Favio Raul Faifer

Large-scale study of the NGC 1399 globular cluster system in Fornax

We present a Washington C and Kron-Cousins R photometric study of the globular cluster system of NGC 1399, the central galaxy of the Fornax cluster. A large areal coverage of 1 square degree around NGC 1399 is achieved with three adjoining fields of the MOSAIC II Imager at the CTIO 4-m telesc ope. Working on such a large field, we can perform the first indicative determination of the total size of the NGC 1399 globular cluster system. The estimated angular extent, measured from the NGC 1399 centre and up to a limiting radius where the areal density of blue globular clusters falls to 30 per cent of the background level, is 45± 5 arcmin, which corresponds to 220 - 275 kpc at the Fornax distance. The bimodal colour distribution of this globular cluster system, as well as the different radial distribution of blue and red clusters, up to the se large distances from the parent galaxy, are confirmed. The azimuth al globular cluster distribution exhibits asymmetries tha t might be understood in terms of tidal stripping of globulars from NGC 1387, a nearby galaxy. The good agreement between the areal density profile of blue clusters and a projected dark-matter NFW density profile is emphasized.

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 Archive of Intermediate Mass Stellar Systems project. These spectroscopically confirmed objects have sizes (∼6 < Re [pc] < 500) and masses (∼2 × 106 < M*/M⊙ < 6 × 109) spanning the range of massive globular clusters, ultracompact 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.

Gemini/GMOS imaging of globular cluster systems in five early-type galaxies★

This paper presents deep high quality photometry of globular cluster (GC) systems belonging to five early-type galaxies covering a range of mass and en vironment. Photometric data were obtained with the Gemini North and Gemini South telescopes in the filter passbands g ′ , r ′ , and i ′ . The combination of these filters with good seeing condition s allows an excellent separation between GC candidates and unresolved field o bjects. In fact, our previously published spectroscopic data indicates a contamination level of only �10 percent in our sample of GC candidates. Bimodal GC colour distributions are found in all five galaxies. Most of the GC systems appear bimodal even in the (g ′ -r ′ ) vs (r ′ -i ′ ) plane. A population of resolved/marginally resolved GC and Ultra Compact Dwarf candidates was found in all the galaxies. A search for the so-called “blue tilt” in the colou r-magnitude diagrams reveals that NGC 4649 clearly shows that phenomenon although no conclusive evidence was found for the other galaxies in the sample. This “blue tilt” translate s into a mass-metallicity relation given by Z / M 0.28±0.03 . This dependence was found using a new empirical (g ′ -i ′ ) vs [Z/H] relation which relies on an homogeneous sample of GC colours and metallicities. This paper also explores the radial trends in both colour and surface density for the blue (metal-poor) and red (metal-rich) GC subpopulations. As usual, the red GCs show a steeper radial distribution than the blue ones. Evidence of galactocentric colour gradients is found in some of the GC systems, being more significant for the two S0 galaxies in the sample. Red GC subpopulations show similar colours and gradients to the galaxy halo stars in their inner region. A GC mean colour-galaxy luminosity relation, consistent with [Z/H]/ LB 0.26±0.08 , is present for the red GCs. An estimate of the total GC populations and specific freq uency SN values is presented for NGC 3115, NGC 3923 and NGC 4649.

A quantitative link between globular clusters and the stellar haloes in elliptical galaxies

