Inner polar gaseous disks: incidence, ages, possible origin
aa r X i v : . [ a s t r o - ph . GA ] J a n **Volume Title**ASP Conference Series, Vol. **Volume Number****Author** c (cid:13) **Copyright Year** Astronomical Society of the Pacific Inner polar gaseous disks: incidence, ages, possible origin
O. K. Sil’chenko and A. V. Moiseev Sternberg Astronomical Institute of the Lomonosov Moscow State University,University av. 13, 119991 Moscow, Russia Special Astrophysical Observatory, Russian Academy of Sciences, 357147Nizhnii Arkhyz, Russia
Abstract.
We review our current knowledge about a particular case of decoupled gaskinematics – inner ionized-gas polar disks. Though more di ffi cult to be noticed, theyseem to be more numerous than their large-scale counterparts; our recent estimatesimply about 10% of early-type disk galaxies to be hosts of inner polar disks. Since inthe most cases the kinematics of the inner polar gaseous disks is decoupled from thekinematics of the outer large-scale gaseous disks and since they nested around very oldstellar nuclei, we speculate that the inner polar disks may be relic of very early eventsof external gas accretion several Gyr ago. Such view is in agreement with our newparadigm of disk galaxies evolution.
1. Introduction
Among gas subsystems with decoupled kinematics, a particular interest is inspired bypolar rings / disks. Firstly, they are beautiful, secondly, they seem to be stable over manydynamic times, and thirdly, they imply certainly accretion of external gas from highlyinclined orbits. Inner polar gaseous disks are less spectacular than large-scale polarrings; however they may be even more numerous though di ffi cult to be detected againstthe bright bulge background in early-type disk galaxies.We note that the first evidence of existence of circumnuclear gas on polar orbitsin the literature was presented by Rubin, Thonnard, & Ford (1977) in their interpreta-tion of the large line-of-sight velocity gradient along minor axis in NGC 3672. FurtherBettoni, Fasano, & Galletta (1990) have claimed inner polar gaseous disk in the south-ern ringed lenticular galaxy NGC 2217. By studying it through long-slit spectroscopy,Bettoni et al. (1990) found visible gas counter-rotation in some slit orientations (notall). Their geometrical scheme for the center of NGC 2217 demonstrated clearly thatthe ionized-gas disk had to be warped in such a way that in the very center it occupiedthe polar plane orthogonal to the bar major axis. Later we found inner polar disks in un-barred early-type spiral galaxies NGC 2841 (Sil’chenko, Vlasyuk, & Burenkov 1997)and NGC 7217 (Sil’chenko & Afanasiev 2000) by obtaining two-dimensional velocityfields for the ionized gas and for the stellar component with the integral-field unit of the6-m telescope, Multi-Pupil Fiber Spectrograph (MPFS). The outer neutral hydrogen inboth spirals is confined to their main symmetry planes and rotates normally. It was apuzzle how a small amount of polar-orbiting gas could reach the circumnuclear regionswithout colliding with the main gaseous disks.1 Sil’chenko and MoiseevNow a few dozens of inner polar gaseous disks / rings are known. Their sam-ples were presented earlier by Corsini et al. (2003) and Moiseev, Sil’chenko, & Katkov(2010); the latest statistics based on the data for 47 inner polar disks collected overliterature is published by Moiseev (2012), and here we review briefly some incidenceproperties.
