The SLUGGS Survey

The SLUGGS Survey


Survey characteristics

In brief, the survey consists of:

Sample: 25 early-type galaxies spanning a range of stellar masses, central stellar kinematics and environment within 30 Megaparsecs (plus 3 bonus galaxies)

Spectroscopy: Calcium Triplet absorption index from Keck / DEIMOS spectra

Imaging: Optical (g, r, i) imaging from Subaru / Suprime-cam covering ~0.5 x 0.5 degrees

Data products: Globular clusters: photometry, radial velocities, CaT indices;
Stellar light: photometric profiles, kinematics, CaT indices

SLUGGS will explore a “representative” sample of 25 early-type galaxies. The sample provides a wide coverage of parameter space: environmental density, Hubble sub-type, central rotation, central stellar cusp slope, isophote shape, ellipticity, velocity dispersion, luminosity. The strategy is to identify the relevant factors that shape the halo characteristics of these galaxies.

Previous work on GC kinematics has generally focused on the most massive galaxies, and had low velocity precision (~50-100 km/s). SLUGGS extends to the realm of ~L* “ordinary” galaxies, with precision of ~10-20 km/s.

Photometric selection of globular candidates, using a color-color diagram. Spectroscopically-confirmed GCs are shown in orange, and star/galaxy “contaminants” with other symbols. In the best cases, the success rate is as high as 97%.

Schematic of spectroscopic mask layout. A DEIMOS ~15x5 arcmin mask outline is shown at the top. SLUGGS typically uses a pinwheel-like pattern for good azimuthal coverage. Globular cluster candidates are the first priority for slit placement, and extra slits are placed to incorporate additional galaxy light.

DEIMOS spectra of globular clusters, showing the Calcium Triplet.

Locations of globular clusters around NGC 1407 with measured velocities.

DEIMOS is used in a quasi-integral field mode. Once the GC light is extracted from the slits, the leftover “sky” light is extracted as galaxy spectra, “for free”. Thus the stellar kinematics and CaT line-strengths can be mapped out in 2-D, to large radii.

SLUGGS demonstration: NGC 3115. Shown are 2-D kinematic maps of stars, and metal-rich and metal-poor GCs.