arXiv subjects

B. K. Gibson

Publications and source records attributed to B. K. Gibson.

At least 127 records · Page 7Linked to original sources

arXiv2006

The Northern HIPASS catalogue - Data presentation, completeness and reliability measures

The Northern HIPASS catalogue (NHICAT) is the northern extension of the HIPASS catalogue, HICAT (Meyer et al. 2004). This extension adds the sky area between the declination range of +2 deg < dec. < +25.5 deg to HICAT's declination range of -90 deg < dec. < +2 deg. HIPASS is a blind HI survey using the Parkes Radio Telescope covering 71% of the sky (including this northern extension) and a heliocentric velocity range of -1,280 km/s to 12,700 km/s . The entire Virgo Cluster region has been observed in the Northern HIPASS. The galaxy catalogue, NHICAT, contains 1002 sources with v_hel > 300 km/s . Sources with -300 km/s < v_hel < 300 km/s were excluded to avoid contamination by Galactic emission. In total, the entire HIPASS survey has found 5317 galaxies identified purely by their HI content. The full galaxy catalogue is publicly-available at .

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arXiv2006

The Radial Velocity Experiment (RAVE): first data release

We present the first data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities and stellar atmosphere parameters (temperature, metallicity, surface gravity) of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). The RAVE program started in 2003, obtaining medium resolution spectra (median R=7,500) in the Ca-triplet region ($λλ$ 8,410--8,795 Å) for southern hemisphere stars drawn from the Tycho-2 and SuperCOSMOS catalogs, in the magnitude range 9<I<12. The first data release is described in this paper and contains radial velocities for 24,748 individual stars (25,274 measurements when including re-observations). Those data were obtained on 67 nights between 11 April 2003 to 03 April 2004. The total sky coverage within this data release is $\sim$4,760 square degrees. The average signal to noise ratio of the observed spectra is 29.5, and 80% of the radial velocities have uncertainties better than 3.4 km/s. Combining internal errors and zero-point errors, the mode is found to be 2 km/s. Repeat observations are used to assess the stability of our radial velocity solution, resulting in a variance of 2.8 km/s. We demonstrate that the radial velocities derived for the first data set do not show any systematic trend with color or signal to noise. The RAVE radial velocities are complemented in the data release with proper motions from Starnet 2.0, Tycho-2 and SuperCOSMOS, in addition to photometric data from the major optical and infrared catalogs (Tycho-2, USNO-B, DENIS and 2MASS). The data release can be accessed via the RAVE webpage: http://www.rave-survey.org.

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arXiv2006

Spitzer/MIPS Infrared Imaging of M31: Further Evidence for a Spiral/Ring Composite Structure

New images of M31 at 24, 70, and 160 micron taken with the Multiband Imaging Photometer for Spitzer (MIPS) reveal the morphology of the dust in this galaxy. This morphology is well represented by a composite of two logarithmic spiral arms and a circular ring (radius ~10 kpc) of star formation offset from the nucleus. The two spiral arms appear to start at the ends of a bar in the nuclear region and extend beyond the star forming ring. As has been found in previous work, the spiral arms are not continuous but composed of spiral segments. The star forming ring is very circular except for a region near M32 where it splits. The lack of well defined spiral arms and the prominence of the nearly circular ring suggests that M31 has been distorted by interactions with its satellite galaxies. Using new dynamical simulations of M31 interacting with M32 and NGC 205 we find that, qualitatively, such interactions can produce an offset, split ring like that seen in the MIPS images.

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arXiv2006

The Galactic Nature of High Velocity Cloud Complex WB

We have detected absorption lines from the High Velocity Cloud Complex WB in the spectrum of the star HE1048+0231. This detection sets an upper distance limit to the cloud of 8.8^{+2.3}_{-1.3} kpc. Non-detection (at >4 sigma confidence) in the star HE1138-1303 at 7.7 +/- 0.2 kpc sets a probable lower limit. The equivalent width of the CaII K line due to the HVC (114.6 +/- 4.4 mA) corresponds to a column density of 1.32 +/- 0.05 x 10^12 cm^-2. Using an HI spectrum from the Leiden/Argentine/Bonn survey, we calculated N(CaII)/N(HI) = 81 +/- 16 x 10^-9. These distance limits imply an HI mass limit of 3.8 x 10^5 Msun < M_HI < 4.9 x 10^5 Msun. The upper distance limit imposed by these observations shows that this HVC complex has a probable Galactic or circum-Galactic origin. Future metallicity measurements will be able to confirm or refute this interpretation.

