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Astron. Astrophys. 360, 439-446 (2000)

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5. Discussion and conclusions

In the previous sections we have pointed out that NGC 4672 is an early-type disk galaxy (most probably an Sa spiral) with a prominent bulge sticking out from the plane of the disk rather than a polar-ring S0 galaxy. In addition to this remarkable orthogonal geometrical decoupling between bulge and disk, the stellar velocity field of NGC 4672 is characterized by a central zero-velocity plateau along the galaxy major axis and by a steep velocity gradient along the minor axis. This means that in the inner regions of the bulge we are in the presence of a stellar core, which is rotating perpendicularly to the disk. We conclude that NGC 4672 has the same morphological (i.e. , a bulge elongated perpendicularly to the disk) and kinematical (i.e. , a stellar velocity gradient along the bulge major axis) properties as those observed recently by Bertola et al. (1999) in the Sa spiral NGC 4698.

As pointed out by Bertola & Corsini (2000), the stellar rotation curve measured along the bulge major axis (corresponding to the disk minor axis) and the bulge minor axis of NGC 4672 and NGC 4698 both show the same radial trends (characterized by an inner velocity gradient and a zero-velocity plateau) which have been observed in the polar-ring elliptical AM 2020-504 along its major axis (corresponding to the ring minor axis) and minor axis, respectively.

AM 2020-504 is considered the prototype of ellipticals with a polar ring. It is constituted by two distinct structures: a mostly gaseous ring and a spheroidal E4 stellar component with their major axes perpendicular each other. It has been extensively studied by Whitmore et al. (1987, 1990), who first revealed the presence of a velocity gradient along the major axis of the spheroidal component, and by Arnaboldi et al. (1993a,b), who showed that the inner velocity gradient of the stars is followed at larger radii by a velocity decline to an almost constant zero value. Recent observations presented in Bertola & Corsini (2000) are consistent with these findings but also reveal that along the ring major axis the stars exhibit a zero-velocity plateau followed by a rising velocity curve, as in the cases of NGC 4698 and NGC 4672. Moreover no rotation of the ionized gas is observed along the spheroid major axis, contrary to the prediction of the warped model for the gaseous component by Arnaboldi et al. (1993a).

In spite of their morphological differences the early-type disk galaxy NGC 4672, the Sa spiral NGC 4698 and the polar-ring elliptical AM 2020-504 share two common characteristics:

  • (i) The major axis of the disk (or ring) component forms an angle of [FORMULA] with the major axis of the bulge (or elliptical) component, at least at a visual inspection. This orthogonal geometrical decoupling is uncommon among spiral galaxies (Bertola et al. 1991).

  • (ii) The stellar kinematics is characterized by a central zero-velocity plateau followed by a rising velocity curve along the disk (or ring) major axis, and by a steep velocity gradient followed by an almost zero constant velocity along the disk (or ring) minor axis. This indicates the presence of a kinematically isolated core, which is rotating perpendicularly with respect to the disk (or ring) component.

At this point we ask ourselves whether these galaxies also share similar formation processes. For NGC 4698 Bertola et al. (1999) suggested two alternative scenarios to explain the geometrical and kinematical orthogonal decoupling between the inner region of the bulge and the disk depending on the adopted (parametric or non-parametric) photometric decomposition.

If the parametric decomposition is adopted for NGC 4698, its surface-brightness distribution is assumed to be the sum of an [FORMULA] bulge and an exponential thin disk. In this case the entire bulge structure is found to be elongated perpendicularly to the major axis of the disk. The fact that the velocity field of the bulge is characterized by zero velocity along the disk major axis and by a velocity gradient along the disk minor axis suggests that the intrinsic angular momenta of bulge and disk are perpendicular. This orthogonal decoupling of bulge and disk in NGC 4698 has been interpreted as the result of the formation of the disk component at a later stage, due to the acquisition of material by a triaxial pre-existing spheroid on the principal plane perpendicular to its major axis. For NGC 4698, according to the results of the photometric decomposition the peculiar velocity curve observed along the minor axis is representative of the rotation of its bulge. This is also true for NGC 4672, since the bulge contribution dominates the galaxy light along the minor axis in the region where we measured the stellar kinematics. However such a kind of rotation curve (which is quickly rising to a maximum value and then fall to about zero) is unrealistic for bulges, which are generally believed to be isotropic rotators (e.g., Kormendy & Illingworth 1982; Jarvis & Freeman 1985a,b).

If, on the other hand, the non-parametric photometric decomposition is adopted for NGC 4698, the surface-brightness distributions of bulge and disk do not depend on a priori fitting laws but are assumed to have elliptical isophotes of constant flattening. In this case the bulge of NGC 4698 should be round with only the inner portion elongated perpendicularly with respect to the disk major axis. For this reason the presence of an orthogonal decoupled core has been invoked by Bertola et al. (1999) to explain the observed photometric and kinematic properties. This stellar core should be photometrically decoupled from the disk and kinematically decoupled with respect to both bulge and disk. With this in mind, it should be noticed that also the velocity fields of NGC 4672 and AM 2020-504 show the same discontinuity between the rotation of the inner and the outer regions as observed in NGC 4698. In elliptical galaxies such a discontinuity in the velocity field is the signature of the presence of a kinematically-decoupled core. Several formation scenarios have been proposed to explain the origin of isolated cores, but the decoupling between the angular momentum of a core and that of the host galaxy is generally interpreted as the result of a second event (see Bertola & Corsini 1999 for a review).

