S. Ellis

Galaxy and Mass Assembly (GAMA): survey diagnostics and core data release

The Galaxy and Mass Assembly (GAMA) survey has been operating since 2008 February on the 3.9-m Anglo-Australian Telescope using the AAOmega fibre-fed spectrograph facility to acquire spectra with a resolution of R ≈ 1300 for 120 862 Sloan Digital Sky Survey selected galaxies. The target catalogue constitutes three contiguous equatorial regions centred at 9h (G09), 12h (G12) and 14.5h (G15) each of 12 × 4 deg2 to limiting fluxes of rpet < 19.4, rpet < 19.8 and rpet <19.4 mag, respectively (and additional limits at other wavelengths). Spectra and reliable redshifts have been acquired for over 98 per cent of the galaxies within these limits. Here we present the survey footprint, progression, data reduction, redshifting, re-redshifting, an assessment of data quality after 3 yr, additional image analysis products (including ugrizYJHK photometry, S´ersic profiles and photometric redshifts), observing mask and construction of our core survey catalogue (GamaCore). From this we create three science-ready catalogues: GamaCoreDR1 for public release, which includes data acquired during year 1 of operations within specified magnitude limits (2008 February to April); GamaCoreMainSurvey containing all data above our survey limits for use by the GAMA Team and collaborators; and GamaCore-AtlasSV containing year 1, 2 and 3 data matched to Herschel-ATLAS science demonstration data. These catalogues along with the associated spectra, stamps and profiles can be accessed via the GAMA website: http://www.gama-survey.org/

The Millennium Galaxy Catalogue: morphological classification and bimodality in the colour–concentration plane

Using 10095 galaxies (B < 20 mag) from the Millennium Galaxy Catalogue, we derive B-band luminosity distributions and selected bivariate brightness distributions for the galaxy population subdivided by eyeball morphology; Sersic index (n); two-degree Field Galaxy Redshift Survey (2dFGRS) η parameter; rest-(u - r) colour (global and core); MGC continuum shape; half-light radius; (extrapolated) central surface brightness; and inferred stellar mass-to-light ratio. All subdivisions extract highly correlated subsets of the galaxy population which consistently point towards two overlapping distributions: an old, red, inert, predominantly luminous, high central-surface brightness subset; and a young, blue, star forming, intermediate surface brightness subset. A clear bimodality in the observed distribution is seen in both the rest-(u - r) colour and log (n) distributions. Whilst the former bimodality was well established from Sloan Digital Sky Survey data, we show here that the rest-(u - r) colour bimodality becomes more pronounced when using the core colour as opposed to global colour. The two populations are extremely well separated in the colour-log(n) plane. Using our sample of 3314 (B < 19 mag) eyeball classified galaxies, we show that the bulge-dominated, early-type galaxies populate one peak and the bulge-less, late-type galaxies occupy the second. The early- and mid-type spirals sprawl across and between the peaks. This constitutes extremely strong evidence that the fundamental way to divide the luminous galaxy population (M BMGC -5 log h < -16 mag, i.e. dwarfs not included) is into bulges (old red, inert, high concentration) and discs (young, blue, star forming, low concentration) and that the galaxy bimodality reflects the two-component nature of galaxies and not two distinct galaxy classes. We argue that these two components require two independent formation mechanisms/processes and advocate early bulge formation through initial collapse and ongoing disc formation through splashback, infall and merging/accretion. We calculate the B-band luminosity densities and stellar mass densities within each subdivision and estimate that the z ≈ 0 stellar mass content in spheroids, bulges and discs is 35 ± 2, 18 ± 7 and 47 ± 7 per cent, respectively.

The case for OH suppression at near-infrared wavelengths

We calculate the advances in near-infrared (NIR) astronomy made possible through the use of fibre Bragg gratings to selectively remove hydroxyl emission lines from the night sky spectrum. Fibre Bragg gratings should remove OH lines at high resolution (R = 10 000), with high suppression (30 dB) whilst maintaining high throughput (≈90 per cent) between the lines. Devices presently under construction should remove 150 lines in each of the J and H bands, effectively making the night sky surface brightness ≈4 mag fainter. This background reduction is greater than the improvement adapative optics makes over natural seeing; photonic OH suppression is at least as important as adaptive optics for the future of cosmology. We present a model of the NIR sky spectrum, and show that the interline continuum is very faint (≈80 photons s −1 m −2 arcsec −2 μm −1 on the ecliptic plane). We show that OH suppression by high dispersion, that is, ‘resolving out’ the skylines, cannot obtain the required level of sensitivity to reach the interline continuum due to scattering of light. The OH lines must be suppressed prior to dispersion. We have simulated observations employing fibre Bragg gratings of first light objects, highredshift galaxies and cool, low-mass stars. The simulations are of complete end-to-end systems from object to detector. The results demonstrate that fibre Bragg grating OH suppression will significantly advance our knowledge in many areas of astrophysics, and in particular will enable rest-frame ultraviolet observations of the Universe at the time of first light and reionization.

