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Register a new userThe SLUGGS survey: Exploring the metallicity gradients of nearby early-type galaxies to large radii
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Stellar metallicity gradients in the outer regions of galaxies are a critical tool for disentangling the contributions of in-situ and ex-situ formed stars. In the two-phase galaxy formation scenario, the initial gas collapse creates steep metallicity gradients, while the accretion of stars formed in satellites tends to flatten these gradients in the outskirts, particularly for massive galaxies. This work presents the first compilation of extended metallicity profiles over a wide range of galaxy mass. We use the DEIMOS spectrograph on the Keck telescope in multi-slit mode to obtain radial stellar metallicity profiles for 22 nearby early-type galaxies. From the calcium triplet lines in the near-infrared we measure the metallicity of the starlight up to 3 effective radii. We find a relation between the outer metallicity gradient and galaxy mass, in the sense that lower mass systems show steeper metallicity gradients than more massive galaxies. This result is consistent with a picture in which the ratio of ex-situ to in-situ formed stars is lower in less massive galaxies as a consequence of the smaller contribution by accretion. In addition, we infer a correlation between the strength of the calcium triplet feature in the near-infrared and the stellar initial mass function slope that is consistent with recent models in the literature.
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Due to longer dynamical timescales, the outskirts of early-type galaxies retain the footprint of their formation and assembly. Under the popular two-phase galaxy formation scenario, an initial in-situ phase of star formation is followed by minor merging and accretion of ex-situ stars leading to the expectation of observable transitions in the kinematics and stellar populations on large scales. However, observing the faint galactic outskirts is challenging, often leaving the transition unexplored. The large scale, spatially-resolved stellar kinematic data from the SAGES Legacy Unifying Galaxies and GlobularS (SLUGGS) survey are ideal for detecting kinematic transitions. We present kinematic maps out to 2.6 effective radii on average, kinemetry profiles, measurement of kinematic twists and misalignments, and the average outer intrinsic shape of 25 SLUGGS galaxies. We find good overall agreement in the kinematic maps and kinemetry radial profiles with literature. We are able to confirm significant radial modulations in rotational versus pressure support of galaxies with radius so that the central and outer rotational properties may be quite different. We also test the suggestion that galaxies may be more triaxial in their outskirts and find that while fast rotating galaxies were already shown to be axisymmetric in their inner regions, we are unable to rule out triaxiality in their outskirts. We compare our derived outer kinematic information to model predictions from a two-phase galaxy formation scenario. We find that the theoretical range of local outer angular momentum agrees well with our observations, but that radial modulations are much smaller than predicted.
We study mass distributions within and beyond 5~effective radii ($R_{rm e}$) in 23 early-type galaxies from the SLUGGS survey, using their globular cluster (GC) kinematic data. The data are obtained with Keck/DEIMOS spectrograph, and consist of line-of-sight velocities for ~$3500$ GCs, measured with a high precision of ~15 $rm km s^{-1}$ per GC and extending out to $~13 R_{rm e}$. We obtain the mass distribution in each galaxy using the tracer mass estimator of Watkins et al. and account for kinematic substructures, rotation of the GC systems and galaxy flattening in our mass estimates. The observed scatter between our mass estimates and results from the literature is less than 0.2 dex. The dark matter fraction within $5R_{rm e}$ ($f_{rm DM}$) increases from ~$0.6$ to ~$0.8$ for low- and high-mass galaxies, respectively, with some intermediate-mass galaxies ($M_*{sim}10^{11}M_odot$) having low $f_{rm DM}sim0.3$, which appears at odds with predictions from simple galaxy models. We show that these results are independent of the adopted orbital anisotropy, stellar mass-to-light ratio, and the assumed slope of the gravitational potential. However, the low $f_{rm DM}$ in the ~$10^{11}M_odot$ galaxies agrees with the cosmological simulations of Wu et al. where the pristine dark matter distribution has been modified by baryons during the galaxy assembly process. We find hints that these $M_*sim10^{11}M_odot$ galaxies with low $f_{rm DM}$ have very diffuse dark matter haloes, implying that they assembled late. Beyond $5R_{rm e}$, the $M/L$ gradients are steeper in the more massive galaxies and shallower in both low and intermediate mass galaxies.
