NGC 7468 a galaxy with an inner polar disk

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We present our spectroscopic observations of the galaxy NGC 7468 performed at the 6-m Special Astrophysical Observatory telescope using the UAGS long-slit spectrograph, the multipupil fiber spectrograph MPFS, and the scanning Fabry-Perot interferometer (IF

a r X i v :a s t r o -p h /0411457v 1 16 N o v 2004Astronomy Letters,vol.30,No.9,2004,pp.583-592

February 2,2008

Translated from Pis’ma v Astronomicheskii Zhurnal,vol.30,No.9,2004,pp.643-652NGC 7468:a galaxy with an inner polar disk

L.V.Shalyapina 1,A.V.Moiseev 2,V.A.Yakovleva 1,V.A.Hagen-Thorn 1,and O.Yu.Barsunova 11

Astronomical Institute,St.Petersburg State University,Universitetsky pr.28,Petrodvorets,198504Russia 2

Special Astrophysical Observatory,Russian Academy of Sciences,Nizhnii Arkhyz,Karachai-Cherkessian Republic,357147Russia Received January 22,2004Abstract.We present our spectroscopic observations of the galaxy NGC 7468performed at the 6-m Special Astrophysical Observatory telescope using the UAGS long-slit spectrograph,the multipupil ?ber spectrograph MPFS,and the scanning Fabry-Perot interferometer (IFP).We found no signi?cant deviations from the circular rotation of the galactic disk in the velocity ?eld in the regions of brightness excess along the major axis of the galaxy (the putative polar ring).Thus,these features are either tidal structures or weakly developed spiral arms.However,we detected a gaseous disk at the center of the galaxy whose rotation plane is almost perpendicular to the plane of the galactic disk.The central collision of NGC 7468with a gas-rich dwarf galaxy and their subsequent merging seem to be responsible for the formation of this disk.INTRODUCTION Whitmore et al.(1990)included NGC 7468(Mrk 314)in their catalog of polar-ring galaxies (PRGs),candidate PRGs,and related objects as a probable candidate (C-69).The direct images (Fig.1)of this galaxy show an extended low surface brightness base and a bright nuclear region re-solvable into several individual condensations.Lobes are observed along the major axis on the galaxy’s southern and northern sides,which may suggest the existence of a ring;this was the reason why NGC 7468was included in the above catalog.The southern protrusion transforms into a faint bar that ends with a brightening whose dis-tance from the galaxy’s center in the plane of the sky is

～45′′,or ～7kpc.(For the line-of-sight velocity we found,

V gal =2220km s ?1,and H 0=65km s ?1Mpc ?1,the dis-

tance to the galaxy is 34Mpc,which yields a scale of 0.16

kpc in 1′′).

NGC 7468was initially classi?ed as a peculiar ellipti-

cal galaxy (RC3,LEDA).However,according to a detailed

photometric study by Evstigneeva (2000),it should be

considered to be a late-type spiral or an irregular galaxy.

Indeed,the galaxy is rich in gas,as follows from neu-

tral and molecular hydrogen data (Taylor et al.1993,

1994;Richter et al.1994;Wiklind et al.1995).The ra-

tios M HI /L B =0.621and M H 2/L B =0.06(M H 2/M HI =

0.11)derived for NGC 7468are characteristic of galaxies of

late morphological types (Young et al.1989).The galaxy

is an infrared source (L IR /L B ～0.6),suggesting the pres-

2Shalyapina et al.:NGC7468:a galaxy with an inner polar disk

Fig.1.Image of the galaxy NGC7468obtained with the

6m telescope through a?lter centered atλ6622?A;the

UAGS slit locations and the MPFS?eld during observa-

tions are shown.

interaction is believed to be the most probable cause of

the polar-ring formation and because she failed to?nd

another suitable candidate that would con?rm the inter-

action.Meanwhile,Taylor et al.(1993)pointed out that

there is a faint galaxy4.′6(45.5kpc)north of NGC7468

that the authors believe to be a possible companion of

NGC7468.The currently available information is clearly

insu?cient to determine whether NGC7468belongs to

PRGs.The velocity?eld of the galaxy should be stud-

ied with a high spatial resolution.This paper presents the

results of such a study based on optical spectroscopy.

