arXiv subjects

Duncan A. Forbes

Publications and source records attributed to Duncan A. Forbes.

At least 163 records · Page 9Linked to original sources

arXiv2015

The SLUGGS Survey: Multi-population dynamical modelling of the elliptical galaxy NGC 1407 from stars and globular clusters

We perform in-depth dynamical modelling of the luminous and dark matter (DM) content of the elliptical galaxy NGC 1407. Our strategy consists of solving the spherical Jeans equations for three independent dynamical tracers: stars, blue GCs and red GCs in a self-consistent manner. We adopt a maximum-likelihood Markov-Chain Monte Carlo fitting technique in the attempt to constrain the inner slope of the DM density profile (the cusp/core problem), and the stellar initial mass function (IMF) of the galaxy. We find the inner logarithmic slope of the DM density profiles to be $γ= 0.6\pm0.4$, which is consistent with either a DM cusp ($γ= 1$) or with a DM core $(γ= 0)$. Our findings are consistent with a Salpeter IMF, and marginally consistent with a Kroupa IMF. We infer tangential orbits for the blue GCs, and radial anisotropy for red GCs and stars. The modelling results are consistent with the virial mass--concentration relation predicted by $Λ$CDM simulations. The virial mass of NGC 1407 is $\log$ $M_{\rm vir} = 13.3 \pm 0.2 M_{\odot}$, whereas the stellar mass is $\log M_* = 11.8 \pm 0.1 M_{\odot}$. The overall uncertainties on the mass of NGC 1407 are only 5 per cent at the projected stellar effective radius. We attribute the disagreement between our results and previous X-ray results to the gas not being in hydrostatic equilibrium in the central regions of the galaxy. The halo of NGC 1407 is found be DM dominated, with a dynamical mass-to-light ratio of $M/L=260_{-100} ^{+174} M_{\odot}/L_{\odot, B}$. However, this value can be larger up to a factor of 3 depending on the assumed prior on the DM scale radius.

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arXiv2015

Small scatter and nearly-isothermal mass profiles to four half-light radii from two-dimensional stellar dynamics of early-type galaxies

We study the total mass-density profile for a sample of 14 fast-rotator early-type galaxies (stellar masses $10.2<\log M_\ast/M_\odot<11.7$). We combine observations from the SLUGGS and Atlas3D surveys to map out the stellar kinematics in two-dimensions, out to a median radius for the sample of four half-light radii $R_e$ (or 10 kpc), and a maximum radius of 2.0-6.2 $R_e$ (or 4-21 kpc). We use axisymmetric dynamical models based on the Jeans equations, which allow for a spatially varying anisotropy, and employ quite general profiles for the dark halos, and in particular do not place any restriction on the profile slope. This is made possible by the availability of spatially extended two-dimensional kinematics. We find that our relatively simple models provide a remarkably good description of the observed kinematics. The resulting total density profiles are well described by a nearly-isothermal power law $ρ_{\rm tot}(r)\propto r^{-γ}$ from $R_e$/10 to at least 4$R_e$, the largest average deviation being 11%. The average logarithmic slope is $\langleγ\rangle=2.19\pm0.03$ with observed rms scatter of just $σ_γ=0.11$. This scatter out to large radii, where dark matter dominates, is as small as previously reported by lensing studies around $r\approx R_e/2$, where the stars dominate. Our bulge-halo conspiracy places much tighter constraints on galaxy formation models. It illustrates the power of two-dimensional stellar kinematics observations at large radii. It would now be important to test the generality of our results for different galaxy types and larger samples.

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arXiv2015

A SLUGGS and Gemini/GMOS combined study of the elliptical galaxy M60: wide-field photometry and kinematics of the globular cluster system

We present new wide-field photometry and spectroscopy of the globular clusters (GCs) around NGC 4649 (M60), the third brightest galaxy in the Virgo cluster. Imaging of NGC 4649 was assembled from a recently-obtained HST/ACS mosaic, and new Subaru/Suprime-Cam and archival CFHT/MegaCam data. About 1200 sources were followed up spectroscopically using combined observations from three multi-object spectrographs: Keck/DEIMOS, Gemini/GMOS and MMT/Hectospec. We confirm 431 unique GCs belonging to NGC 4649, a factor of 3.5 larger than previous datasets and with a factor of 3 improvement in velocity precision. We confirm significant GC colour bimodality and find that the red GCs are more centrally concentrated, while the blue GCs are more spatially extended. We infer negative GC colour gradients in the innermost 20 kpc and flat gradients out to large radii. Rotation is detected along the galaxy major axis for all tracers: blue GCs, red GCs, galaxy stars and planetary nebulae. We compare the observed properties of NGC 4649 with galaxy formation models. We find that formation via a major merger between two gas-poor galaxies, followed by satellite accretion, can consistently reproduce the observations of NGC 4649 at different radii. We find no strong evidence to support an interaction between NGC 4649 and the neighbouring spiral galaxy NGC 4647. We identify interesting GC kinematic features in our data, such as counter-rotating subgroups and bumpy kinematic profiles, which encode more clues about the formation history of NGC 4649.

