arXiv subjects

Duncan A. Forbes

Publications and source records attributed to Duncan A. Forbes.

At least 235 records · Page 13Linked to original sources

arXiv2006

The Group Evolution Multiwavelength Study (GEMS): the Sample and Datasets

Galaxy groups have been under-studied relative to their richer counterparts -- clusters. The Group Evolution Multiwavelength Study (GEMS) aims to redress some the balance. Here we describe the GEMS sample selection and resulting sample of 60 nearby (distance < 130 Mpc) galaxy groups and our multiwavelength dataset of X-ray, optical and HI imaging. ROSAT X-ray images of each group are presented. GEMS also utilizes near-infrared imaging from the 2MASS survey and optical spectra from the 6dFGS. These observational data are complemented by mock group catalogues generated from the latest LCDM simulations with gas physics included. Existing GEMS publications are briefly highlighted as are future publication plans.

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arXiv2006

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 spectra for 38 GCs associated with this galaxy. Applying the multi-index chi^2 minimisation technique of Proctor & Sansom (2002) with the single stellar population models of Thomas, Maraston & Korn (2004) we derive ages, metallicities and alpha-element abundance ratios. We find several young (2--3 Gyr old) super-solar metallicity GCs, while the majority are old (>10 Gyrs), 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 alpha-element ratio with increasing metallicity is found.

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arXiv2005

Gemini/GMOS Spectra of Globular Clusters in the Leo Group Elliptical NGC 3379

We have obtained Gemini/GMOS spectra for 22 GCs associated with NGC 3379. We derive ages, metallicities and alpha-element abundance ratios from simple stellar population models using the multi-index chi^2 minimisation method of Proctor & Sansom (2002). All of these GCs are found to be consistent with old ages, i.e. >10 Gyr, with a wide range of metallicities. A trend of decreasing alpha-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. (2003) who find a decrease in the velocity dispersion profile as determined from planetary nebulae. Our constant dispersion requires a normal sized dark halo, although without anisotropic models we cannot rigorously determine the dark halo mass. A two-sided chi^2 test over all radii, gives a 2 sigma difference between the mass profile derived from our GCs compared to the PN-derived mass model of Romanowsky et al. (2003). 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 >3 sigma difference between the mass models, and hence favor the conclusion that NGC 3379 does indeed have dark matter at large radii in its halo. (abridged)

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arXiv2005

Bimodal Galaxies and Bimodality in Globular Cluster Systems

Various galaxy properties are not continuous over a large range in mass, but rather reveal a remarkable transition or `bimodality' at a stellar mass of 3 x 10^{10} Mo. These properties include colors, stellar populations, Xray emission and mass-to-light ratios. This behavior has been interpreted as the transition from hot to cold flows by Dekel & Birnboim (2005). Here we explore whether globular cluster (GC) systems also reveal a bimodal nature with regard to this critical mass scale. Globular clusters probe star formation at early epochs in the Universe and survive subsequent galaxy mergers and accretions. We use new data from the ACS Virgo Cluster Survey (Peng etal 2005), which provides a homogeneous sample of the GC systems around one hundred Virgo early-type galaxies covering a range of five hundred in galaxy mass. Their classification of the GC color distributions is taken to examine a key quantity -- the number of GCs per unit galaxy luminosity. Below the critical mass, this quantity (called the GC specific frequency) increases dramatically in its mean value and spread. This increase may be due to regulated star formation in low mass galaxies, which in turn is due to mass loss via winds and the transition from hot to cold gas accretion flows. We also note that above the critical mass, galaxies possess two GC subpopulations (with blue and red mean colors) but below this mass, galaxies reveal an increasing proportion of single (blue) GC systems.

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arXiv2005

Galaxy Groups: Proceedings from a Swinburne University Workshop

We present the proceedings from a 2-day workshop held at Swinburne University on the 24th-25th of May 2005. The workshop participants highlighted current Australian research on both theoretical and observational aspects of galaxy groups. These proceedings include short 1-page summaries of a number of the talks presented at the workshop. The talks presented ranged from reconciling N-body simulations with observations, to the HI content of galaxies in groups and the existence of ``dark galaxies''. The formation and existence of ultra-compact dwarfs in groups, and a new supergroup in Eridanus were also discussed.

