On the derivation of dynamical masses of the stellar clusters in the circumnuclear region of NGC 2903

Abstract

Gas and star velocity dispersions have been derived for four circumnuclear star-forming regions (CNSFRs) and the nucleus of the spiral galaxy NGC 2903 using high-resolution spectroscopy in the blue and far red. Stellar velocity dispersions have been obtained from the Ca ii triplet lines at λλ 8494, 8542, 8662 Å, using cross-correlation techniques, while gas velocity dispersions have been measured by Gaussian fits to the Hβ λ 4861 Å line. The CNSFRs, with sizes of about 100 to 150 pc in diameter, show a complex structure at the Hubble Space Telescope (HST) resolution, with a good number of subclusters with linear diameters between 3 and 8 pc. Their stellar velocity dispersions range from 39 to 67 km s-1. These values, together with the sizes measured on archival HST images, yield upper limits to the dynamical masses for the individual star clusters between 1.8 and 8.7 × 106 M⊙ and upper limits to the masses for the whole CNSFR between 4.9 × 106 and 4.3 × 107 M⊙. The masses of the ionizing stellar population responsible for the H ii region gaseous emission have been derived from their published Hα luminosities and are found to be between 1.9 and 2.5 × 10 6 M⊙ for the star-forming regions, and 2.1 × 105 M⊙ for the galaxy nucleus, therefore constituting between 1 and 4 per cent of the total dynamical mass. In the CNSFR, star and gas velocity dispersions are found to differ by about 20 km s-1 with the Hβ lines being narrower than both the stellar lines and the [O iii] λ 5007 Å lines. The ionized gas kinematics are complex; two different kinematical components seem to be present as evidenced by different widths and Doppler shifts. The line profiles in the spectra of the galaxy nucleus, however, are consistent with the presence of a single component with radial velocity and velocity dispersion close to those measured for the stellar absorption lines. The presence and reach of two distinct components in the emission lines in ionized regions and the influence that this fact could have on the observed line ratios are of major interest for several reasons, among others, the classification of the activity in the central regions of galaxies, the inferences about the nature of the source of ionization for the two components and the derivation of the gaseous chemical abundances.