1 Introduction

Our current understanding of galaxy formation has greatly benefited from the results of N-body modelling of structure formation in the early Universe, which predicts that small objects combine gravitationally to produce the galaxies we see today: a process called hierarchical-clustering-merging (hereafter HCM, cf. Kauffmann et al. 1993). One of the tenets of the HCM paradigm is that galaxies are constantly merging with one another. In the case of elliptical and S0 galaxies, there is ample observational evidence that they are continually subjected to mergers with smaller, neighbouring galaxies (cf. Schweizer 1998).

If the HCM paradigm is universal, spirals are subject to the same formation processes as E's and S0's. Often the fingerprints of such second events reside in the stellar and/or gaseous kinematics of a galaxy rather than in its morphology. This is particularly true if we consider that the most evident "morphological tracers'' of interactions such as peculiar or spindle galaxies make up less than $5\%$ of all objects in any one of the RC3 (de Vaucouleurs et al. 1991), UGC (Nilson 1973) or ESO/Upssala (Lauberts 1982) galaxy catalogues. It is therefore crucial to obtain detailed kinematic parameters of both stars and gas to unveil the relics of accretion or merging events which have occurred in galaxy history. A large fraction of spirals exhibit kinematic disturbances ranging from mild to major, and can generally be explained as the visible signs of tidal encounters (Rubin et al. 1999). In recent years a number of otherwise morphologically undisturbed spirals have been found which host kinematically-decoupled components (KDC's), such as stellar KDC's (Bertola et al. 1999; Sarzi et al. 2000), counter-rotating extended stellar discs (Merrifield & Kuijken 1994; Bertola et al. 1996; Jore et al. 1996), counter-rotating or decoupled gaseous discs (Braun et al. 1992; Rubin 1994; Rix et al. 1995; Ciri et al. 1995; Haynes et al. 2000; Kannappan & Fabricant 2001) and possibly counter-rotating bulges (Prada et al. 1996; but see also Bottema 1999).

Studying the interplay between ionized gas and stellar kinematics allows us to address other issues concerning the dynamical structure of spirals. These include the origin of disc heating and the presence of stellar or gaseous discs in galactic nuclei. Gravitational scattering from giant molecular clouds and spiral density waves are the prime candidates to explain the finite thickness of stellar discs. It is expected that the dominant heating mechanism varies along the Hubble sequence but up to now only two external galaxies have been studied in detail (Gerssen et al. 1996, 2000). The presence in the nuclei of S0's and spirals of small stellar (Emsellem et al. 1996; Kormendy et al. 1996a,b; van den Bosch et al. 1998; Scorza & van den Bosch 1998; van den Bosch & Emsellem 1998) and/or gaseous discs (Rubin et al. 1997; Bertola et al. 1998; Funes 2000) is usually connected to the presence of a central mass concentration. It also appears that the central black-hole mass is very strongly correlated with the stellar velocity dispersion of the host galaxy bulge as recently found by different authors (Ferrarese & Merritt 2000; Gebhardt et al. 2000). This relation is however based on samples which are affected by different biases and therefore new black-hole masses as well as stellar velocity dispersion measurements are needed.

Finally, the comparison of mass distributions derived from stellar and gaseous kinematics has shown that the ionized gas velocity may not trace the circular speed in the central regions of S0's (Fillmore et al. 1986; Bertola et al. 1995; Cinzano et al. 1999) and bulge-dominated spirals (Corsini et al. 1999; Pignatelli et al. 2001). The possible difference between the gas rotational velocity and the gravitational equilibrium circular velocity poses questions about the reliability of mass distributions derived from the direct decomposition of ionized gas rotation curves into the bulge, disc and dark halo contribution (see Kent 1988 for a discussion). This phenomenon has been explained in terms of pressure-supported ionized gas, gas motions which are not confined to the galaxy equatorial plane and drag forces but its cause is still unclear due to the limited statistics and requires further investigation.

All these issues will benefit greatly from a survey devoted to the comparative measurements of ionized gas and stellar kinematics. With this aim we obtained long-slit spectroscopy of a sample of 20 disc galaxies, mostly spirals. We measured the velocity, velocity dispersion, h₃ and h₄ radial profiles of the stellar component and velocity and velocity dispersion radial profiles of the ionized gas along their major axes. In Pignatelli et al. (2001) we present the mass modelling of three galaxies of the sample, the Sa NGC 772 and the Sb's NGC 3898 and NGC 7782.

