arXiv subjects

James T. Allen

Publications and source records attributed to James T. Allen.

12 recordsLinked to original sources

arXiv2016

The SAMI Galaxy Survey: Revisiting Galaxy Classification Through High-Order Stellar Kinematics

Recent cosmological hydrodynamical simulations suggest that integral field spectroscopy can connect the high-order stellar kinematic moments h3 (~skewness) and h4 (~kurtosis) in galaxies to their cosmological assembly history. Here, we assess these results by measuring the stellar kinematics on a sample of 315 galaxies, without a morphological selection, using 2D integral field data from the SAMI Galaxy Survey. A proxy for the spin parameter ($λ_{R_e}$) and ellipticity ($ε_e$) are used to separate fast and slow rotators; there exists a good correspondence to regular and non-regular rotators, respectively, as also seen in earlier studies. We confirm that regular rotators show a strong h3 versus $V/σ$ anti-correlation, whereas quasi-regular and non-regular rotators show a more vertical relation in h3 and $V/σ$. Motivated by recent cosmological simulations, we develop an alternative approach to kinematically classify galaxies from their individual h3 versus $V/σ$ signatures. We identify five classes of high-order stellar kinematic signatures using Gaussian mixture models. Class 1 corresponds to slow rotators, whereas Classes 2-5 correspond to fast rotators. We find that galaxies with similar $λ_{R_e}-ε_e$ values can show distinctly different h3-$V/σ$ signatures. Class 5 objects are previously unidentified fast rotators that show a weak h3 versus $V/σ$ anti-correlation. These objects are predicted to be disk-less galaxies formed by gas-poor mergers. From morphological examination, however, there is evidence for large stellar disks. Instead, Class 5 objects are more likely disturbed galaxies, have counter-rotating bulges, or bars in edge-on galaxies. Finally, we interpret the strong anti-correlation in h3 versus $V/σ$ as evidence for disks in most fast rotators, suggesting a dearth of gas-poor mergers among fast rotators.

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arXiv2016

The SAMI galaxy survey: Galaxy Interactions and Kinematic Anomalies in Abell 119

Galaxy mergers are important events that can determine the fate of a galaxy by changing its morphology, star-formation activity and mass growth. Merger systems have commonly been identified from their disturbed morphologies, and we now can employ Integral Field Spectroscopy to detect and analyze the impact of mergers on stellar kinematics as well. We visually classified galaxy morphology using deep images ($μ_{\rm r} = 28\,\rm mag\,\, arcsec^{-2}$) taken by the Blanco 4-m telescope at the Cerro Tololo Inter-American Observatory. In this paper we investigate 63 bright ($M_{\rm r}<-19.3$) spectroscopically-selected galaxies in Abell 119; of which 53 are early type and 20 galaxies show a disturbed morphology by visual inspection. A misalignment between the major axes in the photometric image and the kinematic map is conspicuous in morphologically-disturbed galaxies. Our sample is dominated by early-type galaxies, yet it shows a surprisingly tight Tully-Fisher relation except for the morphologically-disturbed galaxies which show large deviations. Three out of the eight slow rotators in our sample are morphology disturbed. The visually-selected morphologically-disturbed galaxies are generally more asymmetric, visually as well as kinematically. Our findings suggest that galaxy interactions, including mergers and perhaps fly-bys, play an important role in determining the orientation and magnitude of galaxy's angular momentum.

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arXiv2016

The SAMI Galaxy Survey: extraplanar gas, galactic winds, and their association with star formation history

We investigate a sample of 40 local, main-sequence, edge-on disc galaxies using integral field spectroscopy with the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey to understand the link between properties of the extraplanar gas and their host galaxies. The kinematics properties of the extraplanar gas, including velocity asymmetries and increased dispersion, are used to differentiate galaxies hosting large-scale galactic winds from those dominated by the extended diffuse ionized gas. We find rather that a spectrum of diffuse gas-dominated to wind dominated galaxies exist. The wind-dominated galaxies span a wide range of star formation rates ($-1 \lesssim \log({\rm SFR/M_{\odot} yr^{-1}}) \lesssim 0.5$) across the whole stellar mass range of the sample ($8.5 \lesssim \log({\rm M_{*}/M_{\odot}}) \lesssim 11$). The wind galaxies also span a wide range in SFR surface densities ($10^{-3} \textrm{--} 10^{-1.5}\rm~M_{\odot} ~yr^{-1}~kpc^{-2}$) that is much lower than the canonical threshold of $\rm0.1~M_{\odot} ~yr^{-1}~kpc^{-2}$. The wind galaxies on average have higher SFR surface densities and higher $\rm Hδ_A$ values than those without strong wind signatures. The enhanced $\rm Hδ_A$ indicates that bursts of star formation in the recent past are necessary for driving large-scale galactic winds. We demonstrate with Sloan Digital Sky Survey data that galaxies with high SFR surface density have experienced bursts of star formation in the recent past. Our results imply that the galactic winds revealed in our study are indeed driven by bursts of star formation, and thus probing star formation in the time domain is crucial for finding and understanding galactic winds.

