3. The stellar and ionized gas kinematics
The resulting kinematics of all our galaxies are shown in Fig. 5. The plotted velocities are as observed (no inclination correction is applied). In the following we briefly discuss each individual object. At each radius, () and () are the observed rotation velocities of the stars and the ionized gas, respectively.
NGC 2179. The gas and stellar kinematics respectively extend to (8.6 kpc) and (6.9 kpc) on either side of the nucleus. Outwards of , the stellar and the gas radial velocities are comparable.
Stars. In the inner (1.7 kpc), increases to . At the center ; away from the nucleus it remains high ( ), and possibly even rises.
Gas. has a steeper gradient than , reaching a value of at (1.0 kpc). is strongly peaked ( ) at the center; at it drops rapidly to .
A circumnuclear Keplerian disk of ionized gas has been recently discovered in the center of NGC 2179 by Bertola et al. (1998a) by means of optical ground-based observations. By modeling the motion of the gaseous disk they inferred the presence of a central mass concentration of M.
NGC 2775. The gas and stellar kinematics is measured out to (6.1 kpc) from the center.
Stars. increases almost linearly with radius, up to about 130 at (1.7 kpc); for it remains approximately constant; further out it increases to 185 and then flattens out. At , ; farther out it declines to and in the SE and NW side, respectively.
Gas. The gas behaves differently in the two regions (1.5 kpc) and . The [N II] line (the only emission line detected in both regions, see Fig. 2) shows the presence of two kinematically distinct gas components, named component (i) and (ii). Component (i) rotates with the same velocity as the stars but with a lower velocity dispersion; component (ii) rotates faster than the stars for (1.1 kpc). Both components show up simultaneously in the spectrum only at , where a double peak in the emission line is clearly detected. peaks (160 ) at the center and rapidly drops to off center.
NGC 3281. The stellar kinematics extends to (18 kpc) and (12 kpc) in the SE and the NW side, respectively. The gas kinematics can only be measured within (10 kpc) on each side of the nucleus.
Stars. The stars exhibit a rather shallow rotation gradient: at (2 kpc), . At the center ; off center it decreases to, respectively, in the SE side and in the NW side.
Gas. has a steep gradient, reaching 150 at and then 200 at (4 kpc). At the center 160 , while at it falls to 50 ; for .
IC 724. The stellar kinematics is observed out to (22.7 kpc) and (15.1 kpc) in the SW and NE sides, respectively. The ionized-gas kinematics extends to on each side of the nucleus. For (7.6 kpc), the gas and stellar kinematics are similar. For , is centrally peaked at , remaining lower than .
Stars. increases linearly up to in the inner (3.0 kpc), followed by a drop to between and on both sides of the nucleus; further out it rises to at , and then remains constant. At the center , then off the nucleus it decreases to .
Gas. has a steeper gradient than , peaking at 300 at about ; then it decreases, becoming at . The H shows steeper central RC gradient and lower velocity dispersion than [N II] and [S II] (see Fig. 3). This feature is probably due to the lower : the H absorption does not have the same central wavelength as the emission, and hence it shifts the resulting peak toward higher rotation velocities.
NGC 4698. The stellar and ionized-gas kinematics are measured out to (4.4 kpc) and (5.9 kpc) on each side of the nucleus, respectively.
Stars. In the innermost (0.6 kpc) the stars have zero rotation; at outer radii, is less steep than ; only for (2.3 kpc) are and . The profile of is radially asymmetric: in the SE side it shows a maximum of at (0.5 kpc), then it decreases outwards to and 30 at (1.8 kpc) in the SE and NW sides, respectively. The measured zero rotation plateau is explained by Bertola et al. (1998b) as due to the presence of an orthogonal-rotating bulge.
Gas. increases to 130 in the inner (1.1 kpc); then it increases more gradually reaching 200 at (3.5 kpc), and stays approximately constant farther out. In the inner ( kpc, roughly coinciding with the absorption lines region) has a 75 plateau, while at larger radii it drops to .
NGC 4845. The stellar and ionized-gas kinematics are measured out to (4.4 kpc) in the SW side, and out to (5.7 kpc) in the NE side.
Stars. has a shallower gradient than : it reaches 60 at (0.5 kpc), and further out it increases slowly, reaching the at (3.8 kpc). The velocity dispersion is constant, (in the SW side it drops to 30 for ).
Gas. reaches 180 at (0.9 kpc), to decrease and remain constant at 150 farther out. For (0.6 kpc) , then it rapidly falls to and in the SW and NE sides, respectively; farther out the behavior of is more uncertain, due to a considerable scatter of the measurements from different lines: along the NW side , while along the SE side slowly decreases to .
The RCs and velocity-dispersion profiles of both the ionized gas and the stars in our Sa galaxies show a rich diversity of kinematical properties.
has shallower gradient than at the center, while at the last measured radius, in all our sample galaxies. For NGC 2179, NGC 2775, NGC 3593, NGC 4698 and NGC 4845, the gas RCs remain flat after a monotonic rise to a maximum [whose observed values range between (as for NGC 3593) and (as for NGC 2179, NGC 2775)], or rise monotonically to the farthest observed radius (as for NGC 3281 and IC 724: in the latter after an initial peak at ). Stellar counterrotation and orthogonal rotation have been found in NGC 3593 (Bertola et al. 1996) and NGC 4698 (Bertola et al. 1998b) respectively.
The observed exceeds 100 for several kpc in the innermost regions, peaking at values ranging between 130 (as in NGC 4698) and 210 (as in IC 724); the only exception is NGC 4845 with at all observed radii. There are sample galaxies whose is low at all radii, reaching a central maximum of (NGC 4698, NGC 4845) or remaining flattish at (NGC 3593); and others where increases to either at the very center (as in NGC 2179, NGC 2775) or over an extended radial range around the center (as in NGC 3281, IC 724).
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999