Swinburne Research Theses
The Centre is extremely proud of the achievements of its research students. All of our graduates are listed below, PhD titles, placement after graduation, thesis abstracts, and full thesis texts in .pdf form
 Russell Edwards (PhD) | Pulsar Searching | University of Amsterdam |
| Willem van Straten (PhD) | High-Precision Timing and Polarimetry of PSR J0437-4715 | Netherlands Foundation for Research in Astronomy, Dwingeloo |
| Chris Brook (PhD) | Chemo-dynamical Simulations of the Milky Way | Departement de Physique, Universite Laval, Quebec |
| Craig West (Masters) | Development of disk-based baseband recorders and software correlators for radio astronomy | University of Massachusetts, Department of Astronomy, Amherst |
| Yeshe Fenner (PhD) | Chemical Evolution of Galactic Systems | Harvard-Smithsonian Center for Astrophysics, Cambridge, MA |
| Stuart Gill (PhD) | The Evolution of Dark Halo Substructure | Department of Astronomy, Columbia University, NY |
| Christopher Thom (PhD) | High Velocity Clouds and the Milky Way Halo | Dept. of Astronomy and Astrophysics, University of Chicago, IL |
| Aidan Hotan (PhD) | High-precision Observations of Relativistic Binary and Millisecond Pulsars | University of Tasmania |
| Michael Pierce (PhD) | Spectroscopy of Extra-galactic Globular Clusters | McLennan Magasanik Associates |
| Haydon Knight (PhD) | Pulsar Applications of Baseband Recording | ... |
| Fatma Reda (PhD) | Isolated Elliptical Galaxies | National Research Institute of Astronomy and Geophysics, Cairo, Egypt |
| Agostino Renda (PhD) | The Formation of Stellar Halos in Late-Type Galaxies | ... |
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Pulsar Searching
Russell Edwards, Doctor of Philosophy, March 2001
This thesis reports the results of two pulsar survey projects conducted at the Parkes 64-m radio telescope in New South Wales, Australia.
The first, the Swinburne Intermediate Latitude Pulsar Survey, covered a large region of the southern Galaxy (5 degrees < |b| < 15 degrees and -100 degrees < l < 50 degrees) flanking that of the ongoing Galactic plane survey (\b\ < 5 degrees). We used the 13-feed 20 cm "multibeam" receiver package to achieve this broad sky coverage in a short observing campaign with 14 days total integration time. The survey proved remarkably successful, detecting 170 pulsars, 69 of which were new discoveries.
Eight of the new discoveries possess small periods and period derivatives indicative of "recycling", a hypothesis supported by the fact that six of them are in circular orbits with probable white dwarf companions. Pulse time measurements have revealed that two of the white dwarfs are massive CO and ONeMg dwarfs. The mass of one of them (the companion to PSR J1157-5112) exceeds 1.14 solar masses, providing the most convincing evidence to date for the production of "ultra-massive" ONeMg white dwarfs as the end result of stellar evolution on the asymptotic giant branch (albeit with mass transfer indicated). PSR J1757-5322 also possesses a heavy (> 0.55 solar masses) white dwarf companion, in a close 11 hour orbit. The proximity of the massive companion leads to significant relativistic orbital evolution and the effects of this will be measurable by pulsar timing in the coming decades. Under general relativity, the gravitational wave power radiated from the system is sufficient to cause coalescence in < 9.5 gigayears, an event which will have dramatic and unknown consequences. Such events are possible gamma-ray burst sources, and the remnants could include isolated millisecond pulsars, close eclipsing binaries or pulsar planetary systems. The remaining four pulsar binaries show some discrepancies with the bulk of previously known low mass binary pulsars (LMBPs). PSR J1618-39 is in a 23 day orbit, filling what previously appeared to be a gap in the orbital period distribution. PSR J1745-0952 has a relatively long pulse period (19 milliseconds) and along with PSR J1618-39 (12 milliseconds) may have experienced a different evolutionary history to the majority of previously discovered LMBPs.
