A&A 416, 507-514 (2004)
DOI: 10.1051/0004-6361:20034475

Minor-axis velocity gradients in disk galaxies^,

L. Coccato¹ - E. M. Corsini¹ - A. Pizzella¹ - L. Morelli^1,2 - J. G. Funes S. J.³ - F. Bertola¹

1 - Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 2, 35122 Padova, Italy
2 - European Southern Observatory, 3107 Alonso de Cordova, Santiago, Chile
3 - Vatican Observatory, University of Arizona, Tucson, AZ 85721, USA

Received 8 October 2003 / Accepted 25 November 2003

Abstract
We present the ionized-gas kinematics and photometry of a sample of 4 spiral galaxies which are characterized by a zero-velocity plateau along the major axis and a velocity gradient along the minor axis, respectively. By combining these new kinematical data with those available in the literature for the ionized-gas component of the S0s and spirals listed in the Revised Shapley-Ames Catalog of Bright Galaxies we realized that about $50\%$ of unbarred galaxies show a remarkable gas velocity gradient along the optical minor axis. This fraction rises to about $60\%$ if we include unbarred galaxies with an irregular velocity profile along the minor axis. This phenomenon is observed all along the Hubble sequence of disk galaxies, and it is particularly frequent in early-type spirals. Since minor-axis velocity gradients are unexpected if the gas is moving onto circular orbits in a disk coplanar to the stellar one, we conclude that non-circular and off-plane gas motions are not rare in the inner regions of disk galaxies.

Key words: galaxies: kinematics and dynamics - galaxies: elliptical and lenticular, cD - galaxies: spiral - galaxies: structure

1 Introduction

The ionized-gas velocity curves measured along the disk major axis are commonly adopted to derive the mass distribution of spirals within their optical region (see Sofue & Rubin 2001 for a review). In the past decades the central velocity gradient of the gas rotation curves has been used to constrain the amount and distribution of the visible component in high surface brightness galaxies (e.g. Kent 1986; Persic et al. 1996). During recent years a lively debate concerning the dark matter distribution in the central regions of low surface brightness galaxies has been triggered by the measurement of the inner slope of the ionized-gas rotation curves in a large sample of these galaxies (McGaugh et al. 2001; de Block et al. 2001).

These mass models are based on the hypothesis that ionized gas in central regions is moving onto circular orbits and in the plane of the stellar disk. However, evidence has mounted that the kinematic behavior of the gaseous component in inner regions of disk galaxies is more complex. Indeed ionized gas can have an intrinsic velocity dispersion and its velocity can be lower than the circular speed derived from dynamical models based on the stellar kinematics and photometry (Fillmore et al. 1986; Bertola et al. 1995; Cinzano et al. 1999), a large number of kinematically-decoupled gaseous components rotating in a different plane with respect to that of the stellar disk have been found in early-type disk galaxies (Bertola et al. 1992; Bertola & Corsini 2000; Corsini et al. 2003), and non-circular gas motions have been observed in triaxial bulges (Bertola et al. 1989; Gerhard et al. 1989; Berman 2001). Moreover the vast majority of the gas velocity fields measured by Hubble Space Telescope in the nuclei of nearby spiral galaxies are not regular and possibly affected by non-gravitational forces (Sarzi 2003). This poses new questions about the reliability of gas kinematics to derive the mass density profile in the central regions of galaxies.

The presence of a velocity gradient along the disk minor axis is the kinematic signature that ionized gas is not moving onto circular orbits in a disk which is coplanar to that of the stars. To point out that this is a common phenomenon we have built a compilation of the minor-axis velocity profiles available from long-slit spectroscopy for the disk galaxies listed in the Revised Shapley-Ames Catalog of Bright Galaxies (Sandage & Tammann 1981, RSA hereafter). In addition, we have obtained the minor-axis velocity profiles for 4 spiral galaxies, whose major-axis kinematics are characterized by a remarkable zero-velocity plateau. All these galaxies show a gas velocity gradient along the disk minor axis. Such a peculiar kinematics may be also the signature of an inner polar disk as those we have recently found in early-type spiral galaxies (Corsini et al. 2003).