OBSER V ATIONS AND DATA REDUCTION

We performed our spectroscopic observations of the

galaxy NGC7468at the prime focus of the6-m Special

Astrophysical Observatory(SAO)telescope with the long-

slit spectrograph(UAGS),the multipupil?ber spectro-

graph(MPFS)(Afanasiev et al.2001),and the scanning

Fabry-Perot interferometer(IFP)(see Moiseev(2002)and

the SAO WWW-site1).The table1gives a log of obser-

vations.

The reduction technique that we used was described

previously(Shalyapina et al.(2004).

The long-slit spectra were taken near the Hαline(see

the table1).The Hαand[SII]λλ6716,6730?A emission

lines proved to be brightest in the spectra;in addition,

the[NII]λλ6548,6583?A,HeIλ6678?A lines were observed.

The line-of-sight velocity curves were constructed from all

lines.The accuracy of these measurements is±10km s?1.

The central region of the galaxy was observed with

the MPFS in the green spectral range.This range includes

metal absorption lines(MgIλ5175?A,FeIλ5229?A,FeI+CaI

λ5270?A,etc.)and the Hβand[OIII]λλ4959,5007?A emis-

Shalyapina et al.:NGC7468:a galaxy with an inner polar disk3 Table1.Log of observations of NGC7468

Instrument,date Exposure time,Field Scale,Spectral Seeing Spectral P.A.

date s′′′′/px resolution,?A′′region,?A?eld

MPFS180016x151 4.5 2.04600-6000center

08.12.01

4Shalyapina et al.:NGC 7468:a galaxy with an inner polar

disk Fig.2.NGC 7468:the UAGS line-of-sight velocity curves.

The three bright knots that correspond to condensa-

tions “a”,“b”,and “c”noted by Petrosyan et al.(1978)

are clearly seen in the central part of the image in the

green continuum (Fig.4c).Condensation “c”is brightest.

In the emission lines (Fig.4),condensation “c”is poorly

seen,while condensation “a”is most prominent.This char-

acteristic feature was noted by Petrosyan (1981).THE VELOCITY FIELDS OF THE IONIZED GAS AND STARS Figure 5a shows the large-scale velocity ?eld for the emitting gas,as constructed from the IFP data,suggesting the rotation of the galaxy’s gaseous disk around its minor axis.However,a feature indicating the rotation of the gas around the galaxy’s major axis is clearly seen in the central part of the velocity ?eld.Thus,the IFP data con?rm the

presence of two kinematic subsystems of gas in this galaxy.If the gas is assumed to rotate in circular orbits,then the method of “tilted-rings”can be used (Begeman 1989;Moiseev and Mustsevoi 2000)to analyze the velocity ?eld.The method allows us to determine the positions of the kinematic center and the kinematic axis (the direction of the maximum velocity gradient),to estimate the galaxy’s inclination,and to construct the rotation curve.As we see

from Fig.6b,the location of the kinematic axis changes

with increasing radius:P A dyn ≈120?in the nuclear re-

gion (R ≤6′′)and P A dyn ≈180?on the periphery.This

con?rms the existence of two kinematic subsystems of gas.

The locations of the dynamic axes of the subsystems are

close to those estimated from long-slit spectra.The sub-

system associated with the central region is apparently the

inner disk (ring)rotating around the galaxy’s major axis.

The dynamic centers of the subsystems coincide,but are

displaced by approximately 3′′to the W of condensation

“a”.The heliocentric line-of-sight velocity of the galaxy

Shalyapina et al.:NGC 7468:a galaxy with an inner polar disk

Fig.3.Radial metallicity

distributions.

Fig.4.IFP data:(a)images in the continuum near H α;(b)an image in H α.MPFS data:the brightness distribution (c)in the continuum (λ5400??5500?A )and (d)in the [OIII]λ5007line.The gray scale corresponds to intensities in arbitrary units for panels (a)and (b)and to intensities in erg s ?1cm 2arcsec ?2?A ?1for panels (c)and (d).The pluses mark the position of the brightness maximum in the red continuum.

is V sys =2070km s ?1or,after being corrected for the ro-

tation of our Galaxy,V gal =2220km s ?1.The observed

velocity ?eld is best described by the model of circular

rotation at an inclination of the galactic disk i disk ≈45?

(which is equal to the estimate obtained from the galaxy’s

axial ratio)and that of the inner disk/ring,i ring ≈60′′.