A ninth pulsar discovered in the survey may be recycled. The mean pulse profile of PSR J1411-7404 is exceedingly narrow (approximately 2 degrees) and lies in stark contrast to that of other pulsars of similar pulse period. In the past the only other pulsars known with anomalously narrow profiles were believed (for other reasons) to have been recycled, and this fact in combination with the low period derivative measured in timing analysis of PSR J1411-7404 leads me to suggest that it, too, may have been recycled. If this is the case, its is possible that the recycling took place in a system similar in configuration to the progenitors of the double neutron star systems but that sudden mass loss or an unfavourably oriented kick in the birth event of the second neutron star disrupted the system, leaving an isolated, mildly recycled pulsar.
The second pulsar survey program conducted for this work was a targeted search of southern globular clusters. We used a baseband recording system to provide unprecedented time resolution (typically 25 microseconds). The large number of channels and short sampling interval achievable in software filterbanks, in combination with the ability to coherently remove most of the interstellar dispersion from clusters with previously known pulsars, made use the first to achieve a relatively flat sensitivity response to pulsars of approximately 10^-3.5 - 10 seconds. This characteristic is vital if we are to constrain the true period distribution of millisecond pulsars, an important task in the evaluation of alternative equations of state for nuclear matter. We detected six millisecond pulsars and produced pulse profiles of higher resolution than were previously available. the basic sensitivity of the search was not high enough to detect any new pulsars, however the work demonstrates that the approach is feasible with the use of currently available high-preformance computing resources (such as the Swinburne workstation cluster), and is capable of delivering excellent sensitivity characteristics. It is expected that future searches of this kind, of which this is the first, will achieve the goal of sampling the true pulse period distribution within a few years.
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High-Precision Timing and Polarimetry of PSR J0437-4715
Willem van Straten, Doctor of Philosophy, July 2003
This thesis reports on the recent results of a continuing, high-precision pulsar timing project, currently focused on the nearby, binary millisecond pulsar, PSR J0437-4715. Pulse arrival time analysis has yielded a remarkable series of constraints on the physical parameters of this system and evidence for the distortion of space-time as predicted by the General Theory of Relativity
Owing to the proximity of the PSR J0437-4715 system, relative changes in the positions of the Earth and pulsar result in both annual and secular evolution of the line of sight to the pulsar. Although the changes are miniscule, the effects on the projected orbital parameters are detectable in our data at a high level of significance, necessitating the implementation of an improved timing mode
In addition to producing estimates of astrometric parameters with unparalleled precision, the study has also yielded the first three-dimensional orbital geometry of a binary pulsar. This achievement includes the first classical determination of the orbital inclination, thereby providing the unique opportunity to verify the shape of the Shapiro delay and independently confirm a general relativistic prediction.
With a current post-fit arrival time residual RMS of 130 ns over four years, the unrivaled quality of the timing data presented herein may eventually contribute to the most stringent limit on the energy density of the proposed stochastic gravitational wave background. Continuing the quest for even greater timing precision, a detailed study of the polarimetry of PSR J0437-4715 was undertaken. This effort culminated in the development of a new, phase-coherent technique for calibrating the instrumental response of the observing system.
Observations were conducted at the Parkes 64-m radio telescope in New South Wales, Australia, using baseband recorder technologies developed at York University, Toronto, and at the California Institute of Technology. Data were processed off-line at Swinburne University using a beowulf-style cluster of high-performance workstations and custom software developed by the candidate as part of this thesis.