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arXiv2005

Disk evolution since z=1 in a CDM Universe

Increasingly large populations of disk galaxies are now being observed at increasingly high redshifts, providing new constraints on our knowledge of how such galaxies evolve. Are these observations consistent with a cosmology in which structures form hierarchically? To probe this question, we employ SPH/N-body galaxy scale simulations of late-type galaxies. We examine the evolution of these simulated disk galaxies from redshift 1 to 0, looking at the mass-size and luminosity-size relations, and the thickness parameter, defined as the ratio of scale-height to scale-length. The structural parameters of our simulated disks settle down quickly, and after redshift z=1 the galaxies evolve to become only slightly flatter. Our present day simulated galaxies are larger, more massive, less bright, and redder than at z=1. The inside-out nature of the growth of our simulated galaxies reduces, and perhaps eliminates, expectations of evolution in the size-mass relation.

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arXiv2005

Evidence for chimney breakout in the Galactic supershell GSH 242-03+37

We present new high resolution neutral hydrogen (H I) images of the Galactic supershell GSH 242-03+37. These data were obtained with the Parkes Radiotelescope as part of the Galactic All-Sky Survey (GASS). GSH 242-03+37 is one of the largest and most energetic H I supershells in the Galaxy with a radius of $565 \pm 65$ pc and an expansion energy of 3x10^{53} ergs. Our images reveal a complicated shell with multiple chimney structures on both sides of the Galactic plane. These chimneys appear capped by narrow filaments about 1.6 kpc above and below the Galactic mid-plane, confirming structures predicted in simulations of expanding supershells. The structure of GSH 242-03+37 is extremely similar to the only other Galactic supershell known to have blown out of both sides of the plane, GSH 277+00+36. We compare the GASS H I data with X-ray and H-alpha, finding no strong correlations.

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arXiv2005

Studying Galaxy Formation in Loose Galaxy Groups

We present the results of our HI survey of six loose groups of galaxies analogous to the Local Group. The survey was conducted using the Parkes telescope and the Australia Telescope Compact Array to produce a census of all the gas-rich galaxies and analogs to the high-velocity clouds (HVCs) within these groups down to M(HI) < 10^7 M(sun) as a test of models of galaxy formation. We present the HI mass function and halo mass function of the loose groups and show that they are consistent with those of the Local Group. We discuss the possible role of HVCs in solving the ``missing satellite'' problem and discuss the implications of our observations for models of galaxy formation.

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arXiv2005

The HIPASS Catalogue: III - Optical Counterparts & Isolated Dark Galaxies

We present the largest catalogue to date of optical counterparts for HI radio-selected galaxies, Hopcat. Of the 4315 HI radio-detected sources from the HI Parkes All Sky Survey (Hipass) catalogue, we find optical counterparts for 3618 (84%) galaxies. Of these, 1798 (42%) have confirmed optical velocities and 848 (20%) are single matches without confirmed velocities. Some galaxy matches are members of galaxy groups. From these multiple galaxy matches, 714 (16%) have confirmed optical velocities and a further 258 (6%) galaxies are without confirmed velocities. For 481 (11%), multiple galaxies are present but no single optical counterpart can be chosen and 216 (5%) have no obvious optical galaxy present. Most of these 'blank fields' are in crowded fields along the Galactic plane or have high extinctions. Isolated 'Dark galaxy' candidates are investigated using an extinction cut of ABj < 1 mag and the blank fields category. Of the 3692 galaxies with an ABj extinction < 1 mag, only 13 are also blank fields. Of these, 12 are eliminated either with follow-up Parkes observations or are in crowded fields. The remaining one has a low surface brightness optical counterpart. Hence, no isolated optically dark galaxies have been found within the limits of the Hipass survey.

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arXiv2005

Abundance Anomalies in NGC6752 - Do AGB Stars Have a Role?

We are in the process of testing a popular theory that the observed abundance anomalies in the Globular Cluster NGC 6752 are due to `internal pollution' from intermediate mass asymptotic giant branch stars. To this end we are using a chemical evolution model incorporating custom-made stellar evolution yields calculated using a detailed stellar evolution code. By tracing the chemical evolution of the intracluster gas, which is polluted by two generations of stars, we are able to test the internal pollution scenario in which the Na- and Al-enhanced ejecta from intermediate mass stars is either accreted onto the surfaces of other stars, or goes toward forming new stars. In this paper we focus mainly on the nucleosynthetic yields of the AGB stars and discuss whether these stars are the source of the observed Na-O anticorrelation. Comparing our preliminary results with observational data suggests that the qualitative theory is not supported by this quantitative study.