Here,two values of the angle between the galactic disk

and the plane of the inner disk are possible:?i ≈50?and

?i ≈86?.In the ?rst case,the orbits of the gas are unsta-

ble,and,therefore,noncircular motions must be observed in the velocity ?eld.However,such motions have not been found,so the second value (86?)of the angle between the galactic disk and the plane of the inner disk should be taken.The observed rotation curve of the galaxy constructed from our data (crosses)and from the HI data of Taylor et al.(1994)(triangles)is shown in Fig.6a.At R ≤3kpc,the triangle lies well below the crosses.This is most likely because the 21-cm observations have a low spatial reso-

lution (～3kpc ).At R ≥3kpc,the discrepancy between

6Shalyapina et al.:NGC 7468:a galaxy with an inner polar

disk

Fig.5.(a)Large-scale velocity ?eld,as inferred from IFP;(b)and (c)the velocity ?eld of the stellar and gaseous components for the central region obtained from MPFS:the pluses indicate the positions of condensations “a”and “c”and the cross indicates the position of the dynamic center.

the optical and H I data decreases and does not exceed

10km s ?1.The rotation curve re?ects the motions in the

inner disk/ring in the region 0≤R ≤0.8kpc and the

rotation of the galactic disk farther out.There is no sig-

ni?cant jump in velocity when passing from one kinematic

system to the other,which most likely suggests that the

galaxy’s potential is spherically symmetric on these scales.

We analyzed the observed rotation curve using the

models described by Monnet and Simien (1977).At dis-

tances of 0?3kpc,it is well represented by an exponen-

tial disk model (the dashed line in Fig.6a)with the scale

factor h =0.9kpc.This value is close to the estimates

given by Evstigneeva (2000)and Cairos et al.(2001a),

if these are recalculated to our assumed distance to the

galaxy.Note that the photometric pro?le in the works by

Evstigneeva (2000)was represented by two components:

bulge+disk.However,our rotation curve does not con?rm

the presence of a bulge.If the galaxy actually had a bulge,

then the velocity gradient in the central region would be

much larger than that in our rotation curve.Therefore,

we assume that the increase in brightness compared to

the exponential law observed in the central part of the

pro?les could be due to the existence of regions of active

star formation.The HI data show that the rotation curve

?attens out somewhere at 8?10kpc.Although there may

be a small systematic shift between the optical and radio data,in general,the run of the rotation curve at large dis-tances from the center (R ≥3kpc)can be explained by assuming the presence of an extended spherical isother-mal halo.Its parameters were found to be the following:r c =10kpc,ρ0=0.002M ⊙kpc ?3.The combined theoret-ical rotation curve is indicated in Fig.6a by the solid line.The overall shape of the rotation curve is characteristic of late-type galaxies (Amram and Garrido 2002).The total mass of the galaxy,including the halo,is 2·1010M ⊙,and the mass of the disk/ring assuming its radius to be 0.75kpc is ～4·108M ⊙,or 2%of the galaxy’s mass.The velocity ?elds for the stellar and gaseous compo-nents in the central part of the galaxy NGC 7468are dif-ferent (Figs.5b and 5c).As we see from the ?gure,the ion-ized gas in this region rotates around the major axis,while the stellar velocity ?eld is highly irregular.The velocity range does not exceed 40km s ?1.The observed velocity variations can be explained by the presence of noncircular motions.It should also be noted that the contrast of the absorption features from the old stellar population is low for galaxies of late morphological types;therefore,the ac-curacy of the line-of-sight velocity measurements is lower than that of the usual MPFS observations of earlier-type galaxies,being 10?20km s ?1.We attempted to describe the stellar velocity ?eld using the method of “tilted-rings”with the ?xed parameters P A and i corresponding to those

Shalyapina et al.:NGC7468:a galaxy with an inner polar disk

Fig.6.(a)Rotation curves.1:the resulting curve;2:for the disk component;3:for the halo;4:our data;5:the data from Richter et al.(1990).(b)The change in the location of the dynamic axis with radius.

of the galaxy’s outer disk.The model describes the obser-vations satisfactorily,except for the region near condensa-tion“a”,where large residual velocities(～50km s?1)are observed.We found no rotation around the major axis in the stellar velocity?eld.Therefore,we conclude that the stars and the gas in the galaxy’s central region belong to di?erent kinematic systems.