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Chemo-dynamical Simulations of the Milky Way
Chris Brook, Doctor of Philosophy, March 2004
Using a state of the art galaxy formation software package, GCD+, we model the formation and evolution of galaxies which resemble our own Galaxy, the Milky Way. The simulations include gravity, gas dynamics, radiative gas cooling, star formation and stellar evolution, tracing the production of several elements and the subsequent pollution of the interstellar medium. The simulations are compared with observations in order to unravel the details of the Milky Way's formation. Several unresolved issues regarding the Galaxy's evolution are speci cally addressed. In our first study, limits are placed on the mass contribution of white dwarfs to the dark matter halo which envelopes the Milky Way. We obtain this result by comparing the abundances of carbon and nitrogen produced by a white dwarf-progenitor-dominated halo with the abundances observed in the present day halo. Our results are inconsistent with a white dwarf component in the halo > 5% (by mass), however mass fractions of 1-2% cannot be ruled out. In combination with other studies, this result suggests that the dark matter in the Milky Way is probably non-baryonic. The second component of this thesis probes the dynamical signatures of the formation of the stellar halo. By tracing the halo stars in our simulation, we identify a group of high eccentricity stars that can be traced to now-disrupted satellites that were accreted by the host galaxy. By comparing the phase space distribution of these stars in our simulations to observed high-eccentricity stars in the solar neighbourhood, we find evidence that such a group of stars - a "stellar stream" - exists locally in our own Galaxy. Our next set of simulations demonstrate the importance of strong energy feedback from supernova explosions to the regulation of star formation. Strong feedback ensures that the building blocks of galaxy formation remain gas-rich at early epochs. We demonstrate that this process is necessary to reproduce the observed low mass and low metallicity of the stellar halo of the Milky Way. Our simulated galaxy is shown to have a thick disk component similar to that observed in the Milky Way through an abrupt discontinuity in the velocity dispersion-versus-age relation for solar neighbourhood stars. This final study suggests that the thick disk forms in a chaotic merging period during the Galaxy's formation. Our thick disk formation scenario is shown to be consistent with observed properties of the thick disk of the Milky Way.
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Development of disk-based baseband recorders and software correlators for radio astronomy
Craig West, Masters of Applied Science, March 2004
This thesis details work undertaken in the field of radio astronomy instrumentation. Specific components of the data collection and processing systems used by radio astronomers have been implemented using non-traditional approaches. Traditionally, the correlation of radio astronomy data has taken place on dedicated, specific hardware. This thesis deals with the implementation of equivalent correlators using software running on generic clusters of personal computers - the software approach to radio astronomy. Toward this end a system has been developed that records the raw telescope output onto computer hard drives, allowing easy access to the data on cluster supercomputers.
Part of this thesis describes the design, construction, testing and utilisation of these data recording systems. The correlator software used to process these data on supercomputers is also fully described, including extensive tests of the software and a detailed comparison between its output and the output of an existing hardware correlator. The software correlator is shown to produce output that agrees extremely well with the hardware correlator, verifying its accuracy and performance. Finally, results of on-going scientific investigations that use the software correlators described in this thesis are outlined, illustrating the exibility and usefulness of the software approach to radio astronomy.
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Chemical Evolution of Galactic Systems
Yeshe Fenner, Doctor of Philosophy, March 2005
This thesis explores the chemical signatures of galaxy formation and evolution using a software package designed specifically for this investigation. We describe the development of this multi-zone chemical evolution code, which simulates the spacetime evolution of stars, gas and a vast array of chemical elements within galactic systems. We use this tool to analyse observations of a wide range of astrophysical systems.
The chemical evolution code is first calibrated using empirical constraints from the Milky Way. These simulations help shed light on the nature of the gas accretion processes that fueled the formation of our Galaxy. We demonstrate the importance of low- and intermediate-mass stars in explaining the elemental and isotopic abundance patterns measured in Galactic stars. An intriguing question in astrophysics is whether pollution from intermediate-mass stellar winds is responsible for anomalous abundances in globular cluster stars. We test this scenario by modelling the formation and chemical evolution of a globular cluster.