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arXiv2016

Interpreting the Ionization Sequence in Star-Forming Galaxy Emission-Line Spectra

High ionization star forming (SF) galaxies are easily identified with strong emission line techniques such as the BPT diagram, and form an obvious ionization sequence on such diagrams. We use a locally optimally emitting cloud model to fit emission line ratios that constrain the excitation mechanism, spectral energy distribution, abundances and physical conditions along the star-formation ionization sequence. Our analysis takes advantage of the identification of a sample of pure star-forming galaxies, to define the ionization sequence, via mean field independent component analysis. Previous work has suggested that the major parameter controlling the ionization level in SF galaxies is the metallicity. Here we show that the observed SF- sequence could alternatively be interpreted primarily as a sequence in the distribution of the ionizing flux incident on gas spread throughout a galaxy. Metallicity variations remain necessary to model the SF-sequence, however, our best models indicate that galaxies with the highest and lowest observed ionization levels (outside the range -0.37 < log [O III]/H\b{eta} < -0.09) require the variation of an additional physical parameter other than metallicity, which we determine to be the distribution of ionizing flux in the galaxy.

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arXiv2015

SDSS J1138+3517: A quasar showing remarkably variable broad absorption lines

We report on the highly variable SiIV and CIV broad absorption lines in SDSS J113831.4+351725.2 across four observational epochs. Using the SiIV doublet components, we find that the blue component is usually saturated and non-black, with the ratio of optical depths between the two components rarely being 2:1. This indicates that these absorbers do not fully cover the line-of-sight and thus a simple apparent optical depth model is insufficient when measuring the true opacity of the absorbers. Tests with inhomogeneous (power-law) and pure-partial coverage (step-function) models of the absorbing SiIV optical depth predict the most un-blended doublet's component profiles equally well. However, when testing with Gaussian-fitted doublet components to all SiIV absorbers and averaging the total absorption predicted in each doublet, the upper limit of the power law index is mostly unconstrained. This leads us to favour pure partial coverage as a more accurate measure of the true optical depth than the inhomogeneous power law model. The pure-partial coverage model indicates no significant change in covering fraction across the epochs, with changes in the incident ionizing flux on the absorbing gas instead being favoured as the variability mechanism. This is supported by (a) the coordinated behaviour of the absorption troughs, (b) the behaviour of the continuum at the blue end of the spectrum and (c) the consistency of photoionization simulations of ionic column density dependencies on ionization parameter with the observed variations. Evidence from the simulations together with the CIV absorption profile indicates that the absorber lies outside the broad line region, though the precise distance and kinetic luminosity are not well constrained.

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arXiv2014

Line-driven radiative outflows in luminous quasars

An analysis of ~19500 narrow (<200 km/s) CIV 1548.2,1550.8 absorbers in ~34000 Sloan Digital Sky Survey quasar spectra is presented. The statistics of the number of absorbers as a function of outflow-velocity shows that in approximately two-thirds of outflows, with multiple CIV absorbers present, absorbers are line-locked at the 500 km/s velocity separation of the CIV absorber doublet; appearing as 'triplets' in the quasar spectra. Line-locking is an observational signature of radiative line driving in outflowing material, where the successive shielding of 'clouds' of material in the outflow locks the clouds together in outflow velocity. Line-locked absorbers are seen in both broad absorption line quasars (BALs) and non-BAL quasars with comparable frequencies and with velocities out to at least 20000 km/s. There are no detectable differences in the absorber properties and the dust content of single CIV doublets and line-locked CIV doublets. The gas associated with both single and line-locked CIV absorption systems includes material with a wide range of ionization potential (14-138 eV). Both single and line-locked CIV absorber systems show strong systematic trends in their ionization as a function of outflow velocity, with ionization decreasing rapidly with increasing outflow velocity. Initial simulations, employing CLOUDY, demonstrate that a rich spectrum of line-locked signals at various velocities may be expected due to significant opacities from resonance lines of Li-, He- and H-like ions of O, C and N, along with contributions from HeII and HI resonance lines. The simulations confirm that line driving can be the dominant acceleration mechanism for clouds with N(HI) ~ 10^19 cm^-2.