DISCUSSION

As was noted in the Introduction,the putative polar ring manifests itself in the galaxy’s images as lobes on its northern and southern sides.However,analysis of the ve-locity?eld that we constructed from the IFP data and the velocity?eld derived from neutral hydrogen(Taylor et al. 1994)shows no peculiar features near these lobes.The gas in these regions is involved in the rotation of the galactic disk around the minor axis.Here,there is no kinematically decoupled subsystem characteristic of polar rings,which forces us to reject the assumption that NGC7468is a galaxy with an outer polar ring.The nature of the lobes and the brightening in the south is not quite clear.If we assume,following Taylor et al.(1993),that the galaxy to the north of NGC7468is spatially close to it(according to this paper,the di?erence between their line-of-sight veloc-ities is73km s?1),then these may be considered as tidal structures,as may be evidenced by the deviations of the observed velocities from the circular ones in the region of the brightening(by40km s?1)and in the region of the southern lobe(by approximately20km s?1).On the other hand,these may be weakly developed spiral arms.Their appearance and the color bluer than that for the remain-ing parts of the galaxy(Cairos et al.2001b)can serve as evidence for this interpretation.

At the same time,we found a kinematically decoupled gaseous subsystem,a rotating inner disk,in the central part of NGC7468(～1.5kpc in diameter).The angle between the galactic disk and the plane of the inner disk is 86?.Thus,we can assert that the galaxy has an inner polar disk(possibly,a ring)rather than a bar whose presence was used to explain some of the features in the velocity ?eld constructed from the HI data(Taylor et al.1994).

According to current views(Bournaud and Combes 2003),a polar structure in a galaxy can be formed either through accretion from a neighboring gas-rich galaxy dur-ing their close interaction or through the direct collision of galaxies,which can lead to the complete absorption of the less massive participant of the collision by the more massive galaxy.At the observed orientation of the inner disk,the position of the northern galaxy does not permit us to consider it a donor galaxy.Thus,we conclude that NGC7468collided with a low-mass gas-rich galaxy that was absorbed.As a result,a metal-poor polar gaseous disk was formed,and the observed induced star formation be-gan in the central region of the galaxy(in a ring with a radius of～1.2kpc).The individual condensations in this ring are spaced0.5?1kpc apart.Such distances are ob-served between neighboring gas-dust clouds.The sizes of the condensations do not exceed500pc,in agreement with the sizes of stellar associations and stellar complexes.The intensity ratio of the emission lines suggests the photoion-ization mechanism of the emission in these regions.

Note also that the di?erence between the relative brightnesses of the condensations in the continuum and in the emission lines may be due to the age di?erence be-tween these star-forming regions.Condensation“c”,being relatively brighter in the continuum,may be an older stel-lar complex at a stage when the surrounding gas had al-ready been partially swept up to large distances by strong light pressure light and/or supernova explosions.

CONCLUSIONS

Below,we summarize our results.

(1)Based on the long-slit spectra near Hα,we found a kinematically decoupled gaseous subsystem in the central region rotating around the galaxy’s major axis.

(2)Our analysis of the line-of-sight velocity elds ob-tained using two-dimensional spectroscopy con?rmed the existence of two kinematic subsystems of ionized gas:one of these is the gaseous disk of the galaxy,and the other is an inner disk1.5kpc in size.The angle between the planes of the disks is86?;i.e.,the inner disk is polar.In

8Shalyapina et al.:NGC7468:a galaxy with an inner polar disk

the central region of the galaxy,the stars and the ionized gas belong to di?erent subsystems.(

3)The arc-shaped lobes on the northern and southern sides of the galaxy are not kinematically decoupled to an extent that the presence of the polar ring suggested by Whitmore et al.(1990)be con?rmed.They are probably either tidal structures or weakly developed spiral arms.

(4)The intensity ratio of the forbidden and permitted lines con?rms that the emission in HII regions results from photoionization.The derived metallicity is lower than the solar value(～0.3Z⊙).

(5)The detected inner disk allows the galaxy NGC7468to be classi?ed as belonging to PRGs.The cen-tral collision with a dwarf galaxy and its capture could be responsible for the formation of the inner polar disk. The velocity?eld we derived shows that there is no reason to believe that the arc-shaped lobes on the northern and southern sides of the galaxy,which belong to the putative polar ring(Whitmore et al.1990),are a kinematically de-coupled ring.These are probably either tidal structures or weakly developed spiral arms.

Acknowledgements.We are grateful to the Large Telescopes Program Committee(LTPC)for allocating observational time on the6-m telescope.This study was supported by the Russian Foundation for Basic Research(project nos.02-02-16033and 03-02-06766)and the Russian Ministry of Education(project no.E02-11.0-5).

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