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arXiv2005

On the structure of globular cluster systems in elliptical galaxies

It has long been known that the radial density profiles of globular cluster systems (GCSs) in elliptical galaxies vary with the total luminosities of their host galaxies. In order to elucidate the origin of this structural non-homology in GCSs, we numerically investigate the structural properties of GCSs in elliptical galaxies formed from a sequence of major dissipationless galaxy merging. We find that the radial density profiles of GCSs in elliptical galaxies become progressively flatter as the galaxies experience more major merger events. The density profiles of GCSs in ellipticals are well described as power-laws with slopes ($α_{\rm gc}$) ranging from -2.0 to -1.0. They are flatter than, and linearly proportional to, the slopes ($α_{\rm s}$) of the stellar density profiles of their host galaxies. We also find that the GCS core radii ($r_{\rm c}$) of the density profiles are larger in ellipticals that experienced more mergers. By applying a reasonable scaling relation between luminosities and sizes of galaxies to the simulation results, we show that $α_{\rm gc} \approx -0.36 M_{\rm V}-9.2$, $r_{\rm c} \approx -1.85 M_{\rm V}$, and $α_{\rm gc} \approx 0.93 α_{\rm s}$, where $M_{\rm V}$ is the total $V$-band absolute magnitude of a galaxy.

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arXiv2005

Kinematics of globular cluster systems and the formation of early-type galaxies

We numerically investigate the kinematic properties of globular cluster systems (GCSs) in E/S0 galaxies formed from dissipationless merging of spiral galaxies. The metal-poor globular clusters (MPCs) and metal-rich clusters (MRCs) in the merger progenitors are initially assumed to have spatial distributions consistent with the Milky Way GC system. Our principal results, which can be tested against observations, are as follows. Both MPCs and MRCs in elliptical galaxies formed from major mergers can exhibit significant rotation at large radii ($\sim$20 kpc) due to the conversion of initial orbital angular momentum into intrinsic angular momentum of the remnant. MPCs show higher central velocity dispersions than MRCs for most major merger models. $V_{\rm m}/σ_{0}$ (where $V_{\rm m}$ and $σ_{0}$, are the GCS maximum rotational velocity and central velocity dispersion of respectively) ranges from 0.2--1.0 and 0.1--0.9 for the MPCs and MRCs respectively, within $6R_{\rm e}$ for the remnant elliptical. For most merger remnant ellipticals, $V_{\rm m}/σ_{0}$ of GCSs within $6R_{\rm e}$ is greater than that of the field stars within $2R_{\rm e}$. The radial profiles of rotational velocities and velocity dispersions of the GCSs depend upon the orbital configuration of the merger progenitors, their mass-ratios, and the viewing angle. For example, more flattened early-type galaxies, formed through mergers with small mass ratios ($\sim$ 0.1), show little rotation in the outer MRCs. Two-dimensional (2D) velocity dispersion distributions of the GCSs of merger remnant ellipticals are generally flattened for both MPCs and MRCs, reflecting the fact that the GCSs have anisotropic velocity dispersions (abridged).

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arXiv2005

The Correlation of Metallicity Gradient with Galaxy Mass

A number of previous studies have searched for a correlation between radial metallicity gradients and early-type galaxy mass -- no convincing trends have been found. Here we re-examine this issue with several key enhancements: using total metallicity from studies that have broken the age-metallicity degeneracy, excluding galaxies with young stellar ages (i.e. tho se that have experienced a recent central starburst) and using the K-band to derive galaxy luminosities. We find that Coma cluster galaxies have metallicity gradients which correlate with galaxy mass. Furthermore, gradients have values similar to those of monolithic collapse models. The combination of dissipative formation and energy injection from supernova provides a mechanism for the trends with galaxy mass, however other explanations are possible. Additional high quality observational data is needed to further constrain the gas physics involved in galaxy formation.

