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^^—^^r N6L0-AISTRALIAN OBSERVATORY GAS DYNAMICS IN BARRED SPIRALS, II: NGC 7496 AND 289 V.D. Pence and C.P. BlackMn AAO PP 194 * Suonitted to: Mon.Not.R.astr.Soc. Distribution date: February, 1984 P.O. BOX 296. EPPING. N.S.W.. 2121 PHONE 868-1666. TELEX ASTRO 26230 „. *u~ - .*.SÄ._TNW. - Afto-<?(*-- ^M GAS DYNAMICS IN BARRED SPIRALS, II: NGC 7496 AND 289 W.D.Pence. Anglo-Australian Observatory. P.O. Box 296. Epplng. NSW. Australia. C.P.Blactanan. Department of Astronomy. Edinburgh University. Received t SUMMAKT - The gas velocity fields in the barred spiral galaxies NGC 7496 and 289 have been measured by means of long-slit spectra obtained with the Anglo-Australian Telescope. Pronounced deviations fro« circular motion of the type predicted by recent theoretical mooels are seen in NGC 74961 the isovelocity contours are distorted into a characteristic S-shaped pattern and there is a large velocity gradient across the bar. The velocity field is virtually identical to that of a previously observed barred spiral, NGC 5383 for which a number of models have been published. The nuclear [OUI] emission lines are very asymmetric with a wing extending to about 1000 km s-1 to the blue of the systemic velocity; this wing is only faintly seen in the Balmer lines. NGC 289 has a much smaller bar *nd consequently the noncircular motions are less pronounced. The most obvious effect is that the Xinematic major and minor axes are not perpendicular which is a signature of oval distortions. Both galaxies have a mass (within the 10 outermost velocity measurement) of 9 x iO M0 and M/Lß » 6. One significant difference between the galaxies is that the neutral hydrogen contributes about 20% of the mass of NGC 289 but only about 2% of the mass of NGC 7496. As a further test of the models of gas streaming in barred spirals we have collected the results from 11 previously studied galaxies and show that the appearance of the velocity field depends on the orientation of the bar to tne line of sight, in good agreement with the model predictions. Page 2 1 Introduction This paper continues our study of the gas dynamics in barred spiral galaxies in which we have been looking for observational verification of recent theoretical models of the behaviour of gas in response *:o a rotating stellar bar. In the first paper (Blackman and Pence, 1981), based on spectra taken with the SAAO 19m telescope, we found little evidence of noncircular menions greater than 20 - 30 km s-1 in the barred spirals NCC 2525 and 7741. Since higher resolution seemed to be required to detect the streaming motions, we subsequently obtained better quality data on several more barred spirals using the Anglo-Australian 3.9m telescope. In the previous paper (Pence and Blackman, 1984, hereafter referred to as Paper I) we discussed the results on NGC 6221 which showed a discontinuity of about 150 km s-1 in the gas velocity across the dust lanes on the leading edges of the bar, which provided the firct direct observational proof that the dust lanes mark the location of shock fronts in the gas as predicted by the theore. al models. Now in this last paper in the series, we present the results on two more barred spiral galaxies, NGC 7496 and 289, which provide further support for the models. 2 Observations and Reduction Method The 3.9m Anglo-Australian telescope was used to obtain long-slit spectra during the period* 5-7 August 1981 The observations were made with the RGO spectrograph with 25 cm camera and 1200 V grating (first order) in conjunction with the IPCS detector. Each digitally recorded spectrum consists of 2044 spectral elements, each approximately 0.45A wide, and 80 cross-sections, each separated by 2.3 arcsec on the sky. A slit width of 270M « 1.8 arcsec was used which projects to a spectral resolution of 1.3A. Most spectra were centred on the Ha region, but some spectra near Bfi were also taken to look for any interesting nuclear phenomena. The data reduction procedure was essentially the same as described in Paper I. The basic wavelength calibration of the spectra was performed with the Starlink SPICA software package, then specialized routines were written to find the wavelength of each emission line using a Gaussian profile fitting algorithm. The radial velocity of the gas at each spatial increment along the slit was Page 3 derived from a weighted mean of the red shifts of all the measurable emission lines. The weights were 6 for Ha, 3 for [NII]6583.6A and 1 for the [NII]6548.06A and [SII]6717.0A and 6731.3A lines. Fro« comparison of the independent velocity measurements at the 23 intersection points of the spectra, the mean error of a single observation in NGC 7496 is measured v.o be 5 km s-1. In NGC 289, which has weaker emission, only the Ha and [NII]6583 lines were measured and the mean error of a single velocity measurement is a « 10 km s » derived from the velocities at 16 intersection points. We have also observed the 21cm HI line profile of these galaxies for comparison with the optical velocity field (Pig. 1). -These were obtained with the Parkes 210* radio telescope in Nov. 1981 in collaboration with M. Phillips (vTPlO) and A. Turtle (Univ. of Sydney). 