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arXiv2015

How elevated is the dynamical-to-stellar mass ratio of the ultra-compact dwarf S999?

Here we present new Keck ESI high-resolution spectroscopy and deep archival HST/ACS imaging for S999, an ultra-compact dwarf in the vicinity of M87, which was claimed to have an extremely high dynamical-to-stellar mass ratio. Our data increase the total integration times by a factor of 5 and 60 for spectroscopy and imaging, respectively. This allows us to constrain the stellar population parameters for the first time (simple stellar population equivalent age $=7.6^{+2.0}_{-1.6}$ Gyr; $[Z/\textrm{H}]=-0.95^{+0.12}_{-0.10}$; $[α/\textrm{Fe}]=0.34^{+0.10}_{-0.12}$). Assuming a Kroupa stellar initial mass function, the stellar population parameters and luminosity ($M_{F814W}=-12.13\pm0.06$ mag) yield a stellar mass of $M_*=3.9^{+0.9}_{-0.6}\times10^6 M_{\odot}$, which we also find to be consistent with near-infrared data. Via mass modelling, with our new measurements of velocity dispersion ($σ_{ap}=27\pm2$ km s$^{-1}$) and size ($R_e=20.9\pm1.0$ pc), we obtain an elevated dynamical-to-stellar mass ratio $M_{dyn}/M_*=8.2$ (with a range $5.6\le M_{dyn}/M_* \le 11.2$). Furthermore, we analyse the surface brightness profile of S999, finding only a small excess of light in the outer parts with respect to the fitted Sérsic profile, and a positive colour gradient. Taken together these observations suggest that S999 is the remnant of a much larger galaxy that has been tidally stripped. If so, the observed elevated mass ratio may be caused by mechanisms related to the stripping process: the existence of an massive central black hole or internal kinematics that are out of equilibrium due to the stripping event. Given the observed dynamical-to-stellar mass ratio we suggest that S999 is an ideal candidate to search for the presence of an overly massive central black hole.

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arXiv2015

The stellar accretion origin of stellar population gradients in massive galaxies at large radii

We investigate the evolution of stellar population gradients from $z=2$ to $z=0$ in massive galaxies at large radii ($r > 2R_{\mathrm{eff}}$) using ten cosmological zoom simulations of halos with $6 \times 10^{12} M_{\odot} < M_{\mathrm{halo}} < 2 \times 10^{13}M_{\odot}$. The simulations follow metal cooling and enrichment from SNII, SNIa and AGB winds. We explore the differential impact of an empirical model for galactic winds that reproduces the mass-metallicity relation and its evolution with redshift. At larger radii the galaxies, for both models, become more dominated by stars accreted from satellite galaxies in major and minor mergers. In the wind model, fewer stars are accreted, but they are significantly more metal poor resulting in steep global metallicity ($\langle \nabla Z_{\mathrm{stars}} \rangle= -0.35$ dex/dex) and color (e.g. $\langle \nabla g-r \rangle = -0.13$ dex/dex) gradients in agreement with observations. In contrast, colour and metallicity gradients of the models without winds are inconsistent with observations. Age gradients are in general mildly positive at $z=0$ ($\langle \nabla Age_{\mathrm{stars}} \rangle= 0.04$ dex/dex) with significant differences between the models at higher redshift. We demonstrate that for the wind model, stellar accretion is steepening existing in-situ metallicity gradients by about 0.2 dex by the present day and helps to match observed gradients of massive early-type galaxies at large radii. Colour and metallicity gradients are significantly steeper for systems which have accreted stars in minor mergers, while galaxies with major mergers have relatively flat gradients, confirming previous results. This study highlights the importance of stellar accretion for stellar population properties of massive galaxies at large radii, which can provide important constraints for formation models.