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arXiv2005

Spatially resolved stellar populations in the isolated elliptical NGC 821

We present the analysis of Lick absorption-line indices from three separate long-slit spectroscopic observations of the nearby isolated elliptical galaxy NGC 821. The three data sets present a consistent picture of the stellar population within one effective radius, in which strong gradients are evident in both luminosity-weighted age and metallicity. The central population exhibits a young age of ~4 Gyr and a metallicity ~3 times solar. At one effective radius the age has risen to ~12 Gyr and the metallicity fallen to less than 1/3 solar. The low metallicity population around one effective radius appears to have an exclusively red horizontal branch, with no significant contribution from the blue horizontal branch evident in some globular clusters of the same age and metallicity. Despite the strong central age gradient, we demonstrate that only a small fraction (<10%) of the galaxy's stellar mass can have been created in recent star formation events. We consider possible star formation histories for NGC 821 and find that the most likely cause of the young central population was a minor merger or tidal interaction that caused NGC 821 to consume its own gas in a centrally concentrated burst of star formation 1 to 4 Gyr ago.

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arXiv2005

Extragalactic Globular Clusters: Old Spectroscopic Ages and New Views on Their Formation

We present the results of a meta-analysis of Keck spectra of extragalactic globular clusters (GCs) in a sample of eight galaxies, ranging from dwarfs to massive ellipticals. We infer ages for the metal-poor and metal-rich GCs in these galaxies through comparisons to Galactic GCs. Both subpopulations appear to be no younger than their Galactic counterparts, with ages ~> 10 Gyr. This is the largest sample of galaxies for which ages have been constrained spectroscopically. Our results support the formation of most GCs in massive galaxies at high redshift. We propose a scenario for the formation of GC subpopulations that synthesizes aspects of both accretion and in situ approaches in the context of galaxy formation through hierarchical merging.

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arXiv2005

The fundamental plane of isolated early-type galaxies

Here we present new measurements of effective radii, surface brightnesses and internal velocity dispersions for 23 isolated early-type galaxies. The photometric properties are derived from new multi-colour imaging of 10 galaxies, whereas the central kinematics for 7 galaxies are taken from forthcoming work by Hau & Forbes. These are supplemented with data from the literature. We reproduce the colour-magnitude and Kormendy relations and strengthen the result of Paper I that isolated galaxies follow the same photometric relations as galaxies in high density environments. We also find that some isolated galaxies reveal fine structure indicative of a recent merger while others appear undisturbed. We examine the Fundamental Plane in both traditional R_e, mu_e and sigma space and also kappa-space. Most isolated galaxies follow the same Fundamental Plane tilt and scatter for galaxies in high density environments. However, a few galaxies notably deviate from the plane in the sense of having smaller M/L ratios. This can be understood in terms of their younger stellar populations, which are presumably induced by a gaseous merger. Overall, isolated galaxies have similar properties to those in roups and clusters with a slight enhancement in the frequency of recent mergers/interactions.

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arXiv2005

Old Globular Clusters Masquerading as Young in NGC 4365?

High signal-to-noise, low-resolution spectra have been obtained for 22 globular clusters (GCs) in NGC 4365. Some of these were selected as probable representatives of an intermediate-age (2-5 Gyr), extremely metal-rich GC subpopulation. The presence of such a subpopulation had been inferred from the unusual optical and near-IR color distributions of GCs in this otherwise typical Virgo elliptical galaxy. However, ages derived from Lick indices are consistent with uniformly old mean ages for all GCs in our sample. The metallicities of the clusters show some evidence for a trimodal distribution. The most metal-poor and metal-rich peaks are consistent with the values expected for an elliptical galaxy of this luminosity, but there appears to be an additional, intermediate-metallicity peak lying between them. New Hubble Space Telescope photometry is consistent with this result. A plausible scenario is that in earlier data these three peaks merged into a single broad distribution. Our results suggest that it is difficult to identify intermediate-age GC subpopulations solely with photometry, even when both optical and near-infrared colors are used.