3 RESULTS: HGC 7496 3.1 GENERAL NGC 7496 is a SB(s)b barred spiral (de vaucouleurs, de Vaucouleurs and Corwin, 1976, hereafter RC2) with a pronounced bar about 1 arcmin in length which merges into two spiral arms. There are no companion galaxies of comparable size within a 1 degree radius so there is no possibility of tidal interactions confusing the interpretation of the velocity field. A distance of 16 Mpc is adopted _1 -1 based on H0 * 1O0 km s Mpc which gives an image scale of 78 pc arcsec-1. The analysis of NGC 7496 is based on the 11 spectra listed in Table 1. The mean velocity of the Ho, [Nil] and [Sil] emission lines were measured at 321 positions in the galaxy, covering all the major features, ao shown in Fig. 2 and listed in Appendix A. 3.2 VELOCITY FIELD The velocities from each spectrum were smoothed by a 3 point running mean in the spatial direction and then combined into a 2D map using the telescope coordinates and the intensity of the continuum emission along the slit to find their relative positions within the galaxy. The heliocentric isovelocity contours were traced by hand and are shown in Fig. 2. There is very little room for subjective bias in Page 4 Where the contours should be drawn because of the good spatial co\erage and accuracy of the velocity measurements. The main feature of interest in the figure is the way the isovelocity contours deviate froa the symmetric "spider" diagram appearance expected for circular rotation. Instead they are skewed to lie nearly parallel to the bar showing that large systemic streaming motions are present. Before discussing these noncircular notions further, we first derive the basic kinematic and orientation parameters. 3.3 BASIC PARAMETERS The position angle of the line of nodes usually corresponds to the angle of steepest velocity gradient but in NGC 7496 this is not uniquely defined. In the central region the steepest gradient is nearly perpendicular to the bar whose major axis is at pa « 145*; at larger radii away from the perturbing influence of the bar, the maximum velocity gradient is at pa * 5* ± 5* which we assume is the true pa of the line of nodes. This agrees well with the optical major axis at pa « 0° ± 5* as measured from microdensitometer scans of the image on the SERC J sky survey. The inclination of a barred spiral galaxy is difficult to determine because of the uncertainty in the intrinsic shape of the galaxy. If we follow the usual procedure of assuming <:'iat the outer optical isophotes define a circle in the galaxy plane, then from the axial ratio, b/a * 0.55, we derive an inclination of i - 55". However this implies that the maximum rotational velocity is vmax - Av/2 »in(i) - 85 km s"1 where Av - 140 km s-1 is the observed range in radial velocity. This is unrealistically low for an Sb galaxy, which usually has vnax in the range 200 to 250 km s-1 (Rubin et al., 1982), and leads to an improbably low mass for the galaxy. Since NGC 7496 seems typical of Sb galaxies in most other respects (see the comparison with NGC 53S3 in the following section) we shall reverse the usual procedure and assume 1 that vnax = 225 kms"* , which then implies that the inclination must be about 18*. Assuming that the spiral arms are trailing, then the west side of tht galaxy is nearest to us. Page 5 A» an estimate of the systemic velocity, three of our spectra intersect at the nucleus and give a mean velocity of 1657 ± 2 kms-1• Since the nuclear emission often shows a systematic velocity shift relative to the rest of the galaxy (Heckman et al. 1981) we also used the iterative model fitting procedure used previously for NGC 6221 (Paper I), excluding the points within 25" of the nucleus which are most strongly perturbed by the bar, and found a best fitting systemic velocity of V « 1652. These values are in good agreement with the HI 21cm velocities V(HI) • 1657 ± 8 (Pisher and Tully, 1981) and our own measurement of V(HI) « 1640 ± 5 (Pig.