Globular Clusters as Dynamical Probes of the S0 Galaxy NGC 3115

Abstract

Listen

Translate article

NGC 3115 is a lenticular (S0) galaxy located in the sparse environment of the Leo Spur. Previous investigators found evidence of a massive black hole in its center (J. Kormendy, et al., ApJ, 459, L57 [1996]). NGC 3115 rotates rapidly about its minor axis, perhaps fast enough to account for its flattened bulge (G. Illingworth, & P. L. Schechter, ApJ, 256, 481 [1982]). The rotation has been measured out to a few kiloparsecs from the center (M. Capaccioli, E. Cappellaro, E. V. Held, & M. Vietri, A&A, 274, 69 [1993]). These kinematic features show it is much like a disky elliptical galaxy (J. L. Nieto, & R. Bender, A&A, 215, 266 [1989]). Photometric investigations of the globular cluster system (GCS) have revealed a flattened distribution with a specific frequency not unlike spiral galaxy (D. A. Hanes, & W. E. Harris, ApJ, 304, 599 [1986]). Recently, a study of the red giant branch (RGB) population of NGC 3115 has revealed a bimodal color distribution and a distance modulus of (R. A. W. Elson, MNRAS, 286, 771 (m 2 M) 5 29.8 V [1997]). We have made photometric observations of NGC 3115 in Kron-Cousins V and I using images obtained with the CanadaFrance-Hawaii Telescope. The limiting magnitude in both filters is approximately at the peak of the globular cluster luminosity function (GCLF), which we determine to be . This turnover value, compared with that of TO m 5 22.8 5 0.2 V the Milky Way, implies a distance modulus of TO (m 2 M) 5 V , consistent with distance estimates based on the 30.2 5 0.3 magnitude of the RGB tip. Analysis of the colors re(V 2 I) veals that the GCS has a bimodal color distribution. The red clusters appear more concentrated toward the plane of NGC 3115 than the blue clusters (see Fig. 1). We have also determined the GCS specific frequency to be , typical S 5 2 5 1 N of lenticulars that do not possess bars and in the range of values for spiral systems. The dispersion of GCLF is much like that of a spiral galaxy, with . j ∼ 1.1 To probe the mass of NGC 3115 to larger radii than previous rotation curves, we have measured radial velocities of candidate globular clusters using spectra obtained at the William Herschel Telescope. Radial velocities were determined for 39 unresolved objects using the “fxcor” package in IRAF. Of these, 22 objects were found to be associated with NGC 3115 based on their velocities. We find that the cluster system is rotating with an amplitude of km s at a radius of 14 kpc. V(rot) 5 190 5 20 Removing this coherent motion and applying the projected mass estimator (J. N. Bahcall & S. Tremaine, ApJ, 244, 805 [1981]) implies interior to kpc (assuming 110 M/L 5 19 R 5 14 B 23 a distance of 11 Mpc). This value of M/L is larger than that found at smaller radii, , M/L (3 kpc) 5 6 M/L (9 kpc) 5 10 B B (M. Capaccioli, E. Cappellaro, E. V. Held, & M. Vietri, A&A, 274, 69 [1993]), suggesting that NGC 3115 possesses a dark matter halo. From these spectral and photometric data we find strong evidence that NGC 3115 possesses two separate and distinct cluster populations, a rapidly rotating metal-rich thick disk system and a more slowly rotating metal-poor halo system. The GCS is very much like that of the Milky Way with distinct thick disk and halo components. This appears to support the hypothesis that S0 galaxies in sparse environments may be a transition class from spiral to elliptical galaxies.