arXiv subjects

Duncan Forbes

Publications and source records attributed to Duncan Forbes.

34 records · Page 2Linked to original sources

arXiv2007

Is NGC 3108 transforming itself from an early to late type galaxy -- an astronomical hermaphrodite?

A common feature of hierarchical galaxy formation models is the process of "inverse" morphological transformation: a bulge dominated galaxy accretes a gas disk, dramatically reducing the system's bulge-to-disk mass ratio. During their formation, present day galaxies may execute many such cycles across the Hubble diagram. A good candidate for such a "hermaphrodite" galaxy is NGC 3108: a dust-lane early-type galaxy which has a large amount of HI gas distributed in a large scale disk. We present narrow band H_alpha and R-band imaging, and compare the results with the HI distribution. The emission is in two components: a nuclear bar and an extended disk component which coincides with the HI distribution. This suggests that a stellar disk is currently being formed out of the HI gas. The spatial distributions of the H_alpha and HI emission and the HII regions are consistent with a barred spiral structure, extending some 20 kpc in radius. We measure an extinction- corrected SFR of 0.42 Msun/yr. The luminosity function of the HII regions is similar to other spiral galaxies, with a power law index of -2.1, suggesting that the star formation mechanism is similar to other spiral galaxies. We measured the current disk mass and find that it is too massive to have been formed by the current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was higher in the past. With the current SFR, the disk in NGC 3108 will grow to be ~6.2x10^9 Msun in stellar mass within the next 5.5 Gyr. While this is substantial, the disk will be insignificant compared with the large bulge mass: the final stellar mass disk-to-bulge ratio will be ~0.02. NGC 3108 will fail to transform into anything resembling a spiral without a boost in the SFR and additional supply of gas.

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arXiv2007

HI Mapping of Galaxies in Six GEMS Groups

Here we present Australia Telescope Compact Array HI maps of 16 HI sources in six Group Evolution Multiwavelength Study (GEMS) groups that were previously observed with the Parkes telescope. The higher spatial resolution of the ATCA allows us to clearly identify the optical counterparts for the first time -- most being associated with low surface brightness late-type galaxies. New integrated HI maps and velocity fields for each source are presented. We find several interacting systems; one of which contains three galaxies within a common HI envelope. Extended HI structures in the sample are more consistent with tidal effects than ram pressure stripping. We identify two HI detections with previously uncatalogued optical galaxies, and add a total of six newly identified group members to the NGC 3923, 5044 and 7144 groups.

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arXiv2007

The WiggleZ project: AAOmega and Dark Energy

We describe the `WiggleZ' spectroscopic survey of 280,000 star-forming galaxies selected from a combination of GALEX ultra-violet and SDSS + RCS2 optical imaging. The fundamental goal is a detection of the baryonic acoustic oscillations in galaxy clustering at high-redshift (0.5 < z < 1) and a precise measurement of the equation of state of dark energy from this purely geometric and robust method. The survey has already started on the 3.9m Anglo-Australian Telescope using the AAOmega spectrograph, and planned to complete during 2009. The WWW page for the survey can be found at http://astronomy.swin.edu.au/wigglez

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arXiv2006

Structural Parameters for Globular Clusters in NGC 5128. II: HST/ACS Imaging and New Clusters

We report the first results from an imaging program with the ACS camera on HST designed to measure the structural characteristics of a wide range of globular clusters in NGC 5128, the nearest giant elliptical galaxy. From 12 ACS/WFC fields, we have measured a total of 62 previously known globular clusters and have discovered 69 new high-probability cluster candidates not found in any previous work. We present magnitudes and color indices for all of these, along with rough measurements of their effective diameters and ellipticities. The luminosity distribution of this nearly-uncontaminated sample of clusters matches well with the normal GCLF for giant elliptical galaxies, and the cluster scale size and ellipticity distributions are similar to those in the Milky Way system. The indication from this survey is that many hundreds of individual clusters remain to be found with carefully designed search techniques in the future. A very rough estimate of the total cluster population from our data suggests N_GC = 1500 in NGC 5128, over all magnitudes and within a projected radius R = 25' from the galaxy center.