This paper is organized as follows. The list of the RSA disk galaxies for which the ionized-gas velocity profile has been measured along the disk minor axis is given in Sect. 2. The spectroscopic observations and data analysis of our 4 sample galaxies are described in Sect. 3. The resulting ionized-gas kinematics are discussed in Sect. 4. Our conclusions are presented in Sect. 5.

   
2 Minor-axis velocity profiles of ionized gas of the disk galaxies in the Revised Shapley-Ames Catalog

To get an exhaustive picture of the phenomena related to non-circular and off-plane motions of ionized gas in disk galaxies, we compiled in Table 1 both data from the literature and from the present paper. In Table 1 we listed all the disk galaxies of RSA for which the ionized-gas velocity profiles has been measured along the minor axis by means of long-slit spectroscopy. The table gives the main properties of the galaxies, namely the morphological classification, size, inclination, major-axis position angle, apparent magnitude and recession velocity. The position angle of the observed major and minor axis as well as the related bibliographic reference are reported in Table 1 too.

Minor-axis velocity data have been found in literature for 138 out of 1101 disk galaxies ($12\%$), whose RSA morphological type ranges from S0 to Sm, including barred systems. During this process we realized that the velocity curves of the ionized-gas component have a variety of shapes. For purpose of classification we assigned the minor-axis velocity profiles to five different classes, which we named Z, O, C, G, and I, as follows:

Z:

a zero velocity gradient is observed for the ionized-gas component along the minor axis;

O:

an overall gas velocity profile is observed along the minor axis without zero-velocity points out to the last measured radius;

C:

non-zero gas velocities are measured only in the central regions along the minor axis. The gas velocity profile shows a steep gradient rising to a maximum observed velocity of few tens of ${\rm km\;s}^{-1}$ in the inner few arcsec, then the velocity drops to zero at larger radii;

G:

a central velocity gradient is observed, but the limited radial extension of the data does not allow us to distinguish between overall or centrally-confined non-zero velocities;

I:

there is evidence of non-zero velocities along the minor axis but the velocity is either too poorly detected or too asymmetric to be assigned to the previous classes.

In Fig. 1 we show a prototypic example for each class of minor-axis velocity profile. The classification of the minor-axis velocity profiles of RSA galaxies is given in Table 1.

Figure 1: The ionized-gas velocity profiles measured along the optical minor axis of NGC 4420 ( $\rm PA = 98^\circ$, Fisher et al. 1997), NGC 7213 ( $\rm PA = 34^\circ$, Corsini et al. 2003), NGC 2855 ( $\rm PA = 30^\circ$, Corsini et al. 2002), NGC 4111 ( $\rm PA = 60^\circ$, Fisher 1997), and NGC 6810 ( $\rm PA = 86^\circ$, this paper) are given as prototypical examples for the five classes of minor-axis velocity profiles described in Sect. 2. Errorbars smaller than symbols are not plotted.

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3 Sample, observations and data reduction

NGC 949, NGC 2460, NGC 2541, and NGC 6810 are unbarred spiral galaxies which belong to a sample of 16 bright ( $B_T\leq13.5$) and nearby ( $V_\odot\leq3500$ ${\rm km\;s}^{-1}$) spirals studied by Coccato et al. (2004) to search for a circumnuclear Keplerian disk (Bertola et al. 1998; Funes et al. 2002). The main properties of the sample galaxies are summarized in Table 2. The major-axis rotation curve of their gaseous component is characterized by a remarkable zero-velocity plateau. We decided to obtain spectra along their minor axis to investigate the possible presence of a velocity gradient. The position angles of the major and minor axis were chosen according to de Vaucouleurs et al. (1991, RC3 hereafter) and therefore relate to the orientation of the outermost isophotes. This is confirmed by the isophotal analysis of the images available for the sample galaxies in the NASA/IPAC Infrared Science Archive of the Two Micron All Sky Survey (2MASS) and in the archive by Frei et al. (1996).

Table 2: Parameters of the sample galaxies.

3.1 Kinematic data

The long-slit spectroscopic observations of the sample galaxies were carried out at the Roque de los Muchachos Observatory in La Palma with the Telescopio Nazionale Galileo (TNG), at the European Southern Observatory in La Silla with the New Technology Telescope (NTT), and at the Multiple Mirror Telescope Observatory in Arizona with the Multiple Mirror Telescope (MMT) in 2002. The details about the instrumental setup of each observing run as well as the value of the seeing FWHM as measured by fitting a two-dimensional Gaussian to the guide star are given in Table 3.

