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Gemini Sheds New Light on Dark Matter in NGC 3379

February 16, 2006

Using Gemini observations of globular clusters in NGC 3379 (M105), a team led by PhD student Michael Pierce and Prof. Duncan Forbes of Swinburne University in Australia, have found evidence for normal quantities of dark matter in the galaxy’s dark halo. This is contrary to previous observations of planetary nebulae that indicated a paucity of dark matter in the galaxy.

Figure 1. Normalized globular cluster spectra that have been offset by one unit. These spectra have not been de-redshifted. These sample spectra show the wavelength range that the majority of our spectra cover and display the range of S/N and metallicity present. The spectrum of g1426 shows emission features around 5000 Å and 5890 Å of unknown origin. The planetary nebulae hosting globular cluster, g1420, is also plotted and the emission due to 4959 and 5007 Å [OIII] lines can be seen redshifted to 4971 and 5019 Å.

The observations of 22 globular clusters in the Leo Group elliptical galaxy were made using the Gemini Multi-Object Spectrograph (GMOS) on Gemini North in early 2003. The data were obtained in the GMOS multi-slit mode with exposures of 10 hours on-source at a spectral resolution of FWHM ~4Å over an effective wavelength range of 3800Å-6660Å. The final spectra have a signal-to-noise ratio of 18-58/Å at 5000 Å. The spectroscopic data allowed the team to derive ages, metallicities and α-element abundance ratios for the sample of globular clusters. All of the globular clusters were found to be >~ 10 Gyr, with a wide range of metallicities. A trend of decreasing α-element abundance ratio with increasing metallicity is also identified.

Figure 2. Plots of radial velocity dispersion and M/L ratio using GC and stellar data (from Gebhardt et al. 2000 and Statler & Smecker-Hane 1999). The upper panel shows the velocity dispersion of the GCs. Blue open squares and hashing include the four GCs that that may be associated with NGC 3384, and the red hashing and filled squares when they are excluded. The squares show the binned data and the hashed areas show the range of values using the lowess estimator (with 1% errors). The PN data points from Romanowsky et al. (2003) (circles) and the expected isotropic profile with constant M/L (thick line) are also shown. In the lower panel, the blue line (1% hatching) shows that when the four ambiguous GCs are included the M/L ratio rises slowly. The red line (with 1% hatching) shows that the M/L ratio rises strongly in the outer regions when the four ambiguous GCs are excluded. The horizontal line in the bottom panel is M/L = 6.

Most significantly, including 14 extra globular clusters from Puzia, et al. (2004), the projected velocity dispersion of the globular cluster system was found to be constant with radius from the galaxy center, indicating significant dark matter at large radii in its halo. This result is in stark contrast to the “No/Low Dark Matter” interpretation by Romanowsky, et al. (2003) in the journal Science using observations of planetary nebula that indicated a decrease in the velocity dispersion profile with radius.

Reconciling the two velocity dispersion profiles is possible. Dekel, et al. (2005) recently showed that stellar orbits in the outer regions of merger-remnant elliptical galaxies are elongated and that declining planetary nebula velocity dispersions do not necessarily imply a dearth of dark matter.

Another possibility the authors suggest is that NGC 3379 could be a face-on S0 galaxy (as originally suggested by Capaccioli, et al. 1991). If a significant fraction of the planetary nebulae belong to the disk, this could suppress the line-of-sight velocity dispersion of the planetary nebulae relative to that of the globular clusters that lie in a more spherical halo.

These results are currently in press for publication in the Monthly Notices of the Royal Astronomical Society. The accepted version of the paper is available on astro-ph/0510838

The research team includes members from Australia, USA, Canada, Argentina and the UK:

Michael Pierce, Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn, Australia

Michael A. Beasley, Lick Observatory, University of California, Santa Cruz, CA, USA

Duncan A. Forbes, Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn, Australia

Terry Bridges, Department of Physics, Queen's University, Kinston, ON, Canada

Karl Gebhardt, Astronomy Department, University of Texas, Austin, TX, USA

Favio Raul Faifer, Facultad de Cs. Astronomicas y Geofisicas, UNLP, Paseo del Bosque 1900, La Plata, and CONICET, Agentina & IALP - CONICET, Argentina

Juan Carlos Forte, Facultad de Cs. Astronomicas y Geofisicas, UNLP, Paseo del Bosque 1900, La Plata, and CONICET, Agentina

Stephen E. Zepf, Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA

Ray Sharples, Department of Physics, University of Durham, South Road, Durham, UK

David A. Hanes,  Department of Physics, Queen's University, Kinston, ON, Canada

Robert Proctor,  Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn, Australia