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arXiv2006

Southern GEMS Groups I: Dynamical Properties

Here we present an investigation of the properties of 16 nearby galaxy groups and their constituent galaxies. The groups are selected from the Group Evolution Multi-wavelength Study (GEMS) and all have X-ray as well as wide-field neutral hydrogen (HI) observations. Group membership is determined using a friends-of-friends algorithm on the positions and velocities from the 6-degree Field Galaxy Survey (6dFGS) and NASA/IPAC Extra-galactic Database (NED). For each group we derive their physical properties using this membership, including: velocity dispersions (sigma_v), virial masses (M_V), total K-band luminosities (L_K(Tot)) and early-type fractions (f_early) and present these data for the individual groups. We find that the GEMS X-ray luminosity is proportional to the group velocity dispersions and virial masses: L_X(r_500)\proptoσ_v^{3.11\pm0.59} and L_X(r_500)\propto M_V^{1.13\pm0.27}, consistent with the predictions of self-similarity between group and clusters. We also find that M_V\propto L_K(Tot)^{2.0\pm0.9}, i.e. mass grows faster than light and that the fraction of early-type galaxies in the groups is correlated with the group X-ray luminosities and velocity dispersions. We examine the brightest group galaxies (BGGs), finding that, while the luminosity of the BGG correlates with its total group luminosity, the fraction of group luminosity contained in the BGG decreases with increasing total group luminosity. This suggests that BGGs grow by mergers at early times in group evolution while the group continues to grow by accreting infalling galaxies. (Abridged)

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arXiv2006

Radial Kinematics of Isolated Elliptical Galaxies

Ellipticals in very low density environments are extremely rare but hold important clues about galaxy formation and evolution. In this paper we continue our study of isolated elliptical galaxies, presenting results on the radial stellar kinematics for 13 isolated early-type galaxies. We derive radial rotation velocity, velocity dispersion and hermite terms to ~1 effective radius. We observe a dichotomy in kinematic properties similar to that in the elliptical population as a whole, where low luminosity ellipticals tend to be rotationally supported. For all galaxies the V/sigma ratio increases with radius. We find kinematically distinct cores (KDCs), or velocity substructure, in ~40% of the galaxies for which we have major axis spectra. Such a fraction is similar to that observed for ellipticals in higher density environments. Most galaxies in the sample reveal kinematic evidence for a nuclear disk. The non-relaxed kinematics in several galaxies suggests that they have undergone a merger or accretion event. Isolated ellipticals generally follow the fundamental plane defined by cluster ellipticals -- exceptions being those galaxies with evidence for young stellar populations. Overall, we find isolated ellipticals have similar kinematic properties to their counterparts in higher density environments.

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arXiv2006

Eridanus - A Supergroup in the Local Universe?

We examine a possible supergroup in the direction of the Eridanus constellation using 6dF Galaxy Survey second data release (6dFGS DR2) positions and velocities together with 2MASS and HyperLEDA photometry. We perform a friends-of-friends analysis to determine which galaxies are associated with each substructure before examining the properties of the constituent galaxies. The structure is made up of three individual groups that are likely to merge to form a cluster of mass 7x10^13 Msolar. We conclude that this structure is a supergroup. We also examine the colours, morphologies and luminosities of the galaxies in the region with respect to their local projected surface density. We find that the colours of the galaxies redden with increasing density, the median luminosities are brighter with increasing environmental density and the morphologies of the galaxies show a strong morphology-density relation. The colours and luminosities of the galaxies in the supergroup are already similar to those of galaxies in clusters, however the supergroup contains more late-type galaxies, consistent with its lower projected surface density. Due to the velocity dispersion of the groups in the supergroup, which are lower than those of clusters, we conclude that the properties of the constituent galaxies are likely to be a result of merging or strangulation processes in groups outlying this structure.