Recently, the most detailed abundance pattern ever measured beyond the local universe was obtained for a high-redshift quasar absorption cloud, providing an exciting opportunity to explore early conditions of galaxy formation. We compare the chemical abundances in this distant object with predictions from a series of models, in order to gain insight into the protogalaxy's age and star formation history. We continue investigating the high-redshift universe, turning our attention to the issue of space-time variations in the ne-structure constant, as suggested by quasar absorption-line constraints. An excess abundance of heavy Mg isotopes in the absorbing clouds could partly account for the data, without needing to invoke variations in fundamental constants of nature. An enhanced early population of intermediate-mass stars could lead to such extreme Mg isotopic ratios, but we show that additional chemical consequences of this scenario conflict with observations.
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The Evolution of Dark Halo Substructure
Stuart Gill, Doctor of Philosophy, July 2005
In this dissertation we analyse the dark matter substructure dynamics within a series of high-resolution cosmological galaxy clusters simulations generated with the N-body code MLAPM.
Two new halo nding algorithms were designed to aid in this analysis. The first of these was the "MLAPM-halo- finder" (MHF), built upon the adaptive grid structure of MLAPM. The second was the "MLAPM-halo-tracker" (MHT), an extension of MHF which allowed the tracking of orbital characteristics of gravitationally bound objects through any given cosmological N-body-simulation. Using these codes we followed the time evolution of hundreds of satellite galaxies within the simulated clusters.
These clusters were chosen to sample a variety of formation histories, ages, and triaxialities; despite their obvious differences, we find striking similarities within the associated substructure populations. Namely, the radial distribution of these substructure satellites follows a "universal" radial distribution irrespective of the host halo's environment and formation history. Further, this universal substructure profile is anti-biased with respect to the underlying dark matter profile. All satellite orbits follow nearly the same eccentricity distribution with a correlation between eccentricity and pericentre. The destruction rate of the substructure population is nearly independent of the mass, age, and triaxiality of the host halo. There are, however, subtle differences in the velocity anisotropy of the satellite distribution. We find that the local velocity bias at all radii is greater than unity for all halos and this increases as we move closer to the halo centre, where it varies from 1.1 to 1.4. For the global velocity bias we find a small but slightly positive bias, although when we restrict the global velocity bias calculation to satellites that have had at least one orbit, the bias is essentially removed.
Following this general analysis we focused on three specific questions regarding the evolution of substructures within dark matter halos. Observations of the Virgo and Coma clusters have shown that their galaxies align with the principal axis of the cluster. Further, a recent statistical analysis of some 300 Abell clusters confirm this alignment, linking it to the dynamical state of the cluster. Within our simulations the apocentres of the satellite orbits are preferentially found within a cone of opening angle of approximately 40 degrees around the major axis of the host halo, in accordance with the observed anisotropy found in galaxy clusters. We do, however, note that a link to the dynamical age of the cluster is not well established. Further analysis connects this distribution to the infall pattern of satellites along the filaments, rather than some "dynamical selection" during their life within the host's virial radius.
We then focused our attention on the outskirts of clusters investigating the so-called "backsplash population", i.e. satellite galaxies that once were inside the virial radius of the host but now reside beyond it. We find that this population is significant in number and needs to be appreciated when interpreting empirical galaxy morphology-environmental relationships and decoupling the degeneracy between nature and nurture. Specifically, we find that approximately half of the galaxies with current clustercentric distance in the interval 1 - 2 virial radii of the host are backsplash galaxies which once penetrated deep into the cluster potential, with 90% of these entering to within 50% of the virial radius. These galaxies have undergone significant tidal disruption, losing on average 40% of their mass. This results in a mass function for the backsplash population different to those galaxies infalling for the first time. We further show that these two populations are kinematically distinct and should be observable spectroscopically.
Finally we present a detailed study of the real and integrals-of-motion space distributions of a disrupting satellite obtained from one of our self-consistent high resolution cosmological simulations. The satellite has been re-simulated using various analytical halo potentials and we find that its debris appears as a coherent structure in integrals-of-motion space in all models ("live" and analytical potential) although the distribution is significantly smeared for the live host halo. The primary mechanism for the dispersion is the mass growth of the host. However, when quantitatively comparing the effects of "live" and time-varying host potentials we conclude that not all of the dispersion can be accounted for by the steady growth of the host's mass. We ascribe the remaining differences to additional effects in the "live" halo such as non-sphericity of the host and interactions with other satellites, which have not been modelled analytically.