Table 3: Setup of the spectroscopic observations.

Medium-resolution spectra were taken along both the major and minor axis after centering the galaxy nucleus on the slit using the guiding camera. A comparison spectrum was taken before and/or after every object exposure. The typical integration time of the galaxy spectra were 1800 s and 2700 s. Total integration times and slit position angles of the galaxy spectra as well as the log of observations are given in Table 4.

Table 4: Log of the spectroscopic observations.

Basic data reduction was performed as in Corsini et al. (1999). Using standard ESO-MIDAS routines, all the spectra were bias subtracted, flat-field corrected by quartz lamp and twilight exposures, cleaned from cosmic rays, and wavelength calibrated. The flat-field correction was performed by means of both quartz lamp and twilight sky spectra in order to correct for pixel-to-pixel sensitivity variations and large-scale illumination patterns due to slit vignetting. Cosmic rays were identified by comparing the photon counts in each pixel with the local mean and standard deviation and they were eliminated by interpolating over. Residual cosmic rays were eliminated by manually editing the spectra. We checked that the wavelength rebinning was done properly by measuring the difference between the measured and predicted wavelengths (Osterbrock et al. 1996) for the brightest night-sky emission lines in the observed spectral ranges. The resulting accuracy in the wavelength calibration is typically of 5  ${\rm km\;s}^{-1}$. Instrumental resolution was derived as the mean of Gaussian FWHM's measured for a number of unblended arc-lamp lines of a wavelength calibrated spectrum. The mean FWHM of the arc-lamp lines and the corresponding resolution at H$\alpha$ (Run 1-3) and H$\beta$ (Run 4) are given in Table 3. After the calibration all the spectra were corrected for CCD misalignment. The contribution of the sky was determined from the outermost $\sim$10'' at the two edges of the resulting frames where the galaxy light was negligible, and then subtracted. The spectra obtained for a given galaxy along the same position angle were coadded using the center of the stellar-continuum radial profile as reference.

The ionized-gas kinematics was measured by the simultaneous Gaussian fit of the emission lines present in the spectra (namely [N  II] $~\lambda\lambda6548,6583$, H$\alpha$, and [S  II] $~\lambda\lambda6716,6731$ in Run 1-3, H$\beta$ and [O  III] $~\lambda\lambda4959,5007$ in Run 4). The galaxy continuum was removed row-by-row by fitting a fourth to sixth order polynomial avoiding all the spectral regions with strong emission and absorption features. We fitted in each row of the continuum-subtracted spectrum a Gaussian to each emission line, assuming them to have the same line-of-sight velocity and velocity dispersion (corrected for heliocentric velocity and instrumental FWHM, respectively). In the spectra an additional absorption Gaussian has been added in the fit to take into account for the presence of the H$\alpha$ or H$\beta$ absorption line and the flux ratio of the [N  II] $~\lambda\lambda6548,6583$ lines have been fixed to 1:3. Far from the galaxy center (for $\vert r\vert\ga10''$) we averaged adjacent spectral rows in order to increase the signal-to-noise ratio of the relevant emission lines. Errors on the gas velocity and velocity dispersion were derived from photon statistics and CCD readout noise by means of Monte Carlo simulations.

The line-of-sight velocity and velocity dispersion profiles we measured for the gaseous component along the major and minor axis of the sample galaxies are presented in Fig. 2 and values are reported in Table 5. The line-of-sight velocities of the ionized-gas are the observed ones after subtracting the systemic velocities of Table 2 and without applying any correction for galaxy inclination.

3.2 Photometric data

We retrieved the 2MASS H band images of NGC 949 ( $3\hbox{$.\mkern-4mu^\prime$ }5\times3\hbox{$.\mkern-4mu^\prime$ }5$), NGC 2460 ( $3\hbox{$.\mkern-4mu^\prime$ }0\times3\hbox{$.\mkern-4mu^\prime$ }0$), NGC 2541 ( $13\hbox{$.\mkern-4mu^\prime$ }3\times13\hbox{$.\mkern-4mu^\prime$ }3$) and NGC 6810 ( $5\hbox{$.\mkern-4mu^\prime$ }0\times5\hbox{$.\mkern-4mu^\prime$ }0$) from the NASA/IPAC Infrared Science Archive. The galaxy images were reduced and flux calibrated with the standard 2MASS extended source processor GALWORKS (Jarrett et al. 2000). Images have a spatial resolution of 1'' and were obtained with a typical seeing ${\rm {\it FWHM}\sim1''}$.