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arXiv2014

VEGAS-SSS. A VST survey of elliptical galaxies in the southern hemisphere: analysis of small stellar systems. Testing the methodology on the globular cluster system in NGC3115

We present a study of globular clusters (GCs) and other small stellar systems (SSSs) in the field of NGC3115, observed as part of the VEGAS imaging survey, carried out with the VST telescope. We use deep g and i data of NGC3115, a well-studied lenticular galaxy with excellent scientific literature. This is fundamental to test the methodologies, verify the results, and probe the capabilities of the VEGAS-SSS. Leveraging the large field of view of the VST allow us to accurately study of the distribution and properties of SSSs as a function of galactocentric distance Rgc, well beyond ~20 galaxy effective radii, in a way not often possible. Our analysis of colors, magnitudes and sizes of SSS candidates confirms the results from existing studies, some of which carried out with 8-10m class telescopes, and further extends them to unreached Rgc distances, with similar accuracy. We find a color bimodality for the GC population and a r1/4 profile for the surface density of GCs as for the galaxy light profile. The radial color gradient of blue and red GCs previously found, for instance by the SLUGGS survey, is further extended out to the largest Rgc inspected, ~65 kpc. The surface density profiles of blue and red GCs taken separately are well approximated by a r1/4 density profile, with the fraction of blue GCs being slightly larger at larger Rgc. We do not find hints of a trend for the red GC and for the GC turnover magnitude to vary with radius, but we observe a ~0.2 mag difference in the turnover magnitude of the blue and red GCs subpopulations. Inspecting SSS sizes and colors we obtained a list of UCDs and GC candidates suitable for future spectroscopic follow-up. In conclusion, our study shows the reliability of the methodologies developed to study SSSs in the field of bright early-type galaxies, and the great potential of the VEGAS survey to produce original results on SSSs science.

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arXiv2014

The SLUGGS survey: Globular cluster stellar population trends from weak absorption lines in stacked spectra

As part of the SLUGGS survey, we stack 1137 Keck DEIMOS spectra of globular clusters from 10 galaxies to study their stellar populations in detail. The stacked spectra have median signal to noise ratios of $\sim 90$ Å$^{-1}$. Besides the calcium triplet, we study weaker sodium, magnesium, titanium and iron lines as well as the H$α$ and higher order Paschen hydrogen lines. In general, the stacked spectra are consistent with old ages and a Milky Way-like initial mass function. However, we see different metal line index strengths at fixed colour and magnitude, and differences in the calcium triplet--colour relation from galaxy to galaxy. We interpret this as strong evidence for variations in the globular cluster colour--metallicity relation between galaxies. Two possible explanations for the colour--metallicity relation variations are that the average ages of globular clusters vary from galaxy to galaxy or that the average abundances of light elements (i.e. He, C, N and O) differ between galaxies. Stacking spectra by magnitude, we see that the colours become redder and metal line indices stronger with brighter magnitudes. These trends are consistent with the previously reported `blue tilts' being mass--metallicity relations.

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arXiv2014

The SAGES Legacy Unifying Globulars and GalaxieS Survey (SLUGGS): Sample definition, methods, and initial results

We introduce and provide the scientific motivation for a wide-field photometric and spectroscopic chemodynamical survey of nearby early-type galaxies (ETGs) and their globular cluster (GC) systems. The SLUGGS (SAGES Legacy Unifying Globulars and GalaxieS) survey is being carried out primarily with Subaru/Suprime-Cam and Keck/DEIMOS. The former provides deep gri imaging over a 900 arcmin^2 field-of-view to characterize GC and host galaxy colors and spatial distributions, and to identify spectroscopic targets. The NIR CaII triplet provides GC line-of-sight velocities and metallicities out to typically ~8 R_e, and to ~15 R_e in some cases. New techniques to extract integrated stellar kinematics and metallicities to large radii (~2-3 R_e) are used in concert with GC data to create two-dimensional velocity and metallicity maps for comparison with simulations of galaxy formation. The advantages of SLUGGS compared with other, complementary, 2D-chemodynamical surveys are its superior velocity resolution, radial extent, and multiple halo tracers. We describe the sample of 25 nearby ETGs, the selection criteria for galaxies and GCs, the observing strategies, the data reduction techniques, and modeling methods. The survey observations are nearly complete and more than 30 papers have so far been published using SLUGGS data. Here we summarize some initial results, including signatures of two-phase galaxy assembly, evidence for GC metallicity bimodality, and a novel framework for the formation of extended star clusters and ultracompact dwarfs. An integrated overview of current chemodynamical constraints on GC systems points to separate, in-situ formation modes at high redshifts for metal-poor and metal-rich GCs.