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High Velocity Clouds and the Milky Way Halo
Christopher Thom, Doctor of Philosophy, Feburary 2006
This thesis presents an exploration of stars and gas in the halo of our Galaxy. A sample of 8321 field horizontal branch (FHB) stars was selected from the Hamburg/ESO Survey. The stars make excellent tracers of the Milky Way halo, and we studied the kinematics of a subset of the HES FHB stars, comparing their velocity dispersions to those predicted by several models. Since these stars are intrinsically luminous, hot and numerous they make ideal probes of the distances to high-velocity clouds (HVCs) - clouds of neutral hydrogen gas whose distances are largely unknown and which do not fit simple models of Galaxy rotation. A catalogue of stars which align with the HVCs was developed. High resolution spectroscopy of 16 such HVC probes with the Magellan telescope has yielded a remarkably tight distance constraint to complex WB. This is one of only a handful of such distance limits so far established. Lower distance limits were set for several other clouds. Finally, we have suggested that some of the HVCs may be associated with the accretion onto the MilkyWay of the Sagittarius dwarf galaxy.
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High-Precision Observations of Relativistic Binary and Millisecond Pulsars
Aidan Hotan, Doctor of Philosophy, July 2006
The technique of pulsar timing reveals a wealth of new information when a preci- sion of ~1µs or better is reached, but such precision is difficult to achieve. This thesis describes a series of very high precision timing observations that improve our knowledge of the targeted pulsar systems. We begin by describing a newly-developed baseband recording and coherent dedispersion system (CPSR2), along with a new object-oriented software development environment for pulsar data processing. Data obtained with this new instrument during a 3 year observing campaign at the Parkes 64 m radio telescope are analysed in a number of novel ways.
The mean profile of PSR J1022+1001 is shown to be stable on timescales of a few minutes, in contrast with previously published claims. We obtain a level of precision an order of magnitude better than any previous timing of this pulsar. In addition, we observe dramatic changes in the mean profile of the relativistic binary pulsar J1141-6545, which broadens by ~50% over the time span of our observations. This is interpreted as evidence for secular evolution of the line of sight to the emission cone, caused by General relativistic geodetic precession which tilts the spin axis of the pulsar. High precision CPSR2 observations of the extraordinary double pulsar binary system are presented and we construct calibrated, mean polarimetric profiles for PSR J0737-3039A, in two frequency bands. These profiles provide a reference against which future profile evolution may be detected, given that we expect geodetic precession to alter the observed mean profile on an even shorter time scale than for PSR J1141-6545.
The bulk of this thesis involves timing a selection of millisecond pulsars whose physical characteristics should allow the highest precision to be obtained. We mea- sure several new proper motions and parallax distances. Shapiro delay is used to constrain the inclination angles and component masses of several of the binary sys- tems in our source list. In addition, subtle periodic variations of the orbital parame- ters of two nearby binary millisecond pulsars are detected and attributed to annual orbital parallax, providing additional constraints on their three-dimensional orbital geometries. Future observations of these two sources may lead to more stringent tests of post-Keplerian gravitational theories. Finally, we use the timing residuals of one very stable source (PSR J1909-3744) as a reference against which we time PSR J1713+0747 with a root-mean-square precision of 133 ns, amongst the best timing residuals ever obtained. This result is an important step in the search for long-period gravitational waves using pulsar timing arrays.