2. Incidence
When we analyze all the cases with the inner gaseous disks inclined to the galacticsymmetry planes by more than 45 ◦ , we find that the inclinations of such disks tendstrongly to the strictly polar orientation: about two thirds of all such disks are inclinedby ∼ > ◦ . This is consistent with theoretical claims about stability of the strictly po-lar orientations and instability of the disks inclined by intermediate angles; the latterswould precess until they occupy the polar or co-planar orientation.The inner polar disks – as well as the large-scale ones – prefer to inhabit early-typegalaxies. However while large-scale polar rings are seen mostly around gas-poor E / S0galaxies – about a factor of 3 more often than around spirals, – and it can be explainedby them devoiding the hosts with large-scale coplanar gaseous disks (Reshetnikov et al.2011), the inner polar disks are found in Sa–Sc spiral galaxies in one third of all cases,and large-scale coplanar gaseous disks do not prevent their appearance (see the abovementioned examples of NGC 2841 and NGC 7217); even a few cases are known to befound in very late-type dwarfs. The typical size (radius) of an inner polar disk is 0.2–2kpc; the lower limit is perhaps defined by our restricted spatial resolution. If to considerinner polar disks together with the large-scale relatives, a continuous sequence in theirsizes normalized by a galaxy diameter is observed with a gap at the size ∼ . D .This bimodal distribution can be explained by di ff erent agents of stability for polarstructures: while the external structures are stabilized by the spheroidal (or even triax-ial) potential of halo, the inner disks are usually settled well within the bulge-dominatedarea (Smirnova & Moiseev 2013). In any case, the presence of embedding stabilizingpotential is important. Is it crucial that this three-dimensional potential has to be alsotriaxial as in NGC 2217 (Bettoni et al. 1990)? Moiseev (2012) presents the followingstatistics: among 40 galaxies with the inner polar disks which have the morphologicaltype S0 and later there are 17 galaxies with bars or triaxial bulges. This gives us thefraction of barred galaxies among galaxies with the inner polar disks, only 43% ± / or triaxial-bulge galaxies amongall disk galaxies, 45% (Aguerri, M´endez-Abreu, & Corsini 2009).The list of all known till 2012 inner polar disks by Moiseev (2012) cannot beused to estimate how often the phenomenon can be met: the sample of the hosts ofthe inner polar disks listed there is quite inhomogeneous. To estimate the inner polardisk incidence, we have used the data of the recent integral-field spectroscopic surveyATLAS-3D (Cappellari et al. 2011). The ATLAS-3D sample is volume-limited one andincludes 60 elliptical galaxies and 200 lenticular galaxies (if we classify NGC 2768 asS0). We have taken the raw science and calibration frames from the open Isaac NewtonGroup Archive of the Cambridge Astronomical Data Center and have calculated thestellar and ionized-gas line-of-sight velocity fields. Then the orientation of the rotationplanes for both components in every galaxy was determined by fitting a circular-rotationmodel, and the angles between the rotation planes of the stellar and gaseous componentswere calculated by using the formula (1) from Moiseev (2012). Among 200 S0 galaxiesnner polar disks 3of the ATLAS-3D volume-limited sample, we have found 8 new inner polar gaseousdisks with the inclination to the stellar rotation planes by more than 50 ◦ (taken intoaccount both solutions of the equation (1) of Moiseev (2012), because we don’t knowwhich side of the is nearest to the observer); 12 inner polar disks in the S0 galaxies ofthe ATLAS-3D sample have been already listed in Moiseev (2012). Having in total 20inner polar disks in S0 galaxies of the ATLAS-3D volume-limited sample, we concludethat nearby lenticular galaxies have inner polar disks in 10% of all cases. Our estimaterefers to the totality of S0 galaxies over all types of environments. This incidence ofthe inner polar disks in the early-type disk galaxies, 10%, exceeds greatly the frequenceof the large-scale polar rings, 0.1–0.4% (Reshetnikov et al. 2011).Figure 1 shows a nice example of the newly discovered inner polar disk in thelenticular galaxy NGC 2962 – a member of the ATLAS-3D volume-limited sample.We have observed this galaxy earlier at the Russian 6-m telescope with the integral-field spectrograph MPFS which field of view was 16 ′′ × ′′ , and in the very center,inside R = ′′ , we saw a compact, fastly rotating, nearly edge-on polar gaseous disk.But with the larger field of view of the SAURON, 41 ′′ × ′′ , we are now seeing aswitch of the gas rotation sense at R ≈ ′′ − ′′ : the galaxy possesses two nested polargaseous disks counterrotating each other (Fig. 1).
3. Origin3.1. Is the polar momentum inner or external?
This question may seem to sound strange: if a main baryonic component, stars whichare formed from the own gas of the galaxy, rotates in the galactic disk symmetry plane,how may the polar gas be of local origin? Meanwhile there are intrinsic secular evo-lution mechanisms that produce strongly inclined gaseous disks in the very center of agalaxy, and one of them had been revealed by simulations of Friedli & Benz (1993). Bytracing dynamical evolution of initially retrograde gas in the disk of an isolated barredgalaxy, Friedli & Benz (1993) have found that after about 2 Gyr of angular momentumexchange with the stellar bar the gas inside a few hundred parsec comes to a stronglyinclined plane due to vertical instabilities. Since retrograde motions of stars are alwayspresent in the barred potential (Pfenniger 1984), and since stars drop gas during theirevolution, in principle the inner polar gaseous disks may form in barred galaxies with-out outer donor contribution. Indeed, we have found several cases when the presenceof the inner polar disk in the very center is accompanied by the presence of counterro-tating gas in the more outer disk – e.g. in NGC 7280 (Afanasiev & Sil’chenko 2000;Sil’chenko 2005). But the presence of a bar is necessary. However, the statistics in theprevious Section does not show prevalence of barred galaxies among the hosts of innerpolar disks: less than a half of the hosts of inner polar disks reveal triaxiality of theirinner stellar structures. So we are now inclined to the hypothesis of the external gasaccretion as the dominant mechanism of inner polar disk formation.