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arXiv2014

The SLUGGS Survey: Wide-field Stellar Kinematics of Early-type Galaxies

We present stellar kinematics of 22 nearby early-type galaxies (ETGs), based on two-dimensional (2D) absorption line stellar spectroscopy out to ~2-4 R_e (effective radii), as part of the ongoing SLUGGS Survey. The galaxies span a factor of 20 in intrinsic luminosity, as well as a full range of environment and ETG morphology. Our data consist of good velocity resolution (sigma_inst ~ 25 km/s) integrated stellar-light spectra extracted from the individual slitlets of custom made Keck/DEIMOS slitmasks. We extract stellar kinematics measurements (V, sigma, h_3, and h_4) for each galaxy. Combining with literature values from smaller radii, we present 2D spatially resolved maps of the large-scale kinematic structure in each galaxy. We find that the kinematic homogeneity found inside 1 R_e often breaks down at larger radii, where a variety of kinematic behaviors are observed. While central slow rotators remain slowly rotating in their halos, central fast rotators show more diversity, ranging from rapidly increasing to rapidly declining specific angular momentum profiles in the outer regions. There are indications that the outer trends depend on morphological type, raising questions about the proposed unification of the elliptical and lenticular (S0) galaxy families in the ATLAS^3D survey. Several galaxies in our sample show multiple lines of evidence for distinct disk components embedded in more slowly rotating spheroids, and we suggest a joint photometric-kinematic approach for robust bulge-disk decomposition. Our observational results appear generally consistent with a picture of two-phase (in-situ plus accretion) galaxy formation.

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arXiv2014

The AIMSS Project II: Dynamical-to-Stellar Mass Ratios Across the Star Cluster - Galaxy Divide

The previously clear division between small galaxies and massive star clusters is now occupied by objects called ultra compact dwarfs (UCDs) and compact ellipticals (cEs). Here we combine a sample of UCDs and cEs with velocity dispersions from the AIMSS project with literature data to explore their dynamical-to-stellar mass ratios. We confirm that the mass ratios of many UCDs in the stellar mass range 10$^6$ -- 10$^9$ M$_{\odot}$ are systematically higher than those for globular clusters which have mass ratios near unity. However, at the very highest masses in our sample, i.e. 10$^9$ -- 10$^{10}$ M$_{\odot}$, we find that cE galaxies also have mass ratios of close to unity, indicating their central regions are mostly composed of stars. Suggested explanations for the elevated mass ratios of UCDs have included a variable IMF, a central black hole, and the presence of dark matter. Here we present another possible explanation, i.e. tidal stripping. Under various assumptions, we find that the apparent variation in the mass ratio with stellar mass and stellar density can be qualitatively reproduced by published tidal stripping simulations of a dwarf elliptical galaxy. In the early stages of the stripping process the galaxy is unlikely to be in virial equilibrium. At late stages, the final remnant resembles the properties of $\sim$10$^7$ M$_{\odot}$ UCDs. Finally, we discuss the need for more detailed realistic modelling of tidal stripping over a wider range of parameter space, and observations to further test the stripping hypothesis.

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arXiv2014

Dwarf galaxies in the Perseus Cluster: further evidence for a disc origin for dwarf ellipticals

We present the results of a Keck-ESI spectroscopic study of six dwarf elliptical (dE) galaxies in the Perseus Cluster core, and confirm two dwarfs as cluster members for the first time. All six dEs follow the size-magnitude relation for dE/dSph galaxies. Central velocity dispersions are measured for three Perseus dwarfs in our sample, and all lie on the $σ$-luminosity relation for early-type, pressure supported systems. We furthermore examine SA 0426-002, a unique dE in our sample with a bar-like morphology surrounded by low-surface brightness wings/lobes ($μ_{B} = 27$ mag arcsec$^{-2}$). Given its morphology, velocity dispersion ($σ_{0} = 33.9 \pm 6.1 $ km s$^{-1}$), velocity relative to the brightest cluster galaxy NGC 1275 (2711 km s$^{-1}$), size ($R_{e} =2.1 \pm 0.10$ kpc), and Sersic index ($n= 1.2 \pm 0.02$), we hypothesise the dwarf has morphologically transformed from a low mass disc to dE via harassment. The low-surface brightness lobes can be explained as a ring feature, with the bar formation triggered by tidal interactions via speed encounters with Perseus Cluster members. Alongside spiral structure found in dEs in Fornax and Virgo, SA 0426-002 provides crucial evidence that a fraction of bright dEs have a disc infall origin, and are not part of the primordial cluster population.

