The Globular Cluster System of NGC 1399. II. Kinematics of a Large Sample of Globular Clusters

Abstract

We study the kinematics and dynamics of the globular cluster system of NGC 1399, the brightest elliptical galaxy near the center of the Fornax cluster of galaxies. The observational data consists of medium-resolution spectra, obtained at the Very Large Telescope with FORS2 and the Mask Exchange Unit (MXU). Our sample comprises 468 radial velocities in the magnitude range 20 < m R < 23. This is the largest sample of globular cluster velocities around any galaxy obtained so far. Typical velocity uncertainties are 50 km s À1 , significantly improving on earlier samples. The radial range is 2 0 < r < 9 0 , corresponding to 11 kpc to 50 kpc of galactocentric distance. The shape of the velocity distribution of the sample is compatible with being a Gaussian distribution. However, under moderate error selection, a slight asymmetry is visible between high and low radial velocities. We find bright clusters with radial velocities below 800 km s À1 , while they are not found at the corresponding highvelocity side above 2000 km s À1 . There is the possibility that unbound clusters and/or objects in the foreground contaminate the NGC 1399 cluster sample. Under strong error selection, practically no objects are found with velocities lower than 800 km s À1 or higher than 2000 km s À1 . Since the extreme velocities influence the velocity dispersion considerably, uncertainty regarding the exact value of the dispersion remains. With the above velocity limits, we derive a projected velocity dispersion for the total sample of 274 AE 9 km s À1 which within the uncertainties remains constant over the entire radial range. Without any velocity restriction, it increases to 325 km s À1 . Guided by the bimodal color distribution of clusters, we distinguish between red clusters (CÀR > 1:6) and blue clusters (CÀR < 1:6), and find velocity dispersions for these groups of 255 AE 13 and 291 AE 14 km s À1 , respectively, again radially constant. Any possible rotation of either of these cluster populations is below the detection limit, with the exception of a weak signature of rotation for the blue clusters more distant than 6 0 . Spherical models point to a circular velocity of 419 AE 30 km s À1 , assuming isotropy for the red clusters. This value is constant out to 40 kpc. The inferred dark halo potential can be well represented by a logarithmic potential. A halo of the NFW type also provides a good fit to the observations. The orbital structure of the clusters can only be weakly constrained. It is consistent with isotropy for the red clusters and a slight tangential bias for the blue clusters. Some mass profiles derived from X-ray analyses do not agree with a constant circular velocity within our radial range, irrespective of its exact value. Interpreting the extreme low radial velocities as space velocities of bound clusters near their pericentric distances would require an extension of the cluster system of at least 200 kpc. Implications for formation scenarios of the cluster system are briefly commented on.

Counter-dispersed slitless-spectroscopy technique: planetary nebula velocities in the halo of NGC 1399

“…When we apply the shift with this error to our measured values, we find a mean velocity of 1416±29 km s −1 for our sample of PNe in NGC 1399. This is consistent with Graham et al (1998) where v sys = 1425 ± 9 km s −1 (NED) and Richtler et al (2004) where v sys = 1441 km s −1 for the GCs. Since the offset is constant, our measurements stand alone as relative velocities.…”

Ultra-Compact Stellar Systems in the Fornax Galaxy Cluster

“…The velocity gradient is 57 ± 42 km s −1 deg −1 in R.A. from NGC1399 (V/σ0 = 0.23 ± 0.17). This rotation contrasts with the finding [10] that the inner GC population (2 ′ to 9 ′ radius) shows little or no rotation about NGC1399. Our results suggest a UCD bar/filament structure stretches across NGC1399.…”

The Globular Cluster System of M60 (NGC 4649). II. Kinematics of the Globular Cluster System

“…Recently, with the aid of large telescopes with apertures larger than 4 m, a large sample (N > 150) of GC spectra for one galaxy has been obtained, which can be used for statistically meaningful kinematic studies (e.g., Côté et al 2001Côté et al , 2003Richtler et al 2004;Peng et al 2004). Due to their wide spatial distribution, brightness, and compact size, GCs are useful test particles for tracing the gravitational potential of their host galaxies, and especially of the dark matter halo beyond several effective radii of the galaxy.…”