Kinematics of gas and stars in the circumnuclear star-forming ring of NGC 3351

Abstract

We have measured gas and stellar velocity dispersions in five circumnuclear star-forming regions (CNSFRs) and the nucleus of the barred spiral galaxy NGC 3351. The stellar dispersions have been obtained from high-resolution spectra of the Ca II triplet (CaT) lines at λλ8494, 8542, 8662 A, while the gas velocity dispersions have been measured by Gaussian fits to the Hβ λ4861 A line on high-dispersion spectra. The CNSFRs, with sizes of about 100 to 150pc in diameter, are seen to be composed of several individual star clusters with sizes between 1.7 and 4.9 pc on a Hubble Space Telescope (HST) image. Using the stellar velocity dispersions, we have derived dynamical masses for the entire star-forming complexes and for the individual star clusters. Values of the stellar velocity dispersions are between 39 and 67 km s -1 . Dynamical masses for the whole CNSFRs are between 4.9 x 106 and 4.3 x 10 7 M ⊙ and between 1.8 and 8.7 x 10 6 M ⊙ for the individual star clusters. Stellar and gas velocity dispersions are found to differ by about 20kms -1 with the Hβ lines being narrower than both the stellar lines and the [O III]λ5007 A lines. We have found indications for the presence of two different kinematical components in the ionized gas of the regions. The radial velocity curve shows deviation from circular motions for the ionized hydrogen consistent with its infall towards the central regions of the galaxy at a velocity of about 25 km s -1 . To disentangle the origin of these two components it will be necessary to map these regions with high spectral and spatial resolution and much better signal-to-noise ratio in particular for the O 2+ lines.