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Spectroscopy of Extra-galactic Globular Clusters
Michael Pierce, Doctor of Philosophy, May 2007
The focus of this thesis is the study of stellar populations of extra-galactic globular clusters (GCs) by measuring spectral indices and comparing them to simple stellar population models. We present the study of GCs in the context of tracing elliptical galaxy star formation, chemical enrichment and mass assembly. In this thesis we set out to test how can be determined about a galaxy's formation history by studying the spectra of a small sample of GCs. Are the stellar population parameters of the GCs strongly linked with the formation history of the host galaxy? We present spectra and Lick index measurements for GCs associated with 3 elliptical galaxies, NGC 1052, NGC 3379 and NGC 4649. We derive ages, metallicities and alpha-element abundance ratios for these GCs using the chi-squared minimisation approach of Proctor & Sansom (2002). The metallicities we derive are quite consistent, for old GCs, with those derived by empirical calibrations such as Brodie & Huchra (1990) and Strader & Brodie (2004). For each galaxy the GCs observed span a large range in metallicity from approximately [Z/H]=--2 to solar.
We find that the majority of GCs are more than 10 Gyrs old and that we cannot distinguish any finer, age details amongst the old GC populations. However, amongst our three samples we find two age distributions contrary to our expectations. From our sample of 16 GCs associated with the 1-2 Gyr old merger remnant NGC 1052, we find no young GCs. If a significant population of GCs formed during this merger we would expect those GCs to have low mass-to-light ratios and be included in our sample of bright GCs. We find 4 young GCs in our sample of 38 around NGC 4649, an old massive cluster elliptical. There are no signs of recent star formation and therefore we do not expect any GCs to have formed within the galaxy. These results seem to indicate that the GC systems of elliptical galaxies are not strongly associated with recent field star formation. We find a correlation between the alpha-element abundance ratio and the metallicity for all three samples. Using Thomas, Maraston & Korn (2004) models, we measure much higher alpha abundance ratios for low metallicity GCs than high metallicity GCs. With current data and models we are limited in both the accuracy and the detail with which we can probe this relationship. We suggest that there are some difficulties reconciling measured GC parameters with our expectations and propose some future work which could help to resolve these and other issues.
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Pulsar Applications of Baseband Recording
Haydon Knight, Doctor of Philosophy, August 2007
In this thesis I report on the development and implementation of baseband recording techniques for searches for radio emission from neutron stars. Ultra-bright pulses are a sporadic type of neutron star emission that have intrinsic widths of less than a microsecond. Propagation through the interstellar medium broadens these radio pulses and consequently reduces their peak flux densities. Pre-detection dedispersion can be performed on data taken with baseband recorders to completely mitigate the dispersive aspect of this broadening. However, such ``coherent'' dedispersion is computationally expensive, and so its application has historically been limited to narrow bands and/or short observation times. This thesis describes the first wide-bandwidth searches of large data sets for ultra-bright pulses using pre-detection dedispersion techniques. The equations that determine the efficiency and sensitivity of such searches are presented, and practical data processing algorithms are discussed. The results of large-scale searches for ultra-bright pulses using the Parkes and Green Bank radio telescopes are also presented. These revealed three new emitters -- PSRs J1823-3021A, J0218+4232, and B1957+20. This increases the number of millisecond pulsars documented to emit ultra-bright pulses from two to five. In addition, millisecond pulsars that emit ultra-bright pulses are confirmed to be rare. The ultra-bright pulses from all of the five millisecond pulsar emitters are analysed in further detail. Three of these pulsars emit pulsed X-rays. The phases of the ultra-bright pulses always correlate in phase with the X-ray pulses, but have variable phase relations to ordinary radio emission. Structure in the emission of PSRs B1937+21 and J1824-2452A is seen at timescales as short as 8 and 20ns, respectively. The ultra-bright pulses of PSR J1824-2452A are found to be highly elliptically polarised. Their position angles vary widely. This may mean that local effects are dominating over the global magnetic field during the emission events. However, I also report the likely detection of an ultra-bright pulse from PSR J1823-3021A that consists of two disjoint bursts. As these must originate from widely separated regions of the pulsar magnetosphere, macroscopic excitations seem to occur during ultra-bright emission events. I also describe searches for ultra-bright pulses and periodic emission from neutron stars in globular clusters. The ultra-bright pulse searches, whilst not revealing any new emitters, place new bounds on the existence of energetic millisecond pulsars. The Fourier-domain searches of globular clusters revealed 42 previously known pulsars. One new pulsar is reported -- PSR J0024-7204Z in 47 Tucanae.
