relatively close objects are as yet available for detailed ob- ently caused by the limited spatial resolution of most of servations: most of galaxies from Table 1 are located closer observations, since this scale at D = 30 Mpc corresponds than 30–40 Mpc from us, including three, belonging to the to an angular size of about 2′′. Moiseev: Inner Polar Rings and Disks 7

Fig. 1. The histograms of distribution of the internal polar structures by sizes (left) and morphological types of galaxies (right). All the sample are marked in gray, while the galaxies with bars and triaxial bulges are shaded.

3.3. Morphological Types for the formation of these structures. It is possible that the forming inner polar disk has time to preliminarily “sweep We know that the outer polar rings are as a up” the region of the central kiloparsec. In any case, the rule observed around the early-type galaxies, examples of strongly warped IPSs are known, when the E/S0 Whitmore (1991); Reshetnikov et al. (2011). One gas near the nucleus rotates in the polar plane, and with of the explanations for this is that these galaxies have increasing distance from the center, the orbits fall into the poor own internal gas reserves, hence the gas clouds at plane of the galaxy (see Section 3.4 below). It is possible the polar orbits do not undergo any collisions with the that in some cases the inclined orbits are occupied by the gas in the main galactic plane. To what extent is this true gas from the main plane of the galaxy under the effect of for the internal polar structures? The estimates made by the gravitational potential of the bar (Section 3.5). Note, Coccato et al. (2004) have shown that the remarkable however, that the histograms in Fig. 1 do not show any line-of-sight velocity gradient of gas along the minor axes significant differences in the distribution of barred galax- of galaxies is predominantly observed in the S0 galaxies ies. and early-type spirals, which may indicate that there is a relation between this phenomenon and the presence of a vigorous bulge. However, the authors themselves noted 3.4. Inclined or Polar? that the velocity gradients along the minor axis are not Using the term “polar” with respect to the IPSs, one has always due to rotation in the polar plane. to remember that is not always possible to accurately mea- If we consider only the galaxies with confirmed IPSs, sure the inclination of the plane of the inner disk to the the distribution by morphological type becomes broader outer. It is easy to show (Moiseev (2008)) that this angle (Fig. 1, right). The median average here is T = 0, i.e. ∆i is expressed by the relation: only a half of all galaxies belongs to the S0a type and earlier. And nearly a third of them are the Sb-type ob- cos∆i = ± cos(P A0 − P A1) sin i0 sin i1 (1) jects and later, even including a few Sm and Irr galaxies, +cos i0 cos i1. commonly possessing small bulges. Note that the actual number of late-type galaxies must be underestimated by Most often we only know the parameters of orientation the selection effect, since the large surveys of kinematics of of the outer disk (P A0, i0) and the direction of the major the circumnuclear regions with the MPFS and SAURON axis of the inner structure P A1. However, to determine integral-field-spectrographs were focused primarily on the the inclination angle i1 of the orbits to the line of sight E/S0 galaxies. from the observed kinematics within the model of circular Therefore, we can make a preliminary conclusion that rotation, a detailed velocity field with a large number of for the existence of a circumnuclear polar or inclined disk, independent points is required. Moreover, a stable solu- the morphological type of a galaxy is less significant than tion can usually be obtained only for a notable inclination ◦ for the PRG. Apparently, the effect of collision of gas at of the disk to the line of sight (i1 > 30–40 ). An only ex- the polar orbits with the gas of the main disk is not critical ception is a case of a purely polar disk in a galaxy, seen 8 Moiseev: Inner Polar Rings and Disks

Fig. 2. The distribution by the inclination angle of inner rings and disks to the main galactic disk. The left plot: the entire possible range of solutions for ∆i from (1), the right plot: in the case of two solutions the greater was taken. All the galaxies are marked in gray, while the galaxies with bars and triaxial bulges are shaded.