Early-type (E and S0) galaxies may have assembled via a variety of different evolutionary pathways. Here we investigate these pathways by comparing the stellar kinematic properties of 24 early-type galaxies from the SLUGGS survey with the hydrodynamical simulations of Naab et al. (2014). In particular, we use the kinematics of starlight up to 4 effective radii (R$_e$) as diagnostics of galaxy inner and outer regions, and assign each galaxy to one of six Naab et al. assembly classes. The majority of our galaxies (14/24) have kinematic characteristics that indicate an assembly history dominated by gradual gas dissipation and accretion of many gas-rich minor mergers. Three galaxies, all S0s, indicate that they have experienced gas-rich major mergers in their more recent past. One additional elliptical galaxy is tentatively associated with a gas-rich merger which results in a remnant galaxy with low angular momentum. Pathways dominated by gas-poor (major or minor) mergers dominate the mass growth of six galaxies. Most SLUGGS galaxies appear to have grown in mass (and size) via the accretion of stars and gas from minor mergers, with late major mergers playing a much smaller role. We find that the fraction of accreted stars correlates with the stellar mean age and metallicity gradient, but not with the slope of the total mass density profile. We briefly mention future observational and modelling approaches that will enhance our ability to accurately reconstruct the assembly histories of individual present day galaxies.
We investigate the kinematic properties of nine nearby early-type galaxies with evidence of a disk-like component. Three of these galaxies are located in the field, five in the group and only one in the cluster environment. By combining the kinematics of the stars with those of the globular clusters (GCs) and planetary nebulae (PNe), we probe the outer regions of our galaxies out to $sim$4-6 Re. Six galaxies have PNe and red GCs that show good kinematic alignment with the stars, whose rotation occurs along the photometric major-axis of the galaxies, suggesting that both the PNe and red GCs are good tracers of the underlying stellar population beyond that traced by the stars. Additionally, the blue GCs also show rotation that is overall consistent with that of the red GCs in these six galaxies. The remaining three galaxies show kinematic twists and misalignment of the PNe and GCs with respect to the underlying stars, suggesting recent galaxy interactions. From the comparison with simulations, we propose that all six aligned galaxies that show similar dispersion-dominated kinematics at large radii (>2-3 Re) had similar late ($z<1$) assembly histories characterised by mini mergers (mass-ratio <1:10). The different Vrot/$sigma$ profiles are then the result of an early ($z>1$) minor merger (1:10< mass-ratio <1:4) for the four galaxies with peaked and decreasing Vrot/$sigma$ profiles and of a late minor merger for the two galaxies with flat Vrot/$sigma$ profiles. The three mis-aligned galaxies likely formed through multiple late minor mergers that enhanced their velocity dispersion at all radii, or a late major merger that spun-up both the GC sub-populations at large radii. Therefore, lenticular galaxies can have complex merger histories that shape their characteristic kinematic profile shapes.
Here we present positions and radial velocities for over 4000 globular clusters (GCs) in 27 nearby early-type galaxies from the SLUGGS survey. The SLUGGS survey is designed to be representative of elliptical and lenticular galaxies in the stellar mass range 10 $<$ log M$_{ast}$/M$_{odot}$ $<$ 11.7. The data have been obtained over many years, mostly using the very stable multi-object spectrograph DEIMOS on the Keck II 10m telescope. Radial velocities are measured using the calcium triplet lines with a velocity accuracy of $pm$ 10-15 km/s. We use phase space diagrams (i.e. velocity--position diagrams) to identify contaminants such as foreground stars and background galaxies, and to show that the contribution of GCs from neighboring galaxies is generally insignificant. Likely ultra-compact dwarfs are tabulated separately. We find that the mean velocity of the GC system is close to that of the host galaxy systemic velocity, indicating that the GC system is in overall dynamical equilibrium within the galaxy potential. We also find that the GC system velocity dispersion scales with host galaxy stellar mass in a similar manner to the Faber-Jackson relation for the stellar velocity dispersion. Publication of these GC radial velocity catalogs should enable further studies in many areas, such as GC system substructure, kinematics, and host galaxy mass measurements.
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