The signal-to-noise of the 2MASS image of NGC 2541 was too low to perform a reliable photometric analysis, we therefore retrieved the i band image ( $4\hbox{$.\mkern-4mu^\prime$ }0\times4\hbox{$.\mkern-4mu^\prime$ }0$) obtained by Frei et al. (1996) too. They already performed all the basic steps of data reduction, including bias subtraction, flat-fielding, cleaning of cosmic rays, and subtraction of sky background and stellar sources.

We analyzed the isophotal profiles of the background and star subtracted images by masking the residual foreground stars, and then fitting ellipses to the isophotes with the IRAF task ELLIPSE. We first allowed the centers of the ellipses to vary, to test whether the galaxies were disturbed. Within the errors of the fits, we found no evidence of a varying center. The ellipse fits were therefore repeated with the ellipse centers fixed. The resulting azimuthally averaged surface brightness, ellipticity, and position angle profiles for the sample galaxies are plotted in Fig. 3.

	Figure 2: Ionized-gas kinematics measured along the optical major ( left panels) and minor axes ( right panels) of the 4 sample galaxies. Errorbars smaller than symbols are not plotted.
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	Figure 3: Surface-brightness ( upper panel), ellipticity ( central panel) and position angle ( lower panel) radial profiles for the sample galaxies. Data for NGC 949, NGC 2460, and NGC 6810 and in H band, data for NGC 2541 are in i band.
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4 Ionized-gas kinematics and near-infrared photometry

The following kinematic photometric features are noteworthy in the sample galaxies.

The central velocity gradient along the major axis is zero (NGC 949, NGC 2541, NGC 6810) or at least less steep (NGC 2460) than that we measured for radii larger than $\approx$3''. Further out the gas velocity increases to reach a maximum and then remains constant out to the last observed radius. Non-zero gas velocities are measured along the minor axis of all the sample galaxies, in spite of which would be expected if the gas traced the circular velocity in the disk plane. The gas velocity dispersion remains low ($\la$50 ${\rm km\;s}^{-1}$) except for few radii along the major axis of NGC 2460 and in the inner regions of NGC 6810 where it reaches $\approx$120 ${\rm km\;s}^{-1}$. Qualitatively all these data indicate that the gas is predominantly supported by rotation, and exclude the presence of a dynamically hot gas as found in some bulges (Bertola et al. 1995; Cinzano et al. 1999). We attribute the minor-axis velocity gradients to the presence of kinematically-decoupled component which is not rotating in the plane of the stellar disk. The reversal of the gas velocities measured along the minor axis of NGC 949 and NGC 2460, and the receding velocities observed along the minor axis of NGC 6810 are suggestive of an even more complex gas structure involving a warp and an outflow, respectively.

We observed a large isophotal twist in the inner $\sim$10'' of the sample galaxies which is associated with an increase of ellipticity at small radii. These photometric features can be interpreted as the signature of bulge triaxiality (e.g. Bertola et al. 1991). The bump observed at $\sim$20'' in the surface-brightness profiles of NGC 949 and NGC 6810 suggests the presence of a bar. There is no signature of a bar in the photometric profiles of NGC 2450 and NGC 2541. For radii larger than $\sim$20'' the position angle and ellipticity settle to almost constant values as expected when the disk component dominates the galaxy light. The values of position angle and ellipticity we measured at the farthest observed radius are consistent within the errors with those in B band listed in RC3, apart from the ellipticity of NGC 6810. We attribute the different shape of the outer isophotes of NGC 6810 to the dust lanes crossing the disk (see Panel 145, Sandage & Bedke 1994). For all the sample galaxies the slit position in spectroscopic observations corresponds to the major and minor axis of the disk.