◦ ◦ edge-on (P A0 = P A1 + 90 , i0 = 90 ). The uncertainty ness distribution of ionized (NGC 2855, and NGC 7049) or with the sign of the first term in (1) is due to the fact molecular (Arp220, NGC1068, NGC3227) gas (see refer- that P A and i do not fully characterize the position of ences in Table 1). the plane with respect to the observer—we also need to know the direction of the moment of rotation, i.e. which 3.5. Bars and Triaxial Bulges side of the disk is nearer to us, and which is farther. Hence, ◦ for a number of galaxies with i0 < 90 Table 1 gives both The relationships of internal polar structures with possible alternate solutions of (1). One of the few excep- the nonaxisymmetric gravitational potential have been tions is the polar ring in the Andromeda galaxy (M 31). widely discussed in the literature, starting from the pa- Its relatively large angular scale has allowed the authors per Bettoni et al. (1990), for the first time describing the of Melchior & Combes (2011) to precisely understand how case of a circumnuclear polar disk in a barred galaxy it is oriented with respect to the galactic disk. NGC 2217, oriented virtually perpendicular to the major axis of the bar. Further on, similar arrangements of The angle ∆i was estimated for 27 objects, which is IPSs were found in many other barred galaxies, listed in slightly over a half of the entire sample. The histograms Table 1. It was repeatedly noted that such an arrange- presented in Fig. 2 resemble much the distribution by the ment of the polar disk along the smallest section of the same parameter of the outer polar rings from Whitmore bar—i.e. in one of the main planes of a triaxial potential— (1991). Notwithstanding the above uncertainty with the has to be stable. At that, it is sufficient to have a non- estimation of ∆i, most of the outer inclined structures spherical (triaxial) bulge in a galaxy instead of a large- turn out to be truly polar, i.e. perpendicular to the outer ◦ scale bar (Sil’chenko, 2000; Afanasiev & Sil’chenko, 1999; disks of galaxies. Thus, ∆i> 70 in 23 out of 27 (i.e. 85%) Corsini et al., 2003). cases. Figure 3 presents the distribution of all our sample On the other hand, even if we choose between the two galaxies by the angle ∆ψ between the major axis of the ◦ solutions for ∆i the option, closest to 90 (Fig. 2, right), nonaxisymmetric stellar structure (a bar or a triaxial there are still IPSs located at a more moderate angle ∆i bulge) and the major axis of the IPS (projected on the ◦ ◦ equal to 50 –60 : NGC3599, NGC5014 and NGC6340. galactic plane): Such structures are unlikely to be stable: they have to sin(P Abar − P A0)cos i0 relatively quickly fall into the galactic plane under the ef- ∆ψ = arctan − fect of differential precession. An indirect indication of this cos(P Abar P A0) (2) are the observed warps of gaseous disks in NGC3599, and − − sin(P A1 P A0)cos i0 NGC 6340. Note that similar warps are found in seven arctan − . galaxies of the sample. In many cases, the authors can cos(P A1 P A0) construct a detailed spatial model of such warped disks, Despite the relatively small statistics available, the reproducing not only the kinematics, but also the bright- tendency of polar structures to align orthogonally to the Moiseev: Inner Polar Rings and Disks 9