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arXiv2005

On variations in the fine-structure constant and stellar pollution of quasar absorption systems

At redshifts z_abs < 2, quasar absorption-line constraints on space-time variations in the fine-structure constant, alpha, rely on the comparison of MgII and FeII transition wavelengths. One potentially important uncertainty is the relative abundance of Mg isotopes in the absorbers which, if different from solar, can cause spurious shifts in the measured wavelengths and, therefore, alpha. Here we explore chemical evolution models with enhanced populations of intermediate-mass (IM) stars which, in their asymptotic giant branch (AGB) phase, are thought to be the dominant factories for heavy Mg isotopes at the low metallicities typical of quasar absorption systems. By design, these models partially explain recent Keck/HIRES evidence for a smaller alpha in z_abs < 2 absorption clouds than on Earth. However, such models also over-produce N, violating observed abundance trends in high-z_abs damped Lyman-alpha systems (DLAs). Our results do not support the recent claim of Ashenfelter, Mathews & Olive (2004b) that similar models of IM-enhanced initial mass functions (IMFs) may simultaneously explain the HIRES varying-alpha data and DLA N abundances. We explore the effect of the IM-enhanced model on Si, Al and P abundances, finding it to be much-less pronounced than for N. We also show that the 13C/12C ratio, as measured in absorption systems, could constitute a future diagnostic of non-standard models of the high-redshift IMF.

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arXiv2005

Evolving Disks in Cold Dark Matter Cosmology

Despite having entered the era of "precision cosmology," the formation of galaxies within the favored CDM cosmological paradigm remains problematic. By relating our N-body/smooth particle hydro-dynamical simulation to an extensive range of Galactic and extragalactic observations, we shed light on the formation and evolution of the Milky Way and other late type galaxies. In light of recent observations of the stellar populations of extragalactic thick disks, we examine the proposal that the thick disk formed in a high redshift period characterized by gas rich merging. We show such a scenario to be consistent with color observations. We then follow the evolution of structural parameters of the subsequently formed disk galaxy, from redshift ~1 to the present. Consistent with observation, little evolution in the ratio of scale-height to scale-length is found in our simulated galaxy, despite its somewhat chaotic origins. The simulated galaxy in this report forms part of a larger study of a suite of galaxies, with which these issues are to be studied in detail.

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arXiv2005

Galactic Chemical Evolution Redux: Atomic Numbers 6 < Z < 15

Motivated by the inability of Galactic chemical evolution models to reproduce some of the observed solar neighbourhood distribution of elements (and isotopes) with atomic numbers 6 < Z < 15, we have revisited the relevant stellar and Galactic models as part of an ambitious new program aimed at resolving these long-standing discrepancies. Avoiding the use of (traditional) parametric models for low- and intermediate-mass stellar evolution, we have generated a new, physically self-consistent, suite of stellar models and integrated the nucleosynthetic outputs into GEtool, our semi-analytical galactic chemical evolution software package. The predicted temporal evolution of several light- and intermediate-mass elements (and their isotopes) in the solar neighbourhood - from carbon to phosphorus - demonstrate the efficacy of the new yields in reconciling theory and observation.

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arXiv2005

Stellar abundance gradients in galactic disks. I. Method and spectral line gradients

We describe the technique of absorption line imaging of galaxy disks using the Taurus Tunable Filter on the Anglo-Australian Telescope and demonstrate its sensitivity to the behaviour of spectral features associated with Mg and Fe. Radial profiles of Mg2 and Fe5270 line-strengths are presented for a sample of eight face-on spiral galaxies spanning a range of Hubble types. Signatures of phenomena including merger-induced star formation, HII rings and galactic bars are also reported. This study demonstrates the capacity of tunable filters to measure Mg and Fe line-strengths across the face of spiral galaxies, which can ultimately reveal clues about the star formation history and chemical evolution.

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arXiv2004

On the origin of fluorine in the Milky Way

The main astrophysical factories of fluorine (19F) are thought to be Type II supernovae, Wolf-Rayet stars, and the asymptotic giant branch (AGB) of intermediate mass stars. We present a model for the chemical evolution of fluorine in the Milky Way using a semi-analytic multi-zone chemical evolution model. For the first time, we demonstrate quantitatively the impact of fluorine nucleosynthesis in Wolf-Rayet and AGB stars. The inclusion of these latter two fluorine production sites provides a possible solution to the long-standing discrepancy between model predictions and the fluorine abundances observed in Milky Way giants. Finally, fluorine is discussed as a possible probe of the role of supernovae and intermediate mass stars in the chemical evolution history of the globular cluster omega Centauri.