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arXiv2014

Quasar broad absorption line variability measurements using reconstructions of un-absorbed spectra

We present a two-epoch Sloan Digital Sky Survey and Gemini/GMOS+William Herschel Telescope/ISIS variability study of 50 broad absorption line quasars of redshift range 1.9 < z < 4.2, containing 38 Si IV and 59 C IV BALs and spanning rest-frame time intervals of approximately 10 months to 3.7 years. We find that 35/50 quasars exhibit one or more variable BALs, with 58% of Si IV and 46% of C IV BALs showing variability across the entire sample. On average, Si IV BALs show larger fractional change in BAL pseudo equivalent width than C IV BALs, as referenced to an unabsorbed continuum+emission-line spectrum constructed using non-negative matrix factorisation. No correlation is found between BAL variability and quasar luminosity, suggesting that ionizing continuum changes do not play a significant role in BAL variability (assuming the gas is in photoionization equilibrium with the ionizing continuum). A subset of 14 quasars have one variable BAL from each of Si IV and C IV with significant overlap in velocity space and for which variations are in the same sense (strengthening or weakening) and which appear to be correlated (98% confidence). We find examples of both appearing and disappearing BALs in weaker/shallower lines with disappearance rates of 2.3% for C IV and 5.3% for Si IV, suggesting average lifetimes of 142 and 43 years respectively. We identify 5 objects in which the BAL is coincident with the broad emission-line, but appears to cover only the continuum source. Assuming a clumpy inhomogeneous absorber model and a typical size for the continuum source, we infer a maximum cloud radius of 10^13 to 10^14 cm, assuming Eddington limited accretion.

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arXiv2013

Interpreting the Ionization Sequence in AGN Emission-Line Spectra

We investigate the physical cause of the great range in the ionization level seen in the spectra of narrow lined active galactic nuclei (AGN). Mean field independent component analysis identifies examples of individual SDSS galaxies whose spectra are not dominated by emission due to star formation (SF), which we designate as AGN. We assembled high S/N ratio composite spectra of a sequence of these AGN defined by the ionization level of their narrow-line regions (NLR), extending down to very low-ionization cases. We used a local optimally emitting cloud (LOC) model to fit emission-line ratios in this AGN sequence. These included the weak lines that can be measured only in the co-added spectra, providing consistency checks on strong line diagnostics. After integrating over a wide range of radii and densities our models indicate that the radial extent of the NLR is the major parameter in determining the position of high to moderate ionization AGN along our sequence, providing a physical interpretation for their systematic variation. Higher ionization AGN contain optimally emitting clouds that are more concentrated towards the central continuum source than in lower ionization AGN. Our LOC models indicate that for the objects that lie on our AGN sequence, the ionizing luminosity is anticorrelated with the NLR ionization level, and hence anticorrelated with the radial concentration and physical extent of the NLR. A possible interpretation that deserves further exploration is that the ionization sequence might be an age sequence where low ionization objects are older and have systematically cleared out their central regions by radiation pressure. We consider that our AGN sequence instead represents a mixing curve of SF and AGN spectra, but argue that while many galaxies do have this type of composite spectra, our AGN sequence appears to be a special set of objects with negligible SF excitation.

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arXiv2013

Classification and analysis of emission-line galaxies using mean field independent component analysis

We present an analysis of the optical spectra of narrow emission-line galaxies, based on mean field independent component analysis (MFICA). Samples of galaxies were drawn from the Sloan Digital Sky Survey (SDSS) and used to generate compact sets of `continuum' and `emission-line' component spectra. These components can be linearly combined to reconstruct the observed spectra of a wider sample of galaxies. Only 10 components - five continuum and five emission line - are required to produce accurate reconstructions of essentially all narrow emission-line galaxies; the median absolute deviations of the reconstructed emission-line fluxes, given the signal-to-noise ratio (S/N) of the observed spectra, are 1.2-1.8 sigma for the strong lines. After applying the MFICA components to a large sample of SDSS galaxies we identify the regions of parameter space that correspond to pure star formation and pure active galactic nucleus (AGN) emission-line spectra, and produce high S/N reconstructions of these spectra. The physical properties of the pure star formation and pure AGN spectra are investigated by means of a series of photoionization models, exploiting the faint emission lines that can be measured in the reconstructions. We are able to recreate the emission line strengths of the most extreme AGN case by assuming the central engine illuminates a large number of individual clouds with radial distance and density distributions, f(r) ~ r^gamma and g(n) ~ n^beta, respectively. The best fit is obtained with gamma = -0.75 and beta = -1.4. From the reconstructed star formation spectra we are able to estimate the starburst ages. These preliminary investigations serve to demonstrate the success of the MFICA-based technique in identifying distinct emission sources, and its potential as a tool for the detailed analysis of the physical properties of galaxies in large-scale surveys.