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arXiv2005

An Imaging Study of the Globular Cluster Systems of NGC 1407 and NGC 1400

We present wide-field Keck telescope imaging of the globular cluster (GC) systems around NGC 1407 and NGC 1400 in the Eridanus galaxy cloud. This is complemented by Hubble Space Telescope images from the Advanced Camera for Surveys of NGC 1407 and Wide Field and Planetary Camera 2 images of NGC 1400. We clearly detect bimodality in the GC colour distribution of NGC 1407. The blue GC subpopulation has a mean colour of B-I = 1.61 and a relative contribution of around 40%, whereas the red subpopulation with B-I = 2.06 contributes 60% to the overall GC system. Assuming old ages, this corresponds to [Fe/H] = -1.45 and -0.19. Both subpopulations are intrinsically broad in colour (indicating a range in ages and/or metallicities), with the red subpopulation being broader than the blue. The GC colour distribution for NGC 1400 is less clear cut than for NGC 1407, however, we also find evidence for a bimodal distribution. We find the NGC 1407 red GCs to be 20% smaller in size than the blue ones. We find both GC systems to possess a GC surface density distribution which is largely constant in these inner galaxy regions. We fit isothermal-like profiles and derive GC system core radii of 9.4 kpc for NGC 1407 and 5.8 kpc for NGC 1400. For NGC 1407 we are able to separate the surface density distribution into blue and red subpopulations, giving 17.8 and 7.6 kpc respectively. Outside this central region, the radial profile of the GC surface density is similar to that of the galaxy light for NGC 1407 but it is flatter for NGC 1400. A fit to the GC luminosity function gives a distance modulus of 31.6, which is in good agreement with distances based on the Faber-Jackson relation and the Virgo infall corrected velocity.

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arXiv2004

Group, field and isolated early-type galaxies I. Observations and nuclear data

This is the first paper of a series on the investigation of stellar population properties and galaxy evolution of an observationally homogeneous sample of early-type galaxies in groups, field and isolated galaxies. We present high S/N long-slit spectroscopy of 86 nearby elliptical and S0 galaxies. 8 of them are isolated, selected according to a rigorous criterion. This survey has the advantage of covering a larger wavelength range than normally found in the literature, including [OIII]5007 and Halpha, both important for emission correction. Among the 86 galaxies with S/N>15 (per resolution element, for r_e/8 central aperture), 57 have their Hbeta-index corrected for emission, average correction is 0.190A in Hbeta; 42 galaxies reveal [OIII]5007 emission, of which 16 also show obvious Halpha emission. Most of the galaxies in the sample do not show obvious signs of disturbances nor tidal features in the morphologies, although 11 belong to the Arp catalogue of peculiar galaxies; only 3 of them (NGC750, NGC751, NGC3226) seem to be strongly interacting. We present the measurement of 25 central line-strength indices calibrated to the Lick/IDS system. Kinematic information is obtained for the sample. We analyse the line-strength index vs velocity dispersion relations for our sample of mainly low density environment galaxies, and compare the slope of the relations with cluster galaxies from the literature. Our main findings are that the index-sigma_0 relations presented for low-density regions are not significantly different from those of cluster E/S0s. The slope of the index-sigma_0 relations does not seem to change for early-type galaxies of different environmental densities, but the scatter of the relations seems larger for group, field and isolated galaxies than for cluster galaxies.

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arXiv2004

Probing Spectral Line Gradients Beyond One Effective Radius in NGC 3610

The outer region (0.75--1.25 r_e in the B-band) of the merger-remnant elliptical NGC 3610 is studied using extremely high signal to noise Keck spectra, with a supplementary spectrum of the galaxy center. Stellar population parameters -- age, [Z/H], [$α$/Fe] -- are measured in several apertures along the slit. Using the multi-index simultaneous fitting method of Proctor et al. (2004), no significant stellar population gradients are detected in the outer parts of the galaxy. The overall gradients relative to the galaxy center are consistent with those found in many other early-type galaxies, though the metallicity gradient is much steeper than would be expected if NGC 3610 formed in a major merger event. Standard analysis methods using the H$β$ index are found to produce spurious radially variable gradients.

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arXiv2003

Globular Clusters in Early-Type Galaxies with GMOS

We present recent results from our long-term Gemini/GMOS study of globular clusters in early-type galaxies. To date, we have obtained photometry and spectroscopy for clusters in NGCs 3379, 4649, 524, 7332, and IC 1459. We find a clear bimodality in the NGC 4649 cluster colour distribution, with the fraction of blue/red clusters increasing with galactocentric radius. We derive ages and metallicities for 22 clusters in NGC 3379, finding that most of the clusters appear old (10-15 Gyr); however, there is a group of 4 metal-rich, younger clusters with ages of 2-6 Gyr. The NGC 3379 cluster velocity dispersion decreases with radius, as does the inferred (local) mass-to-light ratio: there is no evidence for a dark matter halo in NGC 3379 based on our globular cluster data.