This paper explores the quantitative connection between globular clusters and the ‘diffuse’ stellar population of the galaxies they are associated with. Both NGC 1399 and NGC 4486 (M87) are well suited for this kind of analysis due to their large globular cluster populations. n n n nThe main assumption of our Monte Carlo based models is that each globular cluster is formed along with a given diffuse stellar mass that shares the same spatial distribution, chemical composition and age. The main globular cluster subpopulations, that determine the observed bimodal colour distribution, are decomposed avoiding a priori parametric (e.g. Gaussian) fits and using a new colour (C−T1)–metallicity relation. The eventual detectability of a ‘blue’ tilt in the colour–magnitude diagrams of the blue globular cluster subpopulation is also addressed. n n n nA successful link between globular clusters and the stellar galaxy halo is established by assuming that the number of globular clusters per associated diffuse stellar mass t is a function of total abundance [Z/H] and behaves as t=γ exp(−δ[Z/H]) (i.e. increases when abundance decreases). n n n nThe simulations allow the prediction of a surface brightness profile for each galaxy through these two free parameters approximation. The γ, δ parameters that provide the best fit to the observed profiles in the B band, in turn, determine several features, namely, large-scale halo colour gradients, globular cluster–halo colour offset, clusters cumulative specific frequencies, and stellar metallicity distributions, that compare well with observations. n n n nThe results suggest the co-existence of two distinct stellar populations characterized by widely different metallicities and spatial distributions. One of these populations (connected with the blue globular clusters) is metal poor, highly homogeneous, exhibits an extended spatial distribution and becomes more evident at large galactocentric radius contributing with some 20 per cent of the total stellar mass. In turn, the stellar population associated with the red globular clusters is extremely heterogeneous and dominates the inner region of both galaxies. n n n nRemarkably, and although the cluster populations of these galaxies exhibit detectable differences in colour distribution, the δ parameter that determines the shape of the brightness profiles of both galaxies has the same value, δ≈ 1.1 to 1.2 ± 0.1.

Gemini/GMOS Spectroscopy of the Spheroid and Globular Cluster System of NGC 3923

We present a technique to extract ultra-deep diuse-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 diuse 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/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 nd the GCs to have old ages > 10 Gyr, [ /Fe] 0.3 and a range of metallicities running from [Z/H] = -1.8 to +0.35. The diuse light of the galaxy is found to have ages, metallicities and [ /Fe] abundance ratios indistinguishable from those of the red GCs.

Gemini/GMOS spectra of globular clusters in the Leo group elliptical NGC 3379

The Leo group elliptical NGC 3379 is one of the few normal elliptical galaxies close enough to make possible observations of resolved stellar populations, deep globular cluster (GC) photometry and high signal-to-noise ratio GC spectra. We have obtained Gemini/GMOS spectra for 22 GCs associated with NGC 3379. We derive ages, metallicities and α-element abundance ratios from simple stellar population models using the recent multi-index χ 2 minimization method of Proctor & Sansom. All of these GCs are found to be consistent with old ages, i.e. 10 Gyr, with a wide range of metallicities. This is comparable to the ages and metallicities that Gregg et al. found a couple of years ago for resolved stellar populations in the outer regions of this elliptical. A trend of decreasing α-element abundance ratio with increasing metallicity is indicated. The projected velocity dispersion of the GC system is consistent with being constant with radius. Non-parametric, isotropic models require a significant increase in the mass-to-light ratio at large radii. This result is in contrast to that of Romanowsky et al., who recently found a decrease in the velocity dispersion profile as determined from planetary nebulae (PN). Our constant dispersion requires a normal-sized dark halo, although without anisotropic models we cannot rigorously determine the dark halo mass. A two-sided χ 2 test over all radii gives a 2σ difference between the mass profile derived from our GCs compared to the PN-derived mass model of Romanowsky et al. However, if we restrict our analysis to radii beyond one effective radius and test if the GC velocity dispersion is consistently higher, we determine a > 3σ difference between the mass models, and hence we favour the conclusion that NGC 3379 does indeed have dark matter at large radii in its halo.

Globular clusters as tracers of stellar bimodality in elliptical galaxies: the case of NGC 1399

Globular cluster systems (GCSs) frequently show a bimodal distribution of cluster integrated colours. This work explores the arguments to support the idea that the same feature is shared by the diffuse stellar population of the galaxy they are associated with. The particular case of NGC 1399, one of the dominant central galaxies in the Fornax cluster, for which a new B surface brightness profile and (B - R KC ) colours are presented, is discussed taking advantage of a recently published wide-field study of its GCS. The results show that the galaxy brightness profile and colour gradient, as well as the behaviour of the cumulative globular cluster specific frequency, are compatible with the presence of two dominant stellar populations, associated with the so-called blue and red globular cluster families. These globular families are characterized by different intrinsic specific frequencies (defined in terms of each stellar population): S n = 3.3 ± 0.3 in the case of the red globulars and S n = 14.3 ± 2.5 for the blue ones. We stress that this result does not necessarily conflict with recent works that point out a clear difference between the metallicity distribution of (resolved) halo stars and globulars when comparing their number statistics. The region within 0.5 arcmin of the centre shows a deviation from the model profile (in both surface brightness and colour) that may be explained in terms of the presence of a bulge-like high-metallicity component. Otherwise, the model gives an excellent fit up to 12 arcmin (or 66.5 Kpc) from the centre, the galactocentric limit of our blue brightness profile. The inferred specific frequencies imply that, in terms of their associated stellar populations, the formation of the blue globulars took place with an efficiency about six times higher than that corresponding to their red counterparts. The similarity of the spatial distribution of the blue globulars with that inferred for dark matter, as well as with that of the X-ray-emitting hot gas associated with NGC 1399, is emphasized. The impact of a relatively inconspicuous low-metallicity population, that shares the properties of the blue globulars, as a possible source of chemical enrichment early in the formation history of the galaxy is also briefly discussed.