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arXiv2012

First Science with SAMI: A Serendipitously Discovered Galactic Wind in ESO 185-G031

We present the first scientific results from the Sydney-AAO Multi-Object IFS (SAMI) at the Anglo-Australian Telescope. This unique instrument deploys 13 fused fibre bundles (hexabundles) across a one-degree field of view allowing simultaneous spatially-resolved spectroscopy of 13 galaxies. During the first SAMI commissioning run, targeting a single galaxy field, one object (ESO 185-G031) was found to have extended minor axis emission with ionisation and kinematic properties consistent with a large-scale galactic wind. The importance of this result is two-fold: (i) fibre bundle spectrographs are able to identify low-surface brightness emission arising from extranuclear activity; (ii) such activity may be more common than presently assumed because conventional multi-object spectrographs use single-aperture fibres and spectra from these are nearly always dominated by nuclear emission. These early results demonstrate the extraordinary potential of multi-object hexabundle spectroscopy in future galaxy surveys.

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arXiv2010

A strong redshift dependence of the broad absorption line quasar fraction

We describe the application of non-negative matrix factorisation to generate compact reconstructions of quasar spectra from the Sloan Digital Sky Survey (SDSS), with particular reference to broad absorption line quasars (BALQSOs). BAL properties are measured for SiIV lambda1400, CIV lambda1550, AlIII lambda1860 and MgII lambda2800, resulting in a catalogue of 3547 BALQSOs. Two corrections, based on extensive testing of synthetic BALQSO spectra, are applied in order to estimate the intrinsic fraction of CIV BALQSOs. First, the probability of an observed BALQSO spectrum being identified as such by our algorithm is calculated as a function of redshift, signal-to-noise ratio and BAL properties. Second, the different completenesses of the SDSS target selection algorithm for BALQSOs and non-BAL quasars are quantified. Accounting for these selection effects the intrinsic CIV BALQSO fraction is 41+/-5 per cent. Our analysis of the selection effects allows us to measure the dependence of the intrinsic CIV BALQSO fraction on luminosity and redshift. We find a factor of 3.5+/-0.4 decrease in the intrinsic fraction from the highest redshifts, z~4.0, down to z~2.0. The redshift dependence implies that an orientation effect alone is not sufficient to explain the presence of BAL troughs in some but not all quasar spectra. Our results are consistent with the intrinsic BALQSO fraction having no strong luminosity dependence, although with 3-sigma limits on the rate of change of the intrinsic fraction with luminosity of -6.9 and 7.0 per cent dex^-1 we are unable to rule out such a dependence.

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arXiv2008

Finding SDSS BALQSOs Using Non-Negative Matrix Factorisation

Modern spectroscopic databases provide a wealth of information about the physical processes and environments associated with astrophysical populations. Techniques such as blind source separation (BSS), in which sets of spectra are decomposed into a number of components, offer the prospect of identifying the signatures of the underlying physical emission processes. Principle Component Analysis (PCA) has been applied with some success but is severely limited by the inherent orthogonality restriction that the components must satisfy. Non-negative matrix factorisation (NMF) is a relatively new BSS technique that incorporates a non-negativity constraint on its components. In this respect, the resulting components may more closely reflect the physical emission signatures than is the case using PCA. We discuss some of the considerations that must be made when applying NMF and, through its application to the quasar spectra in the Sloan Digital Sky Survey (SDSS) DR6, we show that NMF is a fast method for generating compact and accurate reconstructions of the spectra. The ability to reconstruct spectra accurately has numerous astrophysical applications. Combined with improved SDSS redshifts, we apply NMF to the problem of defining robust continua for quasars that exhibit strong broad absorption line (BAL) systems. The resulting catalogue of SDSS DR6 BAL quasars will be the largest available. Importantly, the NMF approach allows quantitative error estimates to be derived for the Balnicity Indices as a function of key astrophysical and observational parameters, such as the quasar redshifts and the signal-to-noise ratio of the spectra.

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