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arXiv2014

The AIMSS Project I: Bridging the Star Cluster - Galaxy Divide

We describe the structural and kinematic properties of the first compact stellar systems discovered by the AIMSS project. These spectroscopically confirmed objects have sizes ($\sim$6 $<$ R$_{\rm e}$ [pc] $<$ 500) and masses ($\sim$2$\times$10$^{6}$ $<$ M$_*$/M$_\odot$ $<$ 6$\times$10$^{9}$) spanning the range of massive globular clusters (GCs), ultra compact dwarfs (UCDs) and compact elliptical galaxies (cEs), completely filling the gap between star clusters and galaxies. Several objects are close analogues to the prototypical cE, M32. These objects, which are more massive than previously discovered UCDs of the same size, further call into question the existence of a tight mass-size trend for compact stellar systems, while simultaneously strengthening the case for a universal "zone of avoidance" for dynamically hot stellar systems in the mass-size plane. Overall, we argue that there are two classes of compact stellar systems: 1) massive star clusters and 2) a population closely related to galaxies. Our data provide indications for a further division of the galaxy-type UCD/cE population into two groups, one population that we associate with objects formed by the stripping of nucleated dwarf galaxies, and a second population that formed through the stripping of bulged galaxies or are lower-mass analogues of classical ellipticals. We find compact stellar systems around galaxies in low to high density environments, demonstrating that the physical processes responsible for forming them do not only operate in the densest clusters.

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arXiv2014

Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies

Both major galaxies in the Local Group host planar distributions of co-orbiting satellite galaxies, the Vast Polar Structure (VPOS) of the Milky Way and the Great Plane of Andromeda (GPoA). The $Λ$CDM cosmological model did not predict these features. However, according to three recent studies the properties of the GPoA and the flattening of the VPOS are common features among sub-halo based $Λ$CDM satellite systems, and the GPoA can be naturally explained by satellites being acquired along cold gas streams. We point out some methodological issues in these studies: either the selection of model satellites is different from that of the observed ones, or an incomplete set of observational constraints has been considered, or the observed satellite distribution is inconsistent with basic assumptions. Once these issues have been addressed, the conclusions are different: features like the VPOS and GPoA are very rare (each with probability $\lesssim 10^{-3}$, and combined probability $< 10^{-5}$) if satellites are selected from a $Λ$CDM simulation combined with semi-analytic modelling, and accretion along cold streams is no natural explanation of the GPoA. The origin of planar dwarf galaxy structures remains unexplained in the standard paradigm of galaxy formation.

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arXiv2014

The SLUGGS survey: Exploring the metallicity gradients of nearby early-type galaxies to large radii

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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arXiv2014

Ultra compact dwarfs in the Perseus Cluster: UCD formation via tidal stripping

We present the results of a Keck/DEIMOS survey of Ultra Compact Dwarfs (UCDs) in the Perseus Cluster core. We confirm cluster membership for 14 UCDs, with radial velocities ~5300 km s$^{-1}$. Two of these confirmed Perseus UCDs have extremely blue colours ($B-R < 0.6$ mag), reside in star forming filaments surrounding NGC 1275, and have likely formed as massive star clusters in the last ~100 Myr. We also measure a central velocity dispersion of a third, UCD13 ($σ_0 = 38 \pm 8$ km s$^{-1}$), the most extended UCD in our sample. We determine it to have radius $R_{e} = 85 \pm 1.1$ pc, a dynamical mass of ($2.3 \pm 0.8)\times10^{8}$ M$_{\odot}$, and a metallicity [Z/H]$= -0.52^{+0.33}_{-0.29}$ dex. UCD13 and the cluster's central galaxy, NGC 1275, have a projected separation of 30 kpc and a radial velocity difference of ~20 km s$^{-1}$. Based on its size, red colour, internal velocity dispersion, dynamical mass, metallicity and proximity to NGC 1275, we argue that UCD13 is likely the remnant nucleus of a tidally stripped dE, with this progenitor dE having $M_{B} \approx -16$ mag and mass $\sim10^{9}$ M$_{\odot}$.