In NGC 4672 and AM 2020-504 the orthogonal geometrical decoupling between bulge and the disk (or ring) components is directly visible due to the high inclination of these galaxies. This geometrical decoupling implies that such objects did not form in a single event, as in the case of polar ring galaxies which are believed to be the result of an accretion event (Steiman-Cameron & Durisen 1982; Schweizer et al. 1983; Sparke 1986) or a merger (Bekki 1998). However it is hard to imagine that in NGC 4672 and AM 2020-504 the orthogonal disk component and the isolated core are the result of two distinct and unrelated accretion phenomena which occurred in different epochs.

Arnaboldi et al. (1993a,b) proposed a mechanism for the joint formation of both the kinematically-decoupled core and the polar ring of AM 2020-504. Following Sparke (1986), they considered the acquisition at a given angle of a large amount of external material by an oblate elliptical galaxy. Self-gravity acts on the gaseous disk, which is initially forming at constant inclination, to transfer angular momentum about the galactic pole from the outer material to that inside. In this way the outer gas moves towards nearly polar orbits forming the observed polar ring, while the inner gas is pushed away from the pole settling onto the equatorial plane of the host galaxy, and subsequently turning into the stars, which now constitute the isolated core. It is interesting to observe that for [FORMULA] the kinematics of AM 2020-504 shows a non-rotating stellar body even if the galaxy has been classified as an E4 elliptical and modeled as an oblate spheroid (Arnaboldi et al. 1993a). Anyway there are a number of boxy E4 galaxies which show, slow or even no rotation of the stellar body, like NGC 1600 (Bender et al. 1994), NGC 5322 (Scorza & Bender 1995), and NGC 5576 (Bender et al. 1994). In particular, the case of NGC 1600 has been discussed recently by Matthias & Gerhard (1999), who concluded that the kinematics of the inner parts is consistent with a mostly radially anisotropic, axisymmetric three-integral distribution function.

This scenario has the attractive property that it explains both the orthogonal geometrical and kinematical decoupling between the spheroid and the ring components, as well as the kinematical isolation of the core observed in AM 2020-504, as the byproduct of an unique second event (represented by the skewed accretion of large amount of external material around an oblate spheroid) and its following dynamical evolution. Bertola & Corsini (2000) suggested that the same acquisition mechanism produced also the kinematically-decoupled cores in the center of the two disk galaxies NGC 4698 and NGC 4672. Therefore NGC 4672 and NGC 4698 are interpreted as the result of the disk accretion in a polar plane of a pre-existing oblate spheroid. Neither a velocity gradient along the disk minor axis nor a geometrical decoupling are expected if the acquisition process produces a disk settled on the equatorial plane of the spheroidal component.

The ionized-gas component displays a different behavior in the inner regions of NGC 4698 with respect to NGC 4672 and AM 2020-504. In NGC 4698 the gas rotation closely matches that of the stars (see Fig. 2 in Bertola & Corsini 2000). The observed kinematics suggests that in the center of NGC 4698 we are facing the ionized gas associated with the kinematically decoupled stellar component giving rise to the velocity gradient measured along the disk minor axis. On the contrary, no significant gas rotation is detected along the disk minor axis of NGC 4672 (Fig. 6) and AM 2020-504 (see Fig. 2 in Bertola & Corsini 2000), indicating that the inner ionized gas resides in the galaxy disk and is not associated with the isolated core. This can be explained if in NGC 4672 and AM 2020-504 the ionized gas settled onto the symmetry plane of the galaxy entirely transformed into stars leaving behind a nearly edge-on gas-depleted region, while in NGC 4698 this transformation is still in progress.

To identify candidates that may show the same features as NGC 4672 and NGC 4698 one should look for disk galaxies with an almost round bulge, whose isophotes appear to be elongated perpendicularly to the galaxy major axis after the subtraction of the disk surface brightness. To a first visual inspection of the photographic plates reproduced on the Carnegie Atlas of Galaxies (Sandage & Bedke 1994) the cases of NGC 2911 (S03(2) or S0 pec, Plate 49), NGC 2968 (amorphous or S03 pec, Plate 49), NGC 4448 (Sa (late), Plate 69) and NGC 4933 (S03 pec (tides), Plate 49) appear worthy of further investigation.

Multi-band photometry could reveal color differences between the stellar populations of the spheroid and possibly of the isolated core (as done by Carollo et al. 1997a,b for kinematically decoupled cores in ellipticals), allowing an estimate of their relative age if combined with high-resolution spectroscopy. Moreover, the latter will give us a detailed overview of the kinematics in the center of these galaxies and the analysis of the line profiles could unveil the dynamical nature of these cores (e.g., the case of the isolated core in the gE galaxy NGC 4365 studied by Surma & Bender 1995).

Several mechanisms have been proposed for bulge formation (see Wyse et al. 1997 for a review) in which the bulge formed before (e.g. by hierarchical clustering merging), at the same time (e.g. in a monolithic collapse) or after the disk (e.g. as a results of a secular evolution of the disk structure). Furthermore, there is evidence that disks and bulges can experience accretion events via infall of external material or satellite galaxies (see Barnes & Hernquist 1992 for a review of the different processes). Up to now none of these pictures is able to reproduce all the observed properties of disk galaxies along the Hubble sequence, leading to the idea that several of the mechanisms outlined in these different scenarios could have played a role in forming a galaxy (Bouwens et al. 1999).

The early-type disk galaxies NGC 4672 and NGC 4698 represent striking examples of galaxies which drastically changed morphology during their history and for which we suggest an inside-out formation process with the disk component accreted around a previously formed spheroid, according to the hierarchical clustering merging paradigm (Baugh et al. 1996; Kauffman 1996).

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000