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arXiv2005

The Evolutionary History of the Elliptical Galaxy NGC 1052

We have obtained Keck spectra for 16 globular clusters (GCs) associated with the merger remnant elliptical NGC 1052, as well as a long-slit spectrum of the galaxy. We derive ages, metallicities and abundance ratios from simple stellar population models using the methods of Proctor & Sansom (2002), applied to extragalactic GCs for the first time. We find all of the GCs to be ~13 Gyr old according to simple stellar populations, with a large range of metallicities. From the galaxy spectrum we find NGC 1052 to have a luminosity-weighted central age of ~2 Gyr and metallicity of [Fe/H]~+0.6. No strong gradients in either age or metallicity were found to the maximum radius measured (~1 kpc). However, we do find a strong radial gradient in alpha-element abundance, which reaches a very high central value. The young central starburst age is consistent with the age inferred from the HI tidal tails and infalling gas of \~1 Gyr. Thus, although NGC 1052 shows substantial evidence for a recent merger and an associated starburst, it appears that the merger did not induce the formation of new GCs, perhaps suggesting that little recent star formation occurred. This interpretation is consistent with ``frosting'' models for early-type galaxy formation. (Abridged)

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arXiv2004

Group, field and isolated early-type galaxies II. Global trends from nuclear data

We have derived ages, metallicities and enhanced-element ratios [alpha/Fe] for a sample of 83 early-type galaxies essentially in groups, the field or isolated objects. The stellar population properties derived for each galaxy corresponds to the nuclear r_e/8 aperture extraction. The median age found for Es is 5.8 +- 0.6 Gyr and the average metallicity is +0.37 +- 0.03 dex. For S0s, the median age is 3.0 +- 0.6 Gyr and [Z/H] = 0.53 +- 0.04 dex. We compare the distribution of our galaxies in the Hbeta-[MgFe] diagram with Fornax galaxies. Our elliptical galaxies are 3-4 Gyr younger than Es in the Fornax cluster. We find that the galaxies lie in a plane defined by [Z/H] = 0.99 log sigma_0 - 0.46 log Age - 1.60. More massive (larger sigma_0) and older galaxies present, on average, large [alpha/Fe] values, and therefore, must have undergone shorter star-formation timescales. Comparing group against field/isolated galaxies, it is not clear that environment plays an important role in determining their stellar population history. In particular, our isolated galaxies show ages differing in more than 8 Gyr. Finally we explore our large spectral coverage to derive log(O/H) metallicity from the N2 indicator and compare it with model-dependent [Z/H]. We find that the O/H abundances are similar for all galaxies, and we can interpret it as if most chemical evolution has already finished in them.

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arXiv2004

The Chemical Properties of Milky Way and M31 Globular Clusters: II. Stellar Population Model Predictions

We derive ages, metallicities and [alpha/Fe] ratios from the integrated spectra of 23 globular clusters in M31, by employing multivariate fits to two stellar population models. In parallel we analyze spectra of 21 Galactic globular clusters in order to facilitate a differential analysis. We find that the M31 globular clusters separate into three distinct components in age and metallicity. We identify an old, metal-poor group (7 clusters), an old, metal-rich group (10 clusters) and an intermediate age (3-6 Gyr), intermediate-metallicity ([Z/H]~-1) group (6 clusters). This third group is not identified in the Galactic globular cluster sample. The majority of globular clusters in both samples appear to be enhanced in alpha-elements, the degree of enhancement being model-dependent. The intermediate age GCs appear to be the most enhanced, with [alpha/Fe]~0.4. These clusters are clearly depressed in CN with respect to the models and the bulk of the M31 and Milky Way sample. Compared to the bulge of M31, M32 and NGC 205, these clusters most resemble the stellar populations in NGC 205 in terms of age, metallicity and CN abundance. We infer horizontal branch morphologies for the M31 clusters using the Rose (1984) Ca II index, and demonstrate that blue horizontal branches are not leading to erroneous age estimates in our analysis. The intermediate age clusters have generally higher velocities than the bulk of the M31 cluster population. Spatially, three of these clusters are projected onto the bulge region, the remaining three are distributed at large radii. We discuss these objects within the context of the build-up of the M31 halo, and suggest that these clusters possibly originated in a gas-rich dwarf galaxy, which may or may not be presently observable in M31.