Figure 4: Distribution of the gas velocity gradient along the optical minor axis of the unbarred ( upper panel) and barred ( lower panel) galaxies we collected in Table 1 according to their RSA morphological classification. The white and black regions identify galaxies with (Classes O, C, and G) and without a minor-axis velocity gradient (Class Z), respectively. The shaded region identify galaxies assigned to Class I. The last bins of the panels include all the galaxies with a morphological type later than Sd and SBd, respectively.

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5 Discussion and conclusions

By combining the new kinematic data of our sample galaxies with those available in the literature for the RSA galaxies we have the minor-axis velocity profiles of the ionized-gas component for a sample of 142 disk galaxies. In Fig. 4 we show the distribution of the gas velocity gradients measured along the optical minor axis these galaxies according to their RSA morphological classification. Although the effective frequency of the presence gas velocity gradients along the minor axis of bright disk galaxies can be properly addressed only with the analysis of a magnitude limited sample, Fig. 4 favors a picture in which this is a widespread phenomenon which is not limited to few peculiar objects: 95/142 ($67\%$) of the studied galaxies shows motions (including irregular ones) along the minor axis.

The presence of a minor-axis velocity gradient in 16/25 ($64\%$) of the available barred galaxies is explained as due to the non-circular (e.g. Athanassoula 1992) and off-plane (e.g. Friedli & Benz 1993) motions induced on the gaseous component by the tumbling triaxial potential of the bar.

We find that 54/117 ($46\%$) of the unbarred galaxies shows a minor-axis gas velocity gradient and the phenomenon is particularly frequent in the earliest morphological types. If we consider also the unbarred galaxies with an irregular velocity profile along the minor axis, the fraction rises to 75/117 ($64\%$). Most of these velocity gradients has to be attributed to the presence of non-detected bars since $\approx$$40\%$ of the spiral galaxies without a strong optical bar reveal a strong bar when observed in near infrared (Eskridge et al. 2000). However, it must be noted that in addition to the presence of a bar other mechanisms can be invoked to explain the observed velocity gradients.

A large fraction of the S0 galaxies (6/16, $38\%$) listed in Table 1 is characterized by an overall gas velocity profile along the optical minor axis. If the gas in all S0 galaxies is of external origin (Bertola et al. 1992; Kuijken et al. 1996) then the observed kinematics can be straightforwardly interpreted as due to accreted gas which is rotating in a warped/inclined disk with respect to the stellar disk (e.g. NGC 4753, Steinman-Cameron et al. 1992). This is a stable configuration in a barred potential when gas is settled onto the so-called anomalous orbits (e.g. NGC 128, Emsellem & Arsenault 1996).

The presence of non-zero velocities confined in the nuclear regions along the minor axis is particularly frequent in bulge-dominated spirals. Since there is no significant trend in the bar fraction as a function of morphology in either the optical or near infrared photometry (Eskridge et al. 2000), this finding suggests that at least some of the minor-axis velocity gradients are closely related to bulge prominence. Indeed the intrinsic shape of bulges is triaxial (Bertola et al. 1991) and two equilibrium planes are allowed for the gaseous component. We are left with two alternative scenarios as viable mechanisms to explain minor-axis velocity gradients in early-type spirals. All the gaseous component lies on the principal plane perpendicular to the bulge short axis and moves onto closed elliptical orbits, which become nearly circular at large radii (de Zeeuw & Franx 1989; Gerhard et al. 1989; Corsini et al. 2003). A central velocity gradient is measured along both the major and minor axis as due to the orientation of the inner elliptical orbits lying on the disk plane and seen at an intermediate angle between their intrinsic major and minor axes. The minor-axis gas velocity profile drops to zero where elliptical orbits become circular. Alternatively, only the outer gaseous component lies on the plane perpendicular to the short axis of bulge, while the inner gas is rotating in the principal plane perpendicular to the long axis of the bulge giving rise to an inner polar disk (Corsini et al. 2003). If this is the case the central velocity gradient observed along the disk minor axis is associated with a central zero-velocity plateau (or at least to a shallower velocity gradient) along the disk major axis.

Acknowledgements

We thank the referee, O. Sil'chenko, for the suggestions which helped us to improve the paper. We are also grateful to J. A. L. Aguerri for useful discussions. This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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Online Material

Table 1: Ionized-gas velocity gradients observed along the minor axis of the disk galaxies listed in RSA Catalog.