This paper is organized as follows. An overview of the properties of the sample galaxies as well as the spectroscopic observations and their data analysis are presented in Sect. 2. The resulting stellar and gaseous kinematic parameters are given in Sect. 3. Conclusions are discussed in Sect. 4. In the Appendix a comparison with published kinematic measurements of the sample galaxies is performed.

   

Table 1: Basic properties of the sample galaxies.

object	type		$B_{\rm T}$	PA	i	$V_{\odot}$	D	scale	R25	$M_{B_{\rm T}}^0$
[name]	[RSA]	[RC3]	[mag]	[$^\circ$]	[$^\circ$]	[ $\rm km\;s^{-1}$]	[Mpc]	[pc/'']	[']	[mag]
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)
NGC 224	Sb	.SAS3..	4.36	55	72	-290	0.7	3.4	95.3	-20.87
NGC 470	Sbc(s)	.SAT3..	12.53	155	52	2370	33.8	163.9	1.4	-20.66
NGC 772	Sb(rs)	.SAS3..	11.09	130	54	2470	35.6	172.7	3.6	-22.21
NGC 949	Sc(s)	.SAT3$\ast$$	12.40	145	58	620	11.4	55.2	1.2	-18.50
NGC 980	...	.L.....	13.20	110	58	5765	80.1	388.2	0.8	-22.95
NGC 1160	...	.S..6$\ast$.	13.50	50	62	2510	36.6	177.4	1.0	-21.01
NGC 2541	Sc(s)	.SAS6..	12.26	165	61	565	8.7	42.2	3.2	-18.13
NGC 2683	Sb	.SAT3..	10.64	44	78	460	5.3	25.6	4.7	-18.99
NGC 2841	Sb	.SAR3$\ast$.	10.09	147	65	640	9.6	46.4	4.1	-20.33
NGC 3031	Sb(r)	.SAS2..	7.89	157	59	-30	1.5	7.2	13.5	-18.46
NGC 3200	Sb(r)	.SXT5$\ast$.	12.83	169	73	3550	43.9	213.1	2.1	-21.53
NGC 3368	Sab(s)	.SXT2..	10.11	5	47	860	9.7	47.1	3.8	-20.14
NGC 3705	Sab(r)	.SXR2..	11.86	122	66	1000	11.4	55.2	2.4	-19.03
NGC 3810	Sc(s)	.SAT5..	11.35	15	45	1000	11.9	56.0	2.1	-19.36
NGC 3898	Sa	.SAS2..	11.60	107	54	1185	17.1	82.9	2.2	-19.85
NGC 4419	SBab:	.SBS1./	12.08	133	71	-200	17.0	82.4	1.7	-19.55
NGC 5064	Sa	PSA.2$\ast$.	13.04	38	64	2980	36.0	174.4	1.2	-21.11
NGC 5854	Sa	.LBS+./	12.71	55	76	1630	21.8	100.7	1.4	-18.90
NGC 7331	Sb(rs)	.SAS3..	10.35	171	70	820	14.7	72.0	5.2	-20.48
NGC 7782	Sb(s)	.SAS3..	13.08	175	58	5430	75.3	364.9	1.2	-21.95

Notes - Column (2): morphological classification from RSA. Column (3): morphological classification from RC3. Column (4): total observed blue magnitude from RC3 except for NGC 980 and NGC 5064 (LEDA). Column (5): major-axis position angle taken from RC3. Column (6): inclination derived as $\cos^{2}{i}\,=\,(q^2-q_0^2)/(1-q_0^2)$. The observed axial ratio q is taken from RC3 and the intrinsic flattening q₀=0.11 has been assumed following Guthrie (1992). Column (7): heliocentric velocity of the galaxy derived at centre of symmetry of the rotation curve of the gas. $\Delta V_\odot = 10$ $\rm km\;s^{-1}$. Column (8): distance obtained as V₀/H₀ with H₀=75 $\rm km\;s^{-1}$ Mpc^-1 and V₀ the systemic velocity derived from $V_{\odot}$ corrected for the motion of the Sun with respect to the Local Group as in the RSA. For NGC 224 and NGC 4419 we assume distances of 0.7 Mpc (Binney & Merrifield 1999) and 17 Mpc (Freedman et al. 1994), respectively. Column (10): radius of the 25 B-mag arcsec^-2 isophote derived as R₂₅ = D₂₅/2 with D₂₅ from RC3. Column (11): absolute total blue magnitude corrected for inclination and extinction from RC3.