Juan Carlos Forte

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 connection between globular cluster systems and the host galaxies

ABSTRA C T A large number of early-type galaxies are now known to possess blue and red subpopulations of globular clusters. We have compiled a data base of 28 such galaxies exhibiting bimodal globular cluster colour distributions. After converting to a common V‐I colour system, we investigate correlations between the mean colour of the blue and red subpopulations with galaxy velocity dispersion. We support previous claims that the mean colours of the blue globular clusters are unrelated to their host galaxy. They must have formed rather independently of the galaxy potential they now inhabit. The mean blue colour is similar to that for halo globular clusters in our Galaxy and M31. The red globular clusters, on the other hand, reveal a strong correlation with galaxy velocity dispersion. Furthermore, in well-studied galaxies the red subpopulation has similar, and possibly identical, colours to the galaxy halo stars. Our results indicate an intimate link between the red globular clusters and the host galaxy; they share a common formation history. A natural explanation for these trends would be the formation of the red globular clusters during galaxy collapse.

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.

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.

The metallicity gradient and distribution function of globular clusters around NGC 1399

We have obtained integrated Washington photometry (C,M, T 1 ) of globular cluster candidates in the field of NGC 1399. This galaxy, the dominant object in the Fornax cluster, shares with NGC 4486 (M87) the property of possessing one of the largest known specific frequencies of globulars. Conflicting evidence on the presence of a B−V color gradient as a function of galactocentric distance has been reported for this cluster system. Here we investigate the existence of a gradient using the C−T 1 index, which is twice as metallicity sensitive as B−V. A small gradient is detected. This color gradient, in the sense that outer clusters are bluer, amounts to −0.1 mag in C−T 1 over a galactocentric range from about 0.5 to 3.5 arcmin, corresponding to a difference of ∼0.2 dex in metallicity

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.

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.

Globular cluster candidates within the Fornax cluster: Intracluster globulars?

We present the results of a search for globular clusters in the surroundings of 15 low surface brightness dwarf galaxies belonging to the Fornax Cluster, which was carried out on CCD images obtained with the C and T 1 filters of the Washington photometric system. The globular cluster candidates show an extended and probably bimodal (C - T 1 ) color distribution, which is inconsistent with the presence of a single population of metal-poor clusters detected in several dwarf galaxies. The surface number density of these candidates shows no concentration towards the respective dwarf galaxies, in whose outskirts they have been identified. On the contrary, if we split the candidates in two groups according to their projected distances to the center of the Fornax Cluster, those located closer to the center show a higher projected density than those located farther from it. These results suggest that the potential globular clusters might not be bound to the dwarf galaxies. Alternatively, these globulars could form part of the very peripheral regions of NGC 1399 (the central galaxy of the Fornax Cluster) or even belong to the intracluster medium.