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arXiv2004

The DEEP Groth Strip Galaxy Redshift Survey. III. Redshift Catalog and Properties of Galaxies

The Deep Extragalactic Evolutionary Probe (DEEP) is a series of spectroscopic surveys of faint galaxies, targeted at the properties and clustering of galaxies at redshifts z ~ 1. We present the redshift catalog of the DEEP 1 GSS pilot phase of this project, a Keck/LRIS survey in the HST/WFPC2 Groth Survey Strip. The redshift catalog and data, including reduced spectra, are publicly available through a Web-accessible database. The catalog contains 658 secure galaxy redshifts with a median z=0.65, and shows large-scale structure walls to z = 1. We find a bimodal distribution in the galaxy color-magnitude diagram which persists to z = 1. A similar color division has been seen locally by the SDSS and to z ~ 1 by COMBO-17. For red galaxies, we find a reddening of only 0.11 mag from z ~ 0.8 to now, about half the color evolution measured by COMBO-17. We measure structural properties of the galaxies from the HST imaging, and find that the color division corresponds generally to a structural division. Most red galaxies, ~ 75%, are centrally concentrated, with a red bulge or spheroid, while blue galaxies usually have exponential profiles. However, there are two subclasses of red galaxies that are not bulge-dominated: edge-on disks and a second category which we term diffuse red galaxies (DIFRGs). The distant edge-on disks are similar in appearance and frequency to those at low redshift, but analogs of DIFRGs are rare among local red galaxies. DIFRGs have significant emission lines, indicating that they are reddened mainly by dust rather than age. The DIFRGs in our sample are all at z>0.64, suggesting that DIFRGs are more prevalent at high redshifts; they may be related to the dusty or irregular extremely red objects (EROs) beyond z>1.2 that have been found in deep K-selected surveys. (abridged)

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arXiv2004

A Wide-Field HI Study of the NGC 1566 Group

We report on neutral hydrogen observations of a ~ 5.5 x 5.5 degree field around the NGC 1566 galaxy group with the multibeam narrow-band system on the 64-m Parkes telescope. We detected thirteen HI sources in the field, including two galaxies not previously known to be members of the group, bringing the total number of confirmed galaxies in this group to 26. Each of the HI galaxies can be associated with an optically catalogued galaxy. No 'intergalactic HI clouds' were found to an HI mass limit of ~ 3.5 x 10^8 Msun. We have estimated the expected HI content of the late-type galaxies in this group and find the total detected HI is consistent with our expectations. However, while no global HI deficiency is inferred for this group, two galaxies exhibit individual HI deficiencies. Further observations are needed to determine the gas removal mechanisms in these galaxies.

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arXiv2004

A Robust Method for the Analysis of Integrated Spectra from Globular Clusters using Lick Indices

We define a method for analysis of the integrated spectra of extra-galactic globular clusters that provides more reliable measures of age, metallicity and alpha-element abundance ratio than have so far been achieved. The method involves the simultaneous fitting of up to 25 Lick indices in a chi-squared-fitting technique that maximises the use of the available data. Here we compare three sets of single stellar population (SSP) models of Lick indices to the high signal-to-noise, integrated spectra of 20 Galactic globular clusters. The ages, [Fe/H] and alpha-element abundance ratios derived from the SSP models are compared to the results of resolved stellar population studies from the literature. We find good consistency with the published values, with an agreement of better than 0.1 dex in all three derived parameters. The technique allows identification of abundance ratio anomalies, such as the known nitrogen over-abundance in Galactic globular clusters, and the presence of anomalous horizontal-branch morphologies. It also minimises the impact on the derived parameters of imperfect calibration to the Lick system, and reduction errors in general. The method defined in this work is therefore robust with respect to many of the difficulties that plague the application of SSP models in general, and is, consequently, well suited to the study of extra-galactic globular cluster systems.

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