the modeling made by Friedli & Benz (1993), the obtained disk is not polar, but inclined at about 45◦ to the plane of the galaxy. Secondly, the proposed mechanisms can obviously not be the main method of polar disk formation, since in this case one should expect an increased number of bars in the galaxies with IPSs. However, out of 40 disk galaxies (S0-type and later) from our sample only 17 possess confirmed bars or triaxial bulges3, which is 43 ± 8% or 33 ± 7%, if we ignore the non-spherical bulges4. This is in good agreement with the known frequency of bars in nearby galaxies, which is about 45% (Aguerri et al., 2009). Therefore, we must conclude that although the bars have an effect on the orientation of polar disks, the existence of a triaxial potential is not absolutely necessary for the existence of IPSs. Surely, the galaxies may have a certain internal triaxiality, barely noticeable to the detached observer (see, e.g., the discussion of this issue in Afanasiev & Sil’chenko (2007)). However, the circumnuclear regions usually look more symmetrical than the Fig. 3. The distributions by the angle between the major outer ones not only in the unbarred galaxies, but also in axis of the polar disk and the bar (or the triaxial bulge). the barred spirals (inside the Lindblad resonances of the bar). In Sil’chenko et al. (2010), we show that NGC 3599 and NGC 3626 have an internal “oval distortion of the disk”, which may point to the formerly existing bars, de- ◦ major axis of the bar (∆ψ = 90 ) is clearly visible. So far, stroyed in the process of secular evolution or under the the literature has detailed calculations of the formation effect of external influence. However, such a scenario is of such inner disks, for instance, via the capture of the unlikely to fit most of the remaining unbarred galaxies. outer gas clouds with the corresponding direction of the orbital moment. Usually the authors use a star-dynamic analogy with polar or warped disks, observed in the 3.6. External Environment global triaxial potential of elliptical galaxies (see discus- Another frequently debated issue is the relationship of sion in Corsini et al. (2003) and Sil’chenko & Afanasiev IPSs with the external environment of galaxies, and the (2004)). In any case, it is most likely that triaxiality of processes of their interaction. By analogy with external the potential is responsible for the stabilization of the in- polar rings, it is reasonable to expect that the circumnu- ternal polar disks in the circumnuclear regions of elliptical clear polar structures can as well be formed as a result of galaxies (NGC 3608, NGC 4552, etc.). Besides the scenario the capture of matter (gas clouds or a dwarf companion) with the capture of gas clouds with the corresponding with the orbital momentum, orthogonal to the moment of orbital moment, the studies Sofue & Wakamatsu (1994) rotation of the galactic disk. Typically, such discussions and Friedli & Benz (1993) are often quoted. The authors considered individual cases, where there are either clear of the former paper made an assumption that the inter- signs of a recent interaction, or vice versa—a galaxy is nal polar disk formed under action of the gravitational isolated and looks unperturbed by any external effects. potential of the bar on the warped gaseous disk of the The examples of an explicit relation of IPSs with the en- galaxy. This leads to the loss of angular momentum in the vironment are: NGC 2655 (Sparke et al., 2008), where the azimuthal plane and its conservation in the polar plane. polar disk of ionized gas is an internal part of a strongly Friedli & Benz (1993) have demonstrated with the aid of warped extended disk of neutral hydrogen with pro- a three-dimensional numerical model that if a part of gas nounced tidal structures in the outer regions, low-contrast in the disk of a galaxy initially rotated in the opposite ripples on the optical images of NGC 474 (Arp, 1966) and direction with respect to the rest of the disk, in the pro- NGC 6340 (Zasov et al., 2008; Chilingarian et al., 2009), cess of secular evolution of the bar the gas clouds occupy stable orbits, strongly