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Isolated Elliptical Galaxies
Fatma Reda, Doctor of Philosophy, Oct 2007
This thesis presents a detailed study of a well defined sample of isolated early-type galaxies. We define a sample of 36 nearby isolated early-type galaxies using a strict isolation criteria. New wide-field optical imaging of 20 isolated galaxies confirms their early-type morphology and relative isolation. We find that the isolated galaxies reveal a colour-magnitude relation similar to cluster ellipticals, which suggests that they formed at a similar epoch to cluster galaxies, such that the bulk of their stars are very old. However, several galaxies of our sample reveal evidence for dust lanes, plumes, shells, boxy and disk isophotes. Thus at least some isolated galaxies have experienced a recent merger/accretion event which may have also induced a small burst of star formation. Using new long-slit spectra of 12 galaxies we found that, isolated galaxies follow similar scaling relations between central stellar population parameters, such as age, metallicity [Z/H] and alpha-element abundance [E/Fe], with galaxy velocity dispersion to their counterparts in high density environments. However, isolated galaxies tend to have slightly younger ages, higher metallicities and lower abundance ratios. Such properties imply an extended star formation history for galaxies in lower density environments. We measure age gradients that anticorrelate with the central galaxy age. Thus as a young starburst evolves, the age gradient flattens from positive to almost zero. Metallicity gradients range from near zero to strongly negative. For our high mass galaxies, metallicity gradients are shallower with increasing galaxy mass. Such behaviour is expected in the remnants of multiple mergers. The metallicity gradients are also found to be correlated with the central age and metallicity, as well as to the age gradients. We generally find flat [E/Fe] gradients. We also examine the Fundamental Plane in both traditional Re, mu_e and sigma space and 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. In conclusion, our results are compatible with an extended merger/accretion history for most isolated elliptical galaxies. However, for those galaxies which show no fine structures nor any young stellar populations, an early formation epoch followed by passive evolution is more probable.
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The Formation of Late-Type Galaxies
Agostino Renda, Doctor of Philosophy, March 2008
Near-field observations may provide tight constraints - i.e. "boundary conditions" - on any model of structure formation in the Universe. Detailed observational data have long been available for the Milky Way (e.g., Freeman and Bland-Hawthorn 2002) and have provided tight constraints on several Galaxy formation models (e.g.: Abadi et al. 2003; Bekki and Chiba 2001). An implicit assumption still remains unanswered though: is the Milky Way a "normal" spiral? Searching for directions, it feels natural to look at our neighbour: Andromeda. An intriguing piece of the "puzzle" is provided by contrasting its stellar halo with that of our Galaxy, even more so since Mouhcine et al. (2005) have suggested that a correlation between stellar halo metallicity and galactic luminosity is in place and would leave the Milky Way halo as an outlier with respect to other spirals of comparable luminosities. Further questions hence arise: is there any stellar halo-galaxy formation symbiosis? Our first step has been to contrast the chemical evolution of the Milky Way with that of Andromeda by means of a semi-analytic model. We have then pursued a complementary approach through the analysis of several semi-cosmological late-type galaxy simulations which sample a wide variety of merging histories. We have focused on the stellar halo properties in the simulations at redshift zero and shown that - at any given galaxy luminosity - the metallicities of the stellar halos in the simulations span a range in excess of ~ 1 dex, a result which is strengthened by the robustness tests we have performed. We suggest that the underlying driver of the halo metallicity dispersion can be traced to the diversity of galactic mass assembly histories inherent within the hierarchical clustering paradigm.
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