Gemini/GMOS imaging of globular clusters in the Virgo galaxy NGC 4649 (M60)

We present Sloan g and i imaging from the Gemini Multi-object Spectrograph (GMOS) instrument on the Gemini North telescope for the globular cluster (GC) system around the Virgo galaxy NGC 4649 (M60). Our three pointings, taken in good seeing conditions, cover an area of about 90 square arcmin. We detect 2151 unresolved sources. Applying colour and magnitude selection criteria to this source list gives 995 candidate GCs. Our source list is greater than 90 per cent complete to a magnitude of i = 23.6, and has little contamination from background galaxies. We find fewer than half a dozen potential ultracompact dwarf galaxies around NGC 4649, Foreground extinction from the nearby spiral NGC 4647 is limited to be A V < 0.1. We confirm the bimodality in the GC colour distribution found by earlier work using Hubble Space Telescope/WFPC2 imaging. As is commonly seen in other galaxies, the red GCs are concentrated towards the centre of the galaxy, having a steeper number density profile than the blue GC subpopulation. The varying ratio of red-to-blue GCs with radius can largely explain the overall GC system colour gradient. The underlying galaxy starlight has a similar density profile slope and colour to the red GCs. This suggests a direct connection between the galaxy field stars and the red GC subpopulation. We estimate a total GC population of 3700 ± 900, with the uncertainty dominated by the extrapolation to larger radii than observed. This total number corresponds to a specific frequency S N = 4.1 ± 1.0. Future work will present properties derived from GMOS spectra of the NGC 4649 GCs.

Gemini/GMOS spectra of globular clusters in the Virgo giant elliptical NGC 4649

NGC 4649 (M60) is one of a handful of giant Virgo ellipticals. We have obtained Gemini/GMOS (Gemini North Multi-Object Spectrograph) spectra for 38 globular clusters (GCs) associated with this galaxy. Applying the multi-index χ2 minimization technique of Proctor and Sansom with the single stellar population models of Thomas, Maraston and Korn, we derive ages, metallicities and α-element abundance ratios. We find several young (2–3 Gyr old) supersolar metallicity GCs, while the majority are old (>10 Gyr), spanning a range of metallicities from solar to [Z/H]=−2. At least two of these young GCs are at large projected radii of 17–20 kpc. The galaxy itself shows no obvious signs of a recent starburst, interaction or merger. A trend of decreasing α-element ratio with increasing metallicity is found.

The globular cluster kinematics and galaxy dark matter content of NGC 3923

From observations with the GMOS multi-slit spectrograph on the Gemini North telescope, we have obtained spectra for 39 globular cluster candidates in the Virgo giant elliptical galaxy NGC 4649 (M60), of which 38 are confirmed globular clusters. The clusters extend out to a radius of 260 ′′ (3.5 effective radii). We find no rotation of the globular cluster system, with an upper limit of v/σ < 0.6 at a confidence level of 95%. The globular cluster velocity dispersion is constant with radius, within the uncertainties. We fit isotropic models to the globular cluster and stellar kinematics; these models yield a M/LV around 16 at 200 ′′ radius (16 kpc), an increase of a factor of two from the central M/L. We also use the mass profile as derived from X-rays to determine the orbital structure. Using axisymmetric orbit-based models and the X-ray mass profile, we find the orbital distribution is close to isotropic within 100 ′′ , and becomes tangentially biased beyond. Furthermore, when using the X-ray profile, we find a better fit to the kinematics compared to using a constant M/L model. Thus, both isotropic and axisymmetric orbit-based models give support for the presence of a dark matter halo in NGC 4649.