inclined to the galactic plane. Olga 3 Sil’chenko and her colleagues have repeatedly emphasized Table 1 lists angle PAbar for all the galaxies, where we managed to find references on the presence of an internal tri- in their numerous papers the cases of simultaneous ob- axial structure. We mainly focused on the studies devoted to servations of polar disks in the inner regions of galaxies a detailed analysis of the morphology of galaxies (including and the counter-rotation of gas–stars or stars–stars in the those using the images in the near-IR, optimal for the search outer regions. This feature is detected in 9 galaxies of our of bars). We believe that such an approach is more correct than list (see the following section). The initial counter-rotating the use of a morphological classification from RC3/NED. component is most likely the result of absorption of a 4 Hereinafter the variance of the binomial distribution is dwarf companion. Note two points, however. Firstly, in specified as an error. 10 Moiseev: Inner Polar Rings and Disks

Table 2. Signs of eﬀects introduced by the environment

Name Optical HI counter-rotation (1) (2) (3) (4) Arp 220 Arp (1966) IC 1548 Sil’chenko & Afanasiev (2008) IC 1689 Reshetnikov et al. (1995) M 31 Ibata et al. (2004) Braun et al. (2009) Mrk 33 Bravo-Alfaro et al. (2004) NGC 253 Davidge (2010) Anantharamaiah & Goss (1996) NGC 474 Arp (1966) NGC 2655 Sparke et al. (2008) Sparke et al. (2008) NGC 2681 Cappellari et al. (2001) NGC 2768 Morganti et al. (2006) NGC 2787 Shostak (1987) NGC 2841 Afanasiev & Sil’chenko (1999) NGC 2855 Corsini et al. (2002) NGC 3227 Arp (1966) NGC 3368 Schneider (1989) NGC 3379 Schneider (1989) NGC 3384 Schneider (1989) NGC 3414 Morganti et al. (2006) Sil’chenko & Afanasiev (2004) NGC 3607 Oosterloo et al. (2010) NGC 3608 Oosterloo et al. (2010) Jedrzejewski & Schechter (1988) NGC 3626 Ciri et al. (1995) NGC 4111 Serra et al. (2012) NGC 4424 Cortés et al. (2006) Chung et al. (2007) NGC 4672 deGrijs & Peletier (2000) NGC 4698 Chung et al. (2009) NGC 5014 Noordermeer et al. (2005) NGC 5850 Higdon et al. (1998) NGC 6340 Zasov et al. (2008); Chilingarian et al. (2009) NGC 7217 Merrifield & KuijkenK (1994) NGC 7280 Afanasiev & Sil’chenko (2000) NGC 7468 Evstigneeva (2000) NGC 7742 Sil’chenko & Moiseev (2006) UGC 5600 Shalyapina et al. (2007) the existence of two kinematic H I components with differ- of recent interaction allows us to conclude that the effects ent laws of rotation in NGC 3414 (Morganti et al., 2006). of external environment do indeed play a major role in the An example of an integrated approach to the prob- formation of internal polar rings and disks. lem is a recent paper, devoted to the statistics of the differences in the kinematics of gas and stars in the ATLAS3D Davis et al. (2011) survey, performed with 4. SUMMARY AND CONCLUSIONS the SAURON integral-field spectrograph. However, firstly, these results only relate to the early-type galaxies, and sec- We compiled the list of galaxies with polar (or strongly ondly, as we have already noted above, not every case of inclined to the main the galactic plane) disks and rings de- “kinematic misalignment” indicates rotation at polar or tected in their inner regions. It is interesting to note that inclined orbits. more than a half (60%) of described structures have been Table 2 contains the data on the galaxies of our sample, discovered or confirmed as a result of observations at the for which there is evidence of a recent (on the scale of less SAO RAS 6-m BTA telescope using the MPFS integral- than 1–2 Gyr) interaction. Column (1) gives the name of field spectrograph, or the SCORPIO instrument in the the galaxy, (2) bears references to papers, indicating