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arXiv2014

The SLUGGS Survey: New evidence for a tidal interaction between the early type galaxies NGC 4365 and NGC 4342

We present new imaging and spectral data for globular clusters (GCs) around NGC 4365 and NGC 4342. NGC 4342 is a compact, X-ray luminous S0 galaxy with an unusually massive central black hole. NGC 4365 is another atypical galaxy that dominates the W' group of which NGC 4342 is a member. Using imaging from the MegaCam instrument on the Canada-France-Hawaii Telescope (CFHT) we identify a stream of GCs between the two galaxies and extending beyond NGC 4342. The stream of GCs is spatially coincident with a stream/plume of stars previously identified. We find that the photometric colours of the stream GCs match those associated with NGC 4342, and that the recession velocity of the combined GCs from the stream and NGC 4342 match the recession velocity for NGC 4342 itself. These results suggest that NGC 4342 is being stripped of GCs (and stars) as it undergoes a tidal interaction with the nearby elliptical galaxy NGC 4365. We compare NGC 4342 to two well-known, tidally stripped galaxies (M32 and NGC 4486B) and find various similarities. We also discuss previous claims by Bogdan et al. (2012a) that NGC 4342 cannot be undergoing significant tidal stripping because it hosts a large dark matter halo.

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arXiv2014

The SLUGGS Survey: Breaking degeneracies between dark matter, anisotropy and the IMF using globular cluster subpopulations in the giant elliptical NGC 5846

We study the mass and anisotropy distribution of the giant elliptical galaxy NGC 5846 using stars, as well as the red and blue globular cluster (GC) subpopulations. We break degeneracies in the dynamical models by taking advantage of the different phase space distributions of the two GC subpopulations to unambiguously constrain the mass of the galaxy and the anisotropy of the GC system. Red GCs show the same spatial distribution and behaviour as the starlight, whereas blue GCs have a shallower density profile, a larger velocity dispersion and a lower kurtosis, all of which suggest a different orbital distribution. We use a dispersion-kurtosis Jeans analysis and find that the solutions of separate analyses for the two GC subpopulations overlap in the halo parameter space. The solution converges on a massive dark matter halo, consistent with expectations from $Λ$CDM and WMAP7 cosmology in terms of virial mass ($\log M_{DM} \sim13.3 M_{sun}$) and concentration ($c_{vir}\sim8$). This is the first such analysis that solves the dynamics of the different GC subpopulations in a self-consistent manner. Our method improves the uncertainties on the halo parameter determination by a factor of two and opens new avenues for the use of elliptical galaxy dynamics as tests of predictions from cosmological simulations. The implied stellar mass-to-light ratio derived from the dynamical modelling is fully consistent with a Salpeter initial mass function (IMF) and rules out a bottom light IMF. The different GC subpopulations show markedly distinct orbital distributions at large radii, with red GCs having an anisotropy parameter $β\sim0.4$ outside $\sim3R_e$, and the blue GCs having $β\sim0.15$ at the same radii, while centrally ($\sim1R_e$) they are both isotropic. We discuss the implications of our findings within the two-phase formation scenario for early-type galaxies.

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arXiv2013

The SLUGGS Survey: The globular cluster systems of three early-type galaxies using wide-field imaging

We present the results from a wide-field imaging study of globular cluster (GC) systems in three early-type galaxies. Combinations of Subaru/Suprime-Cam, CFHT/MegaCam and HST/WFPC2/ACS data were used to determine the GC system properties of three highly flattened galaxies NGC 720, NGC 1023 and NGC 2768. This work is the first investigation of the GC system in NGC 720 and NGC 2768 to very large galactocentric radius ($\sim$ 100 kpc). The three galaxies have clear blue and red GC subpopulations. The radial surface densities of the GC systems are fitted with Sersic profiles, and detected out to 15, 8 and 10 galaxy effective radii respectively. The total number of GCs and specific frequency are determined for each GC system. The ellipticity of the red subpopulation is in better agreement with the host galaxy properties than is the blue subpopulation, supporting the traditional view that metal-rich GCs are closely associated with the bulk of their host galaxies' field stars, while metal-poor GCs reflect a distinct stellar halo. With the addition of another 37 literature studied galaxies, we present a new correlation of GC system extent with host galaxy effective radius. We find a dependence of the relative fraction of blue to red GCs with host galaxy environmental density for lenticular galaxies (but not for elliptical or spiral galaxies). We propose that tidal interactions between galaxies in cluster environments might be the reason behind the observed trend for lenticular galaxies.

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