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arXiv2003

Galaxy Disruption in a Halo of Dark Matter

The relics of disrupted satellite galaxies around the Milky Way and Andromeda have been found, but direct evidence of a satellite galaxy in the early stages of being disrupted has remained elusive. We have discovered a dwarf satellite galaxy in the process of being torn apart by gravitational tidal forces as it merges with a larger galaxy's dark matter halo. Our results illustrate the morphological transformation of dwarf galaxies by tidal interaction and the continued build-up of galaxy halos.

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arXiv1998

Dependence of the Fundamental Plane Scatter on Galaxy Age

The fundamental plane (FP) has an intrinsic scatter that can not be explained purely by observational errors. Using recently available age estimates for nearby early type galaxies, we show that a galaxy's position relative to the FP depends on its age. In particular, the mean FP corresponds to ellipticals with an age of ~10 Gyr. Younger galaxies are systematically brighter with higher surface brightness relative to the mean relation. Old ellipticals form an `upper envelope' to the FP. For our sample of mostly non-cluster galaxies, age can account for almost half of the scatter in the B band FP. Distance determinations based on the FP may have a systematic bias, if the mean age of the sample varies with redshift. We also show that fundamental plane residuals, B-V colors and Mg_2 line strength are consistent with an ageing central burst superposed on an old stellar population. This reinforces the view that these age estimates are tracing the last major episode of star formation induced by a gaseous merger event. We briefly discuss the empirical `evolutionary tracks' of merger-remnants and young ellipticals in terms of their key observational parameters.

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arXiv1998

Star Formation in Southern Seyfert Galaxies

We have produced radio maps, using the ATCA, of the central regions of six southern Seyfert 2 galaxies (NGC 1365, 4945, 6221, 6810, 7582, and Circinus) with circumnuclear star formation, to estimate the relative contribution of star formation activity compared to activity from the active galactic nucleus (AGN). The radio morphologies range from extended diffuse structures to compact nuclear emission, with no evidence, even in the relatively compact sources, for synchrotron self--absorption. In each case the radio to far--infrared (FIR) ratio has a value consistent with star formation, and in all but one case the radio to [FeII] ratio is also consistent with star formation. We derive supernova rates and conclude that, despite the presence of a Seyfert nucleus in these galaxies, the radio, FIR, and [FeII] line emission are dominated by processes associated with the circumnuclear star formation (i.e. supernova remnants and HII regions) rather than with the AGN.

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arXiv1997

Globular Clusters in Elliptical Galaxies: Constraints on Mergers

Understanding the formation of globular clusters will help us to constrain the evolutionary history of galaxies. For mergers on-going today, it appears that the number of newly created proto-globulars varies with the progenitor types and that the percentage increase in the total cluster population is 100% or less. Elliptical galaxies, which are generally thought to be the end-result of such mergers, often show evidence for two populations of globular clusters. This has been taken as support for the merger picture but we show that there are several problems when examined in detail. Strong evidence for an elliptical galaxy with a single population of globular clusters is also presented. Implications for galaxy formation are briefly mentioned.

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arXiv1997

HST Imaging of the Globular Clusters in the Fornax Cluster: NGC 1399 and NGC 1404

The Fornax cluster galaxies NGC 1399 and NGC 1404 are ideal for studying the effects of a cluster environment on globular cluster systems. Here we present new optical imaging of these two galaxies from both the Hubble Space Telescope's Wide Field and Planetary Camera 2 and the Cerro Tololo Inter-American Observatory's 1.5m telescope. The combination of both data sets provides unique insight on the spatial and colour distribution of globular clusters. From B-I colours, we find that both galaxies have a broad globular cluster metallicity distribution that is inconsistent with a single population. Two Gaussians provide a reasonable representation of the metallicity distribution in each galaxy. The metal-rich subpopulation is more centrally concentrated than the metal-poor one. We show that the radial metallicity gradient can be explained by the changing relative mix of the two globular cluster subpopulations. We derive globular cluster surface density profiles, and find that they are flatter (i.e. more extended) than the underlying starlight. The total number of globular clusters and specific frequency are calculated to be N = 5700 +/- 500, S_N = 11.5 +/- 1.0 for NGC 1399 and N = 725 +/- 145, S_N = 2.0 +/- 0.5 for NGC 1404. Our results are compared to the expectations of globular cluster formation scenarios.

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