inter- scanning Fabry-Perot interferometer mode. The study of action with a companion or a merger of a satellite based on statistical properties of various parameters, characteriz- the optical photometry data, (3) gives indications of tidal ing these inner structures allows us to draw the following structures or nearby clouds, observed in HI, (4) contains conclusions. an indication of the presence of a counter-rotating component in the galactic disk. In total such evidence were 1. The stellar-gaseous polar disks and rings are found collected for 33 galaxies, making up 70 ± 7% of the entire in the central regions of galaxies of all morphological IPS sample. Such a high proportion of galaxies with signs types. Moiseev: Inner Polar Rings and Disks 11

2. The vast majority of inner polar structures have the ies from the list. Firstly, sufficiently deep optical images radius of less than 1.5 kpc. This limitation can be as- and H I distributions are not available for all the galaxies. sociated with the stabilizing role of the bulge. Secondly, it is possible that some time after the interaction 3. The innermost regions of these structures are as a with a dwarf companion, the presence of matter at polar rule located in the polar plane, while farther from the orbits of the inner part of the galaxy may turn out to be nucleus we frequently observe a warp—the orbits ap- the only evidence of this event. It is therefore important proaching the galactic plane. to study the stability of internal polar orbits in the real 4. The inner polar structures are equally common in gravitational field of galaxies, comprising the contribution galaxies with and without bars. At the same time, if of the disk, bulge, bar and halo. It would be interesting galaxy has a bar (or a triaxial bulge), this leads to to know whether our preliminary assumption of the ab- the stabilization of the polar disk so that its axis of sence of stable structures of intermediate size between the rotation coincides with the major axis of the bar. central kiloparsec region and the outer boundary of the 5. Seventy percent of galaxies with inner polar structures stellar disk is indeed true (Section 3.2). reveal signs of a recent interaction, pointing to the In principle, in some cases it is possible to form an IPS leading role of the external environment in the for- even without the interaction with the environment. For mation of these peculiar structures. example, it is demonstrated in Schinnerer et al. (2000a,b) that compact (about 100 pc) disks of molecular gas in the It is as yet difficult to estimate the frequency circumnuclear regions of NGC1068 and NGC3227 could of occurrence of circumnuclear polar disks, since our become strongly warped under the effect of the ioniza- sample is composed from very heterogeneous sources. tion cone and radiation pressure from the active nucleus. We can only conclude that since the number of such However, this scenario is clearly not suitable for most of galaxies is one and a half times greater than the galaxies with more extended polar disks and rings. number of kinematically confirmed PRGs, and they are on the average located much closer, their frac- Acknowledgements. The work was carried out with the finan- tion among the fairly bright galaxies should signifi- cial support of the ‘Dynasty ’ Foundation and the Ministry of cantly exceed the known estimates of the occurrence of Education and Science of Russian Federation (state contracts PRGs Whitmore et al. (1990); Reshetnikov et al. (2011) no. 14.740.11.0800, 16.552.11.7028, 16.518.11.7073). Working and amount to at least 3–5%. It is possible that a care- on this paper, we used the NASA/IPAC extragalactic data- ful analysis of