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arXiv2004

The cosmological significance of Low Surface Brightness galaxies found in a deep blind neutral-hydrogen survey

We have placed limits on the cosmological significance of gas-rich low surface-brightness (LSB) galaxies as a proportion of the total population of gas-rich galaxies by carrying out a very deep survey (HIDEEP) for neutral hydrogen (HI) with the Parkes multibeam system. Such a survey avoids the surface-brightness selection effects that limit the usefulness of optical surveys for finding LSB galaxies. To complement the HIDEEP survey we have digitally stacked eight 1-hour R-band Tech Pan films from the UK Schmidt Telescope covering 36 square degrees of the survey area to reach a very deep isophotal limit of 26.5 R mag/sq. arcsec. At this level, we find that all of the 129 HI sources within this area have optical counterparts and that 107 of them can be identified with individual galaxies. We have used the properties of the galaxies identified as the optical counterparts of the HI sources to estimate the significance of LSB galaxies (defined to be those at least 1.5 magnitudes dimmer in effective surface-brightness than the peak in the observed distribution seen in optical surveys). We calculate the contribution of LSB galaxies to the total number, neutral hydrogen density, luminosity density, baryonic mass density, dynamical mass density and cross-sectional area of gas-rich galaxies. We do not find any `Crouching Giant' LSB galaxies such as Malin 1, nor do we find a population of extremely low surface-brightness galaxies not previously found by optical surveys. Such objects must either be rare, gas-poor or outside the survey detection limits.

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arXiv2004

Globular Cluster and Galaxy Formation: M31, the Milky Way and Implications for Globular Cluster Systems of Spiral Galaxies

The globular cluster (GC) systems of the Milky Way and of our neighboring spiral galaxy, M31, comprise 2 distinct entities, differing in 3 respects. 1. M31 has young GCs, ages from ~100 Myr to 5 Gyr old, as well as old globular clusters. No such young GCs are known in the Milky Way. 2. We confirm that the oldest M31 GCs have much higher nitrogen abundances than do Galactic GCs at equivalent metallicities. 3. Morrison et al. found M31 has a subcomponent of GCs that follow closely the disk rotation curve of M31. Such a GC system in our own Galaxy has yet to be found. These data are interpreted in terms of the hierarchical-clustering-merging (HCM) paradigm for galaxy formation. We infer that M31 has absorbed more of its dwarf systems than has the Milky Way. This inference has 3 implications: 1. All spiral galaxies likely differ in their GC properties, depending on how many companions each galaxy has, and when the parent galaxy absorbs them. The the Milky Way ties down one end of this spectrum, as almost all of its GCs were absorbed 10-12 Gyr ago. 2. It suggests that young GCs are preferentially formed in the dwarf companions of parent galaxies, and then absorbed by the parent galaxy during mergers. 3. Young GCs seen in tidally-interacting galaxies might come from dwarf companions of these galaxies, rather than be made a-new in the tidal interaction. There is no ready explanation for the marked difference in nitrogen abundance for old M31 GCs relative to the oldest Galactic GCs. The predictions made by Li & Burstein regarding the origin of nitrogen abundance in globular clusters are consistent with what is found for the old M31 GCs compared to that for the two 5 Gyr-old M31 GCs.

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arXiv2004

Modelling self-pollution of globular clusters from AGB stars

A self-consistent model of the chemical evolution of the globular cluster NGC 6752 is presented to test a popular theory that observed abundance anomalies are due to ``internal pollution'' from intermediate mass asymptotic giant branch stars. We simulated the chemical evolution of the intracluster medium under the assumption that the products of Type II SNe are completely expelled from the globular cluster, whereas material ejected from stars with m < 7 M_sun is retained, due to their weak stellar winds. By tracing the chemical evolution of the intracluster gas, we tested an internal pollution scenario in which the Na- and Al-enhanced ejecta from intermediate mass stars is either accreted onto the surfaces of other stars, or goes toward forming new stars. The observed spread in Na and Al was reproduced, but not the O-Na and Mg-Al anticorrelations. In particular, neither O nor Mg are sufficiently depleted to account for the observations. We predict that the Mg content of Na-rich cluster stars should be overwhelmingly dominated by the 25,26Mg isotopes, whereas the latest data shows only a mild 26Mg enhancement and no correlation with 25Mg. Furthermore, stars bearing the imprint of intermediate mass stellar ejecta are predicted to be strongly enhanced in both C and N, in conflict with the empirical data. We find that while standard AGB stellar models do show the hot H burning that seems required to explain the observations, this is accompanied by He burning, producing primary C, N, Mg and Na (via HBB) which do not match the observations. (Abridged)

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arXiv2004

The Gaseous Trail of the Sagittarius Dwarf Galaxy

A possible gaseous component to the stream of debris from the Sagittarius dwarf galaxy is presented. We identify 4 - 10 x 10^6 Msun of neutral hydrogen along the orbit of the Sgr dwarf in the direction of the Galactic anticenter (at 36 kpc, the distance to the stellar debris in this region). This is 1-2% of the estimated total mass of the Sgr dwarf. Both the stellar and gaseous components have negative velocities, but the gaseous component extends to higher negative velocities. If associated, this gaseous stream was most likely stripped from the main body of the dwarf 0.2 - 0.3 Gyr ago during its current orbit after a passage through a diffuse edge of the Galactic disk with a density > 10^{-4} cm^{-3}. This gas represents the dwarf's last source of star formation fuel and explains how the galaxy was forming stars 0.5-2 Gyr ago.

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