Abstract
Abstract We present a technique to extract ultra-deep diffuse-light spectra from the standard multi-object spectroscopic observations used to investigate extragalactic globular cluster (GC) systems. This technique allows a clean extraction of the spectrum of the host galaxy diffuse light from the same slitlets as the GC targets. We show the utility of the method for investigating the kinematics and stellar populations of galaxies at radii much greater than usually probed in longslit studies, at no additional expense in terms of telescope time. To demonstrate this technique, we present Gemini South Multi-Object Spectrograph (GMOS) spectroscopy of 29 GCs associated with the elliptical galaxy NGC 3923. We compare the measured stellar population parameters of the GC system with those of the spheroid of NGC 3923 at the same projected radii, and find the GCs to have old ages >10 Gyr, [α/Fe]∼ 0.3 and a range of metallicities running from [Z/H]=−1.8 to +0.35. The diffuse light of the galaxy is found to have ages, metallicities and [α/Fe] abundance ratios indistinguishable from those of the red GCs.
Highlights
Of gas with an almost uniform metallicity
We limit ourselves to examining the spheroid of NGC 3923 for rotation and in Section 4.2 to comparing the velocities measured for the spheroid of NGC 3923 and those measured for the globular cluster (GC) examined in the same slits
We present the results of our search for evidence of rotation in the diffuse stellar halo of NGC 3923 by carrying out a non-linear least squares fit to the equation: V(θ) = Vrotsin(θ - θ0) + V0 where V(θ) is the velocity measured for the diffuse light in each slitlet, Vrot is the amplitude of the projected rotation velocity, V0 the systematic velocity of NGC 3923, θ the azimuthal angle of each slitlet relative to the galaxy major axis and θ0 is the position angle of the line of nodes
Summary
Of gas with an almost uniform metallicity. This makes them much simpler to model than the integrated stellar populations of the diffuse light of galaxies, where many star formation events at different epochs greatly complicate analysis of the spectra. The great promise of this approach to simplify the examination of the star formation history of galaxies is dependent on one critical assumption: that the stellar populations of GCs are representative of the field stars that form the bulk of a galaxies’ stellar population. To date little work has been done in comparing the stellar population parameters measured from line index analyses for GCs with those determined for the host galaxy diffuse stellar component. This is because at the galactocentric radii where the GCs selected for spectroscopic analysis are found, the surface brightness of the underlying galaxy halo light is very faint. These results hint at a strong connection between the formation of the spheroid of NGC 3115 and its GC system
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