kinematics of all the galaxies from the base (NED), managed by the Jet Propulsion Laboratory of the ATLAS3D survey, instead of the early-type galaxies alone California Institute of Technology under the contract with the (as in Davis et al. (2011)) will allow to give much more National Aeronautics and Space Administration (USA). The accurate estimates of the fraction of galaxies with IPSs. author thanks the referee, O. K. Sil’chenko for helpful com- In contrast to the external large-scale polar rings, ments, and A. A. and P. A. Smirnovs for their help in editing the text of the paper. in which the numerical modelling succeeds to reproduce the main stages of formation (see examples and references in Bournaud & Combes (2003); Combes (2006)), References such modelling has not yet been performed for the internal structures. However, the provided statistics of the Afanasiev V.L., Sil’chenko O.K., 1991, Astrofiz. Issled. IPS properties indicates that just like the classical PRGs, (Izvestiya SAO), 33, 88 the vast majority of internal polar rings and disks were Afanasiev V.L., Sil’chenko O.K., 1999, AJ, 117, 1725 formed as a result of capture of matter from the ex- Afanasiev V.L., Sil’chenko O.K., 2000, AJ, 119, 126 ternal environment of galaxies. Moreover, in a recent Afanasiev V.L., Sil’chenko O.K., 2007, A&A Trans., 26, work Sil’chenko et al. (2011) the authors, using models 311 grids from the GalMer database, have demonstrated that Aguerri J.A.L., Méndez-Abreu J., Corsini E.M., 2009, as a result of interaction of a giant S0 galaxy with a dwarf A&A, 495, 491 companion rich in gas, a ring of star formation has formed Anantharamaiah K.R., Goss W.M., 1996, ApJ, 466, L13 in the bulge region, strongly inclined to the plane of the Arp H., 1966, ApJS, 14, 1 galaxy. The companion mass ratio is 1 : 10, i.e. it corre- Bertola F., Corsini E.M., 2000, ASP Conference Series. , sponds to the minor merging events. Specific conditions of 197, 115 interaction have to be satisfied, the companion must ini- Bettoni D., Buson L.M., Galletta G., 2010, A&A, 519, 72 tially be at the orbit with the retrograde motions, while Bettoni D., Fasano C., Galletta G., 1990, AJ, 99, 1789 the planes of disks of both companions have to be nearly Bournaud F., Combes F., 2003, A&A, 401, 817 orthogonal. We hope that the development of such models Braun R., Thilker D.A., Walterbos R.A.M., Corbelli E., will allow to better conceive the processes of formation of 2009, 2009 ApJ, 695, 937 IPSs in particular galaxies and reproduce their observed Bravo-Alfaro H., Brinks E., Baker A.J., et al., 2004, AJ, parameters. 127, 264 We should not be confused by the lack of visible de Grijs R., Peletier R.F., 2000, MNRAS, 313, 800 signs of recent interaction in approximately 1/3 of galax- de Zeeuw T., Franx M., 1991, ARA&A, 29, 239 12 Moiseev: Inner Polar Rings and Disks

Cappellari M., Bertola F., Burstein D., et al., 2001, ApJ, Moiseev A.V., Smirnova K.I., Smirnova A.A., 551, 197 Reshetnikov V.P., 2011, MNRAS, 418, 244 Chilingarian I.V., Novikova A.P., Cayatte V., et al., 2009, Morganti R., deZeeuw P.T., Oosterloo T.A., et al., 2006, A&A, 504, 389 MNRAS, 371, 157 Chung A., vanGorkom J., Kenney J.D.P., Vollmer B., Morse J.A., Cecil G., Wilson A.S., Tsvetanov Z.I., 1998, 2007, ApJ, 659, L115 ApJ, 505, 159 Chung A., vanGorkom J.H., Kenney J.D.P., et al., 2009, Noordermeer E., vanderHulst J.M., Sancisi R., et al., 2005, AJ, 138, 1741 A&A, 442, 137 Ciri R., Bettoni D., Galletta G., 1995, Nature, 375, 661 Oosterloo T., Morganti R., Crocker A., et al., 2010, Coccato L., Corsini E.M., Pizzella A., et al., 2004, A&A, MNRAS, 409, 500 416, 507 Pizzella A., Bertola F., Sarzi M.C., et al., 2001, MmSAI., Coccato L., Corsini E.M., Pizzella