To identify a source of gas accretion, we must estimate first of all typical amounts ofgas populating polar orbits. Here a lot of diversity is observed. In some cases the innerpolar ionized-gas disks have their extension into the very outer parts of galaxies whenthey are observed at the 21cm line of the neutral hydrogen – these are the cases, e.g., Sil’chenko and Moiseev
Figure 1. The line-of-sight velocity fields for the stellar and ionized-gas compo-nents in the lenticular galaxy NGC 2962: the upper row presents the data from theMPFS of the Russian 6-m telescope, the bottom raw – our reduction of the SAURONdata. of NGC 3414 (with the inner polar disk found by Sil’chenko & Afanasiev (2004)) or ofNGC 7280 or of UGC 9519 mapped in the neutral hydrogen line by Serra et al. (2012).In the prototype of large-scale polar ring galaxies, NGC 2685, the inner ionized gasis also polar (Sil’chenko 1998). In these cases the total mass of the polar gas can beas large as 10 − solar masses, and the M (HI) / L K ratios resemble those of spiralgalaxies (Serra et al. 2012). In the volume-limited S0-galaxy sample from ATLAS-3D(Cappellari et al. 2011) about one third of all galaxies with the inner polar ionized-gasdisks have polar neutral-hydrogen outer extension. However many galaxies have in-ner polar ionized-gas component and outer coplanar neutral-hydrogen disk; and theyare sometimes also rather gas-rich but their main gaseous components are confined tothe galaxy symmetry planes. Among lenticular galaxies, we can mention NGC 2962where Grossi et al. (2009) have found 1 . × M ⊙ of neutral hydrogen in a disk copla-nar to the stellar one but extending much farther from the center. And certainly evennner polar disks 5 Figure 2. The line-of-sight velocity fields for the stellar (left) and ionized-gascomponents (right) for the spiral galaxy NGC 5850 from our reduction of theSAURON data. more such cases can be found among spiral galaxies with the inner ionized-gas polardisks. An inner ionized-gas polar disk was found in a barred spiral, SB(r)b, galaxyNGC 5850 by Moiseev, Vald´es, & Chavushyan (2004); the stellar and gaseous rota-tions were compared over the 16 ′′ × ′′ field of view of the 6-m telescope IFU MPFS.Now we have calculated larger stellar and gaseous velocity fields by using the archivalSAURON data (Fig. 2). One can immediately see from Fig. 2 that the sense of thegas rotation changes at the radius of 7 ′′ –10 ′′ (1.3–1.8 kpc); the more outer ionized gasrotates together with the stars. And the same orientation of the rotation plane is demon-strated by all the 2 × solar masses of neutral hydrogen measured in NGC 5850 byHigdon, Buta, & Purcell (1998). The same patterns of stellar and ionized gas circum-nuclear kinematics were also presented recently in the paper by Bremer et al. (2013),which is based on VLT observations with the VIMOS IFU. The better spatial reso-lution (comparing with the early observations by Moiseev et al. 2004) has allowed tocalculate precisely the kinematic orientation parameters in the inner disk velocity field.Bremer et al. (2013) claimed that the angle between the inner and outer disks planes isonly 24 ◦ , however the equation (1) from Moiseev (2012) gives also the second solution– 54 ◦ , that corresponds to the case of strongly inclined inner gaseous disk. An interesting case of a spiral galaxy with the inner polar ionized-gas disk having theradius of only 350 pc (Sil’chenko & Afanasiev 2000) is represented by an isolated Sabgalaxy NGC 7217; here we show the recent HST image of the central part of the galaxy(Fig. 3) where the inner ionized-gas polar disk can be seen ‘by eye’ in the narrowphotometric band centered onto the emission lines H α + [NII]. Its neutral hydrogen disk,0 . × M ⊙ , extending to R ≈ Figure 3. The narrow-band emission-line (F658N minus
F814W) image of thecentral part of NGC 7217 obtained with the camera ACS / HST. The dashed lineshows the line of nodes of the galactic stellar disk. the visible gas density is below the gravitational stability threshold (Noordermeer et al.2005). Recently we have studied the origin of the complex structure of NGC 7217in detail (Sil’chenko et al. 2011), and here we discuss this galaxy as a pure key pointrevealing possible formation mechanisms of the inner polar disks.Photometric structure of NGC 7217 can be described as three-tiered: we (Sil’chenko & Afanasiev2000) have separated three exponential segments in its surface-brightness radial pro-file. The innermost segment seen only at R < ′′ (0.8 kpc) may be a pseudobulge;then other two segments represent an antitruncated disk. Our deep long-slit spectro-scopic observations (Sil’chenko et al. 2011) having allowed to measure stellar rotationand line-of-sight velocity dispersion (close to a vertical velocity dispersion because thegalaxy is seen almost face-on) as well as the properties of the stellar populations, haverevealed prominent di ff erences in all respects between two exponential parts of the stel-lar disk. Firstly, the inner part of the disk is substantially thinner than the outer part,and secondly, the mean age of the stellar population in the inner disk is 5 Gyr while thestellar ages in the outer disk, even beyond the starforming ring, is very young, less than2 Gyr. The galaxy being an early-type spiral without a bar, possesses meantime threerings of current star formation (Verdes-Montenegro et al. 1995). Interestingly, the ageof the nuclear stellar population, inside the circumnuclear starforming ring, is very old– older than 10 Gyr. Obviously, despite violent processes of gas radial re-distributionand external gas accretion betrayed by the inner polar disk presence, the gas has neverreached the very center of NGC 7217 for the last 10 Gyr.Having in hands the detailed structure of NGC 7217 and evolutionary sequenceof building elements of this structure, we have tried to fit observational properties ofNGC 7217 with the models provided by on-line service GalMer (Chilingarian et al.2010). We have found that only at least two independent gas-rich minor-merger eventscan provide a full list of properties: the inner polar disk is formed by an accretion ofa gas-rich dwarf from an inclined retrograde orbit, and the outer flaring ringed star-nner polar disks 7forming disk is shaped by merging a prograde-orbiting satellite. The necessity of twominor mergers is due to the fact that minor merging from a retrograde orbit gives aninclined inner gaseous disk but does not thicken the large-scale stellar disk. The latterfeature requires minor merging from a prograde orbit. Since the star formation burst inthe outer disk of NGC 7217 is very young, we conclude that the minor merging froma retrograde orbit was the first event, and minor merging from a prograde orbit was thelast, quite recent one.
4. Ages
The large-scale outer polar rings may be stable in the polar state over a few Gyrs ac-cording to theoretical estimations (e.g. Steiman-Cameron & Durisen 1982) as well asto numerical simulations (Snaith et al. 2012). Stability of their circumnuclear counter-parts is still an open question. However some observational evidences in favour of theirvery long living times also exist: just among lenticular galaxies with the inner polardisks we found very old stellar nuclei, T >
10 Gyr (Sil’chenko & Afanasiev 2004),while over the full sample of nearby lenticular galaxies the typical ages of the stellarnuclei are 2–5 Gyr (Sil’chenko 2006, 2008).The whole evolution of disk galaxies is governed by the regime of external gasaccretion. Recently we (Sil’chenko et al. 2012) have proposed a scenario according towhich all disk galaxies were formed around z ≈ z <
1, most of them started smooth gas accretion and, after having formed thindynamically cold stellar disks, transformed into spirals. In the frame of this scenario, anatural epoch of forming inner polar gaseous disk is very early stages of the accretionera. If the first accretion event was from a highly inclined orbit, an inner polar long-living gaseous disk would form before the main gas accretion in the galactic symmetryplane proceeded. It is the way to obtain a stable system with mutually orthogonal nestedgaseous disks; and then inner polar disks would be relics of very early events of externalgas accretion.
Acknowledgments.
We thank Enrica Iodice and the organizers for the interestingand inspiring conference and for the invitation to present this review. A.M. is grate-ful to the non-profit ‘Dynasty’ Foundation and to the RFBR grant 13-02-00416. Thiscontribution makes use of data obtained from the Isaac Newton Group Archive whichis maintained as part of the CASU Astronomical Data Centre at the Institute of As-tronomy, Cambridge. The ACS images of NGC 7217 were obtained from the HubbleLegacy Archive, which is a collaboration between the Space Telescope Science Institute(STScI / NASA), the Space Telescope European Coordinating Facility (ST-ECF / ESA)and the Canadian Astronomy Data Centre (CADC / NRC / CSA).
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