The Millennium Galaxy Catalogue: on the natural subdivision of galaxies

The distribution of global photometric, spectroscopic, structural and morphological parame- ters for a well-defined sample of 350 nearby galaxies has been examined. The usual trends were recovered demonstrating that E/S0 galaxies are redder, more quiescent, more centrally concentrated and possess larger Sersic indices than late-type galaxies. Multivariate statistical analyses were performed to examine the distribution of all parameters simultaneously. The main result of these analyses was the existence of only two classes of galaxies, corresponding closely to early and late types. Linear discriminant analysis (LDA) was able to reproduce the classifications of early- and late-type galaxies with high success, but further refinement of galaxy types was not reproduced in the distribution of observed galaxy properties. A principal components analysis (PCA) showed that the major variance of the parameter set corresponded to a distinction between early and late types, highlighting the importance of the distinction. A hierarchical clustering analysis (HCA) revealed only two clear natural classes within the parameter set, closely corresponding to early and late types. Early and late types are clearly distinct and the distinction is of fundamental importance. In contrast, late types from Sa to Irr are smoothly distributed throughout the parameter space. A population of galaxies classified by eye as elliptical/lenticular, and exhibiting concentration indices similar to early types were found to have a significant star formation activity. These galaxies are preferentially faint, suggesting they are low-mass systems. Ke yw ords: galaxies: evolution - galaxies: fundamental parameters.

GalaxyCount: a java calculator of galaxy counts and variances in multiband wide-field surveys to 28 AB mag

We provide a consistent framework for estimating galaxy counts and variances in wide-field images for a range of photometric bands. The variances include both Poissonian noise and variations due to large-scale structure. We demonstrate that our statistical theory is consistent with the counts in the deepest multiband surveys available. The statistical estimates depend on several observational parameters (e.g. seeing, signal-to-noise ratio), and include a sophisticated treatment of detection completeness. The JAVA calculator is freely available 1 and offers the user the option to adopt our consistent framework or a different scheme. We also provide a summary table of statistical measures in the different bands for a range of different fields of view. Reliable estimation of the background counts has profound consequences in many areas of observational astronomy. We provide two such examples. One is from a recent study of the Sculptor galaxy NGC 300 where stellar photometry has been used to demonstrate that the outer disc extends to 10 effective radii, far beyond what was thought possible for a normal low-luminosity spiral. We confirm this finding by a re-analysis of the background counts. Secondly, we determine the luminosity function of the galaxy cluster Abell 2734, both through spectroscopically determined cluster membership, and through statistical subtraction of the background galaxies using the calculator and offset fields. We demonstrate very good agreement, suggesting that expensive spectroscopic follow-up, or off-source observations, may often be bypassed via determination of the galaxy background with GALAXYCOUNT.

The jet/wind outflow in Centaurus A: a local laboratory for AGN feedback

We present new radio and optical images of the nearest radio galaxy Centaurus A and its host galaxy NGC 5128. We focus our investigation on the northern transition region, where energy is transported from the similar to 5 kpc (similar to 5 arcmin) scales of the northern inner lobe (NIL) to the similar to 30 kpc (similar to 30 arcmin) scales of the northern middle lobe (NML). Our Murchison Widefield Array observations at 154 MHz and our Parkes radio telescope observations at 2.3 GHz show diffuse radio emission connecting the NIL to the NML, in agreement with previous Australia Telescope Compact Array observations at 1.4 GHz. Comparison of these radio data with our wide-field optical emission-line images show the relationship between the NML radio emission and the ionized filaments that extend north from the NIL, and reveal a new ionized filament to the east, possibly associated with a galactic wind. Our deep optical images show clear evidence for a bipolar outflow from the central galaxy extending to intermediate scales, despite the non-detection of a southern radio counterpart to the NML. Thus, our observational overview of Centaurus A reveals a number of features proposed to be associated with active galactic nucleus feedback mechanisms, often cited as likely to have significant effects in galaxy evolution models. As one of the closest galaxies to us, Centaurus A therefore provides a unique laboratory to examine feedback mechanisms in detail.

Possibility of observable signatures of leptonium from astrophysical sources

The formation of Ps in our Galaxy is well measured, and has led to important and unanswered questions on the origin of the positrons. In principle it should be possible to form analogous systems from mu and tau leptons, viz. true muonium and true tauonium. However the probability of formation for these systems is greatly reduced due to the intrinsically short lifetimes of the mu and tau leptons. Likewise, the decay of the atoms is hastened by the high probability of the constituent particles decaying. Nevertheless, if sufficient numbers of mu and tau pairs are produced in high energy astrophysical environments there may be significant production of true muonium and true tauonium, despite the small probabilities. This paper addresses this possibility. We have calculated the pair production spectra of mu and tau leptons from photon-photon annihilation and electron-positron annihilation in astrophysical environments. We have computed the cross-sections for radiative recombination and direct annihilation of the pairs, and the decay constants for the various allowable decays, and the wavelengths and energies of the recombination and annihilation signatures. In this way we have calculated the probabilities for the formation of true muonium and true tauonium, and the branching ratios for the various observable signatures. We have estimated the expected fluxes from accretion discs around microquasars and active galactic nuclei, and from interactions of jets with clouds and stars. We find that accretion discs around stellar mass black holes in our own Galaxy should have observable signatures at X-ray and gamma-ray energies that are in principle observable with current observatories.

Photonic ring resonator filters for astronomical OH suppression

Ring resonators provide a means of filtering specific wavelengths from a waveguide, and optionally dropping the filtered wavelengths into a second waveguide. Both of these features are potentially useful for astronomical instruments. In this paper we focus on their use as notch filters to remove the signal from atmospheric OH emission lines from astronomical spectra. We derive the design requirements for ring resonators for OH suppression from theory and finite difference time domain simulations. We find that rings with small radii (< 10 μm) are required to provide an adequate free spectral range, leading to high index contrast materials such as Si and Si3N4. Critically coupled rings with high self-coupling coefficients should provide the necessary Q factors, suppression depth, and throughput for efficient OH suppression, but will require post-inscription tuning of the coupling and the resonant wavelengths. The overall prospects for the use of ring resonators in astronomical instruments is promising, provided efficient fibre-chip coupling can be achieved.