A., Bertola F., 2005, 72, 797 A&A, 440, 107 Reshetnikov V.P., Faúndez-Abans M., deOliveira- Coccato L., Corsini E.M., Pizzella A., Bertola F., 2007, Abans M., 2011, Astronomy Letters , 37, 171 A&A, 465, 777 Reshetnikov V.P., Hagen-Thorn V.A., Yakovleva V.A., Combes F., 2006, EAS Publications Series, 20, 97 1995, A&A, 303, 398 Corsini E.M., Pizzella A., Bertola F., 2002, A&A, 382, 488 Rubin V.C., Thonnard N., Ford W.K.Jr., 1977, ApJ, 217, Corsini E.M., Pizzella A., Coccato L., Bertola F., 2003, 1L A&A, 408, 873 Sarzi M., Falcón-Barroso J., Davies R.L., et al., 2006, Cortés J.R., Kenney J.D.P., Hardy E., et al., 2006, , 131, MNRAS, 366, 1151 747 Schinnerer E., Eckart A., Tacconi L.J., 2000a, ApJ, 533, Crocker A.F., Bureau M., Young L.M., et al., et al., 2008, 826 MNRAS, 386, 1811 Schinnerer E., Eckart A., Tacconi L.J., et al., 2000b, ApJ, Davidge T.J., 2010, ApJ, 725, 1342 533, 850 Davis T.A., Alatalo K., Sarzi M., et al., 2011, MNRAS , Schneider S.E., 1989, ApJ, 343, 94 417, 882 Serra P., Oosterloo T., Morganti R., 2012, MNRAS ac- Dumas G., Mundell C.G., Emsellem E., Nagar N.M., 2007, cepted, arXiv:1111.4241 MNRAS, 379, 1249 Shalyapina L.V., Merkulova O.A., Yakovleva V.A., Eckart A., Downes D., 2001, ApJ, 551, 730 Volkov E.V., 2007, Astron. Lett., 33, 520 Evstigneeva E.A., 2000, Astrophysics, 43, 381 Shalyapina L.V., Moiseev A.V., Yakovleva V.A., 2002, Fridman A.M., Afanasiev V.L., Dodonov S.N., et al., 2005, Astron. Lett., 28, 443 A&A, 430, 67 Shalyapina L.V., Moiseev A.V., Yakovleva V.A., et al., Friedli D., Benz W., 1993, A&A, 268, 65 2004, Astron. Lett., 30, 583 Fried J.W., Illingworth G.D., 1994, AJ, 107, 992 Sharp R.G., Bland-Hawthorn J., 2010, ApJ, 711, 818 Gabbasov R.F., Repetto P., Rosado M., 2009, ApJ, 702, Shostak G.S., 1987, A&A, 175, 4 392 Sil’chenko O.K., 2000, AJ, 120, 741 Hagen-Thorn V.A., Reshetnikov V.P., 1995, A&A, 319, Sil’chenko O.K., 2002, Astronomy Letters, 28, 207 430 Sil’chenko O.K., 2005, Astronomy Letters , 31, 227 Higdon J., Buta R., Purcel G.B., 1998, AJ, 115, 80 Sil’chenko O.K., Afanasiev V.L., 2000, A&A, 364, 479 Ibata R., Chapman S., Ferguson A.M.N., 2004, MNRAS , Sil’chenko O.K., Afanasiev V.L., 2004, AJ, 127, 2641 351, 117 Sil’chenko O.K., Afanasiev V.L., 2008, Astronomy Jedrzejewski R., Schechter P.L., 1988, ApJ, 330, L87 Reports, 52, 875 Katkov I., Chilingarian I., Sil’chenko O., et al., 2011a, Sil’chenko O.K., Moiseev A.V., 2006, 131, 1336 Baltic Astronomy, 20, 453 Sil’chenko O.K., Vlasyuk V.V., Burenkov A.N., 1997, Katkov I.Yu., Moiseev A.V., Sil’chenko O.K., 2011b, ApJ A&A, 326, 941 , 740, 83 Sil’chenko O.K., Moiseev A.V., Afanasiev V.L., et al., KrajnovićD., Bacon R., et al., 2008, MNRAS, 390, 93 2003, ApJ , 591, 185 McDermid R.M., Emsellem E., Shapiro K.L., et al., Sil’chenko O.K., Moiseev A.V., Shulga A.P., 2010, AJ, 2006,MNRAS, 373, 906 140, 1462 Melchior A.-L., Combes F., 2011, A&A, 536, 52 Sil’chenko O.K., Chilingarian I.V., Sotnikova N.Ya., Merrifield M.R., KuijkenK K., 1994, ApJ, 432, 575 Afanasiev V.L., 2011, MNRAS 414, 3645 Moiseev A. V., 2008, Astrophysical Bulletin, 63, 201 Sofue Y., Wakamatsu K., 1994, AJ, 107, 1018 Moiseev A. V., 2011, EAS Publications Series, 48, 115 Sparke L.S., vanMoorsel G., Erwin P., Wehner E.M.H., Moiseev A.V., Valdés J.R., Chavushyan V.H., 2004, A&A, 2008, AJ, 135, 99 421, 433 Whitmore B.C., 1991, in Warped Disks, Inclined Rings Moiseev A., Sil’chenko O., Katkov I., 2010, AIP Around Galaxies Ed. by S. Casertano P. D., Sackett Conference Proceedings, 1240, 251 F.H., Briggs (Cambridge University Press), 60 Moiseev: Inner Polar Rings and Disks 13

Whitmore B.C., Lucas R.A., McElroyet D.B., et al., 1990, AJ, 100, 1489 Zasov A.V., Moiseev A.V., Khoperskov A.V., Sidorova